Instrument Engineers' Handbook, Volume 1, Fourth Edition: Process Measurement and Analysis

INSTRUMENT ENGINEERS' HANDBOOK Fourth Edition Process Measurement and Analysis VOLUME I Bela G. Liptak EDITOR-IN-CHIE...

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INSTRUMENT ENGINEERS' HANDBOOK Fourth Edition

Process Measurement and Analysis VOLUME I

Bela G. Liptak EDITOR-IN-CHIEF

ISA-The Instrumentation, Systems, and Automation Society f isn

CRC PRESS Boca Raton London New York Washington, D.C.

This reference text is published in cooperation with ISA Press, the publishing division of ISA—Instrumentation, Systems, and Automation Society. ISA is an international, nonproÞt, technical organization that fosters advancement in the theory, design, manufacture, and use of sensors, instruments, computers, and systems for measurement and control in a wide variety of applications. For more information, visit www.isa.org or call (919) 5498411.

Library of Congress Cataloging-in-Publication Data Instrument engineers’ handbook / Béla G. Lipták, editor-in-chief. p. cm. Rev. ed. of: Instrument engineers’ handbook. Process measurement and analysis. c1995 and Instrument engineers’ handbook. Process control. c1995. Includes bibliographical references and index. Contents: v. 1. Process measurement and analysis. ISBN 0-8493-1083-0 (v. 1) 1. Process control—Handbooks, manuals, etc. 2. Measuring instruments—Handbooks, manuals, etc. I. Lipták, Béla G. II. Instrument engineers’ handbook. Process measurement and analysis. TS156.8 .I56 2003 629.8—dc21

2003048453

his book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microÞlming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of speciÞc clients, may be granted by CRC Press LLC, provided that $1.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1083-0 (v. 1)/03/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. SpeciÞc permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identiÞcation and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2003 by Béla Lipták No claim to original U.S. Government works International Standard Book Number 0-8493-1083-0 (v. 1) Library of Congress Card Number 2003048453 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

© 2003 by Béla Lipták

Dedicated to you, my colleagues, the instrument and process control engineers. I hope that by applying the knowledge found on these pages you will make our industries more efficient, safer, and cleaner, and thereby will not only contribute to a happier future for all mankind but will also advance the recognition and respectability of our profession.


CONTENTS

Contributors xiii Introduction xxi Definitions xxvii Abbreviations, Nomenclature, Acronyms, and Symbols Societies and Organizations li

1

General Considerations 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12

2

1

Flowsheet Symbols and P&I Diagrams 4 Functional Diagrams and Function Symbols 31 Instrument Terminology and Performance 46 System Accuracy 78 Uncertainty Calculations 86 Configuring Intelligent Devices 93 Instrument Installation 100 Instrument Calibration 108 Response Time and Drift Testing 114 Redundant and Voting Systems 126 Instrument Evaluation 136 Binary Logic Diagrams 142

Flow Measurement 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17

xxxix

151

Application and Selection 156 Anemometers 173 BTU Flowmeters for Heat Exchangers 177 BTU Flowmeters for Gaseous Fuels 180 Cross-Correlation Flow Metering 183 Elbow Taps 189 Flow Switches 193 Jet Deflection Flow Detectors 198 Laminar Flowmeters 201 Magnetic Flowmeters 208 Mass Flowmeters, Coriolis 225 Mass Flowmeters—Miscellaneous 237 Mass Flowmeters—Thermal 244 Metering Pumps 251 Orifices 259 Pitot Tubes and Area Averaging Units 277 Polyphase (Oil/Water/Gas) Flowmeters 287 vii


viii

Contents

2.18 2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26 2.27 2.28 2.29 2.30 2.31

3

Level Measurement 401 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21

4

Application and Selection 405 Bubblers 421 Capacitance and Radio Frequency (RF) Admittance 430 Conductivity and Field-Effect Level Switches 445 Diaphragm Level Detectors 449 Differential Pressure Level Detectors 454 Displacer Level Devices 465 Float Level Devices 474 Laser Level Sensors 482 Level Gauges, Including Magnetic 486 Microwave Level Switches 497 Optical Level Devices 500 Radar, Noncontacting Level Sensors 504 Radar, Contact Level Sensors (TDR, GWR, PDS) 508 Radiation Level Sensors 514 Resistance Tapes 526 Rotating Paddle Switches 530 Tank Gauges Including Float-Type Tape Gauges 533 Thermal Level Sensors 544 Ultrasonic Level Detectors 548 Vibrating Level Switches 556

Temperature Measurement 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13


Positive-Displacement Gas Flowmeters 294 Positive-Displacement Liquid Meters and Provers 299 Purge Flow Regulators 307 Segmental Wedge Flowmeter 310 Sight Flow Indicators 313 Solids Flowmeters and Feeders 318 Target Meters 335 Turbine and Other Rotary Element Flowmeters 337 Ultrasonic Flowmeters 357 Variable-Area, Gap, and Vane Flowmeters 362 V-Cone Flowmeter 371 Venturi Tubes, Flow Tubes, and Flow Nozzles 374 Vortex and Fluidic Flowmeters 384 Weirs and Flumes 395

561

Application and Selection 565 Bimetallic Thermometers 590 Calibrators and Simulators 594 Cones, Crayons, Labels, Paints, and Pellets 599 Fiber-Optic Thermometers 604 Filled-Bulb and Glass-Stem Thermometers 610 Integrated Circuitry Transistors and Diodes 620 Miscellaneous and Discontinued Sensors 623 Radiation and Infrared Pyrometers 630 Resistance Temperature Detectors 645 Temperature Switches and Thermostats 657 Thermistors 666 Thermocouples 673

Contents

4.14 4.15

5

Pressure Measurement 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14

6

705

709

Selection and Application 712 Accessories (Seals, Snubbers, Calibrators, Manifolds) Bellows-Type Pressure Sensors 726 Bourdon and Helical Pressure Sensors 731 Diaphragm or Capsule-Type Sensors 736 Differential Pressure Instruments 743 Electronic Pressure Sensors 751 High-Pressure Sensors 762 Manometers 766 Multiple Pressure Scanners 774 Pressure Gauges 779 Pressure Repeaters 785 Pressure and Differential Pressure Switches 790 Vacuum Sensors 795

Density Measurement 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10

7

Thermowells 697 Ultrasonic and Sonic Thermometers

807

Density: Applications and Selection 809 Displacement- and Float-Type Densitometers 816 Hydrometers 823 Hydrostatic Densitometers 826 Oscillating Coriolis Densitometer (Gas, Liquid, and Slurry Services) Radiation Densitometers 836 Ultrasonic Sludge and Slurry Densitometers 841 Liquid/Slurry/Gas Density—Vibrating Densitometers 844 Weight-Based and Miscellaneous Densitometers 852 Gas Densitometers 857

Safety and Miscellaneous Sensors 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19 7.20 7.21 7.22


718

865

Boroscopes 872 Electrical and Intrinsic Safety 875 Electrical Meters and Sensors 889 Energy Management Devices (Peak Load Shedding) 903 Excess Flow and Regular Check Valves 908 Explosion Suppression and Deluge Systems 912 Flame Arresters, Conservation Vents, and Emergency Vents Flame, Fire, and Smoke Detectors 928 Leak Detectors 936 Linear and Angular Position Detection 944 Machine Vision Technology 951 Metal Detectors 955 Noise Sensors 958 Proximity Sensors and Limit Switches 964 Relief Valves—Determination of Required Capacity 973 Relief Valves—Sizing, Specification, and Installation 991 Rupture Discs 1018 Soft Sensors 1030 Tachometers and Angular Speed Detectors 1038 Thickness and Dimension Measurement 1045 Torque and Force Transducers 1051 Vibration, Shock, and Acceleration 1061

920

831

ix

x

Contents

7.23 7.24 7.25

8

Analytical Instrumentation 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.18 8.19 8.20 8.21 8.22 8.23 8.24 8.25 8.26 8.27 8.28 8.29 8.30 8.31 8.32 8.33 8.34 8.35 8.36 8.37 8.38 8.39 8.40 8.41 8.42 8.43 8.44 8.45 8.46 8.47 8.48 8.49


Weather Stations 1077 Weighing Systems: General Considerations Weight Sensors 1101

1084

1127

Analyzer Application and Selection 1144 Analyzer Sampling: Process Samples 1170 Analyzer Sampling: Stack Particulates 1189 Analyzers Operating on Electrochemical Principles 1198 Air Quality Monitoring 1207 Biometers 1222 Biological Oxygen Demand, Chemical Oxygen Demand, and Total Oxygen Demand 1224 Calorimeters 1235 Carbon Dioxide 1242 Carbon Monoxide 1245 Chlorine 1251 Chromatographs: Gas 1258 Chromatographs: Liquid 1289 Coal Analyzers 1295 Colorimeters 1299 Combustibles 1304 Conductivity Analyzers 1316 Consistency Analyzers 1323 Corrosion Monitoring 1329 Differential Vapor Pressure Sensor 1335 Dioxin Analysis 1339 Elemental Monitors 1342 Fiber-Optic Probes 1347 Fluoride Analyzers 1353 Hydrocarbon Analyzers 1358 Hydrogen Sulfide 1364 Infrared and Near-Infrared Analyzers 1369 Ion-Selective Electrodes 1388 Mass Spectrometers 1399 Mercury in Ambient Air 1407 Mercury in Water 1413 Moisture in Air: Humidity and Dew Point 1420 Moisture in Gases and Liquids 1434 Moisture in Solids 1450 Molecular Weight 1457 Nitrate, Ammonia, and Total Nitrogen 1469 Nitrogen Oxide Analyzers 1474 Odor Detection 1480 Oil in or on Water 1486 Open Path Spectrophotometry (UV, IR, FT-IR) 1493 Oxidation-Reduction Potential (ORP) 1506 Oxygen in Gases 1514 Oxygen in Liquids (Dissolved Oxygen) 1526 Ozone in Gas 1536 Ozone in Water 1540 Particulates, Opacity, Dust, and Smoke 1544 Particle Size and Distribution Monitors 1559 pH Measurement 1565 Phosphorus Analyzer 1585

Contents

8.50 8.51 8.52 8.53 8.54 8.55 8.56 8.57 8.58 8.59 8.60 8.61 8.62 8.63 8.64 8.65 8.66

Physical Properties Analyzers—ASTM Methods 1589 Raman Analyzers 1606 Refractometers 1620 Rheometers 1628 Streaming Current or Particle Charge Analyzer 1637 Sulfur-in-Oil Analyzers 1641 Sulfur Oxide Analyzers 1646 Thermal Conductivity Detectors 1653 Total Carbon Analyzers 1658 Toxic Gas Monitoring 1666 Turbidity, Sludge, and Suspended Solids 1680 Ultraviolet and Visible Analyzers 1687 Viscometers—Application and Selection 1700 Viscometers—Laboratory 1708 Viscometers—Industrial 1723 Water Quality Monitoring 1744 Wet Chemistry and Autotitrator Analyzers 1755

Appendix A.1 A.2 A.3 A.4 A.5 A.6 A.7 A.8


1765

International System of Units 1767 Engineering Conversion Factors 1777 Chemical Resistance of Materials 1799 Composition of Metallic and Other Materials Steam and Water Tables 1809 Friction Loss in Pipes 1817 Tank Volumes 1821 Directory of “Lost” Companies 1824

1806

xi

CONTRIBUTORS

The names of the authors of each edition are given at the beginning of each section. Here, all the contributors of all editions of this volume are listed in alphabetical order, showing their academic degrees, titles, and positions they held at the time of making their contributions. The authors who have participated in the preparation in this fourth edition of the Instrument Engineers’ Handbook (IEH) are noted by an asterisk (*) in front of their names, but, because they built on the work of the authors of the previous editions, all authors are listed.

*BUD ADLER

BSEE; Life Member ISA; Director, Business Development, Moore Industries-International, Inc.

ROSS C. AHLSTROM, JR.

BSCh and Math, Executive Vice President, Mentech Inc.

ARTHUR ALSTON

BS, PE, Senior Research Engineer, Chevron Research Co.

MARTIN ANKLIN

PhD, Research Scientist, Endress + Hauser, Switzerland

*RAYMOND ANNINO

PhD, retired Professor and Researcher, formerly with The Foxboro Co.

CATHY APPLE

BSChE, Project Engineer, Micro Motion Inc.

*JAMES B. ARANT

BSChE, PE retired Senior Consultant, formerly with E.I. du Pont de Nemours Co.

*TIBOR BAAN

BME, CEO of Aalborg Instrument and Controls Inc.

ALLAN T. BACON, JR.

BACh, Staff Engineer, Environmental Technologies Group

*STEVEN BAIN

BscEE, PEng, Canada

WENDALL M. BARROWS

Senior Applications Coordinator, Union Carbide Corp.

JAN BARTH

EE, MS, Manager, Industrial Instrument Users Association, The Netherlands

*ERNEST H. BAUGHMAN

PhD, Assistant Professor, University of La Verne, California

JONAS BERGE

Engineer, Smar, Singapore

A. C. BLAKE

EE, Manager, Industrial Instrument Div., Cambridge Instrument Co.

CHRISTOPHER P. BLAKELEY

BSChE, Marketing Manager, Water Treatment, Honeywell Inc.

xiii © 2003 by Béla Lipták

xiv

Contributors

*L JOSEPH BOLLYKY

PhD, PE, President, Bollyky Associates

R. V. BOYD, JR.

BSEE, MSEE, PE, Engineering Supervisor, Saudi Aramco

*WALT BOYES

Principal, Marketing Practice Consultants

AUGUST BRODGESELL

BSEE, President, CRB Systems Inc.

JAMES E. BROWN

BSME, PE, Manager of Engineering, Union Carbide Corp.

THOMAS M. CARDIS

MSCh, Laboratory Manager, ABB Process Analytics

*BOYCE CARSELLA, JR.

BA, Senior Product Manager, Magnetrol International

THOMAS J. CLAGGETT

BSEE, Application Specialist, Honeywell, Inc.

WILSON A. CLAYTON

BSChE, MSME, Chief Engineer, Hy-Cal Engineering

GERALD L. COMBS

PhDCh, Research Chemist, Applied Automation/Hartmann & Braun

VINCENT B. CORTINA

BSChE, MSIM, Business Manager, EG&G Co.

GILES M. CRABTREE

BSEE, PE, Principal Engineer, GIMACA Engineering

H. L. DANEMAN

BChE, PE, Principal, LabPlan

JOHN L. DANIEWICZ

BSEE, MA, Product Manager, TN Technologies Inc.

*RONALD H. DIECK

BS, MS, FISA, President, Ron Dieck Associates, Inc.

LOUIS D. DINAPOLI

BSEE, MSEE, Director, Flowmeter Marketing and Technology, BIF Products of Leeds & Northrup Co.

WOLFGANG DRAHM

PhD, Research Scientist, Endress + Hauser, Germany

*WATSON P. DURDEN

AS, Senior Engineer, Westin Engineering

*MICHAEL PAUL DZIEWATKOSKI

PhD, Applications Manager, Metter-Toledo Ingold

*SUZANNE MARIE EDVI

IIT, Senior Instrument Specialist, Bantrel Inc., Canada

ALBERT D. EHRENFRIED

MS in Instrumentation, President, Metritape Inc.

*HALIT EREN

ME, MBA, PhD, Senior Lecturer, Cutin University, Australia

*GEORG F. ERK

BSME, MSChE, PE, Consultant

JOSEF FEHRENBACH

Dipl. Ing., VEGA Grieshaber GmbH & Co., Germany

KENNETH S. FLETCHER

PhD, Technical Group Leader, Analytical Measurements, The Foxboro Co.

ALBERT P. FOUNDOS

BSChE, MBA, President, Fluid Data Inc.

WALTER F. GERDES

BSEE, PE, Technical Specialist, The Dow Chemical Co.


Contributors

xv

*PEDRO M. B. SILVA GIRÃO

PhD, Professor, Instituto Superior Técnico, Lisbon, Portugal

*IAN H. GIBSON

BSc, Dip. App. Chem., Dip. Chem. Eng, Dip. Inst. Tech., Principal Technical Specialist, Process Control Systems, Fluor, Australia

*RICHARD A. GILBERT

BA, MS, PhD, Professor of Chemical Engineering, University of Florida

ANTHONY C. GILBY

PhD, Research Coordinator, The Foxboro Co.

PAUL M. GLATTSTEIN

BSEE, Senior Electrical Engineer, Crawford & Russell Inc.

JOHN D. GOODRICH, JR.

BSME, Engineering Supervisor, Bechtel Corp.

ROBERT J. GORDON

PhD, Environmental Division Manager, Global Geochemistry Corp.

DAVID M. GRAY

BSChE, Senior Application Specialist, Leeds & Northrup, a Unit of General Signal

*JAMES R. GRAY

BSCh, MBA, Applications Manager, Rosemount Analytical

BHISHAM P. GUPTA

BSME, MSME, PhD, PE, Specialist Supervisor, Saudi Aramco

JOHN T. HALL

BS, Senior Technical Editor, Instrument & Control Systems

CHARLES E. HAMILTON

BSChE, Senior Environmental Specialist, The Dow Chemical Co.

JOHN N. HARMAN III

BSCh, MSCh, PE, Senior Project Engineer, Beckman Instruments

*HASHEM M. HASHEMIAN

MSNE, President, Analysis and Measurement Services Corp.

ROBERT A. HERRICK

BSChE, PE, Consulting Engineer

HEROLD I. HERTANU

MSEE, PE, Senior Vice President, Advanced Engineering Concepts Inc.

CONRAD H. HOEPPNER

BSEE, MSEE, Consultant, Simmons Precision Products Inc.

MICHAEL F. HORDESKI

BSEE, MSEE, PE, Control System Consultant, Siltran Digital

JOEL O. HOUGEN

PhDChE, PE, Consultant, Professor Emeritus, University of Texas

WALTER D. HOULE

BSEE, President, Automation Management International

WILFRED H. HOWE

BSEE, MBA, PE, Chief Engineer, The Foxboro Co.

DAVID L. HOYLE

BSChE, System Design Engineer, The Foxboro Co.

JAY S. JACOBSON

PhD, Plant Physiologist, Boyce Thomson Institute for Plant Research

RAJSHREE R. JAIN

BSChE, Applications Engineer, Capital Controls Co.

ROBERT F. JAKUBIK

BSChE, Manager, Process Control Applications, Digital Applications Inc.

*JAMES E. JAMISON

BSc-ChE, PE, Technical Director, Instrumentation and Process Control Systems, VECO (Canada) Ltd.


xvi

Contributors

*JOHN M. JARVIS

PhD, Manager of Gas Products Engineering, Detector Electronics

HERBERT H. JONES

BS, Principal Applications Engineer, Beckman Instruments Inc.

RICHARD K. KAMINSKI

BA, Senior Instrument Designer, Dravo Engineers and Constructors

DAVID S. KAYSER

BSEE, Senior Instrument Engineer, Texas City Refining Inc.

THOMAS J. KEHOE

BSChE, PE, Manager, Technical Services, Beckman Instruments Inc.

TAMÁS KEMÉNY

ME, EE, PhD, Secretary General, IMEKO International Measurement Confederation, Hungary

CHANG H. KIM

BSChE, Manager, Technical Services, ARCO Chemical Co.

JOHN G. KOCAK, JR.

BA, Consultant

JOHN G. KOPP

BSME, PE, Senior Product Marketing Manager, Fischer & Porter Co.

JOSEF KOZÁK

PhD, Aeronautical Research and Test Institute, Czech Republic

*CULLEN G. LANGFORD

BSME, PE, ISA Fellow, Consultant, Cullen G. Langford Inc.

GEORGE R. LEAVITT

BSME, PE, Consultant

*MARIA T. LEE-ALVAREZ

PhD, Physical Science Teacher, Cincinnati Public School District

*DAVID LEWKO

Senior Analyzer Specialist, Bantrel Co.

TRUMAN S. LIGHT

BSCh, MSCh, PhDCh, Consultant

*BÉLA G. LIPTÁK

MME, PE, ISA Fellow, Consultant, inducted into Control Process Automation Hall of Fame in 2001

DAVID H. F. LIU

BSc, MS, PhD, Principal Scientist, J. T. Baker Inc.

*ANDREW J. LIVINGSTON

BS, MBA, Nuclear Product Manager, Ohmart Vega

HARRY E. LOCKERY

BSEE, MSEE, PE, President, Hottinger-Baldwin Measurements Inc.

DAVID J. LOMAS

Marketing Support Executive, Kent Process Control Ltd.

ORVAL P. LOVETT, JR.

BSCE, Consulting Engineer, Instruments and Control Systems, I. E. du Pont de Nemours Co.

JIRÍ LUKAS

MSC, Scientific Worker, Aeronautical Research and Test Institute of Czech Republic

*JULES J. MAGDA

PhD, ChE, Associate Professor, Dept. of Chemical and Fuels Engineering, University of Utah

DAVID C. MAIR

BCE, PE, Manager, Sales Services, Wallace & Tiernan Div. of Pennwalt Corp.

*RAMASAMY MANOHARAN

PhD, Manager of Sensor Technology, Rosemount Analytical Inc.


Contributors

xvii

FRED D. MARTIN

BS, Analyzer Consultant, Fluid Data, Amscor

THOMAS A. MAYER

BSE, MSE, PE, Senior Development/Research Engineer, PPG Industries

GERALD F. McGOWAN

BSEE, MSEE, Vice President of Engineering, Lear Siegler Inc.

GREGORY K. McMILLAN

BSEPhys, MSEE, Fellow, Monsanto Chemical Co.

*DEAN MILLER

BSME, MBA, Manager of Pressure Relief and Tooling Engineering, Fike Corp.

HUGH A. MILLS

ME, President, Macran Products

CHARLES F. MOORE

BSChE, MSChE, PhDChE, Professor of Chemical Engineering, University of Tennessee

*LEONARD W. MOORE

PE, President and CEO of Moore Industries International Inc.

*GERHARD MURER

Dipl. Eng., Manager of Anton Paar GmbH, Austria

THOMAS J. MYRON, JR.

BSChE, Senior Systems Design Engineer, The Foxboro Co.

*JAMES A. NAY

PE, BSME, Consultant, Retired

S. NISHI

DSc. Research Scientist, National Chemical Laboratory for Industry, Japan

ROBERT NUSSBAUM

BSEE, Senior Instrument Engineer, Crawford & Russell Inc.

*DAVID S. NYCE

BSEE, MBA, Director of Technology at MTS Systems Corp.

RICHARD T. OLIVER

BSChE, MSChE, PhDChE, Senior Design Engineer, The Foxboro Co.

WILLIAM H. PARTH

BS, MS, Senior Instrument Specialist, The Dow Chemical Co.

*SIMON J. PATE

B. Eng., Director of Projects & Systems, Detector Electronics Corp.

*ALMONT V. PAWLOWSKI

BSEE, CSST, PE, Research Associate at Louisiana State University

KENNETH A. PERROTTA

BSCh, Vice President of Technology, Balston Inc.

KURT O. PLACHE

BSChE, PE, Vice-President Marketing, Micro-Motion Inc.

GEORGE PLATT

BSChE, PE, Staff Engineer, Bechtel Power Corp.

DANIEL E. PODKULSKI

BSChE, Senior Instrument Engineer, Chevron Research & Technology

MICHAL PTÁCNÍK

PhD, Aeronautical Research and Test Institute, Czech Republic

DIETER RALL

BSME, MSME, PE, General Manager, Trans-Met Engineering Inc.

M. RAZAQ

PhD, Senior Scientist, Teledyne Analytical Instrument Co.

*MORTON W. REED

PE, PhD, Consultant

JAMES B. RISHEL

BSME, President, Corporate Equipment


xviii

Contributors

HOWARD C. ROBERTS

BAEE, PE, Consultant

*JACK C. RODGERS

PE, Vice President of Nuclear Business at Ohmart/VEGA

*JOHN B. ROEDE

ME, Senior Application Consultant, AMETEK-Drexelbrook

*ALBERTO ROHR

EE, Dr. Eng., Consultant, Vedano al Lambro (MI), Italy

LEWIS B. ROOF

BS, MS, Senior Measurement Engineer, Applied Automation Inc.

GREGORY J. RORECH

BSChE, PE, Principal Engineer, Geraghty & Miller Inc.

STEPHAN RUDBÄCH

MSc, President, Matematica AB, Sweden

*ROBERT S. SALTZMAN

BS, Eng. Phys., Principal of Bob Saltzman Associates

*GARY C. SANDERS

BSEE, MT, FICMT, Director of Engineering Tyco Valves & Controls — Penberthy

ERIC J. SCHAFFER

BSEE, MSEE, Project Engineer, MST Systems Corp.

*NARESH K. SETHI

BS, PhD, Technical Team Leader, BP South, Houston, Texas

*ROBERT E. SHERMAN

BSCh, MSCh, MSBA

DONALD J. SIBBETT

PhD, Vice President, Geomet Inc.

ROBERT SIEV

BSChE, MBA, CE, Engineering Specialist, Bechtel Corp.

MIKHAIL SKLIAR

PhD ChE, Associate Professor, Dept. of Chemical and Fuels Engineering, University of Utah

*KENNETH C. SLONEKER

BSME, V.P., Laboratory Director, Electronic Development Laboratories Inc.

RALPH G. SMITH

BS, MS, PhD, Professor, University of Michigan

*ROBERT J. SMITH II

BSEET, Plant Engineer at Rock-Tenn Co.

JOAN B. STODDARD

PhD, President, Stoddard Productivity Systems Inc.

RICHARD STRAUSS

BSChE, MSChE, Consultant

EUGENE L. SZONNTAGH

MSChE, PhD, PE, Consultant

*JAMES F. TATERA

BS, MBA, Senior Process Analysis Consultant, Tatera Associates Inc.

EDWARD TELLER

PhD, Professor-at-Large, University of California

AMOS TURK

PhD, Professor of Chemistry, City University of New York

*ALAN H. ULLMAN

BS (Chemistry), PhD, Senior Scientist at The Procter & Gamble Co.

*IAN VERHAPPEN

BscEnv, BScCh, PE, Engineering Associate at Syncrude Canada Ltd.

MICHAEL VUJICIC

PE, Director, Industrial Products, Optech Inc.


Contributors

xix

WILLIAM H. WAGNER

BSChE, PE, Staff Engineer at Union Carbide Corp.

*MICHAEL H. WALLER B

ME, SM, ME, Professor at Miami University

WILLEM M. WALRAVEN

ME, M&CE, Head of Evaluation Department Netherlands Organization for Applied Research

NORMAN S. WANER

BSME, MSME, ME, PE, Manager of Training and Development, Bechtel Corp.

JOHN V. WELCH

BSME, MBA, Market Specialist at MKS Instruments Inc.

ALAN L. WERTHEIMER

PhD, Principal Scientist, Leeds & Northrup Co.

GEORGE P. WHITTLE

BSChE, MSChE, PhDChE, PE, Associate Professor, University of Alabama

THEODORE J. WILLIAMS

BS, MSChE, MSEE, PhD, PE, Professor of Engineering, Director of Purdue Laboratory for Applied Industrial Control

ROBERT W. WORRALL

BA, PE, Principal Instrument Engineer, Catalytic Inc.

IRVING G. YOUNG

BS, MS, PhD, Chemist, Advanced Technology Staff, Honeywell Inc.

*JESSE L. YODER

PhD, President, Flow Research


INTRODUCTION

Ours is a very young profession: when the first edition of the Instrument Engineers’ Handbook (IEH) came out, Marks’ Mechanical Engineers’ Handbook was in its fifth edition, and Perry’s Chemical Engineers’ Handbook was in its sixth! Now, as we are starting to work on the fourth edition of the IEH, we are already in a new millenium. But while our profession is young, we are also unique and special. After all, no other engineering profession can claim what we can! No other engineering profession can offer to increase the GDP by $50 billion without building a single new plant, and to do that while increasing safety and reducing pollution. We can do that! We can achieve that goal solely through the optimization of our existing industries. We can increase productivity without using a single pound of additional raw material, without needing a single additional BTU.

THIS FOURTH EDITION During the nearly four decades of its existence, the IEH has become the most widely used reference source of the instrumentation and control (I&C) engineering profession. During this same period, the tools of our I&C profession have changed as control systems were transformed from the early mechanical and pneumatic ones to today’s electronic and digital implementations. During this period, even the name of our profession has changed. Today, some call it automation, while others refer to it by a variety of other names, including instrumentation, process control, I&C, and computer automation. Yet, while we have not been able to agree even on the name of our profession, our experience and our knowledge of control principles has penetrated all the fields of modern science and technology. I hope that the three volumes of the IEH have played a major role in spreading this knowledge and understanding. In 1968, this handbook started out as a three-volume reference set, and, in that respect, no change has occurred. The first volume deals with measurement, the second with control, and the third with digital networks and software systems.

CONTENTS OF THE IEH VOLUMES In this, the first volume, a chapter is devoted to each major measured variable, and a subchapter (section) is devoted to each different method of making that measurement. Some measurements are relatively simple as, for example, the detection of level; therefore, that chapter has only 21 sections. Others, such as analysis, are more varied, and that chapter has 66 sections. The individual sections (subchapters) begin with a flowsheet symbol and a feature summary. This summary provides quick access to specific information on the available sizes, costs, suppliers, ranges, and inaccuracies of the devices covered in that section. This fourth edition updates the information content of the previously published sections, incorporates the new developments of the last decade by the addition of new sections, and broadens the horizons of the work from an American to a global perspective. In this first volume, Process Measurement and Analysis, the emphasis is on measurement hardware, including the detection of flow, level, temperature, pressure, density, viscosity, weight, composition, and safety sensors. The second volume of this set, Process Control, covers control hardware, including transmitters, controllers, control valves, and displays, and it provides in-depth coverage to the theory of control and explains how the unit processes of pumping, distillation, chemical reaction, heat transfer, and many others are controlled. The third volume is devoted to Process Software and Digital Networks. In combination, the three volumes cover all the topics used by process control or instrument engineers.

READERS OF THE IEH Experienced process control engineers are likely to use this reference set either to obtain quick access to specific information or to guide them in making selections. Less experienced engineers and students of instrument engineering are xxi


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Introduction

likely to use this reference work as a textbook. A student might use it to learn about the tools of our profession. To fulfill the expectations of both the experienced and the beginning engineer, the handbook has been structured to be flexible. On one hand, it contains all the basic information that a student needs, but it also covers the most recent advances and provides quick and easy access to both types of information. Quick access to specific topics and information is provided both by the feature summary at the beginning of each section and by an extensive index at the end of each volume.

BIRD’S EYE VIEWS: ORIENTATION TABLES Another goal of this reference set is to assist the reader in selecting the best sensors for particular applications. To achieve this goal, each chapter begins with a section that provides an application- and selection-oriented overview along with an orientation table. The orientation tables list all the sensors that are discussed in the chapters and summarize the features and capabilities of each. If the reader is using this handbook to select a sensor for a particular application, the orientation table allows the narrowing of the choices to a few designs. After the options have been reduced, the reader might turn to the corresponding sections and, based on the information in the feature summaries at the front of each section, decide if the costs, inaccuracies, and other characteristics meet the requirements of the application. If so, the reader might focus in on the likely candidate and read all the information in the selected section.

NEW NEEDS AND EXPECTATIONS As I was editing this reference set for the fourth time, I could not help but note the nature of both the new solutions and the new needs of the process control industry. The new solutions become obvious as you review the contents of the 400 to 500 sections of the 25 or so chapters of this set of handbooks. The new needs are not so obvious. The new needs are the consequences of the evolution of new hardware, new software, and the completely new technologies that have evolved. These needs become obvious only if one is immersed in the topic to the depth and for the duration that I have been. It might speed technological progress if some of these needs are mentioned here.

INTERNATIONAL STANDARDIZATION In earlier decades, it took some time and effort to agree on the 3 to 15 PSIG (0.2 to 1.0 bar) signal pressure range for the standard pneumatic or on the 4 to 20 mA DC standard analog electronic signal range. Yet, when these signal ranges


were finally agreed upon, everybody benefited from having a standard signal. Similarly, the time is ripe for adopting a worldwide standard for a single digital communication protocol. The time is ripe for an internationally accepted digital protocol that could link all the digital “black boxes” and could also act as the “translator” for those that were not designed to “speak the same language.” In so doing, the valuable engineering energies that today are being spent to figure out ways for black boxes to communicate could be applied to more valuable tasks, such as increasing the productivity and safety of our processing industries. Optimization can make our industries competitive once again and contribute not to the export of jobs but to the creation of jobs at home.

MEANINGFUL PERFORMANCE STANDARDS It is also time to rein in the commercial interests and to impose uniform expectations so that all sales literature will provide performance data in the same form. In today’s sales literature, the performance-related terms such as inaccuracy and rangeability are rarely defined properly. Such terms as “inaccuracy” are frequently misstated as “accuracy,” and sometimes the error percentages are given without stating whether they are based on full-scale or actual readings. It is also time for professional societies and testing laboratories to make their findings widely available so that test results can be used to compare the products of different manufacturers. It is also desirable to have the manufacturers always state not only the inaccuracy of their products but also the rangeability over which that inaccuracy statement is valid. Similarly, it would be desirable if rangeability were defined as the ratio between those (maximum and minimum) readings for which the inaccuracy statement is valid. It would also be desirable to base the inaccuracy statements on the performance of at least 95% of the sensors tested and to include in the inaccuracy statement not only linearity, hysteresis, and repeatability, but also the effects of drift, ambient temperature, overrange, supply voltage, humidity, radio frequency interference (RFI), and vibration.

BETTER VALVES The performance capabilities of final control elements should also be more uniformly agreed upon and more reliably stated. This is particularly true for the characteristics, gains, and rangeabilities of control valves. For example, a valve should be called linear only if its gain (Gv) equals the maximum flow through the valve (Fmax) divided by the valve stroke in percentage (100%). Valve manufacturers should publish the stroking range (minimum and maximum percentages of valve openings) within which the gain of a linear valve is still Fmax/100%.

Introduction

Valve rangeability should be defined as the ratio of these minimum and maximum valve openings. Other valve characteristics should also be defined by globally accepted standards in this same manner.

“SMARTER” SENSORS AND ANALYZERS In the case of transmitters, the overall performance is largely defined by the internal reference used in the sensor. In many cases, there is a need for multiple-range and multiple-reference units. For example, pressure transmitters should have both atmospheric and vacuum references and should have sufficient intelligence to switch automatically from one to the other reference on the basis of their own measurement. Similarly, d/p flow transmitters should have multiple spans and should have the intelligence to automatically switch their spans to match the actual flow as it changes. The addition of “intelligence” could also increase the amount of information gained from such simple detectors as pitot tubes. If, for example, in addition to detecting the difference between static and velocity pressures, the pitot tube were also able to measure the Reynolds number, it would be able to approximate the shape of the velocity profile. An “intelligent pitot-tube” of such capability could increase the accuracy of volumetric flow measurements.

IMPROVED ON-LINE ANALYZERS In the area of continuous on-line analysis, further development is needed to extend the capabilities of probe-type analyzers. The needs include the changing of probe shapes to achieve self-cleaning or using “flat tips” to facilitate cleaning. The availability of automatic probe cleaners should also be improved, and their visibility should be increased by the use of sight flow indicators. An even greater challenge is to lower the unit costs of fiber-optic probes through multiplexing and by sharing the cost of their electronics among several probes. Another important goal for the analyzer industry is to produce devices that are self-calibrating, self-diagnosing, and modular in design. To reduce the overall cost of analyzer maintenance, defective modules should identify themselves and should be easily replaceable.

In this sense, most of today’s digital controls are still only “empty boxes.” New software packages are needed to “educate” and to give “personality” to them. Software is needed that, when loaded, will transform a general-purpose unit controller into an advanced and optimized control system serving the particular process, whether it is a chemical reactor, a distillation tower, a compressor, or any other unit operation. This transformation in the building blocks of control systems would also make the manufacturing of digital control hardware more economical, because all “empty boxes” could be very similar.

UNIT OPERATION CONTROLLERS The use of such multipurpose hardware could also provide more flexibility to the user, because a unit controller that was controlling a dryer, for example, could be switched to control an evaporator or a pumping station just by loading a different software package into it. Once the particular software package was loaded, the unit controller would require customization only, which could be done in a menu-driven questionand-answer format. During the customization phase, the user would answer questions on piping configuration, equipment sizes, material or heat balances, and the like. Such customization software packages would automatically configure and tune the individual loops and would make the required relative gain calculations to minimize interaction between loops. It will probably take a couple decades to reach these goals, but to get there, it is necessary to set our sights on these goals now.

COMMON SENSE RECOMMENDATIONS While talking about such sophisticated concepts as optimized multivariable control, it is very important to keep our feet on the ground, keep in mind that the best process control engineer is still Murphy, and remember that, in a real plant, even Murphy can turn out to be an optimist. For that reason, I list the following common sense, practical advice, and recommendations: • •

EFFICIENCY AND PRODUCTIVITY CONTROLLERS • In the area of control, what is most needed is to move from the uncoordinated single loops to optimizing, multivariable envelope, and matrix algorithms. When using such multivariable envelopes, the individual levels, pressures, and temperatures become only constraints, while the overall multivariable envelope is dedicated to maximizing the efficiency or productivity of the controlled process.


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•

•

Before one can control a process, one must fully understand it. Being progressive is good, but being a guinea pig is not. If an outdated control strategy is implemented, the performance of even the latest digital hardware will be outdated. Increased safety is gained through the use of multiple sensors, configured through voting systems or median selectors. If an instrument is worth installing, it should also be worth calibrating and maintaining.

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•

• •

•

Introduction

Constancy is the enemy of efficiency; as the load and feed compositions float, the process variables should also be allowed to change with them. Control loops can be stabilized by replacing their single set points with control gaps. Annunciators do not correct emergencies, they just report problems that the designer did not know how to handle and therefore decided to drop into the laps of the operators. The smaller the annunciator, the better the control system design. A good process control engineer will tell the user what he needs to know and not what he wants to hear. The right time for business lunches is not before receiving the purchase order, but after the plant has started up and is running.

HISTORY OF THE HANDBOOK The birth of this handbook was connected to my own work. In 1962, at the age of 26, I became the chief instrument engineer at Crawford & Russell, an engineering design firm specializing in the building of plastics plants. C&R was growing, and my department had to grow with it. Still, at the age of 26, I did not dare to hire experienced people, because I did not believe that I could lead and supervise older engineers. But the department had to grow, so I hired fresh graduates from the best engineering colleges in the country. I picked the smartest graduates, and I obtained permission from C&R’s president, Sam Russell, to spend every Friday afternoon teaching them. In a few years, not only did my department have some outstanding process control engineers, C&R also saved a lot on their salaries. By the time I reached 30, I felt secure enough to stop disguising my youth. I shaved off my beard and threw away my thick-rimmed, phony eyeglasses. I no longer felt that I had to look older, but my Friday’s notes remained—they still stood in a two-foot high pile on the corner of my desk.

“DOES YOUR PROFESSION HAVE A HANDBOOK?” In the mid-1960s, an old-fashioned Dutch gentleman named Nick Groonevelt visited my office and asked, “What is that pile of notes?” When I told him, he asked: “Does your profession have a handbook?” “If it did, would I be teaching from these notes?” I answered with my own question. (Actually, I was wrong in giving that answer, because Behar’s Handbook of Measurement and Control was already available, but I did not know about it.) “So, let me publish your notes, and then instrument engineers will have a handbook!” Nick proposed, and in 1968 the first edition of the Instrument Engineers’ Handbook (IEH) was published.


In 1968, the Soviet tanks (which I fought in 1956) were besieging Prague, so I decided to dedicate the three volumes of the IEH to the Hungarian and Czech freedom fighters. A fellow Hungarian-American, Edward Teller, wrote the preface to the first edition, and Frank Ryan, the editor of ISA Journal, wrote the introduction. My coauthors included such names as Hans Baumann, Stu Jackson, Orval Lovett, Charles Mamzic, Howard Roberts, Greg Shinskey, and Ted Williams. It was an honor to work with such a team. In 1973, because of the publication of the first edition of the IEH, I was elected the youngest ISA fellow ever.

LATER EDITIONS By the end of the 1970s, the world of process control had changed. Pneumatics were on the way out, and new approaches, such as distributed control systems (DCS) and on-line analyzers, proliferated. It was time to revise the handbook. By 1975, I also had to run my own consulting office, so I could not devote my full attention to updating the handbook. Therefore, I hired Kriszta Venczel to do most of the work, and she did her best by inserting metric units and the like. We got some excellent new contributions, from Ed Farmer, Tom Kehoe, Thomas Myron, Richard Oliver, Phillip Schnelle, Mauro Togneri, and Theodore Williams. The second edition was published in 1982. It was well received, but I knew that it would have been better if I had devoted more time to it. By the mid-1990s, the handbook was ready for another updating edition. By that time, the process control market was becoming globalized, “smart” instruments had evolved, and such hardware inventions as fiber-optic probes and throttling solenoid valves proliferated. Therefore, I stopped teaching at Yale and cut back on consulting to make time to edit the third edition. By the second half of the 1990s, the first two volumes of the third edition, one on measurement and the other on control, were published. At that time, I realized that a third volume was also needed to cover all of the evolving digital software packages, communication networks, buses, and optimization packages. Therefore, it took the last decade of the twentieth century to publish the three volumes of the third edition.

THE FOURTH EDITION Work on the fourth edition of the IEH started in the new millenium, and this first volume on measurement and analysis is the result of this effort. I do hope that, in three to five years, you might hold all three updated IEH volumes in your hands. Now that the fourth edition of the Measurement and Analysis

Introduction

volume has been published, I am starting work on the second volume, which is devoted to process control. This second volume will cover control hardware, including transmitters, controllers, control valves, and displays, and it provides in-depth coverage of both control theory and how the unit processes of pumping, distillation, chemical reaction, heat transfer, and many others are controlled and optimized. My main goal is to expand this last area by both increasing the list of unit operations that we cover and, more importantly, by giving much more emphasis to optimization.

WHY DON’T YOU PITCH IN? I would like to ask you to help me locate the best experts on all five continents for each important unit operation in our processing industries. If you have spent a lifetime learning and understanding the unique personality of a process and have figured out how to maximize its efficiency, don’t keep that knowledge to yourself—share it with us. If you or one of your colleagues would like to participate as a coauthor, please send me an e-mail, and I will send you the table of contents (TOC) of the control volume. If the topic of your interest is not in the TOC, we can add it; if it is, I will consider your offer to update the material that has already appeared in the third edition. Please understand that I am not looking for people with writing skills, I am looking for engineers with knowledge and experience! This is not to say that I will reject college


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professors; naturally, I will not, although I might delete some of their differential equations and bring them down from the frequency domain back into the time domain. Similarly, I will consider the contributions of professional consultants if they do not view the IEH as a forum for self-promotion. I will also consider manufacturers as coauthors if they are able to be balanced and are willing to give credit where credit is due, even if it means crediting their competition. But my favorite coauthor is the plant engineer who is short on words but long on experience. I do not mind getting answers such as, “I don’t know if this is conductivity or ultrasonics, all I know is that it works!” The IEH is written by users and for users, and it is not about fancy packaging— it is about content. So don’t worry about your writing skills, I can help with that. Please help make the fourth edition of the IEH one we can be proud of. Please drop me an e-mail if you want to pitch in. We know that there is no greater resource than the combined knowledge and professional dedication of a well educated new generation. We live in an age in which technology can make a difference in overcoming the social and environmental ills on this planet. We live in an age in which an inexhaustible and nonpolluting energy technology must be developed. It is hoped that this handbook will make a contribution toward these goals and that, in addition, it will improve the professional standing of instrument and process control engineers around the world. Béla Lipták Stamford, Connecticutt ([email protected])

DEFINITIONS

ABSOLUTE (DYNAMIC) VISCOSITY

( µ)

ABSORBANCE (A)

ABSORPTION

ACCUMULATION

ADMITTANCE (A)

ADSORPTION

ALPHA CURVE

AMPACITY

AMPEROMETRIC TITRATION

AMPEROMETRY

Constant of proportionality between applied stress and resulting shear velocity (Newton’s hypothesis). Ratio of radiant energy absorbed by a body to the corresponding absorption of a blackbody at the same temperature. Absorbance equals emittance on bodies whose temperature is not changing. (A = 1 − R − T, where R is the reflectance and T is the transmittance.) The taking in of a fluid to fill the cavities in a solid. The pressure increase over the maximum allowable working pressure of a tank or vessel during discharge through the pressure relief valve. It is given either in percentage of the maximum allowable working pressure or in pressure units such as bars or pounds per square inch. The reciprocal of the impedance of a circuit. Admittance of an AC circuit is analogous to the conductance of a DC circuit. Expressed in units of Seimens. The adhesion of a fluid in extremely thin layers to the surfaces of a solid. The relationship between the resistance change of an RTD vs. temperature. In the European alpha curves, the alpha value is 0.00385 Ω/°C; in the American curves, it is 0.00392 Ω/°C. The current (amperes) a conducting system can support without exceeding the temperature rating assigned to its configuration and application. Titration in which the end point is determined by measuring the current (amperage) that passes through the solution at a constant voltage. The process of performing an amperometric titration. The current flow is

APPARENT VISCOSITY ATTENUATION BACKPLANE

BACKPRESSURE

BALANCED SAFETY RELIEF VALVE

BALLING DEGREES BALUN (BALANCED/ UNBALANCED) BANDPASS FILTER

monitored as a function of time between working and auxiliary electrodes while the voltage difference between them is held constant; in other designs, the current is monitored as a function of the amount of reagent added to bring about titration of an analyte to the stoichiometrically defined end point. Also called constant potential voltametry. Viscosity of a non-Newtonian fluid under given conditions. Same as consistency. Loss of communication signal strength. Physical connection between individual components and the data and power distribution buses inside a chassis. Pressure on the discharge side of a pressure relief valve. This pressure is the sum of the superimposed and the built-up backpressures. The superimposed backpressure is the pressure that exists in the discharge piping of the relief valve when the valve is closed. A safety relief valve with the bonnet vented to atmosphere. The effect of backpressure on the performance characteristics of the valve (set pressure, blow-down, and capacity) is much less than on the conventional valve. The balanced safety relief valve is made in three designs: (1) with a balancing piston, (2) with a balancing bellows, and (3) with a balancing bellows and an auxiliary balancing piston. Unit of specific gravity used in the brewing and sugar industries. A device used for matching characteristics between a balanced and an unbalanced medium. An optical or detector filter that permits the passage of a narrow band of the xxvii


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Definitions

BANDWIDTH

BARKOMETER DEGREES BASEBAND

BAUMÉ DEGREES

BLACKBODY

BLOWDOWN

(BLOWBACK)

BOLOMETER

BONDING

BRIGHTNESS PYROMETER

BRITISH THERMAL UNIT (BTU) BRIX DEGREE

BROADBAND


total spectrum. It excludes or is opaque to all other wavelengths. Data-carrying capacity; the range of frequencies available for signals. The term is also used to describe the rated throughput capacity of a given network medium or protocol. Unit of specific gravity used in the tanning industry. A communication technique whereby only one carrier frequency is used to send one signal at a time. Ethernet is an example of a baseband network; also called narrowband; contrast with broadband. A unit of specific gravity used in the acid and syrup industries. The perfect absorber of all radiant energy that strikes it. The blackbody is also a perfect emitter. Therefore, both its absorbance (A) and emissivity (E) are unity. The blackbody radiates energy in predictable spectral distributions and intensities that are a function of the blackbody’s absolute temperature (Figure 4.11a). A blackbody can be configured as shown in Figure 4.11b. The difference between the set pressure and the reseating (closing) pressure of a pressure relief valve, expressed in percent of the set pressure, bars, or pounds per square inch. Thermal detector which changes its electrical resistance as a function of the radiant energy striking it. The practice of creating safe, highcapacity, reliable electrical connectivity between associated metallic parts, machines, and other conductive equipment. This device uses the radiant energy on each side of a fixed wavelength of the spectrum. This band is quite narrow and usually centered at 0.65 µm in the orange-red area of the visible spectrum. The amount of heat required to raise the temperature of 1 lb of water by 1°F at or near 60°F. A specific gravity unit used in the sugar industry. A communication technique that multiplexes multiple independent signals simultaneously, using several distinct carriers. A common term in the telecommunications industry to describe any channel having a bandwidth greater

than a voice-grade channel (4 kHz). Also called wideband. Contrast with baseband. BTU “DRY” This is the heating value that is expressed on a “dry basis.” The common assumption is that pipeline gas contains 7 lb (or less) of water vapor per million standard cubic feet. BTU “SATURATED” This is the heating value that is expressed on the basis of the gas being saturated with water vapors. This state is defined as the condition when the gas contains the maximum amount of water vapors without condensation, when it is at base pressure and 60°F. BUILT-UP Variable backpressure that develops as BACKPRESSURE a result of flow through the pressure relief valve after it opens. This is an increase in pressure in the relief valve’s outlet line caused by the pressure drop through the discharge headers. BURNING Burning is when the flame does not spread or diffuse but remains at an interface where fuel and oxidant are supplied in proper proportions. CAPACITANCE (C) The amount of charge, in coulombs, stored in a system necessary to raise the potential difference across it by 1 V, represented in the SI unit farad. CAPACITOR DEVICE This device consists of two conductors electrically isolated by an insulator. The conductors are called plates, and the insulator is referred to as the dielectric. The larger the capacitor, the smaller its impedance and the more AC current will flow through it. CHARACTERISTIC The impedance obtained from the outIMPEDANCE put terminals of a transmission line that appears to be infinitely long, when there are no standing waves on the line and the ratio of voltage to current is the same for each point of the line (nominal impedance of a waveguide). CHATTER Rapid, abnormal reciprocating variations in lift during which the disc contacts the seat. CHRONOPOTENProcess in which the potential differTIOMETRY ence between a metallic measuring electrode and a reference electrode is monitored as a function of time. At the measuring electrode, an oxidation or reduction of a solution species takes place. CLOSING PRESSURE The pressure, measured at the valve (RESEAT PRESSURE) inlet, at which the valve closes, flow is substantially shut off, and there is no measurable lift.

Definitions

COAX

COLD DIFFERENTIAL TEST PRESSURE

(CDTP)

COMBUSTION AIR REQUIREMENT INDEX (CARI)

Jargon meaning coaxial cable, consisting of a center wire surrounded by lowK insulation, surrounded by a second, shield conductor. It has the characteristic of low capacitance and inductance to facilitate transmission of high-frequency current. The pressure at which the PRV is adjusted to open during testing. The CDTP setting includes the corrections required to consider the expected service temperature and backpressure. This dimensionless number indicates the amount of air required (stoichiometrically) to support the combustion of a fuel gas. Mathematically, the combustion air requirement index is defined by the equation below: CARI =

CONDUCTANCE (G) CONDUCTIVITY

(g)

CONSISTENCY

CONSTANT BACKPRESSURE

CONVENTIONAL SAFETY RELIEF VALVE

COULOMETRY

CPVC


CRYSTALLOGRAPHY

CURIE (CI)

DATA SERVERS

DEAD BAND

DEFLAGRATION OR EXPLOSION

DEIONIZED

air / fuel ratio s.g.

The reciprocal of resistance in units of Siemens (S, formerly mhos). The reciprocal of resistivity. All solids and liquids have some degree of conductivity. For the purpose of this section, any material above 1 µS/cm will be considered to be conductive (including most metals and water containing any ions). Resistance of a substance to deformation. It is the same as viscosity for a Newtonian fluid and the same as apparent viscosity for a non-Newtonian fluid. Backpressure that does not change under any condition of operation, whether the pressure relief valve is closed or open. A safety relief valve with the bonnet vented either to atmosphere or internally to the discharge side of the valve. The performance characteristics (set pressure, blowdown, and capacity) are directly affected by changes of the backpressure on the valve. Process of monitoring analyte concentration by detecting the total amount of electrical charge passed between two electrodes that are held at constant potential or when constant current flow passes between them. Chlorinated polyvinyl chloride, a lowcost, reasonably inert polymer, used in the construction of some noninsertion sensors. It is easily solvent welded. The maximum temperature range is up to about 225°F.

DEMULTIPLEXING

DESIGN PRESSURE

DETONATION

DEVICE DESCRIPTION

DEW POINT

DIELECTRIC

DIELECTRIC COMPENSATION

DIELECTRIC CONSTANT

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How atoms are arranged in an object; the direct relationship between these arrangements and material properties (conductivity, electrical properties, strength, etc.). A unit of radiation source size corresponding to 37 billion disintegrations per second. A standard interface to provide data exchange between field devices and data clients. The range through which an input can be varied without causing a change in the output. A process in which a flame front advances through a gaseous mixture at subsonic speeds. Refers to water of extremely high purity, with few ions to carry current. If exposed to air for any significant period, it will have a conductivity of about 5 µS/cm because of dissolved CO2. Separation of multiple input streams that were multiplexed into a common physical signal back into multiple output streams. This pressure is equal to or less than the maximum allowable working pressure. It is used to define the upper limit of the normal operating pressure range. A process in which the advancement of a flame front occurs at supersonic speeds. A clear, unambiguous, structured text description that allows full utilization/ operation of a field device by a host/ master without any prior knowledge of the field device. Saturation temperature of a gas–water vapor mixture. An electrical insulator (includes metal oxides, plastics, and hydrocarbons). A scheme by which changes in insulating liquid composition or temperature can be prevented from causing any output error. Requires a second sensor and homogeneous liquid. A dielectric is a material that is an electrical insulator or in which an electric field can be sustained with a minimum of dissipation of power. A unit expressing the relative charge storage capability of various insulators. Full vacuum is defined as 1.0, and all gases are indistinguishable for practical

xxx

Definitions

purposes. TFE has a dielectric constant of 2.0, cold water about 80. There are no related units, because this is the ratio of absolute dielectric constant to that of vacuum. The dielectric values of selected materials are given in Tables 3.3q and 3.14a. DIODE A two-terminal electronic (usually semiconductor) device that permits current flow predominantly in only one direction. DISCONTINUITY An abrupt change in the shape (or impedance) of a waveguide (creating a reflection of energy). DUST-IGNITIONEnclosed in a manner to exclude ignitPROOF able amounts of dust or amounts that might affect performance. Enclosed so that arcs, sparks, and heat otherwise generated or liberated inside of the enclosure will not cause ignition of exterior accumulations or atmospheric suspensions of dust. EFFECTIVE COEFFICI- This is a coefficient used to calculate ENT OF DISCHARGE the minimum required discharge area of the PRV. ELECTROCHEMICAL The changes in voltage or current flow PROCESS that occur between two electrodes in a solution (electrolyte) over time. The oxidation or reduction of the analyte provides data related to concentration. ELECTROLYTIC A probe that is similar to a galvanic probe, PROBE except that a potential is applied across the electrodes, and the electrodes are not consumed. Dissolved oxygen detection is a primary application of this type of probe. ELECTROMAGNETIC A disturbance that propagates outward WAVE (ENERGY) from any electric charge that oscillates or is accelerated; far from the charge, it consists of vibrating electric and magnetic fields that move at the speed of light and are at right angles to each other and to the direction of motion. ELECTRON Electron microscopes are scientific insMICROSCOPE truments that use a beam of highly energetic electrons to examine objects on a very fine scale. EMISSIVITY OR The emissivity of an object is the ratio EMITTANCE (E) of radiant energy emitted by that object divided by the radiant energy that a blackbody would emit at that same temperature. If the emittance is the same at all wavelengths, the object is called a gray body. Some industrial materials change their emissivity with temperature and sometimes with other variables. Emissivity always equals


EQUIVALENT TIME SAMPLING (ETS)

ETHERNET

EXPLOSION-PROOF

FARAD (F)

FEP

FIELDBUS

FIREWALL

FLASH POINT

FLUIDITY

FLUTTER

absorption, and it also equals 1 minus the sum of reflectance and transmittance (E = A = 1 − T − R). A process that captures high-speed electromagnetic events in real time (nanoseconds) and reconstructs them into an equivalent time (milliseconds), which allows easier measurement with present electronic circuitry. A baseband local area network specification developed by Xerox Corporation, Intel, and Digital Equipment Corp. to interconnect computer equipment using coaxial cable and transceivers. All equipment is contained within enclosures strong enough to withstand internal explosions without damage, and tight enough to confine the resulting hot gases so that they will not ignite the external atmosphere. This is the traditional method and is applicable to all sizes and types of equipment. A unit of capacitance. Because this is a very large unit, a unit equal to one trillionth of a farad (called a picofarad, pF) is commonly used in RF circuits. Fluorinated ethylene propylene, a fluorocarbon that is extremely chemically inert, melts at a reasonable temperature, and can be plastic-welded fairly easily. It is difficult to bond with adhesives. The maximum temperature range is limited to the 300°F (150°C) area. An all-digital, two-way, multidrop communication system for instruments and other plant automation equipment. A router or access server designated as a buffer between any public networks and a private network. The lowest temperature at which a flammable liquid gives off enough vapors to form a flammable or ignitable mixture with air near the surface of the liquid or within the container used. Many hazardous liquids have flash points at or below room temperatures. They are normally covered by a layer of flammable vapors that will ignite in the presence of a source of ignition. Reciprocal of absolute viscosity; unit in the cgs system is the rhe, which equals 1/poise. Rapid, abnormal reciprocating variations in lift during which the disc does not contact the seat.

Definitions

FUEL CELLS

GALVANIC PROBE

GRAY BODY

GROSS CALORIFIC VALUE

GROUND

GROUND FAULT PROTECTOR

GUARD

GUIDED WAVE RADAR (GWR)


Cells that convert the chemical energy of fuel and oxygen into electrical energy while the electrode and the electrolyte remain unaltered. Fuel is converted at the anode into hydrogen ions, which travel through the electrolyte to the cathode, and electrons, which travel through an external circuit to the cathode. If oxygen is present at the cathode, it is reduced by these electrons, and the hydrogen and oxygen ions eventually react to form water. A probe for which no external voltage is applied across electrodes; current flows as the cell is depolarized when diffusion of the analyte occurs. Electrodes are consumed during this operation and require periodic replacement. This is an object having an emittance of less than unity, but this emittance is constant at all wavelengths (over that part of the spectrum where the measurement takes place). This means that gray-body radiation curves are identical to the ones shown in Figure 4.11a, except that they are dropped down on the radiated power density scale. The heat value of energy per unit volume at standard conditions, expressed in terms of British thermal units per standard cubic feet (Btu/SCF) or as kilocalories per cubic Newton meters (Kcal/N·m3) or other equivalent units. A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the Earth, or to some conducting body that serves in place of Earth. (See NFPA 70–100.) A device used to open ungrounded conductors when high currents, especially those resulting from line-to-ground fault currents, are encountered. The “electronic guard” (called a shield in some RF level literature) consists of a concentric metallic element with an applied voltage that is identical to the voltage on the conductor that it is “guarding.” This negates the capacitance between the guarded conductor and the outside world. A contact radar technology for which time domain reflectometry (TDR) has been developed into an industrial-level measurement system in which a probe immersed in the medium acts as the waveguide.

HAGEN-POISEUILLE LAW HOME RUN WIRING

HUB (SHARED) HYGROMETER HYGROSCOPIC MATERIAL IMPEDANCE

INFRARED

INTERFACE

INTEROPERABILITY

INTRINSIC SAFETY

KINEMATIC VISCOSITY

(υ)

LAMBDA

LATENCY

LIFT

LINE DRIVER

xxxi

Defines the behavior of viscous liquid flow through a capillary. Wire between the cabinet where the Fieldbus host or centralized control system resides and the first field junction box or device. Multiport repeater joining segments into a network. An apparatus that measures humidity. A material with a great affinity for moisture. Maximum voltage divided by maximum current in an alternating current circuit. Impedance is composed of resistive, inductive, and capacitive components. As in DC circuits, the quantity of voltage divided by current is expressed in ohms (Ω). The portion of the spectrum whose wavelength is longer than that of red light. Only the portion between 0.7 and 20 µm gives usable energy for radiation detectors. (1) Shared boundary; for example, the physical connection between two systems or two devices. (2) Generally, the point of interconnection of two components, and the means by which they must exchange signals according to some hardware or software protocol. A marketing term with a blurred meaning. One possible definition is the ability for like devices from different manufacturers to work together in a system and be substituted one for another without loss of functionality at the host level (HART). Available energy is limited under all conditions to levels too low to ignite the hazardous atmosphere. This method is useful only for low-power equipment such as instrumentation, communication, and remote control circuits. Dynamic viscosity/density = υ = µ/ρ. The desired closed-loop time constant, often set to equal the loop lag time. Latency measures the worst-case maximum time between the start of a transaction and the completion of that transaction. The rise of the disc in a pressure-relief valve. Inexpensive amplifier and signal converter that conditions digital signals to ensure reliable transmissions over

xxxii

Definitions

LOWER EXPLOSIVE LIMIT (LEL)

LOWPASS FILTERS

MANCHESTER

MANUFACTURING RANGE

MAXIMUM ALLOWABLE

extended distances without the use of modems. The lowest concentration of gas or vapor in air at which, once ignition occurs, the gas or vapor will continue to burn after the source of ignition has been removed. Filters that are used to remove highfrequency interference or noise from low-frequency signals. A digital signaling technique that contains a signal transition at the center of every bit cell. A range around the specified burst pressure within which the marked or rated burst pressure must fall. Manufacturing range is not used in ISO standards. The maximum pressure expected during normal operation.

MULTIPLEXING

NARROWBAND PYROMETER

NET CALORIFIC VALUE

NETWORK

OPERATING PRESSURE (MAOP) MAXIMUM ALLOWABLE WORKING PRESSURE

(MAWP)

MECHANICAL EMISSIVITY ENHANCEMENT MICRON

MODEM

MORPHOLOGY


This is the maximum pressure allowed for continuous operation. As defined in the construction codes (ASME B31.3) for unfired pressure vessels, it equals the design pressure for the same design temperature. The maximum allowable working pressure depends on the type of material, its thickness, and the service conditions set as the basis for design. The vessel may not be operated above this pressure or its equivalent at any metal temperature other than that used in its design; consequently, for that metal temperature, it is the highest pressure at which the primary pressure relief valve can be set to open. Mechanically increasing the emissivity of a surface to near-blackbody conditions (using multiple reflection). Equals 001 mm, 10,000 angstroms (Å). A unit used to measure wavelengths of radiant energy. Modulator-demodulator; a device that converts digital and analog signals. At the source, a modem converts digital signals to a form suitable for transmission over analog communication facilities. At the destination, the analog signals are returned to their digital form. Modems allow data to be transmitted over voice-grade telephone lines. The shape and size of the particles making up the object; the direct relationship between these structures and

NEWTON

NONFRAGMENTING DISC

NONINCENDIARY

OIL IMMERSION

OPERATING PRESSURE

their material properties (ductility, strength, reactivity, etc.). A scheme that allows multiple logical signals to be transmitted simultaneously across a single physical channel. Compare with demultiplexing. A radiation pyrometer that is sensitive to only a narrow segment of wavelengths within the total radiation spectrum. Optical pyrometers are among the devices in this category. The measurement of the actual available energy per unit volume at standard conditions, which is always less than the gross calorific value by an amount equal to the latent heat of vaporization of the water formed during combustion. All of the media, connectors, and associated communication elements by which a communication system operates. The internationally accepted unit of force, defined as the force required to accelerate 1 kg by 1 m/sec2. It equals 0.2248 pound-force or about 4 oz. A rupture disc design that, when burst, does not eject fragments that could interfere with the operation of downstream equipment (i.e., relief valves). Equipment that, in normal operation, does not constitute a source of ignition; i.e., surface temperature shall not exceed ignition temperature of the specified gas to which it may be exposed, and there are no sliding or make-and-break contacts operating at energy levels capable of causing ignition. Used for all types of equipment in Division 2 locations. Relies on the improbability of an ignition-capable fault condition occurring simultaneously with an escape of hazardous gas. Concept in which equipment is submerged in oil to a depth sufficient to quench any sparks that may be produced. This technique is commonly used for switchgears, but it is not utilized in connection with instruments. The operating pressure of a vessel is the pressure, in pounds per square inch gauge (PSIG), to which the vessel is usually subjected in service. A processing vessel is usually designed for a maximum allowable working pressure, in PSIG, that will provide a suitable

Definitions

OPERATING PRESSURE MARGIN

OPERATING PRESSURE RATIO OPERATING RATIO OF A RUPTURE DISC

OPTICAL PYROMETER

OVERPRESSURE

PARTIAL PRESSURE

PASCAL-SECOND

(PA · S)

PDVF

(POLYVINYLIDENE FLUORIDE)


margin above the operating pressure to prevent any undesirable operation of the relief device. It is suggested that this margin be approximately 10% or 25 PSI (173 kPa), whichever is greater. Such a margin will be adequate to prevent the undesirable opening and operation of the pressure relief valve caused by minor fluctuations in the operating pressure. The margin between the maximum operating pressure and the set pressure of the PRV. The ratio of the maximum operating pressure to the set pressure of the PRV. (1) The ratio of the maximum operating pressure to the marked burst pressure expressed as a percentage (common U.S. definition). (2) The ratio of the maximum operating pressure to the minimum of the performance tolerance expressed as a percentage (common ISO definition). Also called brightness pyrometer, it uses a narrow band of radiation within the visible range (0.4 to 0.7 µm) to measure temperature by color matching and other techniques. The pressure increase over the set pressure of the primary relief device. When the set pressure is the same as the maximum allowable operating pressure (MAOP), the accumulation is the same as the overpressure. Pressure increase over the set pressure of the primary relieving device is overpressure. We may observe from this definition that, when the set pressure of the first (primary) safety or relief valve is less than the maximum allowable working pressure of the vessel, the overpressure may be greater than 10% of set pressure. In a mixture of gases, the partial pressure of one component is the pressure of that component if it alone occupied the entire volume at the temperature of the mixture. Internationally accepted unit of absolute (dynamic) viscosity. One Pa·s = 1 Newton-sec/m2 = 10 poise = 1000 centipoise. This fluorocarbon has substantially lower temperature limits than the others (250°F or 120°C) and is less inert chemically. It is dissolved by the ketones (acetone, MEK, MIBK) and attacked by benzene and high concentrations of sulfuric acid.

xxxiii

The most insidious enemy is caustic, which causes brittleness and cracking. It has much better toughness and abrasion resistance than the other fluorocarbons, as well as unique electrical properties (K = 8). PE (POLYETHYLENE) A low-temperature insulation that is compatible with a wide range of corrosives but is attacked by most petroleum products. Generally limited to situations where fluorocarbons and chlorocarbons are not allowed, such as the tobacco and nuclear power industries. Maximum allowable temperature is in the 180°F (80°C) area. PEEK (POLYETHER A high-temperature, injection-molded ETHERKETONE) polymer that is chemically quite inert. This material has wide chemical application. Temperature capability is high at 450 to 500°F (225 to 260°C). Avoid any liquids with “phenol” in their names. Adhesive bonding to the molded parts would be difficult. PFA A fluorocarbon that is quite inert chem(PERFLUOROALKOXY) ically, melts at a fairly high temperature, and is easily plastic welded. It can be used up to 550°F (290°C) but, as a probe insulation, it is generally limited to 350°F (175°C) because of bonding limitations with the metal rod. PHASE DIFFERENCE A contact radar technology; unlike TDRSENSOR (PDS) based systems, which measure using subnanosecond time intervals, PDS derives level information from the changes in phase angle. PHOTODETECTOR A device that measures thermal radiation by producing an output through release of electrical changes within its body. They are small flakes of crystalline materials, such as CdS or InSb, that respond to different portions of the spectrum, consequently showing great selectivity in the wavelengths at which they operate. PIXEL (“PICTURE A dot that represents the smallest graphic ELEMENT”) unit of display in a digital image, used in machine vision and camera technology. PLENUM Air distribution ducting, chamber, or compartment. POISE (µ) Unit of dynamic or absolute viscosity (dyne-sec/cm2). POISEUILLE (PI) Suggested name for the new international standard unit of viscosity, the pascal-second. POLAROGRAPHY Process for monitoring the diffusion current flow between working and auxiliary

xxxiv

Definitions

electrodes as a function of applied voltage as it is systematically varied. The concentration of analyte allows for the flow of the diffusion current, which is linearly dependent on the analyte concentration. Polarography can be applied using direct current, pulsed direct current, and alternating current voltage excitation waveforms. Dissolved oxygen determination is an example of an application for which polarography is used. POTENTIOMETRY When no current is passing between electrodes. Examples: ORP, pH, selectiveion electrodes. The electromotive force or potential difference (at zero current) is monitored between the measuring and reference electrodes. POTTING Refers to the use of a potting compound to completely surround all live parts, thereby excluding the hazardous atmosphere; has been proposed as a method of protection. There is no known usage except in combination with other means. PP (POLYPROPYLENE) Similar to PE. Used for low cost and where fluorocarbons and chlorocarbons are excluded. Maximum temperature is in the area of 200°F. PRESSURE-RELIEVING The broadest category in the area of DEVICE pressure-relief devices; includes rupture discs and pressure relief valves of both the simple spring-loaded types and certain pilot-operated types. PRESSURE RELIEF A generic term that can refer to relief VALVE (PRV) valves, safety valves, and pilot-operated valves. The purpose of a PRV is to automatically open and to relieve the excess system pressure by sending the process gases or fluids to a safe location when its pressure setting is reached. PRIMARY STANDARD A measuring instrument calibrated at a national standard laboratory such as NIST and used to calibrate other sensors. PROOF A unit of specific gravity used in the alcohol industry. PROTOCOL Formal description of a set of rules and conventions that govern how devices on a network exchange information. (P)TFE The oldest, highest-temperature, and (TETRAFLUOROmost inert fluorocarbon probe insulaETHYLENE) tion. Extremely difficult to adhesive bond, it is usable up to 550°F (290°C) but, on probes, its temperature limit is determined by the type of bonding to the probe rod (300, 450, or 550°F). This is the most common probe insulation in


the industry. Because it never melts (it disintegrates, producing HF at >600°F), it is difficult to fabricate, is impossible to plastic weld, and exhibits a high degree of microporosity. Can be destroyed by butadiene and styrene monomer. (The “P” in (P) TFE stands for polymerized.) PURGING, This refers to the maintenance of a slight PRESSURIZATION, positive pressure of air or inert gas within VENTILATION an enclosure so that the hazardous atmosphere cannot enter. Relatively recent in general application, it is applicable to any size or type of equipment. QUEVENNE DEGREE A specific gravity unit used in expressing the fat content of milk. RACEWAY A general term for enclosed channels, conduit, and tubing designed for holding wires and cables. RADAR (RADIO A system using beamed and reflected DETECTION radio frequency energy for detecting and AND RANGING) locating objects, measuring distance or altitude, navigating, homing, bombing, and other purposes; in detecting and ranging, the time interval between transmission of the energy and reception of the reflected energy establishes the range of an object in the beam’s path. RADIO FREQUENCY A frequency that is higher than sonic (RF) but less than infrared. The low end of the RF range is 20 kHz, and its high end is around 100,000 MHz. RADIO FREQUENCY A phenomenon in which electromagnetic INTERFERENCE (RFI) waves from a source interfere with the performance of another electrical device. RATED RELIEVING The maximum relieving capacity of the CAPACITY PRV. This information is normally provided on the nameplate of the PRV. The rated relieving capacity of the PRV exceeds the required relieving capacity and is the basis for sizing the vent header system. RATIO PYROMETER See two-color pyrometer. REACTANCE (X) The portion of the impedance of a circuit that is caused by capacitance, inductance, or both. Expressed in ohms. REAR MOUNT A technique for making long inactive sections by mounting the probe on the end of a pipe, with its coax cable running through the pipe to the top of the tank. The coax must survive the process temperature, so it is often of hightemperature construction. REFLECTANCE OR The percentage of the total radiation REFLECTIVITY (R) falling on a body that is directly reflected without entry. Reflectance is

Definitions

RELATIVE HUMIDITY

RELATIVE VISCOSITY

RELIEF VALVE

RELIEVING PRESSURE

REOPENING PRESSURE

RESISTIVE COMPONENT

RESISTIVITY

(ρ)

RICHTER DEGREES ROENTGEN (R)

ROENTGEN EQUIVALENT MAN

(REM) ROOT VALVE RUPTURE TOLERANCE OF A RUPTURE DISC


zero for a blackbody and nearly 100% for a highly polished surface. (R = 1 − A − T, where A is the absorbance and T is the transmissivity.) The ratio of the mole fraction of moisture in a gas mixture to the mole fraction of moisture in a saturated mixture at the same temperature and pressure. Alternatively, the ratio of the amount of moisture in a gas mixture to the amount of moisture in a saturated mixture at equal volume, temperature, and pressure. Ratio of absolute viscosity of a fluid at any temperature to that of water at 20°C (68°F). Because water at this temperature has a µ of 1.002 cP, the relative viscosity of a fluid equals approximately its absolute viscosity in cP. Because the density of water is 1, the kinematic viscosity of water equals 1.002 cSt at 20°C. An automatic pressure-relieving device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the operating pressure. It is used primarily for liquid service. The sum of opening pressure plus overpressure. It is the pressure, measured at a valve’s inlet, at which the relieving capacity is determined. The opening pressure when the pressure is raised as soon as practicable after the valve has reseated or closed from a previous discharge. AC current can be separated into two components; the portion that is in phase with the excitation voltage is the resistive component. The property of a conductive material that determines how much resistance a unit cube will produce. Expressed in units of ohm-centimeters (Ω·cm). A unit of specific gravity used in the alcohol industry. A unit for expressing the strength of a radiation field. In a 1-R radiation field, 2.08 billion pairs of ions are produced in 1 cm2 of air. A unit of allowable radiation dosage, corresponding to the amount of radiation received when exposed to 1 R over any period of time. The first valve off the process. The tolerance range on either side of the marked or rated burst pressure

xxxv

within which the rupture disc is expected to burst. Rupture tolerance may also be represented as a minimum–maximum pressure range. Also referred to as performance tolerance in ISO standards. SAFETY RELIEF An automatic pressure-actuated relieving VALVE device suitable for use as either a safety or relief valve. SAFETY VALVE An automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by rapid and full opening or pop action. It can be used for steam, gas, or vapor service. SAND FILLING All potential sources of ignition are buried in a granular solid, such as sand. The sand acts partly to keep the hazardous atmosphere away from the sources of ignition and partly as an arc quencher and flame arrester. It is used in Europe for heavy equipment; it is not used in instruments. SATURATION A condition in which RF current from a probe to ground is determined solely by the impedance of the probe insulation. Increased conductivity in the saturating medium, even to infinity, will not cause a noticeable change in that current or in the transmitter output. SATURATION The pressure of a fluid when condenPRESSURE sation (or vaporization) takes place at a given temperature. (The temperature is the saturation temperature.) SATURATED A solution that has reached the limit of SOLUTION solubility. SAYBOLT FUROL A time unit referring to the Saybolt visSECOND (SFS) cometer with a Furol capillary, which is larger than a universal capillary. SAYBOLT UNIVERSAL A time unit referring to the Saybolt visSECOND (SUS) cometer. SAYBOLT Measures time for given volume of fluid VISCOMETER to flow through standard orifice; units (UNIVERSAL, FUROL) are seconds. SEALING Excluding the atmosphere from potential sources of ignition by sealing such sources in airtight containers. This method is used for components such as relays, not for complete instruments. SEAL-OFF PRESSURE The pressure, measured at the valve inlet after closing, at which no further liquid, steam, or gas is detected at the downstream side of the seat. SEGMENT The section of a network that is terminated in its characteristic impedance. Segments are linked by repeaters to form a complete network.

xxxvi

Definitions

Term used by NFPA-70 (NEC) to demarcate the point at which utility electrical codes published by IEEE (NESC) take over. Includes conductors and equipment that deliver electricity from utilities. SET PRESSURE The pressure at which a relief valve is (OPENING PRESSURE) set to open. It is the pressure, measured at the valve inlet of the PRV, at which there is a measurable lift or at which discharge becomes continuous as determined by seeing, feeling, or hearing. In the pop-type safety valve, it is the pressure at which the valve moves more in the opening direction as compared to corresponding movements at higher or lower pressures. A safety valve or a safety relief valve is not considered to be open when it is simmering at a pressure just below the popping point, even though the simmering may be audible. SHEAR VISCOMETER Viscometer that measures viscosity of a non-Newtonian fluid at several different shear rates. Viscosity is extrapolated to zero shear rate by connecting the measured points and extending the curve to zero shear rate. SIKES DEGREE A unit of specific gravity used in the alcohol industry. SIMMER (WARN) The condition just prior to opening at which a spring-loaded relief valve is at the point of having zero or negative forces holding the valve closed. Under these conditions, as soon as the valve disc attempts to rise, the spring constant develops enough force to close the valve again. SMART FIELD DEVICE A smart field device is a microprocessorbased process transmitter or actuator that supports two-way communications with a host, digitizes the transducer signals, and digitally corrects its process variable values to improve system performance. The value of a smart field device lies in the quality of data it provides. SPECIFIC HUMIDITY The ratio of the mass of water vapor to the mass of dry gas in a given volume. SPECIFIC VISCOSITY Ratio of absolute viscosity of a fluid to that of a standard fluid, usually water, both at the same temperature. SPECTRAL EMISSIVITY The ratio of emittance at a specific wavelength or very narrow band to that of a blackbody at the same temperature. START-TO-LEAK The pressure at the valve inlet at which PRESSURE the relieved fluid is first detected on the SERVICE


STICTION

STOKE STRESS SUBCHANNEL

SUPERIMPOSED BACKPRESSURE

SWITCHED HUB TEFLON, TFE, FEP, AND PFA

THERMOPILE

TRALLES DEGREE

THROUGHPUT

TIME DOMAIN REFLECTOMETER

(TDR)

TIMEOUT

downstream side of the seat before normal relieving action takes place. Combination of sticking and slipping when stroking a control valve. Unit of kinematic viscosity, υ (cm2/sec). Force/area (F/A). In broadband terminology, a frequencybased subdivision creating a separate communication channel. Variable backpressure that is present in the discharge header before the pressure relief valve starts to open. It can be constant or variable, depending on the status of the other PRVs in the system. A multiport bridge joining networks into a larger network. Most people interchange the name Teflon with TFE. This is completely incorrect but understandable. TFE was the first fluorocarbon polymer to carry the trade name “Teflon” at E.I. DuPont. Dupont chose to use the Teflon trade name for a whole family of fluorocarbon resins, so FEP and PFA made by Dupont are also called Teflon. To complicate the matter, other companies now manufacture TFE, FEP, and PFA, which legally cannot be called Teflon, because that name applies only to DuPont-made polymers. A device that measures thermal radiation by absorption to become hotter than its surroundings. It is a number of small thermocouples arranged like the spokes of a wheel with the hot junction at the hub. The thermocouples are connected in series, and the output is based on the difference between the hot and cold junctions. A unit of specific gravity used in the alcohol industry. The maximum number of transactions per second that can be communicated by the system. An instrument that measures the electrical characteristics of wideband transmission systems, subassemblies, components, and lines by feeding in a voltage step and displaying the superimposed reflected signals on an oscilloscope equipped with a suitable timebase sweep. An event that occurs when one network device expects to hear from another network device within a specified period of time but does not. The resulting timeout

Definitions

usually results in a retransmission of information or the dissolving of the session between the two devices. TOPOLOGY (1) Physical arrangement of network nodes and media within an enterprise networking structure. (2) The surface features of an object—“how it looks” or its texture; a direct relation between these features and the material’s properties (hardness, reflectivity, etc.). TOTAL EMISSIVITY The ratio of the integrated value of all spectral emittance to that of a blackbody. TRANSISTOR A three-terminal, solid state electronic device made of silicon, gallium arsenide, or germanium and used for amplification and switching in circuits. TRANSMITTANCE OR The percentage of the total radiant energy TRANSMISSIVITY (T) falling on a body that passes directly through it without being absorbed. Transmittance is zero for a blackbody and nearly 100 percent for a material such as glass in the visible spectrum region. (T = 1 − A − R, where A is the absorbance and R is the reflectance.) TWADDELL DEGREE A unit of specific gravity used in the sugar, tanning, and acid industries. TWO-COLOR A device that measures temperature as PYROMETER a function of the radiation ratio emitted around two narrow wavelength bands. Also called a ratio pyrometer. UPPER EXPLOSIVE The highest concentration of gas or vapor LIMIT (UEL) in air in which a flame will continue to burn after the source of ignition has been removed. VARACTOR A voltage-sensitive capacitor. VARIABLE Backpressure that varies as a result of BACKPRESSURE changes in operation of one or more


VELOCITY GRADIENT

(SHEAR)

VELOCITY HEAD

WAVEGUIDE

WIDEBAND (TOTAL) PYROMETER

WOBBE INDEX

xxxvii

pressure-relief valves connected into a common discharge header. Rate for change of liquid velocity across the stream—V/L for linear velocity profile, dV/dL for nonlinear velocity profile. Units are V–L = ft/sec/ft = sec−1. The velocity head is calculated as v2/2 g, where v is the flowing velocity, and g is the gravitational acceleration (9.819 m/s2 or 32.215 ft/s2 at 60° latitude). A device that constrains or guides the propagation of electromagnetic waves along a path defined by the physical construction of the waveguide; includes ducts, a pair of parallel wires, and coaxial cable. A radiation thermometer that measures the total power density emitted by the material of interest over a wide range of wavelengths. AGA 4A defines the Wobbe index as a numerical value that is calculated by dividing the square root of the relative density (a key flow orifice parameter) into the heat content (or BTU per standard cubic foot) of the gas. Mathematically, the Wobbe index is defined by the equation below:

WI = CV/ SQ where: WI = the Wobbe index CV = the calorific value SG = the specific gravity

ABBREVIATIONS, NOMENCLATURE, ACRONYMS, AND SYMBOLS

2D 3D

two-dimensional three-dimensional

amp AMPS AMS

A a A Å AA AAS abs AC ACFM

ACL ACMH ACMM ACS ACSL A/D AD ADC ADIS A&E AES AF, a-f AFD AGA3 ai AI a(k) ALARA ALARP alpm alt AM

acceleration (1) area; (2) ampere, symbol for basic SI unit of electric current; (3) admittance angstrom (= 10−10 m) atomic absorption atomic absorption spectrometer absolute (e.g., value) alternating current actual cubic feet per minute; volumetric flow at actual conditions in cubic feet per minute (= 28.32 alpm) asynchronous connectionless actual cubic meters per hour actual cubic meters per minute analyzer control system advanced continuous simulation language analog to digital, also analog-to-digital converter actuation depth analog-to-digital converter approved for draft international standard circulation alarm and event atomic emission spectrometer audio frequency adjustable frequency drive American Gas Association Report No. 3 Adobe Illustrator analog input white noise as low as reasonably achievable as low as reasonably practicable actual liters per minute altitude amplitude modulated or actual measurement

AO AOTF AP APC APDU API °API APM AR ARA ARIMA ARP ASCII AS-i ASIC ASK asym ATG atm ATP ATR AUI aux AWG

ampere; also A advanced mobile phone system asset management solutions or analyzer maintenance solutions analog output acousto-optical tunable filters access point automatic process control application (layer) protocol data unit application programming interface or absolute performance index API degrees of liquid density application pulse modulation autoregressive alarm response analysis autoregressive integrated moving average address resolution protocol American Standard Code for Information Interchange actuator sensor interface application-specific integrated circuit amplitude shift keying asymmetrical; not symmetrical automatic tank gauging atmosphere (= 14.7 psi) adenosine triphosphate attenuated total reflectance attachment unit interface auxiliary American wire gauge

B b °Ba bar barg bbl BCD BCS

dead time balling degrees of liquid density (1) barometer; (2) unit of atmospheric pressure measurement (= 100 kPa) bar gauge barrels (= 0.1589 m3) binary coded decimal batch control system xxxix


xl

Abbreviations, Nomenclature, Acronyms, and Symbols

°Bé BFO BFW bhp °Bk blk BMS BOD bp, b.p. BPCS BPS, b/sec BPSK Bq °Br Btu BWG B2B

Baumé degrees of liquid density beat frequency oscillator boiler feedwater brake horsepower (= 746 W) Barkometer degrees of liquid density black (wiring code color for AC “hot” conductor) burner management system biochemical oxygen demand boiling point basic process control system bits per second binary phase shift keying becquerel, symbol for derived SI unit of radioactivity, joules per kilogram, J/kg Brix degrees of liquid density British thermal unit (= 1054 J) Birmingham wire gauge business to business

C c C °C ca. CAC CAD Cal CAN CARI CATV cc CCD CCF Ccm CCR Ccs CCS CCTV CCW CD cd CD CDDP CDF CDMA CDPD CDT CDTP CEMS


(1) velocity of light in vacuum (3 × 108 m/s); (2) centi, prefix meaning 0.01 coulombs, symbol for discharge coefficient, capacitance Celsius degrees of temperature circa (about, approximately) channel access code computer aided design calorie (gram = 4.184 J); also g-cal control area network or control and automation network combustion air requirement index community antenna television (cable) cubic centimeter (= 10−6 m3) charge-coupled device common cause failure or combination capacity factor cubic centimeter per minute central control room constant current source computer control system or constant current source closed circuit television counterclockwise dangerous coverage factor candela, symbol for basic SI unit of luminous intensity compact disk or collision detector cellular digital data packet cumulative distribution function code division multiple access cellular digital packet data color detection tube cold differential test pressure continuous emissions monitoring system

CENP CE CFA CFM, cfm, ft3/min CFR CF/yr Ci CI CIM CIP

CJ CIP CL1 CLD CLP cm CM CMF CMMS CMPC cmph, m3/h CNI CO CO2 CO2D COD COF COM COTS cpm Co cos cp, c.p.

cps

CPS CPU CPVC CR CRC

CRDS

combustion engineering nuclear power Conformité Européne (European Conformity), applicable to electrical safety Continuous flow analyzer cubic feet per minute (28.32 lpm) Code of Federal Regulations cubic foot per year curie (= 3.7 × 1010 Bq) cast iron computer integrated manufacturing computer aided production or control and information protocol (an application layer protocol supported by DeviceNet, ControlNet, and Ethernet/IP) cold junction clean in place electrically hazardous, Class 1, Division 1, Groups C or D chemiluminescence detector closed-loop potential factor centimeter (= 0.01 m) condition monitoring or communication (interface) module Coriolis mass flowmeter computerized maintenance management system constrained multivariable predictive control cubic meter per hour ControlNet International controller output or carbon monoxide carbon dioxide carbon dioxide demand chemical oxygen demand coefficient of haze component object model commercial off-the-shelf cycles per minute; counts per minute cobalt cosine (trigonometric function) (1) candle power, (2) circular pitch, (3) center of pressure (cp and ctp sometimes are used for centipoise) (1) cycles per second (hertz, Hz); (2) counts per second; (3) centipoise (= 0.001 Pa·S) computerized procedure system central processing unit chlorinated polyvinyl chloride corrosion rate cyclical redundancy check or cyclic redundancy code. (An error detection coding technique based on modulo-2 division. Sometimes misused to refer to a block check sequence type of error detection coding.) cavity ring-down spectroscopy


CRLF CRT Cs CS CSL CSMA/CD CSO CSS cSt CSTR CT

CTDMA CTMP CVAAS CVF cvs CW

carriage return-line feed cathode ray tube cesium carbon steel car seal lock carrier sense, multiple access with collision detection car seal open central supervisory station centistoke continuous-stirred tank reactor cooling tower or the product of C for disinfectant concentration and T for time of contact in minutes concurrent time domain multiple access chemi-thermo-mechanical pulp cold vapor atomic absorption spectroscopy circular variable filters comma-separated variables clockwise

DI dia DIAC DIR DIS DIX

d(k) D(k) DLE DLL DMA DMM DN DO DOAS d/p cell

D d

D DA D/A DAC DACU DAE DAMPS dB DBB DBPSK DC DC, dc DCE DCOM DCS DD

D/DBP DDC DDE DDL

deg DES DFIR DFR DFT DH


(1) derivative, (2) differential as in dx/dt, (3) deci, prefix meaning 0.1, (4) depth, (5) day diameter; also dia and φ or derivative time of a controller data access digital-to-analog device access code data acquisition and control unit differential algebraic equation digital advanced mobile phone system decibels double-block and bleed differential binary phase shift keying diagnostic coverage direct current data communications equipment distributed COM distributed control system data definition or dangerous component failure is detected in leg or a device description written using DDL disinfectants/disinfection byproducts direct digital control dynamic data exchange device description language (an objectoriented data-modeling language currently supported by PROFIBUS, FF, and HART) degree; also ° (π/180 rad) data encryption standard diffused infrared digital fiber-optic refractometer digital Fourier transform data highway

DPD DPDT dpi DQPSK DSL DSP DSR DSSS DT DTC DTE DTGS DTM

DU DVM

xli

discrete (digital) input diameter; also, D and φ dedicated inquiry access code diffused infrared draft international standard Digital-Intel-Xerox (DIX is the original specification that created the de facto Ethernet standard. IEEE 802.3 came later, after Ethernet was established.) unmeasured disturbance measured disturbance data link escape dynamic link library dynamic mechanical analyzer digital multimeter diameter normal, the internal diameter of a pipe in rounded millimeters dissolved oxygen or discrete (digital) output differential optical absorption spectroscopy differential pressure transmitter (a Foxboro trademark) N,N-diethyl-p-phenylenediamine double-pole double-throw (switch) dots per inch differential quadrature phase shift keying digital subscriber line digital signal processing direct screen reference direct sequence spread spectrum dead time (second or minutes) digital temperature compensation data terminal equipment deuterated tryglycine sulfate device type manager (An active-X component for configuring an industrial network component. A DTM “plugs into” an FDT.) dangerous component failure occurred in leg, but undetected digital voltmeter

E e

E E{.} EAI EAM EBCDIC EBR ECD ECKO ECN

(1) error, (2) base of natural (Naperian) logarithm, (3) exponential function; also exp (−x) as in e−x (1) electric potential in volts, (2) scientific notation as in 1.5E − 03 = 1.5 × 10−3 expected value operator enterprise application integration enterprise asset management extended binary code for information interchange electronic batch records electron capture detector eddy-current killed oscillator effective carbon number

xlii


ECTFE EDS EDTA EDXRF E/E/PE E/E/PES EFD e.g. EHC EHM e(k) E.L. Emf, EMF EMI EMI/RFI em(k) EN EPA EPC EPCM EPDM EPS EQ, eq ERM ERP ERW ESD ESN ETFE ETS Exp

ethylene chloro-tetra-fluoro-ethylene (Halar) electronic data sheet (DeviceNet) ethylenediaminetetraacetic acid energy dispersive x-ray fluorescence electrical/electronic/programmable electronic electrical/electronic/programmable electronic system engineering flow diagram exempli gratia (for example) electrohydraulic control equipment health management feedback error elastic limit (1) electromotive force (volts), (2) electromotive potential (volts) electromagnetic interference electromagnetic interference/radio frequency interference process/model error European standard enhanced performance architecture, Environmental Protection Agency engineering-procurement-construction (firm or industry) engineering, procurement, and construction management (companies) ethylene propylene diene terpolymer electronic pressure scanner, Encapsulated PostScript file or emergency power supply equation enterprise resource manufacturing enterprise resource planning, effective radiated power electric-resistance welded emergency shutdown (system), electrostatic discharge electronic serial number ethylene-tetrafluoroethylene copolymer (Tefzel®) equivalent time sampling exponential function as in exp (– at) = e−at; also e

F F F °F FAT FBAP FBD FBG FC FCC FCOR


frequency (also freq.) farad, symbol for derived SI unit of capacitance, ampere-second per volt, A·s/V degrees Fahrenheit [t°C = (t°F − 32)/1.8] factory acceptance testing function block application process (FF) function block diagram fiber bragg grating flow controllers fluid catalytic cracking unit filtering and correlation (method)

FCS FDE FDL FDMA FDS FDT

FE FEED FEGT FEP FES FF-HSE FFT FH Fhp FHSS FI FIA FIC FID FIE FIFO Fig. FISCO fl. fl. oz. FM FMCW FMEA FMEDA FMS FO FOP FOV fp, f.p. FPC FPD FPM, fpm, ft/min fps, ft/s FRC FRM FS, fs FSC FSD FSK FT FTA FTIR FTNIR FTP FTS FTU

frame check sequence fault disconnection electronics fieldbus data link frequency division multiple access flame-detection system field device tool (a Windows®-based Microsoft framework for engineering and configuration tools) final elements front end engineering and design furnace exit gas temperature fluorinated ethylene propylene fixed end system Foundation Fieldbus, high-speed Ethernet fast Fourier transform frequency hopping fractional horsepower (e.g., 1/4-hp motor) frequency hopped spread spectrum flow indicator flow injection analyzer flow indicator controller flame ionization detector flame ionization element first-in, first-out figure fieldbus Intrinsic Safety COncept fluid fluid ounce (= 29.57 cc) frequency modulated frequency modulated carrier wave failure mode and effects analysis failure modes, effects and diagnostic analysis fieldbus message specification or fieldbus messaging services/system fiber optic or fail open fiber-optic probe field of view freezing point fine particle content flame photometric detector feet per minute (= 0.3048 m/m) feet per second (= 0.3048 m/s) flow recording controller frequency response method full scale fail safe controller full scale deflection frequency shift keying Fourier transform fault tree analysis Fourier transform infrared Fourier near infrared file transfer protocol fault-tolerant system formazin turbidity unit


G g G gal GB GbE gbps, GBPS Gc GC g-cal Gd GD GD GEOS Gff GFC GHz GIAC GLR G-M Gm GMR GPH, gph, gal/h Gp GPM, gpm, gal/min GPS g grn GSC GSD GUI GWR Gy

acceleration resulting from gravity (= 9.806 m/s2) or conductivity giga, prefix meaning 109, or process gain or conductance gallon (= 3.785 liters) gigabyte, 1,000,000,000 bytes gigabit Ethernet gigabits per second feedback controller transfer function gas chromatograph gramcalorie, see also cal unmeasured disturbance transfer function measured disturbance transfer function approximate feedforward transfer function model geosynchronous Earth orbit satellite feedforward controller transfer function gas filter correlation gigahertz general inquiry access code gas-to-liquid ratio Geiger-Mueller tube, for radiation monitoring model transfer function giant magneto resistive gallons per hour (= 3.785 lph)

H1

HAD H&RA HART


HSE HSI HTG HTML HTTP HVAC H/W HWD Hz

process transfer function gallons per minute (= 3.785 lpm) global positioning satellite, global positioning system gram green (wiring code color for grounded conductor) gas-solid chromatography Profibus version of an electronic data sheet graphical user interface guided wave radar gray, symbol for derived SI unit of absorbed dose, joules per kilogram, J/kg

(1) height, (2) hour (1) humidity expressed as pounds of moisture per pound of dry air; (2) henry, symbol of derived SI unit of inductance, volt-second per ampere, V·s/A field-level fieldbus; also refers to the 31.25 kbps intrinsically safe SP-50, IEC61158–2 physical layer historical data access hazard and risk analysis highway addressable remote transducer

HAZard and OPerability studies horizontal cross-connect hydrogen cyanide header error check hydrogen fluoride or hydrofluoric acid human factors engineering hardware fault tolerance higher heating value high-integrity protection systems high-integrity pressure protection system host interface system test human-machine interface hexamethyldisiloxane horizontal horsepower (U.S. equivalent is 746 W) high-pressure (or high-precision) liquid chromatography high-speed Ethernet (host-level fieldbus) human system interface hydrostatic tank gauging hypertext markup language hypertext transfer protocol heating, ventilation, and air conditioning hardware height, width, depth hertz, symbol for derived SI unit of frequency, one cycle per second (l/s)

I I

H h H

HAZOP HC HCN HEC HF HFE HFT hhv HIPS HIPPS HIST HMI HMSD hor. HP, hp HPLC

xliii

IA IAC IAE I&C I&E IAQ ibidem IC ICA ICCMS ICMP ICP ID i.e. IEH IETF IIS IL ILD IMC iMEMS in.

integral time of a controller in units of time/repeat instrument air inquiry access code integral of absolute error instrumentation and control or information and control instrument and electrical indoor air quality in the same place integrated circuit, intermediate crossconnect, or inorganic carbon independent computing architecture inadequate core cooling monitoring system internet control message protocol inductively coupled plasma inside diameter id est (that is) Instrument Engineers’ Handbook Internet engineering task force Internet information server instruction list instrument loop diagrams internal model control integrated microelectromechanical system inch (= 25.4 mm)

xliv


InGaAs in-lb I/O I-P IP IPA IPL IPTS IR IS ISAB ISE ISFET ISM ISP IT ITAE ITSE ITT JTU IXC

iridium gallium arsenide inch-pound (= 0.113 N × m) input/output current-to-pressure conversion Internet protocol or ionization potential isopropyl alcohol independent protection layer international practical temperature scale infrared intermediate system ionic strength adjustment buffer integral of squared error or ion selective electrode ion-selective field-effect transistor industrial, scientific, medical Internet service provider or interoperable system provider information technology (as in IT manager or IT department) integral of absolute error multiplied by time integral of squared error multiplied by time intelligent temperature transmitters Jackson turbidity unit interexchange carrier

J J JIT

joule, symbol for derived SI unit of energy, heat or work, Newton-meter, N·m just-in-time (manufacturing)

K k K K

kbs, kbps, kb/sec kBps, kB/sec k-cal kg kg-m KHP kip km KOH Kp kPa kVA kW KWD kWh


kilo, prefix meaning 1000 coefficient, also dielectric constant Kelvin, symbol for SI unit of temperature or process gain (dimensionless), not used with degree symbol kilobits per second kilobytes per second kilogram-calories (= 4184 J) kilogram symbol for basic SI unit of mass kilogram-meter (torque, = 7.233 footpounds) potassium acid phthalate 1000 pounds (= 453.6 kg) kilometers potassium hydroxide proportional gain of a PID controller kilopascal kilovolt-amperes kilowatts kilowatt demand kilowatt-hours (= 3.6 × 106J)

L l L L2F Lab LAN LAS lat lb LC LCD Lch LCM LCSR LD LDA LDP LEC LED LEL LEOS LF LGR LI LIC LIDAR lim lin liq LLC lm ln LNG LO LOC log, log10 LOI long. LOPA LOS LP or LPG LPC LPG lph lpm LPR LQG LRC LRL LRV

liter (= 0.001 m3 = 0.2642 gal), also L (1) length, (2) inductance, expressed in henrys laser two-focus anemometer CIE functions for lightness, red/green, blue/yellow local area network link active scheduler (FF) latitude pound (= 0.4535 kg) level controller or liquid chromatography liquid crystal display CIE functions for lightness, chroma, hue life cycle management loop current step response ladder diaphragm laser Doppler anemometer large display panel local exchange carrier or lower explosive limit light-emitting diode lower explosive limit low Earth orbit satellites linear feet liquid-to-gas ratio level indicator level indicator controller laser induced doppler absorption radar or light detection and ranging limit linear liquid logical link control lumen, symbol for derived SI unit of luminous flux, candela-steradian, cd·sr Naperian (natural) logarithm to base e liquefied natural gas lock open limiting oxygen concentration logarithm to base 10; common logarithm local operation interface longitude layers of protection analysis line of sight liquefied petroleum or propane gas large particle content liquefied petroleum gas liters per hour (0.2642 gph) liters per minute (0.2642 gpm) linear polarization resistance linear quadratic Gaussian longitudinal redundancy check or level recording controller lower range limit lower range value


LTI LVDT LVN lx

m

M

mA MAC MACID MAE MAOP MAP MAU MAWP max. Mb MB Mbps, Mb/sec MBps, MB/sec MC mCi, mC m.c.p. MCR MCT MDBS MDIS m/e med. MEDS MEMS m.e.p. MES MeV MFD MFE mfg mg mho mi MI MIB micro

micron


linear time-invariant linear variable differential transformer limiting viscosity number lux, symbol for derived SI unit of illuminance, lumen per square meter, lm/m2

MIE MIMO MIMOSA

M

MIR MIS ml MLR mm

(1) meter, symbol for basic SI unit of length, (2) milli, prefix meaning 10–3, (3) minute (temporal) (also min) (1) 1000 (in commerce only), (2) mach number, (3) molecular weight; mole; (4) mega, prefix meaning 106 milliampere (= 0.001 A) medium access control medium access control identifier minimum absolute error maximum allowable operating pressure manufacturing automation (access) protocol media access unit maximum allowable working pressure maximum megabit, 1,000,000 bits megabyte, 1,000,000 bytes megabits per second megabytes per second main cross-connect millicuries (= 0.001 Ci) mean candle power main control panel mercury cadmium telluride mobile data base station mobile data intermediate system mass-to-energy ratio medium or median medium Earth orbit satellite microelectromechanical system mean effective pressure manufacturing execution system or mobile end station mega-electron volt mechanical flow diagram magnetic flux exclusion manufacturer or manufacturing milligrams (= 0.001 g) outdated unit of conductance, replaced by siemens (S) mile (= 1.609 km) melt index management information base prefix meaning 10−9; also µ (mu); sometimes (incorrectly) u, as in ug to mean µg [both meaning microgram (= 10−9 kg)] micrometer (= 10−6 m) (term now considered obsolete)

min.

mmf MMI mmpy MMS MOC MODEM MON MOS MOSFET mol mol. MOON mp, m.p. MPa MPC MPFM mph, MPH, mi/h mps, m/s MPS mpy mR mrd mrem MRP ms, msec MS MSA MSB MSD MSDS MT MTBE MTBF MTSO MTTF MTTFD MTTFS MTTR MTU

xlv

minimum ignition energy multiple-input, multiple-output machinery information management open system alliance (1) minute (temporal), also m, (2) minimum, (3) mobile identification number multiple internal reflection management information system milliliter (= 0.001 l = 1 cc) multiple linear regression millimeter (= 0.001 m) or millimicron (= 10−9 m) magnetomotive force in amperes man-machine interface millimeters per year machine monitoring system or manufacturing message specification management of change modulator/demodulator motor octane number metal oxide semiconductor metallic oxide semiconductor field-effect transistor mole, symbol for basic SI unit for amount of substance molecule M out of N voting system melting point megapascal (106 Pa) model predictive control multiphase flowmeter mile per hour (1.609 km/h) meters per second manufacturing periodic/aperiodic services mills per year milliroentgen (= 0.001 R) millirad (= 0.001 rd) milliroentgen-equivalent-man material requirement planning or manufacturing resource planning millisecond (= 0.001 s) mass spectrometer, Microsoft® metropolitan statistical areas most significant bit most significant digit material safety data sheet measurement test methyl tertiary butyl ether mean time between failures mobile telephone switching office mean time to failure mean time to fail dangerously mean time to spurious failure mean time to repair master terminal unit

xlvi


MVC MW MWC MWD

minimum variance controller megawatt (= 106 W) municipal waste combustors molecular weight distribution

OLE OLE_DB OMMS ON OPC

N N

n N0 N-16 NAAQS NAP NAT NB NC, N/C NC NDIR NDM NDT NEC NESC NEXT NIC NIP NIR nm NMR NO, N/O NPS NRM NRZ NS NTC NTP

NTSC NTU NUT

Newton, symbol for derived SI unit of force, kilogram-meter per second squared, kg·m/s2 (1) nano, prefix meaning 10−6, (2) refractive index Avogadro’s number (= 6.023 × 1023 mol−1) nitrogen-16 national ambient air quality standards network access port/point network address translation nominal bore, internal diameter of a pipe in inches normally closed (switch contact) numeric controller nondispersive infrared normal disconnect mode nondestructive testing National Electrical Code National Electrical Safety Code near-end crosstalk network interface card normal incident pyrheliometer near infrared nanometer (10−9 m) nuclear magnetic resonance normally open (switch contact) nominal pipe size, the internal diameter of a pipe in inches normal response mode non-return to zero (refers to a digital signaling technique) nominal pipe size, the internal diameter of a pipe in inches negative temperature coefficient network time protocol or normal temperature and pressure corresponding to 1 atm absolute (14.7 psia) and 0°C (32°F) National Television Standards Code nephalometric turbidity unit network update time

OP-FRIR OP-HC OP-TDLAS OP-UV or ORP OS OSFP OSI OSI/RM OT OTDR oz

P P&ID p Pa

PA PAC PAL PAN P&ID Pas, Pa·s PAS PB PC PCA PCCS PCDD PCDF PCR PCS

O OCD OD ODBC OES oft, OFT ohm OJT


orifice-capillary detector outside diameter or oxygen demand open database connectivity or communication optical emission spectrometer optical fiber thermometry unit of electrical resistance; also Ω (omega) on-the-job training

object linking and embedding object linking and embedding data base optical micrometer for micromachine octane number object link embedding (OLE) for process control open path Fourier-transform infrared open-path hydrocarbon open-path tunable diode-laser absorption spectroscopy open-path ultraviolet orange (typical wiring code color) oxidation-reduction potential operator station or operating system open shortest path first open system interconnect (model) open system interconnect/reference model operator terminal or open tubular optical time domain ounce (= 0.0283 kg)

pct PCTFE PCV PD PDA PDD

piping and instrumentation diagram (1) pressure; (2) pico, prefix meaning 10−12 (3) variable for resistivity pascal, symbol for derived SI unit of stress and pressure, Newtons per square meter, N/m2 plant air path average concentration phase alternating line personal area network piping (process) and instrumentation diagram (drawing) pascal-second, a viscosity unit process automation system (successor to DCS) proportional band of a controller in percent (100%/controller gain) personal computer (usually Microsoft Windows®-based) or pressure controller principal component analysis personal computer control system polychlorinated dibenzo-p-dioxine polychlorinated dibenzo furans principal component regression process control system or personal communication services percent; also% polychlorotrifluoroethylene pressure control valve positive displacement or proportional and derivative personal digital assistant or photodiode array pulsed discharge detector


PDF PDS PDU PDVF PE PED PEEK PEL PES PFA PFC PFD PdM pF PF, p.f. PFA PFD PFD PFDavg PFPD PGNAA PGC pH PHA pi, pl PI P/I PIC PID

PI-MDC PIMS PIP PIR PLC PLS PM PMA PMBC PMD PMF PMMC PMT POPRV PP ppb, PPB ppm, PPM PPP ppt


probability density function, probability of failure or portable document file phase difference sensor protocol data unit polyvinylidene fluoride polyethylene pressure equipment directive poly ether ether ketone permissible exposure level programmable electronic system per-fluoro-alkoxy copolymer procedure functional chart process flow diagram predictive maintenance picofarad (= 10−12 F) power factor perfluoralkoxy (a form of Teflon) process flow diagram probability of failure on demand average probability of failure on demand pulsed flame photometric detector prompt gamma neutron activation analysis process gas chromatograph acidity or alkalinity index (logarithm of hydrogen ion concentration) process hazard analysis Poiseuille, a viscosity unit proportional and integral, or pressure indicator pneumatic to current (conversion) pressure indicating controller or path integrated concentration proportional, integral, and derivative (control modes in a classic controller), or photoionization detector path integrated minimum detectable concentration process information management system process industry practices precision infrared radiometer programmable logic controller physical layer signaling or partial least squares photomultiplier physical medium attachment process model based control photomultiplier detector probability mass function permanent magnet moving coil photomultiplier tube or photometer tube pilot-operated pressure relief valve polypropylene parts per billion parts per million point-to-point protocol parts per trillion

PRC PRD precip PRV PS PSAT PSD PSG PSI psi, PSI, lb/in2 PSIA, psia PSID, psid PSIG, psig PSK PSM PSSR PSTN PSU PSV pt PTB PTC PTFE PUVF PV PVC PVDF PVLO PVHI PWM PWR PZT

xlvii

pressure recording controller pressure relief device precipitate or precipitated pressure relief valve power supply (module) pre-startup acceptance test power spectral density or photosensitive device phosphosilicate glass pre-startup inspection pounds per square inch (= 6.894 kPa) absolute pressure in pounds per square inch differential pressure in pounds per square inch above atmospheric (gauge) pressure in pounds per square inch phase shift keying process safety management re-startup safety review public switched telephone network post-startup pressure safety valve point, part, or pint (= 0.4732 liter) Physikalisch-Technische Bundesanstalt positive temperature coefficient polytetrafluoroethylene (conventional Teflon) pulsed ultraviolet fluorescence process variable (measurement) or the HART primary variable polyvinyl chloride polyvinylidene fluoride process variable low (reading or measurement) process variable high (reading or measurement) pulse width modulation pressurized water reactor lead-zirconate-titanate ceramic

Q q q−1 Q °Q QA QAM QCM QPSK qt QV

(1) rate of flow, (2) electric charge in coulombs, C backward shift operator quantity of heat in joules, J, or electric charge Quevenne degrees of liquid density quality assurance quadrature amplitude modulation quartz crystal microbalance quadrature phase shift keying quart (0.9463 l) quaternary variable

R r r2

radius; also rad multiple regression coefficient

xlviii


R

Ra rad

RADAR RAID RAM R&D RASCI RCU RDP rem rev Re ReD RF, rf RFC RFF RFI RFQ RGA RGB RGM RH RI RIP r(k) RMS, rms ROI ROM RON RPC RPM, rpm, r/min RVP rps, r/sec RRF RRT RS RSA RSS RTD RTO


(1) resistance, electrical, in ohms, (2) resistance, thermal, meter-Kelvin per watt, m·K/W, (3) gas constant (= 8.317 × 107 erg·mol−1,°C−1), (4) roentgen, symbol for accepted unit of exposure to X and gamma radiation (= 2.58 × 10−4 C/kg) radium (1) radius, also r, (2) radian, symbol for SI unit of plane angle measurement or symbol for accepted SI unit of absorbed radiation dose (= 0.01 Gy) radio detection and ranging redundant array of inexpensive disks random access memory research and development responsible for, approves, supports, consults, informed remote control unit remote desktop protocol roentgen equivalent man (measure of absorbed radiation dose by living tissue) revolution, cycle Reynolds number Reynolds number corresponding to a particular pipe diameter radio frequency request for comment (an Internet protocol specification) remote fiber fluorimetry radio frequency interference request for quote residual gas analyzer red, green, blue reactive gaseous mercury relative humidity refractive index routing information protocol set point root mean square (square root of the mean of the square) or rotary mirror sleeves return on investment read-only memory research octane number remote procedure call (RFC1831) revolutions per minute Reid vapor pressure revolutions per second risk reduction factor relative response time (the time required to remove most of the disturbance) recommended standard rural service areas root sum squared resistance temperature detector real-time optimization or operation

RTOS RTR RTS RTS/CTS RTU RUDS RV RWS

real-time operating system remote transmission request ready (or request) to send request to send/clear to send remote terminal unit reflectance units of dirt shade relief valve remote workstation

S s S s2y SAP sat. SAT SAW SC SCADA SCCM SCD SCFH SCCM SCD SCE SCFH SCFM SCM SCMM SCO SCOT SCR SCS SD SDIU SDN SDS SEA sec SER SFC SFD SFF SFI SFR S.G. SHE SHS SID SIF SIG

second (also sec), symbol for basic SI unit of time; also Laplace variable siemens (siemens/cm), symbol for unit of conductance, amperes per volt, A/V sample variance of output y service access point saturated site acceptance test or supervisory audio tone surface acoustic wave system codes supervisory control and data acquisition standard cubic centimeter per minute streaming current detector standard cubic feet per hour standard cubic centimeter per minute sulfur chemilumenesce detector saturated calomel electrode standard cubic feet per hour standard cubic feet per minute (air flow at 1.0 atm and 70°F) station class mark standard cubic meters per minute synchronous connection oriented support coated open tubular (column) silicon-controlled rectifier sample control system component in leg has failed safe and failure has been detected Scanivalve digital interface unit send data with no acknowledgement smart distributed system spokesman election algorithm second, also s sequence of event recorder sequential function chart system flow diagram or start of frame delimiter safe failure fraction sight flow indicator spurious failure rate specific gravity, also sp. gr. standard hydrogen electrode sample handling system system identification digit (number) safety instrumented function special interest group


SIL sin SIS SISO SG, SpG SIL SIS SKU SLAMS SLC slph slpm SMR SMTP S/N SNG SNMP SNR SOAP

SOE SONAR SOP SP SPC SPDT SPL SPRT SPST sq SQC SQL Sr SRD SRS SRV SS SSL SSU std. ST STEL STEP STP

STR SU SUS SV S/W


safety integrity level sine, trigonometric function safety instrumented system single-input single output specific gravity; also sp. gr. safety integrity level safety instrumented system stock keeping units state and local air monitoring stations safety life cycle standard liters per hour standard liters per minute specialized mobile radio simple mail transfer (management) protocol signal-to-noise (ratio) synthetic natural gas simple network management protocol signal-to-noise ratio simple object access protocol (an Internet protocol that provides a reliable streamoriented connection for data transfer) sequence of events sound navigation and ranging standard operating procedure set point statistical process control single-pole, double-pole throw (switch) sound pressure level or sound power level standard platinum resistance thermometer single-pole, single-throw (switch) square, squared statistical quality control structured (or standard) query language steradian, symbol for SI unit of solid angle measurement send and request data with reply safety requirements specification safety relief valve stainless steel secure socket layers Saybolt seconds universal standard structural text short-term exposure limit standard for the exchange of product model data shielded twisted pair, or standard temperature and pressure, corresponding to 70°F (21.1°C) and 14.7 psia (1 atm abs) spurious trip rates security unit or component in leg has failed safe and failure has not been detected Seybold universal seconds secondary variable or safety valve software

xlix

T T T

T1/2 tan TAS tau, τ TBM t/c TC TCD TCP TCP/IP TCV td Td TDLAS TDM TDMA TDR T/E TEM TG Ti TI TIC TIFF TISAB TLV TMP TMR TN TOC TOD TOF TQM TOP TR T/R TRC T.S. TTFM TTP TV

(1) ton (metric = 1000 kg), (2) time, (3) thickness (1) temperature, (2) tera, prefix meaning 10−12, (3) period (= 1/Hz, in seconds), (4) tesla, symbol for derived SI unit of magnetic flux density, webers per square meter, Wb/m2 half life tangent, trigonometric function thallium arsenic selenide process time constant (seconds) tertiary butyl mercaptan thermal coefficient of linear expansion thermocouple, temperature controller, or total carbon thermal conductivity detector transmission control protocol transmission control protocol/internet protocol temperature control valve process dead time (seconds) derivative time (in seconds) of a PID controller tunable diode laser absorption spectroscopy time division multiplexing time division multiple access time domain reflectometry thermoelectric transmission electron microscope thermogravimetry integral time (in seconds) of a PID controller time interval between proof tests (test interval), temperature indicator temperature indicating controller or total inorganic carbon tagged image file format total ionic strength adjustment buffer threshold limit value thermomechanical pulp triple modular redundancy total nitrogen total organic carbon total oxygen demand time of flight total quality management technical and office protocol temperature recorder transmit/receive temperature recording controller tensile strength transit time flow measurement through the probe tertiary variable

l


°Tw TWA

Twaddell degrees of liquid density time weighed average

U u UART UBET UCMM UDP

UEL ufb(k) UFD uff(k) UHF UHSDS u(k) UML UPS UPV URL URV USB UTP UTS UUP UV UV-VIS-NIR

prefix = 10−6, used incorrectly when the Greek letter µ is not available universal asynchronous receiver transmitter unbiased estimation unconnected message manager user/universal data protocol (an Internet protocol with low overhead but no guarantee that communication was successful) upper explosive limit feedback controller output utility flow diagram feedforward controller output ultra-high frequency ultra-high-speed deluge system controller output universal modeling language uninterruptible power supply unfired pressure vessel upper range limit upper range value universal serial bus unshielded twisted pair ultimate tensile stress unshielded untwisted pair ultraviolet ultraviolet-visible-near infrared

V v V

Vac V&V VBA VDF VDT VDU vert. VFD VFIR VHF VIS V-L


velocity volt, symbol for derived SI units of voltage, electric potential difference and electromotive force, watts per ampere, W/A voltage, alternating current verification and validation Visual Basic for Applications vacuum fluorescent display video display tube video display unit vertical variable frequency drive very fast infrared very high frequency visible vapor-liquid (ratio)

VLF V/M VME VMS VOC VR VRML vs.

very low frequency voltmeter Virsa Module Europa (IEEE 1014–1987) vibration monitoring system volatile organic compounds virtual reality virtual reality modeling language versus

W w W

w. WAN Wb WCOT WDXRF WG wh WI WLAN WPAN WS wt

(1) width, (2) mass flow rate (1) watt, symbol for derived SI unit of power, joules per second, J/s, (2) weight (also wt) water wide area network weber, symbol for derived SI unit of magnetic flux, volt-seconds, V·s wall coated open tubular (column) wavelength dispersion x-ray fluorescence standard (British) wire gauge white (wiring code color for AC neutral conductor) Wobbe index wireless local area network wireless personal area network workstation weight, also W

X X XML x-ray XRF XYZ

reactance in ohms extensible markup language electromagnetic radiation with a wavelength Η X

M X State State 1 2

L

t


t

Output signal type is different from that of input signal. * is equal to: E – voltage, A – analog I – current, B – binary P – pneumatic, D – digital R – resistance, H – hydraulic O – electromagnetic, sonic

t

1.2 Functional Diagrams and Function Symbols

45

TABLE 1.2c Continued Mathematical Function Block Symbols (proposed for the next revision of ISA S5.1 [now ANSI/ISA-5.01.01] at the time of this writing) No. 28

Symbol/Function

Equation/Graph

∗HL

Definition Output states are dependent on value of input. Output changes state when input is equal to or lower than an arbitrary low limit or equal to or higher than an arbitrary high limit.

(State 1) M = 1 @ X ≤ L (State 2) M = 0 @ L < X < H (State 3) M = 1 @ X ≥ Η X

High/Low Signal Monitor

M X State 1

H

State 2

State 3

L t

29

T

t

Output equals input that is selected by transfer. Transfer is actuated by external signal.

(State 1) M = X1 (State 2) M = X2 X

X1

M

Transfer

State 2

State 1

X2

t

See statement of permission on page 31.


t

1.3

Instrument Terminology and Performance* B. G. LIPTÁK

(1982, 1995, 2003)

This section was reprinted with format change only from the work titled “Process Instrumentation Terminology (ANSI/ ISA-51.1-1979, Reaffirmed 26 May 1995)” with the permission of The Instrumentation, Systems and Automation Society. This permission is gratefully acknowledged. When using the definitions in this document, please indicate, “This definition is from ANSI/ISA-51.1–1979 (R1993), Process Instrumentation Terminology. Copyright © 1993, ISA—The Instrumentation, Systems and Automation Society.” For information, visit www.isa.org. The purpose of this standard is to establish uniform terminology in the field of process instrumentation. The generalized test procedures described in the section titled “Test Procedures” are intended only to illustrate and clarify accuracyrelated terms. It is not intended that they describe specific and detailed test procedures. This process instrumentation terminology standard is intended to include many specialized terms used in the industrial process industries to describe the use, performance, operating influences, hardware, and product qualification of the instrumentation and instrument systems used for measurement, control, or both. Many terms and definitions relate to performance tests and environmental influences (operating conditions) as further explained in the “Introductory Notes” section. Basically, this document is a guideline to promote vendor/user understanding when referring to product specifications, performance, and operating conditions. Process industries include chemical, petroleum, power generation, air conditioning, metallurgical, food, textile, paper, and numerous other industries. The terms of this standard are suitable for use by people involved in all activities related to process instrumentation, including research, design, manufacture, sales, installation, test, use, and maintenance. The standard consists of terms selected primarily from Scientific Apparatus Makers Association (SAMA) Standard PMC20.1 and American National Standards Institute (ANSI) Standard C85.1. Additional terms have been selected from other recognized standards. Selected terms and definitions have not been modified unless there was a sufficiently valid reason for doing so. New terms have been added and defined where necessary.

This standard is primarily intended to cover the field of analog measurement and control concepts and makes no effort to develop terminology in the field of digital measurement and control.

INTRODUCTORY NOTES Defined terms, where used as a part of other definitions, are set in italics to provide a ready cross-reference. In defining certain performance terms, the context in which they are used has been considered. It is fitting, therefore, that the philosophy of performance evaluation on which these terms are based be explained. Ideally, instruments should be designed for realistic operating conditions (those they are likely to meet in service), and they should be evaluated under the same conditions. Unfortunately, it is not practical to evaluate performance under all possible combinations of operating conditions. A test procedure must be used that is practical under laboratory conditions and, at the same time, will make available, with a reasonable amount of effort, sufficient data on which a judgement of field performance can be made. The method of evaluation envisioned is that of checking significant performance characteristics such as accuracy rating, dead band, and hysteresis under a set of reference operating conditions, these having a narrow range of tolerances. Reference performance is, therefore, to be evaluated and stated in terms of reference operating conditions. Generally, reference performance under reference operating conditions represents the “best” performance that can be expected under ideal conditions. The effect of change in an individual operating condition, such as ambient temperature, atmospheric pressure, relative humidity, line voltage, and frequency, will be determined individually throughout a range defined as “normal operating conditions.” Logically, these can be expected to occur above and below the values of reference operating conditions during field operation. While this approach does not duplicate all actual conditions, where many operating variables may vary simultaneously in random fashion, it does develop data from which

* Used with permission of the Instrumentation, Systems and Automation Society.

46 © 2003 by Béla Lipták

1.3 Instrument Terminology and Performance

performance may be inferred from any given set of operating conditions. The effect of changes in an individual operating condition, all other operating conditions being held within the reference range, is herein called operating influence. There may be an operating influence corresponding to a change in each operating condition. In some cases, the effect may be negligible; in others, it may have significant magnitude. Tabulations of operating influences will usually denote the performance quality level of a given design. Comparisons of reference performance and operating influences for instruments of a given design, or for different designs, will show clearly their relative merits and probable performance under actual operating conditions. Operating Conditions vs. Performance Operating Conditions

Performance

Reference (narrowband)

Reference (Region within which accuracy statements apply unless indicated otherwise.)

Normal (wideband)

Conditional (Region within which the influence of environment on performance is stated.)

Operative Limits (extreme band)

Indefinite (Region within which influences are not stated and beyond which damage may occur.)

SOURCES AND REFERENCES In the preparation of this standard of terminology, many standards and publications sponsored by technical organizations such as the American Society of Mechanical Engineers (ASME), the Institute of Electrical and Electronics Engineers (IEEE), and ISA (formerly called the Instrument Society of America) were studied by the committee, in addition to those listed as principal source documents. These are listed as references. Existing terms and definitions have been used wherever they were considered suitable. In many cases, terms have been extracted from source documents with verbatim definitions. In such cases, permission to quote from the respective source document has been obtained from the organization concerned, as indicated below. Terms defined verbatim are followed by the reference number in parentheses. For example, (4) after a defined term indicates that this term is quoted verbatim from ANSI C85.1, “Terminology for Automatic Control.” In other cases, definitions have been modified in varying degrees to conform with current practice in process instrumentation. These have been noted in parentheses as “Ref.” followed by the reference number. For example, (Ref. 8) indicates that this term is a modified definition of the referenced term in SAMA-PMC 20.1–1973, “Process Measurement and Control Terminology.”


47

An omission or alteration of a note following a definition is not considered to be a modification of the definition and is not identified by the abbreviation “Ref.” Principal source documents used from the many reviewed are as follows: 1) American National Standard C39.4–1966, “Specifications for Automatic Null-Balancing Electrical Measuring Instruments,” published by the American National Standards Institute, Inc., copyright 1966 by ANSI. 2) American National Standard C42.100–1972, “Dictionary of Electrical and Electronics Engineers, Inc., copyright 1972 by IEEE. 3) American National Standard C85.1–1963, “Terminology for Automatic Control,” published by the American Society of Mechanical Engineers, copyright 1963 by ASME. 4) SAMA Standard PMC20.1–1973, “Process Measurement and Control Terminology,” published by Scientific Apparatus Makers Association, Process Measurement and Control Section, Inc., copyright 1973 by SAMAPMC. Copies of the American National Standards referred to above may be purchased from the American National Standards Institute, 1430 Broadway, New York, NY 10018. Copies of the SAMA Standard may be purchased from Process Measurement and Control Section, Inc., SAMA, 1101 16th Street N.W., Washington, DC 20036. DEFINITION OF TERMS Accuracy*. In process instrumentation, degree of conformity of an indicated value to a recognized accepted standard value, or ideal value. (Ref. 4, Ref. 8) Accuracy, measured. The maximum positive and negative deviation observed in testing a device under specified conditions and by a specified procedure. See Figure 1.3a. Note 1: It is usually measured as an inaccuracy and expressed as accuracy. Note 2: It is typically expressed in terms of the measured variable, percent of span, percent of upper range value, percent of scale length, or percent of actual output reading. See section titled “Test Procedures.” Accuracy rating. In process instrumentation, a number or quantity that defines a limit that errors will not exceed when a device is used under specified operating conditions. See Figure 1.3a. Note 1: When operating conditions are not specified, reference operating conditions shall be assumed. Note 2: As a performance specification, accuracy (or reference accuracy) * Throughout this handbook, the term inaccuracy has been used instead of accuracy, because the term relates to the error in a measurement. In the following paragraphs, the accuracy term is used, because this section is being quoted from an ISA standard and not because the author agrees with its use.

48

General Considerations

Output

Maximum Actual Positive Deviation Actual Downscale Calibration Curve Specified Characteristic Curve

High or Positive Permissible Limit of error

Accuracy Rating

Actual Upscale Calibration Curve

Measured Accuracy

Maximum Actual Negative Deviation Low or Negative Permissible Limit of Error Input Span 0

100%

FIG. 1.3a Accuracy.

shall be assumed to mean accuracy rating of the device when used at reference operating conditions. Note 3: Accuracy rating includes the combined effects of conformity, hysteresis, dead band, and repeatability errors. The units being used are to be stated explicitly. It is preferred that a ± sign precede the number or quantity. The absence of a sign indicates a + and a − sign. Accuracy rating can be expressed in a number of forms. The following five examples are typical: (a) Accuracy rating expressed in terms of the measured variable. Typical expression: The accuracy rating is ±1°C, or ±2°F. (b) Accuracy rating expressed in percent of span. Typical expression: The accuracy rating is ±0.5% of span. (This percentage is calculated using scale units such as degrees Fahrenheit, psig, and so forth.) (c) Accuracy rating expressed in percent of the upper range value. Typical expression: The accuracy rating is ±0.5% of upper range value. (This percentage is calculated using scale units such as kPa, degrees Fahrenheit, and so forth.) (d) Accuracy rating expressed in percent of scale length. Typical expression: The accuracy rating is ±0.5% of scale length. (e) Accuracy rating expressed in percent of actual output reading. Typical expression: The accuracy rating is ±1% of actual output reading. Accuracy, reference. See accuracy rating. Actuating error signal. See signal, actuating error. Adaptive control. See control, adaptive.


Adjustment, span. Means provided in an instrument to change the slope of the input–output curve. See span shift. Adjustment, zero. Means provided in an instrument to produce a parallel shift of the input–output curve. See zero shift. Air conditioned area. See area, air conditioned. Air consumption. The maximum rate at which air is consumed by a device within its operating range during steady-state signal conditions. Note: It is usually 3 expressed in cubic feet per minute (ft /min) or cubic 3 meters per hour (m /h) at a standard (or normal) specified temperature and pressure. (8) Ambient pressure. See pressure, ambient. Ambient temperature. See temperature, ambient. Amplifier. A device that enables an input signal to control power from a source independent of the signal and thus be capable of delivering an output that bears some relationship to, and is generally greater than, the input signal. (3) Analog signal. See signal, analog. Area, air conditioned. A location in which temperature at a nominal value is maintained constant within narrow tolerance at some point in a specified band of typical comfortable room temperature. Humidity is maintained within a narrow specified band. Note: Air conditioned areas are provided with clean air circulation and are typically used for instrumentation, such as computers or other equipment requiring a closely controlled environment. (Ref. 18) Area, control room. A location with heat and/or cooling facilities. Conditions are maintained within specified limits. Provisions for automatically maintaining constant temperature and humidity may or may not be provided. Note: Control room areas are commonly provided for operation of those parts of a control system for which operator surveillance on a continuing basis is required. (18) Area, environmental. A basic qualified location in a plant with specified environmental conditions dependent on severity. Note: Environmental areas include air conditioned areas; control room areas, heated and/or cooled; sheltered areas (process facilities); and outdoor areas (remote field sites). See specific definitions. Area, outdoor. A location in which equipment is exposed to outdoor ambient conditions, including temperature, humidity, direct sunshine, wind, and precipitation. (Ref. 18) Area, sheltered. An industrial process location, area, storage, or transportation facility, with protection against direct exposure to the elements, such as direct sunlight, rain or other precipitation, or full wind pressure. Minimum and maximum temperatures and humidity may be the same as outdoors. Condensation can occur. Ventilation, if any, is by natural means. Note: Typical areas are shelters for


operating instruments, unheated warehouses for storage, and enclosed trucks for transportation. (18) Attenuation. (1) A decrease in signal magnitude between two points or between two frequencies. (2) The reciprocal of gain. Note: It may be expressed as a dimensionless ratio, scalar ratio, or in decibels as 20 times the log10 of that ratio. (Ref. 4) Auctioneering device. See signal selector. Automatic control system. See control system, automatic. Automatic/manual station. A device that enables an operator to select an automatic signal or a manual signal as the input to a controlling element. The automatic signal is normally the output of a controller, while the manual signal is the output of a manually operated device. Backlash. In process instrumentation, a relative movement between interacting mechanical parts, resulting from looseness when motion is reversed. (Ref. 4) Bode diagram. In process instrumentation, a plot of log gain (magnitude ratio) and phase angle values on a log frequency base for a transfer function. See Figure 1.3b. (8, Ref. 4) Break point. The junction of the extension of two confluent straight line segments of a plotted curve. Note: In the asymptotic approximation of a log-gain vs. log-frequency relation in a Bode diagram, the value .1

.5

1

5

10

50

100

50

100

2 Break Point

Gain or Magnitude Ratio

1.0 0.5 Corner Frequency

0.1

.05

Phase Angle, Degrees

.02 +60

0

−60

−120

−180 .1

.5

1

Frequency

FIG. 1.3b Typical Bode diagram.


5

10

Cycles Per Unit Time

of the abscissa is called the corner frequency. See Figure 1.3b. (4, 8) Calibrate. To ascertain outputs of a device corresponding to a series of values of a quantity that the device is to measure, receive, or transmit. Data so obtained are used to: 1. Determine the locations at which scale graduations are to be placed 2. Adjust the output, to bring it to the desired value, within a specified tolerance 3. Ascertain the error by comparing the device output reading against a standard (Ref. 3) Calibration curve. A graphical representation of the calibration report. (Ref. 11) For example, see Figure 1.3ff. Calibration cycle. The application of known values of the measured variable and the recording of corresponding values of output readings, over the range of the instrument, in ascending and descending directions. (Ref. 11) Calibration report. A table or graph of the measured relationship of an instrument as compared, over its range, against a standard. (Ref. 8) For example, see Table 1.3gg. Calibration traceability. The relationship of the calibration of an instrument through a step-by-step process to an instrument or group of instruments calibrated and certified by a national standardizing laboratory. (Ref. 11) Cascade control. See control, cascade. Characteristic curve. A graph (curve) that shows the ideal values at steady state, or an output variable of a system as a function of an input variable, the other input variables being maintained at specified constant values. Note: When the other input variables are treated as parameters, a set of characteristic curves is obtained. (Ref. 17) Closed loop. See loop, closed. Closed-loop gain. See gain, closed-loop. Coefficient, temperature/pressure/etc. See operating influence. Cold junction. See reference junction. Common-mode interference. See interference, commonmode. Common-mode rejection. The ability of a circuit to discriminate against a common-mode voltage. Note: It may be expressed as a dimensionless ratio, a scalar ratio, or in decibels as 20 times the log10 of that ratio. Common-mode voltage. See voltage, common-mode. Compensation. In process instrumentation, provision of a special construction, a supplemental device or circuit, or special materials to counteract sources of error due to variations in specified operating conditions. (Ref. 11) Compensator. A device that converts a signal into some function that, either alone or in combination with other signals, directs the final controlling element to

49

50

General Considerations

reduce deviations in the directly controlled variable. See Figures 1.3j and 1.3k for application of “setpoint compensator” and “load compensator.” Compliance. The reciprocal of stiffness. Computing instrument. See instrument, computing. Conformity (of a curve). The closeness to which the curve approximates a specified one (e.g., logarithmic, parabolic, cubic, and so on). Note 1: It is usually measured in terms of nonconformity and expressed as conformity, e.g., the maximum deviation between an average curve and a specified curve. The average curve is determined after making two or more full-range traverses in each direction. The value of conformity is referred to the output unless otherwise stated. See linearity. Note 2: As a performance specification, conformity should be expressed as independent conformity, terminal-based conformity, or zero-based conformity. When expressed simply as conformity, it is assumed to be independent conformity. (8, Ref. 4) Conformity, independent. The maximum deviation of the calibration curve (average of upscale and downscale readings) from a specified characteristic curve positioned so as to minimize the maximum deviation. See Figure 1.3c. (8)

Conformity, terminal-based. The maximum deviation of the calibration curve (average of upscale and downscale readings) from a specified characteristic curve positioned so as to coincide with the actual characteristic curve at upper and lower range values. See Figure 1.3d. (8) Conformity, zero-based. The maximum deviation of the calibration curve (average of upscale and downscale readings) from a specified characteristic curve positioned so as to coincide with the actual characteristic curve at the lower range value. See Figure 1.3e. (Ref. 8) Contact, operating conditions, normal. See operating conditions, normal. Control action. Of a controller or of a controlling system, the nature of the change of the output effected by the input. Note: The output may be a signal or a value of a manipulated variable. The input may be the control loop feedback signal when the setpoint is constant, an actuating error signal, or the output of another controller. (Ref. 4, Ref. 8) Control action, derivative (rate) (d). Control action in which the output is proportional to the rate of change of the input. (8, Ref. 4)

Output Output

Actual Calibration Curve (Average of Upscale and Downscale Readings)

Specified Characteristic Curve

Actual Calibration Curve (Average of Upscale and Downscale Readings)

Upper Range Value

Maximum Deviation

Specified Characteristic Curve

Maximum ± Deviations are Minimized

Lower Range Value

Input

Input Span 0

FIG. 1.3c Independent conformity.


Span 100%

0

FIG. 1.3d Terminal-based conformity.

100%


Control action, floating. Control action in which the rate of change of the output variable is a predetermined function of the input variable. Note: The rate of change may have one absolute value, several absolute values, or any value between two predetermined values. (Ref. 17, “floating action”) Control action, integral (reset) (i). Control action in which the output is proportional to the time integral of the input; i.e., the rate of change of output is proportional to the input. See Figure 1.3f. Note: In the practical embodiment of integral control action, the relation between output and input, neglecting high-frequency terms, is given by

Output

Actual Calibration Curve (Average of Upscale and Downscale Readings) Specified Characteristic Curve

Maximum ± Deviations are Minimized and Equal

Y I /s =± , where 0 ≤ − b 1 X 1 + sD/a a D P s X Y

= = = = = =

1.3(2)

derivative action gain derivative action time constant proportional gain complex variable input transform output transform (4, 8)


b I P s X Y

= = = = = =

proportional gain/static gain integral action rate proportional gain complex variable input transform output transform (4, 8)

See note under control action. Control action, proportional plus integral (reset) plus derivative (rate) (pid). Control action in which the output is proportional to a linear combination of the input, the time integral of input, and the time rate of change of input. See Figure 1.3i. Note: In the practical embodiment of proportional plus integral plus derivative control action, the relationship of output to input, neglecting high-frequency terms, is Y I /s + 1 + Ds =±P , where a > 1.0 ≤ − b 2" Radius Taps (RT), D > 6" Pipe Taps (PT)

D

Orifice

FIG. 2.15a Pressure profile through an orifice plate and the different methods of detecting the pressure drop.


Flow

2.15 Orifices

the cross-sectional areas of pipe and flow nozzle, and the difference of velocity heads given by differential-pressure measurements. Flow rate derives from velocity and area. The basic equations are as follows: V=k

h ρ

2.15(1)

Q = kA

h ρ

2.15(2)

W = kA hρ

2.15(3)

where V = velocity Q = volume flow rate W = mass flow rate A = cross-sectional area of the pipe h = differential pressure between points of measurement ρ = the density of the flowing fluid k = a constant that includes ratio of cross-sectional area of pipe to cross-sectional area of nozzle or other restriction, units of measurement, correction factors, and so on, depending on the specific type of head meter For a more complete derivation of the basic flow equations, based on considerations of energy balance and hydrodynamic properties, consult References 1, 2, and 3. Head Meter Characteristics Two fundamental characteristics of head-type flow measurements are apparent from the basic equations. First is the square root relationship between flow rate and differential pressure. Second, the density of the flowing fluid must be taken into account both for volume and for mass flow measurements. The Square Root Relationship This relationship has two important consequences. Both are primarily concerned with readout. The primary sensor (orifice, venturi tube, or other device) develops a head or differential pressure. A simple linear readout of this differential pressure expands the high end of the scale and compresses the low end in terms of flow. Fifty percent of full flow rate produces 25% of full differential pressure. At this point, a flow change of 1% of full flow results in a differential pressure change of 1% of full differential. At 10% flow, the total differential pressure is only 1%, and a change of 1% of full scale flow (10% relative change) results in only 0.2% full scale change in differential pressure. Both accuracy and readability suffer. Readability can be improved by a transducer that extracts the square root of the differential pressure to give a signal linear with flow rate. However, errors in the more complex square root transducer tend to decrease overall accuracy.


261

For a large proportion of industrial processes, which seldom operate below 30% capacity, a device with pointer or pen motion that is linear with differential pressure is generally adequate. Readout directly in flow can be provided by a square root scale. Where maximum accuracy is important, it is generally recommended that the maximum-to-minimum flow ratio shall not exceed 3:1, or at the most 3.5:1, for any single head-type flowmeter. The high repeatability of modern differential-pressure transducers permits a considerably wider range for flow control where constancy and repeatability of low rate are the primary concern. However, where flow variations approach 10:1, the use of two primary flow units of different capacities, two differential-pressure sensors with different ranges, or both is generally recommended. It should be emphasized that the primary head meter devices produce a differential pressure that corresponds accurately to flow over a wide range. Difficulty arises in the accurate measurement of the corresponding extremely wide range of differential pressure; for example, a 20:1 flow variation results in a 400:1 variation in differential pressure. The second problem with the square root relationship is that some computations require linear input signals. This is the case when flow rates are integrated or when two or more flow rates are added or subtracted. This is not necessarily true for multiplication and division; specifically, flow ratio measurement and control do not require linear input signals. A given flow ratio will develop a corresponding differential pressure ratio over the full range of the measured flows. Density of the Flowing Fluid Fluid density is involved in the determination of either mass flow rate or volume flow rate. In other words, head-type meters do not read out directly in either mass or volume flow (weirs and flumes are an exception, as discussed in Section 2.31). The fact that density appears as a square root gives head-type metering an actual advantage, particularly in applications where measurement of mass flow is required. Due to this square root relationship, any error that may exist in the value of the density used to compute mass flow is substantially reduced; a 1% error in the value of the fluid density results in a 0.5% error in calculated mass flow. This is particularly important in gas flow measurement, where the density may vary over a considerable range and where operating density is not easily determined with high accuracy.

β (Beta) Ratio Most head meters depend on a restriction in the flow path to produce a change in velocity. For the usual circular pipe and circular restriction, the β ratio is the ratio between the diameter of the restriction and the inside diameter of the pipe. The ratio between the velocity in the pipe and the velocity at the restriction is equal to the ratio of areas 2 or β. For noncircular configurations, β is defined as the square root of the ratio of area of the restriction to area of the pipe or conduit.

262

Flow Measurement

Reynolds Number

Coefficient of Discharge Concentric Square Edged Orifice

The basic equations of flow assume that the velocity of flow is uniform across a given cross section. In practice, flow velocity at any cross section approaches zero in the boundary layer adjacent to the pipe wall and varies across the diameter. This flow velocity profile has a significant effect on the relationship between flow velocity and pressure difference developed in a head meter. In 1883, Sir Osborne Reynolds, an English scientist, presented a paper before the Royal Society proposing a single, dimensionless ratio (now known as Reynolds number) as a criterion to describe this phenomenon. This number, Re, is expressed as VDρ µ

2.15(4)

where V = velocity D = diameter ρ = density µ = absolute viscosity Reynolds number expresses the ratio of inertial forces to viscous forces. At a very low Reynolds number, viscous forces predominate, and inertial forces have little effect. Pressure difference approaches direct proportionality to average flow velocity and to viscosity. At high Reynolds numbers, inertial forces predominate, and viscous drag effects become negligible. At low Reynolds numbers, flow is laminar and may be regarded as a group of concentric shells; each shell reacts in a viscous shear manner on adjacent shells, and the velocity profile across a diameter is substantially parabolic. At high Reynolds numbers, flow is turbulent, with eddies forming between the boundary layer and the body of the flowing fluid and propagating through the stream pattern. A very complex, random pattern of velocities develops in all directions. This turbulent mixing action tends to produce a uniform average axial velocity across the stream. The change from the laminar flow pattern to the turbulent flow pattern is gradual, with no distinct transition point. For Reynolds numbers above 10,000, flow is definitely turbulent. The coefficients of discharge of the various head-type flowmeters changes with Reynolds number (Figure 2.15b). The value for k in the basic flow equations includes a Reynolds number factor. References 1 and 2 provide tables and graphs for Reynolds number factor. For head meters, this single factor is sufficient to establish compensation in coefficient for changes in ratio of inertial to frictional forces and for the corresponding changes in flow velocity profile; a gas flow with the same Reynolds number as a liquid flow has the same Reynolds number factor. Compressible Fluid Flow Density in the basic equations is assumed to be constant upstream and downstream from the primary device. For gas or vapor flow, the differential pressure developed results in


Integral

=2% Target Meter (Best Case)

Orifice

102

103

Venturi Tube

Flow Nozzle Target Meter (Worst Case)

10

Re =

Magnetic Flowmeter

Eccentric Orifice

Quadrant Edged Orifice

104

105

Pipeline Reynolds Number 106

FIG. 2.15b Discharge coefficients as a function of sensor type and Reynolds number.

a corresponding change in density between upstream and downstream pressure measurement points. For accurate calculations of gas flow, this is corrected by an expansion factor that has been empirically determined. Values are given in References 1 and 2. When practical, the full-scale differential pressure should be less than 0.04 times normal minimum static pressure (differential pressure, stated in inches of water, should be less than static pressure stated in PSIA). Under these conditions, the expansion factor is quite small. Choice of Differential-Pressure Range The most common differential-pressure range for orifices, venturi tubes, and flow nozzles is 0 to 100 in. of water (0 to 25 kPa) for full-scale flow. This range is high enough to minimize errors due to liquid density differences in the connecting lines to the differential-pressure sensor or in seal chambers, condensing chambers, and so on, caused by temperature differences. Most differential-pressure-responsive devices develop their maximum accuracy in or near this range, and the maximum pressure loss—3.5 PSI (24 kPa)—is not serious in most applications. (As shown in Figure 2.27f, the pressure loss in an orifice is about 65% when a β ratio of 0.75 is used.) The 100-in. range permits a 2:1 flow rate change in either direction to accommodate changes in operating conditions. Most differential-pressure sensors can be modified to cover the range from 25 to 400 in. of water (6.2 to 99.4 kPa) or more, either by a simple adjustment or by a relatively minor structural change. Applications in which the pressure loss up to 3.5 PSI is expensive or is not available can be handled either by selection of a lower differential-pressure range or by the use of a venturi tube or other primary element with highpressure recovery. Some high-velocity flows will develop more than 100 in. of differential pressure with the maximum acceptable ratio of primary element effective diameter to pipe diameter. For these applications, a higher differential pressure is indicated. Finally, for low-static-pressure (less than 100 PSIA)

2.15 Orifices

gas or vapor, a lower differential pressure is recommended to minimize the expansion factor. Pulsating Flow and Flow “Noise” Short-period (1 sec and less) variation in differential pressure developed from a head-type flowmeter primary element arises from two distinct sources. First, reciprocating pumps, compressors, and the like may cause a periodic fluctuation in the rate of flow. Second, the random velocities inherent in turbulent flow cause variations in differential pressure even with a constant flow rate. Both have similar results and are often mistaken for each other. However, their characteristics and the procedures used to cope with them are distinct. Pulsating Flow The so-called pulsating flow from reciprocating pumps, compressors, and so on may significantly affect the differential pressure developed by a head-type meter. For example, if the amplitude of instantaneous differentialpressure fluctuation is 24% of the average differential pressure, an error of ±1% can be expected under normal operation conditions. For the pulsation amplitudes of 24, 48, and 98% values, the corresponding errors of ±1, ±4, and ±16% can be expected. The Joint ASME-AGA Committee on Pulsation reported that the ratio between errors varies roughly as the square of the ratio between differential-pressure fluctuations. For liquid flow, there is indication that the average of the square root of the instantaneous differential pressure (essentially average of instantaneous flow signal) results in a lower error than the measurement of the average instantaneous differential pressure. However, for gas flow, extensive investigation has failed to develop any usable relationship between pulsation and deviation from coefficient beyond the estimate 4 of maximum error. Operation at higher differential pressures is generally advantageous for pulsating flow. The only other valid approach to improve the accuracy of pulsating gas flow measurement is the location of the meter at a point where pulsation is minimized. Flow “Noise” Turbulent flow generates a complex pattern of random velocities. This results in a corresponding variation or “noise” in the differential pressure developed at the pressure connections to the primary element. The amplitude of the noise may be as much as 10% of the average differential pressure with a constant flow rate. This noise effect is a complex hydrodynamic phenomenon and is not fully understood. It is augmented by flow disturbances from valves, fittings, and so on both upstream and downstream from the flowmeter primary element and, apparently, by characteristics of the primary element itself. Tests based on average flow rate as accurately determined by static weight/time techniques (compared to accurate measurement of differential pressure including continuous, precise averaging of noise) indicate that the noise, when precisely


263

averaged, introduces negligible (less than 0.1%) measurement error when the average flow is substantially constant (change 5 of average flow rate is not more than 1% per second). It should be noted that average differential pressure, not average flow (average of the square root of differential pressure), is measured, because the noise is developed by the random, not the average, flow. Errors in the determination of true differential-pressure average will result in corresponding errors in flow measurement. For normal use, one form or another of “damping” in devices responsive to differential pressure is adequate. Where accuracy is a major concern, there must be no elements in the system that will develop a bias rather than a true average when subjected to the complex noise pattern of differential pressure. Differential-pressure noise can be reduced by the use of two or more pressure-sensing taps connected in parallel for both high and low differential-pressure connections. This provides major noise reduction. Only minor improvement results from additional taps. Piezometer rings formed of multiple connections are frequently used with venturi tubes but seldom with orifices or flow nozzles.

THE ORIFICE METER The orifice meter is the most common head-type flow measuring device. An orifice plate is inserted in the line, and the differential pressure across it is measured (Figure 2.15a). This section is concerned with the primary device (the orifice plate, its mounting, and the differential-pressure connections). Devices for the measurement of the differential pressure are covered in Chapters 3 and 5. The orifice in general, and the conventional thin, concentric, sharp-edged orifice plate in particular, have important advantages that include being inexpensive manufacture to very close tolerances and easy to install and replace. Orifice measurement of liquids, gases, and vapors under a wide range of conditions enjoys a high degree of confidence based on a great deal of accurate test work. The standard orifice plate itself is a circular disk; usually stainless steel, from 0.12 to 0.5 in. (3.175 to 12.70 mm) thick, depending on size and flow velocity, with a hole (orifice) in the middle and a tab projecting out to one side and used as a data plate (Figure 2.15c). The thickness requirement of the orifice plate is a function of line size, flowing temperature, and differential pressure across the plate. Some helpful guidelines are as follows. By Size 2 to 12 in. (50 to 304 mm), 0.13 in. (3.175 mm) thick 14 in. (355 mm) and larger, 0.25 in. (6.35 mm) thick By Temperature ≥600°F (316°C) 2 to 8 in. (50 to 203 mm), 0.13 in. (3.175 mm) thick 10 in. (254 mm) and larger, 0.25 in. (6.35 mm) thick

264

Flow Measurement

Vent Hole Location (Liquid Service)

Flow

Drain Hole Location (Vapor Service)

Pipe Internal Diameter

Bevel Where Thickness is Greater than 1/8 Inch (3.175 mm) 45° or the Orifice Diameter is Less than 1 Inch (25 mm)

1/8 Inch (3.175 mm) Maximum 1/8-1/2 Inch (3.175−12.70 mm)

FIG. 2.15c Concentric orifice plate.

Flow through the Orifice Plate The orifice plate inserted in the line causes an increase in flow velocity and a corresponding decrease in pressure. The flow pattern shows an effective decrease in cross section beyond the orifice plate, with a maximum velocity and minimum pressure at the vena contracta (Figure 2.15a). This location may be from 0.35 to 0.85 pipe diameters downstream from the orifice plate, depending on β ratio and Reynolds number. This flow pattern and the sharp leading edge of the orifice plate (Figure 2.15d) that produces it are of major importance. The sharp edge results in an almost pure line contact between the plate and the effective flow, with negligible fluid-to-metal friction drag at this boundary. Any nicks, burrs, or rounding of the sharp edge can result in surprisingly large measurement errors. When the usual practice of measuring the differential pressure at a location close to the orifice plate is followed, friction effects between fluid and pipe wall upstream and downstream from the orifice are minimized so that pipe roughness has minimum effect. Fluid viscosity, as reflected in Reynolds number, does have a considerable influence, particularly at low Reynolds numbers. Because the formation of the vena contracta is an inertial effect, a decrease in the ratio of inertial to frictional forces (decrease in Reynolds number) and the

corresponding change in the flow profile result in less constriction of flow at the vena contracta and an increase of the flow coefficient. In general, the sharp edge orifice plate should not be used at pipe Reynolds numbers under 2000 to 10,000 or more (Table 2.1e). The minimum recommended Reynolds number will vary from 10,000 to 15,000 for 2-in. (50-mm) through 4-in. (102-mm) pipe sizes for β ratios up to 0.5, and from 20,000 to 45,000 for higher β ratios. The Reynolds number requirement will increase with pipe size and β ratio and may range up to 200,000 for pipes 14 in. (355 mm) and 6 larger. Maximum Reynolds numbers may be 10 through 4-in. 7 (102-mm) pipe and 10 for larger sizes.

Location of Pressure Taps For liquid flow measurement, gas or vapor accumulations in the connections between the pipe and the differential-pressure measuring device must be prevented. Pressure taps are generally located in the horizontal plane of the centerline of horizontal pipe runs. The differential-pressure measuring device is either mounted close-coupled to the pressure taps or connected through downward sloping connecting pipe of sufficient diameter to allow gas bubbles to flow up and back into the line. For gas, similar precautions to prevent accumulation of liquid are required. Taps may be installed in the top of the line, with upward sloping connections, or the differentialpressure measuring device may be close-coupled to taps in the side of the line (Figure 2.15e). For steam and similar vapors that are condensable at ambient temperatures, condensing chambers or their equivalent are generally used, usually with down-sloping connections from the side of the pipe to the measuring device. There are five common locations for the differential-pressure taps: flange taps, vena contracta taps, radius taps, full-flow or pipe taps, and corner taps. In the United States, flange taps (Figures 2.15e and 2.15f) are predominantly used for pipe sizes 2 in. (50 mm) and larger. The manufacturer of the orifice flange set drills the taps so

2.125" (54mm) Block Valve

Equalizing Valve

FIG. 2.15d Flow pattern with orifice plate.


FIG. 2.15e Measurement of gas flow with differential pressure transmitter and 3 three-valve manifold.

2.15 Orifices

265

Center of Tees Exactly at Same Level

1/2" Plug Cock

1/2" Line Pipe

FIG 2.15g Corner tap installation.

FIG 2.15f 3 Steam flow measurement using standard manifold.

that the centerlines are 1 in. (25 mm) from the orifice plate surface. This location also facilities inspection and cleanup of burrs, weld metal, and so on that may result from installation of a particular type of flange. Flange taps are not recommended below 2 in. (50 mm) pipe size and cannot be used below 1.5 in. (37.5 mm) pipe size, since the vena contracta may be closer than 1 in. (25 mm) from the orifice plate. Flow for a distance of several pipe diameters beyond the vena contracta tends to be unstable and is not suitable for differential-pressure measurement (Figure 2.15a). Vena contracta taps use an upstream tap located one pipe diameter upstream of the orifice plate and a downstream tap located at the point of minimum pressure. Theoretically, this is the optimal location. However, the location of the vena contracta varies with the orifice-to-pipe diameter ratio and is thus subject to error if the orifice plate is changed. A tap location too far downstream in the unstable area may result in inconsistent measurement. For moderate and small pipe, the location of the vena contracta is likely to lie at the edge of or under the flange. It is not considered good piping practice to use the hub of the flange to make a pressure tap. For this reason, vena contracta taps are normally limited to pipe sizes 6 in. (152 mm) or larger, depending on the flange rating and dimensions. Radius taps are similar to vena contracta taps except that the downstream tap is located at one-half pipe diameter (one radius) from the orifice plate. This practically assures that the tap will not be in the unstable region, regardless of orifice diameter. Radius taps today are generally considered superior to the vena contracta tap, because they simplify the pressure


tap location dimensions and do not vary with changes in orifice β ratio. The same pipe size limitations apply as to the vena contracta tap. Pipe taps are located 2.5 pipe diameters upstream and 8 diameters downstream from the orifice plate. Because of the distance from the orifice, exact location is not critical, but the effects of pipe roughness, dimensional inconsistencies, and so on are more severe. Uncertainty of measurement is perhaps 50% greater with pipe taps than with taps close to the orifice plate. These taps are normally used only where it is necessary to install an orifice meter in an existing pipeline and radius or where vena contracta taps cannot be used. Corner taps (Figure 2.15g) are similar in many respects to flange taps, except that the pressure is measured at the “corner” between the orifice plate and the pipe wall. Corner taps are very common for all pipe sizes in Europe, where relatively small clearances exist in all pipe sizes. The relatively small clearances of the passages constitute possible sources of trouble. Also, some tests have indicated inconsistencies with high β ratio installations, attributed to a region of flow instability at the upstream face of the orifice. For this situation, an upstream tap one pipe diameter upstream of the orifice plate has been used. Corner taps are used in the United States primarily for pipe diameters of less than 2 in. (50 mm).

ECCENTRIC AND SEGMENTAL ORIFICE PLATES The use of eccentric and segmental orifices is recommended where horizontal meter runs are required and the fluids contain extraneous matter to a degree that the concentric orifice would plug up. It is preferable to use concentric orifices in a vertical meter tube if at all possible. Flow coefficient data is limited for these orifices, and they are likely to be less accurate. In the absence of specific data, concentric orifice data may be applied as long as accuracy is of no major concern. The eccentric orifice plate, Figure 2.15h, is like the concentric plate except for the offset hole. The segmental orifice

266

Flow Measurement

Eccentric

45° 45°

45°

45° Eccentric

Zone for Pressure Taps For Gas Containing Liquid or For Liquid Containing Solids

For Liquid Containing Gas

FIG. 2.15h Eccentric orifice plate.

Zone for Pressure Taps

Segmental

20°

20° 45°

45°

45°

45° 20°

QUADRANT EDGE AND CONICAL ENTRANCE ORIFICE PLATES

Segmental

20° R

For Vapor Containing Liquid or For Liquid Containing Solids

or gasket interferes with the hole on either type plate. The equivalent β for a segmental orifice may be expressed as β = a/ A , where a is the area of the hole segment, and A is the internal pipe area. In general, the minimum line size for these plates is 4 in. (102 mm). However, the eccentric plate can be made in smaller sizes as long as the hole size does not require beveling. Maximum line sizes are unlimited and contingent only on calculation data availability. Beta ratio limits are limited to between 0.3 and 0.8. Lower Reynolds number limit is 2000D (D in inches) but not less than 10,000. For compressible fluids, ∆P/P1 ≤ 0.30, where ∆P and P1 are in the same units. Flange taps are recommended for both types of orifices, but vena contracta taps can be used in larger pipe sizes. The taps for the eccentric orifice should be located in the quadrants directly opposite the hole. The taps for the segmental orifice should always be in line with the maximum dam height. The straight edge of the dam may be beveled if necessary using the same criteria as for a square edge orifice. To avoid confusion after installation, the tabs on these plates should be clearly stamped “eccentric” or “segmental.”

For Liquid Containing Gas

Pressure taps must always be located in solid area of plate and centerline of tap not nearer than 20° from intersection point of chord and arc.

FIG. 2.15i Segmental orifice plate.

plate, Figure 2.15i, has a hole that is a segment of a circle. Both types of plates may have the hole bored tangent to the inside wall of the pipe or more commonly tangent to a concentric circle with a diameter no smaller than 98% of the pipe internal diameter. The segmental plate is parallel to the pipe wall. Care must be taken so that no portion of the flange

The use of quadrant edge and conical entrance orifice plates is limited to lower pipe Reynolds numbers where flow coefficients for sharp-edged orifice plates are highly variable, in the range of 500 to 10,000. With these special plates, the stability of the flow coefficient increases by a factor of 10. The minimum allowable Reynolds number is a function of β ratio, and the allowable β ratio ranges are limited. Refer to Table 2.15j for β ratio range and minimum allowable Reynolds number. The maximum allowable pipe Reynolds number ranges from 500,000 × (β – 0.1) for quadrant edge to 200,000 × (β) for the conical entrance plate. The conical entrance also has a minimum D ≥ 0.25 in. (6.35 mm). For compressible fluids, ∆P/P1 ≤ 0.25 where ∆P and P1 are in the same units Flange pressure taps are preferred for the quadrant edge, but corner and radius taps can also be used with the same flow coefficients. For the conical entrance units, reliable data

TABLE 2.15j Minimum Allowable Reynolds Numbers for Conical and Quadrant Edge Orifices Type Conical entrance

Quadrant edge


Re Limits

β

0.10

0.11

Re

25

28

30

33

35

38

40

43

45

48

β

0.20

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.30

Re

50

53

55

58

60

63

65

68

70

73

75

β

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Re

250

300

400

500

700

1000

1700

3300

0.12

0.13

0.14

0.15

0.16

0.17

0.18

0.19

2.15 Orifices

Radius r ± 0.01 r

TABLE 2.15m Selecting the Right Orifice Plate for a Particular Application

45°

Flow

d ± 0.001 d W=1.5 d < D

Appropriate Process Fluid

Reynolds Number Range

Normal Pipe Sizes, in. (mm)

Concentric, square edge

Clean gas and liquid

Over 2000

0.5 to 60 (13 to 1500)

Concentric, quadrant, or conical edge

Viscous clean liquid

200 to 10,000

1 to 6 (25 to 150)

Eccentric or segmental square edge

Dirty gas or liquid

Over 10,000

4 to 14 (100 to 350)

THE INTEGRAL ORIFICE

d ± 0.001 d

Miniature flow restrictors provide a convenient primary element for the measurement of small fluid flows. They combine a plate with a small hole to restrict flow, its mounting and connections, and a differential-pressure sensor—usually a pneumatic or electronic transmitter. Units of this type are often referred to as integral orifice flowmeters. Interchangeable flow restrictors are available to cover a wide range of flows. A common minimum standard size is a 0.020-in. (0.5-mm) throat diameter, which will measure water flow down to 3 0.0013 GPM (5 cm /min) or airflow at atmospheric pressure 3 down to 0.0048 SCFH (135 cm /min) (Figure 2.15n).

< 0.1 D

45° ± 1°

0.084 d ± 0.003 d

Orifice Type

Equal to r

FIG. 2.15k Quadrant edge orifice plate.

Flow

> 0.2d < D

0.021 d ± 0.003 d

FIG. 2.15l Conical entrance orifice plate.

is available for corner taps only. A typical quadrant edge plate is shown in Figure 2.15k, and a typical conical entrance orifice plate is shown in Figure 2.15l. These plates are thicker and heavier than the normal sharp-edge type. Because of the critical dimensions and shape, the quadrant edge is difficult to manufacture; it is recommended that it be purchased from skilled commercial fabricators. The conical entrance is much easier to make and could be made by any qualified machine shop. While these special orifice forms are very useful for lower Reynolds numbers, it is recommended that, for a pipe Re > 100,000, the standard sharp-edge orifice be used. To avoid confusion after installation, the tabs on these plates should be clearly stamped “quadrant” or “conical.” An application summary of the different orifice plates is given in Table 2.15m. For dirty gas service, the annular orifice plate (Figure 2.24a) can also be considered.


267

Low Pressure Chamber Integral Orifice

To Low Pressure Chamber

FIG. 2.15n Typical integral orifice meter.

High Pressure Chamber

From High Pressure Chamber

268

Flow Measurement

Miniature flow restrictors are used in laboratory-scale processes and pilot plants, to measure additives to major flow streams, and for other small flow measurements. Clean fluid is required, particularly for the smaller sizes, not only to avoid plugging of the small orifice opening but because a buildup of even a very thin layer on the surface of the element will cause an error. There is little published data on the performance of these small restrictors. These are proprietary products with performance data provided by the supplier. Where accuracy is important, direct flow calibration is recommended. Water flow calibration, using tap water, a soap watch, and a glass graduate (or a pail and scale) to measure total flow, is readily carried out in the instrument shop or laboratory. For viscous liquids, calibration with the working fluid is preferable, because viscosity has a substantial effect on most units. Calibration across the working range is recommended, given that precise conformity to the square law may not exist. Some suppliers are prepared to provide calibrated units for an added fee.

INSTALLATION The orifice is usually mounted between a pair of flanges. Care should be exercised when installing the orifice plate to be sure that the gaskets are trimmed and installed such that they do not protrude across the face of the orifice plate beyond the inside pipe wall (Figure 2.15o). A variety of special devices are commercially available for mounting orifice plates, including units that allow the orifice plate to be inserted and removed from a flowline without interrupting the flow (Figure 2.15p). Such manually operated or motorized orifice fittings can also be used to change the

Operating 11 9 9A 10 B Bleeder Valve

Grease Gun 23

12

7 6 1 Equalizer Valve

5 Slide Valve To Remove Orifice Plate

To Replace Orifice Plate

(A) Open No. 1 (Max. Two Turns Only) (B) Open No. 5 (C) Rotate No. 6 (D) Rotate No. 7 (E) Close No. 5 (F) Close No. 1 (G) Open No. 10 B (H) Lubricate thru No. 23 (I) Loosen No. 11 (do not remove No. 12) (J) Rotate No. 7 to free Nos. 9 and 9A (K) Remove Nos. 12, 9, and 9A

(A) Close 10 B (B) Rotate No. 7 Slowly Until Plate Carrier is Clear of Sealing Bar and Gasket Level. Do Not Lower Plate Carrier onto Slide Valve. (C) Replace Nos. 9A, 9, and 12 (D) Tighten No. 11 (E) Open No. 1 (F) Open No. 5 (G) Rotate No. 7 (H) Rotate No. 6 (I) Close No. 5 (J) Close No. 1 (K) Open 10 B (L) Lubricate thru No. 23 (B) Close No. 10 B

11 12 9 9A

10 B

23 7

5 6

Flow

Important: Remove Burrs After Drilling

FIG. 2.15o Prefabricated meter run with inside surface of the pipe machined for smoothness after welding for a distance of two diameters from each flange face. The mean pipe ID is averaged from four measurements 3 made at different points. They must not differ by more than 0.3%.


Side Sectional Elevation

FIG. 2.15p Typical orifice fitting. (Courtesy of Daniel Measurement and Control.)

2.15 Orifices

flow range by sliding a different orifice opening into the flowing stream. To avoid errors resulting from disturbance of the flow pattern due to valves, fittings, and so forth, a straight run of smooth pipe before and after the orifice is recommended. Required length depends on β ratio (ratio of the diameter of the orifice to inside diameter of the pipe) and the severity of the flow disturbance. For example, an upstream distance to the orifice plate of 45 pipe diameters with 0.75 β ratio is the minimum recommendation for a throttling valve. For a single elbow at the same β, the minimum distance would be only 17 pipe diameters. Figure 2.15q gives minimum values for a variety of upstream disturbances. Upstream lengths greater than the minimum are recommended. A downstream pipe run of five pipe diameters from the orifice plate is recommended in all cases. This straight run should not be interrupted by thermowells or other devices inserted into the pipe. Where it is not practical to install the orifice in a straight run of the desired length, the use of a straightening vane to eliminate swirls or vortices is recommended. Straightening vanes are manufactured in various configurations (Figure 2.15r) and are available from commercial meter tube fabricators. They should be installed so that there are at least two pipe diameters between the disturbance source and vane entry and at least six pipe diameters from the vane exit to the upstream high pressure tap of the orifice. The installation of the pressure taps is important. Burrs and protrusions at the tap entry point must be removed. (Figure 2.15o). The tap hole should enter the line at a right angle to the inside pipe wall and should be slightly beveled. Considerable error can result from protrusions that react with the flow and generate spurious differential pressure. Careful installation is particularly important when full-flow taps are located in areas of full pipe velocity and in positions that are difficult to inspect.

LIMITATIONS Certain limitations exist in the application of the concentric, sharp-edged orifice. 1. The concentric orifice plate is not recommended for slurries and dirty fluids, where solids may accumulate near the orifice plate (Table 2.15m). 2. The sharp-edged orifice plate is not recommended for strongly erosive or corrosive fluids, which tend to round over the sharp edge. Orifice plates made of materials that resist erosion or corrosion are used for conditions that are not too severe. 3. For flows at less than 10,000 Reynolds number (determined in the pipe), the correction factor for Reynolds number may introduce problems in determining the


269

total flow when the flow rate varies considerably (Figure 2.15b). The quadrant-edged orifice plate is recommended for this application in preference to the sharp-edged plate (Table 2.15m). 4. For liquids with entrained gas or vapor, a “vent hole” in the plate can be used for horizontal meter runs to prevent accumulation of gas ahead of the orifice plate (Figure 2.15c). If the diameter of the vent hole is less than 10% of the orifice diameter, then the flow is less than 1% of the total flow. If this error cannot be tolerated, appropriate correction can be made to the orifice calculation. On dirty service, vent or drain holes are considered to be of little value, because they are subject to plugging; they are not recommended. 5. In a similar fashion, a drain or weep hole can be provided for gas with entrained liquid. However, it is recommended that meters for liquid with entrained gas or gas with entrained liquid services be installed vertically. Normally, the flow direction would be upward for liquids and downward for gases. For severe entrainment situations, eccentric or segmental orifice plates should be used. 6. The basic flow equations are based on flow velocities well below sonic. Orifice measurement is also used for flows approaching sonic velocity but requires a different theoretical and computational approach. 7. For concentric orifice plates, it is recommended that the β ratio be limited to a range of 0.2 to 0.65 for best accuracy. In exceptional cases, this can be extended to a range of 0.15 to 0.75. 8. For large flows, the pressure loss through an orifice can result in significant cost in terms of power requirements (see Section 2.1). Venturi tubes with relatively large pressure recovery substantially decrease the pressure loss. Lo-Loss Tubes, Dall Tubes, Foster Flow Tubes, and similar proprietary primary elements develop 95% or better pressure recovery. The pressure loss is less than 5% of differential pressure (see Figure 2.29f). Elbow taps involve no added pressure loss (see Section 2.6). Pitot tube elements introduce negligible loss. Orifice plates can be sized for full-scale differential pressure ranging from 5 in. (127 mm) of water to several hundred inches of water. Most commonly the range is from 20 to 200 in. (508 to 5080 mm) of water. The pressure recovery ratio of an orifice (except for pipe taps) can be estimated by 2 (1 − β ). 9. For compressible fluids, ∆P/P1 should be ≤0.25 where ∆P and P1 are in the same units. This will minimize the errors and corrections required for density changes in flow through the orifice. 10. The use of vent and drain holes is discouraged, if in order to keep them from plugging, they would need to be large enough to adversely affect accuracy.

Flow Measurement

For Orifices and Flow Nozzles Fittings in Different Planes

For Orifices and Flow Nozzles Fittings in Different Planes

B

A

B A Ells, Tube Turns, or Long Radius Bends

Ells, Tube Turns or Long Radius Bends 40

50

Orifice or Flow Nozzle



10 Diam.

Valves


50

A B Straightening Vane

40

C

40

Re du cin gV alv es

270

B

A'

d


30

g

10

A

o -L

ng

s

Ra

A'

0

70 80 90

20

ck

n

he

ra

be a

nd

S

C top

Op ide W e A-Gat e Valv s alve C-Fo r All V

A - G lo

0

10

0 70 80 90

10 20 30 40 50 60 Diameter Ratio

10 20

30 40 50 60

For Venturi Tubes Based on Data From W.S. Pardoe

0 70 80 90

Diameter Ratio

For Orifices and Flow Nozzles all Fittings in Same Plane

Venturi

For Orifices and Flow Nozzles all Fittings in Same Plane

A

Venturi



D

ato

B 0


10

c

ul R eg

en

end s

or Tu be B

di

C B

0

20

A-

Diameter Straight Pipe

nds

Be

A-

nd

eT u rn ub

A-

n Lo

20

ws bo El

A'

d.

lb o

o ws

rT

Ra

A-E

s

A' 2 Diam. Straightening Vane 2 Diam. Long

30

Be

B

C B A' 2 Diam. Straightening Vane 2 Diam. Long

us

C

30

B

A

B

B

A B

12 Diam.

Straightening Vane 2 Diam. Long

A Straightening Vane

B

20 Orifice or Flow Nozzle

Separator A' C

A-

10

El

bo

w

B 0


s

n ds

. Be 20

A' C

10

B

0 70 80 90

0



A

A

R ad

B

A B

ng

A

A

10 30

eT urn

2 Diam. D = 6 Diam.

Long Radius Bends

C B A' 2 Diam. Orifice or B Flow Nozzle rT ub

B

30

Lo

Drum or Tank

B

C

A-

A

0


.0-.50 Ratio 1. 2. 3. 4. 5. 6.

Tees 45 Ells Gate valves Separators Y-Fittings Expansion JTS

.50-.60 Ratio 1. 2. 3. 4. 5.

.60-.70 Ratio

Tees 1. Gate Valves Expansion JTS 2. Y-Fittings Gate Valves 3. Separator Y-Fittings (If Inlet Neck Separator is One Diam. (If Inlet Neck Long) is One Diam. Lg.)

0 70 80 90

For Orifices and Flow Nozzles with Reducers and Expanders Orifice or Flow Nozzle

0 70 80 90 C A

Fittings Allowed on Outlet Side in Place of Straight Pipe.

20

Venturi

so

D

B

40

B

B

A

.70-.80 Ratio 1. Gate Valve 2. Long Radius Bend

As Required by Preceding Fittings

Straightening Vanes

20

A C

10

B 0 0

10 20

30 40 50 60 Diameter Ratio

FIG. 2.15q Orifice straight-run requirements. (Reprinted courtesy of The American Society of Mechanical Engineers.)



A'

B

LRBs

Ells, Tube Turns, or LRBs

A D A

B

A

B

70 80 90


A

A'

B

2.15 Orifices

271

The Old Approach Before the proliferation of computers, approximate calculations were used, giving only moderate accuracy. These are illustrated below more for historical perspective than as a recommended technique. Figure 2.15s illustrates how orifice bore diameters were approximated, and Table 2.15t lists the maximum air, water, and steam flow capacities for both flange and pipe tap installations at various pressure drops. When using Figure 2.15s, the following equations were used to determine the orifice bore. For liquid flow,

FIG 2.15r Straightening vane.

ORIFICE BORE CALCULATIONS Z=

Accurate flow calibration, traceable to recognized standards and using the working fluid under service conditions, is difficult and expensive. For large gas flows, it is nearly impossible and is rarely done. A major advantage of orifice metering is the ease with which flow can be accurately determined from a few simple, readily available measurements. In particular, for the concentric, sharp-edged orifice, measurement confidence is supported by a large body of experience and precise, painstaking tests. Precise flow calculations are quite complex, although the calculation methods and equations have been well standardized. These calculation methods are thoroughly covered in the references at the end of this section. Most, if not all, of the calculations have been automated using readily available computer software for both volumetric and mass flow calculations.

5.663 ER hG f

2.15(5)

GPM Gt

For steam,* Z=

358.9 ERY lbm/hr

h V

2.15(6)

For gas,* Z=

7727 ERY SCFH

hPf

2.15(7)

GTf

* For steam and gas, h expressed in inches H2O should be equal to or less than Pf expressed in PSIA units.

Pipe Constants

A-2 .110 Curve A-2

.100 Curve A-1 .090

.80

.080

.70

.070

Curve B

.980 .960 .940 .920

d D

.900

.10 .40 .50 .60 .70 .75 .80

.880 .860 .840 0

.60

1

2 3 4 5 6 7 8 Pressure Loss Ratio - x

9 10

.060

.50

.050

.40

.040

.30

.030

.20

.020

.10

.010 .100

Pipe Constant

.957 1.049 1.380 1.500 1.610 1.939 2.067 2.323 2.469 2.900 3.068 3.826 4.026 4.063 4.813 5.047 5.761 6.065

.00543 .00653 .01130 .01334 .01537 .02230 .02534 .03200 .03614 .04987 .0558 .0868 .0961 .0979 .1374 .1511 .1968 .2181

.150

.200

.250

.300

.350

.400

.450

.500

.550

1.020 1.010

Pipe Constant

6.625 7.023 7.625 7.981 8.071 9.750 10.020 10.136 11.750 11.938 12.000 12.090 13.250 14.250 15.250 17.182 19.182

18 - 8 Ever-Dur

Curve C

1.000 .990 -200

.600

Monel

Steel

200 0 400 600 Flowing Temperature - °F.

.650

.700

.750

Orifice Ratio - d D

FIG. 2.15s Orifice bore determination chart (flange taps). © 1946 by Taylor Instrument Companies. (ABB Kent-Taylor Inc.)


Pipe I. D.

Pipe Constant (R) = 0.00593 (I. D.)2

Area Factor - E

Flow Factor - z

.90

Compressibility Factor - Y

A-1 1.00

1.000

Pipe I. D.

800

.800

.2603 .2925 .3448 .3777 .3863 .5637 .5954 .6092 .8187 .8451 .8539 .8668 1.0411 1.2042 1.3791 1.7507 2.1819

272

Flow Measurement

TABLE 2.15t Orifice Flowmeter Capacity Table* Flange and Vena Contracta Taps Liquid

Steam

Gas

Pipe Taps Liquid

Steam

Gas

Pipe Size

Actual Inside Diam. (I.D.) Sched. 40

Maximum Orifice Diam.

Meter Range

Water (SG = 1)

100 PSIG Saturated

Air (SG = 1.0) @ 100 PSIG and 60°F

Water (SG = 1)

100 PSIG Saturated

Air (SG = 1.0) @ 100 PSIG and 60°F

Inches

Inches

Inches

Inches of Water

Gal./Min.

Lb./Hr.

Std. Cu. Ft/Min.

Gal./Min.

Lb./Hr.

Std. Cu. Ft./Min.

0.435

200 100 50 20 10 2.5

10.6 7.5 5.3 3.3 2.4 1.17

338 239 170 107 76 38

119 84 59 37 27 13

15.7 11.2 7.9 5.0 3.5 1.7

506 358 253 160 113 56

178 126 89 57 40 20

0.734

200 100 50 20 10 2.5

30 21.2 15.0 9.5 6.7 3.35

963 682 482 305 216 108

295 239 170 108 76 38

44.8 31.7 22.4 14.2 10.1 5.0

1440 1017 719 455 323 161

507 358 253 160 113 56

1.127

200 100 50 20 10 2.5

70.7 50.1 35.1 22.4 15.8 7.9

2270 1600 1135 718 683 254

796 564 399 253 178 90

105 75 52.7 33.4 23.6 11.8

3380 2390 1690 1070 758 379

1190 844 596 378 267 133

1.448

200 100 50 20 10 2.5

116 83 58.5 37.0 26.1 13.1

3740 2645 1870 1183 840 420

1313 932 658 417 295 148

174 123 87 55 39 19.4

5580 3950 2790 1768 1252 625

1966 1390 983 623 440 220

2.147

200 100 50 20 10 2.5

255 181 128 81.5 57.5 28.8

8240 5830 4125 2610 1843 915

2905 2080 1460 922 653 325

383 271 191 121 86 43

12300 8700 6160 3900 2760 1366

4330 3070 2175 1375 975 485

3.02

200 100 50 20 10 2.5

512 362 255 162 115 57

16400 11600 8170 5180 3670 1820

5780 4090 2890 1830 1290 647

764 540 382 242 172 85

24500 17300 12200 7730 5470 2710

8630 6100 4310 2730 1930 965

3.78

200 100 50 20 10 2.5

800 557 402 253 180 90

25600 18200 12900 8110 5750 2880

9050 6410 4530 2870 2020 1010

1190 845 598 378 268 134

38200 27100 19200 12100 8580 4290

13500 9560 6760 4280 3020 1510

1 2

1

1 12

2

3

4

5


0.622

1.049

1.610

2.067

3.068

4.026

5.047

2.15 Orifices

273

TABLE 2.15t Continued Orifice Flowmeter Capacity Table* Flange and Vena Contracta Taps Liquid

Steam

Gas

Pipe Taps Liquid

Steam

Gas

Pipe Size



Meter Range

Water (SG = 1)

100 PSIG Saturated

Air (SG = 1.0) @ 100 PSIG and 60°F

Water (SG = 1)

100 PSIG Saturated

Air (SG = 1.0) @ 100 PSIG and 60°F

Inches

Inches

Inches

Inches of Water

Gal./Min.

Lb./Hr.

Std. Cu. Ft/Min.

Gal./Min.

Lb./Hr.

Std. Cu. Ft./Min.

6

8

10

12

14

16

18


6.065

7.981

10.020

12.000

13.126

15.000

16.876

4.55

200 100 50 20 10 2.5

1158 820 580 367 258 129

37100 26300 18600 11700 8310 4150

13100 9250 6540 4140 2930 1460

1730 1223 866 547 387 193

55300 39200 27700 17500 12400 6200

19500 13800 9760 6180 4370 2180

5.9858

200 100 50 20 10 2.5

2000 1413 1000 634 447 223

64104 45320 32052 20275 14386 7186

22511 15952 11285 7156 5054 2534

2980 2110 1492 943 668 333

95709 67682 47855 30263 21468 10719

33692 23853 16846 10674 7543 3772

7.5150

200 100 50 20 10 2.5

3150 2230 1578 998 706 352

101020 71481 50510 31950 22671 11324

35475 25138 17785 11277 7964 3994

4700 3325 2355 1487 1052 525

150825 106658 75413 47691 33830 16891

53094 37589 26547 16821 11887 5944

9.0000

200 100 50 20 10 2.5

4520 3200 2270 1430 1012 507

145000 103000 72400 46000 32400 16200

51300 36200 25600 16200 11500 5740

6750 4775 3380 2135 1512 757

216000 153000 108000 68600 48300 24200

76500 45100 38200 24200 17100 8560

9.8445

200 100 50 20 10 2.5

5415 3830 2710 1715 1210 603

173398 122588 86699 54842 38914 19437

60891 43148 30526 19356 13670 6855

8060 5720 4040 2555 1808 900

258887 183076 129443 81860 58068 28994

91135 64520 45567 28873 20404 10202

11.2500

200 100 50 20 10 2.5

7065 5000 3535 2240 1580 788

226442 160089 113221 71619 50818 25383

79518 56347 39864 25277 17852 8952

10520 7460 5275 3335 2360 1175

338084 239081 169042 106902 75832 37865

119014 84258 59507 37705 26646 13323

12.6570

200 100 50 20 10 2.5

8920 6330 4475 2830 1995 995

286324 202424 143162 90558 64256 32095

100546 71248 50406 31962 22573 11320

13320 9270 6675 4220 2985 1485

427489 302305 213744 135172 95885 47876

150487 106539 75243 47676 33693 16847

274

Flow Measurement

TABLE 2.15t Continued Orifice Flowmeter Capacity Table* Flange and Vena Contracta Taps Liquid

Steam

Pipe Taps

Gas

Liquid

Steam

Gas

Pipe Size



Meter Range

Water (SG = 1)

100 PSIG Saturated

Air (SG = 1.0) @ 100 PSIG and 60°F

Water (SG = 1)

100 PSIG Saturated

Air (SG = 1.0) @ 100 PSIG and 60°F

Inches

Inches

Inches

Inches of Water

Gal./Min.

Lb./Hr.

Std. Cu. Ft/Min.

Gal./Min.

Lb./Hr.

Std. Cu. Ft./Min.

20

18.814

24

22.626

14.1105

200 100 50 20 10 2.5

11100 7870 5565 3520 2485 1240

356238 251352 178119 112671 79946 39932

125097 88645 62714 39766 28085 14084

16550 11720 8310 5250 3715 1850

531871 376121 265936 168177 119298 59566

187232 132554 93616 59318 41920 20960

16.9695

200 100 50 20 10 2.5

16060 11375 8035 5090 3590 1795

515222 364250 257611 162954 115625 57753

180927 128206 90703 57513 40619 20369

23950 16960 12000 7585 5375 2675

769238 543978 384619 243233 172539 86150

270791 191710 135395 85790 60628 30314

*

Reproduced by permission of Taylor Instrument Co. (ABB Kent-Taylor).

where E = area factor, determined from curve C on Figure 2.15s R = pipe constant, determined from table on Figure 2.15s G = specific gravity of gas (air = 1.0) Gf = specific gravity of liquid at operating temperature Gt = specific gravity of liquid at 60°F (15.6°C) h = pressure differential across orifice in inches H2O Y = compressibility factor, determined from curve B in Figure 2.15s 3 V = specific volume (ft /lbm), determined from steam tables provided in the Appendix Tf = flowing temperature expressed in °R (°F +460) Pf = flowing pressure in PSIA X = pressure loss ratio defined as h/2Pf A useful simplified form of the mass flow equation [Equation 2.15(3)] is W = 359 Cd 2

hρ 1− β4

2.15(8)

where W = mass flow in lb/h d = orifice diameter in inches h = differential pressure in inches of water; water density 3 assumed to be 62.32 lb/ft , corresponding to 68°F (20°C) 3 ρ = operating density in lb/ft β = ratio of orifice diameter to pipe diameter in pure number C = coefficient of discharge in pure number


This is a modification of the basic equation for mass 2 flow [Equation 2.15(3)] substituting the 359 Cd 1 − β 4 for kA. The constant 359 includes a factor for the chosen units of measurement. The coefficient of discharge is involved with the flow pattern established by the orifice, including the vena contracta and its relation to the differential-pressure measurement taps. An average value of C = 0.607 can be used for flange and other close-up taps, which gives working equation W = 218d 2

hρ 1 – β4

2.15(9)

For full flow taps, C = 0.715, and the equation becomes W = 275d 2

hρ 1 – β4

2.15(10)

These working equations can be used for approximate calculations of the flow of liquids, vapors, and gases through any type of sharp-edged orifice. When using orifices for measurement in weight units, errors in determination of ρ must be considered. (Refer to Chapter 6 for density measurement and sensors.) Accurate determination of density under flowing conditions is difficult, particularly for gases and vapors. In some cases, even liquids are subject to density changes with both temperature and pressure (for example, pure water in high-pressure boiler feedwater measurement).

2.15 Orifices For W, d, h, and ρ given in dimensions other than those stated, simple conversion factors apply. Transfer of ρ in Equations 2.15(8) through 2.15(10) from the numerator to denominator will give volume flow in actual cubic feet per hour at flowing conditions [see Equations 2.15(2) and 2.15(3)]. Beta ratio, and hence orifice diameter, can be calculated from a transposed form of the mass flow Equation 2.15(8).

ORIFICE ACCURACY If the purpose of flow measurement is not absolute accuracy but only repeatable performance, then the accuracy in calculating the bore diameter is not critical, and approximate calculations will suffice. On the other hand, if the measurement is going to be the basis for the sale of, for example, valuable fluids or of large quantities of natural gas transported in high-pressure gas lines, absolute accuracy is essential, and precision in the bore calculations is critical. Some engineers believe that, instead of individually siz6 ing each orifice plate, bore diameters should be standardized. This approach would make it practical to keep spare orifices on hand in all standard sizes. This approach seems reasonable, because the introduction of the microprocessor-based DCS systems means it is no longer important to have round figures for the full-scale flow ranges. If this approach to orifice sizing were adopted, the orifice bore diameters and d/p cell ranges would be standardized, round values, and the corresponding maximum flow would be an uneven number that corresponds to them. If orifice bore diameters are selected from standardized sizes, the actual bore diameter required can be calculated, as is normally done, and the next size from the standard sizes (available in 0.125-in. diameter increments) can be selected. The use of this approach is practical and, although it results in an “oddball” full flow value, that is no problem for our computing equipment. In the past, to increase flow rangeability, the natural gas pipeline transport stations used a number of parallel runs (Figure 2.15u). In these systems, the flow rangeability of the

individual orifices was minimized by opening up another parallel path if the flow exceeded about 90% of full-scale flow (of the active paths) or by closing down a path when the flow in the active paths dropped to a selected low limit, such as 80%. By so limiting the rangeability, metering accuracy was kept high, but at the substantial investment of adding piping, metering hardware, and logic controls for the opening and closing of runs. Another, less expensive, choice was to use two (or more) transmitters, one for high (10 to 100%) pressure drop and the other for low (1 to 10%), and to switch their outputs depending on the actual flow. This doubled the transmitter hardware cost and added some logic expense at the receiver, but it increased the rangeability of orifice flowmeters to about 10:1. As smart d/p transmitters with 0.1% of span error became available, another relatively inexpensive option became obtainable: the dual-span transmitter. Some smart d/p transmitters are currently available with 0.1% of span accuracy, and their spans can be automatically switched by 7 the DCS system, based on the value of measurement. Therefore, a 100:1 pressure differential range (10:1 flow range) can be obtained by automatically switching between a high (10 to 100%) and a low (1 to 10%) pressure differential span. As the transmitter accuracy at both the high and low flow condition is 0.1% of the actual span, the overall result can be a 1% of actual flow accuracy over a 10:1 flow range. Where the ultimate in accuracy is required, actual flow calibration of the meter run (the orifice, assembled with the upstream and downstream pipe, including straightening vanes, if any) is recommended. Facilities are available for very accurate weighed water calibrations, in lines up to 24 in. (61 cm) diameter and larger, and with a wide range of Reynolds numbers. For orifice meters, highly reliable data exists for accurate transfer of coefficient values for liquid, vapor, and gas measurement.

References 1. 2.

Run No. 1 Run No. 2 Run No. 3

3. 4. 5.

Run No. 4 6.

FIG. 2.15u Metering accuracy can be maximized by keeping the flow through 8 the active runs between 80% and 90% of full scale.


275

7.

Miller, R. W., Flow Measurement Handbook, 3rd ed., McGraw-Hill, New York, 1996. ASME, Fluid Meters, Their Theory and Application, Report of ASME Research Committee on Fluid Meters, American Society of Mechanical Engineers, New York. Shell Flow Meter Engineering Handbook, Royal Dutch/Shell Group, Delft, The Netherlands, Waltman Publishing Co., 1968. American Gas Association, AGA Gas Measurement Manual, American Gas Association, New York. Miller, O. W. and Kneisel, O., Experimental Study of the Effects of Orifice Plate Eccentricity on Flow Coefficients, ASME Paper Number 68-WA/FM-1, 10, Conclusions 3, 4, 5, American Society of Mechanical Engineers, New York. Ahmad, F., A case for standardizing orifice bore diameters, InTech, January 1987. Rudbäck, S., Optimization of orifice plates, venturies and nozzles, Meas. Control, June 1991.

276

8. 9. 10. 11. 12.

13. 14.

15.

Flow Measurement

Lipták, B. G., Applying gas flow computers, Chem. Eng., December 1970. Measurement of Fluid Flow in Pipes, Using Orifice, Nozzle, and Venturi, ASME MFC-3M, December 1983. Measurement of Fluid Flow by Means of Pressure Differential Devices, ISO 5167, 1991, Amendment in 1998. Flow Measurement Practical Guide Series, 2nd ed., D. W. Spitzer, Ed., ISA, Research Triangle Park, NC. API, Orifice Metering of Natural Gas, American Gas Association, Report No. 3, American Petroleum Institute, API 14.3, Gas Processors Association GPA 8185–90. Reader-Harris, M. J. and Saterry, J. A., The orifice discharge coefficient equation, Flow Meas. Instrum., 1, January 1990. Reader-Harris, M. J., Saterry, J. A. and Spearman, E. P., The orifice plate discharge coefficient equation—further work, Flow Meas. Instrum., 6(2), Elsevier Science, 1995. Reader-Harris, M. J. and Saterry, J. A., The Orifice Plate Discharge Equation for ISO 5167–1, Paper 24 of North Sea Flow Measurement Workshop, 1996.

Bibliography AGA/ASME, The flow of water through orifices, Ohio State University, Student Eng. Ser. Bull. 89, IV(3).


Ahmad, F., A case for standardizing orifice bore diameters, InTech, January 1987. American Gas Association, Report No. 3, Orifice Metering of Natural Gas, 1985. ANSI/API 2530, Orifice metering of natural gas, ANSI, New York, 1978. ANSI/ASME MFC, Differential Producers Used for the Measurement of Fluid Flow in Pipes (Orifice, Nozzle, Venturi), ANSI, New York, December 1983. ASME, The ASME-OSI Orifice Equation, Mech. Eng., 103(7), 1981. BBI Standard 1042, Methods for the Measurement of Fluid Flow in Pipes, Orifice Plates, Nozzles and Venturi Tubes, British Standard Institution, London, 1964. Differential pressure flowmeters, Meas. Control, September 1991. Kendall, K., Orifice Flow, Instrum. Control Syst., December 1964. Sauer, H. J., Metering pulsating flow in orifice installations, InTech, March 1969. Shichman, D., Tap location for segmental orifices, Instrum. Control Syst., April 1962. Starrett, P. S., Nottage, H. B. and Halfpenny, P. F., Survey of Information Concerning the Effects of Nonstandard Approach Conditions upon Orifice and Venturi Meters, presented at the annual meeting of the ASME, Chicago, November 7–11, 1965. Stichweh, L., Gas purged DP transmitters, InTech, November 1992. Stoll, H. W., Determination of Orifice Throat Diameters, Taylor Technical Data Sheets TDS-4H603.

2.16

Pitot Tubes and Area Averaging Units W. H. HOWE (1969) J. 0. HOUGHEN (1982) B. G. LIPTÁK, M. PTÁCNÁK (1995) B. G. LIPTÁK

FE

(2003) Flow Sheet Symbol

Types

A. Standard, single-ported B. Multiple-opening, averaging C. Area averaging for ducts

Applications

Liquids, gases, and steam

Operating Pressure

Permanently installed carbon or stainless-steel units can operate at up to 1400 PSIG (97 bars) at 100°F (38°C) or 800 PSIG (55 bars) at approximately 700°F (371°C); pressure rating of retractable units is function of the ratings of the isolating value

Operating Temperature

For permanent installations, up to 750°F (399°C) in steel and up to 850°F (454°C) in stainless-steel construction

Flow Ranges

Can be used in pipes or ducts in sizes 2 in. (50 mm) or larger; no upper limit

Materials of Construction

Brass, steel, stainless steel

Minimum Reynolds Number

In the range of 20,000 to 50,000

Rangeability

Usually limited to 3:1

Straight-Run Requirements

Twenty-five to 30 pipe diameters upstream and 5 downstream are required if the pitot sensor is located downstream of a valve or of two elbows in different planes; if straightening vanes are provided, this requirement is reduced to 10 pipe diameters upstream and 5 downstream

Inaccuracy

For standard industrial units: 0.5 to 5% of full scale. Industrial pitot venturies must be individually calibrated to obtain 1% of range performance. Full-traversing pitot venturies under laboratory conditions meeting the National Bureau of Standards can limit the error to 0.5% of actual flow. Inaccuracies of individually calibrated multipleopening averaging pitot tubes, when Reynolds numbers exceed 50,000, are 2% of range. The errors of area-averaging duct units are claimed to be between 0.5 and 2% of span. The errors listed above do not include that of the d/p cell, which is additional.

Costs

The cost of the pitot tube itself in case of a 1-in. dia. averaging tube in stainless-steel  materials is $800 if fixed and $1500 if retractable for hot-tap installation. Hastelloy units for smokestack applications can cost $2000 or more. A local pitot indicator cost $500; a d/p transmitter suited for pitot applications costs about $1,500. Calibration costs are additional and can amount to $1000/tube.

Partial List of Suppliers

ABB Automation Instrumentation (www.abb.com/us/instrumentation) (A) Air Monitor Corp. (www.airmonitor.com) (C) Alnor Instrument Co. (www.alnor.com) (A) Blue White Industries (www.bluwhite.com) (A) Brandt Instruments (www.brandt.com) (C) Dietrich Standard (www.annubar.com) (Annubar—B) Dwyer Instruments Inc. (www.dwyer-inst.com) (B)


278

Flow Measurement

The Foxboro Co. (www.foxboro.com) (pitot venturi—A) Kobold Instruments Inc. (www.koboldusa.com) (B) Meriam Instrument (www.meriam.com) (B) Mid-West Instrument (www.midwestinstrument.com) (delta tube—B) United Electric Controls Co. (www.ueonline.com) (A)

For the measurement of the velocities of fluids, in 1732, Henri de Pitot invented the pitot tube. Pitot tubes detect the flowing velocity at a single point (standard), at several points that lead into an averaging probe (multiported), or at many points across the cross section of a pipe or duct (area-averaging). Their advantages are low cost, low permanent pressure loss, and the capability of inserting the probe-type sensors into the process pipes while the system is under pressure (wet- or hot-tapping). The disadvantages of pitot tube-type sensors are low accuracy, low rangeability, and the limitation of being suitable only for clean liquid, gas, or vapor service unless purged.

p

For a compressible perfect gas for which ργ remains constant during an isentropic change, a similar relation emerges. 2 (γ − 1) v p ( Pt − P) = Pv = ρ γ 2 gc

where γ = ratio of specific heats. Assuming isentropic stagnation at the sensing point of the probe, Pt

∫ P

THEORY OF OPERATION The impact pressure on a body, which is immersed in a moving fluid is the sum of the static pressure and the dynamic pressure. Thus,

2.16(4)

vp

dp = ρ

∫

Vp d V

o

2.16(5)

gc

where, using English units, Vp = velocity of approach, ft/s 2 P = pressure, lbf/ft 3 ρ = fluid density, lbm/ft lbm ft gc = 32.2 2 lbf s

Pt = P + Pv

2.16(1)

where Pt = total pressure, which can be sensed by a fixed probe when the fluid at the sensing point is in an isentropic state (constant entropy) P = static pressure of the fluid whether in motion or at rest Pv = dynamic pressure caused by the kinetic energy of the fluid as a continuum

If density is constant, integration yields

( Pt − P) = Pv =

∫ P

vp

dp = ρ

∫ o

v p dv gc

2.16(2)

P

( Pt − P) = Pv =

( Pt − P) = Pv =


Pv 2p

P

2 gc

2.16(3)

( )

2 (γ − 1) ρ Vp gc γ

2.16(7)

where γ is the ratio of specific heats. To compute the fluid velocity at a particular point, it is necessary to measure the values of both the static pressure

where vp = approach velocity at the probe location ρ = fluid density gc = a constant For a liquid of constant density, integration yields, at a point,

2.16(6)

2 gc

For a compressible perfect gas, the ratio ργ remains constant during an isentropic change, and a similar relation is obtained.

With respect to the energy relation at the isentropic stagnation point of an ideal probe, Pt

( )

p Vp2

Vp

Pt

Vp ∼ √Pt − P

FIG. 2.16a The velocity at a point (in the turbulent flow range) is related to the square root of the pressure difference between total and static pressures.

2.16 Pitot Tubes and Area Averaging Units

279

Purge Gas

Flow Indicator

Thermal Flowmeter

Balancing Needle Valve

Bypass Orifice Adaptor Bushing Impact Opening (Flow Inlet)

0.5" NPT Connection Bushing Flow

Gland Nut

FIG. 2.16b Pitot rotameter with bypass flow entering through impact opening (facing flow) and leaving through static port on opposite side (not shown). (Courtesy of ABB Instruments, formerly Fischer & Porter Co.)

FIG. 2.16c Pitot-tube rangeability can be increased by replacing the d/p cell detector with a thermal flowmeter.

(P) and the total pressure (Pt) at that point (Figure 2.16a), whence VP = C

( Pt − P)0.5 ρ

Pipe Wall

2.16(8)

where C = a dimensional constant.

d Tap

L

PRESSURE DIFFERENTIAL PRODUCED One of the problems with pitot tubes is that they do not generate strong output signals. The d/p cells available are discussed in Chapter 5, under “Pressure Measurement.” The minimum span of a “smart” d/p cell is 0 to 2 in. of H2O (0 to 0.5 kPa). These smart d/p cell units are accurate up to 0.1% of actual span. For narrower differentials, down to 0 to 0.1 in. H2O (0 to 25 Pa), the membrane-type d/p cells can be used. In addition to using d/p cells, one can also install elastic element or manometer-type readout devices, variable-area flowmeters (Figure 2.16b), or thermal flowmeters (Figure 2.16c) as pitot tube detectors. The thermal detector gives the highest rangeability, but it can be used only if the pitot tube is purged. STATIC PRESSURE MEASUREMENT In process fluids flowing through pipes or ducts, the static pressure is commonly measured in one of three ways: (1) through taps in the wall, (2) by static probes inserted into the fluid stream, or (3) by small apertures located on an aerodynamic body immersed in the flowing fluid. 1 2 The data of Shaw (presented by Benedict ) show that errors in the measurement of static pressure are minimal for velocities up to 200 ft/s (60 m/s) if the wall tap dimensions conform to those in Figure 2.16d, where the tap diameter (d) is 0.0635 in., the sensing tube ID ≅ 2d, and the tap lengthto-diameter ratio (l/d) is 1.5 < l/d < 6.


Sensing Tube D

FIG. 2.16d Wall tap for static pressure measurement.

Static pressure errors also depend on fluid viscosity, fluid 1 velocity, and whether the fluid is compressible. Shaw states that, for incompressible fluids flowing in a circular conduit 5 with a pipe Reynolds number of 2 × 10 , an error of about 1% of the mean dynamic pressure may occur using a wall 3 tap with a diameter 1/10th that of the pipe. Rayle mentions that a tap diameter of 0.03 in. (0.75 mm) with a conical countersink 0.015 in. (0.34 mm) deep will ensure nearly true static pressure sensing. Static pressure may also be sensed through a tube inserted into the moving fluid. One configuration is shown in Figure 2.16e. Other static probe designs are also described in the liter2 ature. The aerodynamic probe is a bluff body inserted into the flowing fluid with appropriately located holes on its surface through which pressure signals are obtained. The probe is oriented so that the sensed pressure is a measure of the 2 static pressure. Two configurations taken from Benedict, the cylinder and the wedge, are shown in Figure 2.16f. The probes are rotated until the pressure sensed from each hole is the same or, alternatively, the two taps may be manifolded to obtain an averaged pressure.

280

Flow Measurement

15 d

5d

45°

Flow Direction

d

45°

Four Holes (Dia. = 0.04") Spaced 90° Apart Cylinder

Wedge

Wall

FIG. 2.16f Two shapes of aerodynamic probes used to sense static pressure. FIG. 2.16e Typical static pressure-sensing probe. Impact Pressure Connection Stainless Steel Tubing

Tubing Adapter

Static Pressure Holes Outer Pipe Only

Impact Pressure Opening

FIG. 2.16g Typical pitot tube.

Impact (High Pressure) Connection Packing Nut Stuffing Box

Flow

Static (Low Pressure) Connection Corporation Cock

Static Opening Flow

Of Pipe

Impact Opening

FIG. 2.16h Schematic of an industrial device for sensing static and dynamic pressures in a flowing fluid.


The total pressure develops at the point where the flow is isentropically stagnated, which is assumed to occur at the tip of a pitot tube or at a specific point on a bluff body immersed in the stream. Figure 2.16g illustrates a typical pitot tube, also showing the taps for sensing static pressure. Another variation is shown in Figure 2.16h. SINGLE-PORTED PITOT TUBE Pitot tubes are sensitive to flow direction and must be carefully aligned to face into the flow. This can be difficult if the flow direction is caused to vary by changes in turbulence. The pitot tube is made less sensitive to flow direction if the impact aperture has an internal bevel of about 15° extending about 1.5 diameters into the tube. The characteristics of 2 various designs and orientations are discussed in Benedict. Figure 2.16i illustrates the typical performance of a pitot tube. Pitot venturi and double-venturi elements have been developed to amplify the pressure signals generated by the in-stream velocity sensors, as shown in Figures 2.16j and 2.16k. These elements are intended to remain in a fixed position, so their measurements must be converted to flow rate through calibration, which accounts for the properties of


281

1.0 WATER D = 0.7125 cm. WATER D = 2.855 cm. AIR D = 2.855 cm. AIR D = 0.7125 cm.

0.9

AVERAGE VELOCITY CENTER VELOCITY

AIR D = 1.225 cm.

0.8

0.7

0.6

0.5

1

2

3

4

5

6

7

8

9

10

20

30

40

50

60

70

80 90 100

x 1000 = REYNOLDS NUMBER R0

FIG. 2.16i 7 Center to average velocity ratios in straight and smooth pipes.

Flow

1/2-1"

1/2"

Smooth, Straight Approach Section of at Least 10 Pipe Diameters

FIG. 2.16j A pitot venturi produces a higher differential pressure than the standard pitot tube. FIG. 2.16l Reduction nozzle used to expedite velocity traverses.

To determine the average velocity in a pipe, it is necessary to traverse it with a pitot tube. For circular pipes, such an average is obtained from measurements of (Pt − P) on each side of the cross section at the following locations, expressed in percentage of the diameter measured from the center: FIG. 2.16k The double venturi produces a higher differential pressure than the standard pitot tube. (Courtesy of The Foxboro Co.)

the fluid and the velocity profile (e.g., Reynolds number). To obtain a stable velocity profile, it is recommended that a smooth, straight section of pipe, of a length equaling at least 10 to 15 pipe diameters, be provided both upstream and downstream of the probe.


 2n − 1  N    × 100%,  n = 1, 2, 3K  N  2 

2.16(9)

where N is the number of measurements per traverse. Two measurements normal to each other are recommended. To improve the measurements made near the walls of pipes that are more than 6 in. (150 mm) in diameter, a reduction nozzle is inserted into the pipeline (Figure 2.16l).

282

Flow Measurement

Flow

U Uc

Flow

U Turbulent

Laminar V

FIG. 2.16m The velocity profile becomes flatter as the Reynolds number rises (the flow becomes more turbulent), and the task of the pitot-type flow sensor is to find the insertion depth corresponding to the average velocity (V).

Calibration of Pitot Tubes In high-precision laboratory tests, the pitot tube is traversed across the cross-section of the pipe, thereby establishing the velocity profile that exists in the pipe. In industrial applications, the pitot tube is fixed and measures the flow velocity only at one point on the velocity profile (Figure 2.16m). If the velocity (U ) measured by this fixed pitot tube is not the average velocity (V ), a substantial error will result. This error cannot be easily eliminated because, even if the pitot tube insertion is carefully set to measure the average velocity V under one set of flow conditions, it will still be incorrect as soon as the flow velocity changes. At Reynolds numbers under 1000 (in the fully laminar region), the ratio between the average velocity and the center velocity is 0.5 (V/Uc = 0.5 in Figure 2.16m). In fully developed turbulent flow (Re = 50,000 or more), this same ratio is about 0.81(V/Uc = 0.8). Unfortunately, the velocity profile is affected not only by the Reynolds number but also by the pipe surface roughness and by upstream valves, elbows, and other fittings. To reform the velocity profile, it is recommended to provide a straight pipe length of about 25 pipe diameters between the upstream disturbances and the pitot element. If the data for calculating the Reynolds number is available, and if the pitot tube is installed in a pipe with smooth inner surface, it should be possible to design a microprocessorbased smart pitot tube that measures only the center velocity (Uc) and, based on that reading, accurately calculates the flow under all flow conditions. The National Bureau of Standards calibrates pitot tubes by mounting them on a carriage, which is drawn through stagnant air at a known velocity. Smoke is introduced into the room to verify that the air is stagnant—that there is no turbulence. Such tests have shown that pitot tubes with coefficients very close to unity can be designed. Devices such as pitot-venturies or double venturies can provide flow rate measurements with less than 1% error, but only after extensive in situ calibration for each installation. MULTIPLE-OPENING PITOT TUBES One approach in attempting to overcome the inherent limitation of the pitot tube—that of being a point velocity sensor—was to measure the velocities at several points and average these


PL

PH

A = 3/8" , 7/8" , 11/4" or 2" (9.5, 22, 32, or 51 mm)

Velocity Profile

Average Velocity

High Pressure Profile

Low Pressure Profile

PH

PL DP

Average High (Impact) Pressure

Average Low Pressure

FIG. 2.16n The design of a particular averaging pitot tube. (Courtesy of Dietrich Standard.)

readings. It was argued that, by averaging the velocities measured at four fixed points, for example (see Figure 2.16n), changes in the velocity profile will be detected, and therefore the reading of a multiple-opening pitot tube will be more accurate than that of single-point sensors. The manufacturers of averaging pitot tubes usually claim that the flow coefficient (K) will stay within 2% between the Reynolds numbers of 50,000 and 1,000,000. This is probably so, but it might not be attributable to averaging action but rather to the fact that, in this highly turbulent region, the velocity profile is flat and changes very little. Critics of this device argue that it offers little improvement over the single-opening pitot tube, because it is ineffective at Reynolds numbers below 50,000. This means that it is not applicable for the measurement of a large portion of industrial liquid flows. The other argument made by critics is that the averaging pitot tube openings are too large and,


consequently, these devices are not true averaging chambers; rather, the sensed pressure is dominated by the pressure at the nearest port. For these reasons, further testing by independent laboratories is still needed. The reason for making the ports of the averaging pitot tubes so large is to prevent plugging. Some manufacturers of area-averaging pitot tubes do overcome this limitation by purging, because the small port openings are kept clean by the purge gas, and these units can act as true averaging chambers. Naturally, they can be used only on processes in which the introduction of a purge media is acceptable. One advantage of the averaging pitot tubes that both its manufacturers and its critics agree on is their ability to be installed into operating, pressurized pipelines. This hottapping capability, and the ability to remove the sensor without requiring a shutdown, are important advantages of all probe-type instruments (Figure 2.16o). In calculating the pressure differential produced by an averaging pitot tube, one might use the equations listed in Table 2.16p. For the metric equivalents of the units used in this table, refer to the Appendix. The flow coefficient (K) of the pitot tube varies with its design. The K values of the averaging pitot tube shown in Figure 2.16n are listed in Table 2.16q. The distance “A” used in Table 2.16q is also defined in Figure 2.16n.

Install Drill Thru Valve Inserted

FIG. 2.16o The hot-tap installation of an averaging pitot tube involves the same steps, which are required in installing all “retractable” probe-type instruments. (Courtesy of Dietrich Standard.)

available with circular or rectangular cross sections (Figures 2.16r and 2.16s) and can be mounted in the suction or discharge of fans or in any other large pipes or ducts. These stations are designed so that one total pressure detection port and one static pressure sensing port are located in each unit area of the cross section of the duct, and they each are

AREA-AVERAGING PITOT STATIONS Area-averaging pitot stations have been designed for the measurement of large flows of low-pressure gases. Measurements include the flow rate of combustion air to boilers, airflow to dryers, and air movement in HVAC systems. These units are

TABLE 2.16p Equations for Calculation of the Pressure Differential Produced by the Averaging Pitot Tube Described in Figure 2.16n* Liquid, gas, steam (mass rate of flow)  1 hw =   ρf

  lb /hr  2 m   2  358.94 KD 

Liquid (volume rate of flow) GPM  hw = (Gf )   5.666 KD2 

2

Gas (standard volumetric flow)

hw K D lbm/hr

= = = =

differential pressure, inches of water at 68°F flow coefficient internal pipe diameter, inches pounds mass per hour

GPM = U.S. gallons per minute ACFH = Actual cubic feet per hour SCFH = Standard cubic feet per hour (at 14.73 psia and 60°F)

ρf = flowing density, lbm /ft for gas: 3

 T G   SCFH  2 hw =  f   2  p f   7, 711KD  Gas (actual volume rate of flow) ACFH  hw = ( ρf )  358.94 KD2 

* Courtesy of Dietrich Standard.


283

ρf = 3

pf 14.73

×

520 × .076487 × G Tf

.076487 lbm /ft = air density at 14.73 psia and 60°F Gf = specific gravity of liquid G = specific gravity of gas (molecular weight of air = 28.9644) Tf = temperature of flowing gas in degress Rankine (°R = °F + 460) Pf = flowing pressure, psia

284

Flow Measurement

TABLE 2.16q The Flow Coefficient K for the Averaging Pitot Tube Shown in Figure 2.16n Having the “A” Dimension Also Defined in That Figure* Paper Size Size/Sch

D-in

2" sch 40

Flow Coefficient-K D-mm

3

A = /8"

2.067

52.50

2 /2" sch 40

2.469

62.71

.6026

3" sch 40

1

7

A = /8"

1

A = 1 /4"

A = 2"

5912

3.068

77.93

.6134

3 /2" sch 40

3.548

90.12

.6192

4" sch 40

4.026

102.26

.6235

5" sch 40

5.047

128.19

.6297

6" sch 40

6.065

154.05

.6047

8" sch 40

7.981

202.72

.6173

10" sch 40

10.020

254.51

.6250

12" sch std.

12.000

304.80

.6298

.6186

14" sch std.

13.250

336.55

.6321

.6220

16" sch std.

15.250

387.35

.6349

.6263

18" sch std.

17.250

438.15

.6370

.6296

20" sch std.

19.250

488.95

.6387

.6321

24" sch std.

23.250

590.55

.6411

.6357

.6247

30" sch std.

29.250

742.95

.6435

.6393

.6308

36" sch std.

35.250

42" sch std.

41.250

1047.7

48"

48.00

60"

60.00

72"

72.00

1

895.35

.5934

.6450

.6416

.6346

.6461

.6432

.6373

1219.20

.6445

.6395

1524.0

.6461

.6422

1828.80

.6472

.6439

*Courtesy of Dietrich Standard.

connected to their own manifold. The manifolds act as averaging chambers, and they are also purged to protect the sensing ports from plugging. The straight-run requirement of these units is reduced by the addition of a hexagon-cell-type flow straightener and a flow nozzle in front of the area-averaging flow sensor. This nozzle also serves to amplify the differential pressure produced by the unit (Figure 2.16s). According to the manufacturer, this design (Figure 2.16s) reduces the straight-run requirement of most installations to a range of 0 and 10 diameters. The longest straight run (10 diameters) is recommended when the flow meter is installed downstream of a butterfly valve or a damper. Because these area-averaging pitot stations generate very small pressure differentials, special d/p cells are required to detect these minute signals. One such detector is the membrane-type design (Fig. 2.16t), which can have a span as small as 0 to 0.01 in. H2O (to 2.5 Pa). When such extremely small pressure differentials are detected, the pressure drop in the tubing between the d/p cell and the pitot station must


FIG. 2.16r Installation in rectangular duct of area-averaging pitot tube ensembles for metering the flow rate of gases. (Courtesy of Air Monitor Corp.)


Exit Fitting 1/4 NPTF

Wall

Flow Straightener

a b

Probe Array

Nozzle

285

Gain Adjustment

Membrane

HI Output

Flow

Supply Air Supply Cavity

Orifice

FIG. 2.16t Membrane type d/p transmitter.

Gaskets

FIG. 2.16s The flow straightener and the nozzle serve to reduce the upstream straight pipe-run requirement and increase the pressure differential generated. (Courtesy of Brandt Instruments.)

be minimized. This is achieved by making the connecting tubes short and large in diameter. The pressure differential generated by the flow element shown in Figure 2.16s can be calculated by using the equations in Table 2.16u. For the equivalent SI units for use in these equations, refer to the Appendix. SPECIAL PITOT TUBES FOR PULSATING FLOW The mean velocity measurements of unsteady flows, if made 4 by conventional pitot tubes, are usually inaccurate. In such measurements, one can expect errors in the range of 5 to 30%

of mean total pressure. This measurement can be improved by using specially designed probes when the application involves unsteady or pulsating flows. Figure 2.16v shows a design provided with a low-capacity capillary probe filled with silicon oil. The oil serves to transmit the process pressure to the d/p transducer. This type of probe was developed and is used by Deutsche Forschungs– und Versuchsanstalt für Luft und Raumfahrt in 5 Germany. Figure 2.16w shows another example of a probe designed to measure unsteady flow. This probe was developed in the Aeronautical Research and Test Institute in 6 Czechoslovakia. The main design challenge in this design is to equalize the resistances in the input and output openings of the probe. Also, to protect against resonance during measurement, the natural frequency of the probe must be carefully tuned.

TABLE 2.16u Calculation of Pressure Differentials Generated By Area Averaging Pitot Stations* Equations for Differential Pressure Calculation 2

ACFM  DENS DP =  ×  Area  (1096.845)2 SCFM  DP =   4000.7 × Area 

2

DENS DP = (V ) × (1096.845)2 2

2

M 1  × DP =   60 × Area  DENS × (1096.845)2

Terms Used Area = Cross-sectional area of duct section in ft

2

ACFM = Actual cubic feet per minute DP = Differential pressure in inches w.c. M = Mass flow in pounds per hour SCFM = Standard cubic feet per minute V = Velocity in feet per minute PABS = Absolute pressure in PSIA PATM = Atmospheric pressure in PSI Ps = Static pressure in inches w.c. T = Temperature in degrees F DENS = Density at actual conditions lbs/ft

3

DENSTD = Density at standard conditions lbs/ft *Courtesy of Brandt Instruments.


LO

3

286

Flow Measurement

6. Pitot Port

Pressure Detector

7.

74–34, Deutsche Forschungs–und Versuchsanstalt für Luft–und Raumfahrt-Portz-Wahn, 1974. Neruda, J. and Soch, P., Measurement System with a Pitot Tube, Czechoslovak patent no. 218417. Spink, L. K., Principles and Practice of Flow Engineering, 9th ed., The Foxboro Co., Invensys Systems, Inc., Foxboro, MA, 1967.

Capillary

Bibliography

Oil Supply

FIG. 2.16v On highly pulsating flow measurements a minute flow of silicon oil through a capillary can serve as a pressure-averaging purge.

FIG. 2.16w Pitot tube designed for pulsating flow averaging using tuned natural 6 frequency.

References 1. 2. 3. 4. 5.

Shaw, R., The influence of orifice geometry on static pressure measurements, Fluid Mech., 7, pt. 4, 1960. Benedict, R. P., Fundamentals of Temperature, Pressure, and Flow Measurements, John Wiley & Sons, New York, 1969, 237. Rayle, R. F., Influence of orifice geometry on static pressure measurements, ASME Paper 59-A-234, December 1959. Becker, H. A., Reaction of pitot-tube in turbulent flow, J. Fluid Mech., 69, pt. I, 1974. Weyer, L. I., Bestimmung der zeitlichen Druckmittelwerte in stark fluktuirender Strömmung, inbesondere in Turbomaschienen, Forschungsbericht


The Accuracy of the Pitot Tube Traverse Method of Measuring Pipe Flow at Various Distances up to 30 Diameters Downstream of a Smooth Right-Angled Bend, National Engineering Laboratory Flow Measurement Memo, No. 37, 1969. Andrew, W. G. and Williams, H. B., Applied Instrumentation in the Process Industries, Vol. I, 2nd ed., Gulf Publishing Co., Houston, TX, 1979. Baker, R. C., Flow Measurement Handbook, Cambridge University Press, UK, 2000. Beitler, S. R., Present status of the art of flow measurement in the power industry, ASME Paper No. 68-WA/PTC-7, December 1968. Cushing, M., The future of flow measurement, Flow Control, January 2000. De Boom, R. J., Flow meter evaluation, ISA Conference, Paper #91-0509, 1991. Desmeules, M., Fundamentals of Gas Measurement, Canadian Meter Company, Milton, Ontario, Canada, June 1999. Dietrich, P. D., Primary flow meter, Instrum. Control Syst., December 1968. Eren, H., Flowmeters, Survey of Instrumentation and Measurement, S.A. Dyer, Ed., John Wiley & Sons, New York, 2001. Flow meter survey, Instrum. Control Syst., 42(3), 115–130, 1969, and 42(7), 100–102, 1970. Furness, R. A., Developments in pipeline instrumentation, Pipe Line Rules of Thumb Handbook, 4th ed., Gulf Publishing, Houston, TX, 1998. Hiser, R., Increased functions and reduced costs of differential pressure flowmeters, Meas. Control, September 1990. Ifft, S. A., Custody Transfer Flow Measurement with New Technologies, Saudi Aramco, Saudi Arabia, 1999. Lipták, B. G., Flow measurement trends, Control, June 2000. London, A. V., Less traditional methods of flow measurements, Process Eng., Plant & Control, 47–50, 1968. Malherbe, G. and Silberberg, S., Device for measuring the flow of pulverized control, central electricity generating board, Translation CE 4938 from Automatisme, 13(3) 114–122, 1968. Migliorini, R., Pitot sensors, Meas. Control, September 1991. Miller, R. W., Flow Measurement Engineering Handbook, 3rd ed., McGrawHill, New York, l996. Ower, E. and Pankhurst, R. D., The Measurement of Air Flow, 4th ed., Pergamon Press, London, 1966. Scarpa, T. J., Flow velocity profiles, Meas. Control, September 1992. Spencer, E. A., Flow Measurements at the National Engineering Laboratories, Process Eng. Plant & Control, 53–57, August 1968. Spitzer, D. W., Flow Measurement, 2nd ed., ISA, Research Triangle Park, NC, 2001. Stobie, G. J., Wet gas metering in the real world, Wet Gas Metering Seminar, Paris, 2001. Waring, T., Fundamental of Rotary Meter Measurement, Dresser Canada, June 1999. Yoder, J., Flowmeter shootout, part I and II: new technologies, Control, February and March 2001.

2.17

Polyphase (Oil/Water/Gas) Flowmeters

FY 101A

Gas Oil

FY 101B Water

I. H. GIBSON

FY 101C

(2003)

UT 101

MPFM Flow Sheet Symbol

Design Pressure

Limited by piping design class

Operating Temperature Range

Limited by piping design class, 30 to 270°F (0 to 130°C)

Fluids

Mixtures of liquids, vapors, or gases; typically oil, water, and gas

Cost

Extremely variable, depending on line size, pressure rating, and installation requirements, starting at about $100,000 Subsea equipment commonly twice the price of surface-design units of comparable capacity

Accuracy

Five to 10%, depending on relative proportions of phases


3-Phase Measurements AS (VenturiX, PhaseTester) (www.framoeng.no) Agar Corp. (www.agarcorp.com) Aker-Kvaerner (DUET) (www.kvaerner.com/kop) FlowSys AS (Topflow) FMC Technologies (www.fmctechnologies.com) Framo Engineering AS, Schlumberger (www.framoeng.no) Jiskoot Autocontrol (Mixmeter) (www.jiskoot.com) McCrometer (www.mccrometer.com) Petroleum Software Ltd. (ESMER) (www.petroleumsoftware.co.uk) Roxar AS (Fluenta, MFI) (www.roxar.com) Solartron ISA (Dualstream) (www.solartronisa.com)

Whereas most flow measurements are carefully designed to operate in single phase (gas, vapor, liquid), there are many circumstances in which the ability to distinguish the components of a liquid/vapor/gas mixture (or, indeed, multiple immiscible liquids, vapor, and gas) in the presence of solids is highly desirable. In the production of oil/gas from unpumped petroleum wells, the flow from several thousand meters underground to the surface brings up water (commonly saline), inert gases such as carbon dioxide and nitrogen, water-saturated hydrocarbon gas (methane, ethane), hydrocarbon condensates (propane, butane, and so on), and higher hydrocarbons. The liquids and gases do not normally travel at the same velocity, as the liquids are being dragged to the surface by the expanding gas flow. A wide variety of flow regimes are

possible, depending on the mixture of phases and the geometry. A production facility may have tens of wells, and individual wells in a facility may have different mixes of ownership. This leads a requirement to be able to determine the flow of the economically interesting components from each well as accurately as possible. The traditional method has been to use a small test separator that will separate the gas and the oil and water phases from a single well by gravity while production from the rest of the facility is flowing to production separators. The test separator is fitted with a full array of level and pressure controls and flowmeters (turbine, positive-displacement, orifice, and others) to enable a steady-state operation to be achieved. Commonly, the test separator is operated at a pressure slightly above the production separator to allow the 287


288

Flow Measurement

streams from the test separator to be mixed with the flows out of the production separator. This requires a large amount of room and a complex valving system to enable each well to be tested separately. As pressures are commonly 1000 to 4500 PSI (7000 to 30,000 kPa) or higher, and the temperature is 150 to 270°F (65 to 130°C) at the surface, the test separator is not an inexpensive device. Individual well tests rarely can be scheduled more frequently than monthly as a result of the time taken to stabilize operation at a set of operating conditions. Even with the best of intentions, test separator measurements are notorious for inaccuracies, because quite small amounts of vapor disengaging in the liquid meters can induce errors in the 5 to 10% region. For offshore platforms, where the cost of test separator equipment can exceed on-shore costs by many times (and especially for subsea operations, where operators cannot easily access equipment), the potential savings from an on-line method of measurement have driven the development of a variety of devices over the past 15 years. These employ a wide variety of operating principles, some of which are outlined below. The techniques used differ depending on the ratio of liquid to gas (known as LGR or the inverse, GLR). The extreme end of the LGR (say < 10% by mass) is classified as wet gas.

WET-GAS METERING The standard approach for wet-gas metering treats the fluid as a gas. This uses differential-pressure devices (venturi or orifice) or vortex or ultrasonic meters. Turbine meters are unsatisfactory because even small quantities of high-velocity liquid can damage the meter. The venturi has a considerable advantage over the orifice in that it does not dam liquid behind the device, altering the flow profile. The McCrometer V-cone meters (Section 2.28) are also suitable for this service and have also been used for flow conditioners. Normal ISO 5167 flow conditioners are not recommended for wet-gas service; not only can they dam liquids, they can induce hydrocarbon hydrate formation. Where practical, insulation of the metering run is recommended. In some environments, trace heating may be indicated to prevent hydrate formation in the tapping connections. Pressure tappings should be short and inclined vertically upward to avoid trapping condensate in the connections and to avoid hydrate formation. Process gas chromatographs are not recommended for wet-gas services unless the liquid content is below 0.1%. Venturi Meters The venturi (Section 2.29) is much more rugged than the orifice in wet-gas service and allows for higher differentialpressure operation. With modern transmitters, turndown up


to 10:1 is possible and may be necessary, given that wet-gas behavior is far more variable than that of a pure gas. Design is generally in accordance with ISO 5761–1, 1 although recent work by Reader-Harris et al has shown that the ISO coefficients are not as well defined as claimed. The coefficients are quite sensitive to minor machining variations and, at high Reynolds number operation, the flow may break away from the throat. The coefficient can also be sensitive to the diameter-to-length ratio of the tappings; use of the “triple-T” piezometer connections advocated by ISO 5167 is impractical for wet-gas service, because the downward-facing tappings would fill with liquid. A downstream tapping after the full pressure recovery is achieved is recommended to allow the calculation of liquid content and temperature correction to upstream conditions. Algorithms for Wet-Gas Measurement Differential devices used on wet-gas service will generally overestimate the dry-gas flow rate, and various algorithms have been developed to compensate. The Chisholm and 2 Murdock correlations were developed for orifice plates; the de Leeuw correlation has been developed for venturi meters and is an extension of the Chisholm equations. In addition to the dry-gas flow rate, it is normally required to determine the liquid flow rate, and particularly the condensate flow rate. All three correlations (de Leeuw, Chisholm, Murdock) are based on the Lockhart–Martinelli parameter, which is a function of both liquid flow rate and density. The liquid flow rate can be determined by • • •

Routing the flow to a test separator The use of tracer techniques Sampling

or a combination of these. Since these are all spot-test techniques, modification to the Lockhart–Martinelli equation can be made to work in terms of the dry-gas mass fraction, which is relatively constant, provided the wetness of the gas is constant. None of these techniques is easily useful in subsea conditions, and adaptations have been made to apply dual measurements in series with devices of different geometry that allow the Lockhart–Martinelli relationship to be solved for equal values of liquid/gas. These still require considerable input of process data derived from composition. Theory of Operation of Wet-Gas Metering It has long been known that a gas stream carrying a welldispersed liquid content through a differential flow element (orifice, nozzle, venturi, or V-cone) will develop a higher differential than that due to the corresponding gas flow. If the properties of the gas and liquid are known, it is possible to calculate the relative content of liquid in the gas. The following development has been adapted from the UK DTI Oil and Gas Division’s Guidance Notes for

2.17 Polyphase (Oil/Water/Gas) Flowmeters

Petroleum Measurement under the Petroleum (Production) 2 Regulations.

and Frg is the gas Froude number given by  ρgas  V    Frg =  gas  *   ( gD)   ( ρliquid − ρgas ) 

de Leeuw Wet-Gas Venturi Correlation 3

de Leuuw has shown that the real gas mass flow rate can be derived from the following equations (all equations are based on SI units): Qreal gas =

Quncorrectedgas (1 + CX + X 2 )

2.17(1)

where Quncorrected gas = uncorrected gas mass flow rate as indicated by the venturi meter using the following equation:

Quncorrectedgas = Cgasεπ d

2

(2 ρgas ∆P) 4 1− β4

Vgas can be derived using an iterative method and “seeding” a velocity based on the uncorrected mass flow rate. The first pass equation is

Vgas =

Qliquid Qreal gas

Liquid Mass Flow Rate Correction Algorithm The resultant liquid mass flow rates can be derived from the following equations: Qtotal = Qreal gas + Qliquid

n

2.17(4)

−0.746 Frg

) for Frg ≥ 1.5

n = 0.41 for 0.5 ≤ Frg ≤ 1.5


Qcondensate = Qtotal * ζ condensate

2.17(10)

2.17(11)

Methanol (or glycol) mass flow rate Qmethanol = Qtotal * ζ methanol

2.17(12)

where ζcondensate = condensate mass fraction ζwater = water mass fraction ζmethanol = methanol mass fraction In turn,

where the exponent n is given by n = 0.606(1 − e

2.17(9)

Condensate mass flow rate

Qwater = Qtotal * ζ water

The coefficient C is given by the following equation: n

2.17(8)

2.17(3)

where Qliquid = combined liquid flow rate through the venturi flowmeter ρliquid = combined liquid density

ρ   ρ  C =  liquid  +  gas   ρgas   ρliquid 

ρgasπD2

Water mass flow rate

ρliquid ρgas

*

4 * Quncorrectedgas

For further iterations Quncorrected gas is replaced by consecutive Qreal gas values until the equation converges to a solution.

and X is the Lockhart–Martinelli parameter, which is derived as follows: X=

2.17(7)

where Vgas = superficial gas pipe velocity g = local acceleration due to gravity

2.17(2)

where Cgas = discharge coefficient of the venturi flowmeter in dry gas as determined through calibration ε = expansibility of gas in venturi as defined by ISO 5167-1 d = throat diameter of the venturi flowmeter (corrected for temperature) ρgas = gas density at upstream conditions ∆P = raw differential pressure as measured by the transmitter β = ratio of d to D, the pipe diameter

289

ζ condensate = X * ψ condensate

2.17(13)

2.17(5)

ζ water = X * ψ water

2.17(14)

2.17(6)

ζ methanol = X * ψ methanol

2.17(15)

290

Flow Measurement

where ψcondensate = condensate-to-gas mass fraction ψwater = water-to-gas mass fraction ψmethanol = methanol-to-gas mass fraction (methanol injection is commonly used to suppress hydrocarbon hydrate formation)

where tm = measured temperature P3 = fully recovered downstream pressure P1 = pressure measured at the upstream tapping K3 = downstream to upstream temperature correction exponent

Liquid Density Calculation Algorithm

P3 can be measured using a third pressure tapping or calculated (in bar) from the following empirical equation 4 from Miller:

The liquid density can be calculated as follows:

ρ liquid =

ψ liquid   ψ condensate   ψ water   ρ   +   condensate   ρ water 

ψ  +  methanol    ρ methanol 

2.17(16)

Corrections from the standard API MPMS11.2.1M may be applied. However, for improved accuracy, it is recommended that samples of the condensate be obtained and analyzed to derive specific correction factors. The values of K0 and K1 for crude oil (613.9723 and 0, respectively) are not ideal for condensate, and the alternatives from the standards referred to above may be no better. Water and methanol densities can be derived as follows: 2.17(17)

where t = temperature at the inlet of the venturi A, B, C = water constants (e.g., −0.0001732, −0.1307, 1040) D, E, F = methanol constants (e.g., 0.0000713, −0.3344, 540) The correct values of these methanol and water constants may vary due to salinity or product type. It is therefore advisable to have the liquids analyzed to determine appropriate values. Upstream Temperature Correction and Pressure Recovery The correction for downstream measured temperature to upstream temperature (in degrees centigrade) at the inlet is given by P t = (tm + 273.15) *  3  K3 − 273.15  P1 


2.17(19)

∆ω = ( Aβ 2 + Bβ + C) * ∆P

2.17(20)

where

where ψliquid = total liquid to gas mass ratio ρliquid = density of liquid ρcondensate = density of hydrocarbon condensate ρwater = density of water ρmethanol = density of methanol ρcondensate = is derived from the condensate base density and corrected for temperature and pressure (Ctl and Cpl)

P t = (tm + 273.15) *  3  K3 − 273.15  P1 

P3 = P1 − 10 −3 * ∆ω

2.17(18)

and the constants A, B, and C, for venturies with 7 and 15° exit cone angles, are as follows: 7° cone angle 15° cone angle

A = 0.38 A = 0.59

B = 0.42 B = 0.86

C = 0.218 C = 0.436

Gas Mass Fraction Estimation Using Tracer Techniques The gas mass fraction can be estimated as follows: 1. Perform the tracer flow technique to determine condensate and water flow rates and mass ratios. This uses concentrated oil-soluble and water-soluble fluorescent chemicals, injected upstream and partially recovered by sampling downstream of the flowmeter. 2. Analyze the condensate to determine base density. 3. Sample the gas to determine gas density. 4. Record the total uncorrected gas flow from venturi during the tracer flow technique. 5. Determine the dry “first pass” gas mass fraction and liquid-to-gas ratio based on the recorded uncorrected gas flow and tracer flow results (corrections for methanol injection after completion of tracer technique may be required). 6. Seed values from the last stage into the wet-gas venturi flow calculation to determine a “first pass” corrected gas flow rate. 7. Re-seed this value into the calculation, correcting gas mass ratio and liquid-to-gas ratio. 8. Iterate the process until the corrected gas flow rate converges.  Theory Solartron-ISA Dualstream II The Solartron-ISA Dualstream II, originally developed by British Gas, extends the theory noted above by fitting two dissimilar pressure differential devices in series. These have differing Lockhart–Martinelli characteristics, and the


291

through a separate multiphase flowmeter section. No standards exist as yet to assist engineers in designing multiphase metering systems, and there are wide variations in the methods used to define accuracy and performance. When considering a manufacturer’s performance and accuracy statements, it is essential to understand the implications of accuracies quoted in different ways. There are three common ways in which multiphase meter accuracies are presented:

Mixer

Venturi

2nd DP Device

FIG. 2.17a Solartron—ISA Dualstream II wet gas flowmeter.

combination can determine changes in the gas-to-liquid ratio. The venturi will generate an indicated flow rate. This can be corrected to generate a “true” gas flow rate using the equations in previous sections. The second “DP device” will generate a second indicated flow rate. This flow rate can be corrected to give a “true” gas flow rate using similar relations. A simultaneous equation can be formed from these flow rates: Qg =

Qgi (venturi) 1 + M(venturi)

=

x=

ρ (1 − x ) Cg εg g ρl x Cl

Qgi (2nd Dp)

ρ (1 − x ) Cg 1 + M(2nd Dp) εg g ρl x Cl

2.17(21)

 ρg  Qgi (2nd Dp) M(venturi) − M(2nd Dp)   ρl  Qgi (venturi)   ρg  Qgi (2nd Dp)  ρg  1 − M(2nd Dp) ρ  − Q 1 − M(venturi) ρ   l  gi (venturi)  l  2.17(22)

This enables a continuous estimation of the liquid-to-gas ratio without the necessity for tracer measurements, and it is particularly useful sub-sea, where tracer measurements are impractical with current technology. MULTIPHASE FLOWMETERS The various true multiphase flowmeters can be effective from straight liquid through to 95 or 97% gas. Some meters, such as the Agar MPFM400 and Jiskoot, attain the extreme gas end by using an in-line separator to send the wet gas through a wet-gas meter and the remaining gas, with oil and water,


1. Percent phase volume flow rate 2. Percent total multiphase flow rate 3. Percent gas and liquid flow rate plus absolute uncertainty of water cut in liquid phase Method 1 is favored by metrologists and clearly represents performance as stated. This method may not be the most practical for extreme cases of phase fractionation. Methods 2 and 3, whereas quoting relatively small numbers on the order of 5 to 10% for gas/liquid phase uncertainties and 2 or 3% for percentage water cut, may nevertheless exhibit very large individual phase errors of 100% or more, depending on the absolute value of the percentage water. The first requirement for a mixed liquid phase is to distinguish hydrocarbon from water. If the liquid phase is oilcontinuous, typically water less than 40% liquid hydrocarbon, then dielectric constant measurement at microwave frequencies can determine the water fraction. The dielectric constant of dry hydrocarbon is in the order of 2 to 4, depending on composition, while water is 82, giving a sensitive means of measurement. For higher water content, the measuring element will short out in a water-continuous phase, but density measurement can distinguish water from oil. This requires streamspecific characterization, as the composition and density of the liquid is pressure and temperature dependent. The next requirement is to distinguish the flow of liquid from the flow of gas in a system where the two will try to separate and travel at different velocities. Flow conditioning in these systems is commonly provided by a horizontal branch-in, vertical-out tee, which acts as a mixer. Crosscorrelation flowmeters (see Section 2.5) used by some suppliers apply nuclear techniques to measure the density of the stream twice, a short vertical distance apart, and correlate the fluctuations in density with time; others use electrical characteristics in a similar manner. As it is difficult to distinguish the differing velocities of liquid and gas, it is common to measure the velocity of one phase and use flow-modeling techniques to estimate the velocity of the other phase. Most of the techniques are limited by the liquid/gas ratio (LGR). Lean gas streams, with less than 5% liquid, are difficult to measure given that the density of the gas stream can be comparable with that of the liquid phase; at 3000 PSI 3 (21,000 kPa), the gas phase can be on the order of 200 kg/m 3 with the liquid phase perhaps 600 kg/m. So, a change from 5% liquid to 4% liquid is a 20% variation in liquid, but it will

292

Flow Measurement

OWM

FFD Device

Momentum Meter

Gas Bypass Loop

PD Meter

Flow

Gas Meter

MPFM301

FIG. 2.17b Agar MPFM 400 high void fraction meter.

only change the density by less than 2%. To avoid this, one supplier (AGAR) uses a centrifugal separator to separate much of the gas from a remaining three-phase mixture, measure this separately, and then mixes it back after the other stream has been measured. Removing 80% of the gas enables the multiphase meter section to be much smaller and more sensitive to the valuable liquid hydrocarbon. The true volumetric flow of the multiphase mixture is measured by a positive-

Tronic Subsea Mateable Cable Connector

Radiation Detectors

displacement Oval gear meter, and venturi techniques and microwave water content distinguish the different phases. The orientation of a multiphase flowmeter can strongly influence the multiphase flow regime. In systems with medium to high gas content and low velocity, vertical upflow can find the gas phase unable to continuously sweep the liquid phase forward, and the liquid may recycle backward, leading to metering errors and to slug flow. This offers a significant low-end constraint on flow through devices of fixed geometry, which will differ between devices of similar size. Horizontal-flow installations can show internal segregation, with gas, oil, and water layers traveling at different velocities. Again, maintaining a high velocity helps to mix the phases, but the upstream piping layout may contribute to slugging, which the meter system can do little to correct. Measurement in multiphase flow is notable by widely varying conditions under nominally constant flow. In slug flow, the liquid fraction can vary between almost zero in the region after a liquid slug to almost 100% inside the slug. Significant fluctuations will also be present in annular and churn flow patterns. The pressure drop of a liquid slug passing through a venturi meter can be five times higher than the average pressure drop for the flow; the minimum pressure drop in the same flow, corresponding to the film region, can be 20% of the average. Therefore, a venturi meter would experience pressure drop varying by 25:1 at a nominally steady multiphase production condition. This is one reason for the

Pressure and Temperature Transmitter

Radiation Source Holders

Electronics Compartment

Single-point Clamp

FIG. 2.17c Aker-Kvaerner “DUET” subsea multiphase flowmeter.



293

References 1.

2.

3.

4.

Reader-Harris, M. J., Brunton, W. C, Gibson, J. J., Hodges, D. and Nicholson, I. G., Discharge coefficients of venturi tubes with standard and nonstandard convergent angles, Flow Meas. Instrum., 12, 135–145, 2001. U.K. Department of Trade and Industry (DTI) Oil and Gas Division Guidance Notes For Petroleum Measurement Under The Petroleum (Production) Regulations Issue 6, October 2001. de Leeuw, H., Liquid correction of venturi meter readings in wet gas flow, North Sea Flow Measurement Workshop, Kristiansand, Norway, Paper 21, 1997. Miller, R. W., Flow Measurement Engineering Handbook, Table 6.4, 3rd ed., McGraw-Hill, New York, 1996.

Bibliography

FIG. 2.17d Aker-Kvaerner “DUET” subsea multiphase flowmeter sectional drawing.

attraction of correlation flowmetering techniques, which do not experience such extremes yet require (and see) appreciable fluctuations in physical properties in the short term. To reduce the uncertainty associated with measurement of a parameter that fluctuates over such a wide range, many samples are required over a relatively long measuring period. When a multiphase meter is located at the receiving end of a pipeline, the resulting measurement is influenced by the flow into the line (which may be combined from several wells); the flow patterns developing along the line; the elevation changes along the line, which can trap liquid at low points; the outlet pressure changes; and other fluctuations. As can be imagined, the flow out of such a pipeline varies considerably, and the measuring equipment must be specified to cover the full range of the variation, usually based on inadequate data.


Chisholm, D., Two phase flow through sharp-edged orifices, research note, J. Mech. Eng. Sci., 1977. Couput, J. P., Wet Gas Metering in the Upstream Area: Needs, Applications & Developments, North Sea Flow Measurement Workshop, Gleneagles, Scotland, Paper 6.1, 2000. Jamison, A. W., Johnson, P. A., Spearman, E. P. and Sattary, J. A., Unpredicted behaviour of venturi flow meter in gas at high Reynolds numbers, North Sea Flow Measurement Workshop, Peebles, Scotland, 1996. Murdock, J. W., Two phase flow measurement with orifices, J. Basic Eng., December 1962. Steven, R., An Overview of the Current State of Wet Gas Metering in the Natural Gas Production Industry and Proposals for Future Research, 2nd Annual Course on Practical Developments in Gas Flow Metering—Focus on Cost Reduction, NEL, East Kilbride, Glasgow, 1999. Stobie, G., Wet gas flow measurement in the real world, One Day Seminar on Practical Developments in Gas Flow Metering, National Engineering Laboratory, East Kilbride, Glasgow, 1998. Stobie, G. J., Wet gas metering in the real world—part II, Wet Gas Metering Seminar, Paris, 2001. Van Maanen, H. R. E., Cost Reduction for Wet Gas Measurement Using the Tracer-Venturi Combination, Practical Developments in Gas Flow Metering, Paper 2, NEL, East Kilbride, Glasgow, 1999. Wilson, M. B., The Development and Testing of an Ultrasonic Flow Meter for Wet Gas Applications, Seminar on the Measurement of Wet Gas, East Kilbride, Scotland, 1996. Zanker, K. J., The Performance of a Multi-path Ultrasonic Meter with Wet Gas, North Sea Flow Measurement Workshop, Paper 6.2, Gleneagles, Scotland, 2000.

2.18

Positive-Displacement Gas Flowmeters

FQI Flow Sheet Symbol

R. SIEV

(1969)

G. M. CRABTREE

(1982, 1995)


(2003)

Type of Design

A. Positive-displacement B. High-precision

Design Pressures

Low-pressure designs available from 5 to 100 PSIG (0.34 to 6.9 bars); high-pressure units available up to 1440 PSIG (100 bars)

Design Temperatures

Standard units can be used from −30 to 140°F (–34 to 60°C)


Aluminum, steel, plastics, and synthetic elastomers

Inaccuracy

A. 0.5 to 1% of registration B. 0.5% of actual flow over 50:1 range

Costs

A household gas meter for 250 SCFH (7 SCMH) capacity costs about $150. A 50,000 SCFH (1416 SCMH) capacity, diaphragm-type, displacement-type flowmeter in cast aluminum for natural gas service costs about $5000. For natural gas service a 70,000 SCFH (1983 SCMH) rotary positive-displacement meter in cast aluminum costs about $3000.


American Meter Co. (A) Actaris Metering System (A) Bopp & Reuther (www.burhm.de) (A) Dresser Instrument (Root Meter) (www.dresserinstruments.com) (A) Elster-AMCO (Germany) (A) Invensys Process Systems (www.invensysips.com) (A) Instromet (A) Kimmon Mfg. (Japan) (A) Liqua-Tech Controls Pierburg Instruments Inc. (www.pierburginstruments.com) (B) Ritter (Germany) RMG (Germany) Romet Ltd. (Canada) (A) Schlumberger Measurement Div. (www.slb.com/rms/measurement) (A)

Positive-displacement gas meters measure by internally passing isolated volumes of gas that successively fill and empty compartments with a fixed quantity of gas. The filling-andemptying process is controlled by suitable valving and is translated into rotary motion to operate a calibrated register or index that indicates the total volume of gas passed through the meter. The liquid sealed drum meter is the oldest commercial positive-displacement gas meter (see Figure 2.18a). Developed in the early 1800s, it was used for many years during the gaslight era. This type of meter is still available today and remains one of the most accurate of the displacement-type meters. Applications of the liquid sealed drum meter today 294

JESSE YODER

Outlet Liquid Level

Rotation

Inlet

FIG. 2.18a The liquid sealed drum meter.

2.18 Positive-Displacement Gas Flowmeters

Outlet

Inlet

Outlet

Inlet

FC

BC BDC

2

3

4

a a Chamber 1 is emptying, 2 is filling, 3 is empty, and 4 has just filled.

1

FDC

4

FDC BDC 3

2

b b Chamber 1 is now empty, 2 is full, 3 is filling, and 4 is emptying.

BC

FC

BC FDC

Outlet

Inlet

BC

FC

FC 1

Outlet

Inlet

BDC 1

295

2

BDC

FDC 4

1

3

4

2

3

d

c

d Chamber 1 is now completely filled, 2 is empty, 3 is emptying, and 4 is filling.

c Chamber 1 is filling, 2 is emptying, 3 has filled, and 4 has emptied.

FIG. 2.18b The four-chamber diaphragm meter; FC = front chamber; BC = back chamber; FDC = front diaphragm; BDC = back diaphragm chamber.

include laboratory work, appliance testing, pilot plant measurements, and as a calibration standard for other meter types. Some of the inherent difficulties with the liquid sealed meter, such as changes in liquid level and freezing, were overcome in the 1840s with the development of the diaphragm-type positive-displacement meter. Thomas Glover is credited with inventing the first two-diaphragm, sliding-vane meters in 1843, in England. The early meters were constructed with sheepskin diaphragms and sheet metal enclosures. Today, meters are made of cast aluminum with synthetic rubber-oncloth diaphragms. The principle of operation, however, has remained the same for almost 150 years. However, many material, product design, manufacturing, and calibration changes have occurred during that time.

Larger meters are often rated for flow at 2 in. water column (0.5 kPa) differential. Since most meters are sold to gas utility companies that sell natural gas with a specific gravity of approximately 0.6, it may be necessary to determine the flow rating of a diaphragm for other gases. This is accomplished by the following equation: Qn = Qc

( SG)c ( SG)n

2.18(1)

where 3 Qn = new flow rating (ft /h)* 3 Qc = meter rating (ft /h) (SG)c = specific gravity for which meter is rated (usually 0.6) (SG)n = specific gravity of new gas

THE DIAPHRAGM METER The operating principle of the four-chamber diaphragm meter is illustrated in Figure 2.18b. The measurement section consists of four chambers formed by the volumes between the diaphragms and the center partition and between the diaphragms and the meter casing. Differential pressure across the diaphragms extends one diaphragm and contracts the other, alternately filling and emptying the four compartments. The control for the process is through the “D” slide valves that are synchronized with the diaphragm motion and timed to produce a smooth flow of gas by means of a crank mechanism. The crank and valve mechanism is designed and adjusted with no top-dead-center to prevent the meter from stalling. The rotating crank mechanism is connected through suitable gearing to the index, which registers the total volume passed by the meter. The rating of small diaphragm meters is usually specified 3 in cubic feet per hour (0.03 m /h) of 0.6 specific gravity gas that results in a pressure drop of 0.5-in. water column (0.13 kPa).


The inaccuracy of diaphragm positive-displacement meters is typically ±1% of registration over a range in excess of 200:1. This accuracy is maintained over many years of service. Deterioration of meter accuracy is rare unless unusual conditions of dirt, wear, or moisture in the gas are present. ROTARY METERS Rotary meters have one or more rotating parts that implement their measurement operation. Meter design enables them to operate at higher rates of speed than diaphragm meters. For this reason, they can meter higher gas volumes than diaphragm meters. In many cases, rotary meters have built-in temperature compensation to avoid measurement errors based on temperature variations. * For SI units, refer to Appendix.

296

Flow Measurement

There are three types of rotary positive-displacement meters in use today for gas flow measurement:

Displacement Flowmeter M

• • •

Lobed impeller Sliding vane Rotating vane

Gas Flow Low Sensitivity Leaf

The Lobed Impeller The lobed-impeller meter (described in Section 2.19, “PositiveDisplacement Liquid Meters and Provers”) is used for high3 3 volume measurement up to 100,000 ft /h (up to 3000 m /h). This meter has a housing upon which two figure-eight impellers are mounted. The rotation is caused by a pressure differential that is set up across the meter. In this meter, the close clearance of moving parts requires the use of upstream filters to prevent deterioration of accuracy performance. Typically, the inaccuracy of lobed-impeller meters is ±1% over a 10:1 flow range at pressure drops of approximately 0.1 PSI (0.7 kPa). Sliding-Vane Meters A sliding-vane meter has four radial vanes in a single rotating drum that is eccentrically mounted. The rotation of the drum is caused by differential pressure against the vanes. When the drum revolves a single time, four volumes of gas are passed. The meter counts the number of revolutions to provide a readout of total volume.

Displacement Transducers

High Sensitivity Leaf

Zeroing Solenoids

FIG. 2.18d High-precision displacement flowmeter for gas service. (Courtesy of Pierburg Instruments.)

the meter by the vanes, which are passed from inlet side to outlet side through the gate. Gears synchronize the motion of the vanes and gate. Typical inaccuracy for the rotating vane meter is ±1% over a 25:1 range at pressure drops of 0.05 in. of water column (0.013 kPa).

HIGH-PRECISION GAS FLOWMETER Rotating-Vane Meters The rotating-vane meter, as illustrated in Figure 2.18c, is an improvement on the lobed-impeller meter. Here, four compartments formed by the vanes rotate in the same direction as a rotating gate. The fixed volumes of gas are swept through

Gate Recess

V2

Inlet Port A

Outlet Port B

V1 Annular Measuring Chamber

FIG. 2.18c The rotating-vane meter.


For the high-precision measurement of airflows in engine test rigs, positive-displacement flowmeters are used. High precision and high rangeability are achieved by eliminating the pressure drop and thereby eliminating the slip or leakage flows. This is achieved by providing a motor drive for the displacement element and using it to introduce only as much driving energy as is needed to keep the pressures at the inlet and outlet of the meter equal (Figure 2.18d). This flowmeter uses high-sensitivity leaves to detect the pressure differential and displacement transducers to detect the deflection of the leaves. The flowmeter is also provided with automatic rezeroing capability through periodic solenoid isolation of the high-sensitivity leaves. This flowmeter is claimed to provide a reading with only a 0.25% error over a 50:1 range and a 0.5% error over a 100:1 range. The meter is designed for ambient operating temperatures and 30 PSIG (2 bars) operating pressures. The different models of this flowmeter can detect air or gas flows from 0.3 to 1500 ACFM (0.6 to 2500 ACMH). APPLICATION NOTES All displacement gas meters can be used to measure any clean, dry gas that is compatible with the meters’ construction materials and flow and pressure ratings. Dirt and moisture are the worst enemies of good meter performance; inlet filtering should be used when indicated. Since all gases change

2.18 Positive-Displacement Gas Flowmeters

Steel Cable Thermometer-Air Temperature in Bell

Room Temperature Thermometer Fits in this Bracket Oil Temperature Thermometer

297

Turnbuckle for Aligning Bell Adjustable Roller Guides at Top and Bottom of Bell Trip for Operating Magnified Scale

Gas Flow

Magnified Scale for Close Reading Prover Pressure Scale Prover Scale Quick Acting Outlet Valve

Air Valve

FIG. 2.18e The construction of a meter prover.

volume with pressure and temperature changes, these sources of possible error should be controlled or compensated. The national standard cubic foot of fuel gas is at 14.73 PSIA and 60°F; significant deviation from these values should be accounted for in measuring standard gas volumes. At elevated pressures and lower temperatures, a deviation from the ideal gas laws occurs, requiring the application of a compressibility factor to the measured volumes.

pare rates of flow rather than fixed volumes and typically have inaccuracy ratings from ±0.15 to ± 0.5%.

ADVANTAGES The chief advantages of positive-displacement flowmeters for gas applications are their high accuracy and wide rangeability. The chief disadvantages of these meters are maintenance costs and the fact that wear can degrade their performance.

TESTING AND CALIBRATION Bibliography The testing (or proving, as it is called in the gas utility industry) of gas meters is usually done using a special type of gasometer referred to as a prover. The construction of a meter prover is shown in Figure 2.18e. An accurately calibrated “bell” of cylindrical shape is sealed over a tank by a suitable liquid. The lowering of the bell discharges a known volume of air through the meter under test to compare the volumes indicated. Meter provers are typically supplied to 3 3 discharge volumes of 2, 5, and 10 ft (0.06, 0.15, and 0.3 m ), and larger provers of several hundred cubic foot capacity are in use by meter manufacturers and gas utility companies. The volumetric inaccuracy of meter provers is on the order of ±0.1% as determined by physical measurement and comparison with more accurate volumetric standards. Other standards used to calibrate gas meters are calibrated orifices and critical flow nozzles. These devices com-


Bailey, S. J., Fit meter to stream, flow loop for top performance, Control Eng., 29(6), 87, 1982. Beck, H. V., Displacement Gas Meters, Singer American Meter Div., Philadelphia, PA, 1970. Berghegger, H. W., Diaphragm meter capacity ratings, Gas, 44, 51, 1968. Bernitt, C. C., Holmes, H. H. and Stevenson, J. R., New developments in displacement metering, Pipe Line Ind., 28; 29, 38, 40, July 1968. Considine, D. M., Encyclopedia of Instrumentation and Control, McGrawHill, New York, 1971. Crabtree, G. M., Guide to gas meters, Pipeline & Gas J., undated reprint. Delaney, L. J., Rotary and diaphragm displacement meters, Instrum. Control Syst., 114, November l962. Desmeules, M., Fundamentals of Gas Measurement, Canadian Meter Company, Milton, Ontario, Canada, June 1999. Evans, H. J., Turbo-Meters—Theory and Applications, Rockwell International, Pittsburgh, PA, 1968. Fluid Meters, Their Theory and Application, 6th ed., American Society of Mechanical Engineers, New York, 1971.

298

Flow Measurement

Hall, J., Flow monitoring applications guide, Instrum. Control Syst., 41, February 1983. Hall, J., Solving tough flow monitoring problems, Instrum. Control Syst., February 1980. Jasek, A. W., Mechanical displacement meter prover for gas meters, Gas, 41, 52, August 1967. Lief, A., Metering for America, Appleton-Century-Crofts, New York, 1961. Lomas, D. J., Selecting the right flowmeter, Instrum. Tech., May 1977. Miller, J., High Accuracy Transmitters for Custody Transfer of Natural Gas, ISA Conference, Paper #91–0520, 1991. Miller, R., Flow Measurement Engineering Handbook, McGraw-Hill, New York, 1996. O’Rourke, E. L., The select compact gas meter, American Gas Association, Operating Section Proc., Arlington, VA, 1991. Perrine, E. B., Displacement gas meters, Instrum. Control Syst., 127, February 1966.


Staff, Rotary positive displacement meter, Pipeline Eng., 38 (pipeline handbook), 121, March 1966. Steuernagle, R. L., Diaphragm Meter Design & Operation, Technical papers, Appalachian Gas Measurement Short Course, Robert Morris College, Coraopolis, PA, 1990. Upp, E. L., Fluid Flow Measurement, Gulf Publishing Co., Houston, TX, 1993. Welch, J. V., Trends in low gas flow metering, InTech, February 1991. Waring, T., Fundamentals of Rotary Meter Measurement, Dresser Canada, Missisauga, Ontario, Canada, June 1999. Yoder, J., Flowmeter shootout, part I: new-technology flowmeters, Control, February 2001. Yoder, J., Flowmeter shootout, part II: traditional-technology flowmeters, Control, March 2001. Yoder, J., Flowmeter shootout, part III: how users choose, Control, April 2001.

Positive-Displacement Liquid Meters and Provers R. SIEV

(1969)

J. B. STODDARD

(1995)

B. G. LIPTÁK

FQI

~

2.19

Flow Sheet Symbol

(1982, 2003)

Types of Designs

A. Impeller, propeller, turbine B. Nutating disc C. Oval gear, C-1 if toothless D. Piston E. Rotating vane F. Viscous helix G. High-precision, specialized, low-flow, and so on H. Prover

Design Pressure

To 3000 PSIG (21 MPa)

Design Temperature

From −450°F (−268°C) to 560°F (293°C)

Strainer Required

Yes


Bronze, cast iron, aluminum, steel, stainless steel, Monel , Hastelloy , and plastics

Size Range

0.25 to 16 in. (6 to 406 mm)

Flow Range

From 0.01 GPH to 20,000 GPM (0.04 l/h to 75 m /m)

Rangeability

From 3:1 to >100:1 (for specialized designs), 10:1 about average

Inaccuracy

Ranges from ±0.1 to ±2% of actual flow; typical average error ±1/2% of actual flow, dropping as size increases

Cost

A 1 in. (25 mm) bronze disk-type water meter with 2 to 3% error costs about $750. Plastic-piston meters for laboratory applications in 1- and 2-in. (25- and 50-mm) sizes are $600 and $1200.





3

A 1-in. (25-mm) toothless oval meter for high-viscosity service in type 316 SS is $2500. A 1-in. (25-mm) oval flowmeter for LPG service with ductile iron housing complete and valve, vapor eliminator, and register with printer, about $3000. A 2-in. (50-mm) piston meter, in steel construction, having 0.5% error and provided with register, preset valve, and ticket printer costs about $5000 to $6000. A 6-in. (150-mm) flanged, bi-rotor meter for fuel oil service with ductile iron preset valve, impulse contactor, large dial register, and ticket printer costs about $12,000 to $15,000. Prover costs range from $50,000 to $300,000 depending on size, materials of construction, and control accessories. Partial List of Suppliers*

Badger Meter Inc. (www.badgermeter.com) (B, D) Barton Instrument Systems LLC (www.barton-instruments.com) (A, E) Brooks Instrument (www.emersonprocess.com) (A, C, D, F, H)

* The most popular are Brooks Instrument, Smith Meter Inc., and Badger Meter Inc.


300

Flow Measurement

Cole-Parmer Instrument Co. (www.coleparmer.com) (D) Daniel Measurement and Control (www.danielind.com) (E, H) Dresser Instrument (www.ashcroft.com) (A) Flow Technology Inc. (www.ftimeters.com) (H) Kobold Instruments Inc. (www.koboldusa.com) (F, G) Liquid Controls Inc. (www.lcmeter.com) (A, E) Max Machinery Inc. (www.maxmachinery.com) (C, D, F) Omega Engineering Inc. (www.omega.com) (C1) PLU of Pierburg GmbH (www.pierburg-instruments.de) (G) Schlumberger Measurement Div. (www.slb.com/rms/measurement) (A, B, D) Smith Systems Inc., (www.smith-systems-inc.com) (A, C, D, E)

Positive-displacement meters split the flow of liquids into separate known volumes, the size of which are based on the physical dimensions of the meter. The meters act as counters or totalizers of the number of these volumes as they pass through. These mechanical meters have one or more moving parts in contact with the flow stream and physically separate the fluid into increments. Energy to drive the moving parts is extracted from the flowing stream itself, resulting in a pressure loss through the meter. The error of these meters depends on the clearances between the moving and stationary parts. The smaller the clearance and the longer the length of the leakage path, the better the precision of the meter. For this reason, meter accuracy tends to increase with larger meter sizes.

OVERVIEW Positive-displacement meters for liquids are among the most widely used volumetric flow sensors for batch-size measurement applications and when fluid is bought and sold on a contract basis. A wide variety of meters, covering a broad spectrum of requirements, are available. Their good accuracy, wide rangeability, and ready availability warrant their consideration when selecting a volumetric meter. These flowmeters are especially useful when the fluid to be measured is free of any entrained solids. A typical example is the measurement of water delivered to homes, factories, office buildings, and so forth. On the other hand, some designs are well suited for viscous liquid applications. Wear on parts, with the resulting change in clearance dimensions, introduces the major source of error over the service life of the meter. Leakage error increases with dropping process fluid viscosity but remains relatively constant with time. In larger meters, temperature variations and the resulting change in fluid density and viscosity must also be taken into consideration. Positive-displacement meters provide good accuracy (±0.25% of flow) and high rangeability (15:1). They are repeatable to ±0.05% of flow. Some designs are suited for high- or variable-viscosity services (up to or even exceeding 100 cSt). They require no power supplies for their operation (only for remote transmission) and are available with a wide variety of readout devices. Their performance is virtually unaffected by upstream piping configuration. Positive-dis-


placement meters are excellent for batch processes, mixing, and blending applications. These meters are simple and easy to maintain by regular maintenance personnel using standard tools. No specially trained crews or special calibration instruments are needed. On the other hand, because of the close tolerances of the moving parts, they are subject to wear and maintenance, and recalibration is required at frequent intervals. On corrosive services, this may result in high costs. Positive-displacement meters require relatively expensive precision-machined parts to achieve the small clearances that guarantee their high accuracy. From this it follows that the liquids metered must be clean, because wear can rapidly destroy precision. Contaminant particle size must be kept below 100 microns, and most of these meters are not adaptable to the metering of slurries. Positive-displacement flowmeters are expensive in larger sizes or in special materials. They can be damaged by overspeeding and can require high pressure drops. In general, they are not suited for dirty, nonlubricating, or abrasive services.

ROTATING LOBE AND IMPELLER (TYPE A) In this type of meter, two lobed impellers rotate in opposite directions within the housing (Figure 2.19a). They are geared together to maintain a fixed relative position, so a fixed volume of liquid is displaced by each revolution. A register is geared to one of the impellers. These meters are normally built for 2- to 24-in. (50- to 610-mm) pipe sizes, and their capacities (upper limits of their ranges) range from 8 to 17,500 GPM (30.4 to 66,500 l/min). The advantages of this design include good repeatability (0.015%) at high flows, the availability of a range of materials of construction, and high operating pressures (1200 PSIG or 8300 kPa) and temperatures (400°F or 205°C). B

B

A

FIG. 2.19a Rotating lobe meter.

A

A

2.19 Positive-Displacement Liquid Meters and Provers

Division Plate

301

Inlet Port

Outlet

FIG. 2.19b Rotating impeller flowmeter. (Courtesy of Flowdata Inc.)

The disadvantages include loss of accuracy at low flows because of the large size, heavy weight, and high cost. The rotating impeller design is illustrated in Figure 2.19b. It has only two moving parts: the two impellers, which are made out of wear-, abrasion-, and corrosion-resistant thermoplastics. Operation is based on a proximity switch sensing the passage of magnets that are implanted in the impeller lobes and transmitting the resultant pulses to a counter. Units are available from 0.125- to 4-in. (3- to 100-mm) sizes with up to 3000 PSIG (21 MPa) pressure and 0 to 400°F (205°C) temperature ratings. The design is suited for high-viscosity operation, and the claimed precision and rangeabilities are also high.

Disc

Ball

Inlet

Outlet

FIG. 2.19c Nutating disk meter.

NUTATING DISK (TYPE B) The nutating disk meter is used extensively for residential water service. The moving assembly, which separates the fluid into volume increments, consists of an assembly of a radially slotted disk with an integral ball bearing and an axial pin (see Figure 2.19c). This part fits into and divides the metering chamber into four volumes—two above the disk on the inlet side and two below the disk on the outlet side. As the liquid flows through the meter, the pressure drop from inlet to outlet causes the disk to wobble, or nutate. For each cycle, the meter displaces a volume of liquid equal to the volume of the metering chamber minus the volume of the disk assembly. The end of an axial pin, which moves in a circular motion, drives a cam that is connected to a gear train and to a totalizing register. This flowmeter measures the liquids with an error range of about ±1 to 2% of actual flow. It is built only for smaller pipe sizes. Its temperature range is from −300 to 250°F (–150 to 120°C), and its maximum working pressure rating is 150 PSIG (1034 kPa). On cold water service, the capacity ranges are approximately as follows: Size

Capacity

0.5 in. (13 mm)

2 to 20 GPM (7.5 to 75 l/min)

1 in. (25 mm)

5 to 50 GPM (19 to 190 l/min)

1.5 in. (38 mm)

10 to 100 GPM (38 to 380 l/min)

2 in. (51 mm)

16 to 160 GPM (61 to 610 l/min)


FIG. 2.19d Oval-gear flowmeter. (Courtesy of Brooks Instrument.)

OVAL-GEAR FLOWMETERS (TYPE C) A special variety of the rotating-lobe flowmeter is made using oval-geared metering elements. In this design, shown in Figure 2.19d, a precise volume of liquid is captured by a crescent-shaped gap, which is formed between the housing and the gear. This volume is then carried to the outlet, and this movement causes the gears to rotate an output shaft through which the register operates. In new condition, when the slippage between the oval gears and the housing is small, and when both the flow rate and viscosity are high (>1 GPM and >10 cP, respectively), these flowmeters can operate at errors as low as 0.1% of actual flow. At lower flow rates, the relative proportion of the “slip” leakage increases, and so accuracy drops to about 0.5% of actual flow. Viscosity variations will also affect the slip flow. If a meter was

302

Flow Measurement

calibrated using water, a fluid with a viscosity of 1 cps, it will have a 1.2% high error if the viscosity rises to 100 cps. These flowmeters are available in sizes from 0.25 to 16 in. (6 to 406 mm). When the viscosity of the process fluid is between 1.5 and 10 cps, they can handle flow ranges from 0.05 to 0.5 up to 250 to 5000 GPM (from 0.2 to 2 up to 950 to 19,000 l/min). They are available in a wide range of construction materials, including brass, carbon steel, type 316  stainless steel, and Alloy 20 . Operating pressure ratings are available up to 1450 PSIG (10 MPa) and operating temperatures up to 560°F (293°C). The servo version of this meter has been introduced to completely eliminate slip leakage in smaller sizes (0.2 to 40 GPH or 0.8 to 150 l/h). In this design, the servomotor drives the oval-gear elements at a speed that eliminates the pressure drop across the meter and keeps the outlet pressure the same as the inlet. This eliminates the motivating force, which causes the slip flow and therefore increases accuracy at low flows or under variable viscosity conditions. A smooth, toothless oval gear design is also available in 1-in. (25-mm) size with screwed connections. It can handle viscosities up to 100 cSt and is rated for 3000 PSIG (21 MPa) and 450°F (232°C). Its linear range is 2 to 25 GPM (7.5 to 94 l/min). If it is used with a nonlinear Hall-effect pickup, its range is claimed to increase to 0.02 to 25 GPM (0.075 to 94 l/min). The meter is made of type 316 stainless steel, its inaccuracy is within its linear range is 0.25% of actual flow, and it is provided with accessories for remote readouts, both analog and digital.

Slide Valve

Piston

Inlet

FIG. 2.19e Reciprocating piston meter.

PISTON DESIGNS (TYPE D) Reciprocating Piston The oldest of the positive-displacement meters, this meter is available in many forms, including multi-piston meters, double-acting piston meters, rotary valves, and horizontal slide valves. Figure 2.19e shows the schematic of a reciprocating piston meter. Here, a crank arm is operated by the reciprocating motion of the pistons, and this motion drives the register. These meters are widely used in the petroleum industry and can reach the precision of ±0.2% of actual flow. Another version of this meter is shown in Figure 2.19f. In this design, the liquid enters the cylinder on the left, forcing the piston down by lever action of the control plate. As a result, the piston on the right is forced up, discharging the liquid first into the inner portion of the valve, then down through the center of the meter and out through the meter discharge outlet. Oscillating Piston The moving portion of the oscillating piston meter consists of a slotted cylinder that oscillates about a dividing bridge that separates the inlet port from the outlet port. Spokes connect


FIG. 2.19f Cutaway of reciprocating piston meter with two opposing pistons.

this cylinder to a pin located on the axis of the cylinder. As the cylinder oscillates about the bridge (Figure 2.19g) the pin makes one rotation per cycle. This rotation is transmitted to the gear train and registers either directly or magnetically through a diaphragm. This meter, in addition to being in common usage for the measurement of domestic water has the capability of handling clean viscous and/or corrosive liquids. This type of flowmeter is normally used in smaller pipelines (2 in./50 mm or below) to measure low flow rates.


Inlet

Outlet

303

Vane Vane Slot

Housing

Rotor Inlet Position 2

Position 1

Outlet

FIG. 2.19h Rotating-vane meter.

Chamber 1

2

4

5

Abutment Rotor

3

Piston

Position 3

Position 4

FIG. 2.19g Oscillating piston meter.

Measurement errors are in the range of ±1% of actual flow. Metering accuracies are increased by reducing the clearance spaces to 0.002 in. (5 microns). Such small clearances do necessitate pre-filtering the entering fluid in order to remove larger particulars. The cases are usually made of cast iron, bronze, or steel, while the chamber and piston materials are usually made of bronze, aluminum, and Ni-Resist. Iron and bronze meters are good for up to 150 PSIG (1034 kPa) and 200°F (93°C), while steel meters can be used up to 400 PSIG (2760 kPa) and 300°F (149°C).

6

FIG. 2.19i Six-phase metering cycle of a rotary displacement-type flowmeter.

ROTATING VANE (TYPE E) This flowmeter has spring-loaded vanes that seal increments of liquid between the eccentrically mounted rotor and the casing (Figure 2.19h) and transport it from the inlet to the outlet, where it is discharged as a result of the decreasing volume. This type of meter is widely used in the petroleum industry and is used for such varied services as gasoline and crude oil metering, with ranges from a few gallons per minute 3 of low-viscosity clean liquids to 17,500 GPM (66.1 m /m, or 25,000 bbl/h) of viscous particle-laden crude oils. Precisions of ±0.1% of actual flow are normal, and ±0.05% has been achieved in the larger meters. This instrument is built from a variety of materials and can be used at temperatures and pressures up to 350°F (177°C) and 1000 PSIG (6.9 MPa). Another rotary design is illustrated in Figure 2.19i. Here, an abutment rotor operates in timed relation with two displacement rotors and at half their speed. FIG. 2.19j Viscous helix flowmeter. (Courtesy of Fluidyne Instrumentation.)

VISCOUS HELIX (TYPE F) The helix flow transducer (Figure 2.19j) is a positivedisplacement device utilizing two uniquely nested, radically pitched helical rotors as the measuring elements. Close machining tolerances ensure minimal slippage and thus high


accuracy. The design of the sealing surfaces provides a ratio of longitudinal to lateral sealing to minimize pressure drop, especially with high-viscosity liquids. The large inlet size of the progressive cavity allows for the passage of gels, fines, agglomerates, and even undissolved

304

Flow Measurement

or hydraulically conveyed solids. The meter can measure flow rates from 0.5 to >4000 GPM (2 to 15,000 l/min). This flow sensor is available in sizes from 1.5 in. to 10 in. (38 to 250 mm) and can operate at temperatures up to 600°F (315°C) and at pressures up to 3000 PSIG (21 MPa). It is a high-pressuredrop device requiring a minimum of 10 PSID (69 kPa) for its operation at full flow. Its turndown can reach 100:1, and its metering error is claimed to be under 0.5% of actual flow. Available design variations include versions that are heated to maintain line temperatures for metering melted solids or polymers. Also available are units in sanitary construction. This meter is suited for high-viscosity (over 1000 cps) and for slurry services. The straight-through design with no pockets is also available to simplify cleaning. It is recommended that the process fluids be filtered by mesh size 30 filters before they enter this flowmeter.

HIGH-PRECISION AND SPECIALIZED (TYPE G) For the high-precision measurement of fuel and alcohol flows in engine and carburetor test rigs and other applications, specialized positive-displacement flowmeters are often used. Their high precision and high rangeability are achieved by eliminating the pressure drop and thereby eliminating the slip or leakage flows. This is achieved by providing a motor drive for the displacement element and using it to introduce as much pumping energy as is needed to equalize the pressures at the inlet and outlet of the meter (Figure 2.19k). This flowmeter uses a high-sensitivity piston to detect the pressure differential and utilizes photoelectric sensors to detect the position of the piston. The flowmeter is also provided with a variable-speed controller, which adjusts the drive speed whenever the pressure differential is other than zero. Because the response time of the system is less than 0.5 sec, the flowmeter is able to follow most dynamic flow transients or can be used on short-duration tests. This flowmeter is claimed to provide a reading with only 0.25% error over a 50:1 range and a 0.5% error over a 100:1 range. The meter is designed for ambient operating temperatures and for up to 150 PSIG (10 bars) operating pressures.

The different models of this flowmeter can detect diesel, gasoline, and alcohol flows from 0.04 to 40 GPH (0.15 to 150 l/h). Because vapor lock is a common problem in fuel flow metering, the unit is provided with a vapor separator.

PROVERS (TYPE H) All flowmeters that consist of moving and stationary parts that rub against each other (such as positive-displacement and turbine type flowmeters) require periodic recalibration. This is necessary because the clearance space and the slip or clearance flow through that space increase with wear. Recalibration can be done by removing the flowmeter from the pipeline and sending it to a calibration laboratory, or it can be done in line. The flow provers that allow for inline recalibration without interruption of the process flow are described below. As shown in Figure 2.19l, provers consist of a smoothwalled, precalibrated displacement chamber and a barrier piston within it. Usually, a follower rod is attached to the back side of the piston, which is connected to position sensors. The calibrated flow rate is obtained by dividing the volume of the prover with the time it takes to displace its volume. This calibrated flow rate is then compared to the reading of the flowmeter being calibrated. To minimize the disturbance to the process flow, inline ballistic flow provers have been developed. In these units (Figure 2.19l), the piston is constructed so that it will not disrupt the flow in the line. Therefore, the prover can be permanently installed in an operating pipeline, upstream or downstream of the flowmeter being calibrated. The poppet valve within the piston assembly allows for the piston to be withdrawn to the start position after a calibration run while the process flow continues undisturbed. Both portable (Figure 2.19m) and permanently installed provers are available, and the calibration can be manual or automatic. The repeatability of provers is around 0.02% of the actual flow if the seals are tight. It is recommended to periodically check the seals by closing a tight shutoff valve downstream of the prover and applying nitrogen pressure to the upstream

Flow Tube 1st Detector DC Motor

Displacement Flowmeter

Differential Pressure Detection Piston

Displacer

M

Flow

Calibrated Volume

FIG. 2.19k High-precision positive displacement flowmeter. (Courtesy of PLU, Pierburg GmbH.)


FIG. 2.19l Prover operation.

2nd Detector


Skid mounting for truck bed use

305

Vertical Mount

Skid mounting for ease of portability

Skid/Trailer use Two Axle

FIG. 2.19m Portable prover assemblies. (Courtesy of Brooks Instrument.)

face of the piston. If this results in any movement of the piston, the seals need maintenance. Provers are available for up to 3000 PSIG (21 MPa) operating pressure and 165°F (74°C) operating temperature; they can detect flow rates from 0.001 GPM (0.004 l/min) to 20,000 GPM (75,000 l/min). The calibrated displacement volume of provers can range from a fraction of a gallon to several hundred gallons. Large provers can fit on truck beds or trailers (Figure 2.19m). ACCESSORIES AND INTELLIGENT ELECTRONICS Standard accessories for positive-displacement include strainers; air release assemblies, which remove all the vapors from the flow stream before it enters the meter; automatic batch shutoff valves, which provide two-stage closure for full and dribble flow operation; temperature compensators; manual and/or automatic ticket printers; and pulse generators for remote indication, totalization, and other forms of data monitoring and/or control. In addition to the totalizer-type digital readout registers, flow rate indication can also be provided. Impulse contactors are also available to actuate predetermining counters or to serve as electrical interlocks that actuate flow ratio systems, pumps, valves, solenoids, alarms, printers, sampling devices, and so on. Pneumatic pulse generators are still available and sometime used in explosion-proof areas for interfacing with pneumatic batch controllers.


The intelligent positive-displacement meters are usually provided with magnetic or Hall-effect-type pickup and frequency outputs from solid-state pulse transmitters. The frequency outputs can be sent to central computers or DCS/PLC systems over the data highways and can also be converted to 0- to 10-VDC or 4- to 20-mA analog signals. In household utility applications, there is substantial economic justification for substituting a telemetering system, operated either on the telephone lines or by radio, replacing the current system (human meter readers). It is also feasible to combine the readings of electric, water, and gas meters of a household into a single transmitter and to transmit that information to the appropriate utilities without the need for a meter reader to visit the home or apartment. The economic advantages of this type of metering is not only in labor savings but also in the speed and frequency at which the data can be obtained and used for billing or other purposes. Bibliography Baker, R. C., Flow Measurement Handbook, Cambridge University Press, UK, 2000. Barnes, G., Pipeline metering with liquid positive displacement, Adv. in Instrum., 39, ISA, 1984. Blasso, L., Flow measurement under any conditions, Instrum. Control Syst., February 1975. Clark, W. J., Flow Measurement, Pergamon Press, New York, 1967.

306

Flow Measurement

Cushing, M., The future of flow measurement, Flow Control, January 2000. Desmeules, M., Fundamentals of Gas Measurement, Canadian Meter Company, Milton, Ontario, Canada, June 1999. Eren, H., Flowmeters, Survey of Instrumentation and Measurement, S.A. Dyer, Ed., John Wiley & Sons, New York, 2001. Francisco, E. E., Flowmeter proving using the dynamic transfer technique, Meas. Control, April 1993. Furness, R. A., Developments in pipeline instrumentation, Pipe Line Rules of Thumb Handbook, 4th ed., Gulf Publishing, Houston, TX, 1998. Hall, J., Solving tough flow monitoring problems, Instrum. Control Syst., February 1980. Hayward, A. J., Choose the flowmeter right for the job, Process. J., 1980. Hendrix, A. R., Positive displacement flowmeters, InTech, December 1982. Husain, Z. D., Flowmeter Calibration and Performance Evaluation, ISA Conference, Paper #91–0508, 1991. Ifft, S. A., Custody Transfer Flow Measurement with New Technologies, Saudi Aramco, Saudi Arabia, 1999.


Laskaris, E. K., The measurement of flow, Automation, 1980. Lipták, B. G., Flow measurement trends, Control, June 2000. Lomas, D. J., Selecting the right flowmeter, Instrum. Tech., 1977. Positive displacement flowmeters, Meas. Control, October 1991. Miller, R. W., Flow Measurement Engineering Handbook, 3rd ed., McGrawHill, New York, 1996. Renda, L., Flowmeter calibration, Meas. Control, February 1993. Spink, L. K., Principles and Practice of Flow Engineering, 9th ed., The Foxboro Co., Invensys Systems, Inc., Foxboro, MA, 1967. Spitzer, D. W., Flow Measurement, 2nd ed., ISA, Research Triangle Park, NC, 2001. Spitzer, D. W., What affects flowmeter performance, InTech, February, 1993. Watson, G. A., Flowmeter types and their usage, Chartered Mech. Eng. J., 1978. Waring, T., Fundamentals of Rotary Meter Measurement, Dresser Canada, June 1999. Yoder, J., Flowmeter shootout, part I and II: new technologies, Control, February and March 2001.

2.20

Purge Flow Regulators

FI

ROTAMETER W/VALVE

E. L. SZONNTAGH

(1995)

B. G. LIPTÁK

(2003)

FICV

PURGE FLOW REGULATOR

Flow Sheet Symbol

Applications

Purge flow regulators serve the regulation of low flow rates of air, gas, or liquids. They are most often used in air bubblers or in purging electrical housings (in explosionproof areas) and in purging optical windows of smokestack analyzers. Water and liquid purge meters are most often applied to protect process connections from plugging.

Purge Fluids

Air, nitrogen, and liquids

Operating Pressure

Up to 450 PSIG (3 MPa)


For glass tube, up to 200°F (93°C)

Ranges

Range is from a minimum of 0.01 cm /min for liquids and from 0.5 cm /min for gases. A 0.25-in. (6-mm) glass tube rotameter can handle 0.05 to 0.5 GPM (0.2 to 2 l/min) of water or 0.2 to 2 SCFM (0.3 to 3 cmph) of air.

Inaccuracy

Generally, 2 to 5% of range (laboratory units are more accurate)

Costs

A 150-mm glass-tube unit with 0.125-in. (3-mm) threaded connection, in type 316 stainless steel, and a 16-turn high-precision valve will cost about $300; the same with an aluminum frame and a standard valve is about $125. Adding a differential-pressure regulator of brass or aluminum construction costs about an additional $150 (in  stainless steel, about $500). For highly corrosive services, all-Teflon , all-PTFE, allPFA, and all-CTFA units are available that, when provided with valves, cost from $500 with 0.25-in. (6-mm) to $1500 with 0.75-in. (19-mm) connections.


Aalborg Instruments & Controls Inc. (www.aalborg.com) ABB Automation Instrumentation Division (www.abb.com/us/instrumentation) Blue-White Industries (www.bluwhite.com) Brooks Instrument (www.emersonprocess.com) Cole-Parmer Instrument Co. (www.coleparmer.com) Dwyer Instruments Inc. (www.dwyer-inst.com) Key Instruments (www.keyinstruments.com) King Instrument Co. (www.kinginstrument.co.com) Krohne Inc. (www.krohne.com) Matheson Instruments (www.mathesoinstruments.com) Omega Engineering Inc. (www.omega.com) Penberthy (www.penberthy-online.com) USFilter/Wallace & Tiernan Products (www.wallaceandtiernan.usfilter.com)

3

Purge flows are low flow rates of either gases or liquids. They serve to protect pressure taps from plugging or being contacted by hot or corrosive process fluids. Inert gas purging can also serve to protect electrical devices from becoming ignition sources by maintaining a positive pressure of incombustible gases inside their housings. In the case of analyzers, purging protects the cleanliness of the optics.

3

DETECTION OF LOW FLOWS The low flow rates of purge media can be detected by a variety of devices. They include capillaries, miniature orifices, metering pump, positive-displacement, thermal, and variable-areatype flow sensors. Most of these devices are detailed in other sections of this chapter. Capillary flow elements (Section 2.9) 307


308

Flow Measurement

are ideal for the measurement of low flow rates. They can also be combined with thermal flowmeters to provide flow regulators with higher precision and higher rangeability—but also higher cost (Section 2.13). Integral orifices (Section 2.15) can also be used in both gas and liquid flow measurement, whereas positive-displacement meters and metering pumps are most often used to detect the flows of liquids (Sections 2.14 and 2.19). In addition, the second volume of the Instruments Engineers’ Handbook (“Process Control”) includes a section that describes self-contained flow regulators. Only one purge flow regulator design is not covered in other parts of this three-volume handbook: the rotameter-type purge meter. This is the least expensive and most widely used purge meter design, and it is described in this section.

Orifice 0.020" (0.51mm)

PURGE ROTAMETERS Purge flowmeters are widely used devices and are probably the most widely used form of the variable-area flowmeter, the rotameter. These meters are inexpensive and are intended for the measurement and control of low flow rates. Most purge meters are used on inert gas or water services at low flow rates, where measurement accuracy is not critical. These units are reasonably repeatable, which is all that is required in many purge applications where, as long as a low flow rate is maintained, it is not critical to know how much it is. The flow rates through the purge meters are adjusted by needletype throttling valves as shown in Figure 2.20a. The metering needle valves are usually multiple-turn units provided with long stems. The opening around their

FIG. 2.20b Fine-adjustment needle valve with vernier scale. (Courtesy of Swagelok Co.)

TABLE 2.20c Gas Properties under the Standard Conditions of 29.92 in. of Mercury and 70°F (760 mm of Mercury and 21°C) Gas

3

Density (lb/ft )

µ Viscosity Micropoises

Specific Gravity

Air

0.0749

181.87

1.000

Argon

0.1034

225.95

1.380

193.9

Helium

0.0103

End Fitting

Hydrogen

0.0052

88.41

0.0695

0.138

Tube Adaptor Spring Ball Check Valve

Nitrogen

0.0725

175.85

0.968

Oxygen

0.0828

203.47

1.105

Carbon Dioxide

0.1143

146.87

1.526

Tube Adaptor

Protection Shield

Metering Tube Float Tube Adaptor End Fitting

Needle Valve Meter Body

FIG. 2.20a Purge rotameter with integral needle valve.


needle-shaped plugs is very small and can approach capillary dimensions. Figure 2.20b shows a high-precision needle valve provided with a vernier-type scale that allows a more accurate setting of the valve opening. The dual scale increases the precision and reproducibility of setting by subdividing the smallest reading of the first scale onto the second. The flow rate through these devices is a function of the opening in the valve, the pressure differential across that opening, and both the density and the viscosity of the purge media. Table 2.20c provides information on the density and viscosity of a number of purge gases. When the purge flowmeter is combined with a differentialpressure regulator (Figure 2.20d), it becomes a self-contained flow controller. The purge flow is fixed by adjusting springs 1 and 2 for a particular pressure difference, usually in the range of about 60 to 80 in. (150 to 200 cm) of water. This constant pressure drop (P2 − Po) is than maintained across

2.20 Purge Flow Regulators

Bibliography

Flow @ Po Outlet Pressure

Tube

Spring #1

Float

Diaphragm P2 Regulator Valve

Flow @ Pi Inlet Pressure

Flow Control Valve (v) Spring #2

FIG. 2.20d Purge flow regulator consisting of a glass tube rotameter, an inlet needle valve, and a differential pressure regulator. (Courtesy of Krone Inc.)

the flow control valve (V). The configuration in Figure 2.20d maintains the outlet pressure (Po) constant by compensating for any variation in the inlet pressure Pi by changing the regulator valve opening. Other purge flowmeter designs are also available that work in a reverse configuration by keeping the inlet pressure Pi constant and allowing the outlet Po to vary. In these designs, the constant pressure drop across the valve (V) is maintained to equal (Pi − P2) instead of (P2 − Po) being kept constant. The gas flows through purge flow controllers are usually adjustable in a range of 0.2 to 2 SCFH (6 to 60 slph). The error or inaccuracy is usually 5% of full scale over a range of 10:1. The standard pressure and temperature ratings are 150 to 300 PSIG (1 to 2 MPa) and 212 to 572°F (100 to 300°C).


309

Baker, R. C., Flow Measurement Handbook, Cambridge University Press, UK, 2000. Blasso, L., Flow measurement under any conditions, Instrum. Control Syst., February 1975. Cheremisinoff, N. P., Fluid Flow, Ann Arbor Science Publishers, Ann Arbor, MI, 1982. Cross, D. E., Rotameter calibration nomograph for gases, Instrum. Tech., 53–56, April 1969. Cushing, M., The future of flow measurement, Flow Control, January 2000. Des Marais, P. O., Variable-area meter for viscous service, Instrum. Control Syst., August 1961. Desmeules, M., Fundamentals of Gas Measurement, Canadian Meter Company, Milton, Ontario, Canada, June 1999. Factory Mutual Loss Prevention Data Sheet 7–59, Inserting and Purging of Tanks, Process Vessels and Equipment, Norwood, MA, 1977. Hall, J., Solving tough flow monitoring problems, Instrum. Control Syst., February 1980. Instrument Society of America, Recommended Practices RP16.1, RP16.2, RP16.3, RP16.4, RP16.5, and RP16.6. (These documents deal with the terminology, dimensions, installation, operation, maintenance, and calibration of rotameters.) Lomas, D. J., Selecting the right flowmeter, Instrum. Tech., May 1977. Miller, R. W., Flow Measurement Handbook, 3rd ed., McGraw-Hill, New York, 1996. Polentz, L. M., Theory and operation of rotameters, Instrum. Control Syst., June 1961. Purging Principles and Practices, Report XK0775, American Gas Association, Washington, DC, 1990. Rotameters/variable-area flowmeters, Meas. Control, September 1991. Spitzer, D. W., Flow Measurement, 2nd ed., ISA, Research Triangle Park, NC, 2001. Standard on Explosion Prevention Systems, NFPA 96, National Fire Prevention Association, Boston, MA. Standard on Purged and Pressurized Enclosures for Electrical Equipment, NFPA 496, National Fire Prevention Association, Boston, MA. Sydenham, P. H. et al., Introduction to Measurement Science and Engineering, John Wiley & Sons, Chichester, England, 1989. U.S. Chemical Safety and Hazard Investigations Board, Summary Report on Accident at Union Carbide Hahnville Louisiana Plant, Report PB99–159972, Washington, DC, 1998. Waring, T., Fundamentals of Rotary Meter Measurement, Dresser Canada, June 1999. Yoder, J., Flowmeter shootout, part I and II: new technologies, Control, February and March 2001.

2.21

Segmental Wedge Flowmeter B. G. LIPTÁK

Receiver

FT

(1995, 2003)

Flow Sheet Symbol

Applications

Clean, viscous (down to Rd no. = 500) solids containing fluids, gas, and steam

Sizes

0.5- to 24-in. (12- to 610-mm) diameter pipes

Designs

In the smaller sizes (0.5 to 1.5 in.), the wedge can be integral; for larger pipes, remote seal wedges are used with calibrated elements

Wedge Opening Height (H)

From 0.2 to 0.7 of pipe inside diameter

Pressure Drops

25 to 200 in. H2O (6.2 to 49.8 kPa)


Carbon, type 316 SS, Hastelloy , Monel wetted parts; special wedge materials like tungsten carbide are also available. Sealing gasket can be silicate ceramic filled TFE. Chemical tee gasket up to 645°F (340°C) can be graphite.

Design Pressure

300 to 1500 PSIG (20.7 to 103 bars) with remote seals, up to 3000 PSIG (21,000 kPa) in 1-in. size and below

Design Temperature

In sizes 1.5 in. and below, 300°F (148.9°C); higher temperature designs are available from –40 to 700°F (–40 to 370°C) but have been used in higher-temperature processes up to 850°F (454°C).

Inaccuracy

Error, if uncalibrated, is 5% of actual flow. When the elements are individually calibrated, the error drops to 0.5 to 0.75% of actual flow; to this one should add the d/p cell error contribution of about 0.25% of full scale. The total error over a 3:1 flow range is usually not more than 2% of actual flow

Cost

A 3-in. (75-mm) calibrated stainless-steel element with two stainless chemical tees and with an electronic d/p transmitter provided, and provided with remote seals, is about $4000.


ABB Automation Instrumentation Division (www.abb.com/us/instrumentation)



The shape of the flow opening of a segmental wedge flowmeter is similar to that of a segmental orifice except that the obstruction to flow is less abrupt (more gradual). Also, the sloping entrance somewhat resembles the shape of the various flow tubes. The wedge flowmeter was primarily designed for slurry applications. Its main advantage is its ability to operate at 1 Reynolds numbers as low as 500 to 1000. This is in contrast with sharp-edged orifices, venturies, and flow nozzles, where the square root relationship between flow and pressure drop requires a Reynolds number well above 10,000 (see Figure 2.1f). For this reason, wedge flowmeters can measure lowvelocity and viscous fluid flows. In this regard, the wedge 310 © 2003 by Béla Lipták



flowmeter capability is similar to that of the conical or quadrant edge orifices. For pipe diameter sizes under 2 in. (50 mm), the segmental wedge flow element is made by cutting a V-notch into the pipe and accurately welding a solid wedge in place. In sizes over 2 in., the wedge is fabricated from two flat plates that are welded together before insertion into the spoolpiece (Figure 2.21a). On clean services, regular pressure taps are used and located equidistant from the wedge (Figure 2.21a). On viscous or dirty services, or on applications where the process fluid contains solids in suspension, “chemical tees” are installed upstream and downstream of the wedge flow element (Figure 2.21b).

2.21 Segmental Wedge Flowmeter

For Smaller Pipes

H

311

For Larger Pipes D

High Pressure Tap

Transmitter

Low Pressure Tap

Flow

LO

Seal Element

HI

Seal Element

Flow Flow Area Wedge Flange

FIG. 2.21a 1,2 The segmental flowmeter designed for clean fluid service.

Wedge Flow Element

Chemical Tee

Chemical Tee

FIG. 2.21b 1 Segmental wedge flowmeter designed for corrosive or slurry service.

TABLE 2.21c Segmental Wedge Flowmeter Capacities in GPM Units* (Courtesy of ABB Instruments – previously ABB Kent-Taylor) Approximate Differential Pressure Inches H2O Pipe Size

H/D

†

20 in.

40 in.

60 in.

100 in.

120 in.

160 in.

0.2 0.3 0.4 0.5

3.43 5.75 8.30 11.0

4.80 8.14 11.7 15.6

5.90 9.95 14.4 19.1

7.66 12.9 18.6 24.6

8.40 14.1 20.4 27.0

9.70 16.3 23.4 31.2

0.2 0.3 0.4 0.5

6.80 12.8 22.7 32.2

9.70 18.2 32.1 45.5

11.9 22.2 39.3 55.8

15.8 28.7 50.7 72.0

16.8 31.4 55.5 79.0

19.4 36.4 64.2 91.0

2 in.

0.2 0.3 0.4 0.5

12.2 20.3 34.0 50.9

17.2 28.7 48.0 72.0

21.1 35.2 58.9 88.0

27.2 45.4 76.0 114.0

29.9 49.7 83.4 125.0

34.6 57.5 96.3 144.0

3 in.

0.2 0.3 0.4 0.5

26.4 44.5 75.5 113.0

37.4 62.5 107.0 160.0

45.6 77.0 131.0 196.0

59.0 99.5 169.0 252.0

64.6 109.0 185.0 277.0

74.6 126.0 214.0 320.0

4 in.

0.2 0.3 0.4 0.5

49.5 76.1 127.0 192.0

70.0 108.0 180.0 272.0

86.0 132.0 220.0 332.0

111.0 170.0 284.0 430.0

121.0 187.0 311.0 470.0

140.0 216.0 360.0 544.0

6 in.

0.2 0.3 0.4 0.5

86.4 185.0 294.0 444.0

122.0 262.0 416.0 628.0

150.0 320.0 509.0 768.0

193.0 414.0 657.0 994.0

212.0 454.0 720.0 1089.0

244.0 524.0 831.0 1255.0

8 in.

0.2 0.3 0.4 0.5

173.0 311.0 475.0 659.0

244.0 440.0 671.0 930.0

300.0 539.0 824.0 1140.0

388.0 695.0 1060.0 1470.0

425.0 761.0 1165.0 1610.0

490.0 880.0 1340.0 1860.0

1 in.

1–1/4 in.

*The units in the table can be converted as follows: 1.0 in. H 2O = 249 Pa, 1.0 GPM = 3.785 lpm, 1.0 inch = 25.4 mm. †The H/D values shown above represent ratios between segmental opening height and the pipe diameter (Fig. 2.21a).


312

Flow Measurement

On these tees, chemical seal elements are installed flush with the pipe, eliminating pockets and making the installation self-cleaning. The seals are usually made of corrosionresistant materials and are also suited for high-temperature services. Some applications have been reported in which the operating conditions reached 3000 PSIG (210 bars) and 2 850°F (454°C). The segmental wedge flowmeters are usually calibrated on water. The pressure drop detected by d/p transmitter is interpreted on the basis of these calibration curves. The measurement error is a function of the precision of the calibration and some users report performance on slurry service within 2 2.5 and 3.5% of actual flow. The flow capacities of these sensors are listed in Table 2.21c. Based on the above, one might conclude that the segmental wedge flowmeter fills the need for corrosion-resistant slurry flowmeters that are capable of operating at high process pressures and temperatures, but only if the accuracy and rangeability requirements for the measurement are not high.

References 1.

Owen, R. E., Segmental Wedge Flow Element, ABB Kent-Taylor, Newcastle upon Tyne, UK.


2.

Malone, D. P., Slurry flow measurement: a case history, InTech, November 1985.

Bibliography Baker, R. C., Flow Measurement Handbook, Cambridge University Press, UK, 2000. Cushing, M., The future of flow measurement, Flow Control, January 2000. De Boom, R. J., Flow Meter Evaluation, ISA Conference, Paper #91-0509, 1991. Differential pressure flowmeters, Meas. Control, September 1991. Furness, R. A., Developments in pipeline instrumentation, Pipe Line Rules of Thumb Handbook, 4th ed., Gulf Publishing, Houston, TX, 1998. Hall, J., Flow monitoring applications guide, Instrum. Control Syst., 41, February 1983. Husain, Z. D., Flowmeter Calibration and Performance Evaluation, ISA Conference, Paper #91-0508, 1991. Krigman, A., Flow measurement: some recent progress, InTech, April 1983. Krigman, A., Guide to selecting non-intrusive flowmeters, InTech, December 1982. Lipták, B. G., Flow measurement trends, Control, June 2000. Miller, R. W., Flow Measurement Engineering Handbook, 3rd ed., McGrawHill, New York, 1996. Rusnak, J., The fundamentals of flowmeter selection, InTech, April 1989. Spitzer, D. W., Flow Measurement, 2nd ed., ISA, Research Triangle Park, NC, 2001. Yoder, J., Flowmeter shootout, part I and II: new technologies, Control, February and March 2001.

2.22

Sight Flow Indicators

FG Flow Sheet Symbol

D. S. KAYSER

(1982)

B. G. LIPTÁK

G. G. SANDERS

(1995)

(2003)

Design Pressure

To ANSI 600# standard (≈1400 PSIG [9.6 MPaG] material dependent)

Design Temperature

To 500°F (260°C) standard

Materials of Construction*

Windows: soda-lime glass, tempered or annealed borosilicate glass, aluminosilicate glass, quartz, polycarbonate, acrylic 



Body: bronze, iron, carbon steel, stainless steel, duplex steel, Monel , Hastelloy ,  Alloy 20 Cb-3, and so forth 



Gasketing: Buna-N, Viton A, Neoprene , polyethylene, polypropylene, PTFE, graphite, fibrous, PTFE sandwiched fibrous, and so on Sizes

0.25 to 16 in. NPS/BSP/DIN/JIS

Cost

A 0.25-in. NPS bronze/brass unit with PTFE rotator is $175; 1-in. NPS all-stainless, screwed unit is $440, with flanged connections, $600


Archon Industries Inc. (www.archonind.com) Brooks Instrument Div. of Emerson (www.emersonprocess.com) Dwyer Instruments Inc. (www.dwyer-inst.com) Eugene Ernst Products Co. (www.eepproducts.com) John C. Ernst Co. (www.johnernst.com) ERDCO Engineering Corp. (www.erdco.com) Jacoby Tarbox (www.clark-reliance.com) The Johnson Corp. (www.joco.com) Kenco Engineering (www.kenco-eng.com) Kobold Instruments Inc. (www.koboldusa.com) OPW Engineered Systems (www.opw-es.com) J. G. Papailias Co. (www.papailias.com) Penberthy-Tyco Valves and Controls LP (www.tycovalves.com) Plast-O-Matic Valves Inc. (www.plastomatic.com) Pressure Products Company Inc. (www.pressureproducts.com) Schutte and Koerting (www.s-k.com) L.J. Star Inc. (www.ljstar.com) Tokheim Corp. (www.tokheim.com)

Sight flow indicators (SFIs, a.k.a. flow glasses) provide a window into a pipe when visual inspection of a process fluid is necessary. Figure 2.22a shows five standard designs. DESIGN VARIATIONS The plain design is used to observe physical characteristics of the process fluid; it is not meant to provide full pipe flow indication. Four types of indicating devices are incorporated

when flow indication is desired. A metallic flapper design is used in transparent or slightly opaque liquids, and a lightweight polymer/glass flapper is for low-flow gaseous service. Flow direction must be vertically upward or horizontal. Some indication of relative flow velocity can be made by observing the angular position of the flapper; some manufacturers place decals on the window that approximate flow quantity (based on impact force × area) (Figure 2.22b). Bidirectional flappers are hinged in the center of the SFI body ®

* Registered trademarks of Inco Alloys International Inc., Hayes International Inc., Carpenter Technology Corp., and E.I. du Pont de Nemours & Co. are noted by .


314

Flow Measurement

Plain

Flapper

Drip

Rotator

Ball

FIG. 2.22a Basic sight flow indicator and four types of flow indicators.

FIG. 2.22b Flapper-type sight glass provided with a scale for approximation of flow.

FIG. 2.22c Bidirectional flapper. (Courtesy of Dover Corp., OPW Div.)

(Figure 2.22c) for horizontal use only. The drip-tube design is used for intermittent or extremely slow flows such as distillation; flow direction should be vertically downward or horizontal. The rotator (a.k.a., propeller, paddle, or paddlewheel, usually made of white virgin PTFE) is used with dark or opaque process fluids. The rotator is placed close to the window, and its motion is easily detected. Flow through the rotator design may be in any orientation. Very high flow rates will rapidly destroy a rotator, so it is advisable to oversize the SFI (to reduce flow velocity) or use a different indicator if high flow rates are likely. Caged balls provide an indicator that is more sensitive to low flow than the flapper and will withstand high flow velocities that would destroy a rotator.


All indication devices have a minimum flow velocity for first indication. The rotator-style indicator creates relatively high pressure drops (almost an order of magnitude greater than other indicators) due to internal flow redirection required for operation (Figure 2.22d). Figure 2.22e shows the cross section of a typical flanged sight flow indicator. The assembly consists of the body, glasses, sealing and cushion gaskets, covers, and bolting. It is similar in many respects to the transparent level gauge discussed in Chapter 3. Soda-lime glass (like window panes) should be considered only for nonsevere applications. Standard industrial glass is borosilicate, rated to 500°F (260°C) for flow glass applications. It has good resistance to mechanical and thermal shock. For higher temperatures, special glass must be specified such as aluminosilicate, up to 800°F (425°C); fused silica or quartz allows ratings in excess of 1000°F (535°C). Tempered glass is not recommended for fluorine, hydrofluoric acid, or phosphoric anhydride service, because corrosion causes uneven stresses and eventual failure. Annealed glass is a better choice, because it signals the approach of failure by turning cloudy. To reduce point compression stress on the glass, a design is available that forms glass inside a metallic compression ring. Bolting stresses are applied to the metallic ring, not to the glass. For steam service, mica shields help protect the glass from corrosion; for fluoride/phosphoric or caustic service, consider PCTFE shields. Polymer-coated glass is also available. Polycarbonate (PC) can be used instead of glass. Acrylic (PMMA) may be used if high impact strength and abuse resistance is required, but acrylic scratches easily. Polymer bodies are available for low pressure, near ambient temperature applications. SFI bodies may be obtained in almost any castable alloy. Metallic bodies can be lined with a variety of polymers enabling a properly specified SFI to be used in almost any corrosive service. Use caution when specifying a lining if the SFI is used in vacuum service; not all lining materials are adherent. The non-wetted bolting and covers are normally steel but may be obtained in different materials depending on anti-corrosion and temperature requirements.

2.22 Sight Flow Indicators

Rotator Indicator 100 90 80 70 60 50 40

Pressure Drop PSI

30 20 1/4"

3/8"

1/2" 3/4"

1"

1.1/4" 1.1/2" 2"

3"

4"

10 9 8 7 6 5 4 3 2

1 1

2 3 4 5 6 7 8 9 10 Flow GPM Water

20

30 40 50 60 80 100

200 300

500 700 1000

Flapper or Drip Tube Indicators & Plain 10 9 8 7 6 5 4

Pressure Drop PSI

3 2 1/4"

3/8"

1/2"

3/4"

1"

1.1/4" 1.1/2"

2" 3"

4"

1 0.8 0.7 0.6 0.5 0.4 0.3 0.2

0.1 1

2

3

4 5 6 7 8 9 10

20

30 40 50 60 80 100

200 300

Flow GPM Water

FIG. 2.22d Sight flow indicator pressure drops on water application. (Courtesy of Dover Corp., OPW Div.)


500

700 1000

315

316

Flow Measurement

Bolt Glass Cover

Gaskets Chamber

FIG. 2.22e Cross-section of sight flow indicator.

Glass Cushion Radial Seal Secondary Glass Backup Seal Radial Seal Primary Glass

Cap Dual Glass Adaptor Primary Glass Cushion Indicator Main Boot

Pressure

FIG. 2.22f Cross-section of a dual window assembly. (Courtesy of Dover Corp., OPW Div.)

DUAL-WINDOW AND FULL-VIEW DESIGNS Figure 2.22f shows the cross section of a dual-window assembly. This assembly improves the safety of an SFI in two ways. In high-temperature service, the thermal gradient across each glass is reduced, and the outer glass protects the inner glass from thermal shock caused by splashes of cold water, e.g., slant rain or snow. If the outer or inner glass breaks, there is a chance that the remaining glass may contain the process until the assembly can be repaired. Dual-window SFIs may be ordered with third-party safety approvals. To further enhance safety due to abuse or vandalism, or if the process fluid is hazardous or toxic, protective sheaths are recommended on either standard or dual-window SFIs. In smaller pipe sizes, pressure ratings are available up to 3000 PSIG (20.6 MPa). Gasketing may be any available type except those that would cause pressure risers on the glass (e.g., graphite with tanged stainless-steel inserts). Final SFI pressure and temperature ratings may depend on the gasketing material selection. Special designs are available that can be used in sanitary services such as food/pharmaceutical processing. Available accessories include safety jackets, illuminators, insulation blankets, and spray nozzles/rings and wiper blades for cleaning the glass in place. Most SFI bodies are designed for the inclusion of instrumentation taps, e.g., thermowells. Several types of process analyzers require plain flow glass so the operator can see the probe or other sensing element in the process. Instrumentation using visible, IR, or UV light has been adapted to SFIs


FIG. 2.22g 90° elbow sight flow indicator.

using attenuation or backscatter to noninvasively monitor chemical processes. Elbow-style SFIs, both 45 and 90° (Figure 2.22g), have tapping bands available to allow use as elbow differential-pressure flowmeters. Standard SFI bodies are made with a cross-sectional area expansion from piping bore to accommodate full-bore-size vision. If minimum flow profile distortion is desired, full view SFIs (Figure 2.22h) are designed to maintain piping bore diameter. These are essentially constructed of a spoolpiece with tubular glass either radially or end compression sealed forming the body. They are available plain (360° viewing) or in an armored version for breakage protection, but the armoring reduces visibility. These are normally used for vertically upward flow and are available flanged or threaded.

2.22 Sight Flow Indicators

317

FIG. 2.22i Threaded sight window.

FIG. 2.22h Flanged full view and armored full view SFI.

A variation of the standard SFI called a sight window (Figure 2.22i) is one window face of an SFI with either flanged or threaded connections designed to fit a piping tee or coupling. CONCLUSION Sight flow indicators offer an inexpensive means of viewing process material inside a pipe to detect flow or to note process characteristics such as color, turbidity, or other properties that


might indicate process deterioration or equipment malfunction. SFIs are also used in secondary services such as condensate pot installations. Nonetheless, SFI use is limited to primary industrial process areas, because it is difficult to estimate flow rate through a flow glass, and a hazard is created if the glass breaks. They are more commonly used in utility services associated with industrial processing.

Bibliography Green, C. R., Tank sight glasses, Chem. Eng., September 1978. Sunderhous, C. A., Sight indicators allow positive flow check, Machine Design, October 1985.

2.23

Solids Flowmeters and Feeders

Meter WT

R. SIEV

(1969)

D. C. MAIR

(1982)

B. G. LIPTÁK

(1995, 2003)

To Receiver

Belt Type

1 KW

Set pt

WY

w

HC

Tare Bias

WT

Meas. WC

I S

1/ P Loss in Weight Flow Sheet Symbols


Types of Designs

A. Accelerator B. Belt-type gravimetric C. Volumetric, capacitance D. Impulse or impact E. Loss-in-weight F. Switch (Section 2.7) G. Dual-chamber H. Cross-correlation (Section 2.5) I. Nuclear J. Microwave

Capacities

A. 1000 to 80,000 lbm/h (450 to 36,000 kgm/h) 3 3 B. Up to 180,000 lbm/h (80,000 kgm/h) or up to 3600 ft /h (100 m /h) 3 3 C. Up to 3600 ft /h (100 m /h) D. 3000 to 3,000,000 lbm/h (1400 to 1,400,000 kgm/h) E. Determined by hopper or duct size F. Unlimited on–off G. 1000 to 300,000 lbm/h (450 to 140,000 kgm/h) H. Unlimited I. Same as B J. Unlimited on pulverized coal applications

Costs

$1000 to $2000 (F) Around $4000 (C) $4000 to $6000 (A, D) $5000 to $20,000 (B, H) $15,000 to $30,000 (E, G, I)

Inaccuracy

±0.5% of rate over 10:1 range (B [digital], G) ±0.5% to ±1% of full scale (I) ±1% of rate over 10:1 range (E) ±1 to ±2% of full scale (D) ±2 to ±3% of full scale (A, F) ±2 to 4% of full scale (C)


ABB (www.abb.com) (C).

2.23 Solids Flowmeters and Feeders

319

Air Monitor Corp. (www.airmonitor.com) (J) Babbitt International Inc. (www.babbittlevel.com) (D) Cardinal Scale Mfg. (www.ardinalscale.com) (B) Cutler-Hammer, Thayer Scale Div. (www.cutlerhammer.eatoncom) (B, D, E) DeZurik/Copes–Vulcan, a Unit of SPX Corp. (www.dezurikcopesvulcan.com) (A) Endress+Hauser Inc. (www.us.endress.com) (B, C, D, F, H) Fairbanks Scales (www.fairbanks.com) (B) ICS Advent (www.icsadvent.com) (E) Kay-Ray/Sensall (www.thermo.com) (I) Kistler-Morse Corp. (www.kistlermorse.com) (B) M-System (www.m-system.com) (B) Milltronics Inc. (www.milltronics.com) (B, D) Monitor Technologies LLC (www.monitortech.com) (F) Ohmart/VEGA (www.ohmartvega.com) (I) Technicon Industrial Systems (www.technicon.com) (G)

Properly Designed Bins

Poorly Designed Bins

Arching

Plug-Flow

Rat-Hole

Mass-Flow

FIG. 2.23a Good bin design is a critical requirement for a successful solids metering installation. Air Operated Gate Air Vent to Dust Collector

S

Many types of solids flowmeters are currently available. The majority depend on some method of weighing, but others utilize a variety of other phenomena ranging from various forms of radiation to impact force determination, and from dependence on electrical properties to centrifugal force. The conditions and properties of the flowing solids have a major impact on the type of flowmeter required. For example, the flow rate of coal can be measured by microwave detectors or belt feeders. This choice is a function of the coal being pulverized and whether it is pneumatically conveyed. Before undertaking a discussion of solids flowmeters, we will discuss associated process equipment such as solids storage devices and the feeders that bring the solids from the storage vessel. Because keeping solids in motion and preventing arching and rat-holing in the supply bins are serious problems, the description of feeders will be preceded by the topic of feeder accessories.

Timer

LSH

Vibrator

SOLIDS HANDLING EQUIPMENT The bin, the feeder, and the solids flowmeter should be designed in an integrated manner, taking into account the characteristics (density, particle size, moisture content, temperature, or hazardous properties) of the solids. For example, the bed depth on a belt must be less than the height of the skirts (to avoid spillage), but it must be at least three times the maximum lump size to guarantee stable solids flow. Coarse materials (+60 mesh) or wet ores are likely to bridge or rat-hole in the bin (Figure 2.23a) and require vibrators and special feeders. Similarly, aerated, dry, and fine solids (–200 mesh) are likely to either free-flow or be compacted and thereby plug the standard rotary vane or screw feeders. Changing the pitch or inserting additional flights can alleviate flushing. Vibrators usually also help, although in some cases they might worsen the situation by packing the solids. In general, the addition of high-amplitude and low-frequency vibrators or air pads and the use of mass flow bins (steep walls at 10 to 30° from the vertical) tend to improve material flow.


Timer Inlet Flexible Connection to Feeder

LSL

LAL

Manual Shutoff Gate

FIG. 2.23b Deaerating surge hopper.

Hoppers and Accessories A surge hopper, when located between the storage hopper and feeder inlet, provides a means of deaerating the solids. This guarantees that the solids can be fed, using a gate-controlled belt feeder, without causing flooding. The solid feed into the surge hopper is controlled by bin level switches (LSL and LSH in Figure 2.23b), which maintain the solids level within an acceptable zone by on–off control of the hopper supply gate valve. The hopper inlet device may be a rotary vane feeder, screw conveyor, or a knife gate with suitable actuator.

320

Flow Measurement

If the required feed rate is constant or nearly so, the bin switches are located so as to provide a hopper capacity that is equivalent to about 2 min retention time when operating at the design feed rate. In cases in which the material may compact in the hopper and interrupt the supply to the feeders, excess retention time is undesirable. If the feed rate is varied, an adjustable timer is incorporated in the level control circuit to adjust the time setting for keeping the hopper feed valve closed. This timer is started by the upper bin level switch (LSH), which simultaneously closes the bin supply valve when the material contacts the probe. This condition is maintained until the timer runs out and reopens the supply valve, which than stays open until the high-level detector is once again reached. In this arrangement, the low-level switch (LSL in Figure 2.23b) serves only as a low-level alarm, which is used to shut down the feeder. Such shutdown is usually desirable to prevent loss of the plug of material ahead of the belt feeder. If the solids easily aerate, the loss of a plug of deaerated material can cause production delays, because a new supply of deaerated material has to be obtained first. Some materials will deaerate in the surge hopper without the need for vibration. Other materials require that the hoppers be furnished with electric or pneumatic vibrators. The required frequency and duration of vibration varies with solids characteristics and the vibrators therefore are provided with the means for adjusting these variables. All manufacturers recommend that a feeder or meter be isolated from sources of vibration, and some include shock mounts with each machine. Inlet and discharge flexible connections to isolate the equipment from vibration and pipe strain in the material inlet and outlet ducting are also recommended. Material Characteristics A number of common materials, of which sulfur is an example, will compact unless kept in almost continuous motion. Others will compact even while in motion if placed under the pressure of a relatively low head of material. In these applications, it is necessary to use small surge hoppers and use level switches that keep the head of material on the feeder belt low. The retention time of these small hoppers is on the order of a few seconds, and external vibration is not used. The discharge flow pattern of a belt feeder varies with belt speed and material characteristics. A granular free-flowing material such as sugar will flow smoothly off the belt even at low belt speeds. Other materials having a high angle of repose coupled with a tendency to compact will drop off the end of the belt in lumps, especially at low belt speeds. This results in erratic feed rates and in short-term blend errors when part of multifeeder systems. The discharge flow pattern can be markedly improved by equipping the feeder with a material distributor. This device consists of a blade located across the full width of the belt at the discharge end of the feeder and vibrated by an electric or pneumatic vibrator. The blade is located so that it almost touches the belt and the material is directed across it. This vibration causes the solids


to be spread out into a ribbon and to smoothly stream off the belt. Unlike liquids, which exhibit predictable flow behavior, solids flow characteristics are extremely difficult to evaluate on any basis other than an actual trial. For this reason, most manufacturers maintain a test and demonstration facility in which samples of a potential customer’s solids samples can be fed by various test feeders equipped with various volumetric feed sections. Recognizing that a wealth of experience with commonly used materials can very often permit a feed section recommendation without the need for testing, it also should be noted that even a minor change in the properties of a material can drastically change its feeding characteristics. These changes might be in particle size or particle shape but can also be caused by the entrainment of air, which occurred during pneumatic conveying prior to the solids entry into the feeder, or by the addition of an additive to the preblended solids. Many installations involve feeding directly into processes that may be under low pressure or that may discharge corrosive vapors back through the feeder discharge ducting. If pressures are very low, the feeder can be purged with inert gas, or a rotary valve can be installed in the ducting. The rotary valve body should be vented to remove process vapors from the valve pockets before they reach the inlet or feeder discharge side of the valve. If the valve is not vented, blowback resulting from the release of pressure in the rotor pockets can cause discharge flow pattern disturbances and, in extreme cases, affect the feeder weigh section. The valve is vented into a dust or vapor collecting system via a vent port in the side of the valve rotor housing. Taking Samples Feeder manufacturers base their performance guarantees on taking a timed sample, weighing it, and comparing the result with the setpoint of the feeder. This requires some means of sampling, which are available either as sample trays, which are inserted into the feeder discharge stream for a predetermined period and then weighed, or as flap valves, which temporarily divert the discharge stream from the process duct into a sampling container. The flap-type valve is generally preferred, because the tray-type sampler is suitable only for low feed rates. Sampling normally involves the taking of 10 consecutive 1-min samples and comparing the average sample weight to the setpoint. Another advantage of the flap-type sampler is that it is faster acting, and the sample weights obtained are thus more accurate. Each feeder or meter is usually supplied with a test weight or drag chain, which may be used to check the calibration of the device without actually running material. The weight is usually selected to match the full scale of the weight-sensing mechanism. Such test weight is also useful in aligning the control setpoints in multifeeder master–slave systems prior to running any material. In such systems, the test weight can be applied to the master feeder, and the resultant output signal can be sent to the ratio station setpoints of the slave feeders.


Feeder Designs A gravimetric feeder consists of a weight-rate measuring mechanism coupled with a volumetric feed rate control device. The vertical gate volumetric regulator, which is perhaps the most popular, is not suitable if the solids have large particle size, are fibrous, are irregularly shaped, or tend to flow like a fluid because of fine particle size. Because of this wide variation of solids properties, a variety of feeders have been designed as described in the following paragraphs. Vertical-Gate The vertical-gate gravimetric feeder is available in a variety of sizes to produce typical material ribbon widths of 2 to 18 in. (50 to 457 mm) and to regulate up to 6 in. (152 mm) of material depth on the weigh belt. Gate actuators may be electromechanical or pneumatic, or they may use computer-controlled electric servomotors or stepping motors. Manually adjustable gates are also available. The vertical gate has a typical depth control range of 10:1 and is generally suitable for materials that are not fluidized and that have a particle size not larger than about 0.125 in. (3.175 mm). Larger particles will not flow smoothly under the lip of the gate, thus resulting in an irregular belt load. This may require excessive damping of the belt load transmitter output, which will have an undesirable effect on both control accuracy and sensitivity. In addition to producing undesirable control characteristics, rangeability will be decreased as particle size increases. As a rule of thumb, the minimum gate opening should be approximately three times the maximum particle size for solids having irregularly shaped particles of random size. This 3:1 ratio may be reduced somewhat if the material is homogeneous and particles do not tend to interlock and tumble while in motion (typically, if particle shape approaches that of a sphere). Rotary-Vane Figure 2.23c shows a rotary-vane feeder, which can be provided with a variable-speed drive and conventional or computer controls. Such a feeder is used as the volumetric feed section in instances in which the material is

Rotation

aerated or has a low bulk density. Rotary feeders are not recommended for handling solids with large particle sizes or if the solids are sensitive to abrasion by the feeding device. In solids-blending applications, it is possible to operate several feeders in parallel or in cascade from the same setpoint. Similarly to the vertical gate feeder, the rotary-vane feeder is not suitable either for handling fibrous or stringy materials, because sticky or hygroscopic materials tend to clog the pockets of the rotor. The sizing of pocket shape and depth is based on the required volumetric flow rates and material characteristics. Volumetric capacity is regulated by rotor speed, but if the speed is too high, rotor pockets won’t completely fill as they pass under the inlet opening, and volumetric output may decrease if rotor speed exceeds the optimum. Therefore, care must be taken in determining a maximum practical rotor speed. The rotary-vane feeders therefore have limitations when used on applications involving free-flowing powders or materials having small particle size but, unlike the vertical gate, they can handle low-density or aerated materials. The rotary feeder should be separately mounted from the gravimetric meter and should be interconnected by means of a flexible connection to prevent transmittal of vibration from the rotary feeder to the weight-sensing meachanism. Figure 2.23c also shows a manually positioned leveling gate, which is located ahead of the weighing section. This device levels the irregular feed pattern created by a rotary feeder and produces a more consistent feed to both the weighing section and eventually to the process. The shutoff gate at the feeder inlet serves the isolation of the feeder from the material supply during inspections or servicing. Screw Feeders The feeder element in this device is a screw whose rotary motion delivers a fixed volume of material per revolution (Figure 2.23d). The screw is located at the bottom of a hopper so that its inlet is always flooded with solids. Screws grooved in one direction discharge material at one end only. Screws grooved in opposite directions from the middle deliver material at both ends. Rotation of the screw

Manual Shutoff Gate

Rotary Feeder Inlet

Rotary Vane Feeder

Variable Speed Transmission Motor

Belt Motion Feeder Belt

Constant Speed Belt Drive Motor

Manually Positioned Leveling Gate

FIG. 2.23c Gravimetric feeding system utilizing a rotary vane volumetric feeder controlled by a belt-type gravimetric meter.


321

Belt Type Gravimetric Meter

322

Flow Measurement

Hopper

Vibratory Pan Feed Chute

Hopper Screw

Casing

Shaft for Gear or Sprocket

Electromagnetic Power Unit To User

FIG. 2.23d Screw feeder.

can discharge material into receiving vessel(s), at one or both ends of the screw. A variable-speed screw feeder can feed control lowdensity or aerated materials. The screw section can be made as long as is necessary to prevent the material from flooding through it. Screw feeders have also been successfully used on fibrous solids and on powdered materials, which tend to cake. The major advantage of the screw feeder, compared to a rotary vane feeder, is that custom-built screw feeders can be provided with extremely large inlet openings to facilitate the entry of fibers and coarse lumps into the conveying screw. When the solids have a tendency to cake or clog the screw, the double-ended version of the screw feeder can be oscillated laterally. This oscillation imparts lateral forces that assist in moving the solids through the unit by alternately moving the material first toward one end and then the other. To assure an accurate feed, the hopper on the inlet side of the feeder must be designed to provide a uniform supply of material to the feed screw. Vibrators can be added to the hopper to keep the solids agitated and to prevent caking and bridging. Feeder drives are usually electric motors. If the drive is a constant-speed unit, the feed rate is adjustable over a 20:1 range by means of a mechanical clutch that varies the on–off operating time per cycle. In this case, if the feed rate is set at 75%, the screw feeder will be operating 75% of the time or 75% of a clutch revolution. The addition of an analog or digitally controlled variable-speed drive can extend the rangeability of the unit to 200:1. Vibratory Feeders Vibratory feeders are used in gravimetric feeding systems to handle solids with particles that are too large to be handled by screw, rotary-vane, or vertical-gate feeders, or in operations where the physical characteristics of the solid particles would be adversely affected by passage through these volumetric feeding devices. The discharge flow pattern of a vibrating feeder is extremely smooth and thus is ideal for continuous weighing in solids flow metering applications. The vibratory feeder (Figure 2.23e) consists of a feed chute (which may be an open pan or closed tube) that is moved back and forth by the oscillating armature of an electromagnetic driver. The flow rate of the solids can be controlled by adjusting the current input into the electromagnetic driver of the feeder.


To User

FIG. 2.23e Vibratory feeder.

Hanger Rod

Hopper

Turnbuckle

Skirt Board

Chute

Shaker Pan Disk Crank Connecting Rod

Rails

Wheels

To User

FIG. 2.23f Shaker feeder.

This input controls the pull of the electromagnet and the length of its stroke. Vibratory feeders are well suited for remote computer control in integrated material-handling systems. The vibratory feed chute can be jacketed for heating or cooling, and the tubular chutes can be made dust tight by flexible connections at both ends. The vibratory feeders can resist flooding (liquid-like flow) and are available for capacity ranges from ounces to tons per hour. Shaker Feeders The shaker feeder (Figure 2.23f) consists of a shaker pan beneath a hopper. The back end of the shaker pan is supported by hanger rods. The front end is carried on wheels and is moved by a crank. As the pan oscillates, the material is moved forward and dropped into the feed chute. In most units, the number shaking strokes is kept constant while the length of the stroke is varied. The angle of inclination of the shaker varies from about 8° for freely flowing solids to about 20° for sticky materials. If arching is expected in the hopper, special agitator plates are installed in the hopper to break up the arches. The shaker feeder is rugged and self-cleaning, and it can handle most types of solids regardless of particle size or condition. Roll Feeder Roll feeders are low-capacity devices used for handling dry granules and powders (Figure 2.23g). The feeder consists of a feed hopper, two feed rolls, and a drive unit. Guide vanes in the hopper distribute the material and provide agitation by oscillation. The feed rolls form the material into


323

GRAVIMETRIC FEEDERS Hopper Hopper Agitator Guide Vanes Feed Slide

Feed Rolls Motor

Access Door

Feed Rolls Side View

Front View

FIG. 2.23g Roll feeder.

Early Belt Feeder Designs

Gear Hopper Hopper

Adjustable Gate

Revolving Plate

Bearing Skirt Boards

To User

FIG. 2.23h Revolving plate feeder.

a uniform ribbon, and the feed rate is controlled either by means of a slide that varies the width of the ribbon or by means of a variable-speed drive. The rangeability is typically 6:1 when using the feed slide and 10:1 when variable-speed drives are used. For materials that tend to cake or bridge in the hopper, agitators can be provided to maintain the material in a free-flowing state. Revolving-Plate Feeders Revolving-plate feeders (Figure 2.23h) consist of a rotating disk or table (usually horizontal), which is located beneath the hopper outlet. The table is rotated and, as it rotates, fresh material is drawn from the hopper while the solids that the feeder discharges are scraped off by skirt boards. The feed rate is controlled by adjusting the height of the gate or positioning the skirt board. Revolving-plate feeders handle both coarse and fine materials. Sticky materials are also handled satisfactorily, because the skirt boards are able to push them into the chute. This type of unit cannot handle materials that tend to flood. A variation of the revolving plate feeder utilizes rotating fingers to draw feed material from the bin. Revolving-plate feeders can also be equipped with arch-breaker agitators in the conical throat section of the hopper.


Belt feeders are compact factory-assembled devices that use belts to transport the material across a weight-sensing mechanism. In the case of solids flowmeters, the flow of solids is uncontrolled, and the load on the constant speed belt is measured as an indication of the solids flow rate. The flow rate of solids on a simple gravimetric feeder can be regulated by a vertical or rotary gate, screw, or other volumetric control device. More accurate control methods are based on varying the belt speed or adjusting both the belt speed and the belt loading. (Although this volume of the Instrument Engineers’ Handbook is devoted only to measurement, in connection with gravimetric belt feeders, it is also necessary to touch upon the topics of regulation and control, which will be discussed in much more detail in the second volume.)

Figure 2.23i illustrates the forerunner of most modern belt feeders. It consists of a constant-speed belt coupled to a gate that modulates the solids flow rate so that the belt load is balanced by an adjustable poise weight. This feeder is unique in its simplicity but is inferior to the more modern designs for the following reasons: 1. The entire feeder is weighed rather than only a portion of the belt. Consequently, the ratio of live load to tare weight is low. In addition, the mechanical friction in the pivots results in a low sensitivity in the belt loaddetection system. 2. This is a proportional-only controller, because the opening of the gate control element is proportional to the belt load error. Much as a float-operated levelcontrol valve cannot maintain the level at setpoint if valve supply pressure or tank draw-off vary, this feeder cannot maintain the solids flow rate if the bulk density of the solids changes.

Inlet Chute

Control Gate

Rate Setting Poise Weight

Pivot

Constant Speed Conveyor Belt

FIG. 2.23i Early belt-type mechanical gravimetric feeder.

324

Flow Measurement

Gate Actuator And Clutch Unit (Lowers Gate) (Raises Gate)

Magnet - Mercury Switch Belt Load Belt Load Error Setpoint Detector Indicator

Rate Setting Poise Weight

Gate Actuator Control Signal

Flexure Supported Weight Decks

Figure 2.23j illustrates another early electromechanical gravimetric feeder design. Here, the belt load is balanced by a poise weight on a mechanical beam, which also carries a magnet. If the beam is not balanced, the magnet energizes one or the other of two clutches via a pair of mercury switches, which are energized by the magnet. These clutches actuate and establish the direction of travel of the gate-positioning mechanism. The gate modulates the belt loading to keep it constant and matched with the belt load set by the poise weight on the balance beam. This feeder will maintain the belt loading regardless of changes in material density and subject only to the volumetric control limits of the gate. In this design, the belt load setpoint can be indicated by a mechanical counter that is geared to the beam poise weight drive. A second counter can be geared to the belt drive, which can give the total length of belt travel. The total weight of solids fed can thus be calculated by multiplying the readings of the two counters. In more up-to-date versions of this design, remote setpoint and the measurement signals are provided, along with automatic shutdown, after the desired total weight of material has been fed. Gate position-actuated adjustable limit switches can be provided to activate alarms that can indicate either the stoppage of the supply of solids to the feeder or the overtravel of the control gate resulting from abnormally low material density. Feed Rate Control The feed rate of all belt-type gravimetric feeders is a function of the belt speed and the unit loading of the belt. If belt speed is expressed in feet per minute and belt loading in pounds of solids per foot of belt, the solids flow is obtained as Flow rate = (Belt speed) (Belt loading) = 1bm/min 2.23(1) In the case of the constant-speed belt feeders previously discussed, the flow rate of solids is directly proportional to


6"

18"

Constant Speed Belt Drive Feed rate

t=

Belt Travel Totalizer

FIG. 2.23j Belt-type electromechanical gravimetric feeder.

Belt Load Signal WT

t= Belt Drive

WRC

1 6

1 8

Belt Load Signal, 12 FPM Belt Speed

Feeder Discharge Rate to Process, 12 FPM Belt Speed 3

Belt Load Signal, 2 FPM Belt Speed

t= 4

t=1

Feeder Discharge Rate to Process, 2 FPM Belt Speed 1

1

3

1

5

3

7

1

1

0 1 1 8 14 Minutes 8 4 8 2 8 4 8 Elapsed time after belt load step change —“t” Minutes

FIG. 2.23k Open loop response to a step change in belt loading.

belt loading. Another method of flow rate adjustment is to vary the belt speed while maintaining the belt loading constant. The third option is to vary both the belt speed and the belt loading, in which case the flow rate is obtained as in Equation 2.23(1). Belt Load Control of Constant-Speed Belts A standard constant-speed belt feeder, provided with a pneumatic gate actuator, is shown in Figure 2.23k. The length of the weighing section and the distance from the end of weighing section to the end of belt are approximately the same as those in an actual feeder. The response shown in Figure 2.23k is not precisely depicted, because it assumes instantaneous gate response and does not consider the controller lags, but these effects are minor in comparison to the effect of the belt transportation lag, which is the major source of concern in using constant-speed belt feeders. The uppermost curve shows the response of the belt load signal to a step change in belt loading if the belt is moving at a speed of 12 ft/min. The dashed line below represents the instantaneous feeder discharge rate at the end of the feeder belt. This is the solids flow rate that the process downstream of the feeder receives. By reviewing the top line, one can conclude that some effect of the stem change in belt loading is sensed almost immediately after the step change, because the control gate is located at the upstream edge of the weighing section. At the 12-ft/min belt speed, the full length of the weighing section will be covered by the new level of solids in 18/144 = 1/8 min after the step change. Yet, at that time, the feeder is still discharging at the rate, that existed prior to the step change, and an additional 1/24 min is required to transport the material to the end of the belt—a distance of 6 in. If the belt speed is 2 ft/min, the corresponding feeder response will be as described by the lower pair of curves in


Figure 2.23k. In this case, it will take a full minute before the downstream process starts receiving the new solids flow rate after a step change in belt loading is made. Such response times might be tolerable by some single-feeder processes, but not all. Belt Speeds and Blending In continuous blending operations, the instantaneous blend ratio must be continuously maintained, so acceptability of constant-speed feeders is more limited. We can conclude from the data in Figure 2.23k that, if two feeders having belt speeds of 12 ft/min and 2 ft/min were controlled from a common belt loading signal, and a step change occurred in that signal, the result would be a temporary upset in the actual blend ratio. This upset would start 10 sec after the change in the belt loading setpoint and would persist for a period of 50 sec, at which time the original blend ratio would be restored. Therefore, blend ratios that are obtained from two or more constant-speed gate feeders cannot be maintained unless the belt speeds of all feeders are identical. This is a serious limitation, because, in blending application, it is rarely possible to size a number of feeders that are delivering different solids flow rates so that they all have the same belt speed. If the solids flow characteristics permit it, one can increase the belt speed by decreasing the width of the material ribbon on the belt, but this does not satisfactorily solve the problem in most applications. The blend ratio upsets can be reduced if the feeders are cascaded in a master–slave relationship wherein the step change in the belt load is first applied to the master feeder’s gate actuator, and its belt load signal is used to control the gate actuator of the slave feeder. One should always select the slow speed feeder as the master, because slaving the lowspeed feeder to the high-speed one will only increase the duration of the upset in blend ratio. Computer studies indicate that the upsets in blend ratio will be minimized if the belt speed of the slave feeder is 1.5 times that of the master. Belt Speed Selection Guidelines 1. In single-feeder applications, optimal response is obtained by selecting the maximum possible belt speed commensurate with the characteristics of the material being fed and with the belt load limits established by the feeder manufacturer. 2. In continuous blending applications involving two or more feeders of identical speed, the upsets in blend ratio caused by step changes in loading will be minimized if the feeders are controlled in parallel from a common loading-rate signal. 3. In continuous blending applications, where the constantspeed belt feeders have different speeds, the upset in blend ratio can be minimized by arranging the individual feeders in a cascaded (master–slave) configuration and selecting the lowest-speed feeder as the master. The upsets in blend ratio will be minimized if the speed of the slave is 1.5 times that of the master.


325

Varying the Belt Speed The main advantage of belt speed control over belt load control is that the solids flow to the process changes almost simultaneously with a change in belt speed setpoint. The use of speed control in multifeeder blending applications eliminates the blend ratio error that was caused by the differential transport lag, typical of constantspeed feeders. In variable speed blending systems, a common speed signal is applied in parallel to manipulate the speeds of all feeders, increasing or decreasing the total throughput of the blended solids. The ratio of any ingredient in the total blended product can be modified by changing either the belt load or the belt speed of the corresponding feeder. The latter method is preferred if the ratio has to be changed while the system is operating, because the changing of belt loading during operation will cause a temporary blend error due to the transport lag between the control gate and the process. If a continuous integrator is used, it will accurately register the total solids flow, no matter if the blend ratio was manipulated by changes in belt loading or in belt speed. Limitations of Belt Speed Control While the manipulation of the belt speed guarantees fast response to setpoint changes and eliminates the transport response error in blending, it also has some disadvantages. 1. One disadvantage relative to constant-speed feeders is that the variable-speed design does not provide feed rate readout. Therefore, the feed rate must be calculated by multiplying the belt speed times the belt loading. 2. In multifeeder blending systems every change in the blend ratio requires a change in the belt loading or in the speed ratio setpoint to one or more of the feeders. This, in turn, will change the total throughput to the process unless a master speed adjustment is made to compensate. To overcome the above limitations, it is necessary to measure both the belt speed and the belt loading and, based on these two measurements, calculate the total solids flow rate, which then can be compared to a single setpoint representing the required feed rate. Figure 2.23l illustrates such a control configuration. In the older, pneumatic version of this control system, the belt speed rangeability was 10:1. In the electronic version, where silicon-controlled rectifier (SCR) drives are utilized, the rangeability of speed variation is at least 20:1. In Figure 2.23l, the feeder is equipped with a fixed gate. This is acceptable in all applications where the material density is constant enough that the adjustment rangeability of the belt speed drive can accommodate all variations in both density and gravimetric feed rate. If the density variation is substantial, or if the feeder is to be used on a variety of materials having different bulk densities, the rangeability of belt speed adjustment might be insufficient. In such cases, a secondary or slave control loop is added to manipulate belt loading.

326

Flow Measurement

Feedrate Setpoint FRC

Belt Speed Transmitter

Detected Feedrate

WAH/L

Alarm Computing Relay

WI WSH

Indicating Hi - Lo Alarm WSL Switch Unit

FY ST

Manual Gate

Source Housing

Source

“A” Frame Construction Belt Belt Loading

WT

Conveyor Speed (Belt Length/Hour) Transducer Mass Totalizer Flow

Detector Amplifier

Multiplier Belt Speed

FIG. 2.23l Speed-controlled belt feeder with both set-point and measurement in feed rate units.

Precision of Weighing Weighing accuracy is the highest if the belt loading is maximized. This, in turn, will maximize the live load to dead load ratio. Gravimetric belt feeders are sized to handle the maximum required solids feed rate when the belt drive is operating at near maximum speed and the belt loading is at about 90% of maximum, based on the minimum expected material density. To allow accurate setting of the manual gate position, a belt load indicator is desirable. To remind the operator that the manual gate opening needs to be readjusted because of changes in solids density, belt loading alarms are recommended. Such high and low alarm switches (LSH, LSL), as shown in Figure 2.23l, can simultaneously actuate audible alarms and initiate computer printouts.

FIG. 2.23m Nuclear belt scale supported by A-frame. (Courtesy of Kay-RaySensall.)

Manual Gate Preset to Provide Approx. 90% Belt Load

WT

DC Motor

SCR Control

vdc

I/V

ma

FIC


Standardizer with Totalizer

Pulse

Pulse

Pulse %

Nuclear Belt Loading Detectors Belt loading can also be measured by detecting the radiation absorption of a discrete length of material. In all other respects, the nuclear belt scales are similar to gravimetric belt scales except that the load cells are replaced by nuclear densitometers. These devices have been used successfully not only on belt feeders but also on screw, drag chain, and vibrating feeders. The radiation source can be cesium 137, cobalt 60, or americium 241. The radiation source is usually placed above the belt and is supported on either side by a C- or A-frame (Figure 2.23m). In this configuration, the radiation detector is located below the belt and receives a radiation intensity that is inversely proportional to the mass of solids on the conveyor. Nuclear belt scales are suited for such hard-to-handle services as hot, abrasive, dusty, and corrosive materials. If the moisture content, bulk density, and particle size of the solids are all constant, they can measure the belt loading within an error limit of 0.5% of full scale when the belt load is high (70 to 100% of full scale). On the other hand, if dissimilar solids are intermixed and measured by the same scale, the differences in radiation absorption characteristics can result in substantial errors. For nuclear belt scales, the minimum required belt 2 2 loading is about 2.5 lb/ft (12 kg/m ), and these units are not recommended for belt runs that are shorter than 10 min or for belt loadings that are below 10% of full scale.

Continuous Integrator with Photoelectric Pulse Generator

WI

Master Oscillator

Pulse

Ratio Setting Stations %

To Additional Control Station

FIG. 2.23n Belt-type gravimetric feeder with digital controls.

Digital Control The continuous integrator at the bottom right of Figure 2.23l totalizes the quantity of solids delivered by multiplying belt travel times belt loading. The instantaneous rate of integration is the rate of feeding the solids. Therefore, if the continuous integrator was provided with a feed rate transmitter, the belt speed transmitter (ST) and feed rate relay (FY) in Figure 2.23l could be eliminated, and the feed rate signal from the integrator could be sent directly to the feed rate controller (FRC). Figure 2.23n describes this arrangement, which has been developed for use in commercial digital control systems. The digital control system is theoretically without error, because the pulses generated by the master oscillator in Figure 2.23n must be matched by those derived from the pulses generated by the integrator transmitter on the feeder. Laboratory evaluations and field tests have shown that the feeding precision based on weighed samples vs. total integrator pulses is better than 0.5% of feed rate over a 10:1 feed rate range.


Digitally controlled gravimetric feeders are utilized in situations involving a number of materials that must be blended in a wide variety of frequently changed formulations. High accuracy, high speed, ease of formula change, and centralized control characterize the digital control system. Although the cost of the feeder and its associated digital control is perhaps 50% higher than the cost of a feeder with conventional analog controls, digital control is widely used in continuous blending systems, particularly in the food industry. Digital systems are superior to analog ones, because each pulse represents a specific increment of weight. Therefore, a pulse rate of 100 pulses per minute, for example, with a pulse value of 2 lb, signals a solids flow of 200 lb/min. The pulses are totalized on both the measurement and the setpoint side, so errors due to temporary starvation or overcharge, common in analog systems, cannot occur in digital ones. Another advantage of the digital system is the flexibility of the microprocessor, which can easily and quickly be reprogrammed, for example, for operating like a mass flowmeter or being part of a blending system. The microprocessors also provide the capability for automatic recalibration and retention, for future reference, of the corrections that were applied at each test. The microprocessor-operated units are also capable of functioning in several modes, such as in start-up, predetermined fixed flow, or flow-ratio modes. They can have a variety of ratio or cascade configurations, logic interlocks, input and output signals (BCD, serial, analog), displays, printers, and memory units. They can receive their setpoints from other systems and also can receive stop/start signals as a function of other operations in the plant. They can operate as PID loops with dead time compensation utilizing such algorithms as “sample” and “hold,” and, finally, they can operate as batching units with remote resets. Batch vs. Continuous Charging Digital control systems are available in two basic arrangements: one for batching systems, the other for continuous feeding systems. In the batching version, the master oscillator in conjunction with a timer delivers a total number of pulses that are proportional to the desired total weight of solids. The pulse frequency is adjusted to vary the duration of the batch preparation period. The pulses are applied as the setpoint to the feed rate controllers (FIC in Figure 2.23n) via ratio setting stations for ingredient ratio. The feed rate measurement pulses are generated by the photoelectric pulse generator, which is driven by the feeder integrator. These pulses are sent to the feed rate controller after being scaled and standardized. The controller compares the setpoint and measurement pulse frequencies and adjusts the feed rates as required by varying belt drive speed. In the batch controller version, a memory feature is also included so that the feeder continues running until it has generated the total number of pulses that equal the total pulses received as the setpoint by the feed rate controller from its ratio station. In a multifeeder batching system, this feature may result in feeders shutting down at different times, but the batch blend ratio will be correct.


327

FIG. 2.23o Vertical gravimetric feeder.

In continuous systems, another version of controller is used. It includes a pacing feature, which paces down all the feed rates if the feed rate of one feeder drops. Therefore, if the controller cannot correct a decrease in feed rate of one feeder, the corresponding controller will “gate” the output of the master oscillator and thus will pace down the feed rates of the other feeders to maintain blend ratio. When the faulty feeder corrects or is corrected, all feeders are automatically returned to normal control, and the master oscillator continues to set the feed rate. If the faulty condition persists for some predetermined period, an alarm is activated. Vertical Gravimetric Feeders A vertical gravimetric feeder is illustrated in Figure 2.23o. An agitator rotor within the supply bin guarantees a “live” bin bottom. The process material enters through a hole in the top cover of the pre-feeder and is swept through a 180° rotational travel by the rotor vanes until it is dropped into the discharge pipe. The solids are weighed along with the rotary weight feeder as it transports the solids to the outlet. The advantages of this feeder include its convenient inlet–outlet configuration; its sealed, dust-tight design; and its self-contained nature wherein all associated control instruments are also furnished. After calibration, ±0.5% of full scale performance can be expected if a 5:1 rangeability is sufficient. At a 20:1 rangeability, the error, if the unit is calibrated, is ±1% of full scale. The main disadvantages of this design are that the unit has a limited capacity and can only handle dry and free-flowing

328

Flow Measurement

powders with particle diameters under 0.1 in. (2.5 mm). Large foreign objects cannot be tolerated in the process material, nor can damp or sticky solids that might cake or refuse to flow freely. LOSS-IN-WEIGHT FLOWMETERS One continuous loss-in-weight feeder design is illustrated in Figure 2.23p. In this system, the weight of the solids in the hopper is counterbalanced by a poise weight, which travels on the scale beam and is retracted at a constant rate. The controller modulates the speed of the rotary feeder so as to maintain the rate of retraction of the poise weight constant. The balance of the beam is maintained by increasing the rate of solids discharge if the weight of solids in the hopper exceeds that of the poise weight or decreasing the rate if it does not. Instead of a rotary feeder, the modulated control device can be a rotary screw feeder or a vibratory feeder. The loss-in-weight systems are suitable for handling liquids and slurries as well as solids, because the weight-sensing section of the system is a tank or silo rather than a horizontal belt surface, which is open on all sides. Manufacturers of such units claim that if the delivery time period is short, their feeder gives better precision than other continuous feeders, because in their case the weight is measured ahead of the solids discharge device. Therefore, if an error in flow rate exists, it is corrected before the material leaves the feeder and enters the process. Continuous Operation In this configuration, the supply hopper or tank is suspended off one or more load cells. Tension cells are preferred to minimize the errors caused by nonsymmetrical loading. The controller detects the weight sensed by the load cell(s) and subtracts it from its setpoint, which is generated by a programmer. In other words, the programmer generates a signal corresponding to a fixed reduction rate of the total weight in the hopper, and this

signal becomes the setpoint. The difference between the weight of material in the hopper and the programmed setpoint weight is continually sensed, and the flow rate of the material exiting from the hopper is regulated to keep them in balance. The hopper must be periodically refilled, and this filling cycle must be initiated before the hopper is completely empty. Consequently, a “heel” always remains in the hopper and serves to minimize the shock on the load cells at the beginning of the filling cycle. The filling operation is controlled by a differential gap controller and a material supply valve, gate, or feeder (not shown in Figure 2.23p). When the weight of material in the hopper drops to the preset “heel” weight, the differential gap controller starts the filling cycle and at the same time either “locks” the discharge flow regulating device in its last position or closes it. When hopper weight reaches a high limit (corresponding to the filled condition), the differential gap controller stops the filling cycle and restarts the feeding cycle by returning control of the discharge regulator to the loss-in-weight control system. During the filling cycle, the feeding system is operating on a volumetric rather than on a gravimetric basis; hence, filling is accomplished as rapidly as possible. It is desirable to design these system such that the refill cycle is a small portion of the total cycle time. Equipment Hermetically sealed load cells are used that withstand not only dust and corrosion but are also compensated for temperature and barometric pressure changes. To withstand shock loading, the load cells should also be designed to withstand overloads of 150% of rating or more. If straingauge-type load cells are used, their power supplies should not only be closely regulated, but they should also be compensated for supply voltage variations. For loss-in-weight applications, tension-type cells are preferred, because the compression-type strain gauge load cells are sensitive to side load forces, which can be generated either by thermal expansion of the structure or by nonsymmetrical hopper loading.

% Full Weight

Weighing Hopper Mounted on a Scale

Refill

Refill Control

100

Stop Refill

80 60 40 Scale Beam

Poise Drive Rotary Feeder To User

Variable Speed Positioner

20

Start Refill

0

Rate = 1% Per Minute

0

FIG. 2.23p Continuous loss-in-weight feeder.


30

60

90

120

150

180

210

Time (Min.)


The weigh hoppers are often supplied by the user rather than by the supplier of the loss-in-weight feeding system. Their design criteria should not only include capacity and structural strength considerations but should also aim for minimum weight, because the tare weight should be minimized for maximum weighing sensitivity. The material discharge regulator can be a control valve if the material is a liquid or slurry. Solids can be controlled by a rotary vane, belt, or vibrating feeders or by positioned knife gate valves. The choice is based on the required feed rate and on the physical characteristics of the process material. System Sizing In designing a loss-in-weight feeder system, the most important component is the hopper or tank. On the one hand, the hopper should be as large as possible, because the larger the hopper, the longer will be the running cycle and less frequent the filling cycle. On the other hand, for a particular feed rate (loss-in-weight rate), the system accuracy will decrease as the weight of the hopper and its contents increases. Therefore, a compromise is needed between these conflicting considerations. It is recommended that the hopper be sized to hold the equivalent of about 15 min of discharge or approximately 15 times the maximum pounds-per-minute flow rate. The “heel” should equal 1/3 of the total hopper capacity, and the size of a charge during a refill cycle should be set to 2/3 of the total hopper capacity. The refill cycle should be completed in about 1 min or in less than 10% of the total cycle time.

A B

A B

Empty A Fill B

CONCLUSION The loss-in-weight feeders are not truly continuous weight rate control systems, because the gravimetric rate control is interrupted during the refill cycle. As a consequence, high accuracy totalization of the charge is not possible, although counters are available to indicate the number of times the hopper has been refilled. The loss-in-weight systems are not used to feed easy-tohandle, free-flowing materials, because the belt-type gravimetric feeders are less expensive and suited for those applications. Loss-in-weight systems are usually considered for hard-to-handle liquid and slurry services. When no flowmeter or metering pump is available to detect or control the flow of a highly viscous, nonconductive, corrosive, or abrasive liquid, it is then that they are considered, and many highly satisfactory applications have been reported.

DUAL-CHAMBER GRAVIMETRIC FEEDER The feeder illustrated in Figure 2.23q consists of two independently weighed hoppers. While the solids are being discharged from weigh hopper A, hopper B is being filled by the feed of fresh solids. When chamber B has filled up to its target weight (while the weight of hopper A is tared off), the feed is switched to hopper A, and hopper B is weighed prior to its contents being discharged into the process. Once chamber B has been weighed, its contents are discharged into the

A B

Fill B to Target Weight Tare Off A

FIG. 2.23q Dual-chamber gravimetric feeder. (Courtesy of Technicon Industrial Systems.)


329

A B

Switch Feed to A Weight B, Add Weight to Total

Empty B Fill A

330

Flow Measurement

Junction Box Load Cell

Cover Material Flow

Horizontal Force

Load Cell Sensing Plate Dimensions for 100/150 (4"/6") Model Nominal Sizing Only

FIG. 2.23r Cylindrical impulse flow element. (Courtesy of Milltronics Inc.)

process. After each discharge, the corresponding weight is added to the total weight that has previously been discharged. The weighing cycle shown in Figure 2.23q is computer controlled. The only moving parts of the system are the diverter at the top and the two discharge gates at the bottom of the chambers. Because the hoppers are relatively small, their contents can be weighted accurately. The measurement error is usually about 0.5% of actual flow. Because the chambers are filled and emptied on a cycle period of around a minute, the discharged solids flow appears to be almost continuous. Where space is limited, the small size and vertical flow pattern of the equipment can also be of advantage. This dual-chamber gravimetric feeder is suited for the measurement of free-flowing bulk solids and can be utilized as a continuous solids flowmeters or as batch recipe executors.

DYNAMIC SOLIDS FLOWMETERS Whereas the previously discussed devices measure the flow rate while the solids are stationary on a belt or in a hopper, the devices described here measure the flow of falling or moving solid streams. These units detect either the forces needed to initiate the dynamic state by accelerating the solids or the forces resulting from the impact of the falling solids. Impulse-Type Solids Flowmeter When a stream of solids strikes a plate or a cylindrical surface at an angle, the resulting horizontal force component relates to its mass flow rate. The flowmeter illustrated in Figure 2.23r operates on the basis of this principle. The meter housing is manufactured from steel or stainless steel, and the sensing plate is made out of stainless steel. The units can handle freeflowing powers or granular and pelletized solid materials of up to 0.5 in. particle size.


The manufacturer claims both a very high sensitivity and wide rangeability (100:1). The smallest capacity unit is claimed to have a range of 300 to 30,000 lb/h (130 to 13,000 kg/h), and the largest unit can handle flows up to 650,000 lb/h (300,000 kg/h). The standard units can be operated at 140°F (60°C) temperature, but special units are available for operation at up to 450°F (232°C). Metering precision is claimed to be 1% of full scale. (If full scale is defined as the maximum flow the unit can handle, then at maximum turn-down, a unit with 100:1 rangeability will experience 100% error.) Microprocessor-based computer controls are available to integrate this flowmeter into batching or other automated material handling systems. The principles of impulse and momentum detection have been used in liquid flowmeters such as the target, drag-body, and angular momentum designs. Their operation is based on Newton’s second law of motion and on the conservation of momentum. These principles have also been successfully applied to solids flow measurement. Figure 2.23s illustrates a design in which solid particles fall by gravity on a calibrated spring-loaded plate, the displacement of which is a function of the mass flow rate of the solids. A position transmitter is used to continuously detect the force caused by the falling particles. Both of these solids flow transmitters (Figures 2.23r and 2.23s) can be used in continuous weighing applications. They can also be used in flow monitoring and control applications for batch or continuous services. Almost all types of solids can be measured by impulse-type flowmeters, including sugar, salts, cement, and ores. Accelerator-Type Flowmeter In this design, the solids stream enters the “accelerator” section of the meter by gravity (Figure 2.23t). The accelerator is driven at constant speed and, as the entering solids are


Particle Mass (m)

331

To Transmitter or Counter

h Air Purge

B FH

FV

F

0

0 FV

FH

0F

The horizontal component of the impact force on the plate is directly proportional to the flow rate of material over the plate

FIG. 2.23u Volumetric solids flow detector.

FIG. 2.23s Impulse flowmeter. (Courtesy of Endress+Hauser Inc.)

flowmeter is fairly high (25:1), but so is the measurement error (around 2% FS). Volumetric Flowmeters

Accelerator

The designs of volumetric solids flowmeters include the positive-displacement screw impellers, but if reasonable accuracy is desired, they should only be used to measure uniformsize solids such as lead shot. The operation of this type of instrument is similar to that of the turbine- or propeller-type liquid flowmeters except that a helical vane is used instead of the turbine. As the flow of the falling granular material rotates the vane, a flexible cable transmits the rate of rotation to a counting mechanism mounted outside of the pipe or duct (see Figure 2.23u). This counting device can be a mechanical counter mounted directly onto the piping, or it can be a transmitter for remote monitoring and/or control. In this design, the transmitter output is determined by the position of a slotted cam. The cam is positioned by the balance between two rotations: the rotary motion produced by a synchronous motor and the rotary motion of the flexible cable. The vane element is installed in the vertical position, and its bearing surfaces are protected by an air purge. To obtain acceptably accurate flow measurement, the instrument must be calibrated using the same process material, which it will measure after final installation. The measurement error of this flowmeter is around 3% of full scale, and its rangeability is about 10:1.

FIG. 2.23t Accelerator-type solids flowmeter.

CROSS-CORRELATION SOLIDS FLOWMETERING accelerated, they create a corresponding torque on the motor. Variations in this torque are detected by a torque transducer and amplified so that the transmission signal becomes directly proportional to the mass flow rate of solids. The unit is designed for use on a wide range of materials, including powders, granules, pellets, and irregular solids as well as liquid slurries. The measurement rangeability of this


The concept of cross-correlation is based on tagging, which is the oldest of all flowmetering techniques. It consists of injecting some particles, a dye, a chemical, a radioactive material, or a pulse of any other form and measuring the time it takes for such a tag to travel a known distance. Cross-correlation flowmeters also detect the time of transit but, instead of tagging the process

332

Flow Measurement

Name Plate 5.3" (135 mm) 3.3" (84 mm)

5 To 10 × Dia. (Internal) Outlet Run

3.7" (94 mm) Electronics Cable Connection 20.4" (52mm)

FIG. 2.23w Microwave solids flow switch. (Courtesy of Endress+Hauser Inc.)

10 To 20 × Dia. (Internal) Inlet Run

FIG. 2.23v Solids flowmeter of the cross-correlation type. (Courtesy of Endress+Hauser Inc.)

fluid, they look at a noisy process variable and detect the time of travel of the recognizable noise pattern. If the noise pattern exists long enough to pass both detectors, computers are fast enough to recognize and interpret the readings. The general subject of cross-correlation flowmetering is covered in more detail in Section 2.5. Here, we concentrate only on solids flow detection. A variety of sensors have been evaluated for this application, including gamma radiation, ultrasonic, and photometric designs. Figure 2.23v shows a solids flow detector that employs capacitance sensors. While further development is needed before reliable performance data can be reported, this metering technique does have potential, because it is not limited by hostile environments or by the characteristics of the solids being metered.

SOLIDS FLOW SWITCHES Solids flow switches are used to detect abnormal flow conditions that result from either a flow or a no-flow condition. These can include detection of plugging or blockages, loss of feed, bridging in bins, overflowing of cyclones, rupture of bag filters, and the like. These switches should be both inexpensive and sensitive, because the amount of flow resulting from, for example, a bag rupture is not substantial. One solids flow switch (the Triboflow) that can detect such flows consists


of a probe that collects the static charges of solid particles passing over its surface. Microwave flowmeters of the continuous type are used to measure the flow of pulverized solids such as coal. Microwave switches detect the flow of solids by detecting only the motion or the absence of it. In a microwave motion detector, the transducer emits a 24-GHz signal into the flowing solid stream and analyzes the reflected frequency (Doppler effect) to determine the speed of the moving object that reflected it. The switch sensitivity is adjustable, so it may be used to trip at a velocity as low as 6 in./min (15 cm/min) when the pipe is full or at a velocity of one particle every 5 sec in a freefalling, gravity flow system. Units are available in aluminum or stainless steel. They can be connected to a pipe by a coupling or flange (Figure 2.23w) or can look through windows or nonmetallic walls without any openings. The units are intrinsically safe and can be used at working pressures up to 15 PSIG (1 bar). The switch can also observe motion at a distance of several feet from the detector and can tolerate the buildup of 0.5 in. of nonconductive coating or 0.1 in. of conductive coating.

MASS FLOW MEASUREMENT OF PULVERIZED COAL To obtain the mass flow of pulverized coal being transported to a burner, one needs to know both the concentration (in mass density) and the velocity of the coal in the burner pipe. The main advantage of the technique described below is that it does not require in situ calibration, the use of isokinetic sampling, or rota-probing. Detecting Mass Concentration The concentration of the pulverized coal is measured using low-power, low-frequency microwaves, with each burner’s pipe functioning as its own unique waveguide. Since the coal flow in all pipes served by the same mill has the same fuel source, variables such as moisture content, fineness, coal type, and so on are the same for all pipes. Therefore, the only


Reflector Rods

333

Sensors

Coal Flow

λ/4

λ

FIG. 2.23x Standard Sensor and Rod Arrangement. (Courtesy of Air Monitor.) Sensor Distance 40 − 60 cm

Coal Flow Transmitter

Receiver Signal 2 y(t) = x(t−T)

Signal 1 x(t)

Cross Correlation Method

PF Velocity = Distance ∆t

FIG. 2.23y Cross-correlation configuration. (Courtesy of Air Monitor.)

pipe-to-pipe variable is the dielectric load, i.e., the concentration of the pulverized fuel in the section of pipe being measured. Starting with the measured microwave transmission characteristic of each empty pipe, variations in the dielectric load caused by changing coal concentration produce corresponding shifts in measurement frequency, resulting in quantifiable values that are reported as the absolute coal density in each pipe. The concentration measurement is performed by two sensors aligned parallel with the longitudinal axis of the pipe; one functions as the microwave transmitter, and the other operates as the receiver, as shown in Figure 2.23x. Located upstream and downstream from the sensors are pairs of reflector rods—abrasion resistant, electrically conductive rods that prevent the microwave signal from leaving the measurement area and then being reflected back in the form of microwave noise. Measuring the Coal Velocity The velocity of the pulverized coal is measured by the crosscorrelation method, which is conceptually depicted in


Figure 2.23y. The same two sensors used for the measurement of coal concentration have a known separation distance. Stochastic signals created on the pair of sensors by the charged coal particles are nearly identical but are shifted by the time the pulverized coal needs to get from one sensor to the other. As the distance between the sensors is fixed, the velocity of the pulverized coal in the pipe can be accurately calculated. Bibliography AWWA Standard for Quicklime and Hydrated Lime, American Water Works Association, New York, 1965. Baker, R. C., Flow Measurement Handbook, Cambridge University Press, UK, 2000. Beck, M. S. and Plaskowsk, A., Measurement of the mass flow rate of powdered and granular materials in pneumatic conveyors using the inherent flow noise, Instrum. Rev., November 1967. Colijn, H. and Chase, P. W., “How to install belt scales to minimize weighing errors,” Instrum. Tech., June 1967. Cross, C. D., Problems of belt scale weighting, ISA J., February 1964. Cushing, M., The future of flow measurement, Flow Control, January 2000. Digitally controlled coal weigh feeder, Power Eng., 1978. Eren, H., Flowmeters, in Survey of Instrumentation and Measurement, S. A. Dyer, Ed., John Wiley & Sons, New York, 2001. The Flowmeter Industry, 3rd ed., Venture Development Corp., Natick, MA, 1991. Grader, J. E., Controlling the flow rate of dry solids, Control Eng., March 1968. Jenicke, A. W., Storage and Flow of Solids, Bulletin 123, Utah Engineering Experiment Station, University of Utah, Salt Lake City, UT, 1964. Johanson, J. R. and Colijn, H., New design criteria for hoppers and bins, Iron and Steel Eng., October 1964. Kirimaa, J. C. J., Cross-Correlation for Pulp Flow Measurement, ISA/93 Conference, Chicago, IL, September 1993. Linn, J. K. and Sample, D. G., Mass Flow Measurement of Solids/Gas Streams Using Radiometric Techniques, Report SAND-82–0228C, U.S. Department of Energy, Washington, DC, 1982. Lipták, B. G., Flow measurement trends, Control, June 2000. Mass, force, load cells, Meas. Control, October 1991. McEvoy, L. D., Control systems for belt feeders, InTech, February 1968. Mersh, F., Speed and Flow Measurement by an Intelligent Correlation System, Paper #90–0632, 1990 ISA Conference, New Orleans. Miller, R. W., Flow Measurement Engineering Handbook, 3rd ed., McGrawHill, New York, 1996. Nolte, C. B., Solids flow meter, Instrum. Control Syst., May 1970. Solids flowmeter works without obstructing flow, Chem. Eng., September 1972. Spitzenberger, R. M., Long-term accuracy of digital weigh feeders, Chem. Process., April 1974.

334

Flow Measurement

Spitzer, D. W., Flow Measurement, 2nd ed., ISA, Research Triangle Park, NC, 2001. Stepanoff, A. J., Gravity Flow of Bulk Solids and Transportation of Solids in Suspension, John Wiley & Sons, New York, 1969. Van den Berge, H., Weighing on-the-fly keeps the process moving, Cont. Eng., 23(9), 52.


Vines, G. L., Digital weigh feeders automate refractory production, Brick & Clay Record, June 1974. Yoder, J., Flowmeter shootout, part I and II: new technologies, Control, February and March 2001. Zanetti, R. R., Continuous proportioning for the food industry, Instrum. Tech., March 1971.

2.24

Target Meters

FT TARGET

W. H. HOWE (1966, 1982) W. H. BOYES (2003)

B. G. LIPTÁK

(1995) TO RECEIVER Flow Sheet Symbol

Design Pressure

Up to 3000 PSIG (20.70 MPa) with strain-gauge type

Design Temperature

Up to 300°F (150°C) through 600°F (315°C) with strain-gauge type; special units up to 1200°F (649°C)

Sizes

0.5 to 8 in. (12.5 to 203 mm) with standard and up to 48 in. (1.2 m) pipes with the probe design

Fluids

Liquids, gases, and steam; also handles two-phase flows

Flow Range

From 1 GPM (3.785 l/min), 1 SCFM (28 l/min), 3 lb/h (1 kg/h) to practically any value using the probe design

Inaccuracy

±0.5% of full scale for standard and to ±5% of full scale for probe type


Usually carbon or stainless steel; PVC targets and internals available

Cost

$1500 to $10,000 as a function of size, design, and materials of construction

List of Suppliers

Aaliant Division of Venture Measurement (www.venturemeas.com) J.W. Sweet Co.

Material buildup in front of orifice plates can cause both measurement errors and plugging when the process stream is a liquid slurry or a gas carrying wet solids. The annular orifice and the target flowmeter were introduced to solve this problem by providing an annular opening for the solids to pass through. This design is no longer marketed, and the only type of target flowmeter sold today is the drag-body-type target meter. The drag-body target flowmeter (Figure 2.24a) detects the impact forces produced by the flowing fluid by means of strain-gauge circuitry. This unit is available in standard configurations (Figure 2.24b) and is also available in retractable probe designs (Figure 2.24c), which are used in larger pipe sizes in which it is desirable to withdraw the sensor periodically for cleaning without opening the process line. The target meter is applied in a number of fields for measurement of liquids, vapors, and gases. It allows unimpeded flow of condensates and extraneous material along the bottom of a pipe while allowing unimpeded flow of gas or vapor along the top of the pipe. It has given consistent, dependable service on “difficult” measurements such as hot, tarry, sediment-bearing fuels to a pipe still where no other headtype meter has proved successful. There are no differentialpressure connections to “freeze.” This is useful in steam flow

Strain Gage Force Transducer Force Lever Arm

Flexure

Pipe Drag Disc

Flow (Force)

FIG. 2.24a The drag-body flowmeter.

measurement in exposed locations and for liquids that congeal at ambient temperature in pressure connections. Units are available for service up to 700°F (371°C), which is useful in steam service up to 200 PSIG (14 bars) pressure. 335


336

Flow Measurement

DRAG-BODY DESIGN

Flanged 37° Flare Tube

Wafer

MNPT

FIG. 2.24b Standard target meter configurations.

Drag-body targets are empirically derived, and a wide variety of sizes and shapes are available, according to the suppliers. Combined with wide-range force measurement transducers, a wide selection of full-scale flow rates is provided. The manufacturers provide calibration data. The flow range through a particular-size meter can be varied by changing the target size and by replacing or readjusting the transducer. Repeatability of output is good. Calibration accuracy includes not only the uncertainty of the primary element but also the characteristics of the transducer and the precision of the transducer adjustment. As is the case with some other proprietary devices, test data is unavailable for determination of flow coefficients from physical dimensions for different process fluids and operating conditions. On the other hand, target meters with accurate water flow calibration over almost any range of Reynolds numbers can be obtained. Transfer characteristics to other fluids based on Reynolds number are reliable. Because the transducer and the primary element are calibrated as a unit, overall accuracy of calibrated target meters is better than that of orifice-type systems.

Bibliography

Fixed Retractable

FIG. 2.24c The insertion target flowmeter.


Aaliant Div. of Venture Measurement, Mark, V., Target Strain Gauge Flow Meter Installation, Operation and Maintenance Manual M711, Revision B, 2001. Blasso, L., Flow measurement under any conditions, Instrum. Control Syst., February 1975. Cushing, M., The future of flow measurement, Flow Control, January 2000. Eren, H., Flowmeters, in Survey of Instrumentation and Measurement, S.A. Dyer, Ed., John Wiley & Sons, 2001, 568–580. The Flowmeter Industry, 1985–1990, 2nd ed., Venture Development Corp., Natick, MA, 1986. Hall, J., Solving tough flow monitoring problems, Instrum. Control Syst., February 1980. Miller, R. W., Flow Measurement Engineering Handbook, 3rd ed., McGrawHill, New York, l996. Spink, L. K., Principles and Practice of Flow Engineering, 9th ed., The Foxboro Co., Invensys Systems, Inc., Foxboro, MA, 1967. Spitzer, D. W., Flow Measurement Practical Guide Series, 2nd ed., ISA Press, Research Triangle Park, NC, 2001. Stapler, M., Drag-Body flowmeter, Instrum. Control Syst., November 1962.

2.25

Turbine and Other Rotary Element Flowmeters J. G. KOPP (1969) J. B. ARANT

D. J. LOMAS

(1982)

B. G. LIPTÁK

(1995)

FI FE

Flow Sheet Symbol

(2003)

Types

A. Turbine flowmeters A-1. Single-Rotor A-2. Dual-Rotor B. Propeller, impeller, and shunt-flow types C. Insert, probe, or paddlewheel designs

Services

Relatively clean liquids, gases, and vapors (some units for gas service are also covered in Section 2.2)

Sizes

A-1. 3/16 to 24 in. (5 to 610 mm) in flow-through designs A-2. 0.25 to 12 in. (6.12 to 294 mm) in flow-through designs B. Impeller designs available from 3 to 72 in. (75 mm to 1.8 m) C. Paddlewheel units available for up to 12 in. (305 mm) pipes; insertion turbine probes not limited by pipe size, can also be used in open channels

Outputs

Generally, linear frequency outputs are provided, but 4- to 20-mA DC can also be obtained through conversion

Operating Pressure

A-1. 1500 PSIG (10.3 MPa) in standard and 5000 PSIG (34.5 MPa) in special designs A-2. ANSI 150 PSIG (1.03 MPa) up to ANSI 1500 PSIG (10.3 MPa) B. Impeller designs usually designed for 150 PSIG (1 MPa) C. Plastic paddlewheel units operable up to 200 PSIG (1.4 MPa) at ambient temperatures

Pressure Drops

A. Usually, one velocity head or about 3 to 5 PSIG (20 to 35 kPa) B. Usually less than 1 PSID (7 kPa) for the impeller types C. Negligible


A-1. −58 to 300°F (−50 to 150°C) in standard and −328 to 840°F (−200 to 450°C) in extended pickup designs A-2. −440 to 840°F (−268 to 450°C) B. Up to 160°F (71°C) for the impeller design C. The plastic paddlewheel units operable at up to 220°F (105°C) if operating pressure is 100,000. Flow nozzles are used at high pipeline velocities (100 ft/s or 30.5 m/s), usually corresponding to Re > 5 million. Critical flow venturi nozzles operate under choked conditions at sonic velocity.

Costs

Flow nozzles are less expensive than venturi or flow tubes, but cost more than orifices. ASME gas flow nozzles in aluminum for 3- to 8-in. (75- to 200-mm) lines cost from $300 to $1000. Epoxy-fiberglass nozzles for 12- to 32-in. (300- to 812-mm) lines cost from $1000 to $3000. The relative costs of Herschel venturies and flow tubes in different sizes and materials are given below: 6-in. Stainless Steel

8-in. Cast Iron

12-in. Steel

Herschel venturi

$9000

$6000

$6500

Flow tube

$4000

$2500

$3200

2.29 Venturi Tubes, Flow Tubes, and Flow Nozzles


ABB Automation Instrumentation Division (www.abb.com/us/instrumentation) (B) ABB Water Meters Inc. (www.jerman.com/abbmeter.html) (B) Badger Meter Inc. (www.badgermeter.com) (A, B) BIF Products (A, B, C) Daniel Measurement and Control (www.danielind.com) (A, C) Flow Technology Inc. (www.ftimeters.com) (A) Fluidic Techniques Inc. (A) Preso Industries (A, B) Primary Flow Signal Inc. (A, C) Tri-Flow Inc. (A) West Coast Research Corp. (www.members.aol.com/wescores) (A)

Venturi tubes, flow nozzles, and flow tubes, like all differential pressure producers, are based on Bernoulli’s theorem. General performance and calculations are similar to those for orifice plates. In these devices, however, there is continuous contact between the fluid flow and the surface of the primary device, in contrast to the pure line contact between the orifice plate edge and main flow. The surface finish of the devices can have some effect on the meter coefficient, although the venturi tube has a relatively constant coefficient, seldom varying more than a fraction of 1%. Modern precision manufacturing techniques allow much greater accuracy of the coefficient for venturi tubes and flow nozzles computed from dimensions, and the coefficients are only moderately less reliable than those for orifice plates. The C (meter coefficient) values for venturi tubes and flow nozzles have been well established by years of test data and are tabulated in sources such as the handbook called Fluid 1 Meters—Their Theory and Application. In general, this is not true of the proprietary flow tubes, and flow calibration is required to establish the actual meter coefficient. Meter coefficients for venturi tubes and flow nozzles are approximately 0.98 to 0.99 and for orifice plates average about 0.62. Therefore, almost 60% (98/62) more flow can be obtained through these elements for the same differential pressure.

THE CLASSIC VENTURI The venturi tube, as designed by Clemens Herschel in 1887 and described in Reference 1, is shown in Figure 2.29a. It consists of Convergent Entrance Cylindrical Inlet

Throat Divergent Outlet

Annular Chambers

FIG. 2.29a 1 Classic Herschel venturi with annular pressure chambers.


375

• •

• •

A cylindrical inlet section equal to the pipe diameter A converging conical section in which the crosssectional area decreases, causing the velocity to increase with a corresponding increase in the velocity head and a decrease in the pressure head A cylindrical throat section where the velocity is constant so the decreased pressure head can be measured A diverging recovery cone where the velocity decreases and almost all of the original pressure head is recovered

The unrecovered pressure head is commonly called head loss. The classic venturi is always manufactured with a castiron body and a bronze or stainless-steel throat section. At the midpoint of the throat, six to eight pressure taps connect the throat to an annular chamber so that the throat pressure is averaged. The cross-sectional area of the chamber is 1.5 times the cross-sectional area of the taps. Because there is no movement of fluid in the annular chamber, the pressure sensed is strictly static pressure. Usually, four taps from the external surface of the venturi into the annular chamber are made. These are offset from the internal pressure taps. Throat pressure is measured through these taps. This flow meter is limited to use on clean, noncorrosive liquids and gases, because it is impossible to clean out or flush out the pressure taps if they clog up with dirt or debris. The flow coefficient for the classic venturi is 0.984, with an uncertainty tolerance of ±0.75%. SHORT-FORM VENTURIES In the 1950s, in an effort to reduce costs and laying length, manufacturers developed the second-generation, or short-form, venturi shown in Figure 2.29b. There were two major differences in this design. The internal annular chamber was replaced by a single pressure tap or, in some cases, an external pressure averaging chamber, and the recovery cone angle was increased from 7 to 21°. The short-form venturi can be manufactured from cast iron or welded from a variety of materials as compatible with a given application. The flow coefficient for the short-form venturi is 0.985, with an uncertainty tolerance of ±1.5%. The pressure taps are located one-quarter to one-half pipe diameter upstream of the inlet cone and at the middle of the throat section. A piezometer ring is sometimes used for differential pressure measurement. This consists of several holes

Flow Measurement

in the plane of the tap locations. Each set of holes is connected in an annulus ring to give an average pressure. Venturies with piezometer connections are unsuitable for use with purge systems used for slurries and dirty fluids, because the purging fluid tends to short circuit to the nearest tap holes. Piezometer connections are normally used only on very large tubes or where the most accurate average pressure is desired to compensate for variations in the hydraulic profile of the flowing fluid. Therefore, when it is necessary to meter dirty fluids and use piezometer taps, sealed sensors that mount flush with the pipe and throat inside wall should be used. These sensors function as independent measuring devices at each tap connection, yet they function together to read differential pressure only while automatically compensating for static pressure changes within the pipe. Single-pressure-tap venturies can be purged in the normal manner when used with dirty fluids. Because the venturi tube has no sudden changes in contour, no sharp corners, and no projections or stagnant areas, it is often used to measure slurries and dirty fluids that tend to build up on or clog other primary devices. Venturies are built in several forms. These include the standard long-form or classic venturi (Figure 2.29a), a modified short form where the outlet cone is shortened (Figure 2.29b), an eccentric form (Figure 2.29c) to handle mixed phases or to minimize buildup of heavy materials, and a rectangular form (Figure 2.29d) used in ductwork. If a rectangular venturi is substantially square, it is customary to converge–diverge all four sides with angles the same as for the circular form. Where duct width differs from height, the High Pressure Tap Low Pressure Tap

Throat Inlet

Inlet Cone

FIG. 2.29b Short-form venturi tube.

FIG. 2.29c Eccentric venturi tube.


Outlet Cone

FIG. 2.29d Rectangular venturi tube. 30 Loss % Differential

376

15°

20

Out

let C

one

7° Outlet Cone

10

0 0.2

0.3

0.4

0.5 Beta

0.6

0.7

0.8

FIG. 2.29e Venturi pressure loss.

short sides are kept parallel, with the long sides converging– diverging. A converging angle of 21° and a diverging angle of 15° give satisfactory operation. Throat length should be equal to minimum throat height or width, whichever is smaller. Tap locations are the same as for the circular form. The angle of convergence, which may range from 19 to 23°, is the classical value established by Herschel in 1887. This angle is not particularly critical, and 21 ± 1° is commonly used. The recovery cone provides pressure recovery with its smooth flow transition. The classic long cone form is 7.5° ± 0.5° on the divergence, but up to 15° is allowed, and the sharper angle allows the short-form version to be fabricated. The 15° outlet cone sacrifices a modest amount of pressure recovery (Figure 2.29e). The venturi pressure loss of 10 to 25% is the lowest of the standard primary head measurement elements. The long-cone form develops up to 89% pressure recovery at 0.75β ratio, decreasing to 86% at 0.25β ratio. The short-cone form develops up to 85% recovery at 0.75β, decreasing to 75% at 0.25β ratio. As an example of the power savings to be obtained in an energy-short era, an added pressure recovery of 50 in. (1270 mm) H2O differential pressure can represent a 10-HP savings in a 24-in. (610-mm) water line flowing at a velocity of 6 ft/s (1.829 m/s). For a comparison of various head-meter elements from the pressure recovery point of view, see Figure 2.29f.

10

80

20

A

70

30 ASME Flow Nozzle

60

40

50

50

40

60

30 20

70

Standard Venturi

10

Low Loss Venturi 0.1

0.2

0.3

Two Elbows not in Same Plane B

Partially Throttled Gate Valve, Globe Valve, Check Valve

12 Diam. B

Open Wide alve V Gate

Venturi

80

Long Form Venturi

e

Same Plan

Elbows in

Venturi

Orifice Plate Recovery-Percent of Differential

Unrecovered Pressure Loss-Percent of Differential

Pressure Loss Curves 90

er

uc

A 90

d Re

B

Proprietary Flow Tube 0.4 0.5 0.6 0.7 Beta (Diameter) Ratio

0.8

0.9

FIG. 2.29f Pressure loss curves.

0.40

0.60

0.80 0.40 Diameter Ratio, β

r

ande

Exp

0.60

377

6 4 2 0 26 24 22 20 18 16 14 12 10 8 6 4 2 0 10 8 6 4 2 0



0.80

FIG. 2.29g Venturi piping requirement.

Installation A venturi tube may be installed in any position to suit the requirements of the application and piping. The only limitation is that, with liquids, the venturi is always full. In most cases, the valved pressure taps will follow the same installation guidelines as for orifice plates. Upstream piping should be as long as needed to provide a proper velocity profile (Figure 2.29g). However, in most installations, shorter upstream piping is required than for orifices, nozzles, or pitot tubes, because the venturi hydraulic shape itself provides some flow conditioning. Often, the combined length of a venturi and its upstream piping is less than the overall amount of piping required for an orifice or nozzle. Figure 2.29h shows typical upstream pipe diameters required for various elements at 0.7β ratio and one elbow upstream. Straightening vanes can be used upstream to reduce the inlet pipe length. 1 In Fluid Meters, the ASME recommends the use of tubular straightening vanes (19 tubes and 2 diameters long) upstream of the venturi to reduce the inlet pipe length. The vane installation should have a minimum of 2 diameters upstream and 2 diameters downstream before entering the venturi. There is no limitation on piping configuration downstream of the venturi except that a valve should be no closer than two diameters. Valves on other devices that protrude into the flow stream should not be mounted upstream of the venturi, if possible. Flow Calculations The American Society of Mechanical Engineers Fluid Research Committee has adopted a general coefficient of discharge of 0.984 for the classic rough-cast entrance cone


Orifice Plate or Flow Nozzle

Flow

ASME Type Standard Form Venturi

Low Loss Venturi

Proprietary Flow Tube Type

1 2 3

4 5 6 7

8 9 10 11 12 13 14 15 16 17 18 19 20 Pipe Diameters

FIG. 2.29h Typical installation piping comparison.

venturi tube from 4 in. (100 mm) through 32 in. (813 mm) and for β ratios between 0.3 and 0.75. For tubes with machined entrance cones, the general coefficient is 0.995. The Reynolds number must be 200,000 or greater. Approximate flow rates can be calculated from a working equation,

W = 353d 2

hρ 1− β4

2.29(1)

378

Flow Measurement

and, for approximate venturi tube design,

β=

1 4 1 + 125, 000hρD W2

2.29(2)

4

For Reynolds numbers between 50,000 and 200,000, substitute 344 for 353. Below 50,000, reliable data are not available. It should be noted that, it contrast to an orifice, a decrease in Reynolds number results in a decrease of flow corresponding to a given differential pressure. Other correction factors, such as temperature coefficient of expansion, gas expansion factor, and so on, are similar to those for orifices and flow nozzles.

FLOW TUBES There are several proprietary primary-head-type devices that have a higher ratio of pressure developed to pressure lost than a venturi tube (Figure 2.29i). They are all considerably more compact than the classical venturi tube, with its long recovery cone, although the short-from venturi can come close to some types of these tubes. These designs are available in cast iron, can be welded from various materials, and in some cases can have insert-type units in fiberglass-reinforced plastic or metal. The flow coefficient ranges from 0.9797 for an all-static-tap “near venturi” design to 0.75 for an all-corner-tap “flow tube” design. All of these proprietary units are available in the United States except the Dall tube, which was developed in England. All of these tubes vary in contour used, tap locations, and differential pressure and pressure loss for a given flow. All have

B.I.F. Universal Venturi

Badger Lo-Loss Flow Tube

FIG. 2.29i Proprietary flow tubes.


Dall Tube

Gentile or Bethlehem Flow Tube

a laying length less than 4 diameters long. The shortest are the corner tap designs, with lengths equaling 2 to 2.5 diameters. A flow tube is broadly defined by the ASME as any differential-pressure-producing primary whose design differs from the classic venturi. Flow tubes fall into three main classes, depending on the hydraulic position of the inlet and throat pressure tap. Type 1 has static pressure taps at both the inlet and outlet, Type 2 has a corner tap in the inlet and a static tap in the throat, and Type 3 has a corner tap at both the inlet and outlet. The classic venturi had static pressure taps that provided a section in which the velocity is not changing direction and is parallel to the pipe wall. A corner tap senses pressure in a section where the velocity is changing direction and is not parallel to the pipe wall. Figure 2.29i shows examples of several flow tubes. Type 3 flow tubes can be useful in larger sizes because of their shorter lay length, but they may also require longer upstream pipe runs for proper performance. They can be subject to coefficient change due to variations in Reynolds number, line size, and beta ratio; manufacturers can provide data on these effects. The B.I.F. Universal Venturi is the product (Type 1) that most closely approaches the Herschel design classic venturi. The inlet cone has two vena contracta angles that condition the fluid as it enters the throat. This is claimed to reduce the sensitivity to upstream piping configuration and give higher accuracy. Also claimed are a stable coefficient (0.9797) that is unaffected by internal surface roughness, lower Reynolds number application (90,000), low head loss (4 to 18%), and extensive documentation including expansion factors. Whereas flow tubes can be useful in larger sizes because of their shorter lay length, they may also require longer upstream pipe runs than the venturi for proper performance and thus lose any real advantage. They can be subject to coefficient change with viscosity and Reynolds number; manufacturers can provide data on these effects. None has the smooth contour and resistance to clogging of the venturi meter; however, some are claimed to operate satisfactorily on wastewater and sewage flow measurement. In general, these devices are available in 4-in. (100-mm) and larger sizes up to 48 in. (1219 mm). There is little justification for their use in small-flow, small-pipe applications. In the larger sizes, their installed cost may be less than that of the venturi tube. Accuracy depends basically on the manufacturer’s calibration data. Derivation of the flow coefficient by extrapolation from theory and tests on smaller sizes is much less direct than in the simple structure of the venturi tube; actual flow calibration, particularly in sizes above 24 in. (610 mm), can be difficult and expensive. Although these devices generally have a better pressure recovery than the venturi (expressed as a percentage of the differential), most flow tubes have a lower coefficient of discharge (less efficient). As a result, there is often very little difference in the actual head loss.


In selecting a primary flow element, the possible advantages of slightly lower pressure loss and shorter laying length of the flow tubes should be carefully weighed against the metering accuracy and established flow data available on the Herschel-form venturi. The ASME recommends that, if a proprietary flow tube is used, it should be calibrated with the piping section in which it is to be used and over the full range of flows to which it will be subjected. The only possible exception to this is the Universal Venturi, and it must be carefully evaluated. The background of extensive tests under a wide range of conditions that support orifice meters does not exist for these proprietary devices.

nozzle. The ASME Fluid Meters Research Committee has investigated various configurations and has developed the geometry for these nozzles based on the required beta ratio for the application. High-beta nozzles are recommended for diameter ratios between 0.45 and 0.80. Low-beta nozzles are recommended for diameter ratios between 0.20 and 0.50. For beta values between 0.25 and 0.5, either design may be used. The difference between the two nozzles is basically a flattening of the ellipse in the high-beta-ratio version. The power test code, PTC-6, requires that the low-beta-ratio version be used in their test section for turbine acceptance. Both types of nozzles may be either welded in the pipeline or provided with a holding ring for mounting between flanges. The latter design, shown in Figure 2.29k, is preferred when frequent inspection of the nozzle is required. Nozzles may be manufactured from any material that can be machined; typically, they are fabricated from aluminum, fiberglass, stainless steel, or chrome-moly steel. Modern manufacturing methods and fluid contact surface finishes on the order of 6 to 10 µin result in more predictable nozzle coefficients and highly repeatable data. The standard surface finish is 16 RMS. Flow nozzle inaccuracy of ±1% is standard with ±0.25% flow calibrated. The standard coefficient, as published in Reference 1, is 0.9962 with correction factors for beta ratio and throat Reynolds number. ASME gives an uncertainty of ±−2% for nozzles having a beta ratio between 0.2 and 0.8 5 6 and throat Reynolds numbers between 1 × 10 and 2.5 × 10 .

FLOW NOZZLES There are two types of flow nozzles. The 1932 ISA nozzle is a European design that has not seen use in the United States. A special variation known as a venturi nozzle is a hybrid combination of a 1932 ISA nozzle inlet profile combined with the divergent cone of a venturi tube. The common nozzle used in the United States is the so-called long-radius or ASME flow nozzle. This nozzle comes in two versions, known as low-beta-ratio and high-beta-ratio designs. This flow nozzle, shown in Figure 2.29j, is a metering primary whose shape consists of a quarter ellipse convergence section and a cylindrical throat section. In the United States, the nozzle generally used is the long-radius ASME flow

r2

r2 r1

379

r2

Lt d

D

D

r1

Lt

D3

d

r1 ∆

D

D3

D3

d

Lt T

t

t

t

=

FIG. 2.29j ASME nozzle construction.


45 ° Detail Nozzle Outlet

Low β Nozzle with Throat Taps r1 = d 5/8 d r2 2/3 d Lt = 3/4 d dt = 1 1/4 d t = 1/4 d t2 = 1 1/2" 1/8" ∆ 1/4" T = 1/4 d =

t/2 t

Low β Nozzle β 0.5 r1 = d 5/8 d r2 2/3 d 0.6 d Lt 3/4 d 1/8" t 1/2" 1/8" t2 0.15 D

t2

=

r1 = 1/2D r2 = 1/2 (D − d) Lt 0.6 d OR 1/3D 2t D − (d + 1/8") 1/8" t2 0.15 D

t2 d

=

High β Nozzle β 0.45

10 °

=

t2

dt

380

Flow Measurement

D ± .1D

Flow

D

.5 ± .1D

d

FIG. 2.29k Typical nozzle installation.

Most ASME nozzles are calibrated as meter sections 20 pipe diameters long. As with venturi tubes, the uncertainty of calibrated units depends on the uncertainty of the hydraulic laboratory. Generally, one could expect ±0.25% uncertainty on the calibration. The outlet or discharge side of the nozzle is normally beveled and is one of the more critical points of manufacture. Where the 10° back angle meets the throat bore, the edge must be sharp. Particular care must be taken to avoid taper and out-of-roundness of the throat. Flow nozzles are made in various configurations. The most common is the flange type (Figure 2.29k), but others are the holding-ring type, the weld-in type, and the throat-tap type. Differential pressure measurement taps are commonly located one pipe diameter upstream and one-half pipe diameter downstream from the inlet face (U.S. practice), except for the throat-tap type, which has a special downstream construction. The PTC-6 nozzle uses throat taps to sense the low pressure and standard pipe wall taps for the high pressure. Application Considerations Tap installation precautions are the same as for orifice plates. The preferred installation position for flow nozzles is horizontal, but they can be installed in any position. However, a vertical downflow position is preferred for wet steam or for gases and liquids with suspended solids. In general, upstream and downstream piping requirements are similar to those required for orifices. Because of the width, nozzles installed between flanges are difficult to remove. Common practice is to provide a flange in the downstream piping to allow the nozzle to be removed as part of a spool section for inspection at regular intervals. Sometimes, inspection openings are placed just upstream of the nozzle so that frequent inspections can be made without removing the nozzle from service. Flow nozzles are particularly suited for measurement of steam flow and other high-velocity fluids, fluids with some solids, wet gases, and similar materials. Because the exact contour is not critical, the flow nozzle can be expected to retain good calibration for a long time under erosion or other


hostile conditions. Because of its streamlined contour, it tends to sweep solids or moisture through the throat and is far superior to orifice plates in these services. Because tap geometry and contour is critical to maintaining calibration, it is not recommended to use flow nozzles on slurries or dirty fluids. A flow nozzle will pass about 60% more flow than an orifice plate of the same diameter and differential pressure. It also has the advantage of operating acceptably over a wide beta ratio range of 0.2 to 0.8. For the same flow and differential pressure, the flow nozzle has a similar but slightly lower pressure loss than an orifice plate. This becomes apparent when it is recognized that the area of the throat and the velocity in the throat of a flow nozzle must be approximately the same as the area of flow and velocity at the vena contracta following an orifice so as to develop the same differential pressure from the same flow. The slightly lower pressure loss of the flow nozzle is due to its streamlined entrance. On the other hand, because the ASME flow nozzle does not utilize a recovery cone, the permanent head loss can still be as much as 40% of the differential pressure (Figure 2.29f ). In an effort to reduce these losses, particularly in applications for PTC-6 testing of turbines, a recovery cone may be added. In this design, the permanent head losses can be substantially reduced. The actual amount of reduction should be determined through testing. Although nozzles should be used at Reynolds numbers of 50,000 or above, data are available for Re down to 6000, so it is possible to use nozzles with more viscous fluids. Still, work published by the ASME Fluid Research Committee suggests that the most stable flow coefficients are seen at 6 throat Reynolds number of 1 × 10 . For throat Reynolds 5 numbers below 1 × 10 , the shift in flow coefficient can be as high as 6%. Flow nozzles have very high coefficients of discharge, typically 0.99 or greater. Using a typical value of 0.993, approximate flow rates can be calculated from a working equation, hρ 1− β4

W = 358d 2

2.29(3)

and, for approximate flow nozzle design,

β=

1 4 4 1 + 128, 000hρD 2 W

2.29(4)

CRITICAL-VELOCITY VENTURI NOZZLES One of the most accurate ways to measure gas flow is to cause “choked” flow (sonic velocity flow) through a venturi nozzle. Critical-velocity venturi nozzles are also used as secondary flow standards in calibrating other flowmeters. The nozzle can


be the ASME long-radius, elliptical inlet, wall-tap nozzle; the ISA 1932 nozzle; or the ASME throat-tap nozzle used in steam turbine testing. One of the highest rangeability and most accurate gas flowmeters has been devised by combining the sonic venturi nozzles with the digital control valve (Figure 2.1j).

381

Operation and calibration of venturi tubes over a period of many years has resulted in extensive documentation. As a result, most manufacturers will guarantee a standard design inaccuracy of ±0.75% of actual flow. This can be reduced to ±0.25% by calibration at a recognized hydraulics laboratory. Modern manufacturing techniques have led to predictable discharge coefficients and a repeatability of ±0.2% for venturi tubes of the same size and design. For very small (32 in. or 813 mm) venturi tubes, and for very high (>2,000,000) or very low (2 MHz), sometimes referred to as antenna loading. This technique requires an insulated probe and significant distance to ground. It measures the eddy current loss in the area surrounding the probe, which is directly proportional to the volume (level within the electric field) of liquid and also the conductance of the liquid. (No moving parts are employed.) Conductance (point-DC or low-frequency). When conductive material touches any part of the bare metal probe, it signals HIGH. Above an initial threshold, any conductance value works. Oil coating or disruption of the path to ground (such as a plastic-coated tank) defeats the instrument. (No moving parts are employed.) Microwave switches. These devices sense the difference in dielectric between gas (1.0) and the process material, generally >2.0. Generally, there is a sender on one side of the vessel and a receiver on the other. (No moving parts are employed.) Radar. Various types of antennas are used to generate an electromagnetic pulse or wave (moving at the speed of light), which is reflected by an abrupt change in dielectric constant. Numerous electronic schemes are used to determine the distance that the reflection represents. (No touching, no moving parts are employed.) TDR (time domain reflectometry). In this case, the instrument sends an electromagnetic wave or pulse (at the speed of light) down a probe, and the pulse is reflected by the process. It is possible to sense more than one reflection point, allowing the measurement of total level and interface with a single instrument. As with radar, various techniques are used to determine what distance the reflections represent. (No moving parts are employed.)


Mechanical Contact Diaphragm (point). This is primarily a sensor for granular solids. Movement of the diaphragm, caused by process granulars (S.G. >0.5) pressing on it, closes a mechanical switch. A more sensitive version employs an electrically excited, vibrating diaphragm that is damped by the presence of process solids. The resulting electrical change is used to switch a relay. Dip stick. This is the world’s oldest level measurement technology. It can involve the use of a stick or a tape, with or without a sensitive paste, to determine the level of a specific liquid. It is highly labor intensive. Floats (cable connection). The mechanics of cable retraction and hang-up due to various causes are the biggest problem. When the equipment is new, it provides excellent accuracy in storage applications. Floats (inductively coupled). Inductive sensing of float location eliminates the cable mechanics, but float hang-up is still a problem in some applications. Accuracy in storage applications is excellent. Floats (magnet/reed relay). The switches employed require no power. Floats can hang up or sink, but there is no problem with mechanical connections. The resolution of transmitters is limited by number of reed switches per foot. Floats (magnetostrictive pulse sensing). This is much like the inductive float position sensing, except the permanent magnet in the float produces the reflection of a magnetostrictive pulse in a physically isolated, ferromagnetic tape. Paddlewheel (point). A rotating paddle in a dusty atmosphere has an inherent failure mechanism. It can be used only in granular solids. The presence of material stops the paddle’s motion, causing a change in motor current and relay closure. Plumb bobs (yo-yos). Dust buildup on the cable, dust in the bearings, and potential for trapping the plumb bob under incoming solids have made this long-time standard obsolete. It is used only for granulars. Resistance tape. This is an accurate but delicate sensor for liquid storage tanks. The mechanical force from the measured liquid shorts out the submerged segment of the top-to-bottom precision resistor. Changes in density have a minor effect. Sonic/ultrasonic. Most of these switches use a sonic path across a gap of selected width. The presence of gas bubbles or solid particles in the gap can interfere with their operation. The transmitters are quite accurate but require a consistent speed of sound in the “air” space, freedom from spurious echoes, and a process material that produces a strong sonic reflection. Condensation and dust buildup on the transducer are problematic. The transmitter won’t work in vacuum. Frequencies are selected for

3.1 Application and Selection

the application, not the range of human hearing. All these instruments are “sonic,” but not all are “ultrasonic.” (No continuous touch is involved, and no moving parts are employed.) Vibration (point). Using a fork or a single vibrating rod, these devices are now available for solids or liquids. They operate on a modification of the vibration character, switching a relay when submerged in the process material. Coating and packing materials can be a problem. They tend to be delicate because of the sensitivity required. Optical Lasers. Lasers constitute the best way to measure coal in silos. They are not susceptible to spurious reflections as are radar and sonic devices. They require a clear optical path and reflectance rather than transmittance from the process material. (No continuous touch is involved, and no moving parts are employed.) Optical (photocell) switches. Generally, these are quite limited by coating and temperature. An optical switch has the virtue of isolation from the process material but requires that the isolating medium be optically and process compatible. (No continuous touch is involved, and no moving parts are employed.) Level (sight) gauges. A sight gauge is a simple mechanism with complex limitations. Liquids that coat obscure the actual level. The level indication most trusted by operators (“seeing is believing”). A temperature differential between the tank and glass, a classic boiler glass problem, causes incorrect indication. (No moving parts are employed.)

TANK ACCESS Existing tanks often present a challenge to placing the measuring instrument in the correct location to perform properly. Glass-lined and coded pressure vessels provide no possibility of adding or enlarging any penetrations. If an external standpipe proves to be troublesome as a result of plugging or thermal differential, the level instrument needs direct access to the tank. The simplest possibility is to place a spare nozzle of sufficient diameter and short length on top of the tank. Failing that, there is always a chance of “teeing” into the vent pipe or pressure relief line. If there is a manway on top of the tank, the cover can be removed and a nozzle welded on in the shop. There are ways to sneak a continuous sensor into a tank from a side nozzle, but this usually entails a bit of plumbing ingenuity and customarily reduces the maximum height that can be measured. Obviously, a d/p transmitter can be mounted on a tank bottom nozzle, but it could also accept an RF probe mounted upside down. Most switch technologies have provision for vertical or horizontal entry. The refining


413

and fuel storage industries are competent to “hot-tap” a tank while the level is above the new nozzle. This approach definitely requires a sensor that can be inserted through a block valve under pressure. For new tanks, regardless of the level transmitter selected, a wise precaution is to add a spare 8-in. (200-mm)* and a spare 2-in. (50-mm) nozzle to the top of the tank. If there is a problem in the measurement, or whenever the process is modified, this will allow the installation of nearly any level transmitter. The smaller nozzle allows for the addition of an overfill switch. The nozzle length should be as short as possible (4 to 6 in. or 100 to 150 mm) as compatible with required bolting space.

APPLICATIONS Level measurement applications can be broadly grouped in terms of service as atmospheric vessels and pressurized vessels. With the exception of liquefied gases, accounting-grade measurements are made in atmospheric vessels. These are a quantum leap in precision from the process control or material scheduling class of measurement. Atmospheric Vessels Liquid level detection in atmospheric vessels rarely presents a serious problem. The most common problems are caused by high temperature or heavy agitation. Instrumentation generally can be selected and installed so that it is removable for inspection or repair without draining the vessel. With few exceptions, a level indicator located at eye level, combined with the available digital communication technologies, eliminates the necessity for the operator or instrument technician to climb the vessel. Most of the transmitters (with the exception of d/p types) are available as top-mounted designs, eliminating the possibility of a spill if the instrument or nozzle corrodes or ruptures. Most vented-to-atmosphere vessels can be manually gauged. It is always comforting to know that such a simple procedure as manual gauging is available to calibrate or verify an instrument output. Various float types can be used in low-volume storage tanks, underground tanks, transport tankers, and other applications outside of the processing area. Solids level measurement also is generally done in atmospheric tanks, but, in this case, the specifier has fewer available level detecting devices and less installation flexibility. Devices that are suitable for point level detection of solids include the capacitance/RF, diaphragm, rotating paddle, radiation, vibration, microwave, and optical types. Some level switches must be located at the actuation level; this can lead to accessibility problems. Except for the radiation-type device, it also means that a new connection must be provided * Or 4-in. (100-mm) in horizontal cylinders.

414

Level Measurement

if the actuation point is raised or lowered. Paddle, vibration, and RF sensors can be extended at least 10 ft (3 m) from the top, and RF allows the switching point to be adjusted electrically. Solids that behave unpredictably can cause serious measurement problems. If the solid is not free flowing, sensing should be limited to an area beyond the expected wall buildup. If it can bridge or rat-hole, particular care must be taken in the location and installation of the level switch. Continuous level measurement of solids can be made by yo-yo (automatic plumb-bob), laser, nuclear, RF, TDR, radar, and sonic instruments. The yo-yo was formerly most popular, but its problems with its moving parts in dusty bins have spurred the use of stationary devices. These designs are generally top mounted, but all can be equipped with ground-level or remote readouts. Density variation and angle of repose are inherent in the granular solids. Both can cause inaccuracy of the level measurement, which is a substantial multiple of the instrument’s laboratory error specification. As with the switches, good performance requires that the solids be free flowing. These measurements will all be suitable for material scheduling functions. If an inventory grade measurement is required (definitely a weight measurement), load cells are used. Load cells are covered in Chapter 7.

Pressurized Vessels Point level detection of liquids in a pressurized vessel can be made using one of ten types of level sensors. For clean services in industrial processing plants, preference has traditionally been given to the externally mounted displacer switch. This unit is rugged and reliable, it has above-average resistance to vibration, and its actuation point can be easily changed over a limited range. There are a number of cases in which microwave, sonic, capacitance, and float switches are considered if they are installed so that they can be removed for repair without venting the vessel to the atmosphere. Conductivity switches are used in water services to 700°F (370°C) and 3000 PSIG (21 MPa). Optical and thermal dispersion switches have no moving parts, are inexpensive, and are used on clean services. Continuous liquid level detection in pressurized vessels is subdivided into clean and hard-to-handle processes. For clean services requiring local indication only, the traditional choice is the armored sight gauge. Even when a transmitted signal is required, many users specify that transmitters be backed up with a sight gauge for use in calibration and to allow that the process can run manually if the transmitter is out of service. Nevertheless, the need for a sight gauge should be carefully evaluated, as it can be a weak point (personnel hazard) in high-pressure processes and can become plugged in sludge and slurry services. In hazardous services, magneticfloat level gauges can be used. Preferences for clean service transmitters vary from industry to industry. Petroleum refiners have traditionally preferred the externally mounted displacer transmitter but


have recently discovered that much related maintenance and rebuilding can be avoided by using electronic sensing. The existing rugged “cages” can be retrofitted with lowermaintenance instruments. Strength is important in the petroleum industry, because a break at the instrument connection could cause a hydrocarbon spill above the autoignition temperature. The low-side (vapor-phase) connection of these cages does not require a chemical seal. This reduces maintenance requirements and eliminates possible inaccuracies that a d/p transmitter might produce. Most refinery processes are compatible with carbon or alloy steel materials, which are readily available in all sensor designs. In other chemical processing industries, first consideration usually goes to the d/p transmitter when a level signal is required. It is reliable and accurate (provided that specific gravity is constant), and many modifications are available for unique services. The major problem with the d/p transmitter, when used for level measurement on pressurized vessels, is in handling the low-pressure tap. If the low side of the d/p cell can be connected directly to the vapor space of the vessel, the problem is eliminated, but this is rarely the case. Normally, the low-pressure leg must be filled with a seal oil or with the process material. If a seal oil is used, the oil must be compatible with the process. If the leg is filled with the process material, the process fill must not boil away at high ambient temperatures. In either case, ambient temperature variations will change the density of the fill, which can cause inaccuracies in the level reading. The liquid seal also requires frequent inspection. Low-pressure-side repeaters and chemical seals are also available, but although they eliminate the seal problem, they introduce inaccuracies of their own and increase the purchase cost. Despite this, d/p cells are successfully used in a wide range of applications and can be considered whenever the span to be measured is greater than 60 in. (1.5 m). Other devices, such as capacitance/RF, nuclear, sonic, radar, and TDR technologies, are in use for level measurement in pressurized vessels, especially where level indication must be independent of density. Accounting Grade (Tank Gauging) Accounting-grade measurements are made in both atmospheric and pressurized vessels. The need for accuracy in accounting-grade installations can be demonstrated as follows. A typical 750,000-barrel American Petroleum Institute (API) storage tank has a diameter of 345 ft (105 m), and it 3 takes some 8000 gallons (30 m ) to raise the level 1 in. (25 mm). A level measurement error of 1 in. (25 mm) would therefore 3 indicate that 8000 gallons (30 m ) have been gained or lost. In the case of hydrocarbon storage tanks, the accumulation of water at the bottom must be factored into the measurement, or errors equivalent to several inches of product could result. This is no small matter, particularly if the level measurement is used as a basis for custody transfer of the product. Substantial effort has been put into the development of storage


415

by these newer technologies. For custody transfer, dip tapes are still probably the most common measurement. The manual approach has the advantage of measuring the water under organic products at relatively minor additional cost. In this case, the inaccuracy risk is the very real possibility of human error, either in the measurement itself or in the volume abstracted from the strapping table.

P3

HIU P2

P1 RTD

Fieldbus

FIG. 3.1d A Hydrostatic Tank Gauge applied to a pressurized, spherical tank. (Courtesy of The Foxboro Co.)

tank gauging systems that have good reliability, high accuracy, and high resolution. These efforts have been relatively successful, and the user can be confident of obtaining satisfactory results if adequate attention is given to installation details. Every bit as critical as the instruments installed is an accurate, up-to-date strapping table. Because tanks settle and sag over time, it should be updated after the first two years of service. Tanks that are 20 years old often use a strapping table that was created before they saw the first batch of product. The use of differential-pressure transmitters (Figure 3.1d) for hydrostatic tank gauging (HTG) is one of the popular methods to make these high-accuracy measurements. Pressure 1 minus pressure 2 (P1 − P2) divided by the distance between them produces the density information. The pressure P1 is divided by the density to obtain the level. The level is entered into the strapping table for the particular tank to obtain the volume of liquid. In the case of nonvented tanks, P3 is subtracted from P1 before making the division by density. Although it is often neglected, the water level beneath the organic should be entered into the strapping table, and the resulting volume subtracted to obtain net product volume. Radar is another favored technology for obtaining the 0.125-in. (3-mm) accuracy usually required for these applications. In that method, the actual level is measured directly and entered into the strapping table to obtain volume. This may appear to be a more straightforward approach, but measuring to this accuracy from the top of a tall tank has other mechanical considerations such as roof deflection and thermal tank expansion. The float and servo-operated plumb bob that were formerly the top-mounted standards are being replaced


Sludge and Slurries A number of level-switch designs are suited for hard-tohandle service in pressurized vessels. In making a selection, one would first decide if a penetrating design is acceptable (Figure 3.1e). The use of such a level switch usually implies that the tank will have to be depressurized, or sometimes even drained, when maintenance is required. If penetration is not allowed, then only nuclear, clamp-on sonic, or microwave (for fiberglass or plastic tanks) devices can be considered. When a level transmitter is selected for a hard-to-handle service, the radiation type or the load cell might seem to be obvious choices, but licensing and regulatory requirements in the case of radiation, and high costs of both, tend to make them choices of last resort. The installation cost of load-cell systems can be reduced by locating the strain gauge elements directly on the existing steel supports (Figure 3.1f). There are, of course, applications in which almost nothing can be used other than such expensive devices as the nuclear-type level gauge. One example of such an application is the bed level in a fluidized-bed type of combustion process. If the accuracy of purging taps is insufficient, there is little choice but to use radiation gauges.

FIG. 3.1e An optical or sonic gap switch for water/sludge interface. (Courtesy of Thermo MeasureTech.)

416

Level Measurement

FIG. 3.1f Steel support-mounted strain gauges (see Chapter 7) can be calibrated by measuring the output when the tank is empty, and again when it is full. (Courtesy of Kistler-Morse.)

On slurry and sludge services, d/p units are most likely to exhibit large errors due to density variation. The required extended-diaphragm type of differential pressure transmitter eliminates the dead-ended cavity in the nozzle where materials could accumulate and brings the sensing diaphragm flush with the inside surface of the tank. The sensing diaphragm can be coated with TFE to minimize the likelihood of material buildup. One of the best methods of keeping the low-pressure side of the d/p transmitter clean is to insert another extended diaphragm device in the upper nozzle. This can be a pressure repeater (Figure 3.1g), which is capable of repeating either vacuums or pressures if it is within the range of the available vacuum and instrument air supplies. Outside of these pressures, extended-diaphragm types of chemical seals can be used (Figure 3.1h) if they are properly compensated for ambient temperature variations and sun exposure. Other level transmitters that should be considered for hard-to-handle services include the capacitance/RF, laser, radar, sonic, and TDR types. Foaming and surface disturbances due to agitation tend to interfere with the performance of radar, laser, and sonic units. Capacitance probes and TDR probes stand a better chance of operation in these services. They can withstand some coating or can be provided with probe cleaning or washing attachments. Radar transmitters perform accurately and reliably on paper pulp and other applications that coat and clog. Foaming, Boiling, and Agitation In unit operations such as strippers, the goal is to maximize the rate at which the solvents are boiled off against the constraint of foaming. In other processes, the goal is to maintain


1:1 Repeater

Pv

To Controller

DifferentialPressure Transmitter

FIG. 3.1g The clean and cold air output of the repeater duplicates pressure (Pv) of the vapor phase.


Capillary

Filled Elements To Controller

DifferentialPressure Transmitter

FIG. 3.1h Chemical seals with temperature compensation and extended diaphragm protect a d/p transmitter from plugging and chemical attack.

a controlled and constant thickness of foam. In these types of processes, one must detect both the liquid–foam interface and the foam level. The detection of the liquid level below the foam is the easier of the two level-measurement tasks, because the density of the foam tends to be negligible relative to the liquid. A d/p transmitter installation (Figure 3.1h) will measure the hydrostatic weight of the foam, disregarding most of its height. Different industries tend to use different sensors for measuring the foam–liquid interface. In Kraft processing, for example, radiation detectors are used to detect that interface in the digester vessel. RF (capacitance) and TDR transmitters and conductance and RF switches make excellent foam level measurements as long as the foam is conductive (in fact, only very specialized RF switches can differentiate between conductive foam and liquid). The continuous measurement of insulating foam level is more difficult and, for that reason, some people will circumvent its measurement by detecting some other process parameter that is related to foaming. These indirect variables can be the vapor flow rate generated by the stripper, the heat input into the stripper, or just historical data on previous batches of similar size and composition. If direct foam level measurement is desired, it is easier to provide a point sensor than a continuous detector. Horizontal RF switches generally operate successfully if density is sufficient to produce a


417

dielectric constant in the foam that is greater than 1.1 (vacuum and gases are 1.0). In the case of heavier foams, vibrating or tuning fork switches and beta radiation gauges have been used; in some cases, optical or thermal switches have also been successful. Boiling will change the hydrostatic weight of the liquid column in the tank due to variable vapor fraction. As the rate of boiling rises, the relative volume of bubbles will also increase, and therefore the density will drop. Density rises as the rate of boiling is reduced. Density also varies with level as bubbles expand on the way up. Therefore, the measurement of hydrostatic head alone can determine neither the level nor the mass of liquid in the tank. This problem is common when measuring the water in nuclear, boiling-water reactors (BWRs) or in the feedwater drums of boilers. Hightemperature capacitance/RF transmitters can do the feedwater job, but the fluorocarbon insulation is not applicable to nuclear reactors. A standpipe with a series of 10 to 20 horizontal conductance sensors is very common in these applications. If only level indication is required, then the refractiontype level gauge is sufficient, given that it shows only the interface between water and steam. These “external” strategies require the temperature to be equal with the tank to be useful. Some agitators prevent the use of probe-type devices, because they leave no room for them, and they also challenge the use of sonic and radar transmitters unless programmed to ignore the agitator blades and sense the rough surface. Glass-lined reactors are a classic enemy of probes, as they usually have heavy agitation, and the lining prevents support or anchoring. A probe, broken due to fatigue, can cause very expensive damage in these vessels. Radar transmitters with “tank mapping” software are quite suitable as long as the dielectric constant is greater than 2 (most common). Agitation usually does not affect the performance of the displacer and d/p-cell-type level sensors, which are external to the tank. They can measure level in the special case, where the specific gravity is constant. Of the two, the d/p cell is preferred, because it is looking at the liquid inside the tank and not in an external chamber, where its temperature and therefore its density can be different. Of course, the primary reason for heavy agitation is to keep unlike components mixed, which implies variable specific gravity.

Interface Measurement When detecting the interface between two liquids, we can base the measurement on the difference of densities (0.8:1.1 is a typical ratio), electrical conductivity (1:1000 is common), thermal conductivity, opacity, or sonic transmittance of the two fluids. Figure 3.1i illustrates the difference in typical separator response between the conductivity sensors and the density sensors. One should base the measurement on whatever process property gives the largest stem change between the upper and the lower fluid. If, instead of a clean interface,

418

Level Measurement

Conductivity µS/cm 1

600

1200

1800

2400

3000

8

(Oil)

7 Level (FT.)

6

5 Visual Emulsion

Electrical Interface 4

= Conductivity

3

= Specific Gravity

2

(Water)

1

Bottom of Tank

0 0.8

.86

.92 .98 Specific Gravity

1.04

1.1

FIG. 3.1i Graph of density (bubblers, d/p, displacer, nuclear) and conductivity (capacitance, conductance, TDR) versus level in a typical heavy crude/ water separator.

Transmitter Crystal

Receiver Crystal

FIG. 3.1j Sonic interface level switch. (Courtesy of Thermo MeasureTech.)

there is a rag layer (an emulsion of the two fluids) between the two fluids, the interface instrument cannot change that fact (it cannot eliminate the rag layer). If the separator and its control system are properly designed, the emulsion can be kept out of both separated products. Interface-level switches are usually of the optical (Figure 3.1e), capacitance, displacer, conductivity, thermal, microwave, or radiation designs. The unique sonic switch described in Figure 3.1j utilizes a gap-type probe that is installed at a


10° angle from the horizontal. At one end of the gap is the ultrasonic source, and at the other is the receiver. The instrument depends on the acoustic impedance mismatch between the upper and lower phases. When the interface is in the gap, it will attenuate the energy of the sonic pulse before it is received at the detector. This switch is used in detecting the interface between water and oil or other hydrocarbons. Of course, this is no way to control the interface, because, once outside, it could be above or below the gap. It is suitable as a backup to an interface control system. D/P transmitters can continuously detect the interface between two liquids, but, if their density differential is small, it produces only a small pressure differential. Changes in density typically produce 5 to 10 times the error on an interface calibration that they do on a single-liquid calibration. A major limitation is that the range of interface movement must cause a change that is as great as the minimum d/p span. If the difference in conductivity is at least 100:1, such as in case of the dehydrating of crude oil, continuous capacitance or TDR probes make excellent interface transmitters. Interface between two insulating liquids (a rare situation) can be accomplished with TDR but is unreliable using capacitance. Sonic transducers lowered into the brine layer of oil or liquefied gas storage caverns (Figure 3.1k) can measure the interface


To Receiver

Brine

Hydrocarbon Ground Level

Hydrocarbon Cavity

Interface

Brine

Transducer

FIG. 3.1k A unique, bottom-up, sonic interface measurement.

between brine and hydrocarbon by shooting up from the bottom. On clean services, float and displacer-type sensors can also be used as interface-level detectors. For the float-type units, the trick is to select a float density that is heavier than the light layer but lighter than the heavy layer. With displacertype sensors, it is necessary to keep the displacer flooded with the upper connection of the chamber in the light liquid phase and the lower connection in the heavy liquid phase. By so doing, the displacer becomes a differential density sensor and, therefore, the smaller the difference between the densities of the fluids, and the shorter the interface range, the smaller the force differential produced. To produce more force, it is necessary to increase the displacer diameter. The density of the displacer must be heavier than the density of the heavy phase. In specialized cases, such as the continuous detection of the interface between the ash and the coal layers in fluidized bed combustion chambers, the best choice is to use the nuclear radiation sensors. Liquid/solid interface measurements are extremely demanding, and the only general successes have been achieved with nuclear or sonic sensing. The sonic sensor must always be submerged, because a gas phase will either disrupt the measurement entirely or appear to be the solid. In special noncoating cases, optical sensors have worked without frequent cleaning.


419

Bibliography Akeley, L. T., Eight ways to measure liquid level, Control Eng., July 1967. Andreiev, N., Survey and guide to liquid and solid level sensing, Control Eng., May 1973. API Guide for Inspection of Refinery Equipment, Chapter XV, Instruments and Control Equipment, American Petroleum Institute, Washington, DC. API Recommended Practice 550, Manual on Installation of Refinery Instruments and Control Systems, Part I, Process Instrumentation and Control, Section 2, Level, American Petroleum Institute, Washington, DC. Bacon, J. M., The changing world of level measurement, InTech, June 1996. Bahner, M., Level-measurement tools keep tank contents where they belong, Environ. Eng. World, January–February 1996. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June–July, 1997. Bailey, S. J., Level sensors 1976, a case of contact or non-contact, Control Eng., July 1976. Belsterling, C. C., A look at level measurement methods, Instrum. Control Syst., April 1981. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Boyes, W. H., The changing state of the art of level measurement, Flow Control, February 1999. Buckley, P. S., Liquid level measurement in distillation columns, ISA Trans. 12(1), 45–55, 1973. Caldwell, A. B., Process control series: liquid and solid level sensors, Eng. Mining J., May 1967. Carsella, B., Popular level-gauging methods, Chemical Process., December 1998. Cho, C. H., Measurement and Control of Liquid Level, ISA, Research Triangle Park, NC, 1982. Considine, D. M., Process instrumentations; liquid level measurement systems; their evaluation and selection, Chemical Eng., February 12, 1968. Considine D. M., Fluid level systems, in Process/Industrial Instrumentation and Control Handbook, 4th ed., McGraw-Hill, New York, 1993, 4.130–4.136. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Cornane, T., Continuous level control, Measurement and Control, April 1997. Cusick, C. F., Liquid level measurement, Instrumentation, 22(1), 22–7, 1969. Early, P., Solving old tank gauging problems with the new hydrostatic tank gauging technology, Adv. Instrum., 42, 1987. Ehrenfried, A., Level gaging, Meas. Control, April 1991. Engineering Outline; level measurement, Engineering, October 6, 1967. Entwistle, H., Survey of Level Instruments, ISA Conference, Anaheim, CA, Paper #91-0484, 1991. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001. Glenn, L. E., Tank gauging—comparing the various technologies, in ISA Conf. Proc., Anaheim, CA, Paper #91–0471, 1991. Hall, J., Level monitoring; simple or complex, Instrum. Control Syst., October 1979. Hall, J., Measuring interface levels, Instrum. Control Syst., October 1981. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. ISA Directory of Instrumentation, ISA, Research Triangle Park, NC. Johnson, D., Taking your lumps, Control Eng., June 1995. Johnson, D., What the devil is that level, Control Eng., June 1996. Johnson, D., Doing your level best, Control Eng., August 1997. Johnson, D., Process instrumentation’s utility infielder, Control Eng., November 1998. Johnson, D., Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001. Johnson, D., Level sensing in hostile environments, Control Eng., August 2001.

420

Level Measurement

King, C. and Merchant, J., Using electro-optics for non-contact level sensing, InTech, May 1982. Koeneman, D. W., Level among layers (accurately determining interface), Control Eng., August 1998. Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Lanini, L. and Schneider, L., The dawn of new tank gauging system, Adv. Instrum., 42, 155–161, 1987. LaPadula, E. J., Level measuring methods, ISA J., February 1965. Lawford, V. N., How to select liquid-level instruments, Chemical Eng., October 15, 1973. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Lerner, J., Selecting a continuous level measurement system for your operation, Powder and Bulk Solids, 19, March 1991. Level measurement and control, Meas. Control, 142–161, April 1999. Liptak, B. G., Instrumentation to measure slurries and viscous materials, Chemical Eng., January 30, 1967.


Liptak, B. G., On-line instrumentation, Chemical Eng., March 31, 1986. Merritt, R., Level sensors for custody transfer? Control, November 2001. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Owen, T., Overcoming obstacles in solids level measurement, Control, February 1998. Paris, T. and Roede, J., Back to basics, Control Eng., June 1999. Parker, S., Selecting a level device based on application needs, in 1999 Fluid Flow Annual, Putman Publishing, Itasca, IL, 1999, 75–80. Paul, B. O., Seventeen level sensing methods, Chemical Process., February 1999. Sholette, W., Pick the proper level measurement technology, Chemical Eng. Progress, October 1996. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

3.2

Bubblers

LI XFI

D. S. KAYSER (1982) B. G. LIPTÁK C. G. LANGFORD (2003)

N2

(1969, 1995)

Flow Sheet Symbol

Application

Level, interface, and density; open or closed, pressurized tanks and vessels

Operating Pressure

Limited only by the pressure of the available purge gas supply


As required; limited only by materials in contact with the process; has been used on such high-temperature processes as coal gasification

Materials

Limited only by the availability of exposed pipe materials

Costs

$200 for the simplest local indicator installation, can reach $5000 for installations requiring special materials and remote transmission

Inaccuracy

Function of error in the readout and other parts used, ±0.05% to ±2.0% of full scale

Range

Unlimited as long as purge gas supply pressure exceeds that of the process


Bubbler-type level detector packages can be assembled from components described in other sections, such as variable area flowmeters described in Section 2.27, various types of pressure sensing and display devices described in Chapter 5

Prepackaged bubbler assemblies are also available from

Aalborg Instruments & Controls (www.aalborg.com) ABB Fischer & Porter (www.abb.com) Blue-White Industries (www.bluwhite.com) Brooks Instrument Div. of Emerson (www.emersonprocess.com) Dwyer Instruments Inc. (www.dwyer-inst.com) Flowmetrics Inc. (www.flowmetrics.com) Krohne America Inc. (www.krohneamerica.com) McMillan Co. (www.mcmillancompany.com) Omega Engineering Inc. (www.omega.com) Porter Instrument Co. (www.porterinstrument.com) U.S. Filter Wallace & Tiernan Inc. (www.usfwt.com)

INTRODUCTION Many industrial accidents are caused by incomplete or inaccurate level information. Bubblers serve to solve that problem in an inexpensive and reasonably reliable manner. The operation of an air bubbler is similar to blowing air into a glass of water with a straw. The more water is in the glass, the harder one needs to blow. Bubbler-type level sensors have been in use for as long as compressed air has. If the air pressure entering the dip pipe is greater than the hydrostatic head of the process fluid in the tank, the air will bubble out at the bottom of the pipe.

Figure 3.2a illustrates an air bubbler installation for an open (atmospheric) tank with various purge controls. The transmission line should be sloped toward the tank so that, if the purge is lost and process vapors enter the transmission tube, the condensate will drain back into the vessel. If the readout device must be below tank level, a condensate trap can be installed as shown by the dotted line. The purge supply pressure should be at least 10 PSI (69 kPa) higher than the highest hydrostatic pressure to be gauged. The purge flow rate is kept small and relatively 3 constant at about 1 SCFH (500 cm /min), so there will be no significant pressure drop in the dip tube. Usually, the purge 421


422

Level Measurement

PCV

Transmission Line Dip Pipe

PI

A

N2

LI

dPCV

ss

PCV

B

FI

FI

PI

N2

C

PI

FI

N2

LI dPCV

D

PCV

dPCV

FI

Remotely Located Components

N2

Purge Control Variations

dPCV

ss

SS

FI

N2

E

FI

Equalizing Line Transmission Line

PCV

PI

H2 O PI LI

F

Dip Tube

Manometer

FIG. 3.2a Variations of air bubblers for atmospheric tanks.

media is air or inert gas, although liquids can also be used. Several methods of gas purge controls are shown in Figure 3.2a to illustrate some of the installation considerations. In most traditional installations such as system A, nitrogen supply pressure is regulated to a value corresponding to a pressure that is higher than the hydrostatic head when the tank is full. The purge flow rate is adjusted by a needle valve and is sent through a sight feed bubbler, which allows visual inspection of the actual flow. This system allows for detection of levels up to 10 ft (3 m). If higher levels are to be detected, system B, which uses a rotameter instead of a sight feed bubbler, can be considered, because a rotameter can withstand higher pressures. In systems A and B, as the liquid level varies, the downstream pressure also will vary, thereby causing variations in the purge flow rate. Since the purge pressure at the readout device is the sum of hydrostatic head and the dynamic pressure drop in the dip tube, variations in purge flow will cause errors. To correct this condition, a differential-pressure control valve can be installed across the fixed restriction of the needle valve as shown in system C. This will cause the purge flow to be uniform regardless of the liquid head. If the process material can build up or plug the dip tube, either as a result of loss of purge gas or because of the nature of the fluid, an aerator selector switch may be installed as shown on system D to allow for periodic blowing out of the transmission line. System E can be considered in remote locations where gas purge media is not available and a bubbler is desired instead of a liquid purge for level detection. Here, water is jetted across a gap while air is aspirated into the stream and compressed. The air–water mixture enters the dip tube, where the small amount of water runs down the inside of the bubbler tube while the pressure of the escaping


FIG. 3.2b Air bubbler installation for pressurized tanks.

air is detected as a measure of level. Such a setup would be in service only when the operator wanted to make a level reading, so the water would not flow into the vessel continuously. System F shows a more common approach for remote bubbler installations where a small hand pump is used to compress the purge air. For tanks that operate under pressure or vacuum, the installation of a bubbler indicator becomes slightly more complex, because the liquid level measurement is a function of the difference between two bubbler pressures. Because of the differential measurement involved, the readout device can be a manometer or other type of differential-pressure detector. Figure 3.2b shows one of these installations. All of the previously discussed variations apply to both pressure and vacuum installations.

GENERAL The bubbler detects the hydrostatic pressure in a vessel and displays it in a more convenient location. The pressure of an inert gas is used to transport the level information to this more convenient location. Bubbler-type level detection has been in use since compressed air became available. As illustrated in Figure 3.2c, after the air fills the dip pipe, its pressure inside the dip tube will equal the hydrostatic head of the process fluid outside the dip tube, and the excess air that is introduced will bubble out at the bottom of the tube. If the tank is not open to the atmosphere, a second pressure tap is required to provide a reference pressure from the vapor space. The dip tube can enter from the top or side of

3.2 Bubblers

Purged Pressure Reference for Non-Vented Tank

423

FI

LI

A

Isolation Valves as Required

Typical for all: Isolation Valve, Dirt Filter, and Check Valve

FI

Filter

B

Alternate Flow Control and Indicator

Alternative Pressure Tap

FIG. 3.2c Bubbler-type level local indicator system.

the tank as long as it extends below the minimum level that is to be detected. As shown in Figure 3.2a, various combinations of valves, check valves, needle valves, and flow indicators may be required for various applications. Of the bubbler design options, the blow-back dip tube is the simplest and usually the least expensive. Here, piping or tubing is provided to bring the purge gas pressure to the level display or transmitter. The bubbler, because of its simplicity, is inexpensive and robust while being easy to maintain or adopt to changing process conditions. The display portion of the system is not wetted by the process fluid and is in a convenient location. Calibration and replacement of the level readout or transmitting devices is also safe and convenient if, before service, the isolating valves (Figure 3.2c) are fully closed, and the standard safety precautions (see Chapter 7) are observed. If a short dip tube is inserted horizontally in the side of the tank, the dip tube will be easier to access and support. Figure 3.2d illustrates that a tap can be provided for cleaning (rodding out) the accumulated deposits. In this design any plugging or dirt accumulation can be mechanically removed by inserting a rod into the dip tube. If it is desired to clean out the dip tube while the process is in operation (or if the tank is full or pressurized), packing glands are provided, and a “captive rod” is permanently installed to allow clearing the dip tube at any time.


Replaceable dip tubes, with or without packing glands, have also been used on the more difficult applications. Other options, such as dual or self-washing purges, will also be discussed later in this section. In addition, jacketed dip tubes are also available and have been successfully used in applications in which condensation or freezing is a concern (Figure 3.2e). One of the advantages of the bubbler-type level measurement is that their readings are not affected (or affected only very slightly) by foam and by variations in pressure or composition of the vapor space above the liquid. These changes, particularly foaming, can interfere with many other types of level detectors, as was shown in Table 3.1b. On the other hand, process phenomena that change the density of the liquid (bubble formation, boiling) will result in “understating” the level, because a drop in density reduces the hydrostatic head. Purge Gas Air and nitrogen are the most commonly used purge gases. Other gases can also be used if, for some reason (such as their available maximum pressure being insufficient), these cannot be used. The measurement itself is as reliable as the availability of the purge gas supply. The flowing gas also

424

Level Measurement

PS Air Header

Level Tap

PI

h

PI

Auto Switching Valve

Gas Cylinder

FIG. 3.2f Purge gas supply system with automatic backup. Full Bore Ball Valve (Typical)

Plugged RodOut Tap

h

FIG. 3.2d Side entering dip pipe (tube) installations.

Level Tap

serves to keep the inside of the dip tube dry and clean. Proper functioning requires that the purge air or gas pressure be higher than the maximum process pressure plus the maximum friction drop anticipated within the dip tube. Reliability is improved with increasing supply pressures. Some bubbler-type level packages include an air pump to generate the purge pressure and use a manometer to indicate the level. The danger here is that any loss of air due to air leakage or pump failure will result in a false low-level indication. Therefore, it is more reliable to supply the bubblers from the central air supply of the plant. For critical applications, bottled gas, typically nitrogen, is used as a backup for the air supply. Pressure-operated pneumatic valves can provide automatic switching of the gas supply without electrical connections (Figure 3.2f). A lowpressure detector switch can also be used and in that case; its contact not only can switch to the backup gas supply, it can also initiate and alarm so that plant operators will be aware of the loss of air.

SIZING CALCULATIONS

Jacket Connections

FIG. 3.2e Jacketed dip pipe (tube) installation.


The bubbler is fundamentally a mass or weight detector, because the pressure it senses is a function of both the liquid height and density. Therefore, the pressure of the purge gas reflects level only if the liquid density (composition and phase) is constant. Bubbler applications include level control, inventory management, custody transfer, overflow protection, flow rate smoothing, and pump suction protection. For inventory control or for accounting purposes, the information desired is not the volume but the mass of the liquid. Chemical reactions are also based on mass, and even fuels that are sold to end users by volume (gasoline, fuel oil, natural gas) are often sold commercially by mass. As discussed in Section 3.18, they are sold by

3.2 Bubblers

“standard” volume, and this apparent volume is corrected for density (or temperature) difference from the standard. A narrow-range dip tube mounted near the top of a tank can provide accurate overflow protection because, over that small range, the density correction is insignificant.

FI

Typical for all: Isolation Valve, Dirt Filter, and Check Valve LI

Mass and Level Level is inferred from the pressure (H) measured by the bubbler. Equation 3.2(1) shows how this hydrostatic head is calculated. H = (h) (ρ) (G/Gc)

Filter

3.2(1)

where H = hydrostatic head h = vertical depth ρ = average fluid density over depth G = local gravity Gc = units conversion factor, not required with SI units The total mass of liquid in the tank is obtained by Equation 3.2(2). M = (H) (A)

FI Filter h

FIG. 3.2g The measurement of density by bubblers.

3.2(2)

where H = hydrostatic head A = cross-sectional area of vessel The mass calculations must be corrected for any variations in cross-sectional area over the range of interest. The oil industry uses the term strapping to refer to the process of calibrating a tank. Internal devices, construction tolerances, and even the deformation of a large storage tank with level variations affect the accuracy of any level gauge. Actual test data is required for reliable measurement accuracy. The prudent user will not calibrate the level measurement to 100% of the tank height but will allow for errors and for changes in density. If a tank is calibrated for 100% of full tank level for a heavy liquid of, say, specific gravity of 1.2, it will overflow if used on water with a specific gravity of 1.0. If the liquid cannot freely overflow, the hydrostatic pressure will build up inside the vessel and create a lifting force on the top while pushing the walls out. As a result, the sideto-bottom joints might fail. The Hydrostatic Tank Gauge (HTG) Density can be measured by detecting the pressure difference from two dip tubes immersed in the liquid, with their bottom ends vertically separated by a fixed distance “h” in Figure 3.2g. Where needed, a third pressure, the vapor-space pressure above the liquid, is also measured and can be used to determine the density of the liquid. If both level and density are known, one can determine the mass in the tank. All three values (volume, mass, and density) can be reported for different uses. Improved accuracy in pressure transmitters has made it possible to install hydrostatic tank gauges (HTG), which are illustrated in Figure 3.6e.


425

Interface Level Signal A

B

C

Low Limiter

ρ1

Interface Level

ρ2

FIG. 3.2h Density compensated interface detection with bubbler tubes.

If there are two immiscible (nonmixing) liquids, then the height of the interface above the more dense liquid may be inferred by the HTG. Note that the difference in pressures in the two tubes may be small and require a very accurate detector and a large suppression of zero. Note also that the interface measurement is affected by changes in density, which can be caused by changes in composition or density. Density As shown in Figure 3.6e, the differential pressure can also be a measure of density. Figure 3.2h shows how bubblers can also be used to correct the level for variations in density

426

Level Measurement

or to measure interface or other hydrostatic-head-related variables. Calibration The bubbler differential pressure can be calibrated in inches or millimeters of level or in regular pressure units, but it is absolutely vital to have good records of the units used and of all the conversion factors. For high precision at very high operating pressures, it might also be necessary to correct for the weight of the highly compressed gas column in the bubbler. Another factor to consider is the thermal expansion and contraction of the vessel and the dip tube caused by atmospheric or process temperature variations. In addition, pressure changes and gravity forces caused by level variations should be considered.

FLOW RATE AND PLUGGING CONSIDERATIONS Minimum Purge Flow Rate For accurate level signals and to keep the inside of the dip tube dry, it is necessary to provide a sufficient mass flow rate of purge gas to keep the dip tube full, even during a high rate of level or vessel pressure increase. These required rates can be calculated by first calculating the total volume of the tank corresponding to each inch of level change and, after that, determining the corresponding mass of air in this volume. The difference in this mass divided by the time for the pressure or level to change is the average mass flow rate required. A typical conservative value commonly used for atmospheric tank level detection is 0.5 SCFH, or 50% of full range on a 0 to 1 SCFH range rotameter.

Dip Tube Diameter Selection Dip tube sizing is determined both by the pressure drop through it, but mostly by the required mechanical strength of the system. Up to a length of 8 ft in un-agitated tanks (or 5 ft in agitated ones), the size of the dip tube should typically be 1 in. diameter, Schedule 40 pipe. Longer dip tubes must be supported (Figure 3.2i). In case of even longer dip tubes in agitated tanks, in addition to the guide support, a 2-in. (51mm) pipe sheath is added as shown in Figure 3.2j.

Upsets and Plugging The most common complaint about dip tubes is plugging, whereby the purge flow is lost or is inadequate. Under these conditions, the process liquids may rise up inside the dip tube and coat the walls. Over time, the coating will accumulate and restrict the flow of the purge gas. The beginning of plugging can be detected by slightly changing the purge flow rate and observing whether a change in level follows. The probability of plugging in saturated salt solution services increases as the area of the tip of the dip tube is reduced. To unplug the dip pipe, we can apply “rodding out”

Level Tap Head Pressure Tap for Non-Vented Tanks

Maximum Purge Flow Rate The tubing must be large enough to keep the pressure drop between the air or gas supply regulator and the end of the dip tube at a negligible value. Most users specify a minimum of 3/8-in. (10-mm) OD tubing and, preferably, 1/2-in. (12mm) OD tubing or piping should be used. To test the maximum flow limit of a bubbler installation, make a small change in purge flow rate and observe the effect. The level readout should not change as a result. Most problems with excessive pressure drops are caused by damaged or deformed tubing or to partially closed valves. Sometimes, when excessive leakage from the system is noticed, maintenance technicians will respond by increasing the airflow sufficiently to keep the level detector in operation. The resulting problem is that the level signal then will vary not only with level but also with the purge flow rate. On high-vacuum processes, the low density, and therefore high specific volume of the purge gas, will cause high gas velocities and will also increase the probability of leakage.


1 Inch (25 mm) Pipe (Example) Guide Required for Over 5 ft (300 mm), for Agitated Vessel, Over 8 ft (2500 mm), to 25 ft (7500 mm) for Non-Agitated.

Support from Tank Wall Notes: Dimensions and SI Equivalents are Only Suggested and Nominal

FIG. 3.2i Supporting long dip tubes.

3.2 Bubblers

427

FI

Purge Gas

Liquid Solvent FI

2 Inch (50 mm) Pipe Sheath 1 Inch Pipe (25 mm) Dip Tube Drill 41/2 Inch Diameter (12 mm) Holes, 8 Inches (200 mm) from Top Bracket for 12 ft (3600 mm) to 36 ft (10,000 mm) 11/2 Inch (40 mm) Pipe

NOTES: Dimensions and SI Equivalents are Only Suggested and Nominal

FIG. 3.2j Sheath and bracket supports for dip tubes.

using a twist drill that has been welded to a rod so that the end can be drilled clear. Special bubbler system designs can be used when the process liquid is a saturated salt solution and the salts are deposited inside the dip tube as the purge gas dries the solution. In saturated salt solution service, one can use water or a solvent to dissolve the deposits (Figure 3.2k). In viscous fluid services, solvents are used at flow rates of about 1 GPH to keep the dip tube clean. The solvent purge flow rate should be enough to maintain high humidity within the dip tube and to wash out any salts or solids. When measuring the level of caustic, a common practice is to add a secondary purge of low-pressure steam, which is introduced very close to the point at which the dip tube enters the tank. The steam flow is restricted by an orifice union, typically bored for 0.125 in. (3 mm). The theoretical design conditions in a plant can drastically differ from real-life and upset conditions. The prudent design engineer understands that, at one time or another, every instrument will be exposed to the pressure at which the system relief valve is set to open. On the other hand, the minimum pressure for a closed system is full vacuum. One should also consider the plant’s safety in terms of the quality and availability of experienced personnel if, for example, the upset occurs at 3 A.M. on a weekend. When a plant upset or system overhaul occurs, process equipment designed to operate at a high vacuum will be exposed to positive pressures as attempts are made to unplug


FIG. 3.2k Alternate liquid purge can clean bubbler dip tubes.

piping or valves. As high-temperature processes are shut down, they might suddenly develop high vacuums, because their vapors cool and condense. Pumps, blowers, and other equipment are all cycled on and off because of stuck check valves or other system components. A power outage can also cause a loss of control and, on top of all that, human beings are not fully predictable, either. Some people, under an upset condition, will react with great confidence and do exactly the wrong thing. INSTALLATION DETAILS There are two fundamentally different approaches to the installation of bubblers, as illustrated in Figures 3.2l and 3.2m. Figure 3.2l illustrates a transmitter mounted on the top of the tank and therefore has a shorter purged tubing run. This makes the system less prone to plugging but also results in a less convenient access for maintenance. In Figure 3.2m, where the transmitter is mounted at ground level, the purged tubing runs are longer and more prone to plug, but access is more convenient. The ground-mounted installation is usually provided with drip legs that, during upsets, can capture any liquid that might leave the tank. The drip legs can thereby protect the transmitter. In either case, the most important consideration is to maintain the adequate and reliable flow of purge gas. As was shown in Figure 3.2a, it is also advisable to prevent the blocking of the flow of purge gas when the dip pipe rests on the bottom of the tank. A simple solution is to cut the end at about a 45° angle to prevent blocking. On the other hand, this author has

428

Level Measurement

d/p Transmitter

Purge Panel

d/p Transmitter

Purge Panel

Drip Leg Drain Valve

FIG. 3.2l D/P transmitter mounted on the top of a tank.

not found that V-notches cut in the end of the tube will reduce the size of the bubbles or damp the small bump in pressure as each bubble escapes. This writer also believes that maintaining the purge flow rate at a constant value is not essential. Pressure and/or Flow Regulators Figure 3.2a shows a pressure regulator, but it is not necessarily true that a purge gas pressure regulator is necessary to stabilize the purge flow rate; some designs use only a flow rate regulator. For a properly designed system, the flow velocity in the tubing must be low enough that any reasonable change in flow will have practically no effect on the gas pressure at the tip of the dip pipe. If pressure changes with purge rate, then something is wrong with the installation, and the problem should be addressed. The most likely cause is that the flow rate is restricted at the wrong place. It is apparent to this writer that reducing the purge gas pressure is not the preferred practice, because it increases the probability of the loss of purge flow. This can lead to plugging or to measurement errors. The standard air supply regulator has a built-in overpressure vent to protect pneumatic instruments. However, in bubbler service, this vent allows the process to back into the dip tube if the process pressure exceeds the setpoint of the overpressure vent. Therefore, the “nonbleed” regulators should be used on bubbler installations, which will not vent and will therefore make the full air supply pressure available as the flow is reduced to nearly zero. Nonbleed regulators are available but are not always used, because of concern that they might be accidentally installed on other pneumatic instruments as instrument air supply regulators.


Plug

FIG. 3.2m Ground-level mounted d/p transmitter installation.

The purge gas supply pressure is commonly far higher than the pressure in the dip tube, and the pressure drop across the flow-restricting needle valve is large. This results in a nearly constant purge flow rate, even if the pressure of the air supply or in the dip tube changes. Yet, the full supply pressure remains available if needed. This is similar to the constant-current concept used in some electronic circuits. The only remaining reason for having a pressure regulator is to protect the pressure indicator, but, if there is no pressure regulator, no gauge is needed. A modern d/p-cell type of pressure transmitter will withstand the full instrument air supply pressure without damage. For the above reasons, it is this writer’s view that the traditional installations shown in Figures 3.2a and 3.2b can be simplified by eliminating the pressure regulators and by making sure that all instruments and tubing components will withstand the maximum possible supply pressure. DIAPHRAGM-TYPE DIP TUBE For some applications where the normal dip tube is not acceptable, the “dry” dip tube or plug-proof dip tube is used, as described in connection with Figure 3.5d in Section 3.5. All that has been related here about purge supplies and sensors also applies to these installations except that a barrier is provided to prevent process liquids from backing up into the dip tube. The diaphragm is not perfectly flexible and does offer same resistance, so a small offset will exist in the resulting signal pressure. There is a choice of diaphragm sizes, and

3.2 Bubblers

the larger diaphragms will have smaller offsets, whereas the smaller diaphragms are less likely to fail.

SAMPLE CALCULATIONS Level Detector Calibration Example If one is to calibrate the range of the level detector (r) in inches of water column ("WC) for a 25-ft-high tank containing a liquid of 0.85 specific gravity (SG), which corresponds 3 3 to 62.4 lb/ft or 999.8 kg/m , and it was decided that the tank will be considered full at 80% of tank level. Range (r) = 25 × 12 × 0.8 × 0.85 = 204 "WC (5182 mm WC) 3.2(3) If the same level detector range is to read in units of PSI, Range (r) = (25 × 0.8 × 0.85 × 62.4)/144 = 7.36 PSI 3.2(4) If a tank contains oil (SG = 0.8) and water (SG = 1.0) and we want to detect the movement of the interface in inches of WC over a range of 10 in., Range (r) = (1.0 − 0.8) 10 = 2.0 "WC (50.8 mm WC) 3.2(5) Density Detector Calibration Example If one is to detect the average density of a 10" layer of liquid in a tank by measuring the differential pressure across that layer, which can contain any mixture of oil (SG = 0.8, density = 3 3 49.92 lbm/ft ) and water (SG = 1.0, density = 62.4 lbm/ft ), the d/p cell range is: Range (r) = 10 (1.0 − 0.8) = 2 "WC = 50.8 mm WC 3

= 49.92 − 62.4 lbm/ft

3.2(6)

CONCLUSION The bubbler remains a valuable tool in level measurement due to its low cost, simplicity, and flexibility. It is also valuable as an inexpensive and easily installed backup overflow protector. For some specialized measurements, such as interface detection between oil and water, the capacitance gauge is more popular because of its higher sensitivity and better performance. When designing a difficult level-measurement system, a prudent design engineer might do well to specify spare nozzles for installing a bubbler as a backup on a vessel if more


429

modern level sensors could fail to work properly. Successful bubbler applications include polymers, tars, salts, and other difficult fluids. Failures resulting from dirt and plugging can be simpler to live with using a bubbler than with floats or other devices that have moving parts. The advantages of the HTG system (Figure 3.6e) can also be realized with the diptube type of bubbler detectors. There has been some environmental concern that, with a bubbler, the purge gas that enters the tank also has to be removed, but these purges are a very small portion of the total gas and vapor flow that must be removed anyway. For example, in the case of large storage tanks, the gas displaced during each cycle of emptying and filling is usually more than the volume of the gas used for purging at 1 SCFH for a week. The main advantage of air bubbler systems is their simplicity and the ease with which the readout device can be relocated to just about any convenient location. For remote tank farms where compressed air is not available, one of the simplest methods of level detection is to use a small hand pump and a gauge. Bubblers are widely used in the wastewater and food industries and in some bulk storage applications, but they have lost some of their earlier popularity in the processing industries, where the trend seems to be favoring nonflowing, solid-state electronic devices.

Bibliography API Recommended Practice 550, Manual on Installation of Refinery Instruments and Control Systems, Part I, Process Instrumentation and Control, Section 2, Level, American Petroleum Institute, Washington, DC. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June-July 1997. Berto, F. J., The Accuracy of Oil Measurement Using Tank Gaging, ISA, Research Triangle Park, NC, 88–1561. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Cornane, T., Continuous level control, Measurement and Control, April 1997. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 7, 2001. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Johnson, D., Level sensing in hostile environments, Control Engineering, August 2001. Level measurement and control, Meas. Control, April 1991. Luyts, J. and Marcelo, L. D., Fieldbus HTG System Measuring On-Line Concentration, ISA, Research Triangle Park, NC, 1998. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Piccone, R. P., Combining technologies to compute tank inventory, Sensors, October 1988. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

3.3

Capacitance and Radio Frequency (RF) Admittance

LT CA To Continuous Receiver To On-off Receiver

D. S. KAYSER

(1982)

B. G. LIPTÁK

(1969, 1995)

J. B. ROEDE

(2003)

LS CA Flow Sheet Symbol


Service

Point level control (using contact and proximity sensors) and continuous level transmission for liquids, granular solids, and liquid–liquid interface

Design Pressure

Routinely to 1000 PSI (7 MPa), others to 5000 PSI (35 MPa), specialized applications to 20,000 PSI (140 MPa)

Design Temperature

500°F (260°C) maximum with insulated sensors; 1000°F (540°C) bare metal, sealed to 200 PSI (30 kPa); 2000°F (1100°C) bare metal at atmospheric pressure

Excitation

Less than 10 V @ 10 kHz to 1 MHz

Wetted Materials

Type 316 SS and TFE for common models, with options for CPVC, FEP, PE, PEEK,    PFA, PP, PVDF, urethane, Hastelloy , Inconel , Monel , nickel, titanium

Span

2 to 3 in. (50 to 75 mm) of insulating liquid to 1000 ft. (300 m) for immersion probes and 0.1 in. (2.5 mm) to 10 in. (250 mm) with proximity sensors

Inaccuracy

Horizontal, less than the diameter of the probe rod Vertical, less than 0.1 in. (2.5 mm) for bare single points in conducting material, roughly 1% of maximum active length for all insulated probes in conducting or interface service, 3% in insulating liquids (or 0.5% with dielectric compensation), roughly 5% for granular insulating solids with constant density and composition, and 2 to 5% for conducting granular solids

Dead Band

Unmeasurable with analog instruments and horizontal probes; dependent on A/D resolution with digital continuous instruments; small and application dependent on vertical single points, but optional dead band adjustment is available

Temperature Coefficient

Extremely variable depending on (a) probe insulation and degree of probe-to-sheath bonding in conducting materials, and (b) composition and density variation in insulating liquids

Damping and Time Delay

Adjustable time delay of 0 to 30 sec is included on most single-point controls; adjustable time constants up to 30 sec are available on most analog transmitters, and digital instruments offer zero to several minutes

Cost

$200 to $800 for single-point controls; $500 to 1500 for two-wire level transmitters; all with type 316 SS and TFE wetted parts; increased cost with exotic metals, hermetic seals, flange mounting, longer insertion length, digital output, dielectric compensation, extended press and temp, and longer inactives

Vendors (partial list)

ABB Process Automation Instrumentation Div. (www.abb.com/us) AMETEK Drexelbrook (www.drexelbrook.com) Arjay Engineering Ltd. (www.arjayeng.com) Babbitt International Inc. (www.babbittlevel.com) Bindicator (www.bindicator.com)

3.3 Capacitance and Radio Frequency (RF) Admittance

431

BinMaster (www.binmaster.com) Delavan Inc. Delta Controls Corp. (www.deltacnt.com) Endress+Hauser Inc. (www.us.endress.com) FMC Invalco (www.fmcinvalco.com) GLI International (www.gliint.com) HiTech Technologies Inc. (www.hitechtech.com) K-Tek Corp. (www.ktekcorp.com) Lumenite Control Technology Inc. (www.lumenite.com) Magnetrol International (www.magnetrol.com) Monitor Technologies LLC (www.monitortech.com) Monitrol Manufacturing Co. (www.monitrolmfg.com) Omega Engineering Inc. (www.omega.com) Penberthy (www.penberthy-online.com) Princo Instruments Inc. (www.princoinstruments.com) Robertshaw Industrial Products Div., an Invensis Co. (www.robertshawindustrial.com) Scientific Technology Inc. (www.automationsensors.com) Systematic Controls (www.systematiccontrols.com) Vega Messtechnik AG (www.vega-g.de)

INTRODUCTION Characteristic of probe-type sensors, the RF probes operate by applying a constant voltage to a metallic rod and monitoring the current that flows. This current is proportional to the admittance or capacitance (if conductivity is absent) from the metallic rod to a second electrode. Because the tank wall is the most convenient second electrode, most instruments monitor current to ground (which is usually connected through the probe mounting). The obvious difference between conductance and RF probes is the frequency of that constant voltage. Whereas conductance types use DC or low-frequency AC, the RF items usually operate in the range of 0.1 to 1.0 MHz (although special applications can operate at 15 kHz or even lower frequencies). RF probes are connected to their associated electronic units with coaxial cable except when the electronics are integrally mounted on the head of the probe. The classic shortcoming of capacitance probes is false HI level indications caused by conductive coatings that connect the above-level sensing element to ground (or the actual process level). Since 1970, solutions to this problem have existed in all but the heaviest, high-conductance process situations. In the case of single-point level switches, the answer is electrogeometric. In the continuous level transmitters, the approach is purely electrical. Because of the solid, no-moving-parts construction, there is very little to deteriorate or fail once an RF probe is installed. Compatibility with process liquids is the most obvious obstacle to a satisfactory life span. This is no problem with common, well-documented reagents at temperatures below 150°F (65°C). At higher temperatures, the increased chemical activity and accelerated permeation experienced by polymers can produce unexpected results. Abrasion of metals and insulators is another cause of shortened life that must be anticipated. Baffles to protect the sensor from high-velocity


solids, combined with judicious location, can minimize this danger. Probe failure in heavily agitated tanks can be avoided by attention to structural considerations. In the case of rigid probes, the most likely cause of breakage is fatigue failure caused by eddies rapidly pushing the probe in one direction and then in the opposite. A support, with an insulated bushing, near the tip of the probe greatly reduces the possibility of such a failure. Flexible cable types can wrap around an agitator and fail in minutes if not adequately anchored to the tank structure. If they are anchored without removing slack, they can whip back and forth, causing insulation failure and eventual breakage. Intermediate supports are possible in highly agitated service using insulated bushings. Beware of thick, conductive coatings that can cause substantial errors at each support point. These “shortcuts” to ground can defeat the electrical coating rejection in the worst cases. Correct structural design is the responsibility of the system designer, not the probe supplier. Most suppliers can give rules of thumb for their probes and provide structural details of the construction. It may require a consultant who is skilled in fluid dynamics and mechanics to arrive at a sure configuration in a highly agitated vessel. Process instruments that depend on electrical characteristics of the process material are at a disadvantage, given that the electrical character is of little interest to most instrument users. Fortunately, exact values of conductivity (g) are never required, and changes in relative dielectric constant (K) are more important than the precise value. In most cases, classification as conducting or insulating goes a long way toward successful application. Within the conducting category, it is sufficient to know that, except for completely deionized water, aqueous solutions will be conductive. On the insulating side, it is generally sufficient to understand that most liquids will have a K of 2 or greater. The main exceptions are liquids that would be gases at room temperature (with the notable

432

Level Measurement

except for ammonia, for which K > 15) and atmospheric pressure. Not only do these fluids tend to have K less than 2, they tend to have much higher temperature coefficients of K than insulators that are normally liquids. This section is divided into single-point and continuous transmitter categories to reduce confusion. The RF sensors are somewhat unique in that single-point switches have a substantial performance advantage over the continuous type in terms of accuracy, temperature capability, coating rejection, self-checking, and reliability. It seems that, in many cases, the transmitters are thought to be the superior approach to control, and they obviously are necessary to obtain a proportional band. In many cases, strategically placed singlepoint units are a superior route to precise and reliable process control. Considering that any instrument can fail on occasion, single-point probes provide highly reliable backup, with selftest capability, and are superior to any transmitter.

Center Rod Connection Process Seal

Inactive Length

Insertion Length

Weld

Mounting Flange Grounded Inactive Section Metal Rod Polymer Insulation Polymer Plug

FIG. 3.3b An insulated, two-terminal probe with grounded inactive section.

TYPES OF PROBES The most basic probe configuration is a metal rod. The rod is insulated from a metal mounting element that connects it to the process vessel via threads into a half coupling or a flange that mates with one on a tank nozzle. The insulated junction of rod and mounting includes whatever seal is required between the process and outside world. The next step in complexity includes an insulating coating on the rod, (Figure 3.3a) that isolates it chemically as well as electrically from the process. Insulated probes should have the insulation securely bonded to the metal rod over the entire range of service temperature. This bonding ensures that process pressure changes

Center Rod Connection Process Seal Mounting Flange Metal Rod Insertion Length

Polymer Insulation

Polymer Plug

FIG. 3.3a Insulated two-terminal probe.


will not compress air space and change the calibration. Bonding is also important to minimize permeation, which is present to some slight degree with all polymers. The addition of a tight-fitting metal tube over part of the insulation, welded to the mounting element (Figure 3.3b), will make the covered section of the probe “inactive,” because it will always see the same impedance to ground, regardless of the process material on the outside. A more sophisticated way to “inactivate” a section of the probe is to use the widely known electronic guard principal. A tight-fitting metal tube, insulated both from the rod and the mounting element (Figure 3.3c), with a voltage identical to that on the rod, will not only deactivate that section of the probe but will also negate the rod to mounting capacitance. These are sometimes referred to as three-terminal probes, because there is now a rod connection, a ground connection, and a guard (sometimes called a shield) connection, as opposed to the two-terminal rod and ground style connections. An additional variation is the probe that carries its own intrinsic ground reference. This can be a larger concentric tube welded to the mounting element (Figure 3.3d), with bleed holes at the top to avoid compressing gas as the liquid rises in a closed chamber. Perforated tubes, insulated and bare ground rods, as well as structural cages (Figure 3.3e) are also available for various conditions of agitation, temperature, and chemical compatibility. The sensors, which use a ground wire wrapped in a helix directly on the probe insulation, are unstable, unreliable, and facilitate coating. A three-terminal probe, with a plate welded on for greater capacitance (Figure 3.3f), makes an excellent proximity sensor. The field from the guard can even be used to direct or focus the field from the sensing element and determine the region of sensitivity. In proximity measurements, the


Center Rod Connection Guard Connection


Process Seal Mounting Flange Polymer Insulation

Guard Length

Guard Element Polymer Insulation

Insertion Length

Metal Rod

FIG. 3.3c A three-terminal probe that employs the “electrical guard.”


Weld Bleed Holes Insertion Length

Mounting Flange Concentric Ground Tube Metal Rod Polymer Insulation

433

Mounting Flange Welds

Insertion Length

Grounding Cage Metal Rod

Polymer Insulation

Polymer Plug

FIG. 3.3e A two-terminal probe with a cage, for grounding in viscous liquids.

Center Rod Connection Guard Connection Process Seal

Mounting Flange Polymer Insulation Guard Element Polymer Insulation Proximity Plate

FIG. 3.3d A two-terminal probe with intrinsic, concentric ground reference.

FIG. 3.3f A three-terminal probe with plate for proximity sensing.

capacitance change (proportional to sensor area) is typically very small but stable, because it is effectively looking at the air capacitance between sensor and process material. Even minute capacitance changes due to temperature or stress in the mounting area could be catastrophic with an unguarded probe. The distance between the probe and process material should be as short as feasible. The capacitance produced is inversely proportional to this distance. This means that beyond 10 in. (250 mm), the capacitance change becomes extremely hard to detect. The error produced by splashing and condensation buildup on the plate is fairly benign—never more than

the actual coating thickness, and usually less. Spans of 2 or 3 picofarads (pF) have been used successfully. Such small ranges are the result of level change in shallow pans, with limited space for proximity plates. The electronic guard principle can be employed in an infinite variety of geometries. A typical example is the flat plate probe (Figure 3.3g), which mounts flush with the tank wall. This is very effective in reducing abrasion, eliminating bending of low-level sensors under heavy mechanical loads, and eliminating sparks from static discharge. The monitored current flows from the center plate, and the guard surrounds


434

Level Measurement

Connection Enclosure

Electronics or Connection Enclosure

Mounting Holes Polymer Insulator Sensing Plate

Added Tee

Guard Element

Vent Inactive Section

Gasket Surface

FIG. 3.3g A three-terminal flush mounting probe. Probe

FIG. 3.3i Vent pipe serving dual function for tank access.

Hermetic Seal

Flange Facing

Probe Insulation

FIG. 3.3h Probe with plastic-welded process seal. (Courtesy AMETEKDrexelbrook.)

it, interrupting a path to ground through any coating on the face of the probe. For the ultimate in seal reliability and hermeticity, plastic welding offers unique benefits. By plastic welding a polymer flange facing to the probe insulation (Figure 3.3h), a highpressure seal is formed (and has been used to 5000 PSI [35 MPa] with the correct flange rating), with the normal process seal acting as a backup. This type of weld is not possible using TFE, which does not melt, but most other polymers are candidates. It also precludes the use of thin probe insulation. The flange facing should be used as the only gasket, because it must be pinched between the flange faces. Only raised face flanges provide the correct sealing, so RTJ and flat-faced flanges are not applicable.

impossible, because the tank is pressure coded, requires arduous inerting, or cannot be taken out of service. One very simpleminded and geometrically demanding approach is to use a bent probe inserted through a side entry so as to get its active area to the desired measurement location. A flange mounting is usually mandatory to avoid “screwing,” which would cause the bent probe to rotate inside the tank like an airplane propeller. Its angular location, once the thread is tightened, would also be a question. If a suitable nozzle is available, it must be short enough to get the bend in the probe “around the corner” before it binds. If the measuring leg of the probe is relatively long, it must not hit the opposite wall of the vessel before the bend allows it to pivot. Another drawback to the bent probe is the requirement that any insulation be flexible enough to tolerate the bending process without tearing or splitting. This limits the selection to soft, thick insulation and therefore relatively low capacitance (60 to 100 pF/ft). Many strategies allow us to avoid the bent-probe trap by using a little bit of imagination. Every tank or bin should have a vent or pressure relief pipe. It is usually possible to tee that pipe so that the vent or relief goes off to the side and a probe has straight access into the process (Figure 3.3i). This does not affect the pressure code of the vessel, since it is external plumbing. Other ways to avoid the “pretzel probe” include the following: • • •

MOUNTING AND TANK ENTRY • One of the most frustrating barriers to good RF probe application is inadequate or misplaced tank access on existing tanks. Often, adding a nozzle or even a half-coupling is


•

Angle mount a straight probe from the side. Install the probe from the bottom up. Use a grounded inactive section to get through various impediments. “Dog-leg” a pressure gauge to give the probe a straight shot. Cut a hole in the building’s roof or ceiling in lieu of headroom.


435

Isolation Valves

Probe

Cage

FIG. 3.3j Side arm (cage) mounting, when there’s no other way. (Courtesy of Robertshaw Controls Co.)

• • •

Use “rear mount” piping to plumb through a dust collector or a side of the tank. Add a stand pipe (sidearm) parallel to the tank (Figure 3.3j). Tee into a fill pipe (this needs detailed application analysis).

Ingenuity will always trump bent probes for performance, cost, and efficiency. ELECTRONIC UNITS Electronic units for single-point instruments are available in line-powered configurations for 24 VDC; 120 V, 50 to 60 Hz AC; and 230 V, 50 to 60 Hz AC. They also can be obtained with “universal power” capability that allows them to use any of these plus 130 VDC. The output from these units is generally a set of double-pole, double-throw relay contacts. Some of the DC-powered instruments use an NPN or PNP output transistor to effect the switching. There are also two-wire, loop-powered versions that offer intrinsic safety and automatic self-checking. The signal from these instruments is a high or low current within the 4- to 20-mA range. Various types of automatic calibration are available for these instruments, but they all encounter certain conditions that prevent them from being calibrated properly under every possible condition. Regardless, the calibration of single-point instruments is hardly rocket science.


Level transmitters are primarily loop powered, although line-powered items are available from some suppliers. The traditional analog 4- to 20-mA instruments have been giving way to those with microprocessors on board. Most of the digital instruments are capable of communication using one of the HART, Honeywell, or fieldbus protocols. This allows interrogation and modification of the instrument by means of a digital communicator. One of the most popular features of these instruments is their ability to calibrate on any two points in the range. It is possible to enter the correct output (in milliamps or level units) at an existing low level and enter a high point days later. The instrument locks the input/output curve into those two points. The signal from the instrument is either analog 4- to 20-mA or digital. The digital mode allows more than one transmitter to use a single loop. The limited power available at the transmitter, combined with the multitude of calculations being executed, causes digital instruments to be considerably slower than the analog ones. Response times of 3 to 4 sec are possible, whereas analog instruments are capable of responses within 100 to 300 ms. For small tanks with high fill and drain rates, the digital instruments might not be an option. A hybrid instrument is the “multipoint” control. It is essentially an analog instrument with internal, adjustable pick-offs and multiple relay outputs. It is useful for sump control where several pumps might be involved, but the absence of an analog output makes calibration lengthy. Each pick-off must be adjusted with the level at the desired point on the probe. This type of instrument is specified for HI, HI-HI,

436

Level Measurement

and HI-HI-HI points at times. This doesn’t make much sense, because one failure kills the whole operation.

probes for switch points close to the bottom and top of a tank rather than a vertical 2 or 3 in. (50 or 75 mm). Conducting Process Materials

SINGLE-POINT SWITCHES The typical capacitance switch employs a vertical or horizontal probe, either polymer insulated or bare metal, projecting some distance into the process vessel with the center rod insulated from the mounting (Figure 3.3k). With any gas or vacuum (K = 1.0) on the probe, there will be a minute RF current flowing from the center rod to the metallic tank wall. When a liquid or granular solid covers the center rod, the current from there to ground will increase. This change in current is detected by the electronic unit, which switches the output. When the process material drops below the probe, the RF current decreases and, hopefully, returns to its previous low-level state. If the process material is thick and conductive, any coating remaining on the insulator between probe and mounting will maintain a higher RF current level and can cause the switch to continue signaling a high level. The answer to false high-level indications is based on the classic electronic guard principle. By interposing a third metallic element (shield) with an identical RF voltage between the center rod and the mounting (Figure 3.3c), no current can flow from center rod to mounting. The shield element supplies whatever RF current the coating demands, but this current is not included in the measurement. “Length is strength” in capacitance probes. Longer active lengths translate to more substantial capacitance changes in insulating processes, and longer interelement insulators allow the instrument to reject higher conductance coatings. This means using horizontal

Center Rod Connection Process Seal Mounting Flange

Polymer Insulation Insertion Length


Insulating Process Materials The insulating materials (oils, solvents, resins) are subtler in their effect on the switching circuit. A horizontal probe produces a sharp capacitance change over the thickness of the center rod. Any portion that is in a nozzle should be inactive, either with the guard or a grounded inactive. With a vertical probe, capacitance increases gradually as more of the active length is covered. The switching point is adjustable even if the probe is bare metal. Good practice requires that the probe be at least 2 in. (50 mm) into the process material at the desired switching point. Attempting to get switching at the tip of the probe will lead to unreliable performance, because the slightest change in probe mounting or electronics can cause a constant high-level signal. Dielectric constant (K) is relative to the absolute dielectric of a vacuum (K of gases barely differs). This means that the capacitance (and the proportional RF current) in air will be doubled in gasoline (K = 2). It will be multiplied by 20 in ethanol (K = 20). It will only be raised by 60% in liquid carbon dioxide (K = 1.6). Insulating coatings are a very minor problem. Just avoid bridging to a grounded part. Plastic, Concrete, or Fiberglass Tanks and Lined Metal

Metal Rod

FIG. 3.3k Bare metal, two-terminal probe.

Conducting materials (aqueous, metals, and most forms of carbon) carry the ground potential of the tank walls right to the probe. If bare metal, the probe will signal “high level” the instant it contacts the process. If it is insulated, the switching point will depend on the capacitance setting of the instrument. With a vertical, insulated probe, it is possible to adjust the level at which the switching takes place by varying the capacitance setting of the instrument. A horizontal probe of either type allows no level adjustment other than by relocating the mounting. Coating is a serious consideration in all conducting materials, and the guard-type construction is usually required. With bare metallic probes mounted vertically, the plain capacitance probe will do as long as the process never reaches the mounting area and no crystallization or heavy condensation occurs. When using the guarded probes, the guard should be long enough to project well into the vessel, beyond nozzles and potential wall buildup.

The absence of metallic contact for a ground reference is seldom a problem using single-point RF probes in conducting process media. The probe will indicate high level whenever it touches the process. Unlike metallic tanks (or those with metal pipes, pumps, or grounding rods), it must be tuned to a value less than the capacitance-to-ground value of the tank. Most tanks have at least 10 pF to ground, which is a perfectly adequate level for a reliable measurement. Probes with a


guard element should not be mounted horizontally in ungrounded tanks with conductive contents. The guard can drive the entire process at the same voltage as the center rod, hence there is no current flow and no high-level signal. In the case of an insulating medium in a fiberglass tank, even metal pipes offer little help to the miniscule capacitances. The only sure answer is a vertically mounted probe with its own metal ground rods or concentric tube. This is also a good precaution against RFI from walkie-talkies and other sources, which can cause false high-level signals. Metal tanks that are rubber or plastic lined and steelreinforced concrete vessels, on the other hand, represent excellent RF grounds. The capacitance from process to metal structure is very high as compared with the capacitance produced by an insulating medium. This makes it look like a short circuit to the RF current. It is excellent for conducting media, too, but it still requires the avoidance of horizontal probes with driven guards. Underground concrete or fiberglass tanks (except in desert conditions) present a similar ground configuration. Concrete structures of cinder block with no vertical steel reinforcing bars are completely ungrounded and perform exactly the same as fiberglass tanks. Interface Electrical sensing is the premier method of detecting the interface between an insulating and a conducting process medium. The typical margin between the conductivity of organic and aqueous phases is greater than 1000:1. The measurement is completely independent of temperature and density variation. Probes may be mounted vertically or horizontally. Vertical probes should be inactive down to about 6 in. above the desired interface control point. This can be accomplished by use of the electronic guard, a grounded inactive, or a short probe mounted from the rear (rear mount) on the end of a suitable pipe. The instrument will indicate high level as soon as conductive material contacts the tip of the bare probe. In the case of heavy oil separators, it may be desirable to use a probe with sharp edges machined into the tip (Figure 3.3l). This will ensure good contact between water and steel in spite of oil coating. An interface detector with bare metal rod should be tuned to the maximum capacitance level of which it is capable. Horizontal probes should be bare metal with relatively long insulators between probe and mounting (ground). Typical proportions, to maximize the margin between insulating and conducting phases, would be 12 in. (300 mm) overall length with a 10-in. (250-mm) long insulator. The use of sharp edges machined on the tip of the probe is also advantageous in this orientation. This is one situation in which the electrical guard is of no advantage (coatings are usually insulating rather than conducting) and can actually be a drawback if the guard drives the entire conducting phase at the same potential as the probe. This phenomenon is usually a product of insufficient grounding, but it has been observed in metal tanks.


437

Metal Rod

Sharp Edges

FIG. 3.3l Oil shedding tip for interface with viscous organic phase.

GRANULAR SOLIDS The best approach for establishing the high level in large silos is usually via a flexible cable probe with a long, thin weight at the bottom. It can be mounted close to the fill point (Figure 3.3m) and negate most of the angle of repose that side-mounted sensors encounter. The flexible aspect allows it to swing out of the way when struck by incoming solids. The actual switching should take place on the weight that will be least vulnerable to abrasion. In insulating materials, at least 6 in. of weight should be covered at the desired switching point. Conducting materials, of course, will switch as soon as they touch the tip of the probe. Incoming material will not cause false high-level indication as long as it is in free fall. This is because there is a very small air capacitor between each grain and all the others. All this series air capacitance means that any deviation from normal air capacitance is a negligible quantity. If the incoming material is compressed (especially conducting materials), it IS possible that it could cause a trip, so the probe should be located accordingly. Rigid probes, located near the fill, are suitable for smaller bins and lighter duty.

Fill

Top of Silo

Electronics or Connection Enclosure Half Coupling

Cable

Insertion Length Process Granular

Weight

FIG. 3.3m High-level probe for heavy granulars in large silos.

438

Level Measurement

Horizontal probes mounted in the bin wall should be of the “guarded” variety to avoid false high-level alarms— especially for low-level service. The primary concern is bending or abrasion, which becomes more acute as the depth and density of the material increases. Short, fat probes have much better lifespan, and it is possible to use a flat-plate probe (Figure 3.3g) mounted flush with the wall for optimal life.

CONTINUOUS TRANSMITTERS Probes are most commonly mounted vertically from the top of the tank. Angle mounting through the side of a tank is possible, as is mounting up from the bottom. When tank penetrations are at a premium, it is often possible to “tee” into a vent or drain line for probe mounting. There are even cases in which probes have been successfully “teed” into fill nozzles, but this requires considerable application analysis to predict the effect of incoming material. The insertion length of the probe should not extend beyond the desired measuring range by more than 5%. An active probe that is not producing signal can still produce errors. Conducting Liquids The main concept required to understand this class of applications is called saturation. Assume that an insulated probe is immersed in deionized water with minimal conductivity

(Figure 3.3n). The addition of drops of hydrochloric acid to the vessel will gradually increase the conductivity. The output will also rise as more RF current flows from probe to ground. Eventually, the conductivity will be high enough that the resistance (R) from probe to tank wall is negligible compared to the capacitive impedance of the probe insulation (C5). Further increases in conductivity will make no observable difference in the RF current and, therefore, the output (i.e., the RF current has reached its saturation point). This is the concept that makes the RF transmitter an instrument rather than a lab curiosity. A probe that is not saturated will have a calibration that is a function of two variables (conductivity and level), just as a d/p transmitter calibration is a function of density and level. It is possible to adjust the threshold of saturation by adjusting insulation thickness (hence capacitance) and excitation frequency. Raising the impedance of the insulation lowers the threshold, and vice versa. Saturation thresholds from 0.1 to 1000 µS/cm can be accomplished. A reasonable question might be, “Why not just use the 0.1 µS/cm threshold at all times?” The answer is “coating rejection.” The higher impedance that correlates with a low saturation threshold is less able to ignore high-conductivity coatings. In fact, a large mismatch may allow the coating to saturate the probe so that the output reflects the actual level plus coating length. In general, the path to best conducting coating rejection lies in the highest probe capacitance and excitation frequency that will allow saturation by the process material

A1 A2 Ce

B

C1

Ce

C1

C4

C2

C5

C3

Ka C4

C2

L

R = Any Value

C5

I

C3 Ce = C1 +

Kp

C2 C4 C2 + C4

+

C3 C5 C3 + C5

R

FIG. 3.3n Electrical representation of an insulated probe in conductive liquid. (Courtesy of The Foxboro Co.)



being measured. A special case for coating rejection is one in which the process liquid is conductive but can dry on the probe, forming an impervious, insulating coating. An example is latex paint. The liquid is quite conductive, but the dried paint adds to the insulation thickness of the probe, decreasing its capacitance and changing the calibration. The solution is to use a thick (low-capacitance) probe insulation so that thin additions will have negligible effect on calibration. By using the highest possible frequency, conductive coating rejection will also be maximized, and the resulting accuracy will be the best possible compromise. Complex impedance (or its inverse, admittance) measurement allows the electronic unit to measure two variables: the capacitive component of RF current and the resistive component of RF current. The actual liquid level that saturates the probe produces a pure capacitive current. The conductive coating, because it is relatively thin, has a much higher resistance than the bulk liquid, so it will produce both capacitive and resistive phase current. By subtracting the resistive component from the capacitive, the effect of conductive coating on the output will be decreased. In fact, once the coating is longer than a nominal value, the two RF current phases equate, and the effect of coating is precisely zero. Maximum coating error occurs when the coating length is relatively short and its resistance therefore is low. The maximum error is a fixed, predictable number of inches for a given probe capacitance, excitation frequency, coating thickness, and conductivity. This means that percent inaccuracy due to coating will be greater on short ranges than on long ones. In other words, length is strength.

Insulating Liquids Probes for measuring insulating liquids should generally include their own parallel ground reference to guarantee a uniform distance to ground through the process liquid. Linearity, sensitivity, and immunity to RFI are enhanced by a concentric ground tube (Figure 3.3d) or other construction. In some cases, it is efficacious to use a metallic tank wall, baffle, or ladder to furnish the required parallel ground reference. This is frequently the case in pharmaceutical or beverage applications where a concentric tube interferes with any clean-in-place function. It is also common in tall tanks that require flexible cable sensors and with slurries, which can accumulate solids and plug the ground reference. Changes in dielectric constant will cause a change in calibration. If composition is constant, temperature will be the only concern for variation of K. Because K is proportional to the number of molecules between probe and ground, output (as with a d/p transmitter) will be proportional to density and, hence, the weight of process material. For cases in which constant composition and a reasonably narrow temperature range are not possible, a transmitter with dielectric compensation is available. By using such an instrument, it is possible to obtain a level signal that is independent of dielectric, and


439

Concentric Shield (Ground Reference) Level Sensor Insertion Length (I.L.)

Measured Level Reference (Composition) Sensor

5.5"

FIG. 3.3o A probe with a second element for dielectric compensation. (Courtesy of AMETEK-Drexelbrook.)

even conductivity, when tanks are not dedicated. This technology demands two conditions: 1. A short inactive section at the bottom of the probe, approximately 6 in. (150 mm) 2. Homogeneity of the process liquid (no stratification) The compensation is accomplished by making two independent measurements. The first measurement is made with a short sensor (composition probe) below the tip of the level probe (Figure 3.3o). This segment is assumed always to be covered by the process liquid. Its capacitance is proportional to the electrical character of the process material. The second measurement uses the level probe. Its output is proportional to the level times the electrical character of the process fluid. By dividing the output of the level probe by the output of the composition probe, the transmitter output becomes independent of the electrical properties. Please note that conductive coatings can cause large inaccuracies in these particular transmitters. Even so, conducting liquids that do not coat will be measured with accuracy equal to the insulating ones. For the sake of utility, both probe segments are usually provided, coaxially, in the same assembly. It is also possible to use two completely separate probes, with the composition probe mounted horizontally below the tip of the level probe. Inaccuracy can be limited to 0.5% over a wide range of electrical values and a wide temperature range. Continuous Liquid–Liquid Interface Two elements are key to successful interface level measurement: immunity to total level variation and maximum margin between the signal contribution of the organic and aqueous

440

Level Measurement

phases. Making the probe inactive down to a point that always will be covered by liquid can negate variation in total level. Of course, packed vessels and separators on overflow do not exhibit the problem. The primary item to minimize the signal from organics is to maximize the distance to ground. Low K is another positive item. The only practical way it can be varied is by better separation to exclude the high K aqueous content. Concentric ground references, small-diameter stilling wells, and external standpipes, by bringing the ground closer, make the measurement more sensitive to changes in the organic phase. The ideal mounting is in the center of the tank, with no metal nearby. Maximizing the aqueous contribution means using a probe insulation with the highest feasible capacitance. A good margin would be one in which the aqueous contributes 300 pF/ft and the organic 8 pF/ft. Regardless, many very satisfactory separators in the heavy oil fields operate with an 80to 30-pF/ft margin. The limitations on maximum probe capacitance include fragility (thinner coatings have higher capacitance), temperature (TFE and PFA have the lowest K of common polymers and highest temperature capability), and chemical compatibility (again, the low-K insulators are most inert chemically). The question of an intermediate emulsion or rag layer adds a bit of confusion to the measurement. Users often desire a measurement at the bottom of the rag, but that area is generally very close to the aqueous in conductivity. The point sensed by the continuous probe is always that at which the emulsion reverses from organic continuous (aqueous drops in organic matrix) to aqueous continuous (organic drops surrounded by aqueous liquid). In liquids with distinctly different colors, users often believe that the instrument is not functioning correctly. The visual interface typically does not coincide with the electrical interface except in the rare case of perfect separation. This means that sight glasses are useless for calibration of interface transmitters. Sample taps are more valuable, but only if the electrical conductivity of the sample, rather than color and viscosity, is observed. Users often lose sight of the fact that the instruments cannot separate the components. The first criterion for accurate interface control is good separation. This may entail more or better emulsion-breaking chemicals, longer residence time (less throughput), higher temperature, or additional mechanical or electrical aid to coalescence. The idea that certain instruments can produce lower organic in the aqueous stream, less water in the organic stream, and raise throughput is total baloney. In many cases, separators are run above the design throughput, so something needs to be done to accelerate separation if product quality is to be maintained. For best results, the probe should be located as far from the inlet as possible, where the best separation will have occurred. The most frequent use of interface transmitters is in horizontal separators, where the total level is usually maintained by overflow, and the interface is maintained by controlling the aqueous dump valve. A whole other class of application has been largely ignored because of the difficulty in getting


the probe to the interface area. That application, sometimes called water bottoms, measures the water level beneath organic products (usually fuels) in large storage tanks. Typically, this involves measuring a few inches of water at the bottom of 40-ft (15-m) or taller tanks. Dropping a short probe, attached by a cable or pipe, down from the top of the tank is pretty straightforward unless the tank has a floating roof. The most successful floating-roof applications have used a 45° hot-tapped nozzle near the bottom of the tank. Because the tank is seldom empty, it generally requires a probe to be inserted through a sliding seal under pressure. Granular Solids The most common problem in granular measurements is caused by moisture variation in insulating materials, which represent the majority of granular solids. Since water has a nominal K = 80, and the typical solids are in the K = 2 area, small variations in water content can produce large percentage changes in K – 1.0 and, hence, the slope of the input/output curve. If the particular granular produces a similar change in conductivity, subtracting the resistive phase of RF current from the capacitive component can compensate moisture variation over a particular range of water content. The same instrument that provides electronic coating rejection, for example, compensates grains of wheat, corn, and rice. These whole grains produce capacitive and conductive RF current components that are affected equally by moisture variation in the 8 to 20% water range. By subtracting the resistive component from the capacitive, the effect of these changes is eliminated. At the opposite end of the moisture spectrum, very dry (600°F), it is difficult to fabricate, impossible to plastic weld, and exhibits a high degree of microporosity. Can be destroyed by butadiene and styrene monomer.

TECHNOLOGY The purpose of this section is to help the more technically inclined to understand just what is happening in a particular application. It is not necessary to personally make good use of this information. Your friendly vendor will be glad to take the necessary steps to ensure a good result with the instruments you purchase. Analysis of RF probe circuits requires little more than a knowledge of AC circuits. The simplest situation is the bare probe mounted in the center of a vertical, cylindrical tank (Figure 3.3q). The expression for C2 and C3 is the formula for a concentric capacitor and can also be used to calculate the capacitance of probe insulation. (To do so, use K = 2 for TFE, PFA, FEP, PE, and PP. Use K = 8 for PVDF.) The length units are inches, and the diameters can be any units as long as both are the same. A good approximation of C1 is 20 pF. For rough estimates of air capacitance in tanks that are not cylindrical and/or not concentric, 0.35 pF/in. is a good rule of thumb. If the probe is within 1 ft of the wall, it will be higher. A probe in the center of the tank is least sensitive to position. Capacitance increases as the distance to ground is reduced. In the center of the tank, motion toward one wall increases the distance to the opposite wall, so the capacitance is nearly constant. Close to the wall, the capacitance increases rapidly with any reduction in the distance. The capacitance produced by any insulating liquid will be its K times the air capacitance. The insulated probe complicates the calculation because of the nonlinear combination of series capacitors (Figure 3.3n). The equation shown is correct for insulating liquids. For conducting liquids (assuming saturation), the third term is reduced to C5 , because the low resistance represents a short circuit to ground. In the case of a liquid–liquid interface, the organic phase would substitute for the air space shown. The K of the organic phase must be measured based on an upper phase sample at operating temperature. The amount of aqueous phase held in the organic will be a function of temperature as well as throughput. Table 3.3p is a list of nominal dielectric constants, which can be used to make rough estimates of what capacitance will result from a particular probe geometry.

444

Level Measurement

A B Probe

Ce

Insulator

C1

Ce

Vessel C2

C1

C3 Ka

R∼∞

C2

L Ce = C1 + C2 + C3 =

I

Kp

C3

= C1 +

0.614 Ka (L-I) log10 A/B

+

0.614 Kp I log10 A/B

R∼∞

FIG. 3.3q Electrical representation of a bare probe in an insulating liquid. (Courtesy of The Foxboro Co.)

CONCLUSION Early capacitance probes had the advantages of simplicity, relatively low cost, corrosion resistance, and a lack of moving parts. In addition, the proximity design required no contact with the process fluid. On the other hand, these early capacitance probe designs were subject to errors resulting from changes in the dielectric constant of insulating process fluids, to errors resulting from the tank geometry and fiberglass construction, and to conductive coating buildup on the probes. In the newer designs, these problems have been largely solved by increasing the operating frequency, incorporating a phase detector component in the electronic circuits, and modifying the design of the sensors and their guards. These improvements have made the capacitance/RF admittance type probes a powerful means of level detection. The only types of measurements that are still excluded with these instruments are interfaces between two conductive liquids or the liquid–solid interface. A major maintenance problem is water in the head assembly that results from incorrect conduit routing and venting. The head may be potted with two-component RTV to eliminate this problem.

Bibliography Andreiev, N., Survey and guide to liquid and solid level sensing, Control Eng., May 1973.


API Recommended Practice 550, Manual on Installation of Refinery Instruments and Control Systems, Part I, Process Instrumentation and Control, Section 2, Level, American Petroleum Institute, Washington, DC. Belsterling, C. A., A look at level measurement methods, Instrum. Control Syst., April 1981. Capacitance method for liquid-depth measurement, Electron. Power, December 1967. Dinkel, J. A., Universal capacitance probe liquid level measuring system, Rev. Sci. Instrum., November 1966. Duncan, J. and Dutton, W., Capacitance probe confirms presence of liquid NH3 when unloading, CIM Bull., January 1978. Hall, J., Measuring interface levels, Instrum. Control Syst., October 1981. Herbster, E. J. and Roth, J. H., How to gage by capacitance, ISA J., June 1965. Lawford, V. N., How to select liquid-level instruments, Chemical Eng., October 15, 1973. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Mital, P. K., Capacitor sensor monitors stored liquid level, Electronics, October 30, 1967. Morris, H. M., Level instrumentation from soup to nuts, Control Eng., March 1978. Preshkov, V. P., Capacitance liquid helium level indicator, Cryogenics, April 1969. Proximity sensors, capacitive, Meas. Control, February 1991. Ritz, G., Choosing the right solids level sensor, Control, January 1994. Schonfeld, S., Capacitance gaging checks spacecraft fuel level, Hydraulics and Pneumatics, April 1967. Schuler, E., A Practical Guide to RF Level Controls, Drexelbrook Engineering Co., Horsham, PA, 1989. Tavis, R., How to mount a capacitance level-sensing probe in your vessel, Powder and Bulk Solids, 19, April 2000. Weiss, W. I., Capacitance level control, ISA J., November 1966.

3.4

Conductivity and Field-Effect Level Switches D. S. KAYSER

(1982)

B. G. LIPTÁK

(1969, 1995, 2003)

LS To On - Off Receiver

COND.

Flow Sheet Symbol

Applications

Point detection or differential detection of level of conductive liquids or slurries having dielectric constants of 20 or above. For electric switches, the maximum fluid resistivity is 20,000 Ω/cm; electronic types can work on even more resistive fluids. Field-effect probes are used on both solids and liquids, and they can be conductive or nonconductive.

Design Pressure

Up to 3000 PSIG (21 MPa) for conductivity probes, 100 PSIG (6.9 bars, or 0.69 MPa) for field conductivity probes

Design Temperatures

Operating ranges for conductivity-type level switches are from −15°F (−26°C) to 140°F (60°C) for units with integral electronics and from −15°F (−26°C) to 1880°F (982°C) for units with remote electronics. Field-effect probes can operate at up to 212°F (100°C).


Conductivity probes are made out of type 316 stainless steel, Hastelloy , titanium,  or Carpenter 20 rods with Teflon , Kynar, or PVC sleeves. Housings are usually made of corrosion-resistant plastic or aluminum for NEMA 4 and 12 service. The field effect probes are available with a Ryton™ probe with aluminum housing.

Probe Lengths

Conductivity probes with 0.25 in. (6 mm) solid rods are available in lengths up to 6 ft (1.8 m); 1/16-in. (2-mm) stainless-steel cables can be obtained in lengths up to 100 ft (30 m). The length of field-effect probes is 8 in. (200 mm).

Sensitivity

Adjustable from 0 to 50,000 Ω for conductivity probes

Inaccuracy

0.125 in. (3 mm)

Cost

From $100 to $500. The typical price of an industrial conductivity switch is about $300.


AMETEK Drexelbrook (www.drexelbrook.com) Bindicator (www.bindicator.com) B/W Technologies (www.gasmonitors.com) Conax Buffalo Technologies (www.conaxbuffalo.com) Endress+Hauser Inc. (www.us.endress.com) Lumenite Control Technology Inc. (www.lumenite.com) Magnetrol International (www.magnetrol.com) Monitor Technologies LLC (www.monitortech.com) National Instruments (www.ni.com) Omega Engineering Inc. (www.omega.com) Rosemount Inc. (www.rosemount.com)



Conductance is the inverse of resistance (G = 1/R). The conductivity of a substance is defined as the conductance (G) of cylindrical column of that substance, which is 1.0 cm long 2 and 1.0 cm in area. The units of conductivity are in siemens

per centimeter (S/cm) or in mhos (siemens and mhos are different terms for the same unit). The ability of a liquid to conduct electricity is a function of the number of charged ions in that solution. Deionized pure water has very high 445


446

Level Measurement

resistance, but liquids such as sewage water, sea water, and city water are quite conductive. In fact, the conductivity of most liquids is much higher than that of air. Therefore, if an electrical circuit is closed by the substance surrounding the tip of a probe, the resulting current flow will be much higher when the probe is submerged in the liquid that when it is the air space above it. Conductivity-type level switches discriminate between air and liquid by that method.

CONDUCTIVITY-TYPE LEVEL SWITCH The process fluid itself closes an electric circuit when the air–liquid interface rises to contact the level probe (Figure 3.4a). The dual-tip probe of this design eliminates the need for grounded metallic tanks and can be used to detect both levels and interfaces between conductive and nonconductive liquids. The current flow is at the microampere level, which

removes the hazards of shock and sparking. The sensitivity of the switch is adjusted to match the conductivity of the process liquid. In grounded metallic tanks, the level probes are usually installed vertically. If the liquid in the tank is agitated or is turbulent for other reasons, it is desirable to employ two electrodes in detecting a single level point (Figure 3.4b). The small vertical distance between the two probes provides a dead band or neutral zone, which protects against cycling in case of splashing inside the tank. A 0- to 20-sec time delay is also available serving the same purpose. Figure 3.4c illustrates a conductivity level probe installed in a grounded tank. One electrode is shown above the liquid level on the left side of the sketch. The circuit on the left, therefore, is open, no current is flowing through the relay coil, and the load contact is open. When the liquid level rises, as shown on the right side of the sketch, a conductive path between the electrode and the grounded tank is established,

24 V Alarm T1

117 V

FIG. 3.4a Explosion-proof conductivity switch with dual-tip sensor operates at microampere current levels and eliminates the need for a grounded metallic tank. (Courtesy of Revere Corp. of America.) To Low Level Alarm

To High Level Alarm Electrodes Power Input 115V 50/60Hz

Load Contact

Load Contact

External Cables Relay

Relay Conductive Fluid A.C. Supply Transformer

FIG. 3.4b Dual-probe conductivity switch. (Courtesy of Endress+Hauser.)


FIG. 3.4c Single-point conductivity switch.

A.C. Supply Transformer

3.4 Conductivity and Field-Effect Level Switches

closing the circuit through the relay coil. Energizing the relay closes the load contact, which, in turn, can operate pumps, solenoid valves, and other processing equipment. In this system, the liquid in the tank acts as a switch in the relay circuit. Although electromechanical relays are shown in the figure, these days, the solid-state relays are more commonly used. If the tank is fabricated of fiberglass or other insulating material, the switching circuit is configured between the sensing probe and a reference probe (Figures 3.4a and 3.4b). Pump Alternator Circuit Because these switches are available in a variety of configurations, they can be used for the on–off control of one piece of equipment or for the staged control of several pieces of equipment. When two pumps are installed in the same on–off service, it is desirable to automatically alternate the pumps so that they will wear evenly and so that “hot starting” of pump motors is reduced. Figure 3.4d shows how one level switch with two conductivity probes can be used in conjunction with an electromechanical alternator to cycle the pumps. As level rises, the LSL contact of probe 1 will close, because this contact is operated by the lower probe. The control relay CR remains de-energized with its contact CR-2 closed and contacts CR-1 and CR-3 open. When the level rises to a higher level, the LSH contact of probe 2 closes, thereby energizing CR, which, in turn, closes contacts CR-1 and CR3 and opens CR-2. The relay CR will remain energized after the level drops below and the LSH contact opens, because CR-1 is still closed. With CR energized, CR-3 is closed, and

Neutral

110 Volt LSH

LSL CR

CR−1

CR−2

CR−3

Position Switch A

2

B Switch Motor 1 Sequence Switch M1

M2

FIG. 3.4d Pump down alternator circuit. M1 = first pump; M2 = second pump.


447

the circuits that are connected to contact A of the position switch and to contact 1 of the sequence switch are energized. The circuit connected to contact A starts the alternator switch motor, which, in turn, moves the position switch to contact B. This de-energizes the switch motor, because the CR-2 contact is held open by CR, which is energized. The circuit through contact 1 of the sequence switch energizes M1, which is the starter coil for the motor associated with the pump 1, and the first pump starts. When the level falls below the level of the LSL probe tip, relay CR is de-energized, which opens contacts CR-1 and CR-3 and opens CR-2. Dropping out CR-3 stops the motor (M1) associated with the first pump. The closing of CR-2 energizes the switch motor, which steps to move the sequence switch to contact 2 and moves the position switch back to contact A. As a consequence, on a subsequent rise in level, M2 will be energized, and the second pump will start. This is the most basic of the pump-down alternator circuits. A similar circuit can be designed for pump-up applications, and a wide variety of additional control requirements can be met if the numbers of probes and relays are increased. Advantages and Limitations The advantages of the conductivity switch include low cost, simple design, and elimination of moving parts in contact with the process material. Conductivity-type level switches can also be used to detect the level of moist bulk solids. The threeprobe-element design can also provide differential level control. The disadvantages include the possibility of sparking when the liquid level is close to the tip of the probe. Such phenomena are eliminated in the solid-state designs, which are rated for intrinsic safe operations. The conductivity switch is also limited to conductive (below 108 Ω resistivity) and noncoating process applications. One should also consider the possible harmful side effects of electrolytic corrosion of the electrode. Electrolysis can be reduced, but not eliminated, by using AC currents. Conductivity switches are rarely used in chemical processing services. However, they are routinely used in the food, paper, and wastewater industries and in other waterlevel applications, including those on steam drums operated at up to 3000 PSIG (21 MPa). Specialized applications also include the measurement of the level of molten glass, which is a conductor at elevated temperatures.

FIELD-EFFECT LEVEL SWITCHES As illustrated in Figure 3.4e, the field-effect probe creates a field between a metallic cap cast into a Ryton probe and the metal in the tank or in the probe gland and probe housing. When a conductive or nonconductive liquid, slurry, or solid material (with electrical characteristics different from that of air) breaks the field lines, the high-frequency current increases, and the relay trips. The probe should be installed horizontally

448

Level Measurement

probe form the impact of the flowing solids and to protect the electronics from direct sunshine.

Metallic Gland Metallic Cap

High Frequency Quadratic Field Lines

Bibliography

Wrong

Right

Wrong

Right Sunshield

FIG. 3.4e The field effect level switch and its installation. (Courtesy of Endress+ Hauser Inc.)

(as shown in Figure 3.4e), with a small downward angle, if installed on slurry services. The Ryton probe material is unaffected by solvents if the operating temperature is under 212°F (100°C). The probe is 8 in. (200 mm) long and requires a 1.5-in. NPT tank connection. As illustrated in Figure 3.4e, it is advisable to protect the


Andreiev, N., Survey and guide to liquid and solid level sensing, Control Eng., May 1973. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June–July, 1997. Belsterling, C. A., A look at level measurement methods, Instrum. Control Syst., April 1981. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Cho, C. H., Measurement and Control of Liquid Level, ISA, Research Triangle Park, NC, 1982. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Hall, J., Measuring interface levels, Instrum. Control Syst., October 1981. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Johnson, D., Level sensing in hostile environments, Control Engineering, August 2001. Lawford, V. N., How to select liquid-level instruments, Chemical Eng., October 15, 1973. Level measurement and control, Meas. Control, April 1991. Noltingk, B. E., Instrumentation Reference Book, 2nd ed., ButterworthHeinemann, Oxford, UK, 1996. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Parr, E. A., Industrial Control Handbook, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1995. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

3.5

Diaphragm Level Detectors D. S. KAYSER (1982) B. G. LIPTÁK J. B. ROEDE (2003)

LI D To On-Off Receiver

(1969, 1995) LS D

Flow Sheet Symbol

Service

Level transmitter in water wells, open tanks with immersed sensor; tank-mounted diaphragm pressure transmitter and repeaters; level switch for solids in bins and silos

Design Pressure

Switches, atmospheric; transmitters, submersible to 500 PSI (3.5 MPa); others to 5000 PSI (35 MPa)

Temperature

−40°F (−40°C) to 180°F (80°C) typical

Wetted Materials

Switches: aluminum, SS, neoprene, and various polymers. Submersibles: SS, TFE,  neoprene, Buna-N, and polyethylene. Tank mounted: type 316 SS, Hastelloy ,   Inconel , and Monel .

Ranges

Single point for solids; up to 700 ft of water for the transmitters

Inaccuracy

Single points are repeatable to about 1 in. (25 mm) on solids; transmitters are accurate to 0.5 to 1.0%, based on pressure. Level inaccuracy depends on the specific gravity of the liquid.

Cost

Switches, $150 to $500; submersibles, $350 to $600; tank mounted, $1000 to $2500

Partial List of Vendors

AMETEK-Gulton Statham Products (www.ametekpower.com) AMETEK-PMT Products (www.ametekusg.com) Anderson Instrument Co. (www.andinst.com) Barton Instrument Systems LLC (www.barton-instruments.com) Bindicator (www.bindicator.com) Delta Controls Corp. (www.deltacnt.com) Druck Inc. (www.druck.com) Fisher-Rosemount Systems (www.emersonprocess.com) Foxboro Eckardt GmbH, an Invensis Co. (www.foxboro-eckardt.com) Honeywell (www.acs.honeywell.com) Kimray Inc. (www.kimray.com) King Engineering Corp. (www.king-gage.com) Monitor Technologies LLC (www.monitortech.com) Monitrol Bin Level Mfg. Co. (www.monitrolmfg.com) Pressure Systems (www.pressuresystems.com) Scientific Technologies Inc. (www.automationsensors.com) Sensotec Inc. (www.sensotec.com) Siemens Energy and Automation-Process Industries Div. (www.sea-siemens.com) Vega Messtechnik AG (www.vega-g.de) Viatran Corp. (www.viatran.com) Yokogawa Corp. of America (www.yca.com)


450

Level Measurement

All diaphragm detectors operate on the simple principle of detecting the force exerted by the process material against the diaphragm. The designs discussed below include diaphragm switches for liquid and solid services and diaphragm devices for continuous liquid level detection and transmission in tanks or wells.

DIAPHRAGM SWITCHES FOR SOLIDS For solid service, diaphragm switches can be selected from a number of design variations. Devices with mercury switches can be used with materials having a bulk density of more than 3 3 30 lb/ft (48 kg/m ), whereas units with microswitches are used for lower-density services. Some of the most sensitive diaphragm switches will actuate with as little as 6 oz (171 g) of force on the diaphragm. The differential of a single diaphragm can be as high as 8 in. (200 mm), meaning that the switch will close its circuit when the solids rise to the top of the diaphragm and will open the circuit when they drop 8 in. The lower the solid density, the larger the diaphragm area required. Units are available with 4- to 10-in. (100- to 250-mm) diameter diaphragms. As illustrated in Figure 3.5a, there are three ways to install these detectors. They can be suspended

on a support pipe to provide for quick adjustment of the switch position, they can be mounted on the inside wall of thickwalled silos, or, as is most commonly done, they can be externally mounted on thin-walled bins and silos. The mounting location should always be selected to guarantee the free flow of solids to and from the diaphragm area. As shown in Figure 3.5a, diaphragm-type switches for solids can serve several purposes. Switch 1 protects against overfilling, switch 2 signals low supply level, and switch 3 indicates choke-up in the screw conveyor. Diaphragm 4 detects overfeeding the elevator boot, and diaphragm 5 detects plugging of the elevator discharge spout. Diaphragm switches 6 and 7, in the storage silo, will signal extreme level conditions. Switch 6 can interlock the material feed and shut it down when the storage silo is full. In newer designs, the diaphragm itself is vibrated by built-in piezoelectric elements and, when the solids level rises up to the diaphragm, the resulting load on the diaphragm decreases the amplitude of vibration. This change in amplitude is used to trigger the level switch. This solids level switch (Figure 3.5b) is smaller, lighter, and more sensitive than the earlier designs, and its stainless-steel diaphragm (in an ABS resin body) is more rugged than the rubber diaphragms of the direct pressure-sensing units. The vibration Bin Wall

Diaphragm 1 Elevator

Diaphragm

Supply Bin

2 5 6 3

Feeder

Storage Silo Bin Wall

Conveyor

7

4

Diaphragm

FIG. 3.5a Use of diaphragm switches in solid services.


Gasket

3.5 Diaphragm Level Detectors

80 (3.15")

451

Piezo-Electric Element

64 (2.52")

Diaphragm

FIG. 3.5b Vibrating diaphragm-type solids level switch. (Courtesy of Scientific Technologies Inc.)

of the diaphragm at 200 to 400 Hz reduces the probability of material sticking to the diaphragm and it also increases its sensitivity. The switch will actuate when it is 50% covered by materials with specific gravities exceeding 0.5, such as flour (0.48 to 0.55), polyethylene pellets (0.56), rice (0.58), PVC pellets (0.76), or wheat (0.77). On lighter materials, such as instant coffee (0.22) and copier toner (0.49), the diaphragm will actuate when it is 80% covered. To eliminate cycling, a 1- to 3-sec time delay is provided.

DIAPHRAGM SWITCHES FOR LIQUIDS Figure 3.5c shows how diaphragm switches can be used to detect liquid level by sensing the pressure of a captive air column in a riser pipe beneath the diaphragm. An 8-in. (203-mm) head of liquid above the inlet of the riser pipe generally compresses the air sufficiently for switch actuation. The unit can handle a maximum of 60 ft (18 m) of liquid. The diaphragm is in contact with the captive air but not with the process. The liquid rises in the dip pipe enough to compress the enclosed mass of gas to match the level-caused pressure outside the dip pipe minus the liquid rise inside the dip tube. Physical dimensions are important. Sensitivity increases as the wetted portion of the dip tube increases and decreases in proportion to the enclosed air volume. These units can be used only on atmospheric tanks and should be considered only for applications where low cost is desired and accuracy is not a critical consideration.


DIAPHRAGM-TYPE LEVEL SENSORS AND REPEATERS Figure 3.5d illustrates two versions of the continuous level detector, both limited to atmospheric tanks and to applications where low cost is more important than quality or accuracy of measurement. The diaphragm box unit, shown on the left side of the sketch, is similar in operation to the previously discussed riser pipe diaphragm switches except that the diaphragm isolates the captive air from the process fluid. The unit consists of an air-filled diaphragm box connected to a pressure detector via capillary tubing. Correct function depends on a large volume displacement by the diaphragm, with negligible spring constant. As the level rises above the slack diaphragm, the liquid head pressure compresses the captive air spring. The air pressure in the capillary tubing is sensed by a pressure element and displayed as an indication of level. A one-to-one pressure repeater is illustrated on the right side of Figure 3.5d. With this unit submerged in the vessel, the static head of the liquid exerts an upward force on the diaphragm that increases as the level rises. The air supply pressure on the other side of the diaphragm opposes the upward force. The force caused by the rising level moves the diaphragm toward a bleed orifice, thus restricting its flow to atmosphere and causing the air pressure to build up until it equals the static head pressure. When the forces on the two sides of the diaphragm are equal, the unit is in equilibrium. The speed of response of the unit is changed by an adjustable restriction that, if opened, will increase sensitivity by allowing

452

Level Measurement

Vent LI

Air Supply

LI

Diaphragm Box

Flexible Diaphragm

1:1 Repeater Vent Air Supply

Level Detector

Low Level

Adjustable Restriction

8" Min.

Diaphragm

FIG. 3.5c Diaphragm switches in liquid service.

more air to flow onto the diaphragm. Air supply to the unit should be regulated at a pressure of 3 to 5 PSIG (0.2 to 0.3 bar) in excess of the maximum hydraulic head to be repeated. The pressure repeater can be submerged, as shown in Figure 3.5d, or mounted on a nozzle near the bottom of an atmospheric tank (Figure 3.5e). These flange-mounted units are available with stainless-steel diaphragms and steel or stainless-steel bodies. They can operate with up to 160 PSIG (11 bars) air supply and can repeat hydrostatic pressures up to within 5 PSI (0.35 bars) of the air supply pressure. The air consumption of these repeaters is under 0.2 scfm (5.7 slpm). Other repeater designs, such as the extended diaphragm version, are discussed in the following section (Section 3.6), because they are used to complement differential pressure transmitters in level-measurement applications.

ELECTRONIC DIAPHRAGM LEVEL SENSORS Most diaphragm-type pressure detectors and pressure transmitters can also be used to detect level by measuring the weight of a column of liquid in an atmospheric tank. In the


FIG. 3.5d Diaphragm box for coupling bottom-of-tank pressure to a pressure gage (left); and 1:1 pressure repeater (right) for continuous pneumatic transmission of liquid level.

unit shown in Figure 3.5f, the pressure applied to the diaphragm is transferred to a fill fluid that also fills the inner cavities. A straight-axis, twisted Bourdon tube is cantilevered from the process side to convert pressure of the fill fluid to proportional, rotary motion at its free end. A rotary variable differential transformer (RVDT) detects the rotation and converts it to an electrical signal. This particular sensor is available with ranges from 0 to 100 in. (0 to 2.5 m) of water column up to 0 to 300 PSIG (0 to 20.7 bars) and has a 150% overpressure protection over its range. Other means of transduction include strain gauges bonded directly to the diaphragm, silicon diaphragms with the strain gauges etched and diffused into the side away from the process, and capacitive sensing of the diaphragm by an internal parallel plate. Diaphragm-type electronic pressure transmitters are available in all stainless-steel sanitary designs and are used in the food industry. They are available with 0–20 to 0–100 ft (0–6 to 0–30 m) ranges and with 4- to 20-mA DC output signals, and they are suited for process temperatures between 25 and 225°F (–4 and 107°C). They can provide level measurement accuracies (assuming constant specific gravity) of 0.5% of full-scale reading.

3.5 Diaphragm Level Detectors

Pilot Pressure

Diaphragm-type electronic transmitters can also be submersed to detect levels in wells or in open water bodies. They are available with ranges up to 700 ft (213 m) and can be connected by cable to their readout instruments. Accessories generally supplied for these lake and well installations include digital readouts with a variety of packaging types, battery/solar cell power packs, power supplies for various line power voltages, multiple high- and low-level current relays, and optional analog (4- to 20-mA DC) or digital (RS-232, HART, Honeywell protocols) outputs.

Metal Diaphragm

Bibliography

Detecting Nozzle

7 1/2" (190 mm)

Process Pressure Instrument

2 11/16" (68 mm)

FIG. 3.5e Pressure repeater for mounting on 4-in. (200-mm) tank nozzles. (Courtesy of Siemens Energy and Automation.)

Belsterling, C. A., A look at level measurement methods, Instrum. Control Syst., April 1981. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001. Hall, J., Measuring interface levels, Instrum. Control Syst., October 1981. Imsland, T., Connecting d/p elements for level sensing, Instrum. Control Sys., November 1975. Johnson, D. Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001. Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Lawford, V. N., Differential pressure instruments: the universal measurement tools, Instrum. Technol., December 1974. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Level measurement and control, Meas. Control, April 1991. Robinson, C., Hydrostatic tank gaging: what it is, where it’s used, what’s available, InTech, February 1988. Slomiana, M., Using differential pressure sensors for level, density, interface, and viscosity measurements, Instrum. Technol., September 1979. Submersible pressure transducers improve accuracy of pump tests, Water World, May 1998.

Diaphragm Bourdon Tube

Signal Generator

FIG. 3.5f Electronic diaphragm level sensor with twisted bourdon/RVDT transduction.


453

3.6

Differential Pressure Level Detectors LI

B. G. LIPTÁK

(1969)

D. S. KAYSER

(1982)

J. A. NAY

(1995, 2003) LT FG Flow Sheet Symbol

Design Pressure

To 10,000 PSIG (69 MPa)

Design Temperature

To 350°F (175°C) for differential-pressure (d/p) cell and to 1200°F (650°C) for filled systems; others to 200°F (93°C); standard electronics generally limited to 140°F (60°C)

Range

Differential-pressure cells and indicators are available with full-scale ranges as low as 0 to 5 in. (0 to 12 cm) H2O. The higher ranges are limited only by physical tank size, since d/p cells are available with ranges over 433 ft H2O (7 MPa or 134 m H2O).

Inaccuracy

±0.5% to 2% of full scale for indicators and switches. For d/p transmitters, the basic error is from ±0.1% to 0.5% of the actual span. Added to this are the temperature and pressure effects on the span and zero. In case of “intelligent” transmitters, the pressure and temperature correction is automatic, and the overall error is ±0.1% to 0.2% of span with analog outputs and may approach ±0.01% with digital outputs.


Plastics, brass, steel, stainless steel, Monel , and special alloys for the wetted parts. Enclosures and housings are available in aluminum, steel, stainless steel, and fiberglass composites, with aluminum and fiberglass being most readily available.

Cost

$200 to $1500 for transmitters in standard construction; $50 to $500 for local indicators. Add $400 to $1000 each for extended diaphragms and up to $1000 for “smart” features such as communications and digital calibration, although many “smart” features may be included in the base price. “Expert” tank systems cost approximately $1500 for each basic transmitter plus $3500 to $4500 each for one or more interface units and $1500 to $4000 for software plus a hand-held communicator and/or permanent connection to an in-house network. Some “expert” functions may be incorporated in an in-house network.


See also Sections 5.6 and 5.7. Any d/p cell may be installed and connected to measure level if it has the appropriate accuracy and rangeability. Similarly, for vented tanks, any suitable gauge pressure device may be adapted to measure level. ABB Instrumentation Inc. (www.abb.com) Barton (www.ittbarton.com) Endress+Hauser Systems & Gauging (www.systems.endress.com) Enraf (www.enraf.com) Honeywell Control Products (www.honeywell.com/acs/cp/index.jsp) Rosemount Inc. (www.rosemount.com) Schlumberger Measurement Div. (www.slb.com/rms/measurement) Smar International Corp. (www.smar.com/products/function.asp) Viatran Corp (www.viatran.com) Yokogawa Corporation of America (www.yca.com)



For a general treatment of differential pressure devices, refer to Section 5.6. For a discussion of electronic pressure and differential pressure instruments, refer to Section 5.7. Liquid level can be measured (inferred) by measuring a differential pressure (d/p) caused by the weight of the fluid 454 © 2003 by Béla Lipták

column in a vessel balanced against a reference. For vessels at atmospheric pressure, the high side of the instrument is connected to the bottom of the vessel, and the low side (reference) is vented to the atmosphere. This method of level measurement is often referred to as hydrostatic tank gauging

3.6 Differential Pressure Level Detectors

(HTG), especially in the bulk liquid industries. For pressurized vessels, the reference side must also be connected to the vessel so that both sides of the instrument equally sense the static pressure changes within the vessel and their differential responds only to the d/p caused by fluid head. The reference can be a column of fluid of fixed height such as a liquid-filled reference leg outside the tank or, as in the case of a bubbler system, a gas-filled reference leg, usually inside the tank. The key requirement is that the reference leg provide (or represent) a constant, known, hydrostatic head. An increase in the tank level from empty (0% level) to full (100% level) can result in a readout of 0 to 100% d/p (direct acting) or in a readout of 100 to 0% d/p (reverse acting). To obtain an accurate measurement using d/p cells, the densities of the process liquid and of the reference leg must be known and either be constant or continuously considered.

SENSING DIFFERENTIAL PRESSURE Differential pressure can be detected by sensing two pressures separately and taking the difference to obtain liquid level. In practice, however, it is generally desirable to use a single pressure-difference sensor so that the static pressure levels are balanced before any measurement errors are introduced. The importance of this consideration can be visualized, for example, on a 0- to 100-in. (0- to 2.5-m) water column (WC) measurement where the expected accuracy is ±0.5 in. (±13 mm) WC. It would be virtually impossible to approach this accuracy if the measurements were made at a static pressure of 1000 PSIG (7 MPa) using two independent sensors. Since 1000 PSI (7 MPa) is equivalent to about 28,000 in. (714,000 mm) WC, the required accuracy of each measurement would be ±0.25 in./28,000 in., or 0.0009% (7 mm/714,000 mm). Nevertheless, several highly accurate HTGs do use two or more transmitters for gauging tanks at atmospheric pressures (Figure 3.6e). For instance, a third transmitter located a fixed distance above the bottom transmitter can be used to sense, in real time, the difference in pressure caused by the density of the fluid. Its sensing line to the tank must always be submerged and full to ensure accurate readings. However, when the tank is being initially filled or completely emptied, a temperature measurement in the fluid may be used to calculate an estimate of density or to help extrapolate from earlier density measurement records. Extended Diaphragms The d/p cell can be modified for use on viscous, slurry, or other plugging applications. Figure 3.6a shows two such designs: the extended and the flat diaphragm d/p cell transmitters. Their principles of operation are the same as those of the conventional d/p cells, except that the high-pressure side of the diaphragm capsule is exposed and extended to be in direct contact with the process. The extended diaphragm version is designed to bolt directly to the vessel nozzle; the


Extension to Match Tank Nozzle Height

Drain

Low-Pressure Connection

455

Diaphragm Capsule High-Pressure Side

Drain

Diaphragm Capsule High-Pressure Side

Low-Pressure Connection

FIG. 3.6a Extended diaphragm d/p cell (left); flat diaphragm d/p cell.

protrusion can be sized to fill the space in the nozzle, placing the diaphragm flush with or slightly inside of the vessel wall. This design completely eliminates dead-ended cavities and is used especially on materials that can freeze at high temperatures or that can deteriorate or discolor if pocketed. This design cannot be serviced without depressurizing and draining the vessel. The flat diaphragm design is normally installed by bolting it directly to a block valve on the vessel nozzle. A fullsize gate or ball valve can allow for service without draining or depressurizing the tank. Because the connection is large, typically 3 in. (76 mm), the process is less apt to bridge or plug the sensing connection. Flat diaphragm cells can also be furnished with a solvent flush or steam-out connection. Both the flat and the extended designs are available with ranges up to 850 in. (2160 mm) with an accuracy of ±0.5% of span. The flat units can withstand 550 PSIG (3.8 MPa) operating pressures when the process is at 350°F (175°C). The maximum process temperature rating for the extended design is 750°F (400°C). Changes in process or ambient temperatures can cause zero shifts, as is the case with many similarly designed instruments. To minimize this effect, the d/p cell should be zeroed at the normal operating temperature, and the exposed body of the transmitter should be insulated. Alternatively, temperature at the connection or within the transmitter may be measured and continuously accounted for in the computation of level. Some suppliers offer the extended and flat diaphragms with Teflon, Viton, or other plastic coating. This coating is intended as a slippery surface to minimize material buildup on the diaphragm. Do not rely on the plastic coatings for corrosion protection unless the supplier states specifically that the coating is so designed. As a general rule, engineers should not rely on coatings for corrosion protection of wetted parts of most process instruments, because, if the coating is nicked during installation, that protection is destroyed.

456

Level Measurement

Chemical Seals Transmitters may also be provided with liquid-filled extension elements (chemical seals). The units shown in Figure 3.6a are used on atmospheric tanks or on pressurized tanks if the low-side connection can be kept clean or sealed. The designs shown in Figures 3.6b and 3.6c are used on pressurized vessels where plugging or corrosion can occur on both the high- and the low-pressure sides. The chemical seal designs are available in a broad range of materials including such metals as tantalum and zirconium. Vessel connection considerations apply to both the high and the low side. As can be seen from Figure 3.6c, the process material contacts both the diaphragm and the process side of the flange. The instrument side of the diaphragm is filled with an oil or other suitable fluid (sometimes including water) and is connected by capillary to the high and low sides of the d/p cell. The differential pressure capabilities of these systems depend on the d/p cell selected. The accuracy of the system will always be worse than the accuracy of the d/p cell itself.

d/p Cell

INTELLIGENT D/P CELLS AND TANK EXPERT SYSTEMS The microprocessor has extended the applications of electronic differential-pressure transmitters. The detection is still accomplished by use of the elastic element (diaphragm or bellows), although the actual elastic element may be very small. The elastic element may actually have some of the electronic and microprocessor circuits attached to, embedded in, or otherwise incorporated directly. The microprocessor can convert the analog readings of deflection to a linearized, highresolution digital signal compensated for temperature effects. Transmitter stability is thus improved, and the time between physical calibrations is often extended to more than a year. “Smart” level transmitters can convert the level readings of spherical or cylindrical tanks into actual volume percentage readings (Figure 3.6d). Intelligent transmitters have also been

Capillary

Liquid Fill

Wafer Element

Sensing Diaphragm

The spring constant of the diaphragms at the chemical seals and the elasticity of the fill fluid will introduce some error, not all of which can be fully compensated. This error becomes more pronounced at very small differential pressure ranges and at high static pressures. A larger and less predictable error can result from the temperature-sensitive nature of the seal and capillary system. Temperature differences between the low and high sides will cause differing amounts of thermal expansion; this will be sensed by the d/p cell as a differential pressure and interpreted as a level change. Because the unequal amounts of expansion can be caused by the process temperatures or by changes in the ambient conditions, it is not always possible to zero-out this error. The pressure and temperature ratings for these filled systems depend on seal design and filling liquid. Seals are readily available that are rated to 1500 PSIG (10 MPa). The fill material is normally a silicone oil that is good to 450°F (232°C). Other fill materials raise temperature ratings to 1200°F (650°C), allowing chemical seal designs to be considered for high-temperature applications. The volume of fluid in the seal, capillary, and d/p cell housing may be significant. Process, environmental, or health sensitivity to accidental leakage of the fill fluid may restrict choices. Distilled water has been successfully used as a fill fluid in some cases.

FIG. 3.6b d/p cell with wafer elements. Level % 100

Capillary Tubing

Sensing Diaphragm

50

d/p Cell Cylindrical Tank Sphere

Liquid Fill

FIG. 3.6c d/p cell with extended chemical seal elements.


0

50

100 Volume %

FIG. 3.6d Intelligent transmitters can automatically convert level readings into volume.


PT Top Pressure for Reference (Unless Tank has Unrestricted Vent) Dedicated or Networked Computer Interface

} Middle Pressure for Density Calc. PT

LT TE LT

Temperature for Density Calc.

PT Bottom Pressure for Density and Hydrostatic Head

FG

457

If the tank contained an oil with a specific gravity of 0.75, then the difference in readings would be 37.5 in. (93.75 mm) of water. Anything that changes the density, including bulk temperature changes and chemical composition changes, will be reflected in the difference in readings between the middle and bottom transmitters. Thus, the density can always be calculated. A temperature reading may be useful as a diverse measurement to verifying expected density or to assist in estimating or extrapolating density as the tank is initially filled or as it is emptied below the middle transmitter.

Drain

FIG. 3.6e Multiple transmitters to provide level, mass, volume, and density data.

combined into tank expert packages that, in addition to level, can also calculate mass, density, and volume based on the measurements from three or more d/p cells and one temperature transmitter as shown in Figure 3.6e. Outputs are available through digital RS485 and RS232 connections and may be networked digitally for multiple remote access to the data. Most manufacturers offer optional digital communication with the transmitter through the output signal wires coincident with the analog output signal. Wireless Ethernet local area networking per IEEE-802b and other wireless technologies is also available. Calibration, range and zero setting, elevation, suppression, and linearization may all be accomplished remotely, although periodic physical calibration to a known NIST traceable standard is still recommended for the most accurate results. In Figure 3.6e, the “TOP” pressure transmitter can be eliminated if the tank is completely vented without any possible vent restrictions. Simple problems, such as a bird’s nest in a vent pipe, may cause unexpected and possibly undetected errors. This is a particular concern under transient conditions where the restricted vent does not allow enough flow to keep up with filling or draining operations. If the top transmitter is eliminated, it may be prudent to install a very low-pressure detector to alert operators of unexpected pressure or vacuum conditions. A better solution would be to use differentialpressure level transmitters (LTs) for the middle and bottom locations as shown on the left in Figure 3.6e. In this case, they should share a common dry reference leg. The bottom and middle transmitters, whether pressure or differential pressure, must be placed a known vertical distance apart. Because horizontal distance does not matter, on large tanks, it would facilitate maintenance if each were placed within reach of stairways, ladders, or platforms existing at the appropriate levels. Density is determined by the difference in readings between the middle and bottom transmitters. For instance, if the transmitters were placed 50 in. (125 mm) apart, and if the tank contains distilled water at 68°F (20°C), the bottom transmitter should read exactly 50 in. (125 mm) of water more than the middle transmitter.


PRESSURE REPEATERS When detecting the level in pressurized vessels, the vapor space pressure must be connected to the low side of the d/p cell to serve as a reference. On hard-to-handle materials, a one-to-one pressure repeater may be used to provide this reference and simultaneously isolate the d/p cell from the process. Repeaters develop an air output pressure equal to the vapor-space pressure. They are inexpensive, but their accuracy is limited. The error in the repeated output pressure increases as the repeated pressure rises. At a pressure level of 40 PSIG (0.27 MPa), the error is 2 in. (51 mm) H2O; at a pressure level of 400 PSIG (2.7 MPa), the error is 20 in. (508 mm) H2O. Obviously, errors of this magnitude are not acceptable for most process level measurements. Repeaters have generally fallen into disuse in favor of high-accuracy digital transmitters made from the same materials.

DRY, MOTION BALANCE DEVICES These differential pressure detectors are also referred to as bellows meters, because they depend on liquid-filled, doubleopposed bellows. Bellows meters are most useful where local indication or recording is required and where compressed air and electric power are not available as energy sources. They can be very sensitive to low differential pressures because of the large area and slack resistance to motion that can be built in. Figure 3.6f illustrates the high- and low-pressure chambers, the range spring, and the drive assembly (a bell crank of sorts) to transfer bellows motion to the readout pointer. The bellows in both chambers and the passage between them are liquid filled. When the unit is installed, the pressure in the high-pressure chamber compresses the bellows so that the liquid flows from it into the low-side bellows. When the low-pressure (or range) bellows expands, it exerts a force against the range spring, which determines the span of the instrument. The linear motion of the range bellows moves the drive lever, mechanically transmitting a rotary motion through the sealed torque tube assembly to the indicator. The output motion from the torque tube assembly is limited to a few degrees of angular rotation. This is sufficient for most

458

Level Measurement

Bimetallic Temperature Compensator

Liquid Fill

Range Spring

chemical seals can be used. Static pressure ratings up to 10,000 PSIG (69 MPa) are available as standard; operating temperature is limited to 200°F (93°C). LIQUID MANOMETERS

High-Pressure Side

Low-Pressure Side

FIG. 3.6f Motion detector d/p indicator.

mechanically driven local indicators or recorders and well within the capabilities of modern electronic motion sensors. If the secondary device imposes a considerable load on the torque tube assembly, however, the accuracy and sensitivity of the unit can be destroyed. For sustained accuracy, bellows meters depend on the repeatability of their mechanical systems, which have proven to be linear within 0.5% to 1% of full range, and down to 0.2% or less when the range can be limited to a small portion of the total available motion. A temperature compensator built into the bellows assembly compensates for the changing volume of the fill liquid resulting from ambient temperature variations. Bellows meters are provided with overrange protection. The operation of one of the protection mechanisms is as follows: The bellows move in proportion to the differential pressure applied across them and in proportion to the spring rate of the bellows plus the resisting springs. When the bellows have moved over their calibrated travel, a valve mounted on the center stem seals against its seat, thereby trapping the fill liquid in the bellows. Because the liquid is essentially incompressible, the bellows are fully supported and will not rupture, regardless of the pressure applied. This overrange protection is furnished in both directions, protecting both bellows. Another design of overrange protectors involves the use of liquid-filled bellows with a number of diaphragm discs and spacer rings between them. As the bellows are subjected to overrange pressures, the diaphragms nest, and the metallic spacer rings form a solid stop, thereby fully protecting the bellows from rupture. Bellows meters can detect full-range pressure differentials at least as low as 20 in. (508 mm) WC and as high as 400 PSIG (2.7 MPa). Measurement of low differentials is limited by the small forces available to actuate the motion detector mechanism. For very high differentials, the limitation is the mechanical strength of the bellows. Standard units are available with steel or stainless-steel housings and stainless-steel or beryllium copper bellows. For corrosive applications, other materials can be obtained or special, high-displacement-volume


These instruments are discussed in detail in Section 5.9. Their design variations include the U-tube, the well, and the float-type manometers. The float designs can provide remote readouts, whereas regular manometers can serve as the readout indicators for bubbler-type level sensors. Where the use of glass is not allowed, digital manometers using magnetic coupling between a float inside a stainless-steel U-tube manometer, and an outside electronic transmitting mechanism can be used. Glass-tube manometers are available with ranges up to 120 in. (3.05 m), which is sufficient for many level applications. The magnetically coupled float manometers are available for high-pressure services, up to 6000 PSIG (41 MPa), and can measure up to 1000 in. (25 m) of water column. Because of the fragile nature of glass, the toxicity of mercury, and the chemical interaction between the manometer filling fluids and the process, manometers are not widely used on process level measurement applications and are mostly restricted in their use to occasional utility services.

LEVEL APPLICATIONS OF D/P CELLS The applications of pressure differential detectors as components in level-measurement loops will be covered in the following paragraphs. The requirements of atmospheric and pressurized tanks and the features of level loops on clean and hard-to-handle process fluids will be discussed separately. Figure 3.6g shows the symbols used for the Force Balance Transmitters

Local Instruments

LI

Bellows Type d/p Indicator, or Pressure Gauge

LT

Standard d/p Cell

1:1 LY

XLI

Manometer

LT

Flat Diaphragm

XFI

Purge Assembly

LT

Extended Diaphragm

FG

Flow (Sight) Glass

LT

d/p Cell w/ Wafer Seal Elements

LT

FIG. 3.6g Symbols for d/p level loops.

d/p Cell w/ Extended Diaphragm Chemical Seals

Flat Diaphragm Type Pressure Repeater

1:1 LY Extended Diaphragm Type Pressure Repeater


Transmitter Installations

LT

3 LT

4 LI

5 XLI

1

XFI

N2

LT

FIG. 3.6h Detection of clean liquid levels in atmospheric tanks by d/p instruments.

3

4 LT

LT

FG

LI

5

LI

LT

6 LI

LT

LI

LI

XFI

N2

FG

LI

LT

N2

7

Alternate d/p Devices LT

XLI

Air

LI

2

1:1 LY

FG Dry Leg

LI

LI

2

Wet Leg

LI

1

1:1 LY

Dry Leg

FG

Local Readouts

Wet Leg

Local Readouts

Air


459

XLI

XLI

LI XLI

XLI

various loop components. In the discussion that follows, the air-bubbler-type d/p cell installations will not be included, as these are covered in Section 3.2.

FIG. 3.6i Measurement of clean liquid levels in pressurized tanks by d/p instruments.

Clean Liquids in Atmospheric Tanks

Clean Liquids in Pressurized Tanks

Unpressurized vessels containing clean liquids are the least demanding as far as level measurement is concerned, because the two most common sources of difficulties (vapor pressure compensation and plugging) are not present. Figure 3.6h shows five tanks equipped with five different types of level devices. The first two are for remote readout, and the others are for local readout. On tank 1, a standard d/p transmitter with screwed connections is shown with its low-pressure side open to the atmosphere. This installation can be made by using a pressure transmitter instead of a d/p transmitter. The pneumatic receiver gauge is normally calibrated for 0 to 100% level. The flat diaphragm-type d/p transmitter is shown on tank 2. Compared to the standard d/p cell, the flat diaphragm-type transmitter is simpler to install, and it is nozzle mounted, requiring no other means of support. It also can be less expensive, because only the diaphragm and the retaining ring are in contact with the process. As a result, only these parts must be made of corrosion-resistant materials whereas, in the standard d/p cell, the entire body is exposed to the process fluids. A flat diaphragm-type pressure repeater can also be used in place of the d/p transmitter, in which case the receiver gauge will sense the actual hydraulic head instead of a 3 to 15 PSIG (21 to 103 kPa) transmitted signal. Tank 3 shows a motion balance local d/p indicator with the low-pressure side vented to atmosphere. The same measurement can be made by using a standard pressure gauge. The level in tank 4 is detected by a manometer. Although this is one of the most accurate and economical devices to use for local readout, the consequences of mechanical damage and proper selection of the filling liquid must be considered. The installation on tank 5 is basically an air bubbler system, which is detailed in Section 3.2. Not shown is a system (usually portable) wherein a miniaturized electronic transmitter is actually lowered, by a cable containing its wiring, into an atmospheric tank and down through the liquid until it reaches the bottom. The operator can tell that it is on the bottom when the level indication stops increasing. This is particularly useful for occasional tank gauging and for measuring the level in wells.

When the level in a pressurized vessel is to be established by hydraulic head measurement, the instrument has to be compensated for the vapor pressure in the tank. This is done by exposing the low-pressure side of the d/p cell to these vapor pressures. Compensation can be achieved by various means. Figure 3.6i shows seven variations of this installation. Tank 1 illustrates a wet-leg application in which the compensating leg is prefilled with a chemically inert liquid that will not freeze or vaporize under operating temperature conditions. The wet-leg installations are used when the process vapors would otherwise condense into the compensating leg, thereby exposing the low-pressure side of the d/p cell to unpredictable hydraulic heads, or when the transmitter must be sealed from corrosive vapors. The prefilled wet leg creates a constant pressure on the low-pressure side of the transmitter. The leg is filled through a seal pot to provide excess capacity. It is desirable to make this seal pot out of a sight flow indicator so that the level of the filling liquid is visible to the operator. The d/p cell can be either the standard or the flat diaphragm design. On tank 2, the same d/p transmitter is installed in a dry-leg system. This is acceptable when the process vapors are not corrosive and condensation at ambient temperatures is not expected. For such applications, a condensate pot is installed below the d/p cell, and it also should incorporate a sight flow indicator so that the operator can visually determine if it is time to drain out any accumulation of condensate. Tank 3 illustrates the use of a flat diaphragm pressure repeater for vapor pressure compensation. The problems associated with range depressor adjustments, corrosion, and condensate accumulation are eliminated by the use of repeaters, but they do add to the total error of the installation. On tanks 4, 5, and 6, the same basic installations (wet-leg, dry-leg, and repeater) are shown in connection with local bellows indicators. Manometers can also be considered in place of the motion balance d/p indicators if mechanical damage, chemical inertness of the filling fluid, and its compatibility with the operating temperatures are previously established. One concern with


460

Level Measurement

Local Readouts


Alternate Sensors in Vapor Space 1:1 LY N2

XFI

1:1 LI

1

LI

LI

2 LT

3 LT

LT

4

5 LT

LI

6

LY

LI XFI

N2

FIG. 3.6j Sensing of hard-to-handle liquid levels in atmospheric tanks by d/p devices.

the dry-leg installation shown in tank 5 is the possibility that, if the vapors in the tank condense, or if the tank is flooded, process liquids will fill the dry leg. This is unsafe because, under these conditions, the d/p cell will signal a low level or an empty tank condition. The installation of a float trap can lower (but not eliminate) this risk by draining the dry leg if the liquid buildup is slow. Tank 7 illustrates a bubbler system with either transmitting or local indicating d/p devices. The advantages, limitations, and drawbacks of such installations have been pointed out in Section 3.2. Hard-to-Handle Fluids in Atmospheric Tanks Level measurement is more difficult when the process fluid is highly viscous, is likely to freeze, contains solids that can settle out, or can gel or polymerize in dead-ended cavities. Figure 3.6j shows six installations that may be considered for these conditions. On tank 1, an extended diaphragm-type force balance transmitter is shown. The diaphragm motion is limited to a few thousandths of an inch, and the nozzle cavity is completely filled by the diaphragm extension. The extended diaphragm transmitter is a good candidate for level measurement of hard-to-handle liquids, provided that the vessel can be drained when the transmitter needs service. Tank 2 is provided with a flat diaphragm transmitter mounted on a pad. The nozzle cavity is reduced but not eliminated. Tank 3 is furnished with liquid-filled chemical seals such as the one shown in Figure 3.6c. This unit will perform as well as the extended diaphragm d/p cell but, because of the liquid-filled capillary system, it is subject to errors caused by temperature variations. A wafer-type, liquid-filled element such as the one illustrated in Figure 3.6b is shown on tank 4. This sensing method combines the disadvantages of 2 and 3; the deadended cavity is not eliminated, and it is subject to temperature errors. However, it can be used on extremely hot processes. On tank 5, the element is the same extended chemical seal as on tank 3, but the readout is a local pressure gauge. An air bubbler is shown on tank 6. Hard-to-Handle Fluids in Pressurized Tanks When the process material in the vessel is hard to handle, it is frequently the case that the vapor space also contains materials such as foam that can build up and plug the sensing


1

FG

Air

LI

2

XFI

3

LT

LT

4

LI

5

FG

LI

LT

6

LT

LT

LI

XFI

Alternate Elements at Bottom LT

LI

LT

LT

LI

LI

XFI

N2

N2

N2

XLI

LI

FIG. 3.6k Detection of hard-to-handle liquid levels in pressurized tanks by d/p instruments.

line of the compensating leg. Figure 3.6k shows six methods for dealing with these applications. Tank 1 shows an extended d/p transmitter in the liquid region and an extended repeater in the vapor space. The use of these devices eliminates all possible plugging problems, because the sensing diaphragms are flush with the inside of the vessel wall. This detecting system will function properly on all except the most difficult crystallizer applications, where the inside wall of the tank might be coated with a layer of crystals. The extended chemical seals shown on tank 2 will provide an installation similar to that on tank 1 and will perform similarly if temperature differences between the wafers or ambient temperature variations do not cause inaccuracies. Tanks 3 and 4 are equipped with extended d/p transmitters, but a purge flow prevents the process vapors from entering the compensating legs. The purge medium can be either liquid or gas and can be applied to both dry- and wet-leg installations. Such systems require additional maintenance and range depressor adjustments, and corrosion or condensate accumulation can cause calibration or reliability problems. Tank 5 shows a local indicator equipped with extended chemical seals. This device is subject to temperature effects and requires large displacement seals to match the displacement of the d/p indicator. For tanks 1 through 5, flat diaphragm elements can also be considered, but it should be realized that they do not completely eliminate the dead-ended nozzle cavities in which material can accumulate. Such designs should be considered only where it is essential to have an isolating valve between the tank and the level device so that it will not have to be drained prior to removal of the instrument. The bubbler system is illustrated on tank 6; either liquids or gases can be used as the purge media. The extended diaphragm transmitter/repeater installation is attractive for pressure vessels containing hard-to-handle materials if isolation valves are not required. Purged installations are also acceptable if the availability of a purge media


and maintenance are both reliable and if the process can tolerate accumulation of the purge flow.

SPECIAL INSTALLATIONS One variation that can be considered is to install the d/p cell in a “reverse” arrangement wherein the high-pressure side is connected to the wet leg. Naturally, this can be done only with nonextended units such as the ones used on tanks 1 and 4 in Figure 3.6i. If this is done, these transmitters will detect the maximum d/p when the tank is empty and the minimum d/p when it is full. On hard-to-handle processes where extended diaphragms are used, a “reverse-acting” d/p cell can be used when one or both sides are protected by chemical seals (such as in tanks 2 and 5 in Figure 3.6k) and the d/p is located near the bottom of the tank. In these cases, the high-pressure side of the d/p cell can be connected to the chemical seal element at the top of the tank. In that case, the maximum reading of the d/p cell occurs when the tank is empty. Boiling Applications Reverse-acting differential pressure level transmitters can be used on boiler or steam drum level applications (Figure 3.6l) where the wet leg is the high-pressure side of the process. When the process fluid condenses into a liquid at ambient temperatures, a wet-leg configuration can be obtained by allowing the condensate to accumulate in the wet leg rather than mechanically filling the wet leg with a slow drip. This can be achieved by installing an uninsulated condensate pot that remains at ambient temperature. Excess condensation from this pot drains back into the tank and therefore maintains a constant height of the reference wet leg. Condensing Chamber

Slope

Steam Drum

HP LT LP (Reverse)

FIG. 3.6l Level measurement in steam drums or on other boiling liquid applications.


461

One should understand that the output signal of such a d/p cell relates not to the level inside the steam drum but to the mass of water inside. If the condensate in the wet leg is cold, the wet-leg density will be substantially greater than that of the boiling fluid inside the drum. In addition, the density inside the drum will be a variable; density will drop as a result of the swelling effect when the steaming rate rises, and it will rise when the steaming rate drops. A level control system that adds colder feedwater to a steaming drum may result in bubble collapse such that the actual level decreases even further. The exact position of the top surface of the boiling fluid may never be determined. A large void fraction will result in a higher surface position without necessarily changing the differential pressure. A “full” tank or drum will not produce zero differential pressure under those conditions. Therefore, the d/p cell output can be converted into a true level reading only if the density and void fraction of the boiling fluid are separately accounted for using other available parameters, such as steaming rate, bulk fluid temperature, and/or discharge pressure (equivalent to temperature for saturated conditions). These are very important considerations when determining safety system setpoints. The shrinkage that may be caused by cold feedwater or a sudden reduction in steaming rate must be accounted for to avoid uncovering hot tubes. The swell that may be caused by an increasing demand transient must be accounted for to avoid liquid carryover. Nevertheless, differential pressure remains a popular method of measuring and controlling level in boiling vessels. Many safe and effective installations exist throughout industry, including many on nuclear and fossil-fueled boilers and steam generators in the power industry. Cryogenic Applications A similar situation exists when the process liquid is very cold, except that major bulk boiling rarely takes place within the vessel itself. The tank may be located inside high-thermal insulation, called a cold box. More often, it is located in a double-walled high-vacuum dewar tank (Figure 3.6m). These applications also involve boiling, but here the liquid nitrogen or other liquefied gas will, by design, boil primarily in external piping or heat exchangers. For sensing lines, this increase in temperature occurs as the liquid-filled pipe is approaching the wall of the cold box or nears the penetration point of the dewar. At some point in the sensing line, the liquid boils, causing a liquid–gas interface. From that point on, the sensing line is filled with vapor. To provide a stable and noisefree level signal, the installation should be such that the boiling will occur at a stable, well-defined point in the sensing line. Boiling should occur in a large-diameter, low-slope section of the sensing line as it approaches the penetration of the vessel outer wall. The line diameter should be large (1 in. or 25 mm), because the interface between the liquid and vapor can be turbulent during consumption transients. This low-slope section should approach the wall of the cold area so that the temperature will be high enough to guarantee

462

Level Measurement

TABLE 3.6n D/P Cell Capsule Capabilities

High Vacuum Insulation Fill

Low Range

Medium Range

Minimum span - in. H2O - kPa

0–2 0–0.5

0–25 0–6.2

0–30 PSID 0–210

Maximum span - in. H2O - kPa

0–150 0–37.5

0–1000 0–250

0–3000 PSID 0–207 bars

Maximum zero suppression Maximum zero elevation Liquefied Gas in Dewar Tank

(maximum span) minus (calibrated span) Minimum span

SPAN, ELEVATION, AND DEPRESSION

LP

LT

HP

FIG. 3.6m Cryogenic level measurement in vacuum-insulated tank.

that the process liquid is in the vapor form under all process and ambient conditions. To account for transient conditions that may temporarily move the interface point, a “dry” loop is often provided in the warmer portion of the sensing line so as to catch and quickly evaporate any temporary liquid carryover. For most cryogenic level measurements, the density of gas in vertical sections of the sensing lines can safely be ignored. However, for argon (a very heavy gas), the effect should at least be calculated at limiting conditions to ensure that the weight will not exceed the desired accuracy of the measurement. Normal Ambient Temperature Bi-phase Applications Differential pressure may also be used for level measurement on large tanks containing gases such as refrigerants or propane under pressure, where a liquid–gas interface exists at normal ambient temperature. In these cases, a dry reference leg is suggested. A few watts of heat tracing along the dry reference leg will ensure that it remains a degree or two above ambient temperature (thus slightly superheated above saturation temperature) to avoid condensation of the gas. Care must be taken during fast filling operations to ensure that major condensation does not collect temporarily in the reference leg as a result of increasing pressure in the tank. Provision of a drip leg with a sight glass at the bottom of the reference leg is suggested.


High Range

All d/p cells can be provided with zero, span, elevation, and depression adjustments, either mechanical or electronic. Table 3.6n shows some typical d/p cell ranges and the available elevation and suppression setting adjustments for each. Whenever the d/p is at an elevation other than the connecting nozzle on an atmospheric tank, the zero of the d/p cell needs to be elevated or depressed. It is important to realize that two zero reference points exist. One is the level in the tank that is considered to be zero (lower range value) when the tank is near empty. The other zero reference point is the point at which the d/p cell experiences a zero differential (zero value of the measured variable). The terms elevation and depression as used in this discussion refer to the zero experienced by the d/p cell (Figure 3.6o). This figure uses mechanical spring adjustments to physically illustrate the relationships. Equivalent electronic adjustments are available. In both cases, the overall available span must be wide enough to encompass the required adjustments. The tension in the elevation spring can be set to cancel out any initial pressure exerted on the high side of the diaphragm capsule. Similarly, the depression spring can be adjusted to compensate for initial forces on the low-pressure side of the d/p cell. The amount of depression setting is limited to the full range of the capsule, whereas the sum of elevation setting and span cannot exceed the full range of the cell. These settings are normally adjusted in the factory if sufficient data are furnished to the manufacturers. If the setting is changed in the field, it will affect the span of the transmitter. Figure 3.6p shows a dry-leg d/p cell installation with the desired minimum and maximum liquid levels noted. The output of the transmitter will be zero when the level is at the minimum and 100% when it is at the predetermined maximum. The span (range) of the cell will be product of liquid density and the distance between minimum and maximum levels desired (X). The elevation spring will be set for the product of density times distance between the minimum level desired and the cell datum (Y). A reference leg is also shown on this sketch, which is convenient for checking the transmitter. Checking is done by temporarily isolating the cell from the tank and filling the reference leg with a known


Upper Range Value

Dry Leg or Repeater

h1

Maximum Level

Reference Leg

h2

Elevated Span

463

X

Minimum Level

Lower Range Value Elevation (Zero Suppression)

HP

Y

Zero Value of the Measured Variable

HP

LP

Liquid Gravity: SG1

LP LT

FG

Tension

FIG. 3.6p Illustration for range elevation; span = X(SG1) and elevation = Y(SG1).

Direct Reverse Compression

FG Maximum Level

LP

HP

SG2 X SG1

Z

Minimum Level

h1 h2

Suppressed Span

Wet Leg

Zero Value of the Measured Variable Suppression or Depression (Zero Elevation) Upper Range Value

Suppression Span

Y LT S = X (SG1) ; D = Ζ (SG2) − Y(SG1)

Lower Range Value

FIG. 3.6o Illustration of elevation and depression in connection with d/p-type level measurement on atmospheric tanks.

gravity fluid. Once this figure is obtained, the repeatability of the unit can be checked periodically. Figure 3.6q shows a wet-leg installation. Span is determined the same way as before (X × SG1). Range depression is calculated as the difference between the hydraulic head in the wet leg (ZSG2) and the range elevation (YSG1) desired. The difference between process and filling fluid densities must be selected such that the depression does not exceed the limit given in Table 3.6n. For example, if the desired minimum level is at the cell datum line (Y = 0), the difference between maximum and calibrated span is 200 in. (5 m) of water column, and the height of the wet leg is 100 in. (2.5 m), then the density of the filling liquid cannot be more than 2. The actual span setting of the cell can be anywhere below the full range.


FIG. 3.6q Illustration for range depression.

INTERFACE DETECTION Figure 3.6r shows the settings for a liquid–liquid interface application. The span for this cell is the product of the density difference of the two liquids (SG2 – SG1) and the distance between the maximum and minimum interface levels (X). The range depression is the difference between the hydraulic head of the filling fluid (Z × SG3) and the sum of the range elevation (Y × SG2) plus the light liquid head over the range of minimum interface to overflow level ([X + V] × SG1). No depression is required if the minimum interface is at the cell datum, the height of the wet leg is the same as the maximum total level, and the filling fluid density (SG3) is the same as the light liquid (SG1). If the minimum interface is at the cell datum and a dryleg system is used, then, instead of depression, the cell must

464

Level Measurement

Bibliography

FG

V = 10" Overflow Max. Interface SG3 = 2.0

SG1 = 1.0

Z = 100"

X = 50" W = 20" Min. Interface HP Y = 10"

SG2 = 2.0

LP LT

S = X(SG2 − SG1); D = Z(SG3) − [Y(SG2) + (X + V)SG1]

FIG. 3.6r Span and depression settings for interface detection.

be elevated by the hydraulic head of the light liquid over the range of cell datum to overflow level (X + V × SG1). The following calculations, which are based on the data shown in Figure 3.6r, will serve as examples. Wet leg hydraulic head = Z(SG3) = 200 in. H2O

3.6(1)

Process side hydraulic head at minimum interface = (V + X)SG1 + Y(SG2) = 80 in. H2O 3.6(2) Process side hydraulic head at maximum interface = V(SG1) + (X + Y)SG2 = 130 in. H2O 3.6(3) Transmitter span = X(SG1 − SG2) = 50 in. H2O 3.6(4) Range depression = Ζ (SG3) − [Y(SG2) + (X + V)SG1] = 120 in. H2O

3.6(5)

To determine the correct output signal from the transmitter at any known interface level, the proper result is simply the percentage of the transmitter span represented by the known level. If W is the known level, then 20 in. represents 40% of the 50-in. span. For a 3- to 15-PSI output, this will be 40% of 12 (= 4.8) plus 3 PSI for the “live zero,” giving a final answer of 7.8 PSIG. Similarly, for a 4- to 20-mA output, the result will be 40% of 16 (= 6.4) plus 4 mA for the “live zero,” to give a final answer of 10.4 mA.


Appleby, S., Practical uses of pressure transmitters to monitor fluid levels, Transducer Technol., February 1987. Berto, F. J., Hydrostatic tank gages accurately measure mass, volume, and level, Oil & Gas J., May 14, 1990. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Binder, J., Becker, K. and Ehrler, G., Silicon Pressure Sensors for the Range 2 kPa to 40 MPa (English ed.), Siemens Components, Germany, April 1985. Blickley, G. J., Level measurement choices, Control Eng., August 1991. Blickley, G. J., Tank gaging transmitter performs more functions, Control Eng., August 1991. Charrier, G. and Dupont, H., A liquid helium level detector, Le Vide les Couches Minces, France, March–April 1984. Early, P., Solving old tank gauging problems with the new hydrostatic tank gauging technology, Adv. Instrum., 42, 143–153, 1987. Eman, J. F. and Gestrich, N., Selecting manometer-type level gauges, Instrum. Control Syst., July 1977. Gillum, D., Industrial Pressure, Level, and Density Measurement, ISA, Research Triangle Park, NC, 1995. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Hydrostatic tank gauging system offers accurate mass measurement, Food Eng., March 1988. Jethra, R. and Cushing, M., Application of dual sensor transmitters in challenging process environments, ISA Technol., October 7, 1977. Johnson, D., Doing your level best, Control Eng., August 1977. Labs, W., Level measurement, pressure methods dominate, Instrum. Control Syst., February 1990. Lanini, L. and Schneider, L., The dawn of new tank gauging system, Adv. Instrum., 42, 155–161, 1987. Mascone, C., New gauging system wins measure of approval, Chemical Eng., 25–29, September 14, 1987. Nef, G. G. and Evans, R. P., Line pressure effects on differential pressure measurement (PWR system), in Proc. 29th Int. Instrum. Symp., ISA, Research Triangle Park, NC, 1982. Piccone, R. P., A case for an HTG hybrid, Instrum. Control Syst., February 1988. Piccone, R. P., Combining technologies to compute tank inventory, Sensors, October 1988. Proctor, A., The gauge comes of age (hydrostatic measuring techniques), Process Eng., December 1987. Reisch, F., Meeting the need for unambiguous PWR coolant level measurement, Nuclear Eng. Int., January 1984. Robinson, C., Hydrostatic tank gaging: what it is, where it’s used, what’s available, InTech, February 1988. Rowe, J. D., Hydraulic tank gaging systems set inventory accuracy standards, Inventory and Control Syst., February 1987. Slomiana, M., Using differential pressure sensors for level, density, interface, and viscosity measurements, Instrum. Technol., September 1979. Waterbury, R. C., Transmitter keys hydrostatic gauging, InTech, July 1990.

3.7

Displacer Level Devices LC

D. S. KAYSER (1982) B. G. LIPTÁK C. G. LANGFORD (2003)

(1969, 1995)

Flow Sheet Symbol

Design Pressure

Set by the flange rating of the chamber or by the maximum working pressure of the displacer, usually up to 100 PSIG (7 bars, 0.7 MPa) for the flexible disc and up to 600 PSIG (41 bars, 4.1 MPa) for the diaphragm-sealed designs. The flexible shaft unit can operate up to 1000 PSIG (69 bars, 6.9 MPa); torque-tube designs are available up to 2500 PSIG (170 bars, 17 MPa); magnetically coupled units can be used up to 6100 PSIG (410 bars, 41 MPa). Verify ratings with the manufacturer.

Design Temperature

Generally in the range of −50 to 451°F (−45 to 230°C). Inconel torque tubes can operate from −350 to 850°F (−212 to 454°C). For electronic transmitters, the temperature of the topworks must be kept below 180°F (82°C). If the process temperature is above 500°F (260°C) or below 0°F (−18°C), thermal insulation barriers or torquetube extensions are usually recommended.


Displacers are available in type 316 stainless steel, Monel , polypropylene, or solid ® Teflon . The hanger cable assemblies can be obtained in type 316 stainless steel, ® ® Monel , and Hastelloy C. The cage (chamber) is usually carbon or stainless steel.

Inaccuracy

Varies widely with application and the instrument, typically 0.5% of full scale.

Range

Standard displacers are available in lengths of 14 to 60 in. (0.35 to 1.5 m). The range of special units can go up to 60 ft (18 m).

Cost

Displacer-type switches cost from $200 to $500, and a 32-in. (0.81-m) electronic transmitter costs about $2500; add $500 to $700 for an external steel chamber.


ABB Instrumentation Inc. (www.abb.com) Delta Controls Corp. (www.deltacnt.com) Dwyer Instruments Inc. (www.dwyer-inst.com) Endress+Hauser Systems & Gauging (www.systems.endress.com) The Foxboro Co. (www.foxboro.com) Magnetrol International (www.magnetrol.com) Masoneilan Operations Dresser Flow (www.masoneilan.com) Norriseal (www.norrisel.com) Schlumberger Measurement Div. (www.slb.com/rms/measurement) Siemens Moore Energy & Automation (www.sea-siemens.com) Yokogawa Corp. of America (www.yca.com)

®

®

INTRODUCTION Archimedes’ (c. 290 to 212 BC) principle states that a body wholly or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. A level or a density instrument is sensitive to the apparent weight of an immersed displacer. If the cross-sectional area of the displacer and the density of the liquid are constant, then a unit change in level will result in a reproducible unit change

in the apparent weight of the displacer. The simplest level device of this type involves a displacer that is heavier than the process liquid and is suspended from a spring scale. When the liquid level is below the displacer, the scale shows the full weight of the displacer. As the level rises, the apparent weight of the displacer decreases, thereby yielding a linear and proportional relationship between spring tension and level. The spring scale can be calibrated as desired. 465


466

Level Measurement

This simple device is limited to applications in open tanks. In practical industrial service, the basic problem is to seal the process from the spring scale or other force-detecting mechanism. This seal has to be frictionless and useful over a wide range of pressures, temperatures, and corrosion conditions. The variations in the design of this seal provide the basis to distinguish the types of displacement detectors that are in use and are discussed below. They are the magnetically coupled switch, the torque tube, the diaphragm and force bar, the spring balance, the flexible disc, and the flexible shaft design. Each of these units operates on Archimedes’ principle but is different as far as its seals are concerned. All of them can be used to detect a liquid–vapor interface, a liquid–liquid interface, and, if the level is constant, the changes in density as well. The flexible disc unit is available as a pneumatic transmitter, and the flexible shaft unit is available as a high-gain pneumatic controller or as a switch. The other designs are available with integral pneumatic or electronic transmitters or controllers.

mounted on a flexible cable attached to a support spring. When the tank is empty, the spring must support the full weight of the displacer. As the level rises, and the displacer becomes immersed in the liquid, the downward force on the support spring is reduced, and a small stem movement occurs to activate the switch. Figure 3.7a also illustrates how the displacer level switch systems might be tested when installed in open tanks and sumps, or on floating roofs. The technician or operator can attach a testing cable as shown on the right side of Figure 3.7a. Once the cable is attached, lifting on the cable simulates a high-level condition, changing the state of the switch to prove the operation of the switch and probably the wiring. In the next section, the operation of different switching contacts and the designs of the different float chambers are described. Although this information is provided for floattype switches, it is also applicable for displacer-type units. When deciding between float-type and displacer-type level switches, the following advantages can be noted for the latter:

DISPLACER SWITCH

1. The maximum differential between high and low settings can be as much as 50 ft (15 m). 2. Level settings are adjusted by moving the displacer(s) to a new elevation on the cable. 3. Moderate surface turbulence is less apt to cause switch chatter, because the cable is in tension. 4. Within broad ranges, fluid density has no effect on the displacer sizing diameter, making units interchangeable between services of varying density simply by changing the support spring. 5. The displacer switch is less apt to cause spurious trips in vibrating services, because the cable is always in tension. This is an important consideration for automatic shutdown systems such as may be used at compressor suction drums.

The major difference between a float level switch and a displacer level switch is that a float moves with the surface, whereas a displacer is partially or totally immersed and moves very little with process changes. Switching arrangements and installation considerations are similar for the displacer and float switches. Float-type switches are covered in the next section, and the discussion here will be limited to the design features of the displacer switch. Figure 3.7a shows a displacer

Testing Cable

High Level Flexible Cable

Low Level

FIG. 3.7a Displacement level switches.


Displacers

The displacer switch is available in many configurations to achieve the desired functionality. Some have multiple switches and can be used for multiple control functions. For example, a single unit can be used to sequence two pumps by actuating both pumps at high level, one at mid level, and neither at low level. The one disadvantage of most implementations of this style of displacer switch is that the support spring is exposed to the process. This limits the switch to applications that are reasonably clean, nonfreezing, and not corrosive to the available spring materials. One way to eliminate the need for a spring is to install the displacer horizontally and connect it to a microswitch through a flexible shaft seal (Figure 3.7b).

TORQUE-TUBE DISPLACERS The torque tube shown in Figure 3.7c uses a tube in torsion to provide the spring function. The hollow torsion tube supports the displacer, which is always heavier than the process fluid, and also provides a frictionless pressure seal. This makes it

3.7 Displacer Level Devices

467

Float-Arm Extension Flexible Shaft 11/2" NPT Limit-Stop Bracket

Float-Arm Displacer Float

Formed Section SPDT Micro Switch

FIG. 3.7b Side-mounted displacer switch. (Courtesy of Siemens Moore Energy & Automation.)

Torque Tube Flange

Torque Arm Torque Arm Block

Torque Rod

Torque Tube

Limit Stop Knife Edge

Nozzle Flapper

Displacer

of a knife-edge bearing support. A limit stop prevents accidental overstressing of the torque tube by limiting the downward motion of the torque arm. The angular displacement of the torque tube and torque arm are the same at the knife edge end of the tube. At the flange end, the tube is anchored in place and does not rotate, but the torque rod is free to rotate the same amount as it did at the knife edge. The angular displacement, which amounts to about 5° or 6°, is linearly proportional to the apparent weight of the displacer and thus to the level or density. With the pressure sealing problem solved, it is a simple matter to convert the angular displacement to a usable electronic or pneumatic analog signal. The ® standard torque-tube material is Inconel , but the torque tube ® ® is also available in stainless steel, Hastelloy , Monel , nickel, or Durimet. Note that the mechanical design requires the technician to be knowledgeable and careful in doing maintenance or repairs. Sizing of Displacers

FIG. 3.7c Torque tube displacer level detector.

possible to transfer the changes in the apparent weight of the displacer through the wall of the pressure vessel into a measuring device. Figure 3.7c is a schematic presentation of the displacer and torque tube. The displacers are typically cylindrical and can be furnished in a wide selection of plastic and alloy materials. Although any length displacer up to 60 ft (18 m) can be obtained, the most common lengths cataloged are 14, 32, 48, and 60 in. (0.3, 0.8, 1.2, and 1.5 m). The 3 3 volume of a standard displacer is 100 in. (1638 cm ), and the diameter is reduced as the length increases. The torque arm connects the displacer to the torque tube and absorbs any lateral forces. Friction is minimized by use


The technique given below for determining the desirable displacer diameter is applicable for all types of buoyant-force detectors, not just the torque-tube design. The displacer diameter sets the weight change of the displacer per level increment. The torque tube is designed to twist a fixed amount for each increment of buoyancy change. Therefore, in selecting the displacer diameter, the torque-tube characteristics, the density of the process fluid, and the level span must be considered. For purposes of this discussion, it will be assumed that the motion of the torque rod will be used to operate a proportional band controller. Proportional band (PB) refers to the response sensitivity of the controller, and it determines the percentage change in output signal in response to a 1% change in the level. A 100% apparent proportional band setting means that the level in the tank has to cover the displacer completely to generate a full output signal, and that

468

Level Measurement

the level has to drop to the bottom of the displacer to generate the minimum output. A controller set to a PB of 100% can be used as a transmitter if the process fluid is water. At 50% apparent proportional band, a level variation of 50% of the displacer length produces minimum to maximum output, and, at 25% setting, a level variation of 25% will generate the minimum to maximum output. The term apparent proportional band is necessary to distinguish the actual band setting on the instrument based on water density at standard temperature from the resulting apparent band related to the density of the process fluid being measured. Figure 3.7d shows the relationship between the two terms as a function of process liquid density. It can be seen from Figure 3.7d that, if the band setting on the instrument is 50%, the output of the controller will change 50% of the level change with a liquid gravity of 1. It will change 100% with a gravity of 0.5, and, with a specific gravity of 0.1, the output will equal 10 times the level change. The process fluid density thus affects the apparent gain of the controller in an inverse linear fashion. Because the weight change per unit level change generated by the displacer is balanced by the torsion spring constant, the characteristics of the torque tube must be considered. The range of a standard torque tube matches the buoyant 3 3 force generated by a 100 in. (1638 cm ) displacer in water for a band setting of 100%. This is equivalent to a force range of 0 to 3.6 lbf (0 to 1.6 kgf). Table 3.7e below lists the force

Actual (Water Based) Proportional Band 100 SG = 1.0 SG = 0.25

SG = 0.1

SG = 0.5

80

SG = 0.2

60 40

ranges of standard and thin-wall torque tubes for one design at various proportional band settings. The thin-wall tube requires one-half as much force for full range operation as the standard-wall tube. Review the technical literature for the actual device used to confirm the actual numerical constants to use. The calculations here are only examples. Interface Measurement In most liquid level measurement applications, the specific gravity of the liquid is 0.5 (or greater), and it generates sufficient force on the torque tube for a full range of output signals, even with the use of a standard volume displacer, unless a very narrow proportional band is required. This may not be true for liquid–liquid interface applications. Difference in buoyant forces is generated only by the density difference between the two fluids, and the displacer must be completely submerged for meaningful readings. Therefore, available forces are smaller, and the sizing of displacers for liquid–liquid interface is more difficult and requires more attention. On interface applications, it is advisable to select a displacer diameter to result in a 100% apparent proportional band when the actual band setting is 20%. This leaves additional adjustment capability to the operator who, if required, can reduce the actual band to 10% (the minimum recommended) to achieve a reduction in apparent band to 50%. An example will illustrate this. Assume an installation in which the specific gravity of the light fluid is 0.98 and of the heavy fluid is 1.02, and the required displacer is 32 in. (812 mm) long. Furthermore, it is desired to generate full controller output over a level interface change of 16 in. (406 mm). To select a displacer diameter suitable for these requirements, it is necessary to calculate the displacer volume that will generate the force range required by a thin-wall tube when the level variation is 16 in. (406 mm). A thin-wall tube at an actual band of 20% requires a full-range force of 0.36 lbf (0.16 kgf). The calculation of displacer volume to generate this force is as follows:

20 0 100 200 300 400 500 600

800

1000

Apparent Proportion Band

FIG. 3.7d Relationship between apparent and actual proportional band.

Volume for 16 in. length = (torque tube force)/( ∆SG)(wt. of 1 in.3 H 2O) = 0.36/(0.004)(0.036) = 250 in.3

3.7(1)

Diameter = (( 4V )/(πL))1/ 2 = [(( 4)(250))/((π )(16))]1/ 2 = 4.5 in. TABLE 3.7e Force Ranges of Standard and Thin-Walled Torque Tubes Actual PB Setting (percent)

Force Range for Std.-Wall lbf (Kgf)

Force Range for Thin-Wall lbf (Kgf)

100

0–3.60 (0–1.6)

0–1.8 (0–0.8)

50

0–1.80 (0–0.8)

0–0.720 (0–0.32)

20

0–0.72 (0–0.32)

0–0.36 (0–0.16)

10

0–0.36 (0–0.16)

0–0.18 (0–08)


3.7(2) If it is required to broaden the actual proportional hand to 40%, full controller output will result from a 32-in. (812-mm) change in the interface. If the band is narrowed to 10%, full controller output will correspond to a level change of 8 in. (203 mm). If a standard-wall torque tube had been selected for the above example, the diameter of the tube would have worked out to about 6.5 in. The sizing method


469

Pressure Vessel

2" or Larger Nozzle " Gate Valve (Vent) or Plug 2" or 3" Standpipe

2" Gate Valve Gate Valve Automatic Gage Cocks or Tees

" Gate Valve (Vent) or Plug 1 " or Larger Gate Valve

1 " or Larger Screwed or Flange Connections

\

" Coupling 6000 LB Tapped One End Only

Overlapping Gage Glasses 1 " or Larger Gate Valve

FIG. 3.7f Side-mounted displacer. (Courtesy of Siemens Moore Energy & Automation.)

given can be used for all displacer applications including density detection. Standard displacer diameters are 3, 4, and 6 in. (76, 102, and 152 mm), but special sizes and designs are also available (Figure 3.7f). Carefully review the engineering specifications or check with the manufacturer for the details that apply to the specific instrument being considered. In a large vessel, the interface may have a very slow but substantial wave motion if the difference between densities is small. The gravity forces are small, and the fluids have little friction to damp out any fluid motion. If no sight port is located so as to make this motion visible, the operator will have the impression that the signal or the control is cycling. Rag Layer Another cause of measurement doubt and confusion is the common situation of a rag layer of material, lighter than the heavy layer and heavier than the light layer, that will accumulate between the two desired layers. If there is no mechanism or procedure to remove this material, it will become ever deeper until it suddenly appears in the upper layer outlet. In continuous processes, even tiny amounts of undesired materials will always accumulate wherever conditions make this possible. Features and Installation Torque-tube level devices can be mounted internally or externally to the vessel. Internal displacers are used where it is possible to drain the tank for level detector maintenance. If the displacer is to be internally mounted, it is good practice to install it inside of a stilling well, which may be fabricated from a piece of pipe. To avoid errors, the standpipe must have a number of vertical slots or holes along its length to permit a free but restrained flow of liquids, and it should have a stop bar across the bottom to prevent the displacer from falling to


Reducer to These Assemblies May be Elbows

" Pipe

" Gate Valve (Drain)

FIG. 3.7g The installation of an external cage displacer on a standpipe with two level gauge sections. (Courtesy of the American Petroleum Institute, API RP 550.)

the bottom of the vessel if it becomes disconnected from the torque tube. For installations in which the vessel cannot be opened and drained to perform maintenance on the displacer, it may be installed in a level chamber mounted outside of the tank and isolated from the process by means of lockable valves. Local safety rules must be observed. A single barrier between vessel and the environment may require special precautions. It has been typical practice to install a level gauge to approximately match the span of the displacer to provide visual observation of the level. Figure 3.7g shows the installation of a displacer transmitter and level gauge mounted on a standpipe or level chamber. Note that the two units have independent isolation and drain valves. Some users prefer to eliminate the installation of breakable sight glasses for environmental and safety reasons. When an external chamber is used, it may be necessary to heat-trace and insulate the level chamber for freeze or fire protection. The density measurement is for the liquid in the chamber and may not represent the vessel contents density if the level chamber temperature is not the same as the vessel bulk temperature. For liquid–liquid interface applications, the standpipe must have three connections to the vessel: one in the heavy liquid layer, one in the light liquid layer, and one in the vapor space that vents back into the vessel. Special torque-tube and displacer designs are available for operating pressures up to 2500 PSIG (17 MPa). For hightemperature installations, the torque-tube material may be the

470

Level Measurement

limiting factor. The spring characteristics of the tube will change at sufficiently high temperatures. Low temperatures have little effect on the spring characteristics of the tube. For ® torque-tube materials, Inconel is suitable for temperatures between −350 and 850°F (−212 and 454°C). All other materials are limited to 500°F (260°C) except bronze, which is rated at 300°F (150°C). Torque-tube extensions will provide substantial thermal isolation for the instrument case. Installation details will also affect the instrument temperature. When the process is at a temperature above 500°F (260°C), finned extensions are recommended; when it is below 0°F (−18°C), plain extensions may be used. The extensions should not be insulated. Additional thermal insulation or thermal radiation barriers can be used to protect the indicating and transmitting portions of the instrument. Note that pneumatic transmitters may have a higher allowable temperature than the electronic version. Jacketed displacer chambers are available for hardto-handle services if field-applied tracing is judged to be inadequate. The process connections on external chamber displacers are normally 1.5 or 2 in. (38 or 51 mm) and often flanged, so they may be used in mildly dirty services. Consider blowdown or drain valves where appropriate and permissible. It is preferred not to use external chambers if there is a likelihood of plugging because of solids accumulation. The torque-tube displacer can be furnished with pneumatic or electronic transmitters or with local control. Controllers are available with gain, integral, and/or derivative control action. An output gauge should be installed on the transmitter signal to indicate level and for troubleshooting. It may also be good practice to install an independent level gauge. The torque tube and displacer design has an excellent field record for accuracy and reliability. Over time, accumulated noncondensable and solids may compromise accuracy, and some verification of proper performance is desirable. The torque-tube seal is virtually trouble-free if properly specified to meet the process requirements. In processes in which level setpoint changes are infrequent, or where speed of response is a concern, displacer units are often used as local level controllers, even if a sophisticated control system is used. The units may also be furnished with transmitter/controller combinations, allowing the advantage of local control with remote process indication.

SPRING-BALANCE DISPLACER This instrument is similar to the torque-tube unit except that the spring function of the torque tube is replaced by a conventional range spring, and the isolation of the process from the instrument is by means of a magnetic coupling. As illustrated in Figure 3.7h, the displacer is suspended in the liquid by means of an extension range spring. As the level in the vessel rises or falls, the buoyancy force on the displacer changes, causing the spring to extend and contract. A magnetic attracting ball attached to the displacer rod rises and falls in response


Magnet

Nozzle

Attraction Ball Enclosing Tube

Air Pilot

Output

Air Supply

Range Spring

Magnetic Coupling with Electronic Output Displacer

FIG. 3.7h The old spring balance, magnetically coupled displacer.

to the displacer movement. The movement is about 1 in. (25 mm) full range. The ball is centered within the enclosing tube, and its movement is nearly frictionless. A follower mechanism moves with the magnetic ball for indication and signal transmission. Other design features and accessories are similar to the ones discussed in the section on torque-tube design. The springbalanced displacers are also available with corrosion-resistant wetted parts and are suitable for operating temperatures between −250°F (−157°C) and 600°F (316°C). Because the interior-to-exterior motion is magnetically coupled, units with very high pressure ratings to 6000 PSIG (41 MPa) are available. The merits and disadvantages of the spring-balanced design are similar to those of the torque-tube units, except that the movement of the displacer is greater, and the range spring is exposed to the process. Greater movement almost always causes faster wear at pivot points. The range spring exposure creates difficulty in installations where vapor space condensation, polymerization, or crystallization is expected, because material buildup on the spring will interfere with proper operation. Inert gas purging of the spring chamber has been used to prevent the process vapors from entering the chamber, but the large flow rates required make this solution impractical.

FORCE-BALANCE DISPLACER The basic mechanism of the classic differential-pressure transmitters illustrated in the previous section can be adapted to produce another family of displacer level devices. Figure 3.7i shows the force-balance type of top-mounted design. Level variations in the vessel cause a proportional change in buoyant


471

V = displacer volume in cubic inches Lw = working length of displacer in inches L = total length of displacer in inches

Seal & Fulcrum

This instrument is available for mounting either inside the vessel or in an external chamber. The latter is used where maintenance must be performed while the tank is under pressure. The limitations of this device are similar to those of the torque-tube design but, in addition, the diaphragm seal is not as rugged as the torque tube, limiting operating pressures to 600 PSIG (4.1 MPa) and operating temperatures to 400°F (204°C).

Force Bar

Hanger Rod

Displacer

FLEXIBLE DISC DISPLACER

FIG. 3.7i Force balance, diaphragm sealed displacement level transmitter. (Courtesy of The Foxboro Co.)

force of the displacer, reducing the apparent weight on the force bar as the level increases. In the side-mounted version, the process is sealed by a diaphragm, which also serves as the fulcrum of the force bar. The buoyant force is transmitted to the balancing rod, which pivots on the range wheel. An increase in level causes a minute movement that, it turn, is detected by the transmitter, which in the past generated a pneumatic signal. Today, the signal is electronic, in the form of either an analog signal or digital data communicated over the bus or network of the plant. This instrument is available with the displacer mounted either inside the vessel or in an external chamber. The latter is used where maintenance must be performed while the tank is under pressure. The limitations of this device are similar to those of the torque-tube design, but the diaphragm seal is not as rugged as the torque tube, and the operating pressure is limited to 600 PSIG (4.1 MPa) with operating temperatures to 400°F (204°C). A variety of materials are available to meet requirements of corrosive services (see Table 3.7j). The displacer sizing procedure for density, interface, or level detection follows the same basic method outlined for the torque-tube units, except that the buoyant force range involved is different. The buoyant force span for one standard unit is 2.90 lbf (1.29 kgf) or more; for the narrow design, it is 1.45 lbf (0.65 kgf) or more. The formula to calculate tile buoyant force is: F = (0.36)(SG)(V)(Lw)/L where F = buoyant force in pounds SG = specific gravity difference


3.7(3)

In case of the flexible disc design, as rising liquid level reduces the apparent weight of the displacer, a force balance mechanism detects this change in weight while maintaining equilibrium. The output signal is directly related to the level in the vessel. Span adjustments are made by changing the total length of the float arm. The float arm is supported by the flexible disc. As a result of the force-balance principle of operation, all components maintain their predetermined position with essentially no movement. This protects the flexible disc from fatigue due to bending. The weight of the displacer and float arm is carried by the flexible disc, and the static pressure in the vessel acts upon the thrust pivots. Limit stops keep the float arm motion within the elastic limits of the disc when the vessel is empty. This design has the same limitations as the other displacer units but has a narrower range of application, because the flexible disc seal limits its use to 100 PSIG (0.7 MPa) operating pressure. At higher pressures, the unit becomes inaccurate, and an increase in error of ±1% of full scale can be expected per 100 PSIG of process pressure. The flexible disc is normally made of stainless steel, but it is also available in ® Monel and nickel. The displacer can be made of a wide variety of materials. The performance of this unit is generally inferior to the other designs covered in this section.

FLEXIBLE-SHAFT CONTROLLERS This unit moves too much to be simply classified as a displacer and too little to be simply a float device. But the limited float motion makes it more nearly a displacer, so it is considered as such here. As with the other displacer designs, the flexibleshaft unit detects the buoyant force of the float with practically no motion involved approximately 1/32 in. (0.8 mm) travel. As shown in Figure 3.7k, the shaft is tubular, with a flattened center section that moves easily in the vertical direction but resists horizontal motion. In the pneumatic-controller version, the extension tongue transmits the float motion to an air pilot.

472

Level Measurement

TABLE 3.7j Data on Displacer Materials, Sizes, and Other Features (Typical)* Metric Displacer Data Maximum Working Pressure

English Displacer Data

L (mm)

316 SS

356

76

6.9

69

14

3

1000

2.95

0.42 and 1.6

610

51

10.3

103

24

2

1500

2.52

0.56 and 2.2

610

76

6.9

69

24

3

1000

1.69

0.25 and 0.98

813

51

10.3

103

32

2

1500

2.28

0.42 and 1.6

®

Monel

Solid PTFE

L(in.)

OD (in.)

Maximum Working Pressure (PSI)

Permissible Process Liquid Relative Density Limits (SG Term)

Material

MPa

bar or kg/cm

Approximate Relative Density of Displacer (Specific Gravity)

OD (mm)

813

76

6.9

69

32

3

1010

1.22

0.18 and 0.7

1016

51

10.3

103

40

2

1500

2.38

0.33 and 1.4

1219

42

10.3

103

48

1.66

1500

2.27

0.39 and 1.6

1270

51

7.6

76

50

2

1100

1.90

0.27 and 1.0

1524

38

10.3

103

60

1.5

1500

2.41

0.39 and 1.6

1524

76

10.3

103

60

3

600

1.31

0.10 and 0.4

1829

33

10.3

103

72

1.31

1500

2.58

0.42 and 1.7

2134

32

10.3

103

84

1.25

1500

2.60

0.40 and 1.6

2438

27

10.3

103

96

1.05

1500

2.96

0.49 and 2.0

2540

32

10.3

103

100

1.25

1500

2.43

0.34 and 1.4

2743

27

10.3

103

108

1.05

1500

2.80

0.44 and 1.8

2743

51

6.7

67

108

2

975

1.64

0.12 and 0.5

3048

25

10.0

100

120

1

1450

2.47

0.44 and 1.8

3810

25

10.0

100

150

1

1450

2.44

0.35 and 1.4

356

76

6.9

69

14

3

1000

1.63

0.42 and 1.6

610

76

6.5

65

24

3

950

1.69

0.25 and 0.98

813

51

10.3

103

32

2

1500

2.28

0.42 and 1.6

1524

51

6.9

69

60

2

1000

1.88

0.22 and 0.89

2134

33

10.3

103

84

1.31

1500

2.60

0.36 and 1.5

3658

25

5.2

52

144

1

750

2.47

0.37 and 1.5

356

76

10.3

103

14

3

1500

2.28

0.42 and 1.6

813

51

10.3

103

32

2

1500

2.28

0.42 and 1.6

1219

41

18.3

183

48

1.6

2650

2.28

0.42 and 1.6

*Courtesy of The Foxboro Co.

Float Arm

Mounting Flange Flexible Shaft

Float

Limit-Stop Bracket

Extension Tongue

FIG. 3.7k Flexible shaft level controller. (Courtesy of Siemens Moore Energy and Automation.)


An increase in level moves the tongue, gradually closing the vent of the pilot and increasing the output signal to the control valve. By turning the mounting flange 180°, the control pilot action can be reversed so that an increase in level will be accompanied by a decrease in the signal to the valve. The unit can also be provided as an on–off switch with either pneumatic or electric output. As with other displacer designs, this device can be used to control interface or fluid density, but it is very limited in adjustment flexibility as compared to the torque-tube design. The flexible shaft unit has a narrow and fixed proportional band without integral action. The ball float has a throttling


range of approximately 0.5 in. (12.7 mm), depending on arm length. This high sensitivity limits application in many services. The flexible shaft level controller can be furnished with pressure ratings to 1000 PSIG (0.9 MPa) and temperature ratings to 700°F (371°C). Construction materials include ® ® stainless steel, nickel, Monel , Hastelloy , and others. The flexible shaft design is generally used in services where near on–off control action is desired. Cost is less than for the torque-tube devices.

CONCLUSION The external-cage-type displacement level transmitters and controllers are very popular in power plants and chemical processes. Applications include close control of level in hightemperature and high-pressure vessels and where the process cannot be shut down for instrument replacement or maintenance. They have lost some ground to electronic level sensors, partly because they are limited to use on clean fluids, because dirt and material buildup on the displacer cannot be tolerated, and partly as a result of cost considerations. They are not confused by foam, vapors, and changes in vapor density of the devices used, the torque-tube type continuous units up to 60 in. (1.5 m) and the displacement-type switches are the most popular.


473

Bibliography Anderson, J., Measuring level with displacers, Instrum. Control Syst., June 1979. API Recommended Practice 550, Manual on Installation of Refinery Instruments and Control Systems, Part I, Process Instrumentation and Control, Section 2, Level, American Petroleum Institute, Washington, DC. Bacon, J. M., The changing world of level measurement, InTech, June 1996. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June/July 1997. Boyes, W. H., The changing state of the art of level measurement, Flow Control, February 1999. Carsella, B., Popular level-gauging methods, Chemical Process., December 1998. Cho, C. H., Measurement and Control of Liquid Level, ISA, Research Triangle Park, NC, 1982. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001. Johnson, D., Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001. Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Level measurement and control, Meas. Control, April 1991. Paris, T. and Roede, J., Back to basics, Control Eng., June 1999. Parker, S., Selecting a level device based on application needs, in 1999 Fluid Flow Annual, Putman Publishing, Itasca, IL, 1999, 75–80. Paul, B. O., Seventeen level sensing methods, Chemical Process., February 1999. Sholette, W., Pick the proper level measurement technology, Chemical Eng. Prog., October 1996.

3.8

Float Level Devices D. S. KAYSER (1982) C. G. LANGFORD

B. G. LIPTÁK

LI

(1969, 1995)

(2003)

LSL To Receiver Flow Sheet Symbol


Types

Switches and transmitters can be mechanically or magnetically coupled and may have single- or multiple-point sensors. Indicators include the tape board gauge.

Design Pressures

Level switch units used in vending-machine type applications are usually limited to 100 PSIG (6.9 bars, 0.69 MPa), while magnetically coupled float level indicators can operate up to 1000 PSIG (70 bars, 7 MPa), and special designs can go up to 2000 PSIG (140 bars, 14 MPa).

Design Temperatures

Standard level switch units used in vending machine-type applications are usually suited for −40 to 225°F (−40 to 117°C); special designs are available for 500°F (260°C) and higher.

Float Materials

Brass, copper, stainless steel, Monel , Hastelloy , polysulfone, polypropylene, and other plastics

Costs

Vending-machine-quality level switches can be obtained for about $50; the cost of a type 316 stainless-steel industrial float switch ranges from $150 to $500, depending on the type of mounting and area classification. A tape-board-type level indicator costs about $500, and a continuous float level transmitter costs from $2000 to $5000.

Inaccuracy

The repeatability of actuation is about 1 in. (25 mm) for level switches and for guidetube and float-type level indicators. The error in most level transmitters is about 1% of full scale. Consult the manufacturer for guidance if higher precision is required.


See also the lists of suppliers in Sections 3.7 and 3.18. Almeg Controls (www.almegcontrols.com/small_h_d.htm) (switch) American Electronic Components (www.aecsensors.com) (tilt switch) Applied Geomechanics Inc. (www.geomechanics.com) (tilt switch) BinMaster (www.binmaster.com) (tilt switch) Bristol Babcock (www.bristolbabcock.com) (transmitter, pneumatic) Custom Control Sensors Inc. (www.ccsdualsnap.com) Custom Switches Inc. (www.custom-switches.com) (switch) Delta Controls Corp. (www.deltacnt.com) (switch) Dryden Aqua (www.drydenaqua.com/Float_switch/float) (tilt switches) Gauging Systems Inc. (www.gaugingsystemsinc.com) Gems Sensors Inc. (www.gemssensors.com) (switch) Harwil Corp. (www.harwil.com) (switch) Hersey Measurement Co. (www.aaliant.com) (transmitter and switch) Innovative Components (www.liquidlevel.com/products.htm) (switches and indicators) ISE Magtech (www.isemagtech.com) (transmitter and switch) Jo-Bell Products, Div. of Celtech Corp. (switch) Kelco Engineering (www.pumpshop.au.com/kelco.htm) (tilt switch) Krohne America Inc. (www.krohneamerica.com) (transmitter and switch) Magnetrol International (www.magnetrol.com) (switch, transmitter) Mercoid Div. of Dwyer Instruments Inc. (www.mercoid.com) (switch) Montech Systems Inc. (www.monitortech.com) (tilt switch)





3.8 Float Level Devices

475

MTS Systems Corp. (www.levelplus.com) (transmitters) National Magnetic Sensors Inc. (www.nationalmagnetic.com) (float switches, reed switch, tape level indicators) Norriseal (www.norriseal.com) (switch, transmitter, controller) Nova Controls (www.novacontrols.com) (tilt switch) Sherman Instruments (www.johnsherman.com/level) Solartron Mobrey Ltd. (www.solartronmobrey.com) SOR Inc. (www.sorinc.com) (switch) TAV Engineering (www.tavengineering.com) (switches) Techmark Corp. (www.tm-techmark.com) (tilt switch) Thomas’ Products Ltd. (www.thomasprod.com) (switch) W. E. Anderson Div. of Dwyer Instruments (www.dwyer-inst.com) (switch)

In addition to this section, other sections in this handbook discuss float-type devices. These include the displacer-type level devices discussed in Section 3.7, the magnetic level gauge covered in Section 3.10, and the various tape level devices discussed in Section 3.18. In addition, the float level control valves, which are covered in Volume 2 of this handbook, also utilize floats for their operation. In addition, floats are combined with capacitance and ultrasonic level detectors. INTRODUCTION The immersion of a theoretically perfect float would have a perfectly constant immersion depth. The immersion of real floats is not perfectly constant because of the work they have to do to move the mechanical components of the instrument. The level devices discussed in this section can be functionally grouped into point-sensing level switches and continuous or proportional level devices such as level indicators, controllers, or transmitters. They can be directly connected or can include isolation or seal devices; if they have seals, one can also group them according to the method of sealing used. The older ball-float gauges depended on stuffing boxes for sealing. The newer designs use magnetic coupling or other, more reliable means to separate the pressurized process from the readout. The float in float level switches and indicators follows the liquid level or the interface level between liquids of differing specific gravities. The commonly understood difference between the float and the displacer is that floats are allowed to move if level or density changes, whereas the displacers do not move—or move very little. This means that the float devices follow the changes in level more closely, while the displacer follows the changes in density more accurately. Standard floats for top mounting are spherical or cylindrical; they are spherical or oblong for side-mounted designs. Spherical floats are available from 3 to 7 in. (76 to 178 mm) in diameter. The smaller-diameter floats are used in higherdensity materials, and the larger ones are used in liquid–liquid interface detection applications and for measuring the level of lower-density materials. Another reason for using larger floats is to provide additional force for operating the associated instrument.


FLOAT LEVEL SWITCHES Figure 3.8a illustrates a simple device that can be used for level indication or switch actuating in atmospheric tanks or sumps. The ball float has a rod connected to it, and the rod motion is used either to indicate level on a gauge board or to trip any combination of high- and low-level switches for alarm or automatic starting and stopping of lift pumps. For some pump applications, the float and rod may be furnished with the pump and can be factory assembled by the pump manufacturer. Figure 3.8b illustrates the operation of the magnetically coupled and spring-loaded float switch. The magnet and switch are assembled on a swinging arm that rotates on a pivot. As the liquid level rises, the float rises with it and lifts the attractor. When the attractor reaches its high position, it pulls in the magnet on the swing arm against the nonmagnetic enclosing tube, thus tilting the mercury switch to the right. The spring is selected so as to provide snap-action switching. When the switch tilts, the middle-to-left leg circuit is broken, and the middle-to-right leg circuit is closed. The magnetic coupling across the enclosing tube isolates the process liquid from the switching elements of the instrument. The swing-arm design can also be arranged to operate one or more dry contact switches. When two switches are

Gauge Board Support and Guide

Limit Switch 5 4 3 2 1

Sump

FIG. 3.8a Float-operated point switch trip for sump level.

Signal for Alarm or Pump Start

476

Level Measurement

Switching Element Sealing Tube

Pivot

Attraction Sleeve

Bias Spring Magnet Float

Level Differential Rising Level Falling Level

FIG. 3.8b Operation of the float-operated level switch with mercury switching element. (Courtesy of SOR Inc.)

FIG. 3.8d Magnetically coupled float level switch.

the float pivot point to prevent the buildup of solids or caking at the pivot. If the boot is used, it lowers the pressure and temperature ratings of the switch. The previous discussion on direct versus external cage mounting also applies to this switch design. Reed-Switch Designs

Guard Cage

FIG. 3.8c Float switches mounted inside external chamber or on the top or side nozzles of tanks.

employed, they can be spaced approximately 1 to 3 in. (25 to 76 mm) apart. One disadvantage is that making changes to the setpoint or the span of this and other float-operated switch designs can be complicated and time consuming. Figure 3.8c shows some of the external top- and sidemounted float switch designs. The switch operating point is the vessel nozzle elevation. These direct-connected installations usually require that the vessel be depressurized and drained for maintenance. If this would cause a plant shutdown or interfere with normal operations, an external cage (chamber) design can be used. The external chamber can be isolated from the vessel by shutoff valves, which make maintenance possible without draining the vessel—if local safety practices permit. If ambient temperatures can cause the process fluid to freeze or gel, the external chamber should be heat traced. Another type of single-point magnetically coupled float switch is illustrated in Figure 3.8d. As the process level rises, the float rises with it, and the float magnet pivots down, which attracts the switch magnet. As it moves up, it changes the state of the electric or pneumatic switch. This design is available with a plastic boot that covers the float magnet and


Some of the earlier reed-switch-based level switches consisted of several moving parts, which contributed to their potential for maintenance problems caused by corrosion and plugging. Figure 3.8e illustrates one such design, containing a spring-opposed shuttle-and-cam mechanism. Here, the level lifts the float, which in turn rotates a cam counterclockwise, thereby moving the permanent magnet into close proximity with the reed switch. As a result, the switch changes its electrical state.

Spring

Permanent Magnet Cam

Float

Reed Switch Low Level

High Level

FIG. 3.8e Magnetically coupled level switch, which is triggered by a floatoperated cam.


Guide Tube

Float

Permanent Magnet

Ball Float

Reed Switch

Reed Switch " NPT (12 mm)

FIG. 3.8f Reed switch-type level sensor which is operated by a plastic float. (Courtesy of Thomas Products Ltd.) Electrical Connections Mounting, Threaded Liquid Flow Openings Reed Switch Float Magnet

FIG. 3.8g Reed switch-type level float.

The main concerns with most such designs are ruggedness, simplicity, and the ability to overcome the resistance of dirt or material buildup that resists float motion. For these reasons, the more successful designs tend to have only one moving component and a relatively large float on a relatively long float arm to maximize the angular momentum generated. An inexpensive float switch used in vending machine applications is illustrated in Figure 3.8f. This unit is available in polypropylene and in polysulfone materials and can be used from −40 to 225°F (−40 to 107°C) and up to 150 PSIG (10.3 bars, 1.03 MPa) operating pressures. In another all-plastic level switch, also used in vending machines, the float and reed switch assembly is contained within a housing chamber that is inserted vertically into the process (Figure 3.8g). The process fluid enters this chamber and lifts the float (which contains the magnet) until it actuates the hermetically sealed reed switch within the stem. It is always wise to inquire into the electrical ratings of a reed switch. Devices that are mounted in the field with long


477

Annular Magnet

FIG. 3.8h Ball float-operated and magnetically coupled reed switch.

cable runs (possibly in a cable tray) may be exposed to induced currents and high voltages resulting from electrical storms. In such cases, voltage surge protection is needed, because such voltage surges can destroy small switches. Float and Guide Tube Designs The float and guide tube design shown in Figure 3.8h is also a magnetically coupled float switch. In its simplest configuration, a reed switch is positioned inside a sealed, nonmagnetic guide tube at the point where the rising or falling liquid level is to actuate the switch. The float, which contains an annular magnet, rises or falls with the liquid level while it is guided by the tube. In the design shown in Figure 3.8h, the switch is normally open and closes when the float reaches the elevation of the switch. The switch closure can be used to sound an alarm or to initiate logic functions such as starting a pump. The switch will reopen when the float falls, or if it rises and the switch cannot detect whether it was closed by a falling or a rising float. As a solution for answering this question, a mechanical stop can be placed on the guide tube to prevent the float from rising above the elevation of the switch. With the stop installed, the switch will stay closed whenever the level is at or above the switch elevation and will open only when the process level falls. Several mechanically stopped floats can be placed along the same tube to provide multiple switching at different levels. Figure 3.8i shows this configuration. Two floats and two switches are used to control a sump pump, which is started on high level and stopped on low. When the level in the sump is below the elevation of the level switch low (LSL), both LSL and level switch high (LSH) are open, and the relay coil R is deenergized. When the level rises above LSL, it closes that switch. If it continues to rise, when it also closes LSH, relay coil R is energized, and the relay contacts R1 and R2 are both closed. Contact R1 is a “hold-in” contact around LSH, whereas the R2 closure starts the pump motor. When the liquid level

478

Level Measurement

To Ladder

110V

Neutral LSH

LSL

R

LSH

To Ladder

R1

R2

LSL

Mechanical Stops

To Pump Motor

Flow

FIG. 3.8i Pump-down control system for operating a single pump.

Conveyor Belt

FIG. 3.8j Tilt switches for liquid (top) and solids service (bottom).

drops below LSH, it will open, but the pump will remain energized, because the still-closed R1 keeps relay R energized. When the level falls below LSL, the opening of that switch will de-energize the relay coil, which in turn sets both R1 and R2 to open. At this point, the pump shuts down, and the system is at the initially described state. By adding more floats and switches to the assembly, more complex control schemes can be devised.

Float

Tilt Switches Figure 3.8j illustrates two commercially available tilt switch designs: the upper one for liquid and the lower one for solids services. In the liquid level detector switch, a mercury switch is enclosed in a plastic casing and is freely suspended from a cable at the desired level. When the liquid reaches the plastic casing, it tilts it, causing the switch to close (or open) an electric circuit, which in turn actuates a warning device or starts a pump. This device is used in sumps and ponds and is limited to atmospheric pressure and ambient temperature applications. If it is installed outdoors above grade, it should be sheltered by a windscreen. The second tilt switch design, shown in the lower half of the figure, is used primarily to detect the presence or absence of solids on a conveyor belt. As long as there is material on the conveyor belt, the switch is tilted up. If the feed to the belt is lost, the switch rotates to its vertical position, and the switch contacts change state, thereby enabling alarm actuation, belt shutdown, or other automated actions to be initiated. Other designs are available that use a steel ball. As it changes position, the ball also operates a switch as the housing is tilted. This design is available for corrosive or pressurized services.


Gauge Board

FIG. 3.8k Tape gauge operated by float and provided with a gauge board for level indication.

Float-Operated Continuous Indicators The simplest and most direct method of float level measurement is illustrated in Figure 3.8k. The unit shown is basically a tape gauge (detailed in Section 3.18). A tape is connected to a float at one end and to a counterweight at the other, thereby keeping the tape under constant tension. The float


moves the counterweight up and down in front of a direct reading gauge board, thereby indicating the level in the tank. The installation shown is typically used on water storage tanks, although it can be used in any processes that are left open to the atmosphere. Float and tape materials are selected to suit corrosion requirements. The instrument range is a function of tape length used, which can be up to 100 ft (30 m). For float devices used in closed tanks, refer to Section 3.18.

Scale

479

Indicator

Pressurized Tank Applications When float-operated devices are used on pressurized tanks, they require a seal between the process and the indicator. In most cases, the float motion is transferred to the indicator by magnetic coupling, but other designs also exist. Where no external power source is available, the float level indicator shown in Figure 3.8l can be used to obtain remote indication. When the level of the tank rises, the float moves up, and the float arm rotates around the float arm pivot, thus pulling the push rod down. This, in turn, operates the stroke lever. The stroke lever turns on its pivot, which is sealed inside the bellows. This motion is carried by linkage to the tank side bellows (A and B). As bellows A is compressed, some of its filling liquid is transferred into receiver bellows C. At the same time, as bellows B is expanded, it draws some of the filling liquid from receiver bellows D. The net result of this action of the differential bellows is to change the position of a remote level indicator, which requires no power supply. Receiver Bellows C

D

Capillary Tubing

Tank Side Bellows A

B

Bellows Seal

Stroke Lever

Push Rod Float Arm

Float

FIG. 3.8l Float gauge provided with liquid-filled bellows and capillary tubing for remote level indication.


Float

FIG. 3.8m Rotameter indicator adopted as a means of displaying level.

These liquid-filled float gauges are available with temperature compensation to eliminate the effect of ambient temperature variations on the measurement. For installations in which the process vapors are not corrosive to copper and the operating pressure is below 15 PSIG (0.1 MPa), the bellows seal shown in the sketch can be eliminated. If the seal is used, it allows the unit to be exposed to 200 PSIG (1.4 MPa) operating pressures while isolating the process vapors from most of the working parts. The bellows seal and other parts that are wetted by the process can be made out of  stainless steel, Monel aluminum, synthetic rubber, or other materials. The capillary tubing between tank side and receiver bellows can be up to 250 ft (76 m) in length. Figure 3.8m shows another simple level indicator. The float can be placed directly in the tank or mounted in an external chamber that is provided with isolating valves. The maximum level range of the device shown, which uses a standard variablearea flowmeter indicator, is 15 in. (381 mm). This limits its use to narrow-span applications. Another variation, used on home heating oil tanks, is designed with both a pivoted and a rotating arm to allow use of the short indicator scale. Materials of construction are either steel or stainless steel, with glass indicating tubes used on nonhazardous, low-pressure services only. In other installations, metal tubes are used with magnetic coupling to drive the scale indicator. This design can be used for operating pressures to 1200 PSIG (8 MPa) and temperatures to 800°F (427°C). The rotameter design can be used for local indication and can be equipped with high and low alarm switches and electronic or pneumatic transmitters. Magnetically Coupled Indicators Continuous level indication can be obtained by placing many closely spaced switches inside a guide tube and detecting which ones are being held closed by the magnet as it is lifted with the ball float (Figure 3.8n). In this design, a voltage divider

480

Level Measurement

Ammeter To Dial Solar Receiver

Black Red

Follower Magnet and Gauging Rod

R0 R1

R0

R2

R0

R3

R0

R4

R0

R5

R0

R6

R0

White

R0

FIG. 3.8n Magnetically coupled level indicator, which detects the status of float operated switches inside a guide tube.

network is configured by connecting resistors R1, R2,…, Rn in series across the power supply. Since the resistors have equal value, the voltage drop across each one will be equal. If the magnet in the ball float closes the third switch from the top, for example, the voltage between R2 and R3 will be impressed on R0, causing a very small current to flow through the ammeter. As the level continues to fall and closes the fourth switch, R4 is inserted into the circuit, and the voltage to R0 will be reduced. The ammeter scale may display the level in percentage or in engineering units. Resistor R0 is specified at a high value so that current flow through the meter is small in comparison with that in the divider. The switches can be placed with a minimum spacing down to 0.25 in. (6 mm), which is the resolution for this type of indicator. As can be expected, replacement of faulty switches is difficult, so the guide tube normally includes spare (redundant) switches installed at each point. Maximum length available for this design is 10 ft (3 m) if the assembly is furnished with the metal guide tube. However, it is possible to obtain a flexible, plastic-jacketed assembly that can be furnished in longer coils. The plastic jacketed assemblies are field installed into stainless steel or other nonmagnetic guide tubes. Solar-powered versions of this design are also available. Figure 3.8o shows how the magnetically coupled float and guide tube design can be used for manual gauging or to operate a direct-reading dial. The sketch to the left shows the same magnet-carrying float and guide tube as described earlier. But here, instead of switch closures, the magnet is used to reposition a gauging rod. As the level moves the float up, the rod projects farther out of the tank, thus indicating the


Float and Magnet To Follower Magnet

Guide Tube

FIG. 3.8o Level display can be at ground level, if float operates a magnetic follower inside a guide tube.

rising level. When the gauge is not in use, the rod can be pushed down to the bottom of the well, and the opening can be capped. This feature makes the gauge attractive for use on transportation tankers. Given the right geometry, this system can also be installed from the bottom so that the reading can be made from under the tank. As shown on the right side of the sketch in Figure 3.8o, this design is also available with a directreading tape-driven indicator. This unit eliminates the need for the operator to climb on top of the tank. The major drawback of this design is that, if the pipe that contains the tape is ruptured, a spill can result unless some other form of spill protection is provided. Another magnetically coupled float design is shown in Figure 3.8p. This gauge has been installed on a fair number of stationary and mobile liquefied petroleum gas and anhydrous ammonia tanks. The vertical motion of the float is converted

Gauge Dial

Mounting Flange

Center Shaft

Gears and Bearing Supports

Support Tube Magnet

Float

FIG. 3.8p Magnetically coupled float-type level indicator.


Dial Magnet

Magnet Shaft

481

level where the buoyancy and its weight are in balance. One version is calibrated to detect the composition of automobile radiator cooling liquid. Another version is calibrated to measure the state of charge of a lead-acid battery. Others can be calibrated for the specific gravity ranges of various liquids and mixtures.

CONCLUSION Float Guiding Slot

FIG. 3.8q Magnetically coupled float gauge with float inside a slotted guide tube.

to rotary motion by the use of gears and pivots. The center shaft, in turn, positions a permanent magnet behind the dial pointer. There are no shaft connections or holes for any purpose through the gauge head, because it is the magnetic coupling that positions the pointer. This magnetic coupling guarantees a leakproof installation and is applicable up to an operating pressure of 1000 PSIG (6.9 MPa). The readout dial can be calibrated for horizontal, vertical, and spherical tanks and, for good readout visibility, the dial diameter can be up to 8 in. (203 mm). Figure 3.8p also illustrates some of the options for locating the dial on the side or top of tanks. The standard material of construction for the wetted parts is stainless steel. As with most float-operated indicator designs, remote-readout configurations are also available. The common automobile gasoline tank gauge is simply a float on an arm. This arm turns a variable resistor, and the change in resistance changes the current that flows through the meter on the dashboard. Figure 3.8q illustrates a level indicator utilizing a slotted tube combined with a magnetically coupled float level indicator. This design can be considered for installations where the shape or the internals of the vessel prevent the use of the rotarytype float gauge. As the float rises, enclosed in a tube that has a guide slot, its angular position is determined by the slot. The rotation of the float turns the shaft of the magnet and the magnet repositions the pointer on the dial, which indicates the level. This device is used primarily on liquid transporters. It has a limited pressure rating and cannot be used in corrosive services.

DENSITY MEASUREMENT Hydrometers are discussed in detail in Section 6.3 but, also being float operated, they are mentioned here. In a hydrometer (Figure 6.3a), a calibrated float with a small-diameter upper section is placed in the process fluid and sinks to a


Float-operated devices are widely used in utility services and as alarm switches. In some applications, the size of the float may limit its use; in other designs, the associated support and guide tubes cannot be accommodated. Other limitations include that the immersed magnets will accumulate pipe scale and other ferrous metal particles from the process. These buildups will eventually interfere with the proper operation of such switches. In all areas of instrumentation, it is safe to assume that trash, dirt, and solids will always accumulate in unwanted places. Many of the float-operated designs have moving parts exposed to the process and therefore should not be used in dirty or plugging services. The ball float switch designs are relatively inexpensive and reasonably reliable. For these reasons, they are used (selectively) in industrial processes and routinely in a wide array of applications outside of the industrial area. Applications of float level instruments include the protection of pumps and compressors by detecting the presence or the absence of liquids in pipes and vessels. Pneumatically operated level switches avoid the problems associated with the availability of electrical power or the issues of electrical hazards. Independent level alarm and interlock switches are used to back up other level measurements for overflow protection and other safety purposes. The large number of float level device suppliers is an indication of the wide variety of available designs, and the user is advised to search for the most current sources and designs.

Bibliography API Recommended Practice 550, Manual on Installation of Refinery Instruments and Control Systems, Part I, Process Instrumentation and Control, Section 2, Level, American Petroleum Institute, Washington, DC. Berto, F. J., The Accuracy of Oil Measurement Using Tank Gaging, ISA, Research Triangle Park, NC, 88–1561. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 7, 2001. Flashlight, sunlight power tank level indicator, Design News, July 9, 1979. Holzhausen, G. R., Using tilt to measure displacement, Meas. Control, September 1993. Level measurement and control, Meas. Control, April 1991. Luyts, J. and Marcelo, L. D., Fieldbus HTG System Measuring On-line Concentration, ISA, Research Triangle Park, NC, 1998. Piccone, R. P., Combining technologies to compute tank inventory, Sensors, October 1988.

3.9

Laser Level Sensors

LT Laser

J. FEHRENBACH

(1995)

M. VUJICIC

To Receiver

(2003)

Flow Sheet Symbol

Applications

Granular solids, nontransparent liquids, slurries, and molten metals

Design Pressure

Unlimited; transmitter outside of tank

Design Temperature

Ambient at transmitter −40 to +50°C (–40 to 122°F) standard; up to 150°C (302°F) with cooling options; process temperatures up to 1500°C (2732°F)


Aluminum, alloys, or plastic enclosures

Inaccuracy

±5 mm (±0.2 in)

Range

0.2 to 250 m (0.5 to 800 ft)

Price

$4000 to $6000 USD

Suppliers

K-Tek Corp. (www.ktekcorp.com) Optech Inc. (www.optech.on.ca) Thermo MeasureTech (www.thermomt.com)

BACKGROUND In recent years, laser-based level sensors have become accepted in industrial process control as a means of obtaining reliable noncontact level measurements in difficult applications. Process conditions or installation methods often preclude the use of more conventional noncontact gauges such as ultrasonic, radar, and nuclear devices, creating both a technical and economic advantage for laser technology. Three types of laser technology are commonly used in the process control market today: pulsed, continuous-wave (frequency-modulated), and triangulation. It is important to be aware of the fundamental operating differences between the technologies and the suitability of each for specific types of applications. Pulsed Laser Sensors (Time of Flight) Pulsed technology is time of flight based; it measures the time for individual laser pulses to travel from the transmitter of the instrument to the target (where the beam is reflected) 482 © 2003 by Béla Lipták

and back to the receiver of the instrument. The distance is then calculated using the time and the speed of light outlined by the formula, D = (C × T )/2

3.9(1)

where D = distance to the target (m) 8 C = speed of light (3 × 10 m/sec) T = time of flight of laser pulse (sec) In general, pulsed lasers are used for most industrial level-monitoring applications because they offer better range and penetration characteristics (through dust and steam) without sacrificing accuracy and repeatability. Pulsed laser level sensors have improved greatly over the past few years with respect to dust penetration and pulse discrimination, allowing them to be used effectively in severe solids and vapor applications that were precluded in the past. Higherquality pulsed laser sensors allow the user to select the return

3.9 Laser Level Sensors

Laser Device Transmitter S E

483

Laser

Receiver CCD-Array

Lens L

t = f(L) 2L C C = Velocity of Light = 3 × 106 m/s

t=

FIG. 3.9a Level can be detected by measuring the time of reflection. Light −9 travels at a speed of about 0.3 m per nanosecond (10 sec).

pulse of interest (first or last) so as to optimize performance in solids applications (see Figure 3.9a).

FIG. 3.9b Triangular laser measurement is used in cameras and in robotics.

PULSED-LASER LEVEL SENSOR Installation

Frequency-Modulated (Continuous-Wave) Sensors Continuous-wave laser technology directs a continuous laser beam at the target. When it hits the target, this beam is phase shifted and subsequently returned to the receiver. The outgoing and incoming beams are compared to determine the degree of phase shift and the distance is calculated on the basis of frequency, wavelength, and phase shift. Continuous-wave lasers are better suited for short-range, extremely high-accuracy, clean-air applications usually found in laboratory-type settings. In terms of industrial applications, they are suitable for more benign applications, such as positioning, and are not usually used for level monitoring. Triangulation Measurement Sensor Triangulation measurement is done at an angle, with a sharply focused laser beam directed toward an object. The luminous spot on the target surface is optically projected to a CCD device, which maps out the intensity distribution of the reflected light. The electronics determine the illuminated CCD zone and the angle to the luminous spot. Because the angle change is reduced with increasing range, the accuracy also drops, making this type of measurement suitable only for a relatively short range. Primary applications for this type of measurement are object positioning and robotics. Because of these limitations, this method is not used for industrial level sensing (see Figure 3.9b).


Pulsed-laser level gauges have many inherent properties that help overcome the limitations of other gauges and allow them to be the alternative of choice for many applications. First and foremost, the laser level gauge is a nonintrusive instrument that provides a flexible mounting arrangement. Process vessel flanges can be closed off with the use of a processrated sightglass (from vacuum rated vessels to 600 class), and the laser level gauge is mounted external to the process. The laser can effectively fire through a quality sightglass to the target with no interruptions to the production process for commissioning, adjustment, or troubleshooting. The fact that the process and gauge do not come in contact makes this approach ideal for applications such as corrosives and acids that would damage an ultrasonic antenna or a radar horn (Figure 3.9c). Common practice is to use spray rings or air curtains to help keep the sightglass or sensor window clean to optimize performance when warranted by application conditions. Clean air or process-compatible gas or fluid can be continuously or intermittently injected to keep the glass surfaces clean and laser beam transmittance losses to a minimum. Higher-quality laser sensors can provide a return signal-intensity output that can be used to control the cleaning process or for general troubleshooting and optimization. Vapor-Space Effects Different gases and varying pressures and temperatures in the vapor space required for passage of the laser pulse do

484

Level Measurement

Inlet Feed Laser Sensor Sightglass Spray Ring Standard Flange 100% Level

0% Level

Fortunately, the change in the index of refraction is minute, so the effect is negligible for industrial sensing applications, and no adjustment is needed for the laser sensor. It is therefore effective for a variety of applications. Table 3.9d shows values for the index of refraction (N ) for some common gases/vapors. The laser is ideal for vacuum applications because, unlike ultrasonic sensors, the laser does not need a medium for propagation. The laser beam itself, produced by a laser diode, is a very narrowly focused beam with minimal divergence (typically 0.3°). This narrow beam is ideal for applications where geometry is tricky, such as tall, narrow silos and vessels with baffles/agitators or support beams acting as obstructions. The fact that the beam is narrow means that a particular spot can be identified as the target, and no baseline echoes or elaborate subroutines are needed to eliminate obstructions as part of the calibration and commissioning procedure. Types of Targets and Angle of Repose

To Process

FIG. 3.9c Installation of the laser level transmitter.

affect the speed of light and thus the level reading. The relationship is defined by Light velocity in a gas/vapor (C ) = Co / N where 8 Co = light velocity in a vacuum (3 × 10 m/sec) N = index of refraction for gas/vapor

3.9(2)

Laser level gauges use extremely short wavelengths of the electromagnetic spectrum. Combined with the narrow beam, this makes them very accurate, even at oblique angles to the target. This translates into the ability to measure to solid targets with varying angles of repose consistently and reliably. Regardless of cone-up or cone-down conditions, the laser will provide accurate measurements to the specific point of contact without the reflection and beam bounce inherent in both ultrasonic and radar gauges. This unique property also makes installation and commissioning much easier and more flexible, as off-center and side mountings become possible.

TABLE 3.9d Index of Light Refraction for a Variety of Gases and Vapors at 0°C and 1 Atmosphere Dry air*

1.000293246

Krypton—Kr

1.0004287

Acetone—(CH3)2CO Acetylene—C2H2

1.0010867

Methane—CH4

1.0004433

1.0006007

Neon—Ne

1.0000672

Ethanol—C2H5OH

1.000870

Ozone—O3

1.000520

Ammonia—NH3

1.000379

Mercury vapor—Hg

1.000940

Argon—Ar

1.00028314

Oxygen—O2

1.00027227

Benzol—C6H6

1.001759

Sulfur dioxide—SO2

1.0006640

Bromine—Br2

1.0011849

Carbon bisulfide—CS2

1.001477

Chlorine—Cl2

1.0007840

Hydrogen sulfide—H2S

1.00065068

Hydrochloric gas—HCl

1.00045127

Nitrogen—N2

1.00029914

Deuterium—D2

1.00013758

Nitrogen dioxide—NO2

1.0005087

Fluorine—F2

1.000206

Carbon tetrachloride—CCl4

1.0017819

Helium—He

1.00003495

Trichlorethylene—C2HCl3

1.001705

Iodine—I2

1.002200

Steam—H2O

1.0002527

Carbon dioxide—CO2

1.0004506

Hydrogen—H2

1.00013937

Carbon monoxide—CO

1.0003360

Xenon—Xe

1.0007055

Xylene—C8H10

1.002135

*Air is defined as 78.08% N2; 20.95% O2; 0.93% Ar; 0.03% CO2; 0.01% H2 and rare gases.


3.9 Laser Level Sensors

One of the most flexible and useful properties of the laser is its ability to measure to a variety of different types of materials. Both solids and liquids can be effectively monitored with no adjustments required for target temperature, material dielectric, density, or other physical properties. Lasers have been used effectively for materials ranging from cryogenic temperatures to molten metals, with equally positive results. Laser Eye Safety In the United States, laser eye safety is regulated by the Food and Drug Administration (FDA). In Europe, it is regulated by the IEC. Both organizations have standards that govern eye safety (CFR1040 and IEC-60825) and these are generally accepted worldwide. The organizations are in the process of harmonizing their standards. Instrument engineers should try to specify and use Class 1 or 2 eye-safe lasers, if possible, to prevent inadvertent exposure to potentially dangerous laser sources. Additionally, engineers should ensure that the specified laser instruments indicate compliance with FDA and/or IEC guidelines for labeling, safety, manufacturing requirements, and power regulation. Class 1 and 2 laser level sensors can be used freely in industrial applications and require no specific registration or training requirements. Laser Power and Ignition Safety Investigations have shown that a particle or surface heated by radiation from a laser can ignite flammable gas or vapors through localized temperature increases or plasma formation. Industrial agency approvals (CE, CSA, FM, UL, and so on) require that laser manufacturers prove that their lasers are powered at below the minimum ignition energy (MIE) of common materials to eliminate ignition hazard. When specifying laser level sensors, engineers should ensure that the instruments have an agency rating for hazardous locations and/or the CE mark. These markings will indicate compliance with industrial requirements and minimize any potential problems in service. Consulting with the laser manufacturer for nonstandard applications is recommended.


485

SUMMARY As laser technology develops, there exists a huge potential for laser level gauges to capture more of the noncontact levelmonitoring market. In North America, there is now an annual shift away from contact gauges to noncontact gauges of 1 to 3%. This shift is the result of the development of better and newer technologies that continue to optimize processes from both a technical and economic standpoint. With their inherent advantages, laser level gauges are particularly well positioned to benefit greatly from this market shift and become even more common as a multipurpose industrial device.

Bibliography Bonfig, K. W. u. a., Technische Füllstandmessung and Grenzstandkontrolle, Expert Verlag, 1990. Fehrenback, J., Instrument Engineers Handbook, 3rd ed., 1994. Food and Drug Administration, Center for Devices and Radiological Health, CFR 1040 and Laser Notice 50, July 2001. International Electrotechnical Commission, Safety of Laser Products, IEC60825, November 2001. Kompa, G., Extended time sampling for accurate optical pulse reflection measurement in level control, IEEC Trans. Instrum. Meas., 33, 1984. Kompa, G., High resolution pulsed laser radar for contour mapping, in Proc. 28th Midwest Symp. Circuits and Sys., August 1984, Louisville, KY, 527–530. Kompa, G., Laser-Entfernungsmesser Hoher Genauigkeit für den Industriellen Einsatz, in Proc. Laser 79 Opto-Electronics, July 1979, Münich, 587–593. Krüger, G., Berührungslose Giesspiegelmessung mit LASER, VDE-Berichte, 509, 1984. Lewis, R. A. and Jonston, A. R., A scanning laser rangefinder for a robotic vehicle, in Proc. 5th International Joint Conference on Artificial Intelligence, Cambridge, SPIE Press, Bellingham, WA, 1977. Loughlin, C., Distance sensing; making light work, Sensor Review, 9, 3, 1989. Manhart, S. and Dyrna, P., Self-Calibration Low Power Laser Rangefinder for Space Applications, SPIE 663, SPIE, Bellingham, WA, 115–121, 1986. Schwarte, R., Performance capabilities of laser ranging sensors, in Proc. ESA Workshop on Space Laser Applications and Technology, Les Diablerets, ESA SP 202, May 1984, 61–67. Schwarte, R., Baumgarten, V., Bundschuh, B., Dänel, R., Graf, W., Hartmann, K., Heuten, F. and Loffeld, G., Implementation of an advanced laser ranging concept, IAF, October 1985. Ussyshkin, V. R., Laser Safety Ignition, Research Results for AccuPulse PRO Level Monitor, September 2001.

3.10

Level Gauges, Including Magnetic LG

D. S. KAYSER

(1982)

B. G. LIPTÁK

(1969, 1995)

D. S. NYCE

(2003)

Flow Sheet Symbol


Types

(A) tubular, (B) circular or bull’s-eye, (C) transparent, (D) reflex, (E) magnetic, (F) magnetostrictive transducer, and (G) gauge glass.

Design Pressure

Maximum pressures for tubular glass gauges range from 1 to 20 bars (0.1 to 2.0 MPa), but caution is needed for applications above 1 bar; circular gauges, 200 bars, with bull’s-eye units up to 690 bars; transparent flat glass gauges, 200 bars; reflex gauges, 250 bars; armored gauges, 400 bars; and magnetic level gauges, 300 bars. Maximum operating pressure depends on maximum temperature. (Note: 1 bar = 0.1 MPa = 14.5 PSI.)

Design Temperature

Maximum operating temperature for tubular glass gauges is 80 to 200°C (176 to 400°F); circular gauges, 250°C (480°F); transparent flat glass gauges, 300°C (570°F); reflex gauges, 300°C (570°F); armored gauges, 350°C (660°F). Magnetic level gauges can handle process temperatures ranging from −196 to 400°C (−320 to 750°F). For all types with gauge glass, maximum operating temperature depends on maximum pressure and gauge length.


Transparent elements are of prestressed borosilicate glass, aluminosilicate glass, or    quartz; other components may be carbon steel, stainless steel, K-monel , Hastelloy C ,   PVDF (Kynar ), PTFE (Teflon ), PVC, and others for special applications.

Range

Tubular glass gauges are usually limited to 1 m maximum indicated length. The length per section for a flat glass transparent gauge (multiple sections can be arranged for a larger combined length) ranges from 100 to 500 mm; reflex, 100 to 500 mm; armored gauge, 250 to 500 mm; magnetic, 250 mm to 3 m.

Inaccuracy

Gauge error is usually a factor of the resolution of graduations on the scale, if provided. The readability can be degraded by foaming, boiling, and other effects that alter density or reduce visibility. The resolution of magnetic flag indicators is controlled by the spacing of the individual flags, typically about 6 mm. Magnetic gauges fitted with magnetostrictive sensor-based indicators can resolve the float position to 0.1 mm or less.

Cost

Excluding pipe stands, fittings, accessories, and so on, the cost per 300 mm (≈1 ft) of various gauge types is as follows: tubular glass gauge, $100; transparent glass gauge, $400; reflex, $300; armored, $450; magnetic, $1500. A circular (welding pad) gauge costs about $100 to $500.

Suppliers

Barksdale GmbH (www.barksdale.de) (E) Clark Reliance Corp./Jerguson (www.clark-reliance.com) (A, B, C, D, E, F) Essex Brass Co. (www.essexbrass.com) (A) Gems Sensors Inc. (www.gemssensors.com) (E) Jogler (www.jogler.com) (AEF) John C. Ernst Co. (www.johnernst.com) (A, B, E) Klinger (www.klinger.com.au) (A, C, D, E) Kobold Instruments Inc. (www.koboldusa.com) (E, F) K-Tek Corp. (www.ktekcorp.com) (E, F) L.J. Star Inc. (www.ljstar.com) (B)

3.10 Level Gauges, Including Magnetic

487

Magnetrol International (www.magnetrol.com) (E) MTS Systems Corporation (www.levelplus.com) (F) Oil-Rite Corp. (www.oilrite.com) (A, B, G) Penberthy (www.penberthy-online.com) (A, B, C, D, E, F) Schott Auer GmbH (www.schott.com) (G) Spectraglass Ltd. (www.spectraglass.co.uk) (G) Wooil Industries (www.wooilind.com) (C, D, G)

INTRODUCTION Many types of devices are called level gauges, including various transducers, sensors, transmitters, and indicators. The detection methods depend on properties such as head pressure, differential head pressure, conductivity, and capacitance, and they employ optical, microwave, RF, radiation, magnetostriction, ultrasonic, and other principles. This section of the handbook, however, deals with liquid level gauges that operate by connecting a measuring chamber in parallel to the vessel being monitored so as to directly indicate the level visually or by the use of a magnetic indicator or transducer. The configurations of glass-type gauges comprise tubular glass, flat glass-transparent (circular and long-form), reflex, armored, and magnetic gauges. The level in the gauge follows the level in the vessel, as illustrated in Figure 3.10a. The process level is directly visible in a glass gauge. Conversely, in a magnetic level gauge, the measuring chamber can be opaque and is typically constructed fully of metal. A float within the measuring chamber of a magnetic gauge floats on the liquid and incorporates a permanent magnet. The float magnet drives an indicator or transducer through

magnetic coupling as shown in Figure 3.10b. Local indication from the magnetic level gauge can be implemented through the use of a magnetic follower, magnet operated flags, or with magnetic switches or a magnetostrictive transducer. Level gauges are often used in process vessels as well as storage vessels. In level gauges providing direct visual indication, the accuracy is limited by the readability of the meniscus of the liquid in the indicating area of the gauge. The readability can be reduced when monitoring foaming, boiling, or inhomogeneous liquids. On visible glass lengths in excess of ≈300 mm, multiple gauge covers are stacked along the length of the measuring chamber. If a single chamber is used with multiple covers as described, there are small spaces over which the view of the level is blocked by the top and bottom edges of the individual gauge covers. In magnetic gauges with a magnetostrictive transducer,

Vessel

High Level

Long-Form Gauge Circular Level Gauges

Low Level

FIG. 3.10a A level gauge (on right) is mounted in parallel to the vessel, extending over the range of level to be indicated. A circular gauge is mounted at the level of interest (left).


FIG. 3.10b A magnetic gauge is shown mounted to a process vessel. A float within the measuring chamber floats on the liquid. A magnet within the float operates the magnetic flags to indicate the level. (With permission from Clarke-Reliance.)

488

Level Measurement

Stud

Glass

Nut Cover Cushion Gasket Body

Shield (Optional)

FIG. 3.10d Construction of a circular gauge. Cutaway view (left) and side view (right).

CIRCULAR TRANSPARENT GAUGE

FIG. 3.10c The traditional tubular glass level gauge is not recommended for most industrial applications.

improved accuracy is possible because of the high resolution that can be provided by a magnetostrictive transducer, and continuous indication is available through the entire range.

TUBULAR GLASS GAUGE A simple tubular glass gauge (Figure 3.10c) comprises a transparent glass tube, seals, end blocks, and guard rods to protect the glass. It is positioned parallel to the vessel along the elevation over which the level is to be indicated and mounted with suitable fittings to retain the pressure as well as to seal the ends of the sight tube. This construction, however, is not well suited for use with dangerous process fluids. An important consideration in gauge selection is that of maintaining the safety of personnel and associated equipment. The difficulty is evident when the process vessel, storage tank, pipeline, and so on are used with high-temperature, high-pressure, corrosive, or other dangerous fluids or steam. If a sight glass tube sustains a fracture of the glass or a leak at the seals, the dangerous fluid can escape and create the potential for a hazardous condition. The single tube design is not recommended for use with toxic materials, pressures above 1 bar (0.1 MPa or 14.5 PSIG), or temperatures above 100°C. Some tubular glass gauge designs have extra protection against breakage, an improvement on the simple guard rods of standard designs. The protection elements may include an outer tube that contains the fluid if the inner tube is fractured, sheet metal protectors, and a wire glass protector surrounding the gauge glass. Even considering these improvements, it is recommended that flat glass, reflex, armored, and magnetic gauges be specified when the process includes hazardous materials, high temperatures, or high pressures.


Circular level gauges (some small versions are also called bull’s-eye gauges) are mounted next to the monitored vessel at the elevation of interest for level indication, as shown in Figure 3.10a. The circular gauge has a limited range and is used when the level variation that must be indicated is small. This type of gauge is typically constructed as shown in Figure 3.10d. The body, glass, and gasket are wetted by the process. A shield can be added to protect the glass from the process fluid if required. Since circular gauges are used to indicate liquid level over a relatively short range, two or more can be used to show high and low levels, and other gauges can be used at selected locations in between. They can also be used to show fluid in motion and fluid color or contamination.

TRANSPARENT GAUGE (LONG FORM) When monitoring levels over a wider range than is practical with a circular gauge, a long-form flat glass transparent gauge can be used. Typically, if a flat glass gauge is not specified as being circular, it is assumed to be long-form, and the term long-form is not added. The construction of a transparent gauge is shown in Figure 3.10e. The measuring chamber retains the fluid and accepts the glass and covers, which are secured with bolts. One or more vision slots are machined into the chamber to allow viewing of the level. Tie bars are areas that may be left (i.e., not machined out) in the vision slot so as to provide higher strength. Gasket material is compressed between the chamber and the glass to prevent direct contact and to provide a seal. Cushions are placed between the glass and the covers to prevent direct glass-to-metal contact. The chamber, glass, and gaskets are wetted by the process. Shields can be optionally installed to protect the glass from the process fluid and/or from the ambient environment (e.g., windblown sand). The gauge is called transparent because there are glass panels both in front of and behind the measuring chamber, with respect to the observer who is reading the gauge. Accordingly, the liquid level indication is illuminated by light coming from behind. This configuration is useful to allow


489

U - Bolt A

Glass Cover

Total Visible Length

Chamber Housing

Nut Cushion Visible Glass =A+B Bolt

B

Chamber Bore

Gasket

Chamber Bore

Shield (Optional) Gasket Chamber Housing

Glass

Tie Bar (Optional)

Cushion Cover

FIG. 3.10e Transparent level gauge construction, front view (left), and cutaway view (right). The front view shows two covers attached to one chamber, and shows the distinction between visible length and visible glass.

visual inspection of the liquid for color and presence of particles, for example, in addition to indicating the level. One drawback is that it requires the availability of some light from behind. Lighting panels (called illuminators) are available for providing this illumination when it is not available from the ambient lighting. Because of the process fluid viewing capability, transparent gauges can be used in applications that require the indication of the interface between two liquids. As indicated in Figure 3.10e, the total visible length is the distance between the uppermost and lowermost possible reading positions. There may be some unreadable areas included, resulting from tie bars and top and bottom edges of covers. The visible glass is the sum of the lengths of unobstructed glass available for viewing.

REFLEX GAUGE Greater visibility of the transition between the liquid level and the gas or vapor above it is provided by the reflex-type glass gauge. The construction, shown in Figure 3.10f, incorporates a glass element in front of the measuring chamber housing, but not behind it. Illumination is only from the front, and level visibility is aided through the use of grooves in the glass where it contacts the liquid being monitored. The gauge glass is smooth on the outside (the side toward the observer). The grooved surface is called the prismatic area. The face of each groove is at a right angle to the faces of adjacent grooves. When the prismatic area is not in contact with a liquid, the groove faces reflect the incoming light. The light is reflected due to the large difference in the index of refraction between the glass and the gas or vapor above the liquid. The incoming light strikes one groove face, is reflected across to the adjacent groove


Nut

FIG. 3.10f Construction of a reflex glass level gauge.

Front View Top View (Gas) Gas or Vapor

Prismatic Area Glass Reflex Gauge Glass

Ray of Light Reflected

Prismatic Area

Top View (Liquid) Liquid

Ray of Light Refracted

FIG. 3.10g Light is directed back toward the observer in the gas or vapor space above the liquid in a reflex gauge (top, left). Where liquid touches the glass (bottom, left), light is not reflected. The figure on the right shows a gauge glass with grooves forming the prismatic area.

face, and then is reflected back to the observer in front of the gauge (see Figure 3.10g). When the prismatic area is in contact with a liquid, there is little difference in the index of refraction between the glass and the liquid, so the light passes through the prismatic area at a slight angle (it is refracted) without being reflected back toward the observer. The combination of these two effects provides an improved visibility of the liquid and an increase in the distance over which the level indication can be viewed. When viewing a transparent or semi-transparent liquid in a reflex gauge, the liquid will appear black or dark because

490

Level Measurement

the light is not reflected back to the viewer from the prism area. The column above the liquid will appear silvery, because the light is reflected back from the glass-to-gas interface at the prism area (e.g., there will be a dark column below the indicated level and a silvery column above the indicated level). An opaque liquid will show its color in the liquid column area (e.g., milk will show a white column below the indicated level, with a silvery column above the indicated level). Because of the reflection of light back to the viewer, reflex gauges are well suited to viewing with a flashlight in low-light areas. Because of the front illumination and the prismatic glass, reflex gauges are recommended for use only with clean, clear, process fluids and when there is no liquid–liquid interface to be viewed. Because of its irregular shape, shields cannot be installed against the prismatic glass, so corrosive fluid service is limited to those with little or no effect on the glass. Armored Gauges Since a gauge of standard construction provides for clamping of the glass between the cover and the measuring chamber, the edges of the glass may be exposed. In an armored gauge, a lip is formed into the cover, and it wraps around and covers the sides of the gauge glass. So, between the chamber and the cover, the edge of the gauge glass is completely covered. The purpose is to protect the glass from possible damage resulting from, for example, accidental impact from a wrench. The front of the cover is also made thick enough to prevent contact with the glass if the same wrench were to hit the front of the cover. Armored gauges are also often made for extreme duty (e.g., vibration, high temperature, or high pressure). They utilize thicker glass and measuring chamber walls and higherperformance gaskets, and they may have shorter spans of visible glass separated by tie bars in the measuring chamber. They are often used in pressure vessels and where no electrical power is available to operate electronic level indicators. Many manufacturers supply only armored-style gauges. Gauge Glass Materials Borosilicate glass is the most common type of gauge glass. It has good chemical resistance up to about 300°C. In addition, transparent shields can be mounted between the glass and the process fluid to protect the glass from corrosive media in transparent level gauges (e.g., the shields can be made of mica or PCTFE). Borosilicate glass is usually tempered to improve its resistance to thermal shock. The tempering process comprises the heating of the glass to the glassy transition point, followed by rapid cooling. This is done during manufacture of the glass to induce mechanical compressive stress in the outer layer, resulting in an increase in the ultimate tensile strength (UTS). The tensile strength is increased, because the compressive stresses must be offset by tensile stresses before


cracks can propagate (cracking is required before breakage of the material). Aluminosilicate glass has a lower coefficient of linear thermal expansion (also called t/c) than borosilicate glass, but it can be used at higher process temperatures of up to −6 425°C. While borosilicate glass has a t/c of 4.5 × 10 /°C, −6 aluminosilicate glass has a coefficient of only 2.0 × 10 /°C. −6 For comparison, the t/c of carbon steel is 23 × 10 /°C, and −6 type 316 stainless steel (SS) is 17 × 10 /°C. Quartz glass has the highest temperature rating (up to −6 530°C) and also has the lowest t/c (0.5 × 10 /°C). It can be used in transparent gauges but is not available in reflex gauges, because it would be difficult to form the grooves. The materials of construction other than quartz determine the maximum operating temperature of a quartz gauge assembly. Extremely flat gasket and cushion seating surfaces are required when using quartz glass to avoid bending and tor sional or point stress. Belleville spring washers are used to control the clamping forces between the glass and its seating area. Fused natural quartz is made by melting naturally occurring crystalline silica. Synthetic fused quartz (or synthetically fused silica) is made by melting man-made silicon dioxide. Design Features It is typical for all glass level gauges to have a reduction in pressure rating as the temperature increases. Manufacturers list representative charts for general guidance in selecting the type of construction and components to meet the application requirements. Once the complete gauge model has been specified, including chamber model, glass type, shields, and so forth, it is important to verify with the manufacturer the resulting temperature vs. pressure curve for that particular configuration. Transparent and reflex gauges are normally limited to a maximum single-gauge length of up to 1.5 m. If a greater total visible length is required, multiple gauges will have to be installed. Overlapping is suggested to allow viewing of levels that would otherwise be blocked by the top and bottom edges of the covers. To reduce problems from boiling or foaming process fluids, a larger-diameter measuring chamber (approximately 50 mm) can be used as shown in the top row of gauge cross sections illustrated in Figure 3.10h. In cryogenic service, the view of the level in standard glass gauges may be blocked by ice accumulation. A nonfrosting lens can be added that consists of a T-shaped plastic lens held against the glass by the cover and extending out in front of the glass. The plastic lens has a relatively low thermal conductivity and is not as cold on the front viewing surface as the face of the gauge glass, which reduces frosting. Heating or cooling of the gauge may be required to keep it at approximately the same temperature as the process fluid in the vessel. The appropriate device may be added externally or mounted in the bore of the chamber.


491

ACCESSORIES 2" Over 6-7 ft

LG Large Chamber Reflex Gauge

LG Multiple Gauge Installation

Non-Frosting Extension

Circular

Reflex Nipple to Valve

Light LG Liquid Heavy Liquid Interface Installation

Standard Welding Pad Gauges

Transparent Externally Heated (or Cooled) Gauges

Stuffing Box Heating Tube

Internally Heated (or Cooled) Gauges

FIG. 3.10h Special level gauge designs and cross sections.

GAUGING INACCURACIES Gauge error usually depends on the resolution of graduations on the scale, if provided. Readability can be degraded by foaming, boiling, and other effects that alter density or reduce visibility. The indicated level can be affected by a difference between the temperature of the gauge and the vessel. If the gauge is cooler than the vessel, the liquid within the gauge may be more dense and therefore indicate low. This can be eliminated by using a heater on the gauge and controlling the gauge temperature to match the vessel temperature. Conversely, the gauge temperature can be reduced, if needed, by adding a cooler in a similar manner. Gauge heating/cooling configurations are shown in Figure 3.10h. In a gauge with a magnetic follower, some error is due to the strong magnetic attraction required between the float magnet and the follower magnet, which induces friction on the walls between them. The resolution of magnetic flag indicators is controlled by the spacing of the individual indicator flags, typically about 6 mm. Magnetostrictive transducer-based indicators can resolve the float position to 0.1 mm or less and do not have a detectable magnetic attraction between the transducer and the float magnet. Foaming or boiling can show false high readings because of an apparent lower specific gravity (SG) of the fluid in the gauge compared to the fluid in the vessel (if the vessel fluid has less foaming or boiling). This can be reduced by using a gauge with a larger chamber diameter, as was shown in Figure 3.10h. Internal condensation can cause readings to become blurred. To prevent this, a heater can be added inside or outside the chamber.


Gauge support brackets are used with long gauges, where the distance between supports is about 1.5 m or longer, to reduce the load placed on nipples and valves. Steel brackets are welded directly to the measuring chamber and then bolted to a support plate on the vessel. On a reflex gauge, the bracket can be welded to the back of the chamber (between sections to avoid interference with cover bolts). On a transparent gauge, suitable welding locations include the right or left side of the chamber, between sections. The brackets are normally welded on at the factory according to customer specifications. Valves and fittings are also needed, of course, for the installation of level gauges. Fittings can be any of several types, including spherical union, flanged, socket weld, solid shank, or NPT union. Spherical unions allow correction of misalignment but may be difficult to seal if they are connected more than once. Valves are normally factory assembled to the gauge. The fitting of unions to the vessel side of the gauge valve will facilitate the initial connection of the gauge. The gauge can be removed without depressurizing the vessel if valves are included on both the gauge and vessel sides of the unions. Gauge valves are normally supplied with ball checks to minimize venting to the atmosphere in case of breakage of a section of the gauge. Teflon coating is a common requirement in the chemical processing industry. The Teflon can be applied by electrostatic spray or by wet spray. It often includes a base coat of PTFE followed by a top coat of PFA, with a minimum total thickness of 0.25 mm. Maximum service temperatures are up to 230°C. The user should specify whether the Teflon coating is required only for the wetted parts or for the complete gauge. An illuminator can be used with transparent gauges to increase visibility in lower-light areas. A lamp is used with a diffuser to illuminate the entire visible glass area. Explosion-proof versions are available for use in hazardous areas. Scales can be engraved or etched in the desired units and are attached to the gauge cover. If the installed scale does not accurately indicate the actual level in the vessel, a calibration can be undertaken. The calibration data can be used to derive a conversion factor between the indicated reading and the actual vessel volume.

APPLICATION-SPECIFIC REQUIREMENTS Low-temperature applications require specific attention to gauge materials. Some materials, particularly ferritic steels, change from their normally tough property to a more brittle behavior with a decrease in temperature. The transition temperatures and toughness vary with different materials. When materials are used under conditions in which brittle behavior

Level Measurement

INSTALLATION The level gauge(s) should be installed so that the full operating range can be observed, including levels found during warm-up and cool-down, and as indicated by switches, displacers, and so on. For multiple gauges, overlap their elevations so the complete range of levels can be seen without the view being blocked by the top and bottom edges of the covers. As shown in Figure 3.10i, this can be done by installing single section gauges on a standpipe. Using a standpipe also can reduce the number of vessel connections and increase the flexibility in the mounting and positioning of the gauges. To obtain the lowest error when viewing a liquid–liquid interface, add a central connection to the vessel in the area of the lighter liquid phase, in addition to the top and bottom connections. This will prevent errors due to an excessive column height of the light phase when the top level of the


1 " or 2" Nozzle Nozzle Size Block Valve

Vessel (Top View)

" Vent (When Used)

1" or Larger Nozzles and Block Valves

" SCH 160 Nipples

may occur, there is a potential that minor internal flaws, which would not present a problem if the material were sufficiently tough, may propagate to failure (see ASTM A352 supple1 ments for more information). Boiler and steam/water gauges must be reliable to ensure the safety of personnel and equipment. High pressures and temperatures require attention to the specification and application of all parts and materials of the gauge, valves, and fittings. Saturated steam tables should be consulted to ensure that the operating pressure and temperature for the application are within the range shown in the tables provided by the gauge manufacturer. Standard glass level gauges may not be suitable for use with steam. Check with the manufacturer of the gauge to find out which models are specifically suited for use with steam at the vessel design pressure/temperature ranges. Shields (usually mica) should be used to protect against etching of the glass (the etching is also called frosting, but this is different from frosting due to accumulation of ice in cryogenic systems). Transparent gauges are used because shields cannot be applied to the prismatic surface of reflex glass. For more information on boiler equipment installation requirements, consult the ASME Boiler and Pressure Vessel Code, as well as codes that are specific to the locality in which the equipment will be installed. Hydrofluoric acid applications require special materials to prevent attack of the gauge glass by the acid. Hydrogen fluoride (HF) is an extremely corrosive gas that becomes hydrofluoric acid in solution with water. Like many acids, it vigorously attacks most metals but also attacks glass and other silica-containing materials. Some fluorinated polymers are designed to protect against hydrofluoric acid, including   ® PCTFE (Kel-F ), ETFE (Tefzel ), and PECTFE (Halar ).  Metals used with HF can be Hastelloy C up to 90°C or K Monel .

Vessel

492

Vent " Or " Gate Valve

Tee or Automatic Gage Cock " Automatic Gage Cocks or " Blocks Valves and Tees Gage Column

Overlap

Drain " Gate Valve Standpipe 1 " or 2" Pipe

Gage Column Elbow to Eliminate Pockets

All Nipples " SCH 160

Standpipe Drain " Gate Valve

Alternate

A Single Gage Column

" or

Gage Column

Drain " Gate Valve B Two or More Gage Columns

FIG. 3.10i A gauge column (left) may incorporate several gauge glass and covers on one chamber. A standpipe (right) allows more flexibility.

light phase is below the upper connection (bottom-left area of Figure 3.10h). An expansion loop can be added to gauges that will operate over a wide temperature range to allow for differences in the coefficient of thermal expansion between the gauge and the vessel (see Figure 3.10j). An expansion loop is also particularly useful in cases where the gauge can be valved off and allowed to cool, while the vessel is still hot.

MAGNETIC LEVEL GAUGES Because the magnetic level gauge does not require direct viewing of the level (i.e., there is no need for glass), the measuring chamber can be opaque, and welded metal construction is normally used. This substantially widens the operating temperature range and increases the ruggedness as compared to chambers using gauge glass. The wide temperature range is possible, because the measuring chamber can have approximately the same coefficient of thermal expansion as the vessel, and there is no glass (which would have a lower t/c) to interface with the metal chamber.


493

FIG. 3.10k This magnetic follower can be mounted alongside the measuring chamber of a magnetic level gauge to indicate the float position. (With permission from Clarke-Reliance.)

Magnetic Followers and Indicators

FIG. 3.10j Installing an expansion loop improves reliability over a wide temperature range.

To allow the magnetic system to operate properly, the chamber metal must be of a nonmagnetic type—usually an austenitic stainless steel such as AISI 316 SS. Because the float in the chamber of a magnetic gauge incorporates a permanent magnet, any reliable method of detecting the location of a magnetic field can be used to show the location of the float and, thus, the level. Three common methods of indicating the float position are the magnetic follower, magnet-operated flags, and magnetostrictive linear position transducers (also called magnetostrictive transducers). Magnetically operated reed switches may require maintenance and lack the long-term reliability needed for many industrial applications. Therefore, they are not presented here. One limitation in magnetic gauges is that the float must have a comparatively thick wall to operate at higher pressures. Due to the related weight, it is more difficult to measure process fluids of low SG (e.g., less than 0.45) when the pressure is high (e.g., over 200 bars/20 MPa).


Magnetic followers and magnetic flag indicators are mounted alongside the measuring chamber of a magnetic gauge. A magnetic follower is shown in Figure 3.10k. A magnetic flag indicator was shown in Figure 3.10b. A permanent magnet inside of the float (within the measuring chamber) lines up with the follower or adjacent flag as the float slides up or down the measuring chamber. In the case of the follower, the position of the follower is read against a scale. Friction of the follower against the adjacent wall is substantial because of the magnetic attraction between the magnet in the follower and the magnet in the float. This causes an additional limitation on resolution, which is observable as a discontinuous motion of the follower in response to level changes. With magnet-operated flags, the magnet in each flag causes it to flip one way as the float passes while moving upward and flip the other way as the float passes while moving downward. The flags can be colored red and white, for example, to show a red line from the bottom of the indicator up to the indicated level and a white line above that. Red, orange, and yellow are often used, because they are highly visible. Fluorescent colors are good, because they are more easily read when using a flashlight. Some limitations include an occasional flag flipping the wrong way and coarse resolution of ≈6 mm. An incorrect flag position is sometimes caused by the float bobbing up and down or by rapid movement of the level. The flag position can be corrected by passing a magnet along it externally. If the actual float position is in question, it can be found by using a compass and watching the needle movement.

494

Level Measurement

The resolution of the flag indicators is limited by the spacing of the magnetic flags and somewhat by the friction of the float against the measuring chamber wall resulting from the attraction between the magnets in the flags and the magnet in the float.

Waveguide

Outer Pipe

Magnetostrictive Transducers Magnetostrictive transducers and associated indicators represent an improvement in the state of the art in level gauges. They can increase the effectiveness of installed magnetic gauges having followers or flags, or they can stand alone in new installations for accurate level gauging without the associated limitations of using glass gauges. The original looppowered magnetostrictive transducer was invented and devel2 oped in 1990 by the author. Magnetostrictive transducers are now available from several manufacturers. A magnetostrictive transducer is a linear device that can detect the location of a magnetic field (provided by the position magnet) that moves along in parallel to the sensing probe of the transducer (see Figure 3.10l). When used with a magnetic level gauge, the magnet within the float acts as the position magnet. A magnetostrictive transducer can be added to a follower or flag-type gauge, but it exhibits the highest performance when used with a simple nonmagnetic metal tube (such as type 316 stainless steel) that contains a float/ magnet assembly. The primary component inside of a magnetostrictive transducer is called the waveguide (see Figure 3.10m). When a current pulse (called the interrogation pulse) is applied to the waveguide circuit, a torsional force is induced into the waveguide at the location of the position magnet. This phenomenon is called the Wiedemann effect. Also, a timer circuit is initiated when the interrogation pulse is applied. The torsional force causes formation of a strain wave that travels at the “speed of sound” in the waveguide material (≈2850 m/sec). When the strain wave arrives at the pickup, it is detected, and the timer is stopped. The elapsed time measured by the timer indicates the location of the position magnet. The time is converted into the desired output signal, usually 4 to 20 mA or a digital communication protocol. Because the diameter of the waveguide is small (less than 0.5 mm), there is no detectable attraction between it and the magnet within the float, and hence there is no float friction due to magnetic attraction against the chamber wall. This is one reason why the resolution and accuracy can be better

FIG. 3.10l A magnetostrictive transducer measures the location of a magnet, called the position magnet. (Courtesy MTS Systems Corporation.)


Float (Moves as Level Changes)

Float Magnet

Magnetic Field from Float Magnet Waveguide Twist (at Intersection of Magnetic Fields) Magnetic Field from Interrogation Pulse

FIG. 3.10m A torsional strain wave (waveguide twist) travels along the waveguide of a magnetostrictive transducer. The wave starts at the location of the float magnet, and its travel time to one end of the waveguide is measured.

than indicated by a follower or flag indicator. The other reason is that a magnetostrictive transducer can resolve as small a position difference as a few microns, but indication resolution is limited by the float system to about 0.1 mm. Figure 3.10n shows the application of a magnetostrictive transducer to a magnetic level gauge. The transducer output can be indicated locally or remotely, on a stand-alone indicator, or as an input to a computer or data acquisition system or other device.

REMOTE READING GAUGES Several schemes for remote reading gauges have been employed for high-pressure and high-temperature applications of up to 3000 PSIG (20 MPa) and 371°C. Remote reading gauges are often needed to meet the requirements of steam boiler codes. Differential pressure, conductivity probe, and circular gauge glass configurations have been used in the past. However, since the mid-1990s, the remote reading capability of a magnetic gauge with magnetostrictive transducer has become a preferred method to consider when power is available for operation of the transducer.


495

head between the level in the chamber and the level in the overflow tube, the latter being equal to the level in the steam drum. The legs are connected to a manometer assembly filled with a colored liquid that is insoluble in water. A transparent gauge is used as a portion of the manometer and located so that the colored liquid in the gauge will rise as the level in the steam drum rises. For accurate level indication, it is important to keep both legs of the system at the same temperature. Also, when specifying this type of gauge, it is important to specify the pressure of the steam drum, because the water in the steam drum may contain bubbles of steam and thus have a significantly lower specific gravity than that in the gauge. This arrangement is useful in applications where power is not available. Conductivity FIG. 3.10n A magnetostrictive transducer is shown mounted to the right side of a magnetic level gauge with a flag indicator. (With permission from Clarke-Reliance.)

Steam Drum Condensing Chamber Overflow Tube

A conductivity probe configuration was sometimes used in boiler systems and composed of a metal chamber with a series of conductivity probes. As the water rose in the chamber, successive probes energized their associated switches and relays. This arrangement had the safety advantage of not requiring a gauge glass, but its complexity was a disadvantage, and its long-term reliability was not as high as that of magnetostrictive transducers now available. It also required power for operation of the conductivity switches, relays, and indicator lights. The electrodes of the conductivity probes were subject to corrosion and routine replacement, and resolution was limited by the spacing of the conductivity sensing electrodes. This method is not recommended for new installations. Circular Gauges Circular gauges and closed-circuit television monitoring have also been used for remote indication; but again, the complexity, coarse resolution, and long-term reliability have no advantage over the currently preferred magnetic gauge with magnetostrictive transducer for remote reading. These methods also required power for operation of the light sources and video monitor.

Colored Liquid

Remote Level Gauge

FIG. 3.10o A differential pressure type of remote reading level gauge is useful when there is no power available.

Differential Pressure A differential-pressure remote reading gauge is shown in Figure 3.10o. The maximum level is fixed by the elevation of the overflow tube in the condensing chamber. The actual level is obtained by measuring the difference in hydraulic


Magnetostrictive Transducers Magnetostrictive transducers used with magnetic gauges now constitute a preferred method of remote indication in applications where power is available for operation of the transmitter (see Figure 3.10n). The measuring chamber is normally a metal tube containing a float that rides on the liquid level so that the center of the float is even with the level. If there is a liquid–liquid interface, a float can be weighted to sink through the lighter liquid and float so that its center is at the interface level. It is also possible to use two floats, the first one riding at a liquid–liquid interface (indicating the level of the heavier phase) and the second one floating on and indicating the level of the lighter phase.


Although power must be provided to operate the magnetostrictive transducer, the advantages of this arrangement over other remote indicator types include increased safety (because no gauge glass is needed), a lack of parts that need recalibration or replacement, and obtainable resolution of better than 1 mm. A wide range of electronic remote indicators are available. The electronic output signal can be directly read by current-loop-compatible indicators and other equipment or fed to data acquisition systems or computers.

496

provide easy means for remote indication and communication interface to data acquisition systems and computers.

References 1. 2.

Penberthy application report 2780, November 2000. U.S. patent number 5,070,485, 1991.

Bibliography CONCLUSION The various types of glass level gauges provide a means to view the liquid level as well as the color and possible contamination of the liquid. For accurate level indication, foaming and boiling must be minimized. The process fluid in the gauge should be at the same temperature (and density) as that in the vessel. Tubular glass gauges are not recommended for hazardous applications such as toxic liquids, high temperature, and high pressure. Magnetic level gauges can provide a higher safety margin when direct viewing of the liquid is not required. Magnetic level gauges with magnetostrictive transmitters yield the lowest errors, where power is available, at a higher cost. They also


API Recommended Practice 550, Manual on Installation of Refinery Instruments and Control Systems, Part I, Process Instrumentation and Control, Section 2, Level, American Petroleum Institute, Washington, DC. Cantieri, W. F., Water Gauge Accuracy, Technical Publication of the Diamond Power Specialty Co., Malvern, PA. Cho, C. H., Measurement and Control of Liquid Level, ISA, Research Triangle Park, NC, 1982. Green, C. R., Tank sight glasses, Chemical Eng., September 25, 1978. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Level measurement and control, Meas. Control, April 1991. Nyce, D. S., Tank gauging advances, Fuel Technol. Manage., January 1997. Nyce, D. S., Magnetostrictive linear position sensors, Fluid Power J., April 1999. Penberthy Application Report 2000, No. 2781, August 1998; and No. 2783.1, August 1997.

3.11

Microwave Level Switches J. L. DANIEWICZ

(1995)

W. H. BOYES

(2003)

Reflection To On-Off LS Receiver MW Beam Breaker LX

LS MW

To On-Off Receiver

Flow Sheet Symbol

Applications

Microwave level switches can be used on liquids and small-particle-size solids if they are noncoating and have a dielectric greater than 10; they are suited for interface as well. The reflection switch is also suited for interface. The reflection switch is suited for denser material; the beam breaker switch is not limited by density or particle size, but it is limited by vessel diameter.

Design Pressure

Up to 400 PSIG (275 kPa) for reflection type; depends on vessel window for beambreaker type

Design Temperature

Up to 400°F (200°C) for reflection type; depends on vessel window for beam-breaker type (up to 600°F [300°C])

Wetted Materials of Construction

TFE, polyphenylene sulfide, or ceramic as standard for reflection type; firebrick, plastic, unleaded glass, quartz, or mica window for beam-breaker type

Range

Immediate proximity for reflection type; up to 100 ft (30 m) horizontal transmission in air for beam-breaker type

Inaccuracy

0.125 in. (3 mm) for reflection type; ±0.25 to 0.5 in. (6 to 13 mm) for beam-breaker type

Cost

$400 to $1200 for beam-breaker type (cost may vary according to cost of transmission window and material)


Reflection type: WADECO Ltd. (www.wadeco.co.jp) Beam-breaker type: Delavan Process Instrumentation Div. of L & J Technologies (www.landj.com) (X-band) STI (www.sti.com) (K-band) Insatech (www.insatech.com) WADECO Ltd. (www.wadeco.co.jp)

This section discusses the use of side-mounted microwave level switches that are used in hard-to-handle solid, liquid–solids interface, and liquid–liquid interface applications. Top-mounted microwave detectors that use the radar technique for continuous level measurement on liquid applications are covered in Section 3.13. Microwave level switches were briefly popular in the early 1990s as a replacement for RF switches and gamma nuclear point level switches. However, due to instability and erratic operation, most manufacturers have ceased making them. There is no current U.S.-based manufacturer of reflection-type microwave point level switches, but the information is included for historical purposes. Typically, the beam-breaker switches are used in solids applications, although they can be used in liquids. Microwave level detectors use electromagnetic radio waves, typically in either the microwave X-band, around 10 GHz, or

the microwave K-band, around 24 GHz. Because these are shortrange measurements, emissions are at very low power levels 2 2 ranging from 0.6 to 32 mW/in. (0.1 to 5 mW/cm ). At these energy levels, health, safety, licensing, and product contamination concerns are minimal, and only solid-state diodes (instead of tubes) are needed to generate and detect the microwaves. Microwaves do not pass through metal walls, but they do pass through fiberglass or plastic tank walls and through windows of plastic, ceramic, or glass that are installed in metal vessel walls. As long as the window material has a relatively low dielectric constant (e.g., less than 4.0), and as long as its thickness is close to an even multiple of a half wavelength, attenuation is minimal. (Wavelength can be calculated by dividing the wavelength in a vacuum by the square root of the window material’s dielectric constant.) 497


498

Level Measurement

Power Input

Microwave Window Microwave Generator

Reflection Microwave Detector

Zero Adjust Reference Leg Output Signal to Electronics

Microwave Sensor

Microwave Window

Mesurement Leg

FIG. 3.11a Reflection detector with balanced bridge (patented).

This permits the use of thick windows to withstand heavy abrasion on solids service and to isolate the sensor from hazardous and toxic liquids on high-pressure service. Another advantage of the microwave technique is that the presence of dust, mist, or nonmetallic foam has negligible effect. The dielectric constant of these compositions is essentially the same air; therefore, their beam reflection and transmission characteristics are similar to those of free space. They appear to be somewhat product and installation sensitive, however, so it is recommended that the engineer be mindful of that fact when specifying one of these switches.

Dielectric Constant (Refs 1&2) ∞

Water

% Reflection 100

290

90

66

80

27

70

14

60

Solids with Water

8

50

Alumina

5

40

Phenolic Resin

3

30

Cereals Sand Paper, Rubber, Asphalt, Sugar

2

20

1.4

10

Alcohols

Gypsum

REFLECTION SWITCHES There is currently only one manufacturer of this type of switch. Several manufacturers use this technique, but it is used for flow/no-flow sensing instead of level sensing. The sensors are quite similar, however, and the information is furnished for completeness. This type of detector, shown in Fig. 3.11a, uses changes in the amplitude and/or phase of the reflected signal to determine material presence. Reflection is proportional to the dielectric constant of the material immediately next to the process window, as shown in Fig. 3.11b. Air, other gases, and foam have a low dielectric constant and return little or no signal. Materials with high dielectric constants, such as water, tend to return almost the entire signal. One type of reflection detector, shown in Fig. 3.11a, compares the return signal to a reference signal in a balanced bridge circuit to provide additional sensitivity. This helps the detector to recognize low-dielectric materials such as plastic pellets (dielectric of 1.1), and it is useful for liquid–liquid interface and liquid–solid interface detection on materials that have a little as 0.1 difference in dielectric constant.


Steel

Oils Hydrocarbons

1.1 1 Liquids

3 0

Air & Other Gases

Fly Ash & Cement Soap Powders Coal

Solids

FIG. 3.11b Microwave reflection characteristics.

On solid applications, the reflection technique is limited to detecting particles with diameters less than 0.25 in. (6 mm) for an X-band detector and to 0.1 in. (2.5 mm) for a K-band detector. Above this size, the particles begin to scatter the

3.11 Microwave Level Switches

Microwave Transmitter

Reflected Beam

Microwave Window

Transmitted Beam Absorbed Beam

499

TABLE 3.11d Maximum Coating That Can Be Tolerated by Microwave-Type Level Switches

Microwave Receiver

Microwave Frequency Band Microwave Window

Maximum Coating Thickness Conductive Coating (e.g., water-based)

Nonconductive Coating (e.g., hydrocarbon-based)

X

.125 in. (.3 cm)

1 in. (2.5 cm)

K

.06 in. (.16 cm)

.5 in (1.25 cm)

CONCLUSION FIG. 3.11c Beam-breaker detector.

beam and reduce the amount of signal that is reflected directly back to the detector.

BEAM-BREAKER SWITCH This type of a detector sends a beam across the measurement zone, as shown in Fig. 3.11c. When air or vapor is in this zone, a strong signal is received at the detector. When process material breaks the beam path, it reduces the signal received at the detector as a result of signal reflection and beam absorption in the material caused by molecular and ionic resonances. Beam-breaker detectors are very small antennas, so the beam’s included angle is fairly wide: about 26° for K-band and 50° for X-band. Thus, alignment is not critical. Although signal amplitude falls off rapidly in proportion to the square of the distance, separation distance can still be up to 100 ft (30 m), which is considerably greater than with ultrasonic or nuclear approaches. On solids services, the detector will not provide repeatable performance if it is expected to detect the top of the solids pile in the tank. Its switching action will be repeatable when the transmitter and receiver are both covered by solids at the time of switching.

Because of their expense, both types of level switches should be considered primarily for the more difficult applications or where a nonintrusive measurement provides major advantages. The beam-breaker type is the more expensive, as it requires two devices to be installed along with separate windows on metal vessels. However, because it can detect large solid particles, and because windows can be selected that are both thick and abrasion resistant, the beam-breaker technique is useful for detecting large and abrasive materials such as coal, minerals, wood chips, and vegetable pulp. It is also useful for detecting very light materials such as dry sawdust and powdered materials in fluidized beds, especially with the K-band design, which is more easily attenuated. Microwave level switches can be useful on granular solids and powders such as limestone, carbon black, and pelletized materials. In such applications, they have advantages in terms of abrasion and coating resistance as well as having no mechanical parts in the vessel that can be broken or pulled off. They should also be considered on difficult-to-handle liquids that are viscous, toxic, or hazardous because the detector is isolated from the vessel contents. The reflection-type microwave switch is also sensitive to liquid–liquid and liquid–solid interfaces. References 1. 2. 3.

COATING EFFECTS Microwave devices can tolerate more coating than ultrasonic or laser units but less than radiation-type level switches. The amount of coating that microwave switches can tolerate is a function of their frequency band (K or X) and of the nature of the coating (conductive or nonconductive). The maximum coating thickness that can be tolerated is given in Table 3.11d.


4.

Microwave Position and Level Control, Delavan Division of L & J Technologies, Hillside, IL, 1994. Boyles, W. H., Instrumentation Reference Book, 3rd ed., ButterworthHeinemann, Woburn, MA, 2002. Handbook of Chemistry and Physics, 63rd ed., CRC Press, Boca Raton, FL, 1982. Kaye, G. W. C., Table of Physical and Chemical Constants and Some Mathematical Functions, 15th ed., Longman, New York, 1986.

Bibliography Lang, H. et al., Smart transmitters using microwave pulses to measure level, ISA Technical Conference, Chicago, September l993. Ramo, S., Whinnery, J. R. and Van Duzer, T., Fields and Waves in Communication Electronics, John Wiley & Sons, New York, l965.

3.12

Optical Level Devices D. S. KAYSER

(1982)

B. G. LIPTÁK

LS

To Receiver

(1995, 2003)

Flow Sheet Symbol

Types

Visible or infrared (IR) light reflection. Noncontacting type, usually for solids. Laser type (discussed in Section 3.9) is used for solids and molten glass. Light transmission designs are usually for sludge level, while light refraction is for clean liquid level services. Fiber optic probes of various designs for many applications are discussed in Section 8.23.

Applications

Point sensor probes for liquid, sludge, or solids (some continuous detectors also available)

Design Pressure

Up to 150 PSIG (10.3 bars) with polypropylene, polysulfone, PVDF, or Teflon , and up to 500 PSIG (35 bars) with stainless-steel probes

Design Temperature

Between 150 and 200°F (66 to 93°C) with plastic probes and up to 260°F (126°C) with stainless-steel probes


Quartz reflectors with Viton A or Rulon seals, mounted in polypropylene, polysul fone, Teflon , polyvinyl fluoride, phenolic, aluminum, or stainless-steel probes

Housings

Can be integral with the probe or remote. Explosion-proof enclosures and intrinsically safe probes are both available. With remote electronics, the fiber optic cable can be from 50 to 250 ft (15 to 76 m) long.

Dimensions

Refraction probe lengths vary from 1 to 24 in. (25 to 600 mm), and the probe diameter is usually 0.5 to 1 in. (12 to 25 mm).

Costs

Light-refraction level switches cost from $150 to $500. Portable sludge level detectors cost $1000. A continuous transmitter for detecting sludge depth or sludge interface costs $4000 and up.


Automata Inc. (www.automata-inc.com) (noncontacting infrared) Bindicator (www.bindicator.com) (IR switch) BTG Inc. (infrared) Conax Buffalo Technologies (www.conaxbuffalo.com) (fiber optic) Enraf Inc. (www.enrafinc.com) (infrared) Gems Sensors Inc. (www.gemssensors.com) (fiber optics) Kinematics & Controls Corp. (www.kcontrols.com) (fiber optic switch) Markland Specialty Engineering Ltd. (www.sludgecontrols.com) (IR for sludge) OPW Div. of Dover Corp. (www.opw-fc.com) Zi-Tech Instrument Corp. (switch)





The operation of optical level detectors can be based on the reflection, transmission, or refraction of conventional, laser, or IR light. Because laser level sensors have already been discussed in Section 3.9, and fiber optic systems will be dealt with in more detail in Chapter 8, we will concentrate here on sensors that operate with visible and IR light. 500 © 2003 by Béla Lipták



LIGHT REFLECTION Reflected visible or infrared light beams can detect the level of liquids or solids, as illustrated in Figure 3.12a. A beam of light is aimed at the surface of the liquid or solids and is reflected back to a light-sensitive transistor, which is located in the same holder as the light source.

3.12 Optical Level Devices

Switch

Signal Conditioning

Light Source

Signal Conditioning

Light Sensor

Light Source

501

Switch

Light Sensor

1" NPT (25mm)

FIG. 3.12a Noncontacting optical level sensor. (Courtesy of Bindicator, Inc.)

Laser Source

Detector

Surface

control unit is calibrated to convert the displacement to an analog signal proportional to level. It can also be furnished with alarm or interlock contacts. For this unit to work properly, the surface of the process level must be clean and reflective. The span is limited to approximately 0.5 in. (12.5 mm). This noncontacting instrument can be used, for example, to monitor the thickness of molten glass as it is formed into sheets. LIGHT TRANSMISSION

FIG. 3.12b A laser beam reflected from the fluid surface is tracked by a servo1 controller detector.

As a point sensor (switch) on reflective, opaque liquids (such as milk) or on solids services, the transistor sensitivity can be adjusted to detect distances from 0.25 in. (6.3 mm) to 12 in. (300 mm). The sensor can be provided with several light-sensitive detectors to permit multipoint switch actuation. The allowable operating temperature range is from −40 to 150°F (−40 to 66°C). Because of its noncontacting design, the switch is suitable for use on corrosive, sticking, and coating process. The operation of the switch is adversely affected by changes in the reflectivity of the process. The reflection of laser light (see Figure 3.9a) is used in some specialized applications. The measurement of the thickness of molten glass is illustrated in Figure 3.12b. Here, a laser source is mounted on one side of the process at an angle of between 15 and 60° from horizontal. The detector is mounted on the other side of the process and at the same angle as the source. As surface level changes, the reflected beam is displaced as shown in the figure. The detector is arranged so that this displacement can be measured, and the


When light is passing through a fixed distance in a fluid, the intensity of light received at the detector can be used to measure the concentration of solids in the liquid. This same principle can also be used to measure sludge level or the interface between sludge and supernatant. The level sensor can be a point-sensing switch (Figure 3.12c) or a continuous sludge depth detector. In one design, the sensor and its electronics are portable, and the sensor is attached to a 30-ft (9-m) cable with which To Controls

FIG. 3.12c Optical sludge level detector.

502

Level Measurement

the operator lowers the sensor into the clarifier until the interface is detected between the low solids supernatant and the high solids settled sludge. Wetted-parts materials are available in nickel plated naval brass. The error in such measurement, assuming that the cable is marked and read correctly, is 1 in. (25 mm). In clarifiers, digesters, or air flotation thickeners, traveling scrapers are often provided at the bottom of the tank. In such installations, the sludge depth or the sludge-supernatant interface can be detected by pivoted probes that are provided with an optical gap. Inside that gap, pairs of IR light sources and detectors are stacked vertically on 3-in. (19-mm) centers. Figure 3.12d illustrates an installation with 64 such LED and detector pairs.

liquid level has dropped down. In that case, the switch will continue to indicate a high level. Therefore, the use of this switch is limited to clean, noncoating services.

N.O. Switch Closes When Probe Tilted or Removed Handrail Pivot

Liquid Probe Falls Back to Normal Position When Scraper Passes

LIGHT REFRACTION Infrared or visible light refracts when sent into a liquid through a submersed prism. Figure 3.12e illustrates a simple and inexpensive use of that principle for level detection. In this design, a light beam is directed along a cylindrical translucent rod that has a 45° bevel at its base. When no liquid is present at the tip, the beam is reflected back to a light-sensitive transistor. As the level rises and covers the tip of the probe, the index of refraction increases, and light escapes into the liquid. This reduces the amount of light received by the transistor and triggers the switching action. The unit is small, lightweight, and available in plastic, brass, aluminum, or stainless-steel construction. It is capable of detecting such small changes in level as 1/16 in. (1.6 mm). The probe has a pressure rating of 100 PSIG (0.69 MPa) and can operate at temperatures between 15 and 250°F (−90 and 121°C). The switch cannot be used in caking or coating liquids. Another limitation is the false level indication that can result if drops of liquid remain of the probe after the

Switch Switch

Light Sensor

Sludge Travelling Scraper

Gray PVC

Clear PVC Window

Infrared L.E.D. Emitter (64 Places)

PhotoTransistor Detector (64 Places)

Probe Gap 25 mm (1 inch) Probe Cross-Section

64 L.E.D/Phototransistor Pairs are Stacked Vertically on 19 mm ( Inch) Centers.

FIG. 3.12d Optical sludge level detector for wastewater treatment processes. (Courtesy of Markland Specialty Engineering Ltd.)

Signal Conditioning

Light Source

Travelling Scraper Tips Up Probe

Signal Conditioning

Switch

Light Source

Light Sensor

45° Refracted Beam Liquid Present

FIG. 3.12e Light-refraction-type level switch. (Courtesy of Bindicator Inc.)


No Liquid Present

Quartz Reflector

3.12 Optical Level Devices

Source Beam

Return Beam

Unclad Fiber

FIG. 3.12f Level detection using fiber optics.

Figure 3.12f shows how optical fibers can be used for liquid level detection. A light beam travels in the illustrated fiber and, as long as no process fluid contacts the fiber, the return beam will have the same intensity as the source beam. As the level rises, and liquid covers some of the fiber, the index of refraction increases, allowing some of the light to escape into the liquid. This reduces the strength of the return beam.

CONCLUSION The refraction-type optical level switches are not used in severe services, because of their pressure and temperature limitations, and because they are not suited for fouling and caking services. The reflection-type designs are used as noncontacting sensors in processes in which the vapor space is clear and the operating pressures are low. Their laser versions can provide high precision on narrow-span applications. The light-transmission-type optical level sensors have been used successfully in the wastewater treatment industry, both as level switches and as level transmitters.

Reference 1.

King, C. and Merchant, J., Using electro-optics for non-contact level sensing, InTech, May 1982.


503

Bibliography Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June-July 1997. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Cho, C. H., Measurement and Control of Liquid Level, ISA, Research Triangle Park, NC, 1982. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001. Fiber optics measure liquid level, Machine Design, June 28, 1973. Fiber optics monitor oil-tank levels, Machine Design, October 21, 1976. Gaige, R. A, The Installation of Digital Control System for a Glass Container Furnace, IEEE/IAS Meeting, Cleveland, OH, 1979. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Johnson, D., Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001. Johnson, D., Level sensing in hostile environments, Control Engineering, August 2001. Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Level sensing with fiber optics, Instrum. Control Syst., January 1977. Mariam, P. L., Measuring level in hostile and corrosive environments, InTech, April 1979. Mathe, A., Apparatus for the Precise Determination of the Level of Glass in a Furnace, Patent no. 1,706,857, August 10, 1926. Murphy, E. F., Laser refection technique for measurement of glass level in a tank, Paper #76–767,1976 Annual Conference, ISA, Research Triangle Park, NC. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Optic liquid level sensor guards oil tanks against overflow, Control Eng., November 1976. Optical switch uses reflection to monitor the level of liquid, Prod. Eng., September 23, 1968. Optical unit prevents liquid overfill, Can. Chemical Process., June 1976. Passe, J., Optical probe for the accurate measurement of liquid levels, Rev. Sci. Instrum., November 1965. Paul, B. O., Seventeen level sensing methods, Chemical Process., February 1999. PFA’s optical characteristics put to use in liquid level sensor, Modern Plastics, May 1979. Safety of Laser Products, IEC-60825, November 2001, International Electrotechnical Commission, Geneva. Sludge level control, Water Pollut. Control, September-October 1985. Taylor, D., An Economical Method of Glass Level Control Utilizing a Laser Beam Generator and a Solid State Detector, Paper #77–838, 1977 Annual Conference, ISA, Research Triangle Park, NC. Ussyshkin, V. R., Laser Safety Ignition, Research Results for AccuPulse PRO Level Monitor, September 2001. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

3.13

Radar, Noncontacting Level Sensors

LE Radar

J. L. DANIEWICZ

(1995)

W. H. BOYES

(2003)

LIT

To Receiver

Flow Sheet Symbol


Instrument Types

Frequency modulated carrier wave (FMCW); pulse

Applications

Noncontact measurement of liquids in tanks. The two basic types are tank farm gauges for inventory control or custody transfer and for process control in tanks and reactors. Interference can be caused by agitators and other metallic surfaces, thick foam, window coatings or condensation, and splashing during filling. Some solids applications depend on instrument type and frequency.

Antenna Designs

Parabolic reflectors, horns, extended horns, internal horns with process seals, dielectric rods

Design Pressure

Up to 1200 PSIG (800 kPa) for suspended antennas, to 300 PSIG (200 kPa) for isolated antennas

Design Temperature

Up to 450°F (230°C) for standard designs, higher temperatures to 750°F (400°C)

Wetted Materials of Construction

For suspended antennas, type 316 stainless steel, aluminum, or Hastelloy with TFL seal; for isolated antennas, a wide choice of plastics; for dielectric rods, generally TFE

Range

Up to 650 ft (200 m) for large antenna sizes of 8, 12, or 18 in. (20, 30, or 45 cm) in diameter; up to 50 ft (15 m) for small antenna sizes of 2, 3, 4, or 6 in. (5, 8, 10, or 15 cm) in diameter when used in free space, and up to 120 ft (35 cm) when used in a stilling well

Inaccuracy

±0.04 to ±0.125 in. (±1 to ±3 mm) for tank farm gauges, from ±1 in. (±25 mm) to ±0.5% of full scale for process-control transmitters

Tolerated Coating

Up to 1 in. (2.5 cm) if the buildup is a nonconductive oil, tar, or wax; up to 0.125 in. (0.3 cm) if the buildup is conductive and water based

Tolerated Turbulence

Up to 3 ft (1 m) waves. FMCW sensors handle turbulence better than pulse sensors do.

Tolerated Foam

Up to 5 to 6 ft (1.5 to 2.0 m) if nonconductive; up to 6 to 12 in. (15 to 30 cm), depending on density, if conductive; negligible source of error

Mist or Spray Effects

Minimal effect for a light mist; full falling curtain of spray will block the beam. If the full spray is horizontal as a result of throwing action from a partially uncovered agitator blade or from a side fill, it will be reported as the level.

Cost

$3500 to $5000 for tank farm gauges, $1500 to $2500 for process-control transmitters



3.13 Radar, Noncontacting Level Sensors

505

Partial List of Suppliers Tank Farm Gauges

Enraf (formerly Enraf-Nonius) (www.enraf.com) (FMCW) Saab Rosemount Tank Control (formerly Saab Tank Control) (www. saabradar.com) (FMCW) L & J Technologies (www.landj.com) (FMCW)

Process Gauges and Transmitters

AMETEK Drexelbrook (www.drexelbrook.com) (pulse) Emerson Process Measurement (former Rosemount) (www.rosermount.com) (FMCW) Endress+Hauser Inc. (www.us.endress.com) (pulse) Krohne (www.krohne.com) (FMCW) Ohmart/VEGA (www.ohmartvega.com) (pulse) Siemens Milltronics (www.milltronics.com) (FMCW, pulse) Siemens AG (www.siemens.com) (FMCW, pulse) Solid Applied Technologies Ltd. (www.solidappliedtechnologies.com) (FMCW) Thermo MeasureTech (former TN Technologies) (www.thermomt.com) (FMCW) Vega Instruments (www.vega.com) (pulse)

Radar level transmitters and gauges use electromagnetic waves, typically in the microwave K- and X-bands (≈ 6 to 28 GHz), to make a continuous liquid level measurement. Most applications have been on liquid service, although several companies report small to moderate success on solids and powders. Emissions are at low power levels, typically less than 2 2 0.1 mW/in. (0.015 mW/cm ), because industrial level measurement typically requires less than a 100-ft (30-m) range, which is a short range for a radar technique. At these energy levels, there are no special health, safety, licensing, or product contamination considerations; only solid-state transistors or diodes (rather than tubes) are needed to generate and detect the microwaves. Radar level instruments are generally line powered, although at least one company now claims a loop-powered instrument. The radar sensor is mounted at the top of the vessel and is aimed down, perpendicular to the liquid surface. This causes the signal that is reflected from the surface to return directly to the sensor. Figure 3.13a shows two types

FIG. 3.13b Pulse radar transmitters. (Courtesy of Endress+Hauser Inc.)

of sensor mountings available for FMCW gauges, whereas Figure 3.13b shows the various types of sensor mountings for pulse systems.

PRINCIPLES OF OPERATION

FIG. 3.13a FMCW radar transmitters. (Courtesy of Thermo MeasureTech Inc.)


Two basic principles of operation exist for continuous level radar transmitters and gauges. Most tank-farm gauges and some process gauges are operated on the frequency modulated carrier wave (FMCW) principle. Other gauges and transmitters, particularly the lowest-cost units, are operated on the pulse principle. Both principles are fundamentally based on the time of flight from the sensor to the level surface to be measured. In the FMCW method, this time of flight is tracked on a carrier wave; in the pulse method, it is the echo return. The latter is analogous to the air sonar principle under which most ultrasonic level sensors operate except that the pulse method operates between 6 and 28 GHz instead of at ultrasonic acoustic frequencies.

506

Level Measurement

PULSE

Send Radar Sweep

Reflected Return Signal

Frequency

F2

Time of Flight

∆F

F1 T1

T2 Time

FIG. 3.13c Radar frequency sweep.

Level 6 ft (2m) Away

Level 30 ft (9 m) Away

FIG. 3.13d FM return signal.

FMCW The time of flight of the reflected signal is measured by controlling the sensor oscillator so that it sends out a linear frequency sweep at a fixed bandwidth and sweep time, as shown in Figure 3.13c. The radar detector is exposed simultaneously to both the send radar “sweep” and to the reflected return signal, which is an “older part” of the radar sweep. The detector output is a frequency signal that equals the difference between the “send” and the “older” signals. This difference in frequency is directly proportional to the time of flight and thus to the distance between the sensor and the liquid level. The result is a frequency-modulated (FM) signal that varies between 0 and more than 200 Hz as the distance varies between 0 and 200 ft (60 m). See Figure 3.13d for oscilloscope views of this signal for different distances. An advantage of this technique is that the process variable information is in the frequency domain instead of the amplitudemodulated (AM) or time difference domain, which allows more accurate signal conversion. This is the same advantage that FM radio has over AM radio. Most tank noise sources are in the amplitude domain, so FM signal processing can ignore them, because accuracy is not affected.


The sensor transmits a pulse of microwave energy and receives a return signal from the material level surface. The transit time of the signal is calculated and used to determine the distance to the level surface. This calculation can then be used to determine the level of the material itself. The amount of return signal received depends on the specific reflective properties of the material level surface as well as signal loss from foam, agitation, and other interferences. Reflectivity can be determined by examining the conductivity and dielectric constant of the material. Generally, conductive products such as water and other water-based liquids (acids, strong bases, and so on) can be measured, regardless of dielectric constant. Nonconductive materials have reflectivity based on the dielectric constant exclusively. Materials with low dielectric constants absorb microwaves and provide much lower reflected signal strength than do materials with high dielectric constants. To make an accurate distance measurement using time-offlight calculations, the velocity of wave travel must be constant or it must be measured. The velocity of radar wave transmission is equal to the speed of light divided by the square root of the medium’s dielectric constant. Fortunately, the dielectric constants of different gases at different pressures and temperatures vary only slightly from that of air and from that of a total vacuum, so measurement errors due to changing tank conditions are very small. Radar waves are similar to laser signals and very different from ultrasonic waves in this regard. Table 3.13e provides a comparison of radar velocity and ultrasonic velocity under varying gas compositions and temperatures. In ultrasonic gauges, the errors caused by velocity changes can be reduced by wet calibration, by temperature compensation, or by compensation using reference targets. The target in this case should be located near the top of the tank, where the vapor space is uniform and no concentration

TABLE 3.13e The Velocity of Sound and of Microwaves (Radar) Do Not Change the Same Amount as a Function of the Substance Through Which They Travel Velocity at 1 Atmosphere Gas Composition

Gas Temp in °C

Radar in million meters/sec*

Ultrasonic in meters/sec=

Dry air

0 100

299.91 299.94

331.8 386.0

Water vapor

100

299.10

404.8

0 50

299.85 299.87

259.0 279.0

Ammonia

0

299.93

415.0

Acetone

0

297.64

223.0

Carbon dioxide

*From References 1 and 2. = From Reference 1.

3.13 Radar, Noncontacting Level Sensors

gradients exist (as they do near the liquid surface). The presence of nonmetallic foams, mists, and dust in the wave path will have little effect on a microwave’s velocity, because the dielectric constant of these media differ little from that of air. Typically, the minimum dielectric constant for FMCW radar instruments is approximately 2. The minimum dielectric constant for pulse radar instruments is approximately 5. One of the significant application issues for radar continuous level measurement in low-dielectric materials is the fact that, at low levels, the reflectivity of the material being measured may be less than the reflectivity of the vessel bottom or sidewall in the case of a non-straight-sidewall vessel. This means that, when the level is close to zero, the return signal from the vessel bottom may be stronger than the return signal from the material level itself. Live zeroes and metallic targets have been used to overcome this application difficulty.

507

panacea, however. Interferences exist. Although the existence of changing vapor and foam in a vessel has less effect on a radar gauge than on an ultrasonic level sensor, the combination of foam and low-dielectric material can cause the radar gauge to read erroneously or to provide an insufficient signal strength on the return signal for the measurement to be made at all. In glass-lined reactors, tuning problems may exist because of the tendency of the glass lining to act as a waveguide. In other reactors, the tank nozzle acts as a waveguide, and special mounting fixtures or antennas may be necessary to deal with nozzles that are not exactly perpendicular to the tank bottom. The existence of agitators and other internal structures must also be taken into account, because they can produce spurious echo effects. Radar provides an excellent alternative to bubblers, ultrasonics, and gamma nuclear in applications of medium difficulty such as fuming acids, asphalt, LNG, tars, and other heavy hydrocarbons and many other tank-farm and process reactor vessel levels.

ACCURACY AND RESOLUTION FACTORS FMCW radar level gauges are the primary tank farm radar devices because of the inherent accuracy in their design. The most accurate units also have the most precise oscillators, and there is a direct correlation between oscillator precision and cost of the component. This essentially means that there is a direct correlation between the highest-accuracy gauges and the most expensive gauges. Pulse transmitters are generally not suitable for tank-farm inventory and custody transfer applications because of the difficulty of producing a highly accurate level measurement with this type of radar transmitter. Pulse transmitters are extremely effective in process tank and reactor level measurements, however, where the limitations on accuracy are less critical and where the smaller antenna designs are essential. In addition, pulse-type transmitters are usually less expensive than FMCW transmitters.

APPLICATION CONSIDERATIONS Since the early 1990s, radar level transmitters and gauges have become more reliable and have become a mature instrument technology. In use, they have replaced many applications previously handled by ultrasonics and by gamma nuclear continuous level systems. Radar gauges are not a


References 1. 2.

Handbook of Chemistry and Physics, 63rd ed., CRC Press, Boca Raton, FL, 1982. Kaye, G. W. C., Table of Physical and Chemical Constants and Some Mathematical Functions, 15th ed., Longman, New York, 1986.

Bibliography Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June 1997. Boyes, W. H., The changing state of the art in level measurement, Flow Control, February 1999. Daniewicz, J. L., Using radar level measurement for increased environmental protection and plant safety, in Proc. ISA International Conference and Exhibit, Anaheim, CA, October 1991. Increasingly sound investments, Control, May 2002. Larson, K., Tank gauges achieve new levels of accuracy, Control, June 1991. Microwave Radar Instrumentation Installation Planning Guide for Continuous Level Measurement, Thermo MeasureTech (formerly TN Technologies), Round Rock, TX, 1991. Oglesby, W. W., Radar measures sticky liquid level, In Tech, December 1991. Oswald, H., High temperature radar sensors, chemie-anlagen+verfahren, September 1998. Parker, S., Diverse uses for level radar, InTech, May 2002. Silverman, S., Field tests prove radar tank gauge accuracy, Oil Gas J., April 23, 1990. Skowaisa, J., Radar level measurement in reaction tanks, Process, September 2001.

3.14

Radar, Contact Level Sensors (TDR, GWR, PDS) D. S. KAYSER

(1982)

B. G. LIPTÁK

Guided Wave Radar LT 001

B. CARSELLA

(1995)

(2003)

Flow Sheet Symbol


Applications

Liquids and solids with dielectric greater than 1.4 (lower with special techniques)

Supply Voltage

Line (24 to 240 VAC) or loop (24 VDC, two-wire)

Design Pressure

Full vacuum to 5000 PSIG (–1 to 345 bar)

Design Temperature

−230 to +750°F (−150 to +400°C)


Metals, 316SS, Hastelloy C, Monel ; plastics, Teflon , Halar , Tefzel

Range

2 to 200 ft (0.67 to 60 m)

Dielectric range

1.4 to 100

Sensor Pull Strength

11,000 lb (4330 kg)

Inaccuracy

Liquids, 0.1 in. (3 mm) or 0.1%, whichever is greater Solids, 1.0 in. (25 mm)

Tolerated Coating

Low-dielectric coating (E < 10); inaccuracy increases with increase in coating dielectric, thickness, and length

Tolerated Foam

Low-dielectric foam (E < 10); inaccuracy increases with increase in foam dielectric, density, and thickness

Tolerated Mist or Spray Effects

High (little effect)

Tolerated Turbulence

Measurement, high (little effect); mechanical, may need probe secured when severe

Area Classification

Nonincendive, intrinsically safe, and explosion-proof

Cost

$750 to $3500


AMETEK Drexelbrook (www.drexelbrook.com) Bindicator (www.bindicator.com) Endress+Hauser Inc. (www.endress.com) K-Tek Corp. (www.ktekcorp.com) Krohne (www.krohne.com) Magnetrol International (www.magnetrol.com) Rosemount (www.rosemount.com) Vega (www.vega.com)











3.14 Radar, Contact Level Sensors (TDR, GWR, PDS)

DEFINITION OF TERMS Characteristic impedance. The impedance that, when connected to the output terminals of a uniform transmission line of arbitrary length, makes the line appear to be infinitely long. A uniform line so terminated will have no standing waves on the line, and the ratio of voltage to current will be the same at any point on the line (nominal impedance of the 1 waveguide). Dielectric. A material that is an electrical insulator or in which an electric field can be sustained with a 1 minimum of dissipation of power. Dielectric constant. A material characteristic expressed as the capacitance between two plates when the intervening space is filled with a given insulating material, divided by the capacitance of the same plate arrangement when the space is filled with air 2 or is evacuated. The dielectric values of selected materials are given in Table 3.14a. Discontinuity. An abrupt change in the shape (or impedance) of a waveguide (creating a reflection of 1 energy). Electromagnetic wave (energy). A disturbance that propagates outward from any electrical charge that oscillates or is accelerated; at a great distance from the charge, it consists of vibrating electric and magnetic fields that move at the speed of light and are at right 1 angles to each other and to the direction of motion. Equivalent time sampling (ETS). The process that captures high-speed electromagnetic events in real time (nanoseconds) and reconstructs them into an equivalent time (milliseconds) that allows easier measurement with contemporary electronic circuitry. Guided wave radar (GWR). A contact radar technology in which time domain reflectometry (TDR) has been developed into an industrial level measurement system; a probe immersed in the medium acts as the waveguide. Phase difference sensor (PDS). A contact radar technology; unlike TDR-based systems, which measure

509

using subnanosecond time intervals, PDS derives level information from the changes in phase angle. Radar. Radio detection and ranging; a system using beamed and reflected radio-frequency energy for detecting and locating objects, measuring distance or altitude, navigating, homing, bombing, and other purposes. In detecting and ranging, the time interval between transmission of the energy and reception of the reflected energy establishes the range of an 1 object in the beam’s path. Time domain reflectometry (TDR). A process in which an instrument (a time domain reflectometer, also abbreviated TDR) measures the electrical characteristics of wideband transmission systems, subassemblies, components, and lines by feeding in a voltage step and displaying the superimposed reflected signals on an oscilloscope equipped with a suitable 1 time-base sweep. Waveguide. A device that constrains or guides the propagation of electromagnetic waves along a path defined by the physical construction of the waveguide; for example, ducts, a pair of parallel wires, or a coaxial 1 cable.

INTRODUCTION Modern contact radar level transmitters owe much of their existence to time domain reflectometry. Although new to the industrial level market, TDR technology has been used for decades. It has been employed for finding breaks in underground cables and the in-wall installations of large buildings. Companies such as Hewlett-Packard, Agilent, and Tektronics supply TDR generators that are optimized for this use. Recent breakthroughs have made this technology less costly and allowed it to consume much less power. Cost-effective looppowered (24-VDC) transmitters are now available in the industrial market. Two basic types of contact radar level measurement need to be discussed: guided wave radar (GWR) and the phase difference sensor (PDS).

THEORY OF OPERATION TABLE 3.14a Dielectric Constant (ε) Values of Selected Media Media

Dielectric @°F (°C)

Propane

1.6 @32 (0)

Kerosene

1.8 @70 (20)

Mineral Oil

2.1 @80 (27)

Phenol

4.3 @50 (10)

Glycol

37 @77 (25)

Water, DI

15 @70 (20)

Water, tap

80 @80 (27) 48 @212 (100)


Guided Wave Radar The fundamentals of GWR come directly from time domain reflectometry, a pulse-sampling technique that characterizes the distributed electrical properties of transmission lines. TDR instruments launch low-amplitude, high-frequency pulses onto a transmission line, cable, or waveguide under test and then sequentially sample the reflected signal amplitudes. Typically, the reflected pulse amplitudes are displayed on a calibrated time scale. In this way, cable impedance changes and discontinuities can be spatially located and assessed.

510

Level Measurement

24 vdc. 4-20 mA Loop Powered

A 500mVAC10:1 B 5VAC10:1 10ms/DIV TRIG:X −2DIV T

Hold Manual

A A reflection is developed off the liquid surface

Transmit Pulse

Air ε = 1

A small amount of energy continues down the probe in a low dielectric fluid, e.g., hydrocarbon

Water

Fiducial

Level

Media ε > 1.4

B

FIG. 3.14b Fundamentals of guided wave radar level measurement.

Remote Ω

Scope Meter

In guided wave radar, the waveguide becomes a probe immersed in the liquid (or dry, bulk medium). The characteristic impedance, ε, of the probe (in air, ε = 1) decreases when a liquid (or dry media) of a higher dielectric displaces the air. The electromagnetic pulses transmitted down the waveguide are reflected at this point of discontinuity, and the reflections are measured by high-speed circuitry in the transmitter head; in this manner, the level is established. The fundamentals of guided wave radar level measurement are illustrated in Figure 3.14b. For typical process media, the dielectric can range from 1.4 to 100. The higher the dielectric constant, the stronger the reflected signal. An oscilloscope trace of guided wave radar showing fiducial (baseline reflection) and strong reflection received from water (a high-dielectric medium) is shown in Figure 3.14c. The oscilloscope trace in Figure 3.14c shows the extremely large reflection created by the high-dielectric water (ε = 80 at 70°F). The small fiducial, or baseline reflection, is the zero point for the GWR measurement. A typical fiducial is ≈200 mV. The large negative level pulse is developed by the reduction in impedance in the waveguide from the presence of the high-dielectric water. The higher the dielectric of the medium, the higher the amplitude of the reflection it creates. In this oscilloscope trace, the high dielectric (ε = 80) water is approximately 2000 mV. Figure 3.14d shows the same oscilloscope trace with a low-dielectric reflection (≈750 mV) added for comparison. The lower-dielectric medium creates a reflected pulse with decreased amplitude; however, the reflection still exists at the same point in time (position) as the pulse with larger amplitude. Variation in the dielectric of the medium, although important to the creation of a good reflection, is not critical to accurate measurement. Theoretically, error can be introduced by variations in the speed of propagation related to vapor space dielectric. The high-frequency electromagnetic pulses travel at the speed


EXT.mV

Coax Probe − Tap Water

FIG. 3.14c Oscilloscope trace of guided wave radar showing fiducial (baseline reflection) and strong reflection received from water (high dielectric medium). A 500mVAC10:1 B 5VAC10:1 10ms/DIV TRIG:X −2DIV T

Hold Manual

A

Decrease in Amplitude Same Level

B Remote Scope Meter

Ω

EXT.mV

Coax Probe − Low Dielectirc Medium (dashed line)

FIG. 3.14d Oscilloscope trace showing reflection amplitude decreasing dielectric constant of liquid yet its position in time (space) remains constant.

of light. The speed of light (c) in a vacuum (ε = 1) travels at 8 186,000 mi/sec or 3 × 10 m/sec. This is calculated as c/ ε

3.14(1)


where c = speed of light ε = dielectric constant of vapor space As Equation 3.14(1) shows, as long as the pulses travel in a vapor space with ε close to 1.00, no significant variation in the speed of propagation is expected. In practice, this is not a consideration for contact radar. Phase Difference Sensors Phase difference sensors are similar to time domain reflectometers (TDRs), except the detection circuit operates in the phase domain rather than the time domain. A high-frequency ( f ) signal travels through parallel conductors at a fixed velocity (V0) until it is partially reflected by the stored material interface, where the sensor impedance changes abruptly. The dielectric constant of the vapor space remains nearly unity, even if dust, vapor, condensate, or foam is present. Therefore, the signal injected into the sensor travels down to the material interface and back at a constant velocity V0. Because of the travel distance of 2(l), there will be a phase difference (β) between the input and the reflected signals, as shown in Figure 3.14e. Contact Radar Systems The contact radar system is composed of two basic components: an electronic transmitter and probe (waveguide). Careful choice of the probe is particularly important for the successful application of contact radar.

Continuously Varying Input Signal δ=

I

V0

Resultant Signal

β

I

Reflected Signal Phase Difference = β = 4 πfl /V0

511

Electronics Contact radar circuitry is accurate and responsive. The most modern devices (particularly guided wave equipment) are inexpensive and operate at low power levels (loop powered). Systems benefit from the confluence of two key issues. 1. There have been significant advancements in highspeed, microwave circuits, including the use of equivalent time sampling (ETS). ETS captures the high-speed electromagnetic pulses in real time (nanoseconds) and reconstructs them in equivalent time (milliseconds), allowing the use of slower, less-complex circuitry. 2. The probe, or waveguide, provides a conductive path for the highly efficient transfer of energy to the medium surface and back. Signal processing needs no duty cycling to conserve energy, and there are no false-target reflections to ignore (eliminating a source of complex signal processing required in through-air transit time devices—for example, radar and ultrasonic). This efficient propagation of energy allows reliable measurement of extremely low-dielectric (>1.4) media. The measurement stability described previously has two key ramifications. 1. A constant speed of light (electromagnetic energy) allows factory calibration; no field calibration is needed—only simple configuration (no level movement is necessary). 2. Accuracy is unaffected by dielectric changes in the medium. Probe (Waveguide) Three basic probe configurations are utilized today: coaxial, twin-element, and single-element. Coaxial probes (waveguides) are the most efficient, providing efficiency similar to that of the standard 75-Ω coaxial cable used for video and data. The twin element is less efficient; it is analogous to the twin-lead antenna cable used before the implementation of coaxial cable. (It is the configuration of choice with phase difference sensors.) The single-element probe is the least efficient but most forgiving in applications in which the media will coat the probe. Figure 3.14f shows

Abrupt Change in Sensor Impedance

Resultant Signal Amplitude f1

f2

Frequency (f) Coaxial

FIG. 3.14e Frequency (phase) difference measurement used to measure the levels in silos. (Courtesy of Bindicator/Venture.)


Twin Element

Single Element

FIG. 3.14f Fundamental probe (waveguide) configurations and energy fields.

512

Level Measurement

the three fundamental probe configurations with their respective energy fields.

elements: dielectric, coating thickness, and coated length. Low-dielectric media cause little error, whereas higher-dielectric media (e.g., water-based substances) can be problematic. The worst error is caused by a thick, high-dielectric coating that runs the length of the probe before the measurement takes place. Viscosity. Thin media (10,000 lb (4000 kg) of pull strength. Overfill. Measuring to the very top of a vessel can be problematic for any transit-time device (radar, ultrasonic). Ambiguous readings or nonmonotonic measurement can be experienced. Care must be taken when encountering this application. Contact radar devices exist that measure into an overfill or “flooded” condition (cage/chamber installations). Hazardous area/Div. 2 implementation. All modern transmitters carry explosion-proof, intrinsically safe and nonincendive (National Electrical Code Division 2, or Div. 2, hazardous area) approvals. GWR is a technology that must inject a small amount of energy into the vessel to accomplish its measurement. For this reason, it is important in CLI/Div. 2 applications to consider what is inside the vessel as well as outside. If the medium to be measured is flammable, an intrinsically safe probe circuit must be used. In a Div. 2 area, this may require the use of a transmitter that is rated explosion proof.

PROBE SELECTION AND APPLICATION Choosing the proper probe (waveguide) is the most important aspect of applying contact radar. The following are the key issues to consider. Temperature/pressure. The most fundamental process parameters to be considered are temperature and pressure. Because the market offers contact radar sensors rated from − 235 to + 750 ° F ( − 150 to +400°C), and full vacuum to 5000 PSIG (345 bar), there are few process applications that cannot be accommodated. Dielectric sensitivity. Coaxial probes are the best choice in applications of low dielectric (ε < 2.0) given their propagation efficiency; butane (ε = 1.4) and propane (ε = 1.6) have been measured very successfully. It is important to note that the dielectric of most media has a tendency to change inversely over temperature; as temperature increases, dielectric decreases. It is useful to understand the dielectric of a medium at the specific temperature of the process. Due to the inverse relationship, higher temperatures are more problematic. Mounting/obstructions/proximity effects. Mounting can be an issue to varying degrees. A coaxial probe suffers from no proximity effects, whereas a singleelement probe is the most sensitive and must be applied carefully. Length. GWR is a contact technology requiring the probe length to be at least as long as the span. This can become an issue for long probes. Shipping costs increase as probe lengths increase. Physically installing a 20-ft (6-m) rigid probe takes some thought; headroom is a key concern. Material compatibility. Probes are constructed of various combinations of metal, plastic (insulator), and, often, O-rings. Type 316 SS is the most common   metal, with Hastelloy and Monel common  options. Various types of Teflon (TFE, FEP, PFA) are used as an insulator more than any other plastic.   Halar (ECTFE) and Tefzel (ETFE) are also used. A common process seal utilizes an O-ring as part of the design and must be considered as a wetted  part. The most common O-ring materials are Viton ,   EPDM, and Neoprene . Kalrez , although more expensive, is specified as an option because of its superior chemical compatibility. Coating/clogging/buildup/bridging. Being based on contact technology, guided wave and phase difference sensors can suffer error caused by various degrees of coating. Error is related to three key


INTERFACE MEASUREMENT In industrial level measurement, interface is defined as the point between two immiscible liquids. Contact radar has the ability to measure both the upper and lower liquids if the following conditions are met: • • •

The upper liquid is a low-dielectric medium (ε < 10) The lower liquid is a higher-dielectric medium (∆ε > 10) The interface is clean and distinct, with no emulsion layer


A small reflection will be generated by the upper (lowdielectric) medium while most of the energy continues down the probe. A larger reflection is generated by the lower (high-dielectric) medium. The key is to accurately compensate for the change in pulse propagation velocity while it is traveling through the upper medium, which has a dielectric that differs from that of the vapor space. Emulsion layers are problematic because, by definition, there is not a distinct difference between two media but, rather, a gradual change from one to another. For this reason, detectable pulses are not generated.

CONCLUSION There is no ideal level measurement technology. In a perfect world, all measurement would be noncontact and even noninvasive. Noncontact radar comes close, but it also has some weaknesses that contact radar overcomes. The new radar technologies, both contact and noncontact, form a team in effective level measurement for many process applications.


513

References 1. 2.

Dictionary of Scientific and Technical Terms, 4th ed., McGraw-Hill, New York, 1989. ISA Dictionary of Measurement and Control, 3rd ed., ISA, Research Triangle Park, NC, 1995.

Bibliography Carsella, B., Guided wave radar—a new era in level measurement, Industrial Process, Prod. Technol., October 1998. Carsella, B., Capacitance level measurement—the end of an era, Industrial Process, Prod. Technol., October 1999. Carsella, B., The liquid level measurement showdown—guided wave radar vs. pressure/DP, Can. Process Equipment Control News, October 1999. Carsella, B., Automating the invisible liquid, Propane Canada, March-April 1999, and Hydrocarbon Processing (UK), January 2000. Cornane, T., Continuous level control—a review of phase tracking, Meas. Control, April 1997. Gray, J. and Hollywood, P. M., Time domain reflectometry tackles tough tank level measurements, I&CS, November 1997. Hollywood, P. M., TDR level measurement, Meas. Control, December 1997. Nemarich, C. P., Time domain reflectometry liquid level sensors, IEEE Instrum. Meas., December 2001. Parker, S., Diverse uses for level radar, InTech, May 2002.

3.15

Radiation Level Sensors B. G. LIPTÁK (1969) D. S. KAYSER (1982) A. J. LIVINGSTON (1995) J. C. RODGERS

To On-Off Receiver LX

LS R

(2003) To Continuous Receiver LX

LT R Flow Sheet Symbol


Applications

Noncontact and nonintrusive level measurement of liquids and solids

Temperature

External detectors are suitable for −40 to 160°F (−40 to 70°C) ambient conditions. Units can be provided with heaters for lower ambient temperatures and with air/water cooling for higher ambient temperatures. Traversing backscatter units can operate at up to 250°F (121°C).

Radiation Sources

Cobalt 60 (5.3 years of half-life), cesium 137 (30 years of half-life), americum 241 (455 years of half-life), and radium 226 (1602 years of half-life). Cesium is used most often, because it decays more slowly; cobalt is selected when the tank walls are thick. Point sources are affected by product density variations, but strip sources are not.

Radiation Exposure

One roentgen is received during one hour spent within one meter of a one-curie (1-Ci) radiation source. A subject receives a dose of one rem (roentgen equivalent man) when exposed to one roentgen in any time period. General public allowable limits are 2 mrem/hr and 100 mrem/year. The dose for an occupational worker is 5 rems/year.

Source Sizes

In external types, usually a few hundred mCi or less; in dry-well-type backscatter units, usually 10 mCi. A curie is generated by 1 g of radium, 0.88 mg of cobalt, or 10 10 11.5 mg of cesium. One Ci equals 3.7 × 10 disintegrations/sec or 3.7 × 10 becquerels (Bq, SI units).

Inaccuracy

For alarm switches, 0.25 in. (6 mm) error can be expected. Errors in continuous units range from 0.125 in. (3 mm) to 1% of span.

Ranges

External units have ranges from 1 in. (25 mm) to 23 ft (7 m) for single stationary units, higher for multiple units, and up to 50 ft (15 m) for motorized units. Traversing backscatter units mounted in dry wells can detect up to a range of 150 ft (45 m).

Costs

Assuming a 10-ft (3-m) diameter vessel with 0.25 in. (6 mm) wall thickness and 1.0in. (25-mm) insulation, the following costs can be expected: on–off alarm, $2240; continuous level transmitter $8000 and up; gamma backscatter-type level switch $9000 to $15,000; neutron backscatter-type level switch, $16,000 to $35,000; continuous (scanning) backscatter units for up to 150 ft (50 m), $60,000.


Barton Instrument Systems LLC (www.barton-instruments.com) Berthold Industrial Systems (www.berthold.com.au) Endress+Hauser Inc. (www.endress.com) Flow-Tech Inc. (www.flowtechonline.com) Imaging & Sensing Technology (www.istimaging.com) Ohmart/VEGA (www.ohmartvega.com) Ronan Engineering (www.ronan.com) Thermo MeasureTech (www.thermo.com)

3.15 Radiation Level Sensors

Radiation at different frequencies can be used for level measurement. Included are ultrasonic, radar (or microwave), laser (infrared light), neutron, and gamma forms of radiation. These different radiation forms are also of different strengths in terms of what materials they can penetrate. Some require nozzles on the tank (most ultrasonic and radar designs), and others need windows (laser) or nonmetallic tanks (microwave). Nuclear radiation can pass through metallic walls. The weaker, less penetrating the signal, the more likely is it to be used in the echo, reflection, or backscatter mode. Gamma radiation is sufficiently penetrating to pass through tank contents, although radiation gauges are also used in the backscatter mode, as discussed below.

RADIATION PHENOMENON Atoms with the same chemical behavior but with a different number of neutrons are called isotopes. Many elements have one or more naturally occurring stable isotopes. For example, the stable isotopes of oxygen are as follows: Stable Isotopes

No. of Protons

No. of Neutrons

Percent Abundance

16

O

8

8

99.76

17

O

8

9

0.04

18

O

8

10

0.20

Most elements also have unstable (radioactive) isotopes. 15 19 Oxygen’s radioactive isotopes are O and O, which have 7 and 11 neutrons, respectively. The unstable isotopes disintegrate to form elements or stable isotopes. Most of the elements that are heavier than lead are also unstable and disintegrate to form lighter elements. Radioactive disintegration is accompanied by the emission of three different kinds of rays. Alpha (α) radiation consists of positively charged particles having two neutrons and two protons. Beta (β ) radiation consists of electrons. Gamma (γ ) radiation consists of electromagnetic waves that are comparable to X-rays. The relative penetrating powers of the three kinds of radiation are approximately in the range of 1, 100, and 10,000 for the alpha, beta, and gamma rays, respectively. The penetrating power of alpha rays is less than 8 in. (203 mm) of atmospheric pressure air. Alpha radiation cannot penetrate the skin. Beta radiation can penetrate approximately 0.25 in. of aluminum. Alpha and beta rays carry an electrical charge and can be deflected by an electric or magnetic field. Because gamma rays have great penetrating power and cannot be deflected, gamma radiation sources are chosen for use in level-detecting equipment. Source Materials The two most commonly used gamma sources are the radioactive isotopes Co 60 (cobalt) and Cs 137 (cesium).


515

Co 60 is produced by bombarding the stable isotope Co 59 with neutrons. When Co 60 decays, it emits beta and gamma radiation to form the stable element Ni 60 (nickel). In similar fashion, when Cs 137 decays, it emits beta and gamma radiation to form the stable element Ba 137 (barium). The Co 60 isotope decays at two different energy levels, 1.173 million electron volts (MeV) and 1.332 MeV, while Cs 137 decays at 0.662 MeV. Cs 137 is one of many fission products of uranium and is obtained when spent fuel rods from nuclear power plants are reprocessed. There are two important points to note about the gamma decay phenomenon. One is that the decay produces electromagnetic energy, which cannot induce other materials to become radioactive. This means that gamma sources can be used around such materials as food and food-grade packaging materials. The second point is that the source loses strength as it decays. The rate of decay is expressed as half-life, the period of time during which the source loses half of its strength. Co 60 has a half-life of 5.3 years; it will decay approximately 12.3% per year. The figures for Cs 137 are 30 years and 2.3% per year. For point or continuous level measurement, source decay does not affect accuracy, but the initial source size should be fashioned so that the installation has a reasonably long useful life. Typically, the gauge electronics compensate for the decay of the source. In rare instances, the isotope Ra 226 (radium) can be used. This material has a half-life of 1602 years and therefore has no appreciable loss of strength over the life of the installation. Units and Attenuation of Radiation The units used to quantify the activity of any radioactive material are the curie (Ci) and the becquerel (Bq). One gram 10 of Ra 226 has 3.7 × 10 disintegrations per second. This rate 10 of activity is defined as 1 Ci or 3.7 × 10 Bq, whether it is produced by Ra or some other source. For most level detection applications, source strengths of 100 millicuries (mCi) or less are satisfactory. The unit of radiation exposure is the roentgen (r), which is defined as the quantity of radiation that will produce ionization equal to one electrostatic unit of charge in one cubic centimeter of dry air under standard conditions. A 1-Ci source will produce a dose of 1 r at a receiver placed 1 m (3 ft) away from the source for 1 h. The dose rate unit is the roentgen/hour (r/hr), a measure of the photons reaching the receiver at a defined distance. Radiation is attenuated when it penetrates liquids or solids, and the rate of attenuation is a function of the density of the material. The higher the density, the more attenuation the shielding material will provide. The following shows how various thicknesses of different materials will provide different levels of attenuation. The various thickness values illustrate the half-value layer concept. Each of the listed thicknesses of each material represent the thickness needed to attenuate or reduce the radiation field by one-half. As can be seen, various isotopes,

516

Level Measurement

due to their decaying energy value, have different half-value layer thicknesses. Material

Cs-137

Dirt/wood

6 in.

8.5 in.

3.75 in.

7.5 in.

Steel

0.5 in.

1 in.

Lead

0.25 in.

0.5 in.

Water/plastic

Vessel Clips and Support Plate

Co-60

Vessel Clip

Radiation Path

U Bolt

For example, a 0.5-in. (13-mm) steel plate will reduce radiation from a Cs 137 source by half. An additional plate will cause another 50% reduction, so the overall reduction caused by a 1-in. (25-mm) plate is 0.5 × 0.5 = 0.25. As would be expected, the amount of radioactive material required to produce 1 Ci of activity depends on the material. One Ci is generated by 1 g of Ra 226, by 0.88 mg of Co 60 or by 11.5 mg of Cs 137. The dose rate also will vary. Assuming a 1-mCi source and a receiver 32 in. (812 mm) away, the dose rates will be 1.3 milliroentgens per hour (mr/hr) for Ra 226, 2.0 mr/hr for Co 60, and 0.60 mr/hr for Cs 137. Radiation field intensity in air can be calculated from the following equation: D = 1000

KmCi d2

Plan View Source and Holder

Detector

Max Liquid Level

45° Max Radiation Path

3.15(1)

where D = intensity, mr/hr mCi = size of source in millicuries d = distance of source in inches K = a constant, 1.3 for Ra 226, 0.6 for Cs 137, and 2.0 for Co 60

Min Liquid Level

" Platform

6 ft 7 ft Elevation

FIG. 3.15a Radiation instrument installation.

SOURCE SIZING In actual installations, radiation must penetrate substances in addition to air, and it is of interest to determine the radiation field intensity after the gamma rays have passed through the vessel walls and process material. The previous equation may be used for this purpose. Figure 3.15a shows a typical installation, and the information below will show how the radiation intensity at the receiver is determined and what levels of operator exposure to radiation may be expected. It will be assumed that the minimum radiation field intensity at the detector should be 2.0 mr/hr when the vessel is empty and that the field should be reduced at least 50% as the vessel is filled. The liquid in the vessel has a specific gravity of 1.0. The calculation is based on the use of a 100 mCi source of Cs 137. With no vessel at all, field intensity at the detector would be D = 1000


KmCi 0.6 × 100 = = 8.50 mr / hr d2 84 2

3.15(2)

With the empty vessel in place, the attenuation through the two 0.5-in. steel walls will be 0.50 × 0.50 = 0.25 (see Figure 3.15a), and the resultant field intensity at the detector is 8.50 × 0.25 = 2.125 mr/hr. When the tank is full, the radiation will have to penetrate 72 in. (1.8 m) of material having a specific gravity of 1 (water). From the previous information, approximately 4 in. of water will reduce the radiation field by 50% (half-value layer). In this example, we would need only approximately 4 in. of water to reduce the radiation field to meet the 50% radiation field reduction requirement. This example will have 72 in. divided by 3.75 in./half-value layer = 19.2 half-value layers between the source and the detector with the vessel full, creating effectively no radiation at the detector. During the empty condition and with the source providing radiation to the detector of approximately 2.125 mr/hr, the radiation field can be calculated for personnel exposures



250

200

150

r

A gamma source radiates electromagnetic energy in all directions, just as a glowing ember radiates heat in all directions. Short-term exposure to high-intensity gamma radiation or long-term accumulative exposure to lower-intensity radiation is known to be hazardous. The degree of hazard, particularly to long-term low-intensity exposure, is a somewhat subjective determination. Therefore, if an error is made, it should be made on the safe side. Radiation sources are formed into ceramic pellets that are placed in a double-walled stainless-steel capsule (double encapsulation). The capsule is contained in a source holder that is constructed so as to allow a radiation beam to escape through a very narrow window (collimation) while it is blocked by shielding in all other directions. For the window, a shutter is provided that can be closed and locked when the source is being shipped or when it is out of service. Source shielding is thick enough to reduce the field intensity 1 ft (305 mm) from the source to 5 mr/hr or less. As was mentioned earlier, the disintegrations that produce gamma rays will also produce beta emissions. Because of their low penetrating power, beta rays cannot escape the stainless-steel capsule that encloses the source. Source holders are designed for a range of source sizes. For example, one holder may be used for sources in the range

In the United States, rules governing safe exposure limits to radioactive materials have been established by the Nuclear Regulatory Commission (NRC). These rules are incorporated in the Occupational Health and Safety Act. Exposure to external radiation is referred to in units of rem (roentgen + equivalent + man). A rem is a measure of the dose to body tissue in terms of its estimated biological effect relative to a dose of 1 r of X-ray. A person receives the dose of 1 rem when exposed to 1 r of radiation in any time period. As illustrated in Figure 3.15b, a person should not receive more than 250 rems over an entire lifetime. The rate at which this exposure is accumulated is also important. It is desirable to keep the yearly dose below 5 rems, and it should definitely not exceed 12 rems per year or 3 rems per quarter. In most industrial processing applications, it is possible to keep operator exposure far below these levels. For each industrial installation, it is essential to estimate the dosage received by personnel working in the vicinity of

s/Y

SAFETY CONSIDERATIONS

Allowable Radiation Exposures

Unsafe

m

Doubling the distance from the detector will decrease the radiation field by four times. Normally, Cesium 137 sources are used over Cobalt 60 sources because of the longer half-life of the cesium isotope. In some instances, such as very thick vessel walls, Co 60, with its higher decaying energy, can offer a solution where CS 137 cannot. The minimum radiation field intensity required at the detector for good performance depends on several variables, including type of detector technology, sizing practice of the manufacturer, acceptable useful life of the gauge, and radiation safety considerations. For proper sizing, the application information listing all materials between the source and detector, their densities, and their thicknesses must be provided to the manufacturer. This would include insulation, thermal jackets outside or inside the vessel, internal structural supports, diffusers, and agitator blades or shafts. All attenuating materials in the path of the radiation beam must be accounted for to make a proper measurement.

Re

R2 = radiation field at any distance d2 from the source R1 = radiation field at a known distance d1 from the source

:5

3.15(3)

te

R2 = R1( d1/ d 2) 2

of 10 to 30 mCi, whereas the next-heavier holder, which provides more shielding, may be used for sources in the range of 31 to 90 mCi. Obviously, if the source being used is at the low end of the range, the field intensity outside of the holder will be less and will approach the maximum 5 mr/hr limit when the source being used is at the top of the range. Source holders are typically constructed of a steel or stainless-steel outer surface, with lead as the shielding material surrounding the stainless-steel capsule. Some manufacturers offer a fire-proof design in which the source holder is a casting of iron material. Since lead has a melting point of approximately 620°F, during a fire, the lead shielding from the source holder could be lost. The use of iron as shielding material raises the melting point to approximately 1400°F, which is the temperature rating of the source capsule.

100

Ra

at any distance from the detector by the inverse square law formula as follows:

517

Safe

50

0 10

20

30

40

50

Operator’s Age

FIG. 3.15b Radiation exposure as a function of time and safety.

60

70

518

Level Measurement

the source, using both the assumed occupancy and the proximity to the source, on a “worst-case” basis. Returning to the installation shown in Figure 3.15a, it is assumed that the occupancy is 25 h per week and that the operator is within 12 in. (305 mm) of the tank during this period; the worst case would be if the operator were working next to the source. Assuming that the holder just meets the requirements for 5 mr/hr at 1 ft, operator exposure would be 5 × 25 = 125 mr per week, or approximately 6.25 rem per 50-week year. This exposure would exceed set limits. The second-worst case would be if the operator were working by the detector when the tank was empty. Here, the field intensity would be approximately 2 mr/hr, and the operator’s weekly and yearly exposure would be 50 mrem and 2.5 rem, respectively. (The second-worst case condition is cited to illustrate the point that the source shutter should always be closed when the tank is empty. The ultimate in bad practice is to allow a maintenance worker into the tank when the source shutter is open. Special interlock systems are available to prevent this.) After making the worst-case calculations, the design engineer should determine what can be done to reduce operator exposure. By implementing the tools of time, distance, shielding, and planning, the design engineer can create a gauge installation that will minimize an operator’s exposure to radiation. These tools are the essential elements of a principle called ALARA, which stands for “as low as reasonably achievable.” By minimizing time around the radiation field, positioning the gauge installation away from traffic areas or using of shielding, and implementing lock-out/tag-out procedures, radiation exposure to personnel can be kept to very low levels.

Nuclear Regulatory Commission The use of radioactive sources for industrial gauging systems is under the jurisdiction of the Nuclear Regulatory Commission (NRC). The gauges are required to be licensed for use. In many states, the NRC has granted permission for individual state regulatory bodies (agreement states) to be the primary contact for the licensing and regulating of the use of radioactive isotopes. The regulations for use, licensing requirements, identification of agreement states, and responsible contacts for emergency notification or basic contacts can be obtained from the NRC web site at www.nrc.gov. The United States is divided into several regions, and the appropriate contact information for each regional office is provided. When submitting a license application to the NRC or an agreement state for use of a radioactive isotope for industrial gauging, the following information is required: • •

Isotope used and source size Manufacturer and model number of both the instrument and the source holder


Stairway

Walkway Occupancy 25 Hrs/wk Detector Locked Cap Source Well Locked Flange

Source Well

24"

Handling Rod

Detector Steel Tank

Walkway

"

Steel Source Well " Wall Thickness

12" Radiation Source (Give Type and Size in mCi) 20' − 0"

FIG. 3.15c Typical radiation instrument installation.

• • •

Description of installation (see Figure 3.15c) Maximum occupancy of area Responsible individual to contact

Depending on the number of radioactive isotopes that will be used at the facility, the licensing process can range from a relatively simple to a more complicated program that incorporates training, detailed procedures for lock-out/tag-out, and other safety measures. In all cases, the end-user is responsible for maintaining documentation and the ongoing testing that is occasionally required to meet the NRC requirements for possession and use of radioactive isotopes. DETECTORS A number of gamma radiation detectors are available, but the three commonly used in conjunction with level detection are the Geiger–Mueller (G–M) tube, the gas ionization chamber, and the continuous level scintillator. Geiger–Mueller Tube The G–M tube has a wire element anode in the center of a cylindrical cathode. The cathode tube is filled with inert gas and sealed. A bias voltage of up to 700 V is applied across the anode and the cathode. Incident gamma radiation ionizes the inert gas so that there is an electrical breakdown between the anode and cathode. The frequency of the breakdown is related to the intensity of the gamma radiation; therefore, field strength can be determined by counting the pulses produced over a given time interval.


519

Anode

Gas

Detector Output

Bias Voltage Cathode

FIG. 3.15d Gas ionization chamber.

FIG. 3.15f Flexible scintillation detector.

Electronics Electrical Signals

ion chamber. With this technology, a crystal, either specially treated plastic or sodium iodide, replaces the tube of inert gas. The crystal, when exposed to gamma radiation, will create photons of light within the crystal structure. The number of photons created will increase as the radiation field increases. A photomultiplier tube senses the photons of light from the crystal and converts the light to an electrical signal in proportion to the amount of light present.

PMT

LEVEL SWITCH APPLICATIONS

Scintillator Gamma Radiation Light

FIG. 3.15e Scintillator technology.

Gas Ionization Chamber The ionization chamber is likewise filled with inert gas and sealed, but, rather than applying a breakdown voltage, a smaller voltage in the range of 6 to 100 V is applied across the chamber from end to end (Figure 3.15d). The exact bias voltage varies among manufacturers and is related to optimal performance of that particular chamber design. When the chamber is exposed to gamma radiation, ionization occurs, and a continuous current in the microampere range is caused to flow. This current is proportional to field intensity. Scintillation The third gamma radiation detector technology is scintillation (Figure 3.15e). This technology is more sensitive to the same given field of radiation as compared to the G–M tube or the


The G–M tube is most commonly used for level switch designs (point detection). The switch detector is arranged so that it sees the full field intensity (tank empty), or it sees little or no field (tank full). Both ion chamber and scintillation detectors are used for continuous level detection. Ionization chambers are made in continuous lengths up to 20 ft (6 m). As shown in Figure 3.15f, the newer flexible scintillation detectors are made in lengths of up to 23 ft (7 m). Geiger– Mueller tubes are furnished in 6-in. (152-mm) and 12-in. (305-mm) lengths and can be stacked to form a continuous detector. The stacked G–M tubes are less expensive than an equivalent length of ionization chamber. However, the G–M tubes are more subject to drift, their performance can deteriorate with time, and they generally require more radiation. A more serious drawback to the use of the G–M tubes is that an exposed (above the liquid level) element can fail altogether. If this happens, it will appear to the receiver that the failed section is covered by the liquid. In a five-section assembly, this would cause a reading that is 20% too high. Typical level switch installations are shown in Figure 3.15g. The most common is shown at the left, where the source and detector are at the same elevation, and the G–M tube detector is horizontally mounted. In this case, the differential between on–off relay action is 0.25 in. (6.3 mm), meaning that a 0.25in. (6.3-mm) rise in liquid level is sufficient to block the source beam and change the state of the switch. If a wider differential is desired, the detector is mounted at an angle to the horizontal. For a maximum differential in this installation,

520

Level Measurement

" Differential

7" Max. Differential HI

S

D

S

D

D1

S

Amplifier (Local or Remote)

HI S

D

Strip Source

φ

D2

LO S

φ

D

φ

FIG. 3.15g On–off radiation switch installations.

the sensor is mounted vertically, producing a differential of 7 in. (178 mm). For even wider differentials, two detectors can be used with a single source. In this case, the maximum differential between high and low level settings can equal the tank diameter when using a 45° source beam collimator. Differentials greater than the tank diameter require two separate sets of sources and detectors. In high-level applications, the G–M tube and switch assembly is normally above the liquid level and therefore exposed to full field intensity. Pulses from the G–M are taken to a trigger circuit that, in turn, continuously resets a time-out relay much like the time-out relay used in a computer watchdog circuit. When rising level blocks the radiation beam, the relay is no longer reset, and the switch changes state; for failsafe operation, the switch would open. The switch circuitry is arranged so that the switch will open on failure of the G–M tube or failure of any of the switch components, and therefore the entire installation may be judged fail-safe. Low-level switching applications are a different matter. In this case, the G–M tube or switch component failure would not be detected, because exposure of the tube to the beam on falling level would not actuate the switch. Where fail-safe design for falling-level applications is required, a test circuit can be installed in the G–M tube and switch assembly to test the switch when the level is high. There are several ways to do this, but, in general, a small source is installed in the detector and used continuously or intermittently to test the integrity of the tube and switching circuitry.

20 ft Max.

FIG. 3.15h Continuous level detection by use of strip source and electronic cell receivers.

Source

Detector

CONTINUOUS LEVEL MEASUREMENT Two methods exist for making continuous level measurements using fixed sources and detectors. One method, using a strip source and strip detector, is illustrated in Figure 3.15h. The second, shown in Figure 3.15i, uses a point source and a strip detector. As shown, the strip source radiates a long, narrow, uniform beam in the direction of the detector. As the level rises a small increment, a corresponding small increment of the detector is screened off. This incremental response is uniform and linear over the entire span, and therefore the signal produced is linear with level change over the entire span, except for small nonlinear end effects near 0 and 100% of span. Moreover, this installation, unlike the point switch installation, is not sensitive to variations in the


Cells

Source

FIG. 3.15i Level detection using two sources and one detector.


specific gravity of the material in the vessel. For example, if the specific gravity of the material varied from 0.40 to 0.70, the source would be sized for 0.40 specific gravity material; material of higher gravity would cause higher attenuation, and this would not affect the performance of the detector. The point source and strip detector installation also works as a very small incremental on–off device insofar as a small level rise blocks off the radiation beam of a corresponding increment of the detector. The incremental changes in level do not produce a uniform change in the coverage of the detector; consequently, this installation produces a nonlinear signal with level change. Not only does the thickness of the material change as the level changes, but the geometry of the fixed portion of the system—the vessel walls, wall-to-detector distances, and the free space—also changes as the level changes. The nonlinearity of this system can be rectified by correcting the detector output electronically by implementing a linearizer curve.

521

Level Indicator

Drive Reel 8 Motor Position Sensor

Support Tape Perforated Tape

Motor Control

Narrow Vessels or Interface For narrow vessels with long measurement spans implementing the point source and strip detector arrangement, it will be necessary to use multiple sources with a single detector, or multiple sources with multiple strip detectors. If the source holder utilizes a 45° collimator where the radiation coming out of the source holder is broadcast from 0 to 45° down from the top of the measurement (see Figure 3.15i), the maximum measurement span from one source is approximately equal to the diameter of the vessel. The second source is located so that the radiation from the first source slightly overlaps with the second source. For the best accuracy, it is important to implement a linearizer curve in the electronics to correct for the changing radiation fields across the measurement span. Radiation gauges can be arranged to detect, either at a point or continuously, solids levels and liquid–liquid interfaces. The accuracy of these installations depends on source size, detector sensitivity, material gravities, and vessel geometry. Sometimes, a system may be needed to continuously monitor a liquid–liquid interface or a liquid or solid level over a long, vertical, straight side. In these cases, strip source or multiple source and strip detectors would be expensive. Figure 3.15j shows how a point source and point detector may be motor driven over a wide span to detect levels of the above type. The motor drive may be set up to continuously hunt the level (that is, undershoot and overshoot), or it may be set up to look for the level on operator demand, as might be required for an inventory. The exact location of the level is determined by the sprocket-driven position sensor.

INSTALLATION NOTES Source holders are furnished with a mounting flange on the window side of the holder (Figure 3.15a). Most older detectors are supplied inside a piece of steel pipe that is capped


S

Source

D

Detector

FIG. 3.15j Continuous high accuracy radiation detector system for accounting installations.

on both ends. The preferred method of installing this equipment is to bolt it to clips that have been welded to the outside surface of the vessel. Figure 3.15a illustrates how this is done. Newer, lightweight detector designs require a simple support system. The installation can be arranged so that the elevation of both the detector and the source can be changed easily. The system shown calls for the beam to pass through the center of the vessel. This is unnecessary and would be undesirable in some cases. If the vessel has a center-mounted agitator, fill nozzle, or other internal obstruction, the source and detector should be located so that the beam radiates across a chord where there are no obstructions. A chord may also be selected on large-diameter vessels to reduce source size (Figure 3.15k). The cable run to the detector should be made in such a way that condensate from the conduit system will not flood the detector.

522

Level Measurement

Detector Source Source

Detector Interface

Gas

Detector

Source Foam

FIG. 3.15k Relative locations of radiation sources and detectors.

Calibration Considerations The technique for calibrating radiation level detectors is very important. The gauge should be calibrated after all vessel construction is completed, including the mounting of external insulation. For optimal accuracy, all attenuating materials such as insulation, heating/cooling jackets, internal baskets, internal support structures, and so on need to be in place before calibration is performed. Radiation detectors do not work on the absolute amount of radiation from the source; rather, the detectors interpret the change in radiation from the 0 level position to the 100% level position. With the source holder open and the vessel empty, the detector’s 0 level condition is determined. Optimally, the vessel is then filled to the position to the 100% full level, and the detector interprets the full vessel. Many times, the vessel cannot be filled, so technicians will simply close the source holder shutter to represent the blockage of radiation to the detector, simulating a 100% full condition. Unless the upper measurement range accuracy is needed, this generally is acceptable. To determine the actual level in the vessel at any given time when using radiation gauges, the technician can use a portable survey meter and run the meter up along the side of the vessel. The top of the level in the vessel will be at the point at which a substantial increase in radiation intensity is detected. Below that level, most of the radiation will be blocked by the process fluid.

Oil

FIG. 3.15l Neutron backscatter-type level or interface level switch has a repeatability of about 1 in. (25.4 mm).

The exception to this is the neutron backscatter gauge. This type of source emits fast neutrons that become slow neutrons after they have passed through or been backscattered by the process material. The neutron detector only “sees” slow neutrons; therefore, the source and the detector can be placed side by side without any interference (Figure 3.15l). The disadvantages of neutron systems are their high costs ($15,000 to $35,000 per point) and the fact that this investment pays for only a single point sensing level switch. When gamma radiation is used in a backscattered radiation sensor, a shield between the source and the detector is essential. These units are less expensive than the neutron backscatter switches but are also more limited in their capabilities. They cannot penetrate vessel walls that are thicker than 0.5 in. (12 mm) and must be mounted right next to the wall to minimize backscattering from the wall. This requirement makes it impossible to place thermal insulation between the wall and the source or the detector; therefore, these units are limited to cold tank applications.

BACKSCATTER DESIGNS Traversing Designs and Density Measurement Radiation does not always travel in a straight line. In any stream of radiation, there is a certain percentage of the stream that will strike some of the atoms of the material in its path so that part of the radiation stream will bounce back or backscatter. The percentage of the radiation that backscatters depends on how much material is in the path and how dense that material is. This method of detection has the advantage that both the source and the detector are on the same side of the vessel. However, one must make sure that there is enough shielding between the source and the detector so that only the radiation backscattered by the process material will be seen by the detector.


To provide continuous level detection based on backscattered radiation, it is necessary to move the source/detector assembly up and down the vessel. In earlier tank-farm level gauges, this was achieved by the operator manually moving a backscatter source/detector assembly up and down on the outside wall of the tank. Automatic traversing is achieved by installing a 2- to 4-in. (50- to 100-mm) dry well inside the tank and by mounting the source/detector assembly inside this well in the form of a traversing plumb-bob. This assembly is connected by a signal cable to the electronics, which are on


Detector Source Can Insert an Isolating Gate Shut-Off Valve Here

FIG. 3.15m Traversing radiation backscatter detector can monitor total and interface levels in addition to drawing a specific gravity profile. (Courtesy of Ohmart/Vega Corp.)

the top of the tank. This cable also serves to lift and lower the assembly under the control of a stepping motor (Figure 3.15m). Such a well-type installation tends to amplify the measurement signal, because the backscattered radiation is received from all around the pipe rather than from a single direction. This increased sensitivity allows the backscatter gauge to detect not only the level but also the density of the tank contents. Because of its density-sensing capability, the traversing backscatter detectors are well suited for interface measurement between two or more layers. If the output of the transmitter is sent to a DCS-based CRT, it can display the location of the different liquid layers, the thickness of the “rag” layers between them, and the density within each layer. Presently, the traversing backscatter units are limited to approximately 120°F (49°C) applications. Another variation


523

of this design is a two-dry-well arrangement in which one well contains the source and the other well the detector. Both the source and detector are scanned up and down in their respective wells to make a transmission measurement of the process between the wells. The required source size (10 mCi to 50 mCi) is much less than for external source units, because the radiation is received from all directions, and because there is no tank wall to look through. The inner surface of the dry well must be clean and smooth, and the pipe itself must be straight to protect against capsule hangup. These units can detect level ranges up to 150 ft (45 m) and are unaffected by tank diameter or wall thickness. ELECTRONICS The detector electronics can be integrally or remotely mounted. In the past, remote-mounted electronics were common. The detector output was a frequency or voltage signal that ran via wire to the remote-mounted electronics, which were generally mounted in the control room or rack room. Mounting the electronics in such a location normally eliminated the need and cost of hazardous qualified enclosures. The electronics received the detector output and provided, normally, a 4- to 20-mA DC output. As a result of costs associated with running field instrument wiring and allocating valuable control room space for field instrument electronics, the current trend is to have the electronics integrally mounted in the top of the detector housing. The detector electronics transmits a 4- to-20 mA DC signal directly to an end-user’s digital control system (DCS) or programmable logic controller (PLC). The 4- to 20-mA DC signal can be a simple 4- to 20-mA analog signal or a 4- to 20-mA DC HART protocol signal. The HART protocol signal superimposes a digital signal, used for calibration and diagnostics, on top of the primary process variable 4- to 20-mA DC signal. This output has proven to be very valuable for end-users, because communication is achieved anywhere along the 4- to 20-mA DC wiring. This can preclude personnel from having to enter a hazardous environment (safety or radiological) to communicate and troubleshoot a gauge. Communication is accomplished through HART universal handhelds, or personal computer electronic software packages from the detector manufacturer. Newer digital processor-based electronic designs can also provide a pure digital FOUNDATION fieldbus output to take advantage of new plant instrument wiring techniques associated with FOUNDATION fieldbus devices. CONCLUSIONS AND TRENDS A trend to expect is continued evolution in digital electronics, as previously discussed, for preventive maintenance considerations. Digital processors hold promise for more complex signal processing, resulting in higher measurement accuracy and stability.

524

Level Measurement

100000

10000

Recuction Factor (NB)

1000

100

+ + +

10 + + + + + + + ñ

1 0

0.5

+ ñ

ñ

1.5

2.0

+ ñ 1.0

Steel Cs-137 Water Co-60

ñ

ñ

2.5

ñ

3.0 3.5 Thickness (Inches)

Lead Cs-137 Steel Co-60 Cement Cs-137 ñ Cement Co-60

ñ

4.0

ñ

4.5

Lead Co-60

ñ

5.0

ñ

5.5

ñ

6.0

6.5

Water Cs-137

FIG. 3.15n Radiation reduction (nb) as a function of the source material, the material, which the narrow beam radiation is passing through and of the thickness of this material. So, for example, if the source is Cs-137, which is passing through a 1″-thick steel plate, nb = 4 and therefore the field intensity will be reduced to 25%.

Detector technology will continue to migrate away from ion chambers to more scintillation-based technology. Higher sensitivity and improved stability of detectors can result in lower radiation field requirements, i.e., smaller-activity isotopes. A very important step in the use of radiation level sensors is the proper sizing of their sources, which generate the


gamma radiation at the detector. As the radiation passes the various layers of process and vessel wall materials, it is attenuated to different degrees. To accurately determine the required source size, reliable information is needed. Such information is provided in Figure 3.15n. Radiation level detection continues to be very appealing for hard-to-handle, toxic, and corrosive processes, because


it does not require vessel wall penetrations. Costs and licensing requirements do limit the number of applications but are not serious impediments to carefully designed systems. Although other level technologies are improving and may erode the number of potential nuclear applications, radiation level detectors continue to solve many critical process level applications.

Bibliography Adams, W. L., The statistics of radiation gaging, InTech, September 1968. Bacon, J. M., The changing world of level measurement, InTech, June 1996. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June–July 1997. Boyes, W. H., The changing state of the art of level measurement, Flow Control, February 1999. Carsella, B., Popular level-gauging methods, Chemical Process., December 1998. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001.


525

Hazard free analysis, Processing, October 1989. Holzschuher, P., Gamma level, Meas. Control, October 1991. Johnson, D., Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001. Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Loftin, R. L., Nuclear level gaging, Instrum. Control Sys., March 1966. McConnell, J. A. and Smuck, W. W., Gamma back scatter technique for level and density detection, Chemical Eng. Prog., August 1967. McKinney, A. H., Radiation techniques for process measurements, Chemical Eng. Prog., September 1960. Paris, T. and Roede, J., Back to basics, Control Eng., June 1999. Parker, S., Selecting a level device based on application needs, in 1999 Fluid Flow Annual, Putman Publishing, Itasca, IL, 1999, 75–80. Paul, B. O., Seventeen level sensing methods, Chemical Process., February 1999. Rowe, S. and Cook, H. L., Nuclear gages for density and level control, Chemical Eng., January 27, 1969. Sholette, W., Pick the proper level measurement technology, Chemical Eng. Prog., October 1996. Thomason, E. M., Design procedure for nuclear level gages, Instrum. Technol., June 1968. United States Nuclear Commission Booklet, NUREG/BR-0133 Revision 1. Williams, J., Tips on nuclear gaging, Instrum. Control Sys., January 1979.

3.16

Resistance Tapes D. S. KAYSER

(1982)

LI Tape

A. D. EHRENFRIED

B. G. LIPTÁK

(1995)

(1969, 2003)

Flow Sheet Symbol

Applications

Liquids including slurries but generally not solids; in addition to level, can measure temperature (see Table 3.16f)

Resolution

0.125 in., which is the distance between helix turns

Actuation Depth (AD)

The depth required to short out the tape varies with the specific gravity of the process fluid (SG) as shown in Figure 3.16c. AD (in inches) = 4/(SG). Therefore, AD at the minimum SG of 0.5 is 8 in. (200 mm). In newer designs, the AD is 5 in. at SG = 1.0.

Temperature Effect

A 100°F (55°C) change in temperature will change the resistance of the nonshorted tape by 0.1%. Temperature compensation is available.

Inaccuracy

0.5 in. if the actuation depth is zeroed out and both AD and temperature are constant. If SG varies, a zero shift based on AD = 4/(SD) will occur. Cold temperature also increases the AD.

Wetted Material

Fluorocarbon polymer film (see Table 3.16f)

Allowable Operating Pressure

From 10 to 30 PSIA (0.7 to 2.1 bars absolute)

Operating Temperature Range

−20 to 225°F (−29 to 107°C)

Costs

The resistance tape unit cost varies with services and tape length. A 10-ft (3-m) tape with breather and transmitter for water service costs from $800 to $1500. The added cost for longer tapes is $30 to $40 per foot, depending on service.

Suppliers

Metritape Inc. (www.consiliumus.com) R-Tape Corp. (www.rtape.com) Sankyo Pio-Tech (www.sankyo-piotech.co.jp)

Resistance tape for continuous liquid-level measurement was invented in the early 1960s, initially for water well gauging and subsequently for marine and industrial usage. The sensor is a flat, coilable strip (or tape) ranging from 3 to 100 ft (1 to 30 m) in length, suspended from the top of the tank. It is small enough in cross section to be held within a perforated pipe (2 to 3 in. diameter), which also supports the transducer and acts as a stilling pipe if the process is turbulent. Within the sensing tape, shown in Figure 3.16a, is a goldfinished nichrome resistance wire, which is helically wound around a stainless-steel base strip and insulated on the edges and back. It is provided with a gold contact stripe on its uninsulated front face. The wounded helix is joined to the conducting base strip at the bottom, and lead wires are brought from the base strip and the top of the wound helix through the sensor top end. A compliant sheath, commonly of fluorocarbon film, 526 © 2003 by Béla Lipták

Protective Channel (Optional)

Corrosion Resistant Jacket Envelope

Wound Resistive Helix

Base Strip

Translucent Heat-Sealed Jacket (Teflon)

FIG. 3.16a Construction of resistance tape level sensor.

Insulation Gold Contact Stripe

3.16 Resistance Tapes

To Breather/Equalizer Sensor Resistance (R) to Readout Device

527

Actuation Depth Inch (mm)

Base Strip Sealed Outer Jacket Liquid Surface

Resistance Helix Unshorted

8 (200) 6 (150)

AD = 4 Inch SG

4 (100)

Actuation Depth (AD) Helix Shorted Below Surface

2 (50)

0

1

2

3

4

Specific Gravity

5

FIG. 3.16c The relationship between actuation depth and specific gravity. FIG. 3.16b Schematic diagram of resistance tape sensor operation.

encloses the helically wound inner sensor and acts as an isolation barrier against the infiltration of liquid and vapor. When surrounded by the process liquid or slurry, the flexible envelope is flattened by its hydrostatic pressure as shown schematically in Figure 3.16b. This presses the resistance helix winding into the gold contact stripe on the base strip.

instrument or accept the resulting error (Figure 3.16c). The actuation depth usually does not change with aging of the tape, although it does increase as the ambient temperature turns cold, as a result of the stiffening of the sheath material. If it is desired to have the tape read the liquid level all the way down to the bottom of the tank, a sump must be provided at that location to extend the resistance tape below the tank’s bottom.

PRESSURE EFFECT ACTUATION DEPTH Because some pressure is required to compress the jacket and thus to short the resistance leg, the tape cannot be shorted out all the way up to the surface of the material. The uppermost electrical contact is made some distance below the surface of the liquid, and this distance, the actuation depth, is a function of the density of the process material. For water, the actuation depth is approximately 4 in. (100 mm); for lighter materials, it is increased in proportion to its reduced density. The recommended minimum specific gravity is 0.5. At the time of calibration, this known, and constant offset should be zeroed out. Because of this zero offset, levels cannot be detected below its value. Sensor resistance, R, measured across the two lead wires, changes 1 Ω for each millimeter of level change and corresponds to the length of unshorted helix above the liquid. An ohmmeter measurement can indicate the distance from tank top down to the point of shorting. Sensor resistance R can be set to equal zero when the tank is full and can equal the full resistance of the helix winding at a liquid level that is low enough that all helix contacts are relieved of pressure and open. Actuation depth is a function of the sum of the spring rate of the winding and of the jacket, and it causes the uppermost helix contact to lag below the liquid surface by about 100 mm of cold water (SG = 1.00). The gauge is zeroed on the basis of the average anticipated SG anticipated for the particular process. If the specific gravity varies, one can either rezero the


To maintain accuracy, the pressure inside the tape jacket must equal the pressure in the vapor space of the tank. For atmospheric tank applications, this is accomplished by venting the tape interior to the atmosphere through a small desiccant dryer (Figure 3.16d). For tanks under pressure or vacuum, Dessicant Cartridge Contains Cleaning & Drying Chemicals

Vapors From Inside Metritape Sensor Sensor Housing

Capillary Breather Coil Long, Small Bore Tubing

Pneumatic Snap Coupling Sensor-toBreather

Ring Nut

In Atmospheric Tanks Breather Tube can be Left Open as Shown Connection to the Vapor Space Pressure in The Tank

Sensor Mounting Nipple

Metritape Sensor Head Tank Top Tank Internal Pressure/Vacuum

FIG. 3.16d Capillary breather/equalizer assembly.

528

Level Measurement

the tape’s internal and external pressures must be equalized. This can be done by installing a direct-connected equalizing line or by mounting a one-to-one pressure repeater on the tank and tubing the output of the repeater to the vent connection on the tape. However, care must be taken to keep moisture and other contaminants from getting inside the tape jacket. The sealed sensor sheath is vented at the top through a capillary breather/equalizer assembly. This allows the inner resistance-tape sensor to “exhale” as tank level rises and “inhale” as tank level falls. Air expelled from the sensor is cleaned and dried as it passes through the desiccant cartridge and is held in the capillary breather coil, which acts as a reservoir. When the liquid level falls and some of the sensor contacts open up, the clean, dry air is returned to the inner sensor. At the same time, the pressure inside the previously extended breather tubing is returned to the tank pressure, thereby equalizing the inner sensor chamber to match the pressure surrounding the sensor. This allows resistance tapes to operate in pressurized or evacuated tanks. A typical installation of the resistance tape sensor inside a protective still pipe is shown in Figure 3.16e. The topmounted housing has a 1.5-in. (38-mm) diameter threaded nipple that is welded in the bottom and to which the sensor head is secured and sealed with a slip nut. The housing serves to hold the sensor head, the capillary breather, the lead-wire connections, and an optional loop-powered transmitter. The stilling pipe can be perforated or slotted if the viscosity or solids content of the process material requires it. A vent hole at the top is always provided. The sensor may be edgeclamped to allow unobstructed gauging of waves or to detect sewage or slurries having very high solids content. It can also be cable-suspended in deep wells or caverns.

Dessicant Cartridge Capillary Breather

Cover Housing Slip Nut

Return to Atmosphere

Sensor Head

Return to Tank Pressure Nozzle/Deckstand Tank Top

LS Sensor Length

Stillpipe Vents 1" Dia. 2 Holes LP Stillpipe Length

Stillpipe Resistance Tape

Tank Bottom

FIG. 3.16e Typical mounting of resistance tape sensor, housing, and still pipe on tank-top nozzle.


Whereas tank pressure or vacuum is directly equalized through the capillary breather/equalizer, the operating range of equalization is limited by the ability of the standard breather to contain pressure or vacuum without leakage. Resistance tape gauges are used in liquid storage tanks that are vented or pressured by an inert gas blanket. The capillary breather/equalizer requires annual inspection for color change in the desiccant cartridge indicator. Replacement of this component every 3 to 5 years is recommended to prevent corrosion of internal sensor contacts. The outer sheath of the resistance tape is wetted by the process material and must resist corrosion and permeation. A laminated polyester sheath is used for water and wastewater applications, and a sheath of heat-sealed fluorocarbon film can be added for use in a wide range of liquid applications, including many chemicals and solvents. Certain chemicals,  including halogen acids, Freon , ammonia, and halogenated hydrocarbons permeate this fluorocarbon film. Thus, their level cannot be measured by this design of resistance tapes. The resistance tape may be provided with built-in temperature detectors for in situ temperature measurement. Combined measurements of liquid level and temperature are accomplished by mounting 1 to 3 resistance temperature detectors at selected elevations on the back of the inner sensor and bringing additional lead wires out the sensor top end. A single tank penetration and cable run can thus serve both level and temperature measurement functions.

TEMPERATURE AND OTHER EFFECTS The unit resistance or resistance gradient, Rg, of a standard tape is 305 Ω/ft, and its temperature gradient, Tg, is 40 ppm/°C. Therefore, if we have a 20-ft. tape in a vessel that is half full (length L of the unshorted tape is 10 ft), and if we want to calculate the error caused in the level measurement if the process temperature changed by ∆T = 100°C, the calculation described by Equation 3.16(1) has to be made. 6

E100 = (Tg)(L)(∆T)/12 = (40/10 ) × 10 × 100/12 = 0.48 in. 3.16(1) A built-in temperature detector can be used to compensate for such shift. Resistance tape devices are limited to moderate temperature applications up to 225°F (107°C) so as to stay within the corrosion and permeation capability of the outer fluorocarbon sheath. Resistance tape cannot operate in a zero-gravity field and cannot be used for liquid–liquid interface measurement. On the other hand, it does have high resistance to mechanical shock and vibration and is therefore well suited for military and seismic services. Resistance tapes are made in standard lengths of integral feet. The height of the stand-off nozzle at the top of the tank can be adjusted to lift the sensor bottom end to 2 in. above tank bottom.

3.16 Resistance Tapes

529

TABLE 3.16f Tape Selection Chart (Courtesy of Consilium US Inc.) Service

Sensor Type

Wetted Materials

Temperature Range

Water, wastewater, sewage

Aquatape

Polyester, epoxy, polypropylene, stainless steel

5° to 140°F (−15° to 60°C)

3–50 ft (1–15 m)

Atmospheric only

Crude oil, petroleum products

Petrotape

Hastelloy C276, Nylon 12, glass-filled polypropylene



5° to 225°F (−15° to 107°C)

3–100 ft (1–30 m)

±2 PSI

Chemicals, solvents

Chemtape

Hastelloy C276 or Teflon, polypropylene



Hastelloy : 5° to 225°F (−15° to 107°C) Teflon: 5 to 140°F (−15° to 60°C)

3–100 ft (1–30 m)

±2 PSI

Water, wastewater, sewage

LA

Polyester, epoxy, glass-filled polypropylene, stainless steel

5° to 158°F (−15° to 70°C)

3–100 ft (1–30 m)

±15 PSI (±1 atm)

Crude oil, petroleum products

LA-HN

Hastelloy C276, Nylon 12



5° to 225°F (−15° to 107°C)

3–100 ft (1–30 m)

±15 PSI (±1 atm)

Chemicals, solvents

LA-HP

Hastelloy C276, polypropylene



−20 to 225°F (−30 to 107°C)

3–100 ft (1–30 m)

±15 PSI (±1 atm)

LA-AF

FEP Teflon, glass-filled polypropylene

The resistance helix winding is approved as being intrinsically safe for use in hazardous, explosive liquid, and vapor applications (Class 1, Div. I, Groups A, B, C, and D) and in dust and grain applications (Groups F and G).

CONCLUSION Resistance tapes measure liquid and slurry levels with only one moving part. The movement is caused by the process medium and produces a resistance output that is stable and independent of most liquid properties. This gauge is suitable for the measurement of levels in liquid storage tanks, sumps, and streams if the pressure is near atmospheric and the temperature is near ambient. It can handle corrosive and slurry-type materials (see Table 3.16f). However, because of limitations caused by actuation depth variation, pressure equalization, and dryer maintenance, their applications should be chosen very selectively.

Bibliography Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June-July 1997. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Cornane, T., Continuous level control, Measurement and Control, April 1997. Ehrenfried, A. D., A Guide to Metritape Gauging, Metritape Inc., Littleton, MA, July 1987.




Level Range

Pressure Range

Ehrenfried, A. D., Level Sensor Key to Dispersed Plant Operation, in Sensors, Helmers Publishing, Peterborough, NH, December 1987. Ehrenfried, A. D., Resistive metritape level/temp gauge for marine closed tank service, in Proc. Second International Conference on Marine Transportation, Gastech Ltd., Monte Carlo, March 1979. Gauging problem liquids with resistance-tape level sensor, in Sensors, Helmers Publishing, Peterborough, NH, August 1989. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Johnson, D., Level sensing in hostile environments, Control Engineering, August 2001. Massachusetts firm has level gauging taped, in Marine Log, SimmonsBoardman, Omaha, NE, October 1989. Metritape Inc., The resistance-tape liquid level sensors, Shipping World and Shipbuilder, Marine Publication International, UK, September 1990. Monitoring liquid level in underground tanks, InTech, ISA Services, Inc., September 1991. Noltingk, B. E., Instrumentation Reference Book, 2nd ed., ButterworthHeinemann, Oxford, UK, 1996. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Parr, E.A., Industrial Control Handbook, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1995. Randall, C., Four case histories: liquid level sensing under extreme conditions, in Sensors, Helmers Publishing, Peterborough, NH, 1991. Randall, C., Level sensing: choosing the course of most resistance, InTech, December 1989. Randall, C., Metritape resistance-tape level sensors help to modernize fuel handling system at Tulsa International Airport, Aviation Ground Equipment Market, Jane’s Information Group, Surrey, UK, November 1989. Resistance-tape slurry level gauging contributes to the mineral concentration efficiency of flotation columns in the mining and milling industry, Can. Process Equipment and Control News, August 1989. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

3.17

Rotating Paddle Switches D. S. KAYSER

(1982)

B. G. LIPTÁK

(1969, 1995)

LS

W. H. BOYES

To Receiver

(2003)

Flow Sheet Symbol

Applications

Solids level switches

Design Pressure

From full vacuum to 100 PSIG (6.9 bars = 0.69 MPa)

Design Temperature

Units with air-cooled shaft extensions can be used on fly ash precipitators where temperatures can range from 450 to 1600°F (232 to 870°C)


Wetted parts can be made of aluminum, steel, or stainless steel or may be lined with  PVC, Kel-F , or Teflon

Housings

Weather-tight or explosion-proof

Inaccuracy

Actuation on the same process material is repeatable within 1 in. (25 mm)

Cost

From $200 to $1200


Bindicator Div. of Venture Measurement Inc. (www.bindicator.com) BinMaster (www.binmaster.com) Conveyor Components Co. (www.conveyorcomponents.com) Monitor Technologies LLC (www.monitortech.com) Nohken, Sick-Maihak (www.maihak.de)

INTRODUCTION Solids level measurement is made difficult by the wide range of solids properties involved. The densities of some pulver3 3 ized materials can be as low as 5 lb/ft (80 kg/m ) while other, sometimes lumpy materials can have densities in excess of 3 3 100 lb/ft (1600 kg/m ). The main purpose of detecting the level of solids is to signal the breakdown in continuous solids flows, which usually occurs as a result of either arching or bridging (the blockage of flow due to the segregation and collection of larger-sized particles) or flushing and “ratholing” (the sudden dumping of large quantities as arches collapse). Tank and bin vibrators are used to maintain uninterrupted solids flows. Some materials (e.g., zinc oxide, lampblack, soda ash, activated carbon, titanium oxide, and hydrated lime) have a higher tendency to arch (bridge). Granular 530 © 2003 by Béla Lipták

materials such as stone, grain, sand, and coal tend to slide more when moist or when the coarse particles are intermixed with finer particles. The design of feeders and bins must take into consideration the flowing and arching properties of the solids. These properties are usually expressed in terms of the angle of repose. The angle of repose is that angle with the horizontal at which the solids will not yet slide off the inclined plane. When this angle is low (20 to 30°), such as with rye, barley, oats, or soybeans, it is relatively easy to transport these solids; however, when the angle of repose is greater than 45°, such as with pulverized coal or pulverized phosphate, the problems of moving these solids becomes more difficult. If the angle of repose varies in a particular bin, the high-level switch should be located based on the maximum possible angle, and the low-level switch should be located based on the minimum.

3.17 Rotating Paddle Switches

531

Switch

Gear Torque Rod

Base Casting

Torque Tube

Clamp Torsion Sleeve Assembly

Wire Paddle

FIG. 3.17a Rotating paddle switch schematic.

ROTATING PADDLE SWITCHES The rotating paddle-type level switch is used to detect the presence or absence of solids in a silo. A small, geared, synchronous motor keeps the paddle in motion at very low speed. When solids are absent, there is no torque on the paddle drive assembly. When level rises to the paddle, it is stopped, and torque is applied to the drive assembly. Detection of the torque is used to actuate a switch that, in turn, can be used for alarm or for control of silo filling or emptying equipment. As shown in Figure 3.17a, one method for torque detection uses a modification of the displacer-type torque tube. When solids are not present (and thus there is no torque on the tube), the entire drive assembly rotates at the speed of the gear in a counterclockwise direction. With solids present, the paddle stops, torque develops on the tube, and the torque rod is forced clockwise relative to the rest of the assembly. This clockwise motion rotates a beveled cam that rises to


FIG. 3.17b Paddle switches can be provided with either flat flanges or flexible mounting plates. (Courtesy of Monitor Technology Inc.)

operate the switch. In some designs, the motor is rated to operate in a continuously stalled condition. In others, the motor is switched off and does not restart until the springopposed torque detector returns to the zero-torque position. A number of paddle designs are available; larger paddle areas are required to generate sufficient torque in lower bulk density materials. Conventional four-blade paddles, 6 in. (152 mm) wide by 2 in. (50 mm) high, are used in materials with 3 3 densities below 20 to 30 lb/ft (320 to 480 kg/m ), whereas vane and wire designs are used in heavier materials. The latter types can be installed through a coupling on the silo wall. The larger paddle designs are furnished with a plate to cover the silo nozzle or cutout through which the paddles are inserted (Figure 3.17b). Pressure and temperature ratings vary, depending on the unit selected. Standard units have pressure ratings from 7.5 PSIG (52 kPa) to 30 PSIG (207 kPa). When the switch is installed on a pressurized silo, the electrical conduit should be sealed to prevent accumulation of dust in the conduit system in the event that the torque tube or seal ruptures. A common temperature rating for standard units is −30 to 200°F (−34 to 93°C) including the switch and motor housing, although higher ratings are available. These level switches will give repeatable performance on the same solids within 1 in. (25 mm), but their mounting location should be carefully selected, giving consideration to the possible variations in the angle of response. High-level switches have to be lowered in the bin as the angle of repose rises. Installations Figure 3.17c illustrates several paddle designs and some methods of installing these switches on silos. Switch locations and immersion lengths are selected after determining the angle of repose of the material and how the orientation and location of the inlet and outlet nozzles will affect average level. Unit A shows a wire design mounted in the side of a silo that may be in the path of falling material. The protective baffle is installed to prevent spurious trips. Unit B shows a conventional top-mounted paddle design with a shaft guard. Unit C is also top mounted and illustrates a rectangular vane design.

532

Level Measurement

B

Paddle switches have the advantage of many decades of application history on different types of services. Their limitations involve the possibility of their rotation being prevented by dirt or rust, their relatively low pressure ratings, and their limited availability for corrosive services.

C

Shaft Guard Protective Baffle

Bibliography

A

D

FIG. 3.17c Rotating paddle solids level switch.

This design, used for heavier material, is located so that the vane will be pinned against the silo wall by the rising level. Unit D is located for low-level detection and depicts a vane design that is suitable for operation in low-density materials.


Andreiev, N., Survey and guide to liquid and solid level sensing, Control Eng., May 1973. Belsterling, C. A., A look at level measurement methods, Instrum. Control Sys., April 1981. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001. Hall, J., Measuring interface levels, Instrum. Control Sys., October 1981. Johnson, D., Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001. Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Lipták, B. G., On-line instrumentation, Chemical Eng., March 31, 1986. Parker, S., Selecting a level device based on application needs, in 1999 Fluid Flow Annual, Putman Publishing, Itasca, IL, 1999, 75–80. Paul, B. O., Seventeen level sensing methods, Chemical Process., February 1999.

3.18

Tank Gauges Including Float-Type Tape Gauges LT

D. S. KAYSER

(1982)

B. G. LIPTÁK

(1969, 1995, 2003)*

To Receiver Flow Sheet Symbol

Applications

Tank-farm liquid level detection for accounting purposes and solids level sensing

Types

A. Float operated, wire-guided, inductively coupled B. Servo-operated float type C. Surface detector (plumb-bob) gauges D. Radiation backscatter design (see Section 3.15) E. Radar tank gauges (see Sections 3.13 and 3.14) F. Hydrostatic tank gauges, HTGs (see Section 3.6) G. Hybrid tank gauges

Design Pressure

Surface-detecting solids gauges are used up to 5 PSIG (0.34 bars), standard wireguided tape gauges are used at 30 PSIG (2 bars), and special designs can be used up to 300 PSIG (20.6 bars)

Design Temperature

Solids level sensors can operate from −4 to 176°F (−20 to 80°C), and tape gauges on liquid tank farms can handle from cryogenic services to up to 300°F (150°C)


Mounting flange and housing can be aluminum; wetted parts can be made of aluminum, steel, stainless steel, nylon, PVC, polyethylene, and other plastics or higher alloys.

Electrical Safety

Tape gauges for liquids can be all mechanical or explosion-proof. Inductively coupled float designs are intrinsically safe. The solids level detector plumb-bobs are available in explosion-proof housings.

Range

Standard wire-guided tape gauges are available up to 100 ft (30 m); plumb-bob-type surface sensors are available up to 200 ft (60 m).

Inaccuracy

Error in solids sensors is about 0.2 ft (61 mm); for liquid service automatic tank gauges (ATG) refer to Table 3.18a.

Cost

Plumb-bob solids level sensors start at about $2000. For liquid service automatic tank gauges, refer to Table 3.18a.


For the radiation backscatter design, see Section 3.15; for radar tank gauges, see Sections 3.13 and 3.14; for hydrostatic tank gauges, see Section 3.6. Bindicator (www.bindicator.com) (C, solids) BinMaster (www.binmaster.com) (C, solids) Endress+Hauser Systems & Gauging (www.systems.endress.com) (A, B, E, F) Enraf Inc. (www.enrafinc.com) (A, B, C) Krohne Inc. (www.krohne.com) (A, B, E) L & J Technologies (www.ljtechnologies.com) (A, B, C, solids) Monitrol Manufacturing Co. (www.monitrolmfg.com) (A, B, C, E, F)

* I would like to give particular thanks to Frank J. Berto for his many and invaluable inputs on the subject of automatic tank gauging.


534

Level Measurement

MTS Systems Corp. (www.mtssensors.com) (A, B) Rosemount-Varec (A, B) Saab Rosemount Tank Control (www.saabradar.com) (C, E)

In this section, we concentrate on types of automatic tank gauge (ATG) designs that are not detailed in the other sections. The radiation backscatter gauges are discussed in Section 3.15, the radar-type tank gauges in Sections 3.13 and 3.14, and the hydrostatic tank gauges in Section 3.6. For that reason, they will be mentioned only briefly. HISTORY OF CUSTODY TRANSFER Tank-farm level measurement, particularly in the oil industry, has been the basis for buying and selling products on a volumetric basis. In the 19th century, oil could not be measured more accurately than about 5%, so producers agreed on the size of the 42-gal barrel, thereby making sure that there would be at least 40 gallons in every barrel. A hundred years later, the precision of custody transfer improved to about 0.5% and, today, if every error source except nonuniformity in the tank’s cross section is carefully eliminated, the error will be about 0.25%. When oil is sold, it can be sold by weight or volume. If sold by the volume, it can be metered or sold on the basis of level measurements. The more advanced and most accurate method is flow metering (see Sections 2.19 and 2.25), which definitely should be used when transferring smaller volumes, and the more traditional is level measurement. One can measure the level manually (which involves climbing to the top of the tank) or automatically, and one can detect the drop in the level of the supply tank (outage) or the rise in the level of the receiving tank (innage). As shown in Table 3.18a, some automatic tank gauges (ATGs) are better suited for outage detection, and others for innage sensing. When measuring outage, we are detecting the distance from the top of the tank to the oil surface. Multiplying this distance times the cross-sectional area of the tank gives the volume of the vapor space, called ullage. Outage detection is less accurate

than innage, because the outage distance has to be compared to a reference height (the mounting location of the gauge), which, if the ATG is supported from the tank shell or roof (Figure 3.18b), varies with oil depth, temperature, and age. Therefore, if the ATG error is 0.125 in., but the gauge mounting can travel 0.5 in., the actual error can reach 0.625 in. This, if the transfer volume is small (e.g., a 2.0-in. change in level), makes the measurement meaningless. The best way to minimize the variation in reference height is to mount the ATG on a properly supported and slotted gauging well. Figures 3.18c and 3.18d illustrate the bottom-supported wells used on floating and fixed roof tanks, whereas Figures 3.18e and 3.18f describe the installation of shell-supported wells on floating and fixed roof tanks. As shown in Table 3.18a, HTGs and most smart cable ATGs detect innage by measuring the distance from the tank bottom to the liquid surface. Here, if the tank is on a solid surface so that its bottom does not move, there is no reference height, because the volume of liquid in the tank is determined by multiplying the innage distance with the cross-sectional area of the tank. 1

TANK GAUGE DESIGNS

As can be seen from Table 3.18a, the largest number of existing ATGs are the float-operated tank gauges, which have been used for more than 50 years. They are the least expensive and least accurate and were developed to reduce the need for the operator to climb the tank for manual “dipping.” Here, as shown in Figure 3.18g, a perforated tape runs up from the float to the top of the tank and then down to the gauge head. For this reason, the float-operated ATGs double the reference height error, because the gauge head is mounted at grade. They also require high maintenance because of the moving parts, although the newer designs have fewer of such parts.

TABLE 3.18a Features of Automatic Tank Gauges (Based on Reference 1) Number in Use in USA

Cost (for 40-ft floating roof tank w/temp. measurement)

It Measures

Accuracy

Float

300,000±

$4000±

Outage

Low

Highest

Servo

10,000±

$6500±

Outage

Good

High

Radar

5000±

$8500±

Outage

Good

Low

HTG

5000±

$8500±

Innage

Low

Low

Smart Cable

5000±

Note 1

Innage

Good

Varies

200±

Note 2

Note 2

Note 2

Note 2

Type of Design

Hybrid

Maintenance Required

Note 1. Costs vary for the different types of Smart Cable systems. A magnetostrictive system for a 40-ft high floating roof tank costs $3500±, including average temperature measurement. Note 2. The cost, measurement method, accuracy, and maintenance of a Hybrid ATG depends on the level and temperature measurement system. A pressure transmitter adds $1500± to the cost of the system.


3.18 Tank Gauges Including Float-Type Tape Gauges

Spring Loaded Top Anchor

535

Automatic Tank Gauge

Typ. Weld

Stilling Well Sliding Guide Guide Pipe

A

A

Slotted Stilling Well

Float or Displacer if Applicable

Snap Type Fastener Typical 18 to 30 Inches 15" Dia. Height to Suit Eye Level Grade

Datum Plate

FIG. 3.18d Bottom-supported gauging well for fixed roof tank.

Weld U Bolt View A−A

FIG. 3.18b Wire-guided float detector installation for low-pressure tanks.

Automatic Tank Gauge Stilling Well Sliding Guide

Slotted Stilling Well Automatic Tank Gauge Pontoon

Stilling Well Sliding Guide Slotted Stilling Well

Float or Displacer if Applicable 18 to 30 Inches

Pontoon Float or Displacer if Applicable Typical 18 to 30 Inches

FIG. 3.18e Shell-supported gauging well for floating roof tank. Datum Plate

FIG. 3.18c Bottom-supported gauging well for floating roof tank.

In Europe, during the last decades, servo-operated tank gauges been used for custody transfer level measurement to detect the outage. They, too, should be mounted on a properly


supported slotted gauging well to minimize the error caused by reference height movement. Their maintenance has also been improved during the past five years by the development of a new gauge head with fewer moving parts. As was discussed in Sections 3.13 and 3.14, radar tank gauges can also be used for custody transfer level measurement. They, too, measure outage, so they should also be mounted on a properly supported slotted gauging well. They have no moving parts, so their maintenance is relatively low.

536

Level Measurement

Transmitter

Automatic Tank Gauge

Storage Vessel

Stilling Well Sliding Guide

Float

Product Level Multiple Temp Sensors

Slotted Stilling Well

Pressure Sensors

Float or Displacer if Applicable

Interface Level

18 to 30 Inches

FIG. 3.18h Level, volume, mass, density, interface, pressure, and temperature can be monitored by the inductively coupled tape system. (Courtesy of the former Sarasota Measurements and Controls.)

Datum Plate

FIG. 3.18f Shell-supported gauging well for fixed roof tank.

Input Shaft

Sprocket Sheave

Float

Foot Wheel

Code Tracks

Inch Wheel

To Gauge Head

FIG. 3.18g Schematic of foot and inch wheel drives.

As discussed in Section 3.20, sonic or ultrasonic tank gauges also measure an echo, but their accuracy is affected by vapor above the product, and they have not been widely used for tank measurement in the oil industry. As already discussed in connection with Figure 3.6e, hydrostatic tank gauges (HTGs) were developed to convert from volume measurement to the detection of mass. HTGs provide mass measurement, limited only by the accuracy of the pressure transmitters and the tank strapping tables. They are not affected by reference height variation, because they measure innage, but the accuracy of their level measurement drops if the tank contents are temperature or density stratified. The smaller and less expensive hydrostatic interface units (HIUs) represent an improvement over the HTGs.


If a cable containing the level-measuring element runs from the bottom to the top of the tank, it can be called a smart cable. Most types have a float that rides up and down the cable. The best types measure innage, so their accuracy is not affected by changes in reference height, they do not require gauging wells, and (as illustrated in Figure 3.18h) they provide a series of resistance bulbs in the cable to simultaneously measure average temperature. Capacitance tank gauges use two capacitance plates. Capacitance varies with level, because the product dielectric constant differs from that of the air or vapor. Inductive tank gauges measure level using a digital position signal generated by the inductive interaction with a transponder in the float (Figure 3.18i). Magnetostrictive tank gauges measure the time of flight of a torsion wave that pulses up and down a ferromagnetic wave guide, where the wave is reversed by a magnet in the float. Resistive tank gauges (see Section 3.15) use a nichrome helix wrapped  around a steel core and covered with a Teflon jacket. The hydrostatic pressure of the product shorts the helix against the core so that the resistance varies with the product level. Hybrid level gauges (as illustrated by Figure 3.6e) are still evolving and, in addition to level, can also detect volume, mass, temperature, and density. In this configuration, level and volume are measured by the ATG, and mass is measured by the pressure sensor. The design is still evolving. Hybrids can measure density without sampling or laboratory analysis. Because of the redundant measurements, they also provide some error checking. ACCURACY 1

According to Berto, quantitatively, the following error contributions can be expected: 1.

The error in manual gauging or the ATG is about ±0.125 in.


537

Pole Teflon

2. Pole Wiper 1. Cover Gasket Rim Seal

Pontoon Polyolefin Excitation Loop

Tank Shell

Conductors (in Gray Code Pattern) Well Signals Induced into Conductors

3. Internal Sleeve

FIG. 3.18j 1 The improved slotted gauge pole.

Transponder Held in Proximity to Tape by the Float Conductors #4 #3 #2 #1Ref

Return

Gray Code

Increment 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

1111 1110 1100 1101 1001 1000 1010 1011 0011 0010 0000 0001 0101 0100 0110 0111

Left

Right

FIG. 3.18i The inductively coupled tape and float assembly and the arrangement for a 16-increment system. (Courtesy of the former Sarasota Measurements and Controls.)

2.

3.

The error in the tank capacity tables, including the effect of tank tilt and hydrostatic pressure, is about ±0.5 in. Tank shells expand as a result of liquid head as a full tank takes the shape of a barrel. This effect is normally included in the calculation of the tank tables,


but the accompanying movement of the lower datum plate or the upper reference point is not included. 4. Tank bottom movement results in undermeasurement (+0.25 in.). This varies with the compressive strength of the soil under the tank. 5. Incrustation may be significant with heavy or waxy products stored in small tanks. It results in overmeasurement. 6. Movement of the gauging well can be ±1 in. Floating roof tanks should be fitted with slotted gauging wells (Figure 3.18j). Vertical movement of the gauging well movement affects outage measurements and causes an error when converting measured outage to innage. 7. Datum plate movement affects innage measurements by ±0.5 in. When a tank is filled and the shell takes a barrel shape, the bottom bulges up adjacent to the shell. Farther inward, the bottom moves down because of hydrostatic pressure. The datum plate should be located 18 to 30 in. from the shell to minimize the effect of bottom movement. 8. Thermal expansion of the tank shell and the gauging well can amount to ±0.125 in. Thermal expansion causes two errors, because both the tank diameter and the tank height change. Tank capacity tables are calculated for one temperature (60°F). They do not correct for the thermal expansion of the tank shell. The amount of error depends on the product temperature and the ambient temperature. 9. Another serous source of errors is poor temperature measurements. With heavy oils, it takes 45 min for a manual temperature measurement using a cup-case, even if the thermometer is continuously moved up and down. This can result in 2 to 3° of error. ATGs are provided with high-performance average temperature sensors, but at a cost of about $2000. 10. In crude oil level measurement, a major error source is the method used in determining the sediment and water content of the oil. Manual sampling is unacceptable,

538

Level Measurement

because it does not provide a representative sample and does not necessarily analyze the sample correctly. Therefore, automatic samplers are recommended. TRADITIONAL TAPE LEVEL SENSORS Tape level detectors can be furnished with local or remote readouts and can facilitate inventory control in multiple tanks and silo installations. Liquid level detector tape gauges include the conventional tape gauges (a wire-guided float or displacer), the inductively coupled float, and the wire-guided thermal sensor. The surface-sensing tape gauges (plumbbobs) used in solids service have a resolution of about 0.1 ft (30 mm) and an error of about 0.2 ft (60 mm). This accuracy on solids level measurement is usually acceptable, as other variables (such as changes in the angle of repose, bridging, rat-holing, and the change in the shape of solids level as the operation is changed from filling to discharging) all cause errors in the correlation between level and volume. As a result of these uncertainties, very precise measurement of the level at a particular point is of no great value. WIRE-GUIDED FLOAT DETECTORS Figure 3.18k shows a wire-guided float detector that has a tape connection to a ground reading assembly. This detector has evolved from the float-operated gauge board. The system shown here is suitable for tanks having an operating pressure to 30 PSIG (0.2 MPa) and a height of up to 60 ft (18 m), although other designs are available that are rated to 300 PSIG (2 MPa). The float is about 15 in. (381 mm) in diameter and can be made of aluminum, stainless steel, or other alloys; it can be hollow or filled with materials such as foam glass. To direct the float, guide wires are connected to top and bottom anchors. The top anchors are normally spring loaded to maintain constant tension on the wires; the bottom anchors are tank clips. Top Guide Wire Anchor

Oil Seal Assembly

A tape runs from the connection on the top of the float, over sheave assemblies, and down to the gauging head, which is outside the tank at eye level. The detail in Figure 3.18k shows the tape passing through an oil seal assembly. This seal can be used for gauge head corrosion protection or to prevent product condensation in the head, provided that the tank is operated at very close to atmospheric pressure. If the tank is pressurized, it is usually desirable to fill the head with the product, particularly if the product is clean and lubricating. The tape, which is perforated, enters the gauge head, runs around a sprocketed counter drive, and is taken up on the tape storage reel. Tape tension is maintained by a secondary (spring-wound) take-up device. The shaft on the counter drive rotates as the float moves the tape up and down, and, by proper gearing, this rotary motion can be used to drive a feetand-inches or metric readout. The counter is located outside of the gauge head, preventing exposure to any liquid fill in the head that may be present. For lower-pressure designs, the shaft extends out of the gauge head through a gland, whereas, for pressures above 30 PSIG (0.2 MPa), the shaft motion is normally taken out of the head through a magnetic coupling. The gauge head can be equipped with several optional devices. A crank assembly mounted on the head permits lifting the float out of the process. Material buildup on the float that hinders smooth operation may make this necessary. Often, the float can be freed by lifting and lowering it. The head also may be equipped with a variety of switch configurations for high and low level alarms or for control-circuitry actuation. Figure 3.18b shows installation details for wire-guided floats in low-pressure tanks. Figure 3.18l shows a high-pressure installation rated to 300 PSIG (2 MPa). At the higher pressures, Tank Roof Gate Valve

Counter Drive

Weld

Weld

Tension System Head 17" Weld

Float

Crank Assembly

FIG. 3.18k Wire-guided float detectors and detail of head.


Tape Storage

Weld

Convenient Eye Level Grade

FIG. 3.18l Wire-guided float detector installation for high pressure tanks.


Tank FarmMonitor

Tank Interface Unit

Field Wiring

FIG. 3.18m The use of tank farm unit controllers increases the safety of operation.

the flat, circular float design is no longer suitable, and one or more spherical floats are used. Connections for a high-pressure installation should be flanged, including those for the top anchor assemblies. Another feature to note in Figure 3.18l is the gate valve with rubber plug that is installed at the tank entry, allowing removal of the gauge head without tank depressurization. One common problem with these level devices is tape hang-up. This can occur if the long guide pipes are not perfectly vertical and the tape rubs against the inside of these pipes. If dirt or corrosion is also present, the resulting friction can hold the tape in place while the float is moving. This has caused accidents in cases where tanks controlled using tape level gauges overflowed because the tape was stuck. One recommended precaution is to install a separate high-level switch. Another recommended precaution is to use a microprocessor-based tank-farm operations controller as an added level of safety (Figure 3.18m). This unit controller can continuously monitor all operations that occur on the tank farm. It knows the capacity of each tank and the pumping rate of each pump, so it can check whether, under a particular filling operation, the level in the tank should be rising at a particular rate. If it should not (because the level transmitter is defective), the controller can sound an alarm or shut the system down. Encoding The wire-guided float detector level must be converted to an electrical signal by using the shaft rotation of the gauge head to drive encoding discs. Figure 3.18g illustrates one conversion method. The input shaft drives the “inches” wheel, and the gear assembly at the left of the sketch drives the “foot” wheel. For purposes of this sketch, the level tape sheave and shaft are set up to rotate 180° for each foot of level change. As the inch wheel completes one-half of a revolution, it steps the foot wheel up or down, corresponding to rising or falling level. Stepping of the foot wheel occurs when the notches on the inch wheel pass the gear. The wheels are coded so that a rotation corresponding to a 0.01-ft level change presents a


539

new and unique digital code to the code take-off assembly. Codes are available for foot, inch, and fraction, or meter and millimeter readouts. Since the principle of operation is the same for all, only the foot, tenths, and hundredths will be covered. The wheel has a number of concentric tracks on it, each track representing one digit of the digital word. The tracks are designed to produce the zero or one information needed for the digital word. This can be done in several ways. One way is to plate portions of the track with a conductor and allow a conducting brush to ride on the track. If the brush is on a conducting portion of the track, a current path will be formed, and a one will be produced. If the brush is on a nonconducting part of the track, the current path will not be formed, and a zero will be produced. Another encoding method is to use optical coupling. Portions of the tracks are plated with a reflecting material, and a light is beamed on the tracks. If the beam hits a reflecting portion of the track, a light-sensitive transistor conducts, thereby producing a one. If the beam hits a nonreflecting portion, the transistor does not conduct, and a zero is produced. Figure 3.18n shows how a modified gray code could be used to produce a digital word that is unique for a given wheel position and thus a given level. (Only 8 tracks are shown instead of the 16-track arrangement that would be required for a ±0.01-ft (0.3-cm) resolution over an 80-ft (24.3-m) span. Also, the tracks are shown as being linear whereas, in the actual configuration, they would be on closed circular tracks. The shaded areas represent conducting portions of the tracks, and the light portions are nonconducting. Thus, if the float were at a level corresponding to 6.4 ft, the code produced would be 10100110. The digital code is produced continuously. It is read by the remote device when the tank gauge is addressed as described in the preceding paragraphs.

0

Track 1 2

3

4

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

Feet × 1

FIG. 3.18n Encoding with a modified gray code.

Track 5 6

Feet × 0.1

7

540

Level Measurement

Remote Readout

Field

+ DC Supply # 50 Series Switch Switch # 00 Switch # 01 Switch # 02 Switch # 03 Switch # 04

Locate at Tank # R-50

R-50A

R-51

R-51A R-52

R-52A R-53

R-53A

50 51 52 53 54

R-54

R-54A

− DC Supply Readout

FIG. 3.18o Multiple tank system cabling.

Temperature Compensation Because liquids expand when heated, variations in the temperature of the material in the tank will affect the level reading. For this reason, it is quite common to take a temperature measurement of the liquid at the same time that the level is gauged, using the data to make a level correction based on temperature change. Equipment is available to accomplish this automatically. The resistance temperature detector (RTD) sensors at the various tanks can be switched into the remote readout at the same time that the level measurement is being made, using the same type of wiring arrangement shown in Figure 3.18o. A wide array of remote temperature and level readout equipment is commercially available. The simplest is the manually operated unit with pushbutton random access to all tanks. These units generally display the number of the tank called, its level, and its temperature. The more complex systems are microcomputer or minicomputer based and can have automatic logging of temperature-compensated level plus other features such as highand high-high-level alarm. Most systems can be readily interfaced with larger computers.

INDUCTIVELY COUPLED TAPE DETECTOR Figure 3.18h illustrates a fixed tape, float-actuated level measuring device. The tape is suspended from the roof of the tank and is anchored to the bottom. The tape is used to guide a float that contains an inductively coupled transducer. The


tape consists of a steel ribbon and a number of insulated conductors encapsulated in a Teflon jacket. In addition to providing mechanical strength, the steel tape is used to provide power to the transducer in the float through inductive coupling. At short intervals, this primary coupling is interrupted, and a secondary inductive coupling from the transducer to the conductors on the tape is established. The conductors are arranged on the tape in coded patterns so that each 0.1-in. (2.5-mm) increment has a unique code. The receiver mounted at the top of the tank reads which conductors have been inductively coupled. From this information, it can determine where along the tape the float is located and thus the elevation of the liquid level. The float is Teflon coated, and the tape-to-float clearance is approximately 0.25 in. (6.3 mm) to minimize float sticking and material buildup on the tape. The receiver can be furnished to transmit an analog signal proportional to level, or it can transmit the digital signal that has already been produced by the tape-and-float assembly, which has a resolution of 0.1 in. (2.5 mm). The conductors on the tape are arranged to produce a gray code digital word. In the gray code, only one digit in the word changes from one word to the next, so only one conductor must change its position from one 0.1-in. (2.5-mm) increment to the next. The number of conductors required increases with the span of the liquid level to be measured. N The span covered is equal to 2 , where N is the number of conductors. Thus, if four conductors are used, the span would be 16 increments, or 1.6 in. (40 mm). If 14 conductors are used, the span would be 16,384 increments, or 135 ft (41 m). In addition, each system requires a reference conductor and a return conductor. Figure 3.18i describes, in schematic form, how four conductors might be arranged on the steel tape to produce the gray code digital word for a 16-increment measuring system. If the conductor is on the right-hand side of the tape, it is inductively coupled to the transducer in the float; if on the left, it is not. The reference wire tells the receiver which side of the tape is the right-hand side. The return conductor is common, completing the circuit for all conductors. As shown in the figure, if the float is at increment 7, conductors 1 and 2 will be inductively coupled, and conductors 3 and 4 will not. Thus, the gray code digital word produced is 0011, which is unique for the particular increment. As previously noted, each additional conductor doubles the preceding span; adding a fifth conductor to the arrangement shown in Figure 3.18i would enable measurement more than 32 increments; a sixth conductor would enable measurement over 64 increments, and so on. The inductively coupled tape level system is intrinsically safe. In addition to accurately measuring the level, it can also determine the density of the process fluid and, based on that information, calculate the mass of the tank contents. Sensors are also provided for pressure, temperature, and interface measurement.


Transmitter

Pulse Generator

Reversible Servo Motor

Drum

Control Cable Thermal Sensors

541

Weight Balance and Micro-Switch

Cable Sensing Bob

Guide Cable Liquid or Solid

FIG. 3.18p Wire-guided thermal sensor. Heavy sensor can also provide temperature profile.

FIG. 3.18q Surface sensor.

WIRE-GUIDED THERMAL SENSOR Because liquid conducts heat better than does vapor, the liquid surface is bracketed by the two vertically displaced sensors. The lower one is cooler than the upper one. Figure 3.18p gives an installation detail for a wire-guided thermal sensor. The sensor, which is heavier than the liquid being measured, is suspended from an armored control cable and guided by a wire attached to the top and bottom of the storage vessel. The control unit detects the position of the sensor relative to the liquid level and issues step-up or step-down commands to the control cable take-up wheel until the lower sensor is in the liquid and the upper sensor is in the vapor. The system remains at rest as long as these conditions are met. When the sensor is moving, each stepping command adds or subtracts a length unit from the previous controller reading so that sensor position, and thus level, is accurately known. The unique feature of this instrument is that the sensor is heavier than the liquid, allowing the unit to be lowered to the tank floor so that the control and counter circuitry can be zeroed. The controller contains a cable tension sensor to signal when the level sensor has hit bottom. A controller subroutine permits automatic zeroing. The sensor also can be equipped with a temperature detector to provide a thermal profile of the tank material. This is useful for accurate correction of level measurement and can also be used to detect temperature inversions in cryogenic services.

SOLIDS LEVEL DETECTORS Although the gauges described here were originally developed for solids level detection, they can also be used for liquid level detection if equipped with a properly designed sounder. As shown in Figure 3.18q, a sounder is suspended


A

B

C

For Solids

For Liquids

FIG. 3.18r Sounder designs.

from the winding drum. The wire tension is continuously detected by the weight balance. A reversible servomotor rotates the drum when a starting signal is received, and it releases the wire until the sounder strikes the solid (or liquid) surface. When this occurs, the tension in the wire slackens, causing the weight balance to actuate a microswitch. After the momentary slackening of the cable, the microswitch reverses the motor and returns the sensing bob to its original reference position. The shape of the sensing bobs (or sounders) varies with the process fluid. Figure 3.18r illustrates some of the typical shapes used on both liquid and solids services. On solids with 3 3 less than 20 lbm/ft (320 kg/m ), the type A sensing bob is used. For higher densities, the type B is recommended, and, for coarser solids, the type C design is the appropriate choice. A pulse generator is coupled to the system to provide an input signal to the counter, which counts down from a preset maximum reference value in steps. When the sounder strikes

542

Level Measurement

Gauge Nozzle

Relative locations of solids inlet and gauge nozzles for rectangular and circular bin cross-sections

Solids Inlet

x/ 6

x Solids Inlet

Gauge Nozzle x/6

FIG. 3.18s Surface sounder installation for solids.

the product surface, the solids level is displayed and, at the same time, the counter is automatically disconnected from the pulse generator. The reading stays on the counter until the next measurement. On receipt of a new start signal, the counter is reset to the maximum reference value, and the measurement cycle is repeated. There are several ways in which the amount of cable paid out can be converted to pulses. In one design, the cable pays out over a measuring wheel with a 6-in. diameter. The measuring wheel drives a five-lobe cam that trips a stationary cam each time a lobe passes by. In this way, ten contact closures are produced for each foot of cable paid out. In another, higher-resolution design, the measuring wheel drives a disc that has 50 radial slots around its circumference. Here, the slots are counted by a light beam and a lightsensitive transistor, and 100 pulses are generated for each foot of cable “paid out.” In either case, the level measurement reference is the top of the tank. In dust-filled atmospheres, a solenoid-operated pneumatic cleaning assembly should be added to ensure reliable operation. As shown at the top of Figure 3.18s, the relative locations of the gauge nozzle and the solids inlet nozzle are important, because the sounder is used to take an average level reading. At no time should there be any contact between the filling system and the sounder. If the inlet nozzle is in the center of the bin, the surface of granular products will tend to take the shape of a cone. The gauge, therefore, should be located to obtain to obtain average level. As shown in the sketch, this requires the nozzle to be one-sixth of the diameter from the bin wall for circular bins. The sketch on the bottom of Figure 3.18s shows the proper location for installations in rectangular bins.

CAPACITANCE AND DISPLACER TAPE DEVICES At least two other tape level detector designs are available. In one, the sensor is suspended on a cable and held a short distance above the liquid level. The distance is sensed by a proximity-type capacitance probe (Figure 3.3f). The control unit monitors the capacitance between the sensor and the


liquid level, repositioning the sensor as the level changes. Sensor position, and thus level, is determined by measuring the amount of cable that has been paid out. In this respect, it is the same as the wire-guided thermal sensor previously described. The second design uses a displacer mounted on the end of a cable. In this design, the displacer is continuously repositioned so that it is always immersed the same amount, say, to 50% of its 0.1-in. (2.5-mm) thickness. Level is determined by the amount of cable paid out. The displacer design has cable weight compensation but is not compensated for changes in liquid density. Both the capacitance and the displacer designs are installed in stilling wells.

MULTIPLE-TANK SYSTEMS As previously mentioned, gauges covered in this section are used in conjunction with remote manually operated and automatically operated multiple-tank gauging systems. Multiple gauging requires cables from each tank to the remote readout. For wire-guided float detectors, the shaft position on the gauge head must be transduced to an electrical signal. The objective in designing the cable system is to wire up all the tank gauges with as few wires as possible, which means that wires must be shared. Figure 3.18o shows a wiring system used to obtain a level reading for any one of five tanks. Eight wires are used. By closing the tens switch #50 (at the top of the figure), one-half of the circuit to the relays at tanks 50 through 54 has been closed. Closure of any unit switch #00 through #04 will complete a circuit through the relay coil associated with the tank that is to be remotely metered. In the figure, switch #02 is closed; therefore, relay R-52 is energized, and relay contact R-52A is held closed. The remote readout can now obtain the level data that is available at the gauge head at tank 52. This technique allows a great number of tanks to be remotely monitored with relatively few wires. For example, a 100-tank installation can be monitored using 22 wires. The 22 wires would be composed of 10 for the tens position, 10 for the units position, and 2 for the signal. A second group of 100 tanks can be picked up by adding only two more wires, one for the 100s-series tanks and one for the 200sseries tanks. When the distance from the tanks to the remote readout is long, a satellite multiplexer may be considered. The satellite multiplexer collects level information for tanks in the immediate vicinity and transmits it, on demand, over two wires to the remote readout. The satellite multiplexing system might be used for a pipeline transmission installation where the various bulk storage facilities are hundreds of miles apart. The switches shown in Figure 3.18o constitute a manually operated multiplexer. These switches can be operated by a data logger or by a computer, making inventory monitoring completely automatic.


The reason for monitoring a tank farm with as few wires as possible is to reduce installation costs. In so doing, the gauge head relays and much of the wiring are run in parallel. This means that a short to ground, an open wire, or a malfunctioning gauge head can disrupt the entire system. Many tank farms are located in corrosive and/or humid environments. Therefore, particular care should be taken in the design and installation of the cable system, especially at the terminals in the gauge heads and junction boxes. Lightning strikes can be another source of trouble, given that most tankfarm cabling systems are run overhead. Surge protection should be installed at each gauge head and at the remote readout. Associated loggers and computers should be electrically isolated. As an aid to troubleshooting a crippled system, isolating switches should be installed so that blocks of gauges can be separated from the system to enable a more rapid location of the fault. CONCLUSION Flow meters and provers offer the most accurate ways to measure standard volumes of liquids. Tank measurement is much more inaccurate when measuring small parcels. In custody transfer of full or nearly full tank volumes, manual gauging is preferred in the U.S., and ATGs are preferred in Europe. In either case, the inherent tank accuracy is a factor, because the filling of a large tank causes the bottom to sink, the shell to bulge, and the top to sink, and changes in temperature also cause changes in tank dimensions. Automatic tank gauging systems are found in almost all tank farms of any size. They enable inventory monitoring at a given time each day. The wire-guided float tape gauge systems are most common on the existing tank farms (Table 3.18a) on liquid services. The design of the remote metering portion of these systems has been improved markedly over the past years, and these systems can be expected to perform satisfactorily if they are properly installed and maintained. To protect against tank-farm accidents such as overfilling, they should be backed up with high-level switches, and computer monitoring should be provided to detect for sensor failures resulting from float or tape hang-up. The inductive tape-and-float systems and the wire-guided thermal capsule systems are more expensive and do not have as great a degree of field exposure as the wire-guided float types. (Table 3.18a). However, the tape-and-float system has fewer moving parts and therefore requires less maintenance. The wire-guided thermal capsule can be zeroed and can also be used for temperature profiling. Some users regard the in-tank electrical circuitry required by these latter designs as a drawback, but intrinsically safe designs should alleviate that concern. The surface sensor design has been used for some time for solids level measurement. There have been reports that the sensor can become buried or detached from the takeup cable, contaminating product or ruining downstream


543

equipment. A more rugged cable and sounder design should overcome this problem. The surface sensor is not highly recommended for liquid level applications, but it is acceptable for solids level measurement if an error of a couple of inches is acceptable. Reference 1.

Berto, F. J., Review of tank measurement, parts 1 and 2, Oil & Gas J., March 3 and March 10, 1997.

Bibliography Avolio, G., Encoders, resolvers, digitizers, Meas. Control, September 1991. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June-July 1997. Berto, F. J., Control program halves crude losses, Oil & Gas J., December 27, 1982. Berto, F. J., Methods for volume measurement using tank gauging devices can be error prone, Oil & Gas J., March 13, 1989. Berto, F. J., Hydrostatic tank gauges accurately measure mass, volume, and level, Oil & Gas J., May 14, 1990. Berto, F. J., Gauging data pose question on stability of reference gauge heights, Oil & Gas J., July 29, 1991. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Cornane, T., Continuous level control, Measurement and Control, April 1997. Detecting tank levels remotely, safely, Chemical Process., July 1970. Entwistle, H., Survey of Level Instruments, ISA Conference, Anaheim, CA, Paper #91-0484, 1991. Floats on pulleys keep track of tank levels, Machine Design, February 12, 1976. Glenn, L. E., Tank gauging—comparing the various technologies, in ISA Conf. Proc., Anaheim, CA, Paper #91–0471, 1991. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Johnson, D., Level sensing in hostile environments, Control Engineering, August 2001. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Mei, K. W., Automatic tank gauges can be used for custody transfer, Oil & Gas J., November 13, 1989. Mei, K. W., Accurate automatic temperature measurement reduces tank volume errors, Oil & Gas J., July 20, 1992. Mei, K. W., Unslotted gauge wells cause tank-level measurement errors, Oil & Gas J., January 30, 1995. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Piccone, R. P., Combining technologies to compute tank inventory, Sensor, October 1988. Sivaraman, S. and Thorpe, W. A., Measurement of tank-bottom deformation reduces volume errors, Oil & Gas J., November 3, 1986. Sivaraman, S. and Holloway, C. J., Method measures cylindrical storage tank reference height variations, Oil & Gas J., December 12, 1988. Sivaraman, S., Field tests prove radar tank gauge accuracy, Oil & Gas J., April 23, 1990. Sivaraman, S. and Sheppard, R., Minimum transferred volume necessary for accuracy when determining custody transfer volumes by tank gauging, Petroleum Rev., August 1991. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

3.20

Ultrasonic Level Detectors

To Continuous Receiver LT US

D. S. KAYSER (1982) J. E. JAMISON

B. G. LIPTÁK

(1969, 1995) To On-Off Receiver

(2003) LS

US Flow Sheet Symbol


Applications

Applications are wetted and noncontacting switch and transmitter applications for liquid level or interface and solids level measurement. They are also used as openchannel flow monitors. Advantages: generally, no contact with the product; suitable for various liquids and bulk products. Disadvantages: product must not produce too much surface foam; not suitable for extremely high pressures and temperatures; not applicable in vacuum service applications.

Design Pressure

Probe switches are used up to 3000 PSIG (207 bars = 20.7 MPa); transmitters are usually used for atmospheric-type service up to 60 PSIG (4.1 bar), but some special units are available for use up to 250 PSIG (17.2 bars)

Design Temperature

Switches from −100 to 300°F (−72 to 149°C) with special units to 750°F (399°C); transmitters from −40 to 302°F (−40 to 150°C)


Aluminum, type 304/316 stainless steel, titanium, Hastelloy B/C, Monel , Kynar,  PVC, CPVC, Teflon (TFE), Tefzel , polypropylene, PVDF, epoxy

Ranges

For tanks and silos (pulse usually travels in vapor space), up to 230 ft (70 m) for some special designs and up to 50 ft (15.3 m) for most standard systems. For wells (usually submerged), up to 2000 ft (600 m).

Inaccuracy

For a horizontal probe switch, 0.040 in. (1 mm). For transmitters, the error varies from 0.2 to 2% of full scale depending on the dust and dew in the vapor space and on the quality of the surface that serves to reflect the ultrasonic pulse.

Costs

Level switches cost from $150 basic to $950 (smart, multipoint); transmitters cost from $650 to $2500, with an average cost around $1200


AMETEK Drexelbrook Company (www.drexelbrook.com) Babbitt International Inc. (www.babbittlevel.com) Bindicator Co. (www.bindicator.com) Bürkert Contromatic Corp. (www.burkert.com) Cole-Parmer Instrument Co. (www.coleparmer.com) Consilium US Inc. (www.consiliumus.com) Contaq Technologies Corp. (www.contaq.com) Cosense Inc. (www.cosense.com) Delavan Process Instrumentation, an L&J Terminologies Company (www.ljtechnologies.com) Delta Controls Corp. (www.deltacnt.com) Devar Inc. (www.devarinc.com) Dwyer Instruments Inc. (www.dwyerinstruments.com) EDO Electro-Ceramic Products (www.edocorp.com) Electronic Sensors Inc. (www.leveldevil.com) Endress+Hauser Inc. (www.us.endress.com) Fischer and Porter, a Unit of ABB (www.abb.com/us)





3.20 Ultrasonic Level Detectors

549

Gems Sensors Inc. (www.gemssensors.com) Gordon Products Inc. (www.gordonproducts.com) Greyline Instruments Inc. (www.greyline.com) HiTech Technologies Inc. (www.hitechtech.com) (fly ash applications) Honeywell (www.honeywell.com/acs/cp) Hyde Park Electronics Inc. (www.hpsensors.com) Introtek (www.introtek.com) Inventron Inc. (www.inventron.com) Kay-Ray/Sensall, subsidiary of TN Technologies Inc. (www.thermo.com) Kistler-Morse Corp. (www.kistlermorse.com) Kobold Instruments Inc. (www.koboldusa.com) Krohne America Inc. (www.krohne.com) (sludge interface) Magnetrol International (www.magnetrol.com) Markland Specialty Engineering Ltd. (www.sludgecontrols.com) (sludge level) Massa Products Corp. (www.massa.com) Milltronics Inc. (a Division of Siemens) (www.milltronics.com) Monitek Technologies Inc. (part of Metrisa Inc) (www.monitek.com) Monitor Technologies LLC (www.monitortech.com) Monitrol Mfg. Co. (www.monitrolmfg.com) Ohmart/VEGA (www.ohmartvega.com) Omega Engineering Inc. (www.omega.com) Penberthy (www.penberthy-online.com) Pepperl+Fuchs (www.pepperl-fuchs.com) Solartron Mobrey Ltd. (www.solartronmobrey.com) Thermo MeasureTech Inc. (www.thermo.com) TN Technologies Inc. (www.tn-technologies.com) Ultrasonic Arrays Inc. (www.ultrasonicarrays.com) (thickness, texture, surface reflectivity) Vega B.V. (www.vega.com) Zevex Inc. (www.zevex.com)

Sonic (up to 9500 Hz) and ultrasonic (10 to 70 kHz) level switches operate either by the absorption (attenuation) of acoustic energy as it travels from source to receiver or by the frequency change of a vibrating diaphragm face, oscillating at 35,000 to 40,000 Hz. Ultrasonic level transmitters operate by generating an ultrasonic pulse and measuring the time it takes for the echo to return. If the transmitter is mounted at the top of the tank, the pulse travels in air (at a speed of 1087 ft/sec at 32°F, or 331 m/sec at 0°C); therefore, the time of travel is an indication of the depth of the vapor space above the liquid in the tank. If the transmitter is mounted on the bottom of the tank, the time of travel reflects the depth of liquid in the tank, and the speed of travel is a function of what that liquid is. In the case of water at 77°F (25°C), an ultrasonic pulse travels at 4936 ft/sec (1505 m/sec).

THE NATURE OF ULTRASOUND To understand the capabilities and limitations of ultrasonic instruments, one must understand the conditions that determine the characteristics of sound: temperature, reflection, propagation, and absorption. Temperature compensation is essential in ultrasonic level measurement, because the velocity of sound is proportional to the square root of temperature and, in case of air, it changes by about 2 ft/sec (0.6 m/sec) for each degree Celsius change in temperature. The speed of


travel rises with temperature, and it amounts to about 0.1% per degree Fahrenheit (0.18% per degree Celsius). To measure the time of travel of the echo of an ultrasonic pulse, it is essential that some of the sonic energy be reflected. Liquids as well as solids with large and hard particles are good reflectors. Fluff and loose dirt have poor reflecting characteristics, because they tend to absorb the sonic pulse. It is also important that the reflecting surface be flat, because the angle of reflection equals the angle of incidence. Therefore, if the sonic pulse is reflected from a sloping surface, its echo will not be directed back to the source, and the roundtrip travel time (time of flight, or TOF ) will not accurately reflect the vertical distance. Irregular surfaces result in diffuse reflection, in which only a small portion of the total echo travels vertically back to the source. The travel (propagation) of sound results in its dispersion (loss of intensity). The intensity of sound decreases with the square of distance; therefore, the echo becomes exponentially weaker as the range of the instrument is increased. The decrease in sound energy is caused not only by dispersion (traveling distance) but also by absorption in the substance through which it travels. For example, an ultrasonic (e.g., 44,000 Hz) sound wave traveling in dry and dust-free 60°F (20°C) air is attenuated by 1 to 3 dB for each 3.3 ft (1 m) of travel. It can be seen from the above that an ultrasonic transmitter is subject to many interferences that will affect the strength of the echo. Many of these physical phenomena are

550

Level Measurement

beyond the control of the instrument manufacturers, although microprocessor applications help compensate for some of the symptoms of the presence of the phenomena. All the transmitter can be expected to do is accurately compute the round-trip time of flight based on the first segment of the echo, provide temperature compensation or heat if condensation in the transducer is a possibility, and provide a strong and well-focused ultrasonic pulse. It cannot change the reflection, propagation, or absorption characteristics of the process. Ultrasonic level devices can be used for both continuous and point measurement. The point detectors—for measurement of gas–liquid, liquid–liquid, liquid–foam, or solid–gas interfaces—can be grouped by design into damped sensor or on–off transmitter categories. They also can be categorized by method of packaging as single-element and two-element units. The continuous level detector designs can be categorized as under-liquid sensors and above-liquid sensors. Most designs use a 20 kHz or higher (up to 70 kHz) oscillator circuit as the ultrasonic signal generator. Some designs incorporate filters, discriminatory circuitry in electronics, or software in microprocessors to prevent false readings that might be caused by random noise. Each of these basic design variations will be discussed separately. LEVEL SWITCHES Damped Vibration Type The devices in this category operate on a principle similar to that of the vibrating reed switch (see Section 3.21). As long as the sensor face is in the vapor space of the tank, it vibrates at its resonant frequency but is damped out when the process material contacts it. Some designs incorporate a piezoelectric crystal in the vibrating tip. Figure 3.20a shows some of A

FIG. 3.20b Dampened vibration ultrasonic level transmitter signals (Design “B” example.)(Courtesy of Endress+Hauser Inc.)

the elements and their installation. Four units are shown in the figure. Design “A” notes the top-entry installation, where the vibrating face is in the vapor space (and is therefore undamped). This design can be repositioned manually or automatically for flexible adjustment of the control point. Design “B” is a unique design in that it does not penetrate the tank wall and thus is not in contact with the process fluid. When the liquid rises to the opposite side of the wall, the transducer is damped (see Figure 3.20b), and a switch action occurs. Designs “C” and “D” show the side-mounted switch elements, one (“D”) damped and the other in an undamped condition (“C”). These units are normally limited to liquid service, because the damping effect of solids is insufficient in most cases. Design “B” can be used on any liquid, while the others are limited to clean, noncoating fluids. Absorption Type

B C

Vibrating Face

D

Tank Wall

FIG. 3.20a Dampened ultrasonic sensors.


Sensor

These ultrasonic switches contain transmitter and receiver elements. The transmitter generates pulses in the ultrasonic range, and the receiver detects these pulses if they are transmitted through the medium in which the probe is located. The transmitter and receiver can be mounted on the same probe, or they can be located on the opposite sides of the tank. Figure 3.20c illustrates some variations of this design. Installation “A” shows a design in which the transmitter and receiver are packaged separately. This design transmits in air; the switch will actuate when the ultrasonic sound beam is interrupted by the rising process material. Reflectors are installed to narrow the sonic beam angle when the distance between source and receiver is more than 10 ft (3 m). Installation “B” is a single-probe design in which the pulses generated by the transmitter will be sensed by the detector only


551

Transmitter

Receiver

A B

FIG. 3.20d Top- and side-mounted level switches for sludge or slurry services. (Courtesy of Delavan Inc.) C D E 10˚

Transmitter Crystal

Receiver Crystal

Transmitter Receiver

FIG. 3.20c Transmitting ultrasonic point sensors. FIG. 3.20e Ultrasonic filler nozzle.

if they are submerged in a non-compressible liquid. The pulses are not transmitted in the vapor space. Design “C,” which is similar to “B,” transmits only in liquid. When fluid is present in the gap of the single probe, the ultrasonic sound beam is received by the detector in the tip and signals the presence of liquids. Probe “D” is a multipoint variation of “C,” allowing the measurement of both high and low levels by the same probe. Interface Detector Design “E” is mounted ≈10° from the horizontal, and it can be used for detection of liquid–liquid interfaces. The ultrasonic sound beam generated by the transmitter crystal will be detected by the receiver crystal if the probe is in only one liquid. If an interface is present in the probe cavity, the interface will reflect the signal, preventing it from reaching the receiver. Ultrasonic level switches can also be used to signal when the light layer becomes too thick or too thin. This is achieved by mounting the ultrasonic level switch on a float that continuously follows the total level in the tank. All of the designs discussed above are applicable to clean liquid service, and none is particularly suitable for slurry or coating services. For slurry or sludge services, it is desirable to separate the source from the receiver by a more substantial distance (Figure 3.20d) and install them so that the slurry material will drip off when the level drops. Only design “A” in Figure 3.20c will detect the level of solids. In connection with solids level detection, it should be noted that these devices, being point sensors, will not take into consideration the angle of repose during filling or emptying, nor will they detect rat holes, arches, or bridging.


Figure 3.20e illustrates a special application of ultrasonic level detection for gasoline filling nozzles. The nozzle itself contains a transmitting and receiving element to detect when liquid reaches the nozzle, at which point flow is shut off. LEVEL TRANSMITTERS The principle of operation of these units is very similar to that of the echometers used to measure the depth of wells. In that design, a blank shell is fired; the time needed for the echo to return is converted to an indication of the depth. The continuous ultrasonic level detector (SONAR) measures the time required for an ultrasonic pulse to travel to the process surface and back. The source is an oscillator-type ultrasonic speaker, and the receiver, in most designs, is a metal disc that is both electrically and mechanically resonant. The transducer can be mounted either below or above the liquid level. Figure 3.20f illustrates some of the design features and possible installations. Installation “A” shows a two-element continuous detector (no longer in use) in which the transmitting A

B

C

FIG. 3.20f Continuous ultrasonic level detectors.

D

552

Level Measurement

To Receiver Brine

Hydrocarbon Ground Level

Casing Brine Pipe

Cable

Hydrocarbon

FIG. 3.20g Adjustable top-mount ultrasonic transmitter for solids level measurement. (Courtesy of Endress+Hauser Inc.)

and receiving transducers are packaged separately. This device transmits in air. The time required to receive the ultrasonic reflection from the surface is the measure of the vapor depth of the space, which is an indirect indication of level. Another, more commonly used version of an “in-air” continuous level system is illustrated in “B.” The transducer and receiver are packaged as a single unit. The transducer generates short bursts of ultrasonic energy and, while the acoustic energy is being produced, the receiver is blanked off. When the ultrasonic waves are on their way, the receiver gate is opened to detect the echo. Mounting the transducers in the vapor space has the advantage that the instrument does not contact the process materials, but it has the disadvantage that some energy is lost in traveling through the vapor space. On liquid level applications, the aiming angle must be within ±5° of the vertical. When measuring levels of solids, the angle of repose should be tested. Current systems incorporate an aiming system to accommodate larger aiming angles (see Figure 3.20g). In installation “C,” the time for the ultrasonic echo is a true indication of level. The transducer can also be mounted on the outside of the tank (“D”), with the added advantage that the sensing element does not penetrate the tank. Design “C” is applicable to continuous detection of clean liquid levels, and designs “A” and “B” also can be used to measure the level of solids. By using several sensors in the same bin, a visual profile can be obtained, showing the angle of repose and indicating if the bin is being filled or discharged. Figure 3.20h shows an interesting application of the continuous ultrasonic level detector. When hydrocarbons are stored in salt dome wells, the hydrocarbon rests on a brine layer. When additional hydrocarbons are pumped in, the brine is


Cavity Interface

Brine Transducer

FIG. 3.20h Interface detector for hydrocarbon storage cavity.

displaced; the hydrocarbons are recovered by displacing them with brine. For reasons of safety, inventory monitoring, and cavity use, it is important to know where the hydrocarbon–brine interface is located. This device is suitable for finding the interface.

MULTI-TANK PACKAGES “Intelligent,” microprocessor-based ultrasonic transmitters can convert the level in cylindrical, horizontally mounted tanks into actual volume of the contents. Microprocessors can be useful in other ways. For example, for better accuracy of measurement, the transducer can be furnished with a fixed target assembly (Figure 3.20i), and the unit can be automatically recalibrated periodically using that reference. Because the length of the reference bar is known, this will give a very accurate reference if the density of the vapor space is uniform and constant. Microprocessor-based ultrasonic level sensors can be used in a multi-tank or multi-silo configuration (Figure 3.20j). This tends to lower their per-tank unit cost because, through multiplexing, some of the electronic and display equipment can


FIG. 3.20i Level transmitter provided with automatic calibration target. (Courtesy of Inventron Inc.)

Scanning Console

1

2

3

4

24

FIG. 3.20j Ultrasonic silo scanning system.

be shared among the 24 or 48 storage tanks. In such packages, the transmitters are wired to an automatic scanning console, which operates the individual display devices (indicators, recorders, alarms, and so forth). The scanning frequencies are individually programmed to match the requirements of the processes in the different tanks.

RECENT DEVELOPMENTS In the mid to late 1990s, a number of new developments occurred in ultrasonic level sensor technology. One such improvement involves the continuous monitoring of the depth of sludge blankets. This ultrasonic sensor is provided with a dual-head assembly and a microprocessor, and it is useful in the wastewater treatment industry. Another development was improved filtering capabilities via microprocessor-implemented fuzzy logic technology against interference reflections and against buildups in tanks over time. Another area of recent advancements involves automatic self-calibration of ultrasonic sensors, which can correct for some


553

of the effects of changing vapor space composition or temperature and can provide more than a single calibration target. The multiple calibration targets are provided in forms of precisely located ridges in a sounding pipe and can result in level measurement inaccuracies within 5 mm over a distance of 30 m. New techniques in ultrasonic level measurement are revolutionary, utilizing the changes in the speed of sound in the 1 tank wall. According to Lynnworth, this speed does change when the other side of the tank wall is wetted and therefore can be used for level measurement in both the transmission and the echo mode of operation. Such units have been tested in pilot applications. The mid to late 1990s saw the incorporation of serial data communications links such as RS485, and the HART protocol became available on several manufacturers’ systems to link the output data to higher-level systems. In the late 1990s and early 2000s, the use of various digital fieldbus technologies became commonplace as the output signal of ultrasonic level transmitters. FOUNDATION fieldbus and Profibus PA and DP are the main fieldbuses currently being employed. Another interesting and recent development in the ultrasonic or time of flight technology is that several manufacturers have designed their systems to allow the use of the same transmitter between both ultrasonic and microwave (RADAR) technologies (see Sections 3.13 and 3.14). This allows upgrades or changes in requirements between the two related devices but still allows inventories of transmitters to be cross-utilized between the different but similar sensor technologies. The trend is to utilize the similar functionality of the two related technologies from a hardware and software/firmware point of view that benefits application users. Other interesting developments are in the area of “smart” local microprocessor-driven displays on transmitters that show measurement integrity information. Using the echo envelope curve representation (signal progression) on the local transmitter display shows signal strength versus distance. The curve indicates anomalies within a tank such as nozzles, weld seams, specific tank internals, and so forth for which compensation is required (see Figure 3.20k). This helps the technician in the field to validate the instrument’s integrity, even in hazardous locations. The embedded microprocessor can compensate for the spurious signals. False echoes can be suppressed to prevent any misinterpretation by increasing the detection threshold at one or more fixed points. If the reflected signal is strong enough, the transmitter will follow the true echo, even during the increased detection threshold. Should the reflected echo signal be smaller than the threshold, the transmitter will “look around” the increased threshold and will hold the output until the true signal appears again. This can be done on both a per-point basis and automatically. In addition, many manufacturers have a troubleshooting menu-based system displaying configuration information, diagnostics, and documentation on the local display. If necessary, this information can also be accessed in remote locations via HART protocols or a digital fieldbus protocol (FOUNDATION fieldbus, Profibus-PA).

554

Level Measurement

Travel Time/Distance

Echo signal

of the ultrasonic pulse, the result can be an error, as the round-trip time of travel might not correspond to the vertical distance between transmitter and level. Therefore, the best guide for using ultrasonic level instruments is past experience on similar installations.

Reference 1.

Lynnworth, L. C., Ultrasonic Measurements for Process Control: Theory, Techniques, Applications, Academic Press, 1989.

Bibliography Echo detection Threshold Envelope

FIG. 3.20k Echo envelope curve and internal tank anomalies. (Courtesy of Endress+Hauser Inc.)

CONCLUSION Ultrasonics now can be considered as one of the traditional methods of level measurement. As probe-type level switches, they can give reliable performance, even on difficult slurry or sludge-type services. The main advantages of ultrasonic transmitters are the absence of moving parts and the ability to measure the level without making physical contact with the process material. In some specialized designs, the penetration of the tank can also be avoided. The reliability of the reading is unaffected by changes in the composition, density, moisture content, electrical conductivity, and dielectric constant of the process fluid. If temperature compensation and automatic self-calibration are included, the resulting level reading can be accurate to 0.25% of full scale. In terms of limitations, the ultrasonic level transmitter is just as good as the echo it receives. The echo can be weak as a result of dispersion (which reduces sound intensity by the square of distance) and absorption (which, in dry air, reduces its energy level by 1 to 3 dB/m). The energy content of the echo will be further reduced if the bin is tall, if the vapor space is dusty, or if it contains foam or other soundabsorbing materials such as water vapors or mists. In addition to the problem of weak echos, another potential problem is the reflective properties and density of the process surface. If that surface is sound-absorbing (fluffy solids), sloping (angle of repose), or irregular, causing a diffused reflection


Andsager, R. L. and Knapp, R. M., Acoustic determination of liquid levels in gas wells, Pet. Technol., May 1967. Bacon, J. M., The changing world of level measurement, InTech, June 1996. Bacon, J. M., D/P versus Other Technologies—The Changing World of Level Measurement, ISA Conference, Paper #95–071, October 1995. Bahner, M., Meeting Spill Prevention Regulations using RF Admittance and Ultrasonic Level Measurement Technologies, ISA Conference, Paper #95–072, October 1995. Berrie, P. G., Ultrasonic measurement of level filling using fuzzy logic, Technica, May 1994. Brovtsyn, A. K., Ultrasonic monitoring of high temperature and aggressive media, Russ. Ultrasonics, 30(1) (English), 2000. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Duncan, D. L., Ultrasonics in Solids Level Measurement, ISA Conference, Paper #91–0485, 1991. Duncan, D. L., Ultrasonic sensors: now an even better choice for solid material level detection, Instrum. Control Sys., November 1998. Entwistle, H., Survey of Level Instruments, ISA Conference, Anaheim, CA, Paper #91-0484, 1991. Glenn, L. E., Tank gauging—comparing the various technologies, in ISA Conf. Proc., Anaheim, CA, Paper #91–0471, 1991. How can we measure level of petroleum sludge? Control, August 1999. Johnson, D., Level sensing in hostile environments, Control Eng., August 2001. Kalmus, H. P., New ultrasonic liquid level gauge, Rev. Sci. Instrum., October 1965. Kaminski, H. K., Sonics measure level, Instrum. Technol., December 1967. Lerner, J., Continuous level measurement: an introduction to 16 basic types, Control, November 1990. Level measurement and control, Meas. Control, April 1991. Liquid levels are measured by ultrasonic signal, Prod. Eng., October 1975. Owen, T., Overcoming obstacles in solids level measurement, Control, February 1998. Reason, J., Ultrasonics: practical plant engineering total for level control, Plant Eng., December 28, 1972. Sholette, W., Eliminating Fugitive Emissions and Environmental Accidents through the Proper Selection of Level Measurement and Control Systems, ISA Conference, Paper #95–115, October 1995. Ultrasonic level measurement of solids and liquids, in Proc. 1996 International Conference on Advances in Instrumentation and Control, October 1996. Ultrasonic multiple-sensor solids level measurements, in Proc. 1994 IEEE Instrumentation and Measurement Technology Conference, Part 2, 1994.


Ultrasonic monitor/sensor controls sludge flow, Water and Sewage Works, June 1978. Ultrasonics pinpoint liquid levels, Automotive Eng., September 1975. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Level sensing key to water treatment, InTech, December 1991. Waterbury, R. C., Level sensing tips towards basics, InTech, May 1994.


555

Waterbury, R. C., Liquid level measurement 101, Control, November 1998. Weiss, W. I., Ultrasonic level controls, ISA J., December 1966. Wolff, J., Ultrasonic in the sewage industry, Water and Sewage Works, June 1973. Yan, H., Ultrasonic sensors and in-situ correction for the monitoring of indistinct level interfaces, in ISCS 2000—Proc. International Society for Optical Engineering, 4077, 2000.

3.21

Vibrating Level Switches D. S. KAYSER

(1982)

B. G. LIPTÁK

(1969, 1995, 2003) LS

To Receiver

Flow Sheet Symbol

Types

A. Tuning fork B. Vibrating probe C. Vibrating reed

Applications

Can be used to detect the level of liquids, slurry or solids

Design Pressure

A. and B. Up to 150 PSIG (10.3 bars = 1 MPa) C. Up to 3000 PSIG (207 bars = 20.7 MPa)

Design Temperature

A. From −45 to 200°F (−43 to 93°C) B. From 8 to 176°F (−10 to 80°C) C. From −150 to 300°F (−100 to 149°C)


Aluminum, steel, stainless steel

Minimum Bulk Density

A. and C. Down to 1.0 lbm/ft (16 kg/m ) B. Requires an apparent specific gravity of 0.2

Inaccuracy

Generally, 0.25 to 0.5 in; the repeatability of type C is 0.125 in. (3 mm)

Cost

$300 to $500


Automation Products Inc. (www.dynatrolusa.com) Bindicator (www.bindicator.com) Endress+Hauser Systems & Gauging (www.systems.endress.com) Monitor Mfg. Technologies LLC (www.monitortech.com) Monitrol Manufacturing Co. (www.monitrolmfg.com) Solartron Inc. (www.solartron.com)

3

Several level switch designs operate by keeping a probe or other element in oscillation or in natural frequency vibration and triggering a relay when the process material in the tank reaches the vibrating element and damps out the vibration. The reed, probe, and tuning fork variations are distinguished only by the frequencies of their oscillation (reed, 120 Hz; probe, 200 to 400 Hz; tuning fork, 85 Hz) and by their dimensions or physical shapes. Their shapes are similar, although the close spacing between the arms of the tuning fork could make it more susceptible to material buildup in sticky services than the others. This is not necessarily so on all types of coatings because, although any buildup changes the natural frequency of vibration, a limited amount of buildup will not collapse the oscillation. The applications of vibrating level switch designs are also similar and include powders of different plastics, toners 556 © 2003 by Béla Lipták

3

for copiers, detergents, powdered sugar, flour, ground or instant coffee, chocolate, dried milk, tea leaves, cosmetic powders; granules of plastic pellets, rice, wheat, beans, carbon, sugar, and salt; plus slurries and liquids.

VIBRATING LEVEL SWITCHES The design of the vibrating reed level switch is illustrated in Figure 3.21a. The unit consists of a driver, paddle, and pickup. The driver coil induces a 120 Hz vibration in the paddle that is damped out when the paddle is covered by the process material. The pickup end contains a permanent magnet and a coil that generates a millivolt output signal when the paddle is vibrating. When the paddle is covered, the signal decreases and as a consequence, a control relay

3.21 Vibrating Level Switches

557

Driver End

Pickup End

"

Node

Paddle

FIG. 3.21a Vibrating reed switch. (Courtesy of Automation Products Inc.)

" Bottom of Paddle

Millivolts (Open Circuit Signal)

Bottom of Paddle

600

400 1

Millivolts (Open Circuit Signal)

1 2

400

200 4 1

0 Inches (Level Rise)

FIG. 3.21b Reed switch actuation ranges for liquid service. (Courtesy of Automation Products Inc.)

is de-energized. A welded pressure seal is made at the node points. The device will detect liquid–liquid, liquid–vapor, and solid–vapor interfaces, because the switch is sensitive enough to detect relatively small changes in the density of the surrounding process material. The characteristics of this switch are described by Figures 3.21b and 3.21c. Figure 3.21b is a plot of millivolt signal strength versus paddle coverage. Curve 1 describes the switch behavior in flour, 2 in water, 3 in polyethylene pellets, and 4 in granular sugar. It can be noted that, in all four cases, the switch will actuate before the paddle is fully covered by the process material. For example, in the case of water (2), switch actuation occurs when the water level is about 0.4 in. (10 mm) above the bottom of the paddle. The curves in Figure 3.21c refer to granular powders at 3 3 various densities. Curve 1 is for 60 lbm/ft (960 kg/m ), 2 is 3 3 3 3 for 50 lbm/ft (800 kg/m ), and 3 is for 40 lbm/ft (640 kg/m ). As would be expected, the lighter the powder, the more level


3

0 0

1 2 1 Inches (Level Rise)

2

FIG. 3.21c Reed switch actuating ranges for heavy solids. (Courtesy of Automation Products Inc.)

3

0 −

2

200

Top of Paddle

Switch Actuation Range

600

Switch Actuation Range

800

Paddle

800

Top of Paddle

Paddle

has to build up before switch actuation occurs. In case of the 3 40 lbm/ft powder (3), the switch will actuate at 1.0-in. level rise, which corresponds to a level buildup of 0.25 in. (6 mm) above the top of the paddle. The vibrating reed level switch can be used to detect both rising and falling levels, and it can be installed in tanks or pipelines. Its use for measuring density and viscosity are discussed in Chapters 6 and 8, respectively. If the process material has a tendency to adhere to the paddle, such buildup can be removed by periodically purging the line through a purge well but, in general, this unit is not recommended for applications where buildup is probable. The sensing wires between probe and receiver should be shielded and grounded at both ends. Supply voltage variations between 105 and 125 V will not interfere with the measurement. When used on wet powders, the vibrating paddle has a tendency to create a cavity in the granular solids. If this occurs, the vibration amplitude will be the same as if the paddle were in the vapor space. Therefore, this level switch should not be used in applications involving wet powders. Where the solids bins are purposely vibrated and the vibration frequency is close to 120 Hz, this method of level measurement is unreliable and therefore should not be considered.

TUNING FORK The tuning fork type of level switch is oscillated at its resonant frequency of about 85 Hz by a piezoelectric crystal located near the head of the fork. Another crystal, also mounted in the head, detects the vibration or the lack thereof. The unit can tolerate some limited amount of buildup and

558

Level Measurement

Note Position Top Index

Repose Angle Wrong

Yes

Right

Protective Rods

No

FIG. 3.21d The installation of tuning fork-type level switches. (Courtesy of Endress + Hauser Inc.)

can also operate at higher than the design temperature if purging is provided to cool the crystals. For mounting recommendations and requirements refer to Figure 3.21d. When the self-sustaining resonant frequency probe is in the vapor space, an electromechanical oscillator is formed, causing the probe to vibrate at its natural mechanical resonant frequency of approximately 85 Hz. One coil drives the probe, and a second coil monitors the vibration and generates a corresponding AC voltage signal. This feedback signal is amplified and reapplied to the drive coil, sustaining the mechanical vibration. When the process fluid rises to cover the probe, it damps the vibration, causing the feedback signal and drive voltages to collapse. At that point, oscillation ceases. A relay located in the control unit detects the state of oscillation and initiates the contact closures. The advantage of the self-sustaining resonant frequency probe design is that material buildup on the probe has only limited effect, because, with a limited amount of coating, the resulting change in the natural frequency of the probe but does not collapse the oscillation. Therefore, the sensor is suitable for some slurry services. This design is also applicable for use on low-density solids level detection down to 3 3 1.0 lbm/ft (16 kg/m ). VIBRATING PROBES In the vibrating plate or vibrating probe level switches, the probe is caused to vibrate by applying piezoelectric energy. Two piezoelectric elements are used, one providing vibration


Dead Stock

FIG. 3.21e The cross-section of a vibrating probe and illustrations of false level indications caused by not probably considering the effects.

(at a frequency of 200 to 400 Hz) and the other receiving it. When the probe is buried by the process material, the vibration decreases, and this decrease is used to trigger the switch. To make it immune to the effects of hopper vibration, the switch has a relatively high resonant frequency. Figure 3.21e illustrates that the sensing probe should be located so that the angle of repose will not cause false level indication when the solids level is low.

CONCLUSION Vibrating reed switches are reliable devices that operate on a readily understood principle. Because they can detect the 3 presence of materials with a bulk density as low as 1.0 lb/ft 3 (0.016 g/cm ), they have a wide range of applications, particularly in solids level detection. Their limitation is that the switch setting cannot be changed without changing the length or the mounting location of the switch.

Bibliography Bacon, J. M., The changing world of level measurement, InTech, June 1996. Bahner, M., A practical overview of continuous level measurement technologies, Flow Control, June-July 1997. Berto, F. J., Technology review of tank measurement errors reveals techniques for greater accuracy, Oil & Gas J., March 3, 1997. Carsella, B., Popular level-gauging methods, Chemical Process., December 1998. Control level under fouling conditions, Hydrocarbon Processing, November 2000. Entwistle, H., Survey of Level Instruments, ISA Conference, Anaheim, CA, Paper #91-0484, 1991. Felton, B., Level measurement: ancient chore, modern tools, InTech, August 2001. Glenn, L. E., Tank gauging—comparing the various technologies, in ISA Conf. Proc., Anaheim, CA, Paper #91-0471, 1991. How can we measure level of petroleum sludge? Control, August 1999. Hughes, T. A., Measurement and Control Basics, 3rd ed., ISA, Research Triangle Park, NC, 2002. Johnson, D., Level sensing in hostile environments, Control Engineering, August 2001. Johnson, D., Checking level: not glamorous, sometimes dangerous, but necessary, Control Eng., August 2001.

3.21 Vibrating Level Switches

Koeneman, D. W., Evaluate the options for measuring process levels, Chemical Eng., July 2000. Liptak, B. G., On-line instrumentation, Chemical Eng., March 31, 1986. Nyce, D. S., Tank gauging advances, Fuel Technology Management, January 1997. Parker, S., Selecting a level device based on application needs, in 1999 Fluid Flow Annual, Putman Publishing, Itasca, IL, 1999, 75–80. Paul, B. O., Seventeen level sensing methods, Chemical Process., February 1999.


559

Sholette, W., Pick the proper level measurement technology, Chemical Eng. Prog., October 1996. Tuning fork notes level of salt in CEGB brine tanks, Process Eng., January 1975. Van de Kamp, W., The Theory and Practice of Level Measurement, 17th ed., Endress+Hauser, Greenwood, IN, 2001. Waterbury, R. C., Liquid level measurement 101, Control, November 1998.

Temperature Measurement

4

4.1 APPLICATION AND SELECTION

565

Introduction 565 Orientation Tables 565 International Practical Temperature Scale 570 Temperature Sensors 572 Nonelectric Temperature Sensors 572 Liquid-in-Glass Thermometers 572 Bimetallic Thermometers 573 Filled System Thermometers 573 Bistate/Phase Change Sensors 573 Electronic Thermometers/Sensors 573 Thermocouples 573 Resistance Temperature Detectors 574 Thermistors 575 Radiation Pyrometers 575 Solid-State Sensors 578 Heat-Flow and Thermal-Conductivity Sensors 578 Intelligent Transmitters and Remote Input/Output (I/O) 579 Fieldbus Structures 580 Advanced Transmitters 580 Temperature Measurement Applications 581 High Temperature Measurement 581 Speed of Response 581 Surface Measurement 581 Measuring the Temperature of Solids 582 Averaging Measurements 584 Narrow Span Measurements 584

Installation Considerations 584 Temperature Transmitters in Place of Direct Wiring 584 Lower Wiring Costs 584 Protect Signals from Plant Noise 584 Stop Ground Loops 586 Reduce Hardware and Stocking Costs 586 Match the Best Sensor to the Application 586 Enhance Accuracy and Stability 586 Simplify Engineering and Prevent Miswiring 586 Ease Future Upgrades 586 Lower Maintenance Time and Expense 586 Avoid Lead Wire Imbalances 587 Calibration/Certification 587 Agency Approvals for Hazardous Areas 587 Safety-Related Applications 587 References 588 Bibliography 588 4.2 BIMETALLIC THERMOMETERS

590

Bimetallic Springs 591 Thermometers 591 Dial Orientation and Size 592 Advantages and Disadvantages 592 Bibliography 592 561


562


4.3 CALIBRATORS AND SIMULATORS Temperature Calibration Baths Simulators 596 Conclusions 596 Bibliography 596

594 595

Introduction 620 Integrated Circuit Temperature Sensors 621 Diode-Type Temperature Sensors 621 Reference 622 Bibliography 622

4.4 CONES, CRAYONS, LABELS, PAINTS, AND PELLETS 599 Introduction 600 Color Indicators 600 Paints and Pellets 600 Crayons 600 Pyrometric Cones 601 The Ceramics Industry 601 Bar and Hole Indicators 602 Engine Test Research 603 References 603 Bibliography 603

4.5 FIBER-OPTIC THERMOMETERS

604

Thermometer Design 605 The Optical Fiber Thermometer (OFT) Sensors 606 Conclusions 607 References 608 Bibliography 608

4.6 FILLED-BULB AND GLASS-STEM THERMOMETERS 610 Glass-Stem Thermometers 611 Filled Thermal Systems 612 Bulbs, Wells, and Capillaries 612 Class I: Liquid-Filled Systems 614 Class II: Vapor Systems 615 Class IIA Systems 615 Class IIB Systems 615 Class IIC Systems 616 Class IID Systems 616 Class III: Gas-Filled Systems 616 Class V: Mercury-Filled Systems 617 Ambient-Temperature Compensation 617 Effects of Bulb Elevation 618 Barometric Errors 619 Conclusions 619 Reference 619 Bibliography 619


4.7 INTEGRATED CIRCUITRY TRANSISTORS AND DIODES 620

4.8 MISCELLANEOUS AND DISCONTINUED SENSORS 623 Self-Measuring Devices 623 Acoustic Time Domain Reflectometry 624 Carbon Resistors 624 Capacitance Cable for Detecting Hot Spots 624 Fluidic Sensors 625 Johnson Noise Thermometer 625 Liquid Crystals 625 Paramagnetic Salts 625 Spectroscopic Temperature Measurement 626 Thermography 626 Fiber Bragg Grating Temperature Sensors 626 Discontinued Temperature Sensors 626 Pneumatic and Suction Pyrometers 626 Suction Pyrometers 626 Pneumatic Pyrometers 627 Quartz Crystal Thermometry 627 Advantages and Disadvantages 628 References 629 Bibliography 629 4.9 RADIATION AND INFRARED PYROMETERS

630

Introduction 631 Theoretical Relationships 632 The Theoretical and Real Targets 633 Emittance, Emissivity 633 Selecting the Radiation Pyrometer 634 Radiation Pyrometer Designs 635 Total Radiation Pyrometers 635 Narrow-Band Pyrometers 635 Ratio Pyrometers 636 Manual Optical Pyrometers 637 Automatic Optical and IR Pyrometers 638 Detectors 638 Thermal Detectors 638 Photo-Detectors 639 Selection 639 Installations and Accessories 640 Advances and New Developments 641 Summary 641 Advantages 642 Disadvantages 642

Contents of Chapter 4

Limited Control Range 663 Zero Energy Band Control 663 Split Range Control 663 Electrical/Electronic Designs 664 Recent Advances 664 Bibliography 665

Definitions 642 Reference 643 Bibliography 643

4.10 RESISTANCE TEMPERATURE DETECTORS

645

Resistance Temperature Detector Basics 647 Detectors and Their Applications 647 Industrial RTD Construction Requirements 648 Platinum RTDs 648 Base-Metal RTDs 649 Balco 649 Copper 649 Measuring the RTD Resistance 649 Two-Wire RTDs 649 Three-Wire RTDs 650 Four-Wire RTDs 651 Sensor Construction 652 Thermowells 653 Installation 653 Transmitters 654 Intelligent Transmitters 654 A/D Converters, Digital Protocols 655 Advantages and Limitations 655 Reference 655 Bibliography 655

4.12 THERMISTORS

666

Introduction 667 Historical Note 667 Resistance-Temperature Characteristic 667 Sensor Types 668 Temperature Measurement 668 Microammeter Readout 668 Wheatstone Bridge 669 Digital Instrumentation 670 Thermistors Combined with Resistors 670 Self-Heating Effect 670 Applications 671 Calibration and Testing 671 Advantages and Limitations 671 References 672 Bibliography 672 4.13 THERMOCOUPLES

4.11 TEMPERATURE SWITCHES AND THERMOSTATS 657 Introduction 658 Electronic Temperature Switches 658 Input from Transmitter 659 Input Directly from Sensor 659 Installation Considerations 659 In The Control Room 659 Field Mounting 660 Availability and Reliability 660 Electromechanical Temperature Switches 660 Features Required for Industrial Applications 661 Safety Considerations 661 Thermostats 661 Electromechanical Designs 661 Pneumatic Designs 662 Pneumatic-Bimetallic 662 Throttling 662 Offset Error 662 Design Features 663 Advanced Features 663 Adjustable Gains or Proportional Bands 663 Dual Set Points 663


673

Theory of Operation 675 Interpreting the Generated Voltage 675 Laws of Intermediate Temperatures and Metals 676 Cold Junction Compensation 676 Multiplexing 678 Hardware Compensation 678 Measuring the EMF Generated 678 Transmitter Location and Noise 679 Intelligent Transmitters 679 Thermocouple Types 679 ISA Types J, S, and T 679 ISA Types B, E, K, R, and N 679 Thermocouple Construction and Protection 682 Measuring Junction Designs 682 Extension Wires 682 Sheath Materials 683 Thermowells 683 Surface Temperature Detectors 684 Specialized Detectors 684 Needle Sensors 684 Suction Pyrometers 685 Installation and Protection 685 Multiple Thermocouples 685 Average Temperatures and Temperature Differences 685

563

564


Thermopiles 685 Thermocouple Burnout 685 Protection Against Noise 686 Normal Mode Noise 687 Calibration, Diagnostics, and Transmission 687 Calibration 687 Diagnostics 687 Transmission 687 Intelligent Transmitters 688 Advantages and Limitations 688 Thermocouple Tables 688 Converting Millivoltage to Temperature 694 Example 1 694 Example 2 695 Converting Temperature to Millivoltage 695 Example 1 695 Example 2 695 References 696 Bibliography 696


4.14 THERMOWELLS

697

Introduction 698 Thermowell Types 698 Protection Tubes 698 Sheaths 698 Thermowell Installation 699 Immersion Depth 699 Thermowell Time Constants References 703 Bibliography 703

700

4.15 ULTRASONIC AND SONIC THERMOMETERS Operating Principle 705 Ranges and Applications 705 Boiler/Furnace Applications 706 Conclusions 707 References 707 Bibliography 707

705

4.1

Application and Selection D. RALL

(1969)

B. G. LIPTÁK

Partial List of Suppliers:

(1982, 1995)

L. W. MOORE, B. ADLER

(2003)

ABB (www.abb.com) Foxboro/Invensys (www.foxboro.com/temp) Honeywell (www.iac.honeywell.com/ichome) Hukseflux (www.hukseflux.com) Kamstrup (www.kamstrup-process.com) Mathis Instruments Ltd. (www.mathis.unb.ca) Moore Industries-International Inc. (www.miinet.com/products/ca_temperature.shtml) Rosemount/Emerson (www.rosemount.com/products/temperature/) Siemens (www.sea.siemens.com) Yokogawa (www.yokogawa.com)

INTRODUCTION Temperature is as fundamental a physical concept as the three basic quantities of mechanics: mass, length, and time. Temperature is an expression that denotes a physical condition of matter. Yet, the idea of temperature is a relative one, arrived at by a number of conflicting theories. Classic kinetic theory depicts heat as a form of energy associated with the activity of the molecules of a substance. These minute particles of all matter are assumed to be in continuous motion that is sensed as heat. Temperature is a measure of this heat. To standardize on the temperature of objects under varying conditions, several scales have been devised. The Fahrenheit scale arbitrarily assigns the number 32 to the freezing point of water and the number 212 to the boiling point of water. The interval is divided into 180 equal parts. The Centigrade or Celsius scale defines the freezing point of the water to be 0, and its boiling point to be 100. In line with the classic theory, some relation to the point where molecular motion is at a minimum had to be established, and the Kelvin scale, using Centigrade divisions, was drawn. Zero Kelvin was determined to be 273.19°C. The Rankine scale places its zero at 459.61°F and uses Fahrenheit divisions in the same arbitrary way in which Lord Kelvin used the Celsius scale. Orientation Tables The range in temperature within the universe varies 18 orders of magnitude. It ranges from the near absolute zero of black space to the billions of degrees in the nuclear fusion process deep within the stars. But the practical range on earth can be

considered as extending from 1°R upward about 5 decades to around 20,000°R. This is still a tremendous range, and no single sensor could possibly cover it. Table 4.1a provides the reader of this handbook with an orientation table containing information on the ranges and other features of the various temperature sensors. Table 4.1b is a conversion table, which is convenient when one has to go from Fahrenheit to Centigrade units and back. Therefore, one of the restrictions on the temperature sensor concerns the temperature range over which it can stay reasonably accurate. Table 4.1c provides the approximate temperature ranges of each sensor type. The many types of sensors are listed on the left, while some of their characteristics are shown horizontally across the top. If it is not known what general type of sensor will do a specific job, the table can help point the way to the right selection. Once the class of sensors has been found, the data in the table will give a rough idea of the applicability of that design. When the possible choices of selection have been narrowed down to a few instrument types, the reader should turn to the corresponding sections of this chapter. In the front of each section there is a listing of range, accuracy, cost, and vendors. Inspecting these briefly, one can determine if the instrument generally meets the requirements or not. If it does, one should read the section for a description of the design and its available variations in detail. If some of the features are unacceptable, one should proceed to the next choice noted in the orientation table (Table 4.1a). Temperature sensors should be selected to meet the requirements of specific applications. Sensor Selection Table 4.1d can 1 assist the reader in this task. If the application engineer determines the required temperature range, the nature of the information required (point or average temperature), and the nature of the process environment, this table can be used to determine 565


566

TABLE 4.1a Orientation Table for Temperature Sensors

Fiber-Optic Filled Elements Liquid Vapor

Glass-Stem Therm. Integrated Circuit Diodes Transistors Misc.— Carbon Resistors

0.5−2

0.1

0.6

0.5−2

1.2

0.5−2

0.25

0.1−2

0.01

0.2−2

0.2

2

1 0.5

Paramagnetic Salts

1 1 1

2

20

2

25

Thermography Pyrometers —Suction Pneumatic

1

5

Sensitivity

N

Maximum Distance to Readout in Feet (0.305m)

Response Time

G

Complete System

Repeatability

G

Linear

Stability

F

Interchangeable

Controller

E

N

N

F

F

E

G

N

1000+

E

F

F

G

40

E

F

G

F

N

200

E

F

G

F

150

50

N

N

F

E

F

F

G

E

E

G

G

G

G

G

E

F

G

G

E

N

F, G

G

G

G

N

N

F

F

F

F

N

E

G, E

G

E, G

G G

G, E G, E

E G

F, G

F

G

F

P

N

F

G

F

P

N

E

F

F

F

0.005

Spectroscopy

2

0.2

Indicator

Recorder

8 1

0.5−2

Liquid Crystals


Fluidic Sensors

Pyrometric Cones

1

Gas Mercury

1−2

Available With

Large

100 200 500 1000 2000 5000 10000

Medium

−450 −300 −100 0

Small

260 538 1094 2760 5538

Above $1000

93

°F

Between $200 and $1000

38

Sensor Size

Cost ($)

Under $200

Color Indicators

−268 −184 −73 −18

Best Attainable in°F (°C = 5/9°F)

°C

% Full Scale or % of Span

Between 100 and 1000°F

Type

Above 1000°F (538°C)

Under 100°F (38°C)

Bimetallic Elements

Temperature Range

Accuracy

N

N N

F

N N

N


Available Span

Radiation Pyrometers— Optical & Ratio

Narrow & Wide Band

Quartz Crystals

1–2

0.5–2

Platinum

Thermistors

Resistance Bulbs—Nickel

0.1

2–10 5

0.2

0.25

0.3

0.15

0.2

0.2

0.02

F

F

G

G

F

F

E

G

E

G

G

E

N

100

N

100

N

1000

G, E

E

G

E

N

N

1000

E

E

G

G, E

N

3000

F

G

E

E

N

N

3000

Thermocouples— Type T

0.1

1.5

G

G

G

G

N

N

3000

Type J

0.1

2.5

G

G

G

G

N

N

3000

Type K

0.1

2.5

G

G

G

G

N

3000

0.1

4

G

G

E

E

N

N

3000

5

G

F, G

E

E, G

N

N

Types R & S

Ultrasonic Terminology

N–No or None E–Excellent G–Good F–Fair

Interchangeable sensor, without recalibration of entire system. System is complete when sensor and readout is sold as a single unit. When several readouts can be used with the same sensor, system is not considered to be complete. Without special compensation.

Recommended Available but not recommended

4.1 Application and Selection 567


F

C

F

C

F

C

F

C

F

C

F

C

–459.4

F

−17.8

0

32

10.0

50

122.0

38

100

212

260

500

932

538

1000

1832

816

1500

2732

1093

2000

3632

1371

2500

4532

−268

–450

−17.2

1

33.8

10.6

51

123.8

43

110

230

266

510

950

543

1010

1850

821

1510

2750

1099

2010

3650

1377

2510

4550

−262

−440

−16.7

2

35.6

11.1

52

125.6

49

120

248

271

520

968

549

1020

1868

827

1520

2768

1104

2020

3668

1382

2520

4568

−257

–430

−16.1

3

37.4

11.7

53

127.4

54

130

266

277

530

986

554

1030

1886

832

1530

2786

1110

2030

3686

1388

2530

4586

−251

–420

−15.6

4

39.2

12.2

54

129.2

60

140

284

282

540

1004

560

1040

1904

838

1540

2804

1116

2040

3704

1393

2540

4604

−246

–410

−15.0

5

41.0

12.8

55

131.0

66

150

302

288

550

1022

566

1050

1922

843

1550

2822

1121

2050

3722

1399

2550

4622

−240

–400

−14.4

6

42.8

13.3

56

132.8

71

160

320

293

560

1040

571

1060

1940

849

1560

2740

1127

2060

3740

1404

2560

4640

−234

–390

−13.9

7

44.6

13.9

57

134.6

77

170

338

299

570

1058

577

1070

1958

854

1570

2858

1132

2070

3758

1410

2570

4658

−229

–380

−13.3

8

46.4

14.4

58

136.4

82

180

356

304

580

1076

582

1080

1976

860

1580

2876

1138

2080

3776

1416

2580

4676

−223

–370

−12.8

9

48.2

15.0

59

138.2

88

190

374

310

590

1094

588

1090

1994

886

1590

2894

1143

2090

3794

1421

2590

4694

−218

–360

−12.2

10

50.0

15.6

60

140.0

93

200

392

316

600

1112

593

1100

2012

871

1600

2912

1149

2100

3812

1427

2600

4712

−212

–350

−11.7

11

51.8

16.1

61

141.8

99

210

410

321

610

1130

599

1110

2030

877

1610

2930

1154

2110

3830

1432

2610

4730

−207

–340

−11.1

12

53.6

16.7

62

143.6

100

212

413

327

620

1148

604

1120

2048

882

1620

2948

1160

2120

3848

1438

2620

4748

−201

–330

−10.6

13

55.4

17.2

63

145.4

104

220

428

332

630

1166

610

1130

2066

888

1630

2966

1166

2130

3866

1443

2630

4766

−196

–320

−10.0

14

57.2

17.8

64

147.2

110

230

446

338

640

1184

616

1140

2084

893

1640

2984

1171

2140

3884

1449

2640

4784

−190

–310

−9.44

15

59.0

18.3

65

149.0

116

240

464

343

650

1202

621

1150

2102

899

1650

3002

1177

2150

3902

1454

2650

4802

−184

–300

−8.89

16

60.8

18.9

66

150.8

121

250

482

349

660

1220

627

1160

2120

904

1660

3020

1182

2160

3920

1460

2660

4820

−179

–290

−8.33

17

62.6

19.4

67

152.6

127

260

500

354

670

1238

632

1170

2138

910

1670

3038

1188

2170

3938

1466

2670

4838

−173

–280

−7.78

18

64.4

20.0

68

154.4

132

270

518

360

680

1256

638

1180

2156

916

1680

3056

1193

2180

3956

1471

2680

4856

−169

–273

−459.4

−7.22

19

66.2

20.6

69

156.2

138

280

536

366

690

1274

643

1190

2174

921

1690

3074

1199

2190

3974

1477

2690

4874

−168

–270

−454

−6.67

20

68.0

21.1

70

158.0

143

290

554

371

700

1292

649

1200

2192

927

1700

3092

1204

2200

3992

1482

2700

4892

−162

–260

−436

−6.11

21

69.8

21.7

71

159.8

149

300

572

377

710

1310

654

1210

2210

932

1710

3110

1210

2210

4010

1488

2710

4910

−157

–250

−418

−5.56

22

71.6

22.2

72

161.6

154

310

590

382

720

1328

660

1220

2228

938

1720

3128

1216

2220

4028

1493

2720

4928

−151

–240

−400

−5.00

23

73.4

22.8

73

163.4

160

320

608

388

730

1346

666

1230

2246

943

1730

3146

1221

2230

4046

1499

2730

4946

−146

–230

−382

−4.44

24

75.2

23.3

74

165.2

166

330

626

393

740

1364

671

1240

2264

949

1740

3164

1227

2240

4064

1504

2740

4964

−140

–220

−364

−3.89

25

77.0

23.9

75

167.0

171

340

644

399

750

1382

677

1250

2282

954

1750

3182

1232

2250

4082

1510

2750

4982

−134

–210

−346

−3.33

26

78.8

24.4

76

168.8

177

350

662

404

760

1400

682

1260

2300

960

1760

3200

1238

2260

4100

1516

2760

5000

−129

–200

−328

−2.78

27

80.6

25.0

77

170.6

182

360

680

410

770

1418

688

1270

2318

966

1770

3218

1243

2270

4118

1521

2770

5018

−123

–190

−310

−2.22

28

82.4

25.6

78

172.4

188

370

698

416

780

1436

693

1280

2336

971

1780

3236

1249

2280

4136

1527

2780

5036

−118

–180

−292

−1.67

29

84.2

26.1

79

174.2

193

380

716

421

790

1454

699

1290

2354

977

1790

3254

1254

2290

4154

1532

2790

5054


C

F

C

F


C −273.1

568

TABLE 4.1b Temperature Conversion Table (When converting any temperature, find the boldface value of the temperature to be converted and look to the left for its °C equivalent or to right for its °F equivalent. Temperatures not listed can be converted using °F = (9°C/5) + 32 or °C = 5(°F − 32)/9.)

−112

–170

−274

−1.11

30

86.0

26.7

80

176.0

199

390

734

427

800

1472

704

1300

2372

982

1800

3272

1260

2300

4172

1538

2800

5072

−107

–160

−256

−0.56

31

87.8

27.2

81

177.8

204

400

752

432

810

1490

710

1310

2390

988

1810

3290

1266

2310

4190

1543

2810

5090

−101

–150

−238

0

32

89.6

27.8

82

179.6

210

410

770

438

820

1508

716

1320

2408

993

1820

3308

1271

2320

4208

1549

2820

5108

−95.6

−140

−220

0.56

33

91.4

28.3

83

181.4

216

420

788

443

830

1526

721

1330

2426

999

1830

3326

1277

2330

4226

1554

2830

5126

−90.0

–130

−202

1.11

34

93.2

28.9

84

183.2

221

430

806

449

840

1544

727

1340

2444

1004

1840

3344

1282

2340

4244

1560

2840

5144

−84.4

–120

−184

1.67

35

95.0

29.4

85

185.0

227

440

824

454

850

1562

732

1350

2462

1010

1850

3362

1288

2350

4262

1566

2850

5162

−78.9

–110

−166

2.22

36

96.8

30.0

86

186.8

232

450

842

460

860

1580

738

1360

2480

1016

1860

3380

1293

2360

4282

1571

2860

5180

−73.3

–100

−148

2.78

37

98.6

30.6

87

188.6

238

460

860

466

870

1598

743

1370

2498

1021

1870

3398

1299

2370

4298

1577

2870

5198

−67.8

–90

−130

3.33

38

100.4

31.1

88

190.4

243

470

878

471

880

1616

749

1380

2516

1027

1880

3416

1304

2380

4316

1582

2880

5216

−62.2

–80

−112

3.89

39

102.2

31.7

89

192.2

249

480

896

477

890

1634

754

1390

2534

1032

1890

3434

1310

2390

4334

1588

2890

5234

−56.7

–70

−94

4.44

40

104.0

32.2

90

194.0

254

490

914

482

900

1652

760

1400

2552

1038

1900

3452

1316

2400

4352

1593

2900

5252

−51.1

–60

−76

5.00

41

105.8

32.8

91

195.8

488

910

1670

766

1410

2570

1043

1910

3470

1321

2410

4370

1599

2910

5270

−45.6

–50

−58

5.56

42

107.6

33.3

92

197.6

493

920

1688

771

1420

2588

1049

1920

3488

1327

2420

4388

1604

2920

5288

−40.0

–40

−40

6.11

43

109.4

33.9

93

199.4

499

930

1706

777

1430

2606

1054

1930

3506

1332

2430

4406

1610

2930

5306

−34.4

–30

−22

6.67

44

111.2

34.4

94

201.2

504

940

1724

782

1440

2624

1060

1940

3524

1338

2440

4424

1616

2940

5324

−28.9

–20

−4

7.22

45

113.0

35.0

95

203.0

510

950

1742

788

1450

2642

1066

1950

3542

1343

2450

4442

1621

2950

5342

−23.3

–10

14

7.78

46

114.8

35.6

96

204.8

516

960

1760

793

1460

2660

1071

1960

3560

1349

2460

4460

1627

2960

5360

−17.8

0

32

8.33

47

116.6

36.1

97

206.6

521

970

1778

799

1470

2678

1077

1970

3578

1354

2470

4478

1632

2970

5378

8.89

48

118.4

36.7

98

208.4

527

980

1796

804

1480

2696

1082

1980

3596

1360

2480

4496

1638

2980

5396

9.44

49

120.2

37.2

99

210.2

532

990

1814

810

1490

2714

1088

1990

3614

1366

2490

4514

1643

2990

5414

4.1 Application and Selection 569


570


TABLE 4.1c Temperature Sensor Accuracy and Range Useful Range °F*

Maximum Range °F*

Accuracy Standard Grade

Accuracy Premium Grade

32 to 1382

−346 to 2192

±4°F or 0.75%**

±2°F or 0.4%

Type K Chromal vs. Alumel

−238 to 2282

−454 to 2502

±4°F or 0.75% >32°F ±4°F or 2% 32°F ±2°F or 1.5% 32°F ±3°F or 1% 2

B

4I

kD

iam

eter

=12

Measurement

n. (

0.6

Disturbance

1m

to 2

)

4 I n. (

0.30 to 0 .61 *From Point of any m) Type of Disturbance (Bend, Expansion, Contraction, etc.)

0 2 3 4 5 6 7 8 9 10 Duct Diameters Downstream from Flow Disturbance* (Distance B)

FIG. 8.3e Minimum number of traverse points for particulate traverses.

eliminates the major aerodynamic interference effects. The probe nozzle is of the bottom-hook or elbow design. It is made of seamless 316 stainless steel or glass with a sharp, tapered leading edge. The angle of taper should be less than 30°, and the taper should be on the outside to preserve a constant internal diameter (ID).

1192

Analytical Instrumentation

For probe lining of either borosilicate or quartz glass, probe liners are used for stack temperatures up to approximately 900°F (482°C); quartz liners are used for temperatures between 900 and 1650°F (482 and 899°C). Although borosilicate or quartz glass probe linings are generally recommended, 316 stainless steel, Incoloy, or other corrosion-resistant metal may also be used. Selecting the Sampling Point The specific points of stack sampling are selected to ensure that the samples collected are representative of the material being discharged or controlled. These points are determined after examination of the process of the sources of emissions and their variation with time. In general, the sampling point should be located at a distance equal to at least eight stack or duct diameters downstream and two diameters upstream from any source of flow disturbance, such as expansion, bend, contraction, valve, fitting, or visible flame. (Note: This eight and two criterion is adopted to ensure the presence of stable, fully developed flow patterns at the test section.) For rectangular stacks, the equivalent diameter is calculated from the following equation: Equivalent diameter = 2(length × width)/(length + width) 8.3(1) Traversing Point Locations Next, provisions must be made to traverse the stack. The number of traverse points is 12. If the eight and two diameter criterion is not met, the required number of traverse points depends on the sampling point distance from the nearest upstream and downstream disturbances. This number may be determined by using Figure 8.3e. The cross-sectional layout and location of traverse points are as follows: 1. For circular stacks, the traverse points should be located on two perpendicular diameters, as shown in Figure 8.3f and Table 8.3g. 2. For rectangular stacks, the cross section is divided into as many equal rectangular areas as traverse points, such that the ratio of the length to width of the elemental area is between 1 and 2. The traverse points are to be located at the center of at least nine and preferably more equal areas, as shown in Figure 8.3f. Pitot Tube Calculation Form The velocity head at various traverse points is measured using the pitot tube assembly shown in Figure 8.3b. The gas samples are collected at a rate proportional to the stack gas velocity and analyzed for carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2). The pitot tube is calibrated by measuring the velocity head at some point in the flowing gas stream with both the Type S pitot tube and a standard pitot tube with a known coefficient. Other data also needed for calculation of the volumetric flow are stack temperature, stack and barometric pressures, and wet-bulb and dry-bulb temperatures of the gas sample at each traverse.


Rectangular Stack (Measure at Center of at least 9 Equal Areas)

R 0.916 R 0.837 R 0.707 R 0.548 R 0.316 R Circular Stack (10-Point Traverse)

FIG. 8.3f Traverse point locations for velocity measurement or for multipoint sampling.

Table8.3h gives the equations for converting pitot tube readings into velocity and mass flow, and a typical data sheet 3 for stack flow measurements. Sampling Velocity for Particle Collection Based on the range of velocity heads, a probe with a properly sized nozzle is selected to maintain isokinetic sampling of particulate matter. As shown in Figure 8.3i, a converging stream will be developed at the nozzle face if the sampling velocity is too high. Under this subisokinetic sampling condition, an excessive amount of lighter particles enters the probe. Because of the inertia effect, the heavier particles, especially those in the range of 3 microns or greater, travel around the edge of the nozzle and are not collected. The result is a sample indicating an excessively high concentration of lighter particles, and the weight of the solid sample is in error on the low side. Conversely, portions of the gas stream approaching at a higher velocity are deflected if the sampling velocity is below that of the flowing gas stream. Under this superisokinetic sampling condition, the lighter particles follow the deflected stream and are not collected, while the heavier particles, because of their inertia, continue into the probe. The result is a sampling indicating high concentration of heavier particles, and the weight of solid sample is in error on the high side. Isokinetic Sampling Isokinetic sampling requires the precise adjustment of the sampling rate with the aid of the pitot tube 9 manometer readings and nomographs such as APTD-0576. If the pressure drop across the filter in the sampling unit becomes too high, making isokinetic sampling difficult to maintain, the filter may be replaced in the midst of a sample run. To measure the concentration of particulate matter, the sampling time for each run should be at least 60 min, and the minimum volumetric rate of sampling should be 30 dry 3 4 scfm (51 m /h).

8.3 Analyzer Sampling: Stack Particulates

1193

TABLE 8.3g a Location of Traverse Points in Circular Stacks Traverse Point Number on a Diameter

Number of Traverse Points on a Diameter

2

4

6

8

10

12

14

16

18

20

22

24

1

14.6

6.7

4.4

3.2

2.6

2.1

1.8

1.6

1.4

1.3

1.1

1.1

2

85.4

25.0

14.6

1.5

8.2

6.7

5.7

4.9

4.4

3.9

3.5

3.2

3

75.0

29.6

19.4

14.6

11.8

9.9

8.5

7.5

6.7

6.0

5.5

4

93.3

70.4

32.3

22.6

17.7

14.6

12.5

10.9

9.7

8.7

7.9

5

85.4

67.7

34.2

25.0

20.1

16.9

14.6

12.9

11.6

10.5

6

95.6

80.6

65.8

35.6

26.9

22.0

18.8

16.5

14.6

13.2

7

89.5

77.4

64.4

36.6

28.3

23.6

20.4

18.0

16.1

8

96.8

85.4

75.0

63.4

37.5

29.6

25.0

21.8

19.4

9

91.8

82.3

73.1

62.5

38.2

30.6

26.2

23.0

10

97.4

88.2

79.9

71.7

61.8

38.8

31.5

27.2

11

93.3

85.4

78.0

70.4

61.2

39.3

32.3

12

97.9

90.1

83.1

76.4

69.4

60.7

39.8

13

94.3

87.5

81.2

75.0

68.5

60.2

14

98.2

91.5

85.4

79.6

73.8

67.7

15

95.1

89.1

83.5

78.2

72.8

16

98.4

92.5

87.1

82.0

77.0

17

95.6

90.3

85.4

80.6

18

98.6

93.3

88.4

83.9

19

96.1

91.3

86.8

20

98.7

94.0

89.5

21

96.5

92.1

22

98.9

94.5

23

96.8

24

98.9 a

Percent of stack diameter from inside wall to traverse point.

Heated Compartment (Hot Box) As shown in Figure 8.3a, the probe is connected to the heated compartment that contains the filter holder and other particulate-collecting devices, such as cyclone and flask. The filter holder is made of borosilicate glass, with a frit filter support and a silicone rubber gasket. The compartment is insulated and equipped with a heating system capable of maintaining a temperature around the filter holder during sampling at 248 ± 25°F (120 ± 14°C), or such other temperature as specified by the EPA. The thermometer should measure temperature to within 5.4°F (3°C). The compartment should be provided with a circulating fan to minimize thermal gradients. Ice-Bath Compartment (Cold Box) The ice-bath compartment contains a number of impingers and bubblers. The system for determining stack gas moisture


content consists of four impingers connected in series, as shown in Figure 8.3a. The first, third, and fourth impingers are of the Greenburg–Smith design. To reduce the pressure drop, the tips are removed and replaced with a 0.5-in. (12.5-mm) ID glass tube extending to 0.5 in. (12.5 mm) from the bottom of the flask. The second impinger is of the Greenburg–Smith design with a standard tip. During sampling for particulates, the first and second impingers are filled with 100 ml (3.4 oz) of distilled and deionized water. The third impinger is left dry to separate entrained water. The last impinger is filled with 200 to 300 g (7 to 10.5 oz) of precisely weighed silica gel (6 to 16 mesh) that has been dried at 350°F (177°C) for 2 h to completely remove any remaining water. A thermometer capable of measuring temperature to within 2°F (1.1°C) is placed at the outlet of the last impinger for monitoring purposes. Crushed ice should be added during the run to maintain the temperature of the gas, leaving the last impinger at 60°F (16°C) or less.

1194


TABLE 8.3h Pitot Tube Calculation Sheet STACK VOLUME DATA Stack no. ____________________ Station ____________________ Date ____________________

Page ____________________

Name of firm __________________________________________________________________________________

Point

Reading, in. of H2O

Position, in.

H

Temperature t3, °F

Velocity V3, ft/sec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Totals Average Absolute temperature: Ts = ts + 460 = °R Dry-bulb temperature: td = ____________________ °F

Barometer: Pb = _______________ in Hg

Wet-bulb temperature: tw = ____________________ °F

Stack gauge pressure: _________ in., H2O

Absolute humidity: W = __ lb H2O/lb dry gas Stack area: As = ____________________ ft Component

2

Stack absolute pressure: Ps = ___ in., H2O/13.6 ± Ph ___ in., Hg Pitot correction factor: Fs = ___________

Volume Fraction, Dry Basis × Molecular Weight = Weight Fraction, Dry Basis

Carbon dioxide

44 =

Carbon monoxide

28 =

Oxygen

32 =

Nitrogen

28 =

Average dry gas molecular weight: M = ________ Specific gravity of stack gas: GS =

0.62 M (W + 1) 0.62 × __ × __ = = _________________ 18 + MW 18 + __

(Reference dry air at same conditions) Velocity: Vs = 2.9 Fs

29.92 × TS PS × GS

H = 2.9 × __

29.92 × __ __ × __

H =______________ft./sec.

2

Volume = ______ ft./sec × _____ ft × 60 _____ = _____ cfm. Standard volume = cfm. ×


P 530 530 __ × S = __ × × = __ scfm. TS 29.92 29.92 _

8.3 Analyzer Sampling: Stack Particulates

1195

V2 V1

Gas Stream Isokinetic V1 = V2

V2 V1

Gas Stream

Super Isokinetic V1 >> V2 V1 V2

Gas Stream

FIG. 8.3j Automatic stack train. (Courtesy of ThermoAndersen.)

Sub Isokinetic V1 t1, the continuous addition of Rr, coupled with the Ro present from the redox reaction with Ao, results in a continuously increasing current flow. Curve (b) indicates the response when only the analyte reaction is reversible. The curve from to < t ≤ t1 is identical to the same portion of curve (a) for the same reasons. However, for t > t1, no current is detected because the reagent half-reaction is irreversible. Curve (c) shows the results when the analyte reaction is irreversible but the reagent half-reaction is reversible. Current is not detected until Rr remains in the solution. This case is often called a dead-start titration because the current does not begin until all the analyte has been removed. Figure 8.4i illustrates the design of another membrane-type amperometric sensor.


Cathode

Teflon Membrane

Polarography Polarography is a process for monitoring the diffusion current flow between working and auxiliary electrodes as a function of applied voltage as it is varied systematically. The diffusion current that results is linearly dependent on the concentration of the analyte. Polarography can be applied using direct current, pulsed direct current, or alternating current (AC) voltage excitation waveforms. Dissolved oxygen determination is an example of an application for which polarography is used (Figure 8.4e). Nobel Prize winner Jaroslav Heyrovsky developed direct current polarography in 1922. Polarography is performed by measuring the current flow, which is a function of the systematically varied electrode potential. Different types of polarographies can be performed as a function of the voltage variation patterns, voltage sweep rates, and the choice of working electrode materials. Figure 8.4j shows a typical polarographic response curve. Diffusion Plateau

(E 1 , i 1 ) Current

(b)

2

0

2

Cathode Potential

id

−1.0 V

FIG. 8.4j Polarogram showing typical current vs. cathode potential response. The diffusion current, id; the half-wave potential, E1/2; and the halfwave current, i1/2, are all illustrated.

8.4 Analyzers Operating on Electrochemical Principles

1205

For this curve, the K term in Equation 8.4(1) is defined as 1 /2

K = nFC°(7/3Do)

8.4(4)

and 8.4(5)

where id is the diffusion current. The curve shows an S-shaped current response as the potential at the cathode, the working electrode where analyte reduction occurs, becomes more negative. Current limited by diffusion occurs at cathodic potentials beyond the potential required to initiate reduction. Thus, id is sufficient to reduce the diffusion-supplied analyte concentration to zero. The significance of this is that the diffusion rate is controlled by the concentration gradient established between the concentration at the surface of the working electrode, which is held to zero as a result of the electrode reaction, and the concentration existing in the bulk of the solution. The current observed for the Faradic process of interest is directly proportional to the concentration of the desired analyte. Subsequently, this technique was modified by changing the voltage excitation waveform to alternating current (AC polarography) or to pulsed direct current (pulse polarography or differential pulse polarography); these modifications resulted in increased sensitivity and better discrimination for the detection of the desired analyte in a background matrix of interferences. Advantages One of the valuable aspects of a polarographic determination is that the potential observed at the point i1/2 = id/2 is unique to the analyte in the solution matrix. This occurs because the potential at i1/2 is independent of the reactant concentrations but directly related to the standard potential for the analyte half-reaction. This inflection point potential is defined as the half-wave potential, E1/2, and is a common parameter for qualitative identification of electroactive components in a solution. When the reaction is fast and reversible and the diffusion coefficients of the oxidized and reduced forms of the analyte are virtually identical, E1/2 becomes E°. Another value of polarography is that the diffusion current is linearly related to the concentration of the analyte. Coulometry Coulometry is the process of monitoring analyte concentration by detecting the total amount of electrical charge passed between two electrodes that are held at a constant potential or when constant current flow passes between them. Because the coulomb provides a direct connection through Faraday’s law between reaction current and analyte concentration during a redox process, coulometry offers a direct method of determining analyte concentration.


Current

1/2

(nF/KT)(E − E° − ln(fo /fr(Do /Dr) ) = ln(id − i/i)

t1idt t0

t0

Time

t1

FIG. 8.4k Response curve for controlled potential coulometry, showing the relationship among current, i; time, t; and coulombs, QF , in the Faradic portion of the curve.

Two types of coulometry are possible: constant current and controlled potential. Constant current coulometry depends on analyte oxidation–reduction to support the specified current flow. When the supply of material is insufficient to carry this current, the electrode potential drifts until another reaction begins. The result of this process is a potential time relationship similar to the results obtained in chronopotentiometry, and the amount of charge passed is simply the product of the constant current and electrolysis time. Controlled Potential Coulometry By contrast, controlled potential coulometry is conducted by maintaining a constant electrode potential while a current flow is measured. The number of coulombs is determined by integration of the reaction current as a function of electrolysis time. Figure 8.4k illustrates the current–time relationship for controlled potential coulometry. The choice of integration time is important because the current is the sum of the Faradic component and a capacitative component. As a result of this dual contribution, current integration is not started until the capacitative-charging contribution is minimal.

CONCLUSIONS Although instruments are available for all types of voltametric analysis, currently only the potentiometric, polarographic, amperometric, and coulometric designs are available as online process analyzers. This is likely to change.

1206


Limitations

Bibliography

The limitations of this category of sensors are mostly related to the membranes. One major problem with the thin permeable membranes is the fact that the electrode response to the analyte is directly dependent on the condition of the membrane. Once the potential has been applied across the electrodes and the concentration depletion wave has reached the inner membrane wall, the amount of analyte that reaches the electrode depends on the diffusion of the analyte through the membrane. This situation is usually acceptable because the membrane condition is stable. The membrane itself is also subject to failure, and the surface of the membrane is exposed to the process fluid, which can alter its permeability to the analyte. These conditions make the long-term reliable use of electrolytic probes problematical, because they require constant maintenance and recalibration. Amperometric end points pose another problem. To date, provisions for the various end points possible are not available, and care must be taken when determining analysis end points. Finally, instrumentation for coulometric methods requires that the working electrode be physically separated from the auxiliary electrode so that the reaction products do not migrate to the opposite electrode. Sintered glass discs are used to restrict the migration of reaction products between the electrodes, but they permit electrical conduction through the cell. Table 8.4a provides a summary of voltametric methods and applications.

Adams, V., Water and Wastewater Examination Manual, Chelsea, MI: Lewis, 1990. Bard, A. J. and Faulkner, L. R., Electrochemical Methods, New York: John Wiley & Sons, 1980. Bond, A. and Anterford, D., “Comparative Study of a Wide Variety of Polarographic Techniques with Multifunctional Instruments,” Analytical Chemistry, 44:721, 1972. Bradley, H. J. and Tsai, L. S., Amperometric Titration of Chlorine with a Modified pH Meter, Washington, D.C.: U.S. Department of Agriculture, 1999. Bretherick, L., Bretherick’s Handbook of Reactive Chemical Hazards, Stoneham, ME: Butterworth-Heinemann, 1999. Buonauito, R. P., “Oxygen Analyzers,” Measurements and Control, February 1990. Cali, G. V., “Improvements in pH Control and Dissolved Oxygen Instrumentation for Industrial and Municipal Waste Treatment,” Conference on Application of U.S. Pollution Control Technology in Korea, Seoul, Korea, March 22, 1989. Cardis, T. M., “Managing Data from Continuous Analyzers,” 1992 ISA Conference, Houston, TX, October 1992. Clark, K., “Chlorine Analyzers Cut Costs, Improve Performance,” InTech, May 1998. Evans, R., Potentiometry and Ion Selective Electrodes, New York: John Wiley & Sons, 1987. Flato, J., “The Renaissance in Polarographic and Voltametric Analysis,” Analytical Chemistry, 44:75A, 1972. Galster, H., pH Measurement: Fundamentals, Methods, Applications, Instrumentation, VCH, Weinheim, Germany, 1991. Gray, J. R., “Glass pH Electrode Aging Characteristics,” ISA/93 Technical Conference, Chicago, September, 1993. Hitchman, M. L., Measurement of Dissolved Oxygen, New York: John Wiley & Sons, 1978. Kissinger, P. T. and Heinemann, W. R., Laboratory Techniques in ElectroAnalytical Chemistry, New York: Marcel Dekker, 1984. NIOSH, NIOSH Manual of Analytical Methods: Supplement, 4th ed., 2 vols., Bpi Information Services, London, 1996. Riley, T. et al., Principles of Electroanalytical Methods, 1987. Skoog, D. A., Principles of Instrumental Analysis, Pacific Grove, CA: Brooks/Cole Publishing, 1997. Standard Methods for the Examination of Water and Wastewater, New York: APHA, AWWA, and WPCF, latest edition. Wang, H. Y., “Transient Measurement of Dissolved Oxygen Using Membrane Electrodes,” Biosensors, 4(5):273–285, 1989. Weiss, M. D., “Teaching Old Electrodes New Tricks,” Control, July 1991.

Advances Significant advances have been made by the introduction of equilibrium-state voltametric probes. These have the advantage of analyte sensitivities that are relatively independent of membrane coating and consume so little of the analyte as a result of the measurement process that they are able to work in processes where the sample flow rate past the face of the sensor is small. Reference 1.

Reimmuth, Analytical Chemistry, 32:1509, 1960.


8.5

Air Quality Monitoring R. A. HERRICK, R. G. SMITH

(1974)

B. G. LIPTÁK

(1995, 2003)

Costs:

A battery-operated adjustable-flow air sampling pump costs $500; equipment costs for an audit system might include $6000 for an ozone analyzer, $10,000 for a CO analyzer, $6000 for a pure air generator, $1500 for a methane reactor, and $15,000 for a gas calibrator; a microprocessor-based portable infrared spectrometer with preprogrammed multicomponent identification capability and with space for 10 userdefined standards for calibration, an AC/DC converter, a sample probe, and a carrying case costs $17,000.


AMC (Armstrong Monitoring Corp.) (www.armstrongmonitoring.com) Ametek/Thermox (www.thermox.com) Bran & Luebbe (www.branluebbe.com) Dasibi Environmental Corp. (www.dasibi.com) Delphian Corp. (www.delphian.com) Ecotech (www.ecotech.com.au) EMS (Environmental Monitoring Systems) (www.emssales.com) Enviro Technology (www.et.co.uk) Foxboro-Invensys (www.foxboro.com) Horiba Instrument Inc. (www.nettune.net) Innova Air Tech Instruments (www.inniva.dk) International Sensor Technology (www.intlsensor.com) IT Group (www.theitgroup.com) MSA Instrument Div. (www.msanet.com) Purafil Inc. (www.purafilonguard.com) Sensidyne Inc. (www.sensidyne.com) Servomex Co. (www.servomex.com) Sieger Gasalarm; Siemens Energy & Automation (www.sea.siemens.com) Sierra Monitor Corp. (www.sierramonitor.com) Sigrist-Photometer Ltd. (www.photometer.com) Teledyne Analytical Instruments (www.teledyne-ai.com) Thermo Environmental Instruments (www.thermoei.com) Thermo Gas Tech (www.thermo.com) Yokogawa Corp. of America (www.yca.com)

INTRODUCTION

AIR QUALITY MONITORING SYSTEMS

This section covers the monitoring of ambient air quality. Air sampling is discussed, but the same subject is also covered in more detail in Section 8.2. Particulate and opacity monitoring are not covered here, as those topics are discussed in a later section in this chapter. This section starts with a general discussion of air quality monitoring systems and continues with a description of air quality detecting sensors, automatic monitoring packages, and microprocessor-based portable ambient air analyzers. The section is concluded with a discussion of ambient air sampling for both gases and particulate matter.

There are many options for the types of air quality information that can be collected, and the cost of air quality monitoring systems can vary by orders of magnitude. Only with a thorough understanding of the decisions that must be made based on the information received from the air quality monitoring system can an appropriate selection be made. Regardless of the type of instruments used to measure air quality, the data are only as good as they are representative of the sampling site selected. See Table 8.5a for a list of air quality and meteorological parameters and their measurement ranges, accuracy, and method of analysis. 1207


1208


TABLE 8.5a Ambient Air Monitoring and Meteorological Measurement Parameters Compound

Range

Accuracy

Technique Employed

Oxides of nitrogen

0–20 ppm

0.5 ppb

Chemiluminescence or DOAS open path

Sulfur dioxide

0–20 ppm

0.5 ppb

Fluorescence dual-channel ratiometric phase detection or DOAS open path

Ozone

0–20 ppm

0.5 ppb

Ultraviolet photometrics or DOAS open path

Carbon monoxide

0–200 ppm

50 ppb

Gas filter correlation or DOAS open path

Carbon dioxide

0–100%

50 ppb

Gas filter correlation or DOAS open path

Benzene, toluene, xylene

0–5 ppm

0.5 ppb

Gas chromatography or DOAS open path

Nonmethane hydrocarbons

0–1000 ppm

0.01 ppm

FID or DOAS open path

Methane

0–1000 ppm

0.01 ppm

FID or DOAS open path

Particulates (PM10, TSP, PM2.5)

0–5 g/m

3

0.1 µg/m

Carbon particulates

0–5 g/m

3

0.25 µg/m

Local visual distance

0–16 km

±10%

Nephelometer

Wind speed

0–70 m/sec

0.22 m/sec

Anemometer

Wind direction

0–540°

±3°

Airfoil vane

Ambient temperature

–50 to 100°C

±0.1°C

Solid state thermistor

Relative humidity

0–100%

±2%

Thin film capacitor

Barometric pressure

800–1200 mbar

±1.3 mbar

Solid state transducer

Precipitation

NA

0.1 mm

Net radiometer

Solar radiation

250–2800 nm

9 mV/kWm

2

Pyranometer

Net radiation

250–60000 nm

8 mV/kWm

2

Net radiometer

3

Tapered element oscillating microbalance 3

Thermal CO2 method

Note: NA = not applicable; FID = flame ionization detector. Source: Courtesy of Ecotech.

The simplest air quality monitors are static sensors, which are exposed for a given length of time and are later analyzed in the laboratory. In some cases, these devices provided all the information required. More commonly, a system of automatic instruments measuring several different air quality parameters will be used. With more than a few instruments, the signals from these instruments can be retained on magnetic tape rather than on recorder charts. The most common errors in the design of air quality monitoring systems are poor site location and the acquisition of more data than necessary to accomplish the purpose of the installation. Purpose of Monitoring The principle purpose of air quality monitoring is frequently the acquisition of data for comparison to regulated standards. In the U.S., standards have been promulgated by the federal 1 government and by many of the states. Where possible, these standards have been based on the physiological effect of the air pollutant in question, so that human health is protected. The averaging time over which various concentration standards must be maintained is different for each pollutant. Tables 8.5b and 8.5c are tabulations of the ambient air quality standards promulgated by the federal government.


Impact of Single Sources Some air quality monitoring systems are operated to determine the impact of a single source or a concentrated group of sources of emission on the surrounding area. In this case, it is important to determine the background level, the maximum ground-level concentration in the area, and the geographical extent of the air pollutant impact of the source. When the source is isolated, such as a single industrial plant in a rural area, the design is straightforward. Utilizing meteorological records, which are normally available from nearby airports or from government meteorological reporting stations, a wind rose can be prepared to estimate the direction of principal drift of the air pollutant from the source. Dispersion calculations can be performed to estimate the location of the expected point of maximum ground-level concentration. As a rule of thumb, with stacks between about 50 and 350 ft tall, this point of maximum concentration will be approximately 10 stack heights downwind. The air quality monitoring system should include at least one sensor at the point of expected maximum ground-level concentration. Additional sensors should be placed not less than 100 stack heights upwind (prevailing) to provide a background reading, and at least two or three sensors should be placed between 100 and 200 stack heights downwind to determine the extent

8.5 Air Quality Monitoring

1209

TABLE 8.5b a Primary Air Quality Standards Long-Term Concentration 3 Level(µg/m )

Pollutant

Short-Term Concentration b 3 Level (µg/m )

Particulate matter

75—annual geometric mean

260—maximum 24-h concentration

Sulfur oxides (SO2)

80 (0.03 ppm)—annual arithmetic mean

365 (0.14 ppm)—maximum 24-h concentration

Carbon monoxide

—

10,000 (9 ppm)—maximum 8-h concentration or 40,000 (35 ppm)—maximum 1-h concentration

Photochemical oxidants

—


Hydrocarbons

—

160 (0.24 ppm)—maximum 3-h concentration (6–9 A.M.)

Nitrogen dioxide

100 (0.05 ppm)—mean arithmetic mean

a

Levels of air quality that are deemed necessary, with an adequate margin of safety, to protect the public health. Stated concentration not to be exceeded more than once per year.

b

TABLE 8.5c a Secondary Air Quality Standards Pollutant

Long-Term Concentration 3 Level(µg/m )

Short-Term Concentration b 3 Level (µg/m )

Particulate

60—annual geometric mean

150—maximum 24-h concentration

Sulfur dioxide

60 (0.02 ppm)—annual arithmetic mean


Carbon monoxide

—

10,000 (9 ppm)—maximum 8-h concentration or 40,000 (35 ppm)—maximum 1-h concentration

Photochemical oxidants

—


Hydrocarbons

—

160 (0.24 ppm)—maximum 3-h concentration (6–9 A.M.)

Nitrogen dioxide

100 (0.05 ppm)—maximum arithmetic mean

—

a

Levels of air quality that are deemed adequate to protect the public welfare from any known or anticipated adverse effects. Stated concentration not to be exceeded more than once per year.

b

of the travel of the pollutants from the source in question. If adequate resources are available, sampling at the intersection points of a rectilinear grid with its center at the source in question is advisable. With such a system for an isolated source, adequate data can be obtained in 1 year to determine the impact of the sources on the air quality of the area. There are very few instances where less than 1 year of data collection will provide adequate information because of the variability in climatic conditions on an annual basis. If a study of this type is performed to determine the effect on air quality of process changes, it may be necessary to continue the study for 2 to 5 years to develop information that is statistically reliable. Some air quality monitoring is designed for the specific purpose of investigating complaints concerning an unidentified source. This usually happens in urban situations for odor complaints. In these cases, a triangulation technique is used. By the use of this technique, human observers over a period of days can correlate the location of the observed odor and the direction of the wind. Plotting on a map can pinpoint the


offending source in most cases. While this is not an air pollution monitoring system in an instrumental sense, it is a useful tool in certain situations. Research Needs The research needs of air pollution call for a completely different approach to air quality monitoring. Here, the purpose is to define some known variable or combination of variables. This can be either a new atmospheric phenomenon or the evaluation of a new air pollution sensor. In the former case, the most important consideration is the proper operation of the instrument used. In the latter case, the most important factor is the availability of a reference determination against which the results of the new instruments can be compared. Monitoring in Urban Areas Urban situations are the areas of major interest in air pollution in the U.S., since most of the population lives in urban areas. The most sophisticated and expensive air quality monitoring systems in the U.S. are those of large cities (and one or two

1210


of the largest states) where data collection and analysis are centralized at a single location through the use of telemetry. On-line computer facilities provide data reduction. Three philosophies can be utilized in the design of an urban air quality monitoring system: 1) using location of sensors on a uniform area basis (rectilinear grid); 2) installing location of sensors in areas where pollutant concentrations are expected to be high; and 3) installing location sensors in proportion to population distribution. The operation of these systems is nearly identical, but the interpretation of the results can be radically different. The most easily designed systems are those where the sensors are located uniformly on a geographical basis according to a rectilinear grid. Adequate coverage of an urban area frequently requires at least 100 sensors, so this concept is usually applied only with static or manual methods of air quality monitoring. The location of air quality sensors at points of maximum concentration indicates the highest levels of air pollutants that are encountered throughout the area. Typically, this will include the central business district and the industrial areas on the periphery of the community. Data of this type are extremely useful when interpreted in the context of total system design. One or two sensors are usually placed in clean or background locations, so the average concentration of air pollutants over the entire area can be estimated. The basis of this philosophy is that if the concentrations in the dirtiest areas are below the air quality standards, there will certainly be no problem in the cleaner areas. The design of air quality monitoring systems based on population distribution calls for the placement of air quality sensors in the most populous areas. While this may not include all the high-pollutant-concentration areas in the urban region, it generally encompasses the central business district and is a measure of the levels of air pollutants to which the bulk of the population is exposed. The average concentrations from a sampling network of this type are an adequate description from a public health standpoint. However, this system design may miss some localized high-concentration areas. Agreement on the purpose of air quality monitoring must be reached between the system designer and those responsible for interpreting the data before the system is designed. Sampling Site Selection Once the initial layout has been developed for an air quality monitoring system, specific sampling sites must be located as close as practical to the ideal locations. The major considerations are the lack of obstruction from local interference and the adequacy of the site to represent the air mass of interest, accessibility, and security. Local interference can cause major disruptions to air quality sensor sites. A sampler inlet placed at a sheltered interior corner of a building is most undesirable because of poor air motion. Tall buildings or trees immediately adjacent to the sampling site can also invalidate most readings.


The selection of sampling sites in urban areas is complicated by the canyon effect of streets and by the high density of pollutants, both gaseous and particulate, immediately at street level. In order to be representative of the air mass to be sampled, the purpose of the study must again be reviewed. If the data are to be collected for the determination of areawide pollutant averages, it may be best to locate the sampler inlet in a city park, vacant lot, or other completely open area. If this is not possible, the sampler inlet could be at the roof level of a one- or two-story building so that street-level effects could be minimized. On the other hand, if the physiological impact of the air pollutants is a prime consideration, the samplers should be at or near the breathing level of the people exposed. As a rule of thumb, an elevation of 3 to 6 m aboveground is suggested as optimum. Sampling site location can be different for the same pollutant depending upon the purpose of the sampling. Carbon monoxide sampling is an example. The federally promulgated air quality standards for carbon monoxide include both an 8- and 1-h concentration limit. Maximum 1-h concentrations are likely to be found in a high-traffic-density centercity location. It is unlikely that people would ordinarily be exposed to these concentrations for 8-h periods. When sampling for comparison with the 1-h standard, the sensor should be located within about 20 ft of a major traffic intersection. When sampling for comparison with the 8-h standard, the sensor should be located near a major thoroughfare in either the center-city area or the suburban area, with the sample less than about 50 ft from the intersection. The purpose for two different locations of sampling sites is to be consistent with the physiological effects noted from carbon monoxide exposure and with the living pattern of the bulk of the population. If only one site can be selected, the location described for the 8-h averaging time is preferred. When the sampling instruments are located inside a building and an air sample is to be drawn in from the outside, it is frequently advantageous to utilize a sampling pipe with a small blower to bring outside air to the instrument inlets. This improves sampler line response time. An air velocity of approximately 700 ft/min in the pipe is a good choice to balance the problems of gravitational and inertial deposition of particular matter, where particulates are to be sampled. The sampling site should be accessible to the operation and maintenance personnel. Since most air pollution monitoring sites are unattended much of the time, sample site security is a very important consideration; the risk of vandalism is high in many areas. Static Methods of Air Monitoring Minimum capital cost is attained when static sensors are used to monitor air quality. While averaging times are in terms of weeks and sensitivity is frequently low, there are many cases where static monitors provide the most information for the amount of investment. They should not be rejected out of


Bird Ring (Optional)

Plastic Jar Approx. 7 1 -inch Diameter, 2 8-inch Tall Support Pole

FIG. 8.5d Dust-fall jar.

hand, but should be considered as useful adjuncts to more sophisticated systems. Dust-Fall Jars The simplest of all air quality monitoring devices is the dust-fall jar (Figure 8.5d). This device measures the fallout rate of coarse particulate matter, generally above about 10 microns in size. Dust-fall and odor are two of the major reasons for citizen complaints concerning air pollution. Dust-fall is offensive because it builds up on porches, automobiles, and such, and it is highly visible and gritty to walk upon. Dust-fall seldom carries for distances in excess of 12 mi because these large particles are subject to strong gravitational effects. For this reason, dust-fall stations must be more closely spaced than other air pollution sensors, if detailed study of an area is desired. Dust measurements conducted in large cities in the U.S. in the 1920s and 1930s commonly indicated dust-fall rates in hundreds of tons per square mile per month. These levels would be considered excessive today, as evidenced by the 2 dust-fall standards of 25 to 30 tons/mi /month promulgated by many of the states. While the measurement of low or moderate values of dust-fall does not indicate freedom from air pollution problems, measured dust-fall values in excess 2 of 50 to 100 tons/mi /month in an area are a sure indication of the existence of excessive air pollution. The large size of the particulate matter found in dust-fall jars makes it amenable to chemical analysis or to physical analysis by such techniques as microscopy. These analyses can be useful to identify specific sources. Lead Peroxide Candles For many years, lead peroxide candles had been used in measuring the concentration of sulfur dioxide. These devices are known as candles because they are a mixture of lead peroxide paste spread on a porcelain cylinder that is about the size and shape of a candle. They are normally exposed to the ambient air in the location of interest for 1 month. Sulfur gases in the air react with lead peroxide to form lead sulfate. Sulfate is analyzed according to standard laboratory procedures to give an indication of the atmospheric levels of sulfur gases during the period of exposure.


1211

The laboratory procedure can be simplified by a modification of this technique, which uses a fiber filter cemented to the inside of a plastic petri dish (a flat-bottom dish with shallow walls, used for biological cultures). The filter in the dish is saturated with an aqueous mixture of lead peroxide and a gel (commonly gum tragacanth) and is allowed to dry. These dishes, or plates, are exposed to the ambient air in an inverted position for 1 to 4 weeks. Lead peroxide estimation of sulfur dioxide suffers from inherent weakness. All sulfur gases, including reduced sulfur, react with lead peroxide to form lead sulfate. More importantly, the reactivity of lead peroxide is dependent upon its particle size distribution. For this reason, the results from different investigators are not directly comparable. Nevertheless, a network of lead peroxide plates over an area provides a good indication of the relative exposure to sulfur gases during the particular exposure period. This is useful for determining the geographical extent of sulfur pollution. Other Static Methods The use of a fiber filter cemented to a petri dish has been modified by the use of sodium carbonate rather than lead peroxide for the measurement of sulfur gases. This method also shows some success in indicating the relative concentration of other gases, including nitrogen oxides and chlorides. Relative levels of gaseous fluoride air pollution have been measured using larger filters, e.g., 3 in. in diameter, dipped in sodium carbonate, and placed in shelters to protect them from the rain. With all these static methods, the accuracy is extremely low and the data cannot be converted directly into ambient air concentrations. They do, however, provide a low-cost indicator of relative levels of pollution in a given area. The corrosive nature of the atmosphere has been evaluated, using standardized steel exposure plates for extended periods to measure the corrosion rate. This provides a gross indication of the corrosive nature of the atmosphere. As with other static samplers, the results are not directly related to the concentration of air pollutants. Laboratory Analyses Manual analyses for air quality measurements are those that require the simple to first be collected and then analyzed in the laboratory. Manual instruments provide no automatic indication of pollution levels. The manual air sampling instrument, which is in widest use, is the high-volume sampler shown in Figure 8.5e. With this method, ambient air is drawn through a preweighed filter at a rate of approximately 50 ACFM for 24 h. The filter is then removed from the sampler, returned to the laboratory, and weighed. The gain in weight, in combination with the measured air volume through the sampler, allows for the calculation of particulate mass concentration, expressed in micrograms per cubic meter. Reference methods for nearly all gaseous air pollutants involve the use of a wet sampling train in which air is drawn

1212


Filter Position

Face Plate Adapter Mounting Motor Plate

Adapter

Three-Wire Cord

Gasket Brush Gasket

Retaining Ring

Bolt Back Grommet Housing Tap Plate Ring Assembly

Nut and Bolt

Gasket

Gasket

A.

Nut and Bolt Rotameter

Tubing

Backing Plate

Condenser and Clip

B.

FIG. 8.5e The high-volume air sampler. (A) Illustrates the assembled sampler and its shelter. (B) Shows the components of a typical high-volume air sampler.

through a collecting medium for some period of time. The exposed collecting medium is then returned to the laboratory for chemical analysis. Sampling trains have been developed that allow the sampling of five or more gases simultaneously in separate bubblers. There are also sequential samplers, which automatically divert the airflow from one bubbler to another at preset time intervals. These methods of sampling can be accomplished with a modest initial investment, but the manpower required to distribute and pick up the samples and to analyze them in the laboratory raises the total cost to a point where automated systems may be more economical for long-term studies. AUTOMATIC MONITORING As the need for accurate data that can be statistically reduced in a convenient manner becomes more the rule than the exception, automated sampling systems become more and more desirable. The elements of an automated system include the airflow handling system, the sensors, the data transmission storage and display apparatus, and the data processing facility. The overall system is no more valuable than the weakest link of this chain. Sensors Some of the sensor categories used have been listed in Table 8.5a. The detectors that are available to measure the various individual air pollutants are discussed in other sections of this chapter. The reliability of the output from an air quality sensor is dependent upon its inherent accuracy, sensitivity, zero drift, and calibration. The inherent accuracy and sensitivity are a


function of the design of the instrument and of the principle upon which it operates. Zero drift can be either an electronic phenomenon or an indication of difficulties with the instrument. In those instruments using an optical path, lenses can become dirty. In wet chemical analyzers, the flow rates of reagents can vary, causing a change in both the zero and the span (range) of the instrument. Because of these potential problems, every instrument should have routine field calibration at an interval determined in field practice to be reasonable for the sensor. The calibration of an air quality sensor is frequently accomplished using either a standard gas mixture or a prepared, diluted gas mixture using permeation tubes. In some cases, the airstream entering the sensor is concurrently sampled by a reference wet chemical technique. The operator of air quality sensors should always have an adequate supply of spare parts and tools so that downtime can be held to a minimum. Operator training should, at a minimum, include instruction to recognize the symptoms of equipment malfunction and vocabulary to describe the symptoms to the individual responsible for instrument repair. Ideally, each operator would receive a short training session from the instrument manufacturer or someone trained in the use and maintenance of the instrument, so that he or she can make repairs on-site. Since this is seldom possible in actual practice, the recognition of symptoms of malfunction becomes increasingly important. Data Transmission The output signal from a continuous monitor used in an air quality monitoring system is typically the input to a stripchart recorder, magnetic tape data storage, or an on-line data


transmission bus or highway system. If the output of the air quality transmitter is in analog form, it is suitable for direct input to a strip-chart recorder, but it has to be converted into a digital signal before it can be sent over a data highway. In the case of sensors with a logarithmic output, it may be desirable to first convert this signal into a linear form. Many of the first automated air quality monitoring systems in the U.S. experienced major difficulties with their data transmission systems. In some cases, this was caused by attempts to overextend the lower range of the sensors, which resulted in an unfavorable signal-to-noise ratio. In other cases, the matching of sensor signal output to the data transmission system was poor. With developments of the new data highway protocols, these early difficulties have been overcome. Continuous detectors tend to increase the complexity of the air quality monitoring systems, but data from the continuous monitors can be stored on magnetic tape for later processing and statistical reduction. In case of an on-line system, this can be done instantaneously. When decisions with substantial community impact must be made within a very short time, this real-time capability is likely to be necessary. Data Processing The concentration of many air pollutants has been found to follow a lognormal rather than a normal distribution. In a lognormal distribution, a plot of the logarithm of the measured value more closely approximates the bell-shaped Gaussian distribution curve than does a plot of the numerical data. Suspended particulate concentrations are a prime example of this type of distribution. When this is the case, the geometric mean is the statistical parameter that best describes the population of data. The arithmetic mean is of limited value because it is dominated by a few occurrences of high values. The geometric mean, in combination with the geometric standard deviation, is a complete description of a frequency distribution for a lognormally distributed pollutant. Averaging Times The averaging time over which the sample results are reported is a consideration in the processing and interpretation of air quality data. For sulfur dioxide, air quality standards have been promulgated by various agencies based on annual, monthly, weekly, 24-h, 3-h, and 1-h arithmetic average concentrations. The output of a continuous analyzer can be averaged over nearly any discrete time interval. In order to reduce the computation time, the time interval over which the continuous analyzer output is averaged to obtain a discrete input for the calculations must be considered. If a 1-h average concentration is the shortest time interval of value in interpreting the study results, it is not economical to use a 1- or 2-min averaging time for inputs to the computation program. Displays Caution must be exercised in using strip-chart recorders for the acquisition of air quality data. The experience


1213

of many organizations, both governmental and industrial, is that the reduction of data from strip charts is a tedious chore at best. Many organizations have decided that they did not really need all that data in the first place, once they find a large backlog of unreduced strip charts. Two cautions are suggested by this experience. First, if you do not plan to use the data, do not collect it. Second, be aware of the advantages of magnetic tape data storage followed by computer processing. The visual display of air quality data has considerable appeal to many nontechnical personnel. Long columns of numbers can be deceptive if only one or two important trends are to be shown. The use of bar charts or graphs is frequently advantageous, even though they do not show the complete history of the air quality over the time span of interest. Audits Periodic performance audits are required to validate the accuracy of the air monitoring system. The Code of Federal Regulations (CFR) requires that performance audits be conducted at least once a year for criteria pollutant analyzers operated at State and Local Air Monitoring Stations (SLAMS). The U.S. Environmental Protection Agency (EPA) recommends that each analyzer be disconnected from the monitoring station manifold and be individually connected to the audit, from which it will receive the audit gas of known concentration. The audit gas concentrations are usually generated in a van, using a gas calibrator to dilute multiblend gases with zero air. Each state approaches air pollution monitoring differently. The California Air Resources Board (CARB), for example, has been conducting through-the-probe performance audits of continuous ambient air analyzers since 1981. CARB has the responsibility of overseeing the implementation of the California Clean Air Act and the Federal Clean Air Act in California. Automatic Analyzers The automatic sampling train (AST) packages have been described in Section 8.3. They can automate any of the U.S. EPA Methods or international methods specified by VDI, BS, or ISO. In these packages the microprocessor stores all measurements, diagnoses all inputs, controls the manipulated variables, calculates isokinetic conditions, and reports the results in either a printed form or over the data bus. Infrared Spectrometers Microprocessor-controlled spectrometers are also available to measure the concentrations of a variety of gases and vapors in ambient air. These units can be portable or permanently installed and can serve compliance with environmental and occupational safety regulations. In the infrared spectrometer design, the ambient air is drawn into the test cell by an integral air pump, operating at a flow rate of 0.88 ACFM (25 l/min). The microprocessor selects the appropriate wavelengths for the components of interest, and the filter wheel in the

1214


analyzer allows the selected wavelengths to pass through the ambient air sample in the cell. The microprocessor automatically adjusts the path length through the cell to give the required sensitivity. Because of the folded path length design, the path length can be increased to 20 m (60 ft), and the resulting measurement sensitivity can be better than 1 ppm in many cases. As shown in Table 8.5f, practically all organic and also some of the inorganic vapors and gases can be monitored by these infrared spectrometers. The advantage of the microprocessor-based operation is that the monitor is precalibrated for the analysis of over 100 Occupational Safety and Health Administration (OSHA)-cited compounds. The memory capacity of the microprocessor is sufficient to accommodate another 10 user-selected and user-calibrated gases. Analysis time is minimized because the microprocessor automatically sets the measurement wavelengths and parameters for any of the compounds in its memory. A general scan for contaminant in the atmosphere takes about 5 min, while the analysis of a specific compound can be completed in just a few minutes. The portable units are battery operated for 4 h of continuous operation and are approved for use in hazardous areas. Handheld Indoor Air Quality Monitors Battery-operated, handheld indoor air quality (IAQ) monitors are available for monitoring the air quality in schools, offices, meeting rooms, and greenhouses, and to service heating, ventilation, and air conditioning (HVAC) systems. These units monitor and record the temperature (range: 0 to 50°C = 32 to 122°F), relative humidity (range: 0 to 100%), and the concentration of different selected gases. Plug-in sensors are available for gases, such as CO2, with a range of 0 to 10,000 ppm. Dual-beam infrared radiation is used for making the measurement. The sample flow is drawn by diffusion or is maintained at 100 to 300 ml/m. Sample rates are adjustable from 10 sec to one per day, and data loggers are also available with capacities for up to 50,000 samples.

SAMPLING OF AMBIENT AIR The sampling of process gases and vapors has been discussed in Section 8.2, and some of that discussion is also applicable to ambient air monitoring. Yet ambient air sampling is sufficiently different to justify a separate discussion in addition to what has already been said. All substances in the ambient air exist as either particulate matter or gases and vapors. In general, the distinction is easily made; gases and vapors consist of substances dispersed as molecules in the atmosphere, while particulate matter consists of aggregates of molecules sufficiently large that they are said to behave like particles. Particulate matter (or particulates) are filterable, may be precipitated, and, in still air, may be expected to settle out. By contrast, gases and vapors do not behave in this fashion and are homogeneously mixed with the air molecules.


A substance such as carbon monoxide may exist only as a gas; an inorganic compound like iron oxide may exist only as a particle. Many substances may exist as either particles or vapors, however; additionally, substances that are gases can also be attached by some means to the particulate matter in the air. If sampling is to be intelligently conducted, prior knowledge of the physical state in which a substance exists must be available or else a judgment must be made. Particulate matter, opacity, dust, and smoke analyzers are discussed in a separate section in this chapter. They do not usually collect gases or vapors; hence, an incorrect selection of sampling method may lead to erroneous results. Fortunately, a considerable body of experience has evolved concerning the more common pollutants, and it is not difficult in most cases to select a sampling method reasonably suitable for the substances of interest.

General Air Sampling Problems Certain general observations relative to sampling ambient air must be recognized. For example, the quantity of a given substance contained in a volume of air is likely to be extremely small, and it will necessitate a sample of sufficient size for the analytical method employed to be adequate. Even air, which is heavily polluted, is not likely to contain more than a few milligrams per cubic meter of most contaminants and, more frequently, the amount present will best be measured in micrograms, or even nanograms, per cubic meter. Consider, for example, the air quality standard for par3 3 ticulates, which is 75 g/m . A cubic meter of air, or 35.3 ft , is a relatively large volume for many sampling devices, and a considerable sampling period may be required to draw such a quantity of air through the sampler. If atmospheric mercury analyses are to be attempted, then it must be realized that background levels are likely to be as low as several nanograms per cubic meter and, in general, most substances tend to be of concern at quite low levels in the ambient air. In addition to problems arising from the low concentrations of substances being sampled, other problems include those caused by the reactivity of the substances, changes after collection, and necessitating special measures to minimize such changes. Whenever something is removed from a volume of air by sampling procedures, some alteration of the substance of interest may taken place and analysis may be less informative than desired, or may even be misleading. Therefore, it would be ideal to perform analyses of the unchanged atmosphere if possible, using direct-reading devices, which could give accurate information concerning the chemical and physical states of contaminants as well as concentration information. Such instruments do exist for some substances, and many more are being developed, but conventional air sampling methods are still used in many instances and doubtlessly will continue to be required for some time to come.


TABLE 8.5f Compounds That Can Be Analyzed by the Microprocessor-Controlled Portable Infrared Spectrometer Compound

Range of Calibration (ppm)

Acetaldehyde Acetic acid Acetone Acetonitrile Acetophenone Acetylene Acetylene tetrabromide Acrylonitrile Ammonia Aniline

0–400 0–50 0–2000 0–200 0–100 0–200 0–200 0–20 and 0–100 0–100 and 0–500 0–20

Benzaldehyde Benzene Benzyl chloride Bromoform Butadiene Butane 2-Butanone (MEK) Butyl acetate n-Butyl alcohol

0–500 0–50 and 0–200 0–100 0–10 0–2000 0–200 and 0–2000 0–250 and 0–1000 0–300 and 0–600 0–200 and 0–1000

Carbon dioxide Carbon disulfide Carbon monoxide Carbon tetrachloride Chlorobenzene Chlorobromomethane Chlorodifluoromethane Chloroform m-Cresol Cumene Cyclohexane Cyclopentane

0–2000 0–50 0–100 and 0–250 0–20 and 0–200 0–150 0–500 0–1000 0–100 and 0–500 0–20 0–100 0–500 0–500

Diborane m-Dichlorobenzene o-Dichlorobenzene p-Dichlorobenzene Dichlorodifluoromethane (Freon 12) 1,1-Dichloroethane 1,2-Dichloroethylene Dichloroethyl ether Dichloromonofluoroethane (Freon 21) Dichlorotetrafluoromethane (Freon 114) Diethylamine Dimethylacetamide Dimethylamine Dimethylformamide Dioxane

0–10 0–150 0–100 0–150 0–5 and 0–800 0–200 0–500 0–50 0–1000 0–1000 0–50 0–50 0–50 0–50 0–100 and 0–500

2-Ethoxyethyl acetate Ethyl acetate Ethyl alcohol Ethylbenzene Ethyl chloride Ethylene Ethylene dibromide Ethylene dichloride Ethylene oxide Ethyl ether Enflurane

0–200 0–400 and 0–1000 0–1000 and 0–2000 0–200 0–1500 0–100 0–10 and 0–50 0–100 0–10 and 0–100 0–1000 and 0–2000 0–10 and 0–100


Compound


Ethane Ethanolamine

0–1000 0–100

Fluorotrichloromethane (Freon 11) Formaldehyde Formic acid

0–2000 0–20 0–20

Halothane Heptane Hexane Hydrazine Hydrogen cyanide

0–20 0–10 and 0–100 0–1000 0–1000 0–100

Isoflurane Isopropyl alcohol Isopropyl ether

0–10 and 0–100 0–1000 and 0–2000 0–1000

Methane Methoxyflurane Methyl acetate Methyl acetylene Methyl acrylate Methyl alcohol Methylamine Methyl bromide Methyl cellosolve Methyl chloride Methyl chloroform Methylene chloride Methyl iodide Methyl mercaptan Methyl methacrylate Morpholine

0–100 and 0–1000 0–10 and 0–100 0–500 0–1000 and 0–5000 0–50 0–500 and 0–1000 0–50 0–50 0–50 0–200 and 0–1000 0–500 0–1000 0–40 0–100 0–250 2–50

Nitrobenzene Nitromethane Nitrous oxide

0–20 0–200 0–100 and 0–2000

Octane

0–100 and 0–1000

Pentane Perchloroethylene Phosgene Propane n-Propyl alcohol Propylene oxide Pyridine

0–1500 0–200 and 0–500 0–5 0–2000 0–500 0–200 0–100

Styrene Sulfur dioxide Sulfur hexafluoride

0–200 and 0–500 0–100 and 0–250 0–5 and 0–500

1,1,2,2-Tetrachloro 1,2-difluoroethane (Freon 112) 1,1,2,2-Tetrachloroethane Tetrahydrofuran Toluene Total hydrocarbons 1,1,2-Trichloroethane Trichloroethylene 1,1,2-Trichloro 1,2,2-trifluoroethane (Freon 113)

0–2000 0–50 0–500 0–1000 0–1000 0–50 0–200 and 0–2000 0–2000

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1216


TABLE 8.5f Continued Compounds That Can Be Analyzed by the Microprocessor-Controlled Portable Infrared Spectrometer Compound


Trifluoromonobromomethane (Freon 13B1)

0–1000

Vinyl acetate Vinyl chloride

0–1000 0–10

Compound


Vinylidene chloride

0–20

Xylene (xylol)

0–200 and 0–2000

Sampling for Gases and Vapors The simplest method of collecting a sample of air for subsequent analysis is to fill a bottle or other rigid container with it or, more conveniently, to use a bag made of suitable material. Although it may be relatively easy to sample by this method, the sample size is distinctly limited, and it may not be possible to collect a sufficiently large sample to permit subsequent analysis. Bottles larger than several liters in capacity are awkward to transport, and while bags of any size are conveniently transported when empty, they may be difficult to deal with when inflated. Nevertheless, it may prove more convenient to collect a number of samples in small bags than to take more complex sampling apparatus to a number of sampling sites. If it is possible to analyze for the contaminant of interest by means of gas chromatographic procedures, or by gas phase infrared spectroscopy, for example, then samples as small as a liter or less may be adequate and can be quickly and easily collected by bags. There are several methods of filling rigid containers such as a bottle. One is to evacuate the bottle beforehand and then fill it at the sampling site by drawing air into the bottle and sealing it again (Figure 8.5g). Alternatively, a bottle may be filled with water, which is then allowed to drain and fill with the air. A third method consists of passing a sufficient amount of air through the bottle by using a pumping device until the original air is completely displaced by the air being sampled. Plastic bags are frequently filled by means of a simple, hand-operated squeeze bulb with values on each end (Figure 8.5g) and then connected to a piece of tubing attached to the sampling inlet of the bag. In most cases, this procedure is satisfactory, but care should be taken to avoid contamination of the sampled air by the sampling bulb or possible losses of the constituent on the walls of the sampling bulb. To avoid problems of this kind, it is possible to place the bag in a rigid container such as a box and then withdraw air from the box so that a negative pressure is created, which results in air being drawn into the bag. Bag materials must be selected with care, for some will permit losses of contaminants by diffusion through the walls, and others to the air being sampled. A number of polymers have been studied, and several have been found to be suitable for air sampling purposes. Materials suitable for use as sampling bags include Mylar, Saran, Scotchpak (a laminate of polyethylene, aluminum foil, and Teflon), and Teflon.


Breaking Scratch

Sealed with Wax-Filled Wax-Filled Cartridge Cartridge Breaking Scratch 250 to 300 c.c Capacity

Rubber Cap

250 to 300 c.c Capacity Absorbing Liquid

Evacuated and Sealed Bottles

250 to 300 c.c Capacity

Gas Pipet

Flushing Air Through Bottle

Double-Acting Rubber Bulb Aspirator

FIG. 8.5g Devices for obtaining grab samples.

Even though the bag may be made of relatively inert materials, it is always possible that gas phase chemical reactions will occur, so that after a period of time the contents of the bag may not be identical in composition with the air originally sampled. Thus, a reactive gas like sulfur dioxide or nitric oxide can be expected to gradually oxidize, depending upon the storage temperature. It is generally advisable to perform analyses as soon as possible after collecting the samples. Losses by adsorption or diffusion will also tend to be greater with the passage of time and will occur to some extent even though the best available bag materials have been used. The use of small bags may permit the collection of samples to be analyzed for a relatively stable gas such as carbon


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30 mm 60 mm 30 mm

Simple Bubbler

Spiral Type Absorber

50 mm 390 mm 330 mm

30 25 20 15 10 5

Fritted Absorber

280 mm

30 mm

Ring

500 Graduations

at 100, 250, 500 cc

100

FIG. 8.5h Gas-absorbing vessels.

5 mm 40 mm

A. All Glass Standard and Midget Impingers

monoxide at a number of locations throughout a community of interest, thus permitting routine air quality measurements that might otherwise be inordinately expensive. Absorption The gases or vapors of interest can also be absorbed in a suitable sampling medium. Ordinarily, this medium is a liquid of some kind, but absorption may also take place in solid absorbents or upon supporting materials such as filter papers, which are impregnated with suitable absorbents. Carbon dioxide, for example, may be absorbed in a granular bed of alkaline material, and sulfur dioxide is frequently measured by absorption of reactive chemicals placed on a cloth or ceramic support. A number of gases are also detected by passing them through filter papers or glass tubes containing reactive chemicals, with the immediate production of a color change, which can be evaluated by eye to give a measure of the concentration of the substance of interest. Liquid Absorption Most commonly, however, gases and vapors are absorbed by passing them through a liquid in which they are soluble, or which contains reactive chemicals that will combine with the substance being sampled. Many different absorption vessels have been designed, ranging from simple bubblers made by inserting a piece of tubing beneath the surface of a liquid to rather complex gas-washing devices that are designed to increase the length of time that the air and liquid are in contact with each other (Figure 8.5h). Impingers Probably the most widely used contacting device is the impinger, which is available in several sizes and configurations. An impinger consists of an entrance tube terminating in a small orifice, causing the velocity of the air passing through the orifice to greatly increase. When this jet of air strikes a plate or the bottom of the sampling vessel, an intense impingement or bubbling action occurs, which results in much more efficient absorption of gases from the airstream than would take place if the air was simply bubbled through at low velocity. The two impingement devices most frequently used are the standard impinger and the midget impinger (Figure 8.5i). They are designed to operate best at airflow rates of 1 and


Metal Guard Bent as Shown

3 Rods 5 mm O.D.

10 mm 25 mm

Cap

Tip-I.D. : 2.3−2.4 mm O.D. : 6−8 mm Plate ~ 3mm Thick Polished B. Dimensions, Standard Impinger

5 mm Stopper 15.2 cm

1 cm 11.4 cm

ml 30 25 20 15 10 5

1-mm Orifice

5-mm Mark

2.5 cm

C. Dimensions, Midget Impinger

FIG. 8.5i Standard and midget impingers. 3

0.1 ft /min, or 28.3 and 2.8 l/min, respectively. Using such devices for sampling periods of 10 or 30 min will result in the passage of a substantial amount of air through the devices, thus permitting low concentrations of trace substances to be determined with improved sensitivity and accuracy. Many relatively insoluble gases, such as nitrogen dioxide, are not quantitatively removed by passage through an impinger containing the usual sampling solutions. Fritted Absorber The most useful sampling devices for absorbing trace gases from air are those in which a gas dispersion tube made of fritted or sintered glass, ceramic, or other materials is immersed in a suitable vessel containing the absorption liquid (Figure 8.5h). This device causes the gas stream to be broken into thousands of small bubbles, thus promoting contact between the gas and the liquid with resulting high collection efficiencies in most cases. In general, fritted absorbers are more widely applicable to sampling gases and vapors than are impingers and, in addition, are not as dependent upon flow rates as are impingers. They are standard items available from scientific supply houses and come in various sizes suitable for many sampling tasks. It is often advisable to prefilter the air prior to sampling with a

1218


fritted absorber in order to prevent the gradual accumulation of dirt within the pores of the frit. The use of solid absorbents is not widely practiced in ambient air sampling because the quantity of absorbed gases is usually determined by gravimetric means. With the exception of carbon dioxide, relatively few gases of interest in the atmosphere lend themselves to this type of analysis. Adsorption Adsorption, by contrast with absorption, consists of the retention of gaseous substances by solid adsorbents that, in most cases, do not chemically combine with the gases. Instead, the gases or vapors are held by adsorptive forces and may subsequently be removed unchanged. Any solid substance will adsorb a small amount of most gases, but to be useful as an adsorbent, a substance must have a large surface area and be able to concentrate a substantial amount of gas in a small volume of adsorbent. Most widely used for this purpose are activated carbon or charcoal and activated silica gel. A small quantity of either adsorbent placed in a U-tube or other container through which air is passed will quantitatively remove many vapors and gases from a large volume of air and may subsequently be taken to the laboratory where desorption will remove the collected substance for analysis. Desorption is commonly achieved by heating the adsorbent and collecting the effluent gases or by eluting the collected substance with a suitable organic liquid. In the case of most organic vapors, subsequent analysis by gas chromatographic, infrared, or ultraviolet spectroscopic means is usually most convenient. For some purposes, either silica gel or activated carbon may be used. In other instances, the use of silica gel is undesirable because it also adsorbs water vapor, and a relatively short sampling period in humid air can result in saturation of the silica gel before sufficient contaminant is adsorbed. Charcoal does not adsorb water and hence may be used in humid environments and may be sampled for days or even weeks if the concentration of contaminant is low. The ease of sampling by using adsorbents is offset somewhat by the difficulty of quantitatively desorbing the samples for analysis. When literature data are not available to assist in predicting the behavior of a new substance, it is advisable to perform tests in the laboratory to determine both the collection efficiency and the success of desorption procedures after sample collection. Freeze-Out Sampling Vapors or gases, which are condensable at low temperature, may be removed from the sampled airstream by passage through a vessel that is immersed in a refrigerating liquid. Table 8.5j lists liquids that are commonly used for this purpose, and usually it is desirable to form a sampling train in which two or three coolant liquids progressively lower the air temperature in its passage through the system. All freeze-out systems are hampered somewhat by the accumulation of ice resulting from water vapor and may eventually become plugged with ice. Flow rates through a freeze-out train are necessarily limited also; in order for the air to be cooled to the required degree,


TABLE 8.5j Coolant Solutions for Freeze-Out Sampling Coolant

Temperature (°C)

Ice water

0

Ice and salt (NaCl)

–21

Dry ice and acetone

–78.5

Liquid air

–147

Liquid oxygen

–183

Liquid nitrogen

–196

a sufficient residence time in the system must be provided. For these reasons, and because of the general inconvenience of assembling freeze-out sampling trains, they are not generally used for routine sampling purposes unless no other approach is feasible. However, freeze-out sampling is an excellent means of collecting substances for research studies, inasmuch as the low temperatures tend to arrest further chemical changes and ensure that the material being analyzed will remain in the sampling container ready for analysis after warming. Analysis is most frequently conducted by means of gas chromatographic, infrared, or ultraviolet spectrophotometry, or by mass spectrometric means. Sampling of Particulates Particulate matter is most conveniently removed from air by passage through a filter (Figure 8.5e). Before filtration is used to obtain a sample, however, consideration should be given to the purpose for which the sample is being taken. Many filters collect particulates efficiently, but thereafter it may be impossible to remover the collected matter except by chemical treatment. If the samples were initially collected for the purpose of examining the particles and measuring their size, or noting morphological characteristics, then many filters are not suitable because the particles are imbedded in the fibrous web of the filter and cannot readily be viewed or removed. If the sample is collected for the purpose of performing a chemical analysis, then it is important that the filter itself does not contain significant quantities of the substance for which analysis is required. If the purpose of sampling is to collect a sufficient amount of particulate matter to permit weighing, then it is necessary to select a filter that can be weighed. This can be a problem, because many filtration materials are hygroscopic and change weight appreciably in response to changes in the relative humidity. Filters may be made of many substances and, in fact, almost any solid substance could probably be made into a filter. In practice, however, fibrous substances, such as cellulose or paper, fabrics, asbestos, and a number of plastics or polymerized materials, are generally used. The most readily available filters are those made of cellulose or paper used in chemical laboratories for filtering liquids. Such filter papers come in a variety of sizes and range in efficiency from rather loose filters, which remove only the larger particles, to paper,


which will remove very fine particles with high efficiency. All filters display similar behavior, and ordinarily a high collection efficiency is accompanied by increased resistance to airflow. Air Filters Certain kinds of filtration media are more suited to air sampling than most paper or fibrous filters. Of these, membrane filters are of greatest utility. The commercially available membrane filters combine extremely high collection efficiencies with relatively low resistance to flow. Such filters are not made up of a fibrous mat, but instead are usually composed of gels of cellulose esters or other polymeric substances in such fashion that a smooth surface of predictable characteristics is formed. Such filters contain many small holes, or pores, and may be made to exacting specifications in this regard so that their performance characteristics may be predicted. In addition, the filters are usually of high chemical purity and are well suited to trace metal analyses. Some of the membrane filters can also be rendered transparent and thus permit direct observation of collected particles with a microscope. Alternatively, the filters can be dissolved in an organic solvent, and the particles may be isolated and studied. Most membrane filters are not greatly affected by relative humidity changes and may be weighed before and after use to obtain reliable gravimetric data. Fiberglass Filters Another kind of filter that is widely used in sampling ambient air is the fiberglass filter. These filters are originally made of glass fibers in an organic binder; subsequently, the organic binder may be removed by firing, leaving a web of glass, which is very efficient in collecting fine particles from the air. The principal advantage of using this type of filter is its relatively low resistance to airflow and its virtually unchanging weight regardless of relative humidity. The filters are not well suited to particle size studies, however; additionally, they are not chemically pure, and care must be taken to be certain that a substance for which analysis is to be made will not be contributed in unknown quantities by the filter itself. In the U.S., most of the data relating to suspended particulate matter in our cities have been obtained on filters made of fiberglass and used in conjunction with a sampling device referred to as a high-volume sampler (Figure 8.5e). Many other kinds of filters are available, but most sampling needs are well met by membrane or fiberglass filters. Extensive data concerning the types of filters available and their performance characteristics will be found in the bibliography cited at the end of this section. Impingement and Impaction The impingers previously described in relation to sampling gases and vapors (Figure 8.5i) may also be used for the collection of particles and, in fact, they were originally developed for that purpose. However, in ambient air sampling they are not used, inasmuch as the efficiency of collection tends to be low and unpredictable for the fine particles that may be present in ambient air. The relatively low sampling rates also make them less attractive than filters for


1219

general air sampling, but instances do arise when impingers can be satisfactorily used. When they are used, it is important that the correct sampling rates be maintained, for the collection efficiency of impingers for particles may vary drastically when flow rates are other than optimal. Impactors Of more widespread use in ambient air sampling are devices known as impactors, in which air is passed through small holes or orifices and made to impinge or impact against a solid surface. When such devices are constructed so that the air passing through one stage is subsequently directed onto another stage containing smaller holes, the resulting device is known as a cascade impactor and has the capability of separating particles according to their sizes. Various commercial devices are available. Figure 8.5k portrays one that is widely used and consists of several layers of perforated plates through which the air must pass. Each plate contains a constant number of holes, but the hole size is progressively decreased so that the same volume of air passing through each stage will impinge at an increased velocity. The result is that coarse particles are deposited on the first stage and successively finer particles are removed at each subsequent stage. While the particle size fractions obtained by such instruments are not very accurate, they do perform predictably when the characteristics of the aerosol being sampled are known. In use, a cascade impactor is assembled after scrupulously cleaning each stage and applying a sticky substance

Stage No. Jet Size Jet Velocity Stage 1 0.0465" Dia. 3.54 ft/sec Stage 2 0.0360" Dia. 5.89 ft/sec

Airflow

Medium

Petri Dish

Stage 3 0.0280" Dia. 9.74 ft/sec Stage 4 0.0210" Dia. 17.31 ft/sec Stage 5 0.0135" Dia. 41.92 ft/sec Stage 6 0.0100" Dia. 76.40 ft/sec

FIG. 8.5k Cascade impactor (Andersen sampler).

Gasket

8"

1220


takes place on the central wire; the particles entering the tube are charged and are promptly swept to the walls of the tube where they remain firmly attached. By this means, it is possible to collect a sample for subsequent weighing or chemical analysis. It is also possible to examine the particles and study their particle size and shape, and the intense electrical forces may produce aggregates of particles that are different than those that existed in the sampled air. Electrostatic precipitators are not as widely used as filters for ambient air sampling, because they are generally less convenient and tend to be heavy due to the power pack necessary to generate the required high voltage. Nevertheless, when available, they are excellent instruments for obtaining samples for subsequent analysis, and samples at relatively high flow rates and very low resistance.

or a removable surface on which the particles are to be deposited. After a suitably long period of sampling, during which time the volume of air is metered, the stages may be removed and the total weight of each fraction determined, as well as the fractions’ chemical composition. Such information may be more useful than a single weight or chemical analysis of the total suspended particulate matter without regard to its particle size. Electrostatic Precipitation Particulate matter can be quantitatively removed from air by means of instruments known as electrostatic precipitators. They operate on the same principle as the devices used to remove particulate matter from stack gases prior to discharging into the atmosphere. There are several commercially available electrostatic precipitators that may be used for air sampling. They operate on the same general principle that passing the air between charged surfaces imparts a charge to the solid particles in the air. Therefore, these particles can be collected on an oppositely charged surface or plate. In one of the more widely used commercial devices (Figure 8.5l), a high-voltage discharge is made to occur along a central wire; the collecting electrode is a metallic cylinder that is placed around the central wire, while the air to be sampled is passing through the tube. An intense corona discharge

Thermal Precipitators Whenever a strong temperature gradient exists between two adjacent surfaces, there will be a tendency for particles to be deposited on the colder of these surfaces. Collection of aerosols by this means is termed thermal precipitation and, in practice, several such commercial devices are available. Because thermal forces are so weak, it is necessary to have a rather large temperature difference maintained in a small area.

Central Electrode with Ionizing Wire

Sampling Head

Head Switch Motor Blower

Sampling or Collecting Tube Precipitation Chamber High-Voltage Cable. 5 Conductor, 12 ft Long

Sampling Tube Kit Contains: 4 Sampling Tubes with Plastic Cups, 2 Central Electrodes and 2 Spare Fuses

Retractable Stand

Motor-Blower Connector

High-Voltage Connector High-Voltage Indicator Power Pack

High-Voltage Control Knob

Removable Lid

Pilot Light

FIG. 8.5l Electrostatic precipitator. (Courtesy of MSA Instrument Div.)


Main Switch

Fuse Electrical Supply Cord, 3 Conductor, 15 ft Long


Additionally, the rate of airflow between the two surfaces must be low in order not to destroy the temperature gradient and to permit particles to be deposited before moving out of the collection area. As a result of these requirements, most devices use a heated wire as the source of the temperature differential and deposit a narrow ribbon of particles on the cold surface. Airflows are very small, being on the order of 10 to 25 ml/min. At such low rates, the amount of material collected will normally be insufficient for chemical analysis or weight determinations, but will be ample for examination by optical or electron microscopy. Collection for particle size studies is, in fact, the principal use for thermal precipitation units, and they are well suited to collecting samples for such investigations. Because the collecting forces are gentle, it is generally believed that the particles are deposited unchanged. The microscopic examination gives information that can be translated into data concerning the number of particles and their morphological characteristics. It is convenient also to use a small grid suitable for insertion into an electron microscope as the collecting surface, thus making it unnecessary to perform additional manipulations of the sample prior to examination by electron microscopy. Reference 1.

www.ecotech.com.au.


1221

Bibliography Annual Book of ASTM Standards, West Conshohocken, PA: American Society for Testing and Materials, 2002. ASHRAE, Standard 110–1995, “Method of Testing Performance of Fume Hoods.” Dubois, R. et al., The New Sampling Initiative, 47th Annual ISA Analysis Division Symposium, April 2002. Fussell, E., “Molding the Future of Process Analytical Sampling,” InTech, August 2001, 32. “Gas Detectors and Analyzers,” Measurements and Control, October 1992. Gregg, W., “The Use of Inertial Separators for Sampling,” ISA/93 Technical Conference, Chicago, September 19–24, 1993. McMahon, T. K., “The New Sampling/Sensor Initiative,” Control, August 2001. Laznow, J. and Ponder, T., “Monitoring and Data Management of Fugitive Hazardous Air Pollutants,” ISA Conference, Houston, TX, October 1992. Lodge, J. P., Methods of Air Sampling and Analysis, Chelsea, MI: Lewis Publishers, 1988. Lord, H. C. and Brown, R. V., “Open-Path Multi-Component NDIR Monitoring of Toxic, Combustible or Hazardous Vapors,” paper 91-0401, ISA Conference, Anaheim, CA, 1991. Ness, S. A., Air Monitoring for Toxic Exposures, New York: Van Nostrand Reinhold, 1991. Pevoto, L. F. and Hawkins, L. J., “Sample Preparation Techniques for Very Wet Gas Analysis,” ISA Conference, Houston, TX, October 1992. Sherman, R. E., Process Analyzer Sample-Conditioning System Technology, New York: John Wiley & Sons, 2002. Van den Berg, F. W. J., Hoefsloot, H. C. J., and Smilde, A. K., “Selection of Optimal Process Analyzers for Plant-Wide Monitoring,” Analytical Chemistry, 74(13), 3105–3111, 2002.

8.6

Biometers I. G. YOUNG

(1974, 1982)

B. G. LIPTÁK

(1995, 2003)

Method of Detection:

Photometric measurement of light emitted by chemical reaction

Sample Pressure:

Atmospheric

Sample Temperature:

Ambient

Sample Type:

Grab sample

Materials of Construction:

Glass

Range:

10 to 10 µg of ATP per 10-ml sample of bacterial extract. Sensitively to 10 µg per 10-µl sample. It can be calibrated for number of bacteria per microgram of ATP.

Response:

Laboratory method: minutes after starting reaction

Cost:

About $10,000

Supplier:

Asay Design Inc. (www.assaydesign.com) Bio-Tek Instruments (www.biotek.com) Innovative Imaging Inc. (www.innovative-imaging.com) Sigma-Aldrich (www.sigmaaldrich.com) SP Industries Inc. (www.wilmad-labglass.com)

–7

−7

–2

INTRODUCTION The key wastewater-related measurements, which detect the concentration of the discharged waste stream in terms of its oxygen demand, are discussed in Section 8.7. The sensors discussed in this section measure the biological population and the biological oxidative activity by detecting the amount of adenosine triphosphate (ATP) and the changes in its concentration.

1

ity of the biomass. Thus, it is of great interest to measure the ATP content of samples in the activated sludge process as well as in rivers, lakes, and other receiving waters. Sensitive methods for ATP analysis have been developed based on the observation that the luminescent reaction in fireflies is absolutely dependent on the presence of ATP. The in vitro light-yielding reactions are given in Equations 8.6(1) and 8.6(2): LH 2 + E + ATP[Mg2 + ]E − LH 2 − AMP + PP 8.6(1)

ATP ANALYSIS In a detailed study of the control parameters for the activated sludge process, measurements of great interest are biochemical oxygen demand (BOD) and chemical oxygen demand (COD). BOD and COD reduction, biological population density, and biological oxidative activity are important indicators of this process. It has been found that the amount of ATP is proportional to the viable biomass in a sample. It has also been found that changes in ATP concentration measure the oxidative capabil1222 © 2003 by Béla Lipták

E − LH2 − AMP + O2 → E + PRODUCT + CO2 + AMP + Hv 8.6(2) where LH2 = luciferin E = luciferase enzyme E − LH2 − AMP = enzyme–luciferin–adenosine monophosphate complex PP = pyrophosphate

8.6 Biometers

It is seen that the yield of light quanta (Hv) is in proportion to the amount of ATP present in the sample.

Reference 1.

LUMINESCENCE BIOMETER ATP assay procedures have been developed based on the reactions just described. Briefly, the procedure involves rapid killing of the live bacterial cells and immediate extraction of ATP into aqueous solution. The latter is then treated with firefly lantern extract, and the light emission of the resultant solution is measured with a photometer. The firefly lantern extract and the ATP required for calibration are commercially available. A manually operated instrument is available for the ATP measurement. It is supplied with all the required reagents. A tablet containing buffer and magnesium sulfate is dissolved in water, after which a homogeneous powder of luciferin and luciferase is added. The sample is filtered through a coarse filter to remove solid matter, and the latter is discarded. The filtrate is passed through a bacterial filter to catch all the living bacteria. The bacteria on the filter are treated with butanol, which ruptures the cell walls and releases the ATP. The filtrate is made up to volume with water, and a microliter aliquot is added to the prepared reagent already in a cuvette. The cuvette is then placed in the instrument for reading of its light emission. The light flash is automatically converted to ATP or microorganism concentration per milliliter, depending on how the instrument is calibrated.


1223

Patterson, J. W., Brezonik, P. L., and Putnam, H. D., “Sludge Activity Parameters and Their Application to Toxicity Measurements in Activated Sludge,” Proceedings, Industrial Waste Conference, Purdue University, May 1969.

Bibliography Adams, V., Water and Wastewater Examination Manual, 1990. Annual Book of ASTM Standards, West Conshohocken, PA: American Society for Testing and Materials, 2002, www.normas.com/ASTM/. Dawson, R., Data for Biochemical Research, Oxford: Oxford University Press, 1990. Fresenius, W. et al., Water Analysis, New York: Springer-Verlag, 1988. Matzner, B. A., “Instantaneous Metering Aids Activated Sludge Plant,” Water and Wastes Engineering, August 1976. McNeil, B. and Harvey, L., Fermentation: A Practical Approach, Oxford: IRL Press, 1990. Meyers, R. A., Ed., Encyclopedia of Analytical Chemistry: Instrumentation and Applicationss, New York: John Wiley & Sons, 2000. Molvar, A. E., “Instrumentation and Automation Experiences in WastewaterTreatment Facilities,” EPA Document 600/2-76-198, October 1976. Sherman, R. E., Process Analyzer Sample-Conditioning System Technology, New York: John Wiley & Sons, 2002. Sokal, R. R. and Rohlf, F. J., Biometry, San Francisco: W.H. Freeman, 1994. Stanley, P. E., ATP Luminescence, Oxford: Blackwell Science, 1990. Van den Berg, F. W. J., Hoefsloot, H. C. J., and Smilde, A. K., “Selection of Optimal Process Analyzers for Plant-Wide Monitoring,” Analytical Chemistry, 74(13), 3105–3111, 2002. Vij, D. R., Luminescence of Solids, New York: Plenum Pub. Corp., 1998. “Water and Environmental Technology,” in ASTM Standards, Vols. 11.01 and 11.02, West Conshohocken, PA: American Society for Testing and Materials, published yearly. Zar, J. H., Biostatistical Analysis, New York: Prentice Hall, 1998.

8.7

Biological Oxygen Demand, Chemical Oxygen Demand, and Total Oxygen Demand

To Receiver

AT

I. G. YOUNG

(1974, 1982)

B. G. LIPTÁK

COD Flow Sheet Symbol

A. Biological oxygen demand (BOD) B. Chemical oxygen demand (COD) C. Total oxygen demand (TOD)

Sampling Technique:

Depending on the application, composite flow-averaged or time-averaged samples are often automatically collected for manual introduction to an instrument, in addition to traditional instantaneous grab samples. Continuous-flow automatic sampling is common for instruments that are in continuous operation for process monitoring and control.

Sample Pressure:

Typically collected at near atmospheric pressure

Sample Temperature:

Collected at process or ambient conditions

Suspended Solids:

Varies with the application and instrument. Many applications are interested in the oxygen demand/impact of both the liquid and solids in the sample stream.


Glass, quartz, Teflon, polyethylene, Tygon, polyvinyl chloride (PVC), stainless steel, ceramic

Ranges:

A. 0.1 to 1500 mg/l is typical and higher with dilution B. 5 to 1500 mg/l is typical and higher with dilution or some methods C. 0 to 60,000 ppm

Inaccuracy:

A. 3 to 20% B. 2 to 10% C. 2 to 5%

Response:

A. 3 min to 5 days B. 2 to 15 min C. 3 to 10 min

Costs:

A. $500 to $20,000 B. $8,000 to $20,000 C. $5,000 to $20,000


Bran and Luebbe Inc (www.branleubbe.com) (B) Challenge Environmental Systems Inc. (www.challenge-sys.com) (A) Horiba Instruments Inc. (www.horiba.com) (B) Ionics Inc. (www.ionics.com) (C) Isco Inc. (www.isco.com) (A, B) LAR Analytical Inc. (www.lar.com) (A, B) Respirometry Plus LLC (www.respirometryplus.com) (A) Star Instruments, Inc. (www.startoc.com) (A, B)

A significant portion of the total damage caused by the discharging of wastewater into lakes or rivers is because many


J. F. TATERA (2003)

Types of Measurements:

INTRODUCTION

1224

(1995)

of these discharges deplete the oxygen content of the lake or river and the resulting impact that this depletion has on the lake’s or river’s ecosystem. This oxygen-depleting potential is usually expressed and quantified by biological oxygen

8.7 Biological Oxygen, Chemical Oxygen, and Total Oxygen Demand

OXYGEN DEMAND The oxygen demand of a sample of water is the amount of elemental oxygen required to react with biodegradable material that is dissolved and suspended in the sample. This amount is expressed as milligrams of oxygen per liter of sample. When a population of bacteria causes the oxidation reaction in a population of bacteria, the oxygen required is called the biological oxygen demand. When the oxidation is carried out with a chemical oxidizing reagent such as potassium dichromate, the oxygen equivalent is called the chemical oxygen demand. In a sample of water, oxidation can also be caused by heating of the sample in a furnace in the presence of a catalyst and oxygen. This is called total oxygen demand. If the heating in a furnace occurs in the presence of carbon dioxide, the result is called total carbon dioxide demand (TCO2D). The BOD test is perhaps the most important oxygen demand measurement for the analysis of effluents and receiving waters (streams, lakes, and rivers). Basically, the BOD test measures the amount of oxygen used by microorganisms that feed on organic pollutants in the water under aerobic conditions. In this test, a bacterial culture is added to the sample under well-defined conditions and oxygen utilization is measured. 1,2 Although test procedures are carefully defined, it is difficult to obtain highly reproducible results, and the procedure is subject to the influence of many variables, particularly when the wastewater contains a variety of complex materials. Some of the factors that contribute to variations in BOD results are discussed below. The Seed The seed is the bacterial culture that affects the oxidation of materials in the sample. If the biological seed is not acclimated to the particular wastewater, erroneous results are frequently


obtained. Because different bacterial cultures are used in BOD measurements at different locations, it is not surprising that the results may be inconsistent. Also, recall the intent of the measurement. If the intent is to monitor plant feeds to help control the waste treatment operation, bacteria from the plant waste treatment process may be most useful, as it is probably acclimated to the plant’s typical composition. If the intent is to predict the impact on receiving waters, a more random population may better predict this. pH The BOD results are also greatly affected by the pH of the sample, especially if it is lower than 6.5 or higher than 8.3. Not only is oxidation of the material itself pH dependent, but so is that of the bacterial activity. In order to achieve uniform conditions, the sample should be buffered to a pH of about 7. Temperature Although the standard test condition calls for a temperature of 20°C (68°F), field tests often require operation at other temperatures and, consequently, the results tend to vary unless temperature corrections are applied (Figure 8.7a). Toxicity Toxic materials in the sample, although they may be oxidizable or biodegradable, frequently have a biotoxic or biostatic effect on the biological seed. The presence of toxic materials of this type is indicated by an increase in the BOD value as a specific sample is diluted for the BOD test. Consistent values may be obtained either by removing the toxic materials from the sample or by developing a seed that is compatible with the toxic materials in the sample.

300 30°C

250

BOD, mg/liter

demand (BOD), chemical oxygen demand (COD), or total oxygen demand (TOD) measurements. These instruments measure the amount of oxygen that a liter of wastewater is expected to take from the receiving waters as its pollutants are degraded by oxygen-consuming (aerobic) bacteria. BOD analyzers utilize bacteria to oxidize the pollutants. In COD analyzers, the oxygen demand is usually measured through chemical oxidation and catalytic combustion techniques. TOD analyzers typically oxidize the sample in a catalyzed thermal combustion process and detect both the organic and inorganic impurities in a sample. This section describes the various BOD, COD, and TOD analyzers. The main distinction between the various designs is in the speed at which the measurement is obtained and in the correlation of the resulting readings with manometric BOD tests. Other measurements that are often used for similar applications and correlated to these measurements include total carbon (TC), total inorganic carbon (TIC), and total organic carbon (TOC) analyzers; they are described in Section 8.58.

1225

20°C

200 150 9°C 100 50

10

20

40 30 Time, Days

50

60

70

FIG. 8.7a 3 Progress of BOD at 9, 20, and 30°C (48, 68, and 86°F). The break in each curve corresponds to the onset of nitrification.

1226


Incubation Time The usual standard lab test incubation time is 5 days, although the time required for stabilization (complete biochemical degradation of materials in the water) may take as long as 20 or 30 days. The 5-day results may occur at a flat part of the oxygen demand vs. time curve, or they may occur at a steeply rising portion. Thus, depending on the type of seed and the type of oxidizable material, divergent results can be expected for this reason alone (Figure 8.7a). On-line and more rapid BOD-based testing (like fast BOD and respirometry) typically reduce this time factor into the range of minute to hours. To accomplish this, they avoid long-term equilibrium requirements and usually optimize sample size to bacteria population and other relevant factors to give them a faster analysis with reasonable repeatability. Nitrification In the usual course of the BOD test, the oxygen consumption rises steeply at the beginning of the test owing to attack on carbohydrate materials. Another sharp increase in oxygen utilization occurs sometime during the 10th to 15th day in those samples containing nitrogenous materials (Figure 8.7a). Stated another way, the rate constant for attack on nitrogenous materials is much lower than that for attack on carbonaceous materials, and the demand due to nitrification is not appreciable until most of the carbonaceous material has been 3,4 destroyed. In view of difficulties and variability of the classic BOD determination, a rapid procedure that minimizes or eliminates these problems has been sought for many years. Although other procedures are used, the BOD continues to remain the universal standard method, supported by the force of tradition and the weight of legal authority in many jurisdictions. Thus, those who are concerned with estimating the oxygen depletion load of effluent waters must be thoroughly acquainted with the BOD test and prepared to support other methods by suitable correlation to BOD results. Therefore, although oxygen demand may be measured in a number of ways, the 5-day BOD result is what is meant by oxygen demand in most cases. BIOLOGICAL OXYGEN DEMAND Five-Day BOD Procedure If the BOD of a sample of water at 20°C (68°F) is measured as a function of time, a curve such as the one in Figure 8.7a is obtained. For the first 10 to 15 days, the curve is approximately exponential, but at about the 15th day a sharp increase is noted, which then falls off to a steady BOD rate. Because of the length of time and because the curve does not flatten, a standard test period of 5 days has been adopted universally for the BOD procedure. This is a laboratory procedure requiring some skill and training to obtain concordant


results. The procedure is described in greater detail in the 1,2 literature; only a brief description is given here. A measured portion of the sample to be analyzed is mixed with seeded dilution water so that after 5 days of incubation, the dissolved oxygen (DO) in the mixture is still sufficient for biological oxidation of materials in the sample. Of course, this cannot be known beforehand; consequently, a number of dilutions are run simultaneously for an unknown sample, or experience is used as a guide for well-defined samples. The seeded dilution water contains phosphate buffer (including ammonium chloride), magnesium sulfate, calcium chloride, and ferric chloride, as well as a portion of seeding material. The former group of inorganic materials is frequently referred to as nutrients. The latter group is a suspension of bacteria in water, usually supernatant liquor from a domestic sewage plant. Seeds may also be prepared from soil, developed from cultures in the laboratory, or obtained from receiving water 2 to 5 mi downstream of the discharge. The DO content of the mixture is determined at the start of the test and again after 5 days of incubation at 20°C in a special BOD bottle. DO Determination and Standards The DO may be deter1 mined by the Winkler titration method or instrumentally with a DO membrane electrode. The difference in DO after 5 days is used to calculate the BOD of the original sample. Corrections must be applied for immediate oxygen demand (that due to inorganic reducing materials) and for the oxygen required by the bacteria themselves for sustaining life (endogenous metabolism). A blank sample is run to assist with this. There is no standard against which the accuracy of the BOD test can be measured. The precision of the method is also difficult to ascertain because of the many variables. However, the single-operator precision of the method has been tested using a standard glucose–glutamic acid solution. Using eight different types of seed materials, the singleoperator precision was 11 mg/l at a level of 223 mg/l, or about 5%. It must be recognized that these results were obtained with highly skilled personnel under well-controlled laboratory conditions. Instrumentation A semiautomatic instrument has been designed to measure the BOD of as many as 11 samples. The samples have to be manually placed on the instrument turntable and the controls manually set. Means are provided for automatic re-aeration of those samples in which the DO has fallen to low values. Measurements of the DO are made on a preset time schedule by the polarographic DO sensor. The capability of automatic re-aeration when the DO is low eliminates the need for dilution, leading to improved precision in the BOD results. The instrument consists of a measuring unit (DO probe, aerator, water-sealing mechanism, unplugging mechanism, sample bottle, and turntable) and a control unit, by which all of the operations are programmed. The measuring


Flowmeter Measuring Station

Unplugging Mechanism

Aerator Span Check Solution

Stirrer

To Recorder

Controller Unit

Pump (for Aeration)

Sensor

Water Sealing Mechanism

Sample Washing Bottle Bath

Measuring Device

3 Way Electromagnetic Valve Washing Water Needle Valve Inlet

Turntable

Measuring Unit

Tank (Washing Water)

Exhaust Water

FIG. 8.7b Semiautomatic BOD instrument.

unit is housed in a chamber maintained at 20°C. Means for storing DO data on each sample are supplied, and the BOD is calculated from the DO values as already described. Figure 8.7b illustrates this instrument. Extended BOD Test As reflected in Figure 8.7a, continuation of the BOD test beyond 5 days shows a continuing oxygen demand, with a sharp increase in BOD rate at the 10th day owing to nitrification. The latter process involves biological attack on nitrogenous organic material accompanied by an increase in BOD rate. The oxygen demand continues at a uniform rate for an extended time. Knowledge of oxygen utilization of a polluted water supply is important because 1) it is a measure of the pollution load, relative to oxygen utilization by other life in the water; 2) it is a means for predicting progress of aerobic decomposition and the amount of self-purification taking place; and 3) it is a measure of the oxygen demand load removal efficiency by different treatment processes. As a means for treatment plant control and setting the legal standards for wastewater effluents, the extended test is not used. However, it must be remembered that the 5-day BOD does not represent the TOD load of a receiving water. The dynamics of oxygen removal and replenishment in 3 receiving water is discussed in the literature. Manometric BOD Test In the standard dilution method that was previously described, all the oxygen required must already be inside the BOD bottle, since it is sealed in a gas-tight manner at the initiation of the incubation period, and care is taken to prevent access 5 of air into the sample. In the manometric procedure, the seeded sample is confined in a closed system that includes an appreciable amount of air.


1227

As the oxygen in the water is depleted, it is replenished by the gas phase. A potassium hydroxide (KOH) absorber within the system removes any gaseous carbon dioxide generated by bacterial action. The oxygen removed from the air phase results in a drop in pressure that is removed with a manometer. This fall is then related to the BOD of the sample. Thus, in the manometric method, the DO of the water remains at a moderately high level, close to saturation (9 mg/l at 20°C), whereas in the standard BOD, the DO falls continuously during the 5-day incubation period to values near 1 mg/l. Despite this marked difference in conditions of DO during incubation, results in close agreement are obtained on many samples by the two different procedures. An apparatus is commercially available in which the BODs of five samples can be determined simultaneously by the manometric method. A measured sample of the sewage or wastewater is placed in one of the bottles of the apparatus, and the bottle is connected to a closed-end mercury manometer (Figure 8.7c). Above the water sample a quantity of air is trapped. As bacteria in the sample utilize oxygen, it is replenished by oxygen from the air. The removal of oxygen from the air results in a lowering of the air pressure. The fall in pressure is read on the mercury manometer directly in BOD units, assuming that the original air contained 21% oxygen. The preceding description assumes a sample that is already seeded. Of course, the method can be modified for those samples that require the addition of a bacterial culture. The procedure is carried out manually in the laboratory. In addition to the manipulations already described, it requires reading of the manometer by the laboratory technician.

Vinyl Tubing

Mixing Bottle Cap

Screw Cap (Closed During Operation)

Wick Moistened with Potassium Hydroxide Brown Glass Mixing Bottle Direct Reading BOD Scale Mercury Reservoir Sewage Sample (157 ml)

200

100 2 ml. Mercury

Manometer Sight Glass

FIG. 8.7c Manometric BOD apparatus.

300

0

1228


Entrance Port

Computer

Printer

Vent Solenoid CO2 Absorber

Water Pump

Transducer

Float

Recorder

Air Pump Sample Chamber

Manometer

Sewage Pump

Fill Solenoid

Dilution Water Pump

Reservoir

Air

Oxygen Electrode 1

Oxygen Electrode 2 Circulating Pump

Drain Solenoid

FIG. 8.7d BOD determination by automatic respirometer.

Automatic Recording The manometric method also lends itself to automatic recording of the course of oxygen utilization, since it is possible to monitor the pressure continuously. 6 This has been accomplished in an automatic respirometer now commercially available (Figure 8.7d). The sample, from 1 to 4 l, is introduced into a closed system containing air. Countercurrent circulation of both air and water insures equilibrium between dissolved and gaseous oxygen. A carbon dioxide scrubber is provided in the gascirculation line. A recording manometer detects the utilization of oxygen as the test is run for several hours. Published data indicate a correlation between the respirometer BOD 7 and the standard BOD. Both laboratory and automated online versions of this instrument are available. It must be recognized that BOD is inherently a timeconsuming process and ill-suited to the requirements of process monitoring or control. The shortest period mentioned for the automatic respirometer is 2 h, often too long for an effective control instrument. However, it is an excellent device for longer-term laboratory and process studies, since it can be made to simulate the activated sludge process. Useful fast BOD and respirometric measurements have been reduced to times under an hour and have been useful in the control of several waste treatment operations, though they are more often utilized in a monitoring role. BOD Assessment in Minutes When determining the BOD concentration in groundwaters, it might be acceptable to wait 5 days for the results, but in the control and operation of sewage treatment plants, it is not. The hold-up capacity of industrial and municipal wastewater treatment facilities and the desire for closed-loop control


Bioreactor

FIG. 8.7e BOD assessment is obtained in a few minutes in a continuously circulated bioreactor where the oxygen take-up of the organisms 8 controls dilution.

necessitates the use of a much faster sensor. One such analyzer is described in Figure 8.7e. In this design a bioreactor is filled with a large number of plastic rings, the interior of which are protected against mechanical abrasions and thereby provide a growth surface for organisms. A circulation pump serves to quickly distribute the sewage in the bioreactor and to keep the plastic rings in continuous motion. The sewage concentration (nutrient level) in the reactor is maintained at a constant low value, which results in an oxygen demand of about 3 mg/l. This oxygen demand is measured and maintained constant by detecting the decrease in oxygen concentration at the points where the diluted sewage enters and leaves the reactor. The dissolved oxygen concentration at electrode 2 is kept at a constant value below that at electrode 1. If this difference drops, the sewage concentration is increased; if this difference rises, the concentration is reduced. The sewage concentration (the nutrient concentration) in the bioreactor is adjusted by a computer. It varies the mixing ratio of sewage and dilution water. The total flow from the sewage and dilution water pumps is always 1 l/min, and it is the ratio of the two streams that is modulated. Therefore, this pumping ratio is an indication of the BOD concentration of the sewage sample. Correlation between this fast BOD measurement and the 5-day BOD obtained through conventional methods has been 8 acceptable. Fouling of the piping was found to be minimal, and weekly recalibration of the oxygen electrodes was found to be satisfactory.

8.7 Biological Oxygen, Chemical Oxygen, and Total Oxygen Demand + Oxygen Electrode − Hydrogen Electrode

Switch Electrode

Electrolysis Cell

Electrolyte

Adaptor-Alkali Container

Glass Fiber Wick

CO2

O2 Potassium Hydroxide Solution Sample Sample Bottle

Stirring Magnet

FIG. 8.7f Electrolysis system for measuring BOD.

Electrolysis System for BOD 9,10

Electrolysis of water can supply oxygen to a closed system as incubation proceeds (Figure 8.7f). At constant current, the time during which electrolysis generates the oxygen to keep the system pressure constant is a direct measure of the oxygen demand (by Faraday’s law). An instrument based on this principle permits the running of six samples simultaneously, and its readout gives BOD directly, in milligrams per liter for each sample. After starting the test run, operator attention is not required.

CHEMICAL OXYGEN DEMAND Standard Dichromate COD Procedure 1,2

This laboratory method requires skill and training similar to that required for the BOD test. A sample is heated to its boiling point with known amounts of sulfuric acid and potassium dichromate. Loss of water is minimized by use of a reflux condenser. After 2 h, the solution is cooled, and the amount of dichromate that reacted with oxidizable material in the water sample is determined by titrating the excess potassium dichromate with ferrous sulfate, using ferrous 1,10-phenanthraline (ferroin) as the indicator. The dichromate consumed is calculated as to oxygen equivalent for the


1229

sample and reported as milligrams of oxygen per liter of sample. Interpretations of COD values are difficult, since this method of oxidation is markedly different from the BOD method. Although ultimate BOD values can be expected to agree with COD values, a number of factors may prevent this concordance. Among these, we may mention the following: 1. Many organic materials are oxidizable by dichromate but not biochemically oxidizable, and vice versa. For example, pyridine, benzene, and ammonia are not attacked by the dichromate procedure. 2. A number of inorganic substances such as sulfide, sulfites, thiosulfates, nitrites, and ferrous iron are oxidized by dichromate, creating an inorganic COD that is misleading when estimating the organic content of wastewater. Although the factor of seed acclimation will give erroneously low results on the BOD tests, COD results are not dependent on acclimation. 3. Chlorides interfere with the COD analysis, and their effect must be minimized in order to obtain consistent results. The standard procedure provides for only a limited amount of chlorides in the sample. Despite these limitations, the dichromate COD has been useful in control of wastewater effluents from plants concerned with caustic and chlorine, dyeing and textiles, organic and inorganic chemicals, paper, paints, plating, plastics, steel, aluminum, and ammonia. This is usually accomplished by diluting the sample to achieve a lower chloride concentration and interference. This can be a problem for low COD concentration samples, as the dilution may dilute the COD concentration below the detection level or to levels at which accuracy and repeatability are poor. COD Detector The term COD usually refers to the laboratory dichromate oxidation procedure, although it has also been applied to other procedures that differ greatly from the dichromate method but which do involve chemical reaction. These methods have been embodied in instruments both for manual operation in the laboratory and for automatic operation online. They have the distinct advantage of reducing analysis time from days (5-day BOD) and hours (dichromate, respirometer) to minutes. Automatic On-Line Designs Figure 8.7g illustrates an on-line analyzer available with COD ranges from 0 to 5000 ppm and with measurement cycle times that are adjustable from 10 min to 5 h. The sample flow can be continuous at rates up to 0.25 GPM (1.0 lpm) and can contain solid particles up to 100 microns in size. The automatic COD analyzer periodically takes a 5 cc sample from the flowing process stream and injects it into the reflux

1230


Dilution Zero Calibration H2O Dichromate Acid Solution Solution S

S Sample in 50 to 1.000 Cotten

S

S

S

S

5 cc Sample

S

Flush

Sample Return

Reflux Chamber Out Reflux Condensor

Fail Safe Switch

Colling Water In

Fiber Optics To Electronics Module

Colorimeter

Heater/Reboiler at 150°C Dump Valve

S Drain

FIG. 8.7g Automatic COD analyzer using dichromate reagent and fiber-optic colorimeter detector and providing features of adjustable reflux time and autocalibration. (Courtesy of Ionics Inc.)

chamber after mixing it together with dilution water (if any) and with two reagents: dichromate solution and sulfuric acid. The reagents also contain an oxidation catalyst (silver sulfate) and a chemical that complexes chlorides in the solution (mercuric sulfate). The mixture is boiled at 302°F (150°C) by the heater, and the vapors are recondensed by the cooling water in the reflux condenser. The solution is refluxed for a preset time, during which the dichromate ions are reduced to trivalent chromic ions, as the oxygen-demanding organics are oxidized in the sample. The chromic ions give the solution a green color. The COD concentration is measured by detecting the amount of dichromate converted to chromic ions by measuring the intensity of the green color through a fiber-optic

Regulator Set at 10 PSIG

detector. The microprocessor-controlled package is available with automatic zeroing, calibration, and flushing features. Analyzers utilizing this or other COD wet chemical reaction schemes are generally categorized as flow injection analyzers (FIAs) or continuous-flow analyzers (CFAs), and some of the manufacturers of these types of analyzers do not advertise them as COD analyzers. To them, COD is just one application of their FIA or CFA capability. In one instrument, a 20-µl water sample is manually injected into a carbon dioxide carrier stream and swept through a platinum catalyst combustion furnace. In that furnace, the pollutants are oxidized to carbon monoxide and water, and the water is removed from the stream by a drying tube, after which the reaction products receive a second platinum catalytic treatment. A nondispersive infrared (NDIR) detector is used to measure the concentration of carbon monoxide, and a calibration chart is utilized to convert the readings into COD. An analysis can be completed in 2 min. This instrument is available commercially for manual operation (Figure 8.7h). Data obtained 11 on domestic sewage indicate excellent correlation between this method (frequently called CO2D) and the standard COD. Some other approaches to COD monitoring include methods that utilize ozone and OH–radicals as the oxidizing agents. One ozone-based scheme enriches dilution water with ozone and measures the dissolved ozone concentration of the dilution water and the residual dissolved ozone concentration downstream of the reaction chamber. It then calculates a COD value based on the difference in the two measurements and the ratios of dilution water and sample being delivered to the reaction chamber. It has a response time of 3 min. Another OH–radical approach produces OH–radicals on an electrode, by an electrical current. These OH–radicals are extremely strong oxidizing agents. An electrochemical measurement signal based on the OH–radicals being converted and measured as OH–ions results in a 30-sec analysis with a range of 1 to 100,000 ppm that is said to correlate to the dichromate standard method.

Gas “Purifying” Carbon Furnace

Flow Meter

Check Valve Sample Furnace

Sample Injection Port with Purging Manifold

Control Valve

Bone Dry CO2

Differential Pressure Regulator Drying (Preset) Tube

Infrared Analyzer

FIG. 8.7h COD detection employing combustion in a carbon dioxide carrier and an NDIR sensor.


Connection to External Recorder


One thing to remember is that all of these techniques are based on a chemical oxidation reaction and that all reactions can experience interferences. The nice thing about having several options is that you can pick the one that is most suited to your sample matrix.

TOTAL OXYGEN DEMAND The TOD method is based on the quantitative measurement of the amount of oxygen used to burn the impurities in a liquid sample. Thus it is a direct measure of the oxygen demand of 12,13 of the the sample. Measurement is by continuous analysis concentration of oxygen in a combustion process gas effluent (Figure 8.7i). The oxidizable components in a liquid sample introduced into the combustion tube are converted to their stable oxides by a reaction that disturbs the oxygen equilibrium in the carrier gas stream. The momentary depletion in the oxygen concentration in the carrier gas is detected by an oxygen detector and recorded as a negative oxygen peak. The TOD for the sample is obtained by comparing the recorded peak

height or area to the peak sizes of the standard TOD calibration solutions, e.g., potassium acid phthalate (KHP). Prepurified nitrogen from a cylinder passes through a fixed length of tube permeable to oxygen (usually silicone) into the combustion chamber packed with a solid catalyst, the gas scrubber, and then the oxygen detector. The baseline oxygen concentration is obtained as the nitrogen passes through the temperature-controlled permeation tube and can be varied to accommodate different TOD ranges by changing the nitrogen flow rate and the length of permeation tubing. The combustion chamber is a length of Victor tubing, quartz tube, ceramic tube, or other metal tube that contains a platinum catalyst and is mounted in an electric furnace and held at a temperature of 900°C (1672°F). The aqueous sample is injected into this chamber, and the combustible components are oxidized.

Sample Valves Two basic types of injection valves are in use today, the sliding plate and the rotary sampling valve. Manual injection can be accomplished though a silicone rubber septum, if desired. Figure 8.7j shows the main features of the sliding plate valve. Upon a signal from a cycle timer, the air actuator temporarily moves the valve to its “sample fill” position. At the same time, an air-operated actuator moves a 20-µl sample through the valve into the combustion tube. A stream of oxygenenriched nitrogen carrier gas moves the slug of sample into the combustion tube. Traditionally, sliding plate valves were manufactured from a variety of metals (stainless steel, Hastelloy, Monel, and others), but ceramic sliding plate sampling valves have been developed to provide reduced maintenance for this harsh application. The rotary sample valve is mainly used for on-stream TOD analyzers. Figure 8.7k shows the cross section of the

Sample Valve Before Actuation

Sample Valve Actuated Carrier Gas

Carrier Gas Sample Out

Sample In

FIG. 8.7i Basic components of an on-line TOD analyzer. (Courtesy of Ionics Inc.)


1231

Sample Out

Sample In Reaction Chamber Furnace

FIG. 8.7j Automatic sliding plate liquid sample injection value.

1232


Another type of TOD oxygen detector is a yttrium-doped zirconium oxide ceramic tube that has been coated on both sides with a porous layer of platinum. It is maintained at an elevated temperature and also provides an output that represents the reduction in oxygen concentration in the carrier gas that is a result of the sample’s TOD. A more complete discussion of this detector technology can be found in Section 8.42 (“Oxygen in Gases”) under “High-Temperature Zirconium Fuel Cells.”

Sampling Syringe Assembly Syringe Plunger Lifter

Sampling Head Sample Trough In Out

Syringe Plunger

Cam Ramp

Calibration Analysis is by comparison of peak heights or areas to a standard calibration curve. To prepare this curve, known TOD concentrations of a primary standard (KHP) are prepared in distilled and deionized water. Standard solutions are stable for several weeks at room temperature. Water solutions of other pure organic compounds can also be used as standards. Several analyses can be made at each calibration concentration, and the resulting data are recorded as parts per million (ppm) TOD vs. peak height or area. Older instruments utilize a recorder, and within the normal instrument range, the plot is a straight line. Consequently, a single-concentration calibration of the recorder chart in milligrams per liter TOD can be made. Newer instruments are microprocessor controlled, and the microprocessor stores the calibration results and outputs a result directly as ppm TOD. Interferences

Combustion Tube

FIG. 8.7k Rotary sample valve.

valve, in which a motor continuously rotates a sampling head, which contains a built-in sampling syringe. For part of the time, the tip of the syringe is over a trough that contains the flowing sample. Two or more cam ramps along the rotational path cause the syringe plunger to rise and fall, thus rinsing the sample chamber. Just before the syringe reaches the combustion tube, it picks up a 20-µl sample. As it rotates over the combustion tube, it discharges the sample. Oxygen Detector One type of TOD oxygen sensor is a platinum–lead fuel cell that generates a current in proportion to the oxygen content of the carrier gas passing through it. Before entering the cell, the gas is scrubbed in a potassium hydroxide solution, both to remove acid gases and other harmful combustion products and to humidify the gas. The oxygen cell and the scrubber are located in a temperature-controlled compartment. The fuel cell output is monitored and zeroed to provide a constant baseline. The output peaks are linearly proportionate to the reduced concentration of oxygen in the carrier gas as a result of the sample’s TOD.


Nitrate salts and sulfuric acid will normally decompose under sample combustion conditions as follows: 2 NaNO3 H 2SO 4

900°C Na 2O + 1 1 2 O2 + 2 NO Pt

8.7(1)

900°C H 2O + SO2 + Pt

8.7(2)

1

2

O2

This oxygen release results in a proportionate reduction in the TOD reading. The interference from sulfuric acid can be overcome by neutralizing it with sodium hydroxide. Dissolved oxygen in the aqueous sample also becomes an unknown source of oxygen to the combustion reaction and lowers the TOD readings. But unless special precautions are taken, the standard solutions are also at or near saturation, thus automatically minimizing this potential interference. If absolute TOD levels are required, the oxygen equivalent of the interference should be added to the TOD reading. Oxygen-saturated or air-saturated samples with very low TOD values pose a special problem, which can be circumvented by spiking the sample with a known concentration of a standard solution. The actual TOD will then be the analysis value minus the spiking concentration. Heavy-metal ions give long-term interferences, usually by eventual reduction of the catalyst efficiency. Replacement of the combustion tube and a thorough cleaning of the catalyst


TABLE 8.7l Application Suggestions for TOD Analyzers Application

Purpose

Waste treatment plant influent

Determines loading of plant

Primary, secondary, and plant effluents

Determines efficiency of treatment and TOD load on the receiving waterway

Enforcement programs

Determines pollution levels

Industrial process control

Determines process efficiency and leaks by continuous monitoring

Research and development

Evaluates waste treatment processes

Stream surveillance

Monitors fresh, estuarine, and marine water quality

Municipal or industrial water treatment plants

Monitors quality of influent water prior to treatment

Boiler feed and high-purity water monitoring

Measures TOD with great sensitivity

is the only remedy. Materials like silanes or siloxanes also combust in the reactor and exhibit a positive TOD, but can leave a residue of silica in the reactor that can coat and desensitize the catalyst. The catalyst and the reactor components can be coated when the sample is vaporized and combusted in a fashion that leaves some solids behind in the reactor. The total matrix of the sample should be evaluated before committing to a technique, not just the components being measured. Applications Table 8.7l lists typical applications for the TOD analyzer. Instruments equipped with the rotary or sliding plate sampling valve lend themselves nicely to multistream analysis, since no carryover of the previous sample is present. If the streams have varying levels of TODs, each stream must be diluted to the basic range of the analyzer by appropriate automatic dilution techniques. On the laboratory analyzers, it is best to sample deionized water when the instrument is in a standby condition. CORRELATION BETWEEN MEASUREMENT TECHNIQUES Many regulatory agencies recognize as the basis for oxygendepleting pollution control only the BOD or COD (preferably BOD) measurements of pollution load. The reason for this is that they are concerned with the oxygen-consuming pollution load on receiving waters, which is related to lowering of the DO due to bacterial activity. Thus, if the other methods described are to be used to satisfy legal requirements of the oxygen-consuming pollution load in effluents or to measure BOD removal, it is necessary that a correlation be established between the other methods and BOD. In order to summarize the features of various methods, it is assumed that the BOD is the standard reference method.


1233

The salient features of this method are: 1) a measurement of property (the BOD) of the sample, i.e., the amount of oxygen required for bacterial oxidation of bacterial food in the water; 2) dependence of the oxygen demand on the nature of the food as well as on its quantity; and 3) dependence of the oxygen demand on the nature and amount of the bacteria. Variations in Oxygen Demand Variations in oxygen demand due to variations in the amount (pounds per gallon) of food in the wastewater are expected; we are less able to deal with variations in oxygen demand when the amount of food is constant but changes occur in its BOD requirements. The same observations apply to the bacterial seed. Thus, variation in oxygen demand due to variation in the number of bacteria or their activity, as well as those due to the changes in the nature of their foodstuffs, leads to a systematic or bias error in BOD measurements, which cannot be predicted or corrected for. The standard reference method is therefore inherently variable—one subject to analytical error. Researchers in an 14 interlaboratory comparative study employing a synthetic waste found standard deviations around the mean of ±20% for BOD and ±10% for COD. BOD and COD Correlation 15

Another extensive study concluded the following: 1) A reliable statistical correlation between BOD and COD of a wastewater and its corresponding TOC or TOD can frequently be achieved, particularly when the organic strength is high and the diversity in dissolved organic constituents is low. 2) The relationship is best described by a least squares regression with the degree of fit expressed by the correlation coefficient—this applies to the characterization of individual chemical-processing and oil-refining wastewaters, not to all types of samples across the board. 3) The observed correspondence of COD-TOD was better than that of COD-BOD for the wastewaters. It was difficult to correlate BOD with TOD, particularly when the wastewater contained low concentrations of complex organic materials. 4) The BOD-COD or BOD-TOC ratios of an untreated wastewater are indicative of the biological treatment possible with the particular wastewater. As these ratios increased, higher treatment efficiencies by biological methods in terms of organic removal were noted. BOD and Other Methods Several papers have indicated high correlation between BOD and other methods. This can be achieved when the nature of the pollutant is constant and only its amount changes. For complex and varying mixtures, it is difficult or impossible to obtain good correlations. An interesting example is given in the work of Nelson 16 et al., who discuss a pyrolytic method combined with flame ionization detector (FID) (Section 8.12) detection. Values from the new method agreed with BOD values within ±15%

1234


for BOD values greater than 100 ppm on raw sewage and primary effluent. However, discrepancies of several hundred percent were found when the BOD was 20 ppm or less. These poor results can be attributed to marked variation in biodegradability of carbonaceous products in the secondary effluent, compared with products before treatment, as well as to the very small amount of total material left. The ability to truly correlate one measurement to another requires more than measurements that are measuring oftenrelated properties. Correlation can also be highly dependent on the matrix of the sample being measured. Different measurement techniques often respond differently to different backgrounds and to different interferences. For example, techniques like FID and ultraviolet (UV) absorption (Section 8.61) are being sold to make correlated COD measurements. Obviously, they are not measuring the ability to oxidize the sample. What they are measuring is a generalized organic content of a sample and trying to predict what a COD value of that sample would be, assuming the determined organic content represents it. While this is possible for some very well characterized samples, this approach often fails to adequately handle surprises (process upsets or changes that result in significant sample matrix changes). Many types of analyzers can measure various components that contribute to the BOD, COD, or TOD loading of a sample and, under the right circumstances, they can be reliably correlated to an appropriate BOD, COD, or TOD result. Many of these analyzers offer advantages like continuous or faster analysis and lower purchase, operating, and maintenance costs. The key to instrument selection in this case is to insure that you have selected one that is appropriate for your process, sample, and measurement requirements. If in doubt, usually the safest path is to go with an instrument that is compatible with your sample and most directly measures the property of interest. References 1. 2. 3. 4.

5. 6.

7.

American Public Health Association: Standard Methods for the Examination of Water and Wastewater, 12th ed., New York: APHA, 1965. Industrial Water: Atmospheric Analysis, Book of ASTM Standards, Part 23, Method D 2329-68, Philadelphia: ASTM, 1969. Gair, G. M. and Geyer, J. C., Water Supply and Waste-Water Disposal, New York: John Wiley & Sons, 1954. Ford, D. L., Eller, J. M., and Gloyna, E. F., “Analytical Parameters of Petrochemical and Refinery Waste Waters,” Journal of the Water Pollution Control Federation, Vol. 43, 1971, p. 1713. Tool, H. R., “Manometric Measurement of BOD,” Water Sewage Works, Vol. 114, 1967, p. 211. Arthur, R. M., “An Automated BOD Respirometer,” Proceedings of the 19th Industrial Waste Conference, Part 2, Purdue University, May 1964, p. 628. Arthur, R. M. and Hursta, W. N., “Short-Term BOD Using the Automatic Respirometer,” Proceedings of the 23rd Industrial Waste Conference, Part 1, Purdue University, May 1968, p. 242.


8. Riegler, G., “3-Minute BOD Assessments,” InTech, May 1987. 9. Clark, J. W., New Mexico State University, Engineering Experiment Station, Bill 11, 1959. 10. Young, J. C. and Baumann, E. R., “Demonstration of the Electrolysis Method for Measuring BOD,” Engineering Research Institute, Iowa State University, Ames, IA, progress report 1, March 1970; progress report 2, March 1971. 11. Stenger, V. A. and Van Hall, C. E., “Rapid Method for Determination of Chemical Oxygen Demand,” Analytical Chemistry, Vol. 39, 1967, p. 206. 12. Goldstein, A. L., Katz, W., Meller, F. H., and Murdoch, D. M., Total Oxygen Demand: A New Automatic Instrumental Method for Measuring Pollution and Loading on Oxidation Process, Watertown, MA: Ionics Inc., 1968. 13. Clifford, D. E., “Automatic Measurement of Total Oxygen Demand,” Proceedings of the 23rd Industrial Waste Conference, Purdue University, May 1968. 14. Ballinger, D. G. and Lishka, R. J., “Reliability and Precision of BOD and COD Determinations,” Journal of the Water Pollution Control Federation, Vol. 34, 1962, p. 470. 15. Ford, D. L., Eller, J. M., and Gloyna, E. F., “Analytical Parameters of Petrochemical and Refinery Waste Waters,” Journal of the Water Pollution Control Federation, Vol. 43, 1971, p. 1712. 16. Nelson, K. H., Lysyj, I., and Nagano, J., “Pyrolytic Analyzer Detects Organic Matter in Wastewaters,” Water Sewage Works, Vol. 14, January 1970.

Bibliography Adams, V., Water and Wastewater Examination Manual, 1990. ASTM Standards, Vols. 11.01 and 11.02, “Water and Environmental Technology” West Conshohocken, PA: American Society for Testing Materials, published yearly. Baird, R. B. and Smith, R. K., Third Century of Biochemical Oxygen Demand, Water Environment Federation, 2002. D’Alessandro, P. L. and Characklis, W. G., “Simple Measurement Technique for Soluble BOD Progression,” Water and Sewage Works, September 1972. Davis, E. M., “BOD vs. COD vs. TOC vs. TOD,” Water and Wastes Engineering, February 1971. Dawson, R., Data for Biochemical Research, Oxford: Oxford University Press, 1990. Fresenius, W. et al., Water Analysis, New York: Springer-Verlag, 1988. Geisler, C., Andrews, J. F., and Schierjott, G., “New COD Analysis Arrives,” Water and Wastes Engineering, April 1974. Hill, N. H., “Carbon Analyzers for Contaminants in Water,” InTech, March 1969. McNeil, B. and Harvey, L., Fermentation: A Practical Approach, Oxford: IRL Press, 1990. Ratliff, T. A., The Laboratory Quality Assurance System, New York: Van Nostrand Reinhold, 1989. Safranko, J. W., Schuler, J. D., and Small, J. W., “A Low-Temperature Microprocessor-Controlled TOC Analyzer,” American Laboratory, August 1983. Shaw, A., Mason, L., and Miller, R., “The Use of Online Respirometric Monitoring and Dose Response Testing to Determine the Best Process Alternatives for an Existing Petrochemical Waste Treatment Facility,” Proceedings of the WEFTEC2001 Conference, Water Environment Federation, October 2001. Small, J. W., “New Advances in TOC Analysis,” Pollution Analysis, September 1980. Tool, H. R., “Manometric Measurement of the Biochemical Oxygen Demand,” Water and Sewage Works, June 1967.

8.8

Calorimeters P. FOUNDOS (1982)

AT I

G. LIPTÁK (1995)

D. LEWKO

(2003)

AIE

Heat Value

Flow Sheet Symbol

Type of Designs:

A. Direct measurement by burning of fuel gas B. Inferential by calculation from composition and physical analysis, including chromatography (Section 8.12), mass spectrometry (Section 8.29), etc. C. Special designs such as reaction calorimeters of Mettler-Toledo, designs for the measurement of partial molar heat capacities of biopolymers by CSC, and total absorption calorimeters made by Opal

Applications:

1. 2. 3. 4. 5.

Operation:

a. Continuous b. Cyclic c. Portable

Performance:

(1) Controlled environment (2) Varying ambient (3) High speed of response (4) Inaccuracy ±0.5% of full scale or better (5) Inaccuracy ±1.0% of full scale or better (6) Inaccuracy ±2.0% of full scale or better

Area Classification:

(a) General purpose (b) Explosion-proof

Cost:

Under $10,000 [A, 2/3, a, (3)/(5)/(6)] $10,000 to $15,000 [A, 2, a, (3)/(5)/(6), (b)] $15,000 to $30,000 [A/B, 2/3, a/b, (1)/(2)/(4), (a)/(b)]


Ametek Process & Analytical Instruments (www.thermox.com) Cosa Instrument Corp. (www.cosa-instrument.com) Daniel Measurement and Control (www.danielind.com) Delta Instrument LLC (www.deltainstrument.com) EG & G Chandler Engineering/Ranarex (www.chandlerengineering.com) Galvanic Applied Sciences (www.galvanic.ab.ca)

Custody transfer Process monitoring and control Blending and mixing of fuel gases Gas and liquefied natural gas (LNG) processing Compliance recording

INTRODUCTION This section does not discuss the special calorimeter designs such as reaction calorimeters (Mettler-Toledo), designs for the measurement of partial molar heat capacities of biopolymers

(CSC), or total absorption calorimeters (Opal), but only directs the reader to the web pages of their respective suppliers. Inferential calorimeters, which determine the heat content by calculation from composition and physical analysis, including chromatography (Section 8.12) and mass spectrometry 1235


1236


(Section 8.29), are not covered in detail either, because they are discussed in the above-noted sections. Calorimeters are analyzers that measure the heat value or energy content of gaseous fuels. There are two broad categories of this type of instrument: those that can be considered true calorimeters, because they are actually burning the gas and directly measuring its heating value, and inferential calorimeters, which analyze the composition of the gas or measure a physical parameter to determine the heating value.

TERMINOLOGY Basic terms and definitions used in gas calorimetry are given here and may also be found in the references listed at the end of this section. A summary of the measurement calibration techniques is outlined in Table 8.8a.

Net Calorific Value This the measurement of the actual available energy per unit volume at standard conditions, which is always less than the gross calorific value by an amount equal to the latent heat of vaporization of the water formed during combustion.

Wobbe Index American Gas Association (AGA) 4A defines the Wobbe Index as a numerical value that is calculated by dividing the square root of the relative density (a key flow orifice parameter) into the heat content (or BTU/SCF) of the gas. Mathematically, the Wobbe Index is defined by Equation 8.8(2):

Wobbe index =

calorific value specific gravity

8.8(2)

British Thermal Unit A British thermal unit (BTU) is the amount of heat required to raise the temperature of 1 lb of water by 1°F at or near 60°F.

BTU Dry This is the heating value that is expressed on a dry basis. The common assumption is that pipeline gas contains 7 lb (or less) of water vapor per million standard cubic feet (SCF).

The Wobbe Index accounts for composition variations in terms of their effect on the heat value and specific gravity, which affect the flow rate through an orifice. In essence, the Wobbe Index is a measurement of the available potential heat, and it can be used in conjunction with the gas flow measurement to produce a measurement of heat flow rate (see Section 2.5). In the following paragraphs, some basic terms and gas calorimeter design variations are described. The American Society for Testing Materials (ASTM) Standards listed in the references serve to compliment this information.

BTU Saturated This is the heating value that is expressed on the basis that the gas is saturated with water vapors. This state is defined as the condition when the gas contains the maximum amount of water vapors without condensation, when it is at base pressure and 60°F.

Combustion Air Requirement Index The combustion air requirement index (CARI), a dimensionless number, indicates the amount of air required (stoichiometrically) to support the combustion of a fuel gas. Mathematically, the CARI is defined by Equation 8.8(1): CARI =

Air/Fuel Ratio s.g.

8.8(1)

Gross Calorific Value This is the heat value of energy per unit volume at standard conditions, expressed in terms of BTU per SCF, kilocalorie 3 per cubic Newton meters (Kcal/N.m ), or other equivalent units.


UNITS, ACCURACY, AND OUTPUT SIGNALS When calorimeters are used for custody transfer of natural gas, the unit of measurement is often the BTU. The Natural Gas Policy Act of 1978 (15USC 3311, Supplement 1981) established the BTU as the basic measurement of natural gas for pricing purposes, supplanting the traditional volume bases measurement. As a result, the market demand for custody transfer type calorimeters increased markedly. In Europe and in other countries where the metric system is used, natural gas calorimeters are calibrated in mega-Joule units. The response time can be a critical consideration in selecting the right analyzer for closed-loop control applications. On the other hand, for custody transfer applications, one should maximize accuracy even at the expense of response time, while improved response time even at the expense of accuracy is justified in some critical process control applications. The analog output of the analyzer may represent the gross calorific value (sometimes referred to as upper heating value or gross heating value), the net calorific value (sometimes referred to as lower heating value or net heating value), or the Wobbe Index.

TABLE 8.8a Summary of Calorimeter Features and Specifications a

Operation

Range in Btu Full Scale

Accuracy ±% of Full Scale

Speed of Response (90%)

5

Remote Transmitters

4

Local Readout

3

Performance Ambient Limits °F (°C)

2

72–77 (22–25)

130–3300

0.5

3 min

72–77 (22–25)

120–3600

0.5

15 min

0–100 (–18–38)

Any

0.5

10 min

N/A

N/A

0.5

N/A

50–90 (10–32)

130–3300

1.0

8 sec

0–128 (−18–53)

Any

0.5

10 min

N/A

120–3300

1.0

3.5 min

0–128 (–18–53)

Varies

2.0

N/A

150–3600

2.0

4.5 min

150–3300

2.0

55 sec

130–3300

0.75

8 sec

Any

0.5

10 min

120–3300

1.0

3.5 min

150–3300

2.0

55 sec

Cyclic

1

Continuously

Ex-Proof

General Purpose

Inferential

Type

Directt

Area Class

Empirical Calibration

Application

Standard Sample

Type

GROSS CALORIFIC VALUE Water ∆T

Air ∆T

Gas Chromatograph

Adiabatic Flame Temperature

Airflow Calorimeter

NET CALORIFIC VALUE

Gas Chromatograph Expansion Tube Calorimeter

Specific Gravity Process Chromatograph

Thermopile Calorimeter

60–90 (16–32)

N/A

50–110 (10–43)

0–120 (−18–49)

WOBBE INDEX Airflow Calorimeter

Gas Chromatograph

Expansion Tube Calorimeter

Thermopile Calorimeter

See feature summary at begining of section.


N/A

N/A

1237

a

8.8 Calorimeters

1238


Water Overflow

Non-Condensible Gas Vent

Water Outlet Thermistor

Combustion Chamber Shell Calibration Heater

Water Pressure Regulator

Ignition Electrode Water Capillary Water Inlet Thermistor

Sol V

Water Inlet

Insulation Jacket

Calibration Heater Controller

Sol V

Water Heater Gas Capillary

Sol V

Mixed Gas Tube Sol V

Gas Pressure Regulator Gas Inlet

Gas Capillary Sol V

Sol V

Oxygen Pressure Regulator Oxygen

Oxygen Heater

Gas Heater

FIG. 8.8b 1 Water-temperature-rise-type calorimeter provided with electric heater for direct calibration.

DESIGN VARIATIONS In the following paragraphs, a brief description is given of the various gas calorimeter designs that are used to detect the heating value of gaseous fuels and waste gases.

burned, the amount of electric heat introduced matches the heating value of the gas. This type of calibration can be made at any time and for any reading of the calorimeter.

Air-Temperature-Rise Calorimeter Water-Temperature-Rise Calorimeter One variation of this design is illustrated in Figure 8.8b. Here a constant (capillary-controlled) flow rate of the gas is mixed with a constant flow rate of oxygen (or air) and is burned. The resulting heat of combustion is removed by a constant flow rate of water. Both the flow rates and the temperatures of the entering streams are controlled at a constant value. Thermistors measure the temperature of the water entering and leaving. The temperature difference or temperature rise is therefore a direct measure of the heating value of the burned gas. This calorimeter can be compensated for the variations in barometric pressure and can therefore measure the gross calorific value of fuel or waste gases. Figure 8.8b also shows a calibration heater. During the calibration cycle this electrical resistance heater is turned on while the gas flow is off. Thereby a known accurately determined amount of heat is introduced and the corresponding temperature rise on the water side is measured. When the temperature rise is the same as it was when gas was being


The measurement is accomplished by continuously transferring all the combustion heat of a metered quantity of gas to a metered quantity of air (see Figure 8.8c). The temperature rise of the air is measured and is related directly to gross calorific value of the gas. The unit can be modified so that the heat-absorbing air is not separated from the products of combustion, thus resulting in a more accurate measurement of the net calorific value.

Airflow Calorimeter In this design the variations in the heat released by the continuous burning of the fuel gas are offset by a continuous, varying airflow that maintains the temperature of the products of combustion constant (see Figure 8.8d). Thus, the airflow is correlated to the heat value of gas or to the Wobbe Index. With the addition of a constant-volume gas-metering pump, and compensating for specific gravity variations, the instrument can be calibrated for the net calorific value.

8.8 Calorimeters

Pressure Regulator

Gauge Location

Secondary Air Orifice Cap

Inlet Orifice Sight Plug

Piping

Inlet Bleeder Burner Flame Orifice

Metered Primary Gas Art Orifice Cap

Bleeder Gas Gas

Vent to Outside

1239

(9) Outgoing Air Thermometer Main Burner Flame

Combustion Air Combustion Air Meter Meter Orifice Combustion Air Overflow Weir

Heat Exchanger (A) Incoming Air Thermometer Heat Absorbing Air Outlet Connector Heat Absorbing Air Tank Water Level

Gas Meter

Safety Shut-Off Valve

Overflow Weir Drainpipe Gas and Primary Air

Thermocouple Tube

Heat Absorbing Air Meter

Secondary Air

Drain Manifolds

Burner Water Seal

Condensate Pan

Water Pump

Auxiliary Tank Supply

Products of Combustion

FIG. 8.8c Air ∆T calorimeter.

Ambient Compensating Tubes

Thermal Expansion Element Products of Combustion

Mixing Baffle

Heat Exchange Air

Flapper/ Nozzle Air Orifice

Burner Gas Orifice

Wobre Index =

cv

√ 2g

Air Control Valve Gas

Combustion Air Adjustment Safety Shutdown Solenoid

Air Supply

Precision Gas Regulator

D/P Transmitter

Gas M Calorific Value-CV

FIG. 8.8d Airflow calorimeter.

Residual Oxygen Calorimeter In this design, a continuous gas sample is mixed with dry air at a precisely maintained ratio. The ratio is dependent on the BTU range of the gas to be measured. The fuel–air mixture


is oxidized in a combustion furnace, and the oxygen concentration in the spent sample is measured using a zirconia oxide cell. With the addition of a precision specific gravity cell, the analyzer can be calibrated to provide a measurement corresponding to the calorific value of the gas.

1240


derivative of the flame temperature composition that allows calibration for gross calorific value.

Air Breather

Thermopile Calorimeter Precision Oil-Sealed Bell-Regulator

Air Flow

First Regulator

Pressure Set at 1 Inch (25 mm) H2O Sample Flowing to Expansion Tube Calorimeter Burner

The thermopile calorimeter measures the temperature of the hot products of combustion mixed with a constant volume of air supplied by a fan. The sample to the burner is provided with an orifice bypass, which is needed for specific gravity compensation; thus, the resulting measurement is in terms of net calorific value. To measure the Wobbe Index, the bleed is blocked and the sample goes through the burner.

APPLICATIONS There are five general areas of application:

Pressure Approx. Atmospheric Air Breather

Sample Inlet

FIG. 8.8e Precision regulator used for expansion tube-type calorimeter. (Courtesy of Cosa Instrument Corp.)

Chromatographic Calorimeter A conventional chromatograph can also be used to analyze gas composition (see Section 8.12), and a microprocessor can be used to calculate the heating value and specific gravity of the gas from empirical data held in memory by the microprocessor. This information can be used to calibrate for the gross or net calorific value of the Wobbe Index. Expansion Tube Calorimeter In this design, the gas sample is delivered through a precision regulator system (Figure 8.8e), which is independent of specific gravity and atmospheric pressure. The gas is burned at the base of a differential expansion tube unit that responds to the temperature of the products of combustion and excess air. The differential signal is calibrated as the net calorific value. Modification of the regulator to respond to specific gravity changes allows calibration to the Wobbe Index. Adiabatic Flame Temperature The gross calorific value of fuel gas is proportionate to the ratio of air to a fuel that maximizes the adiabatic flame temperature of the mixture. Therefore, in this design, flows are controlled and two burners are used to obtain the mathematical


1. Custody transfer: Sale or purchase of fuel gas with accuracy being the primary consideration. 2. Process monitoring and control: To effect on-line manual or automatic control of process or efficient burning of the gas by using heat value measurement as one of the measured variables. Such applications include feed-forward control of fuel gas-fired heaters or boilers, stove-firing control, vaporizer control, and synthetic natural gas (SNG) reactor control. In these applications, measurement of heat value is important as a process measurement for efficient control of energy consumption, or to limit the flow of heat to an energysensitive process. This application requires high speed of responses as well as reliability to achieve effective control. 3. Blending and mixing of fuel gas: To obtain a uniform quality gas or to utilize waste or by-product gases by blending them into the main fuel gas. Blending is often used to achieve a desired ratio of streams such as propane/air or blast furnace gas/coke oven gas. However, when by-product or waste gases are to be injected into a stream and they vary significantly in quantity, monitoring of the mixture is necessary to make proper use of the fuel. Speed of response and reliability are essential. 4. Processing of gas and liquefied natural gas (LNG) operations: LNG must be vaporized and conditioned for efficient consumption. Coke oven and blast furnace gases are the main fuels used by the steel industry, and refinery gas is a major by-product that is used in petroleum refining. All these fuel gases are produced in specific processing operations that require monitoring and conditioning for efficient use. 5. Compliance recording: For government-regulated energy transfer by pipeline and utility distributors to the consumers. Accuracy and traceability are essential criteria.

8.8 Calorimeters

SAMPLE CONDITIONING The requirements for the sample conditioning system are dependent on the limitations of the selected analyzer and the minimum, normal, and maximum limits of the process stream being monitored. In many industrial applications, fuel gases are generated as by-products of other processes, and these “off-gases” can be extremely dirty and require sample conditioning. In some cases, special consideration must be given to the dew point of the off-gas when designing the sample conditioning system. Refer to Sections 8.2 and 8.3 for sample system options and design features.

CONCLUSION Gaseous fuel energy is a costly commodity that is being consumed with much more care and efficiency than in the past. The key to its efficient consumption is measuring the available heat of the fuel gas. There are several calorimeters available for reliable on-line measurement of gas heating values. For closed-loop control applications, the designs that provide a speed of response of less than a minute (Table 8.8a) are preferred. In addition, there are several manufacturers that offer calorimeters specifically for custody transfer applications. These analyzers offer improved accuracy at the expense of response time (Table 8.8a).

Reference 1.

Christopher, D. E., “Direct Energy Measurement,” Measurements and Control, December 1991.

Bibliography AGA, Report 3. AGA, Report 8. AGA Gas Measurement Manual, Section 11A.2, “Determination of Heating Value of Gas,” p. 11A2.1. Armstrong, G. T., “Standard Combustion Data for the Fuel Gas Industry,” AGA, 1972.


1241

ASTM Standard D 900-55(70), Calorific Value of Gaseous Fuels by the Water Flow Calorimeter, Philadelphia: American Society for Testing and Materials, 1970. ASTM Standard D 1826-77, Test for Calorific Value of Gases in Natural Gas Range by Continuous Recording Calorimeter, Philadelphia: American Society for Testing and Materials, 1977. ASTM D 1945, “Analysis of Natural Gas by Gas Chromatography,” Philadelphia: American Society for Testing and Materials. Bowles, E. B., “Small Errors in BTU Measurement Can Add Up to Large Losses in Revenue,” Pipe Line & Gas Industry, November 2000. Broadwater, S. R., “Columbia Gas Moves toward Heating-Value Measurement,” Oil and Gas Journal, August 25, 1980. Christopher, D. E., “Direct Energy Measurement,” Measurement and Control, December 1991. Distribution Conference 72-D-76, American Gas Association, Arlington, VA, 1972. Foundos, A. P., “Measuring Heat Release Rate from Fuel Gases,” Instrumentation Technology, Instrument Society of America, 1977. GPA, “Calculations of Gross Heating Value, Relative Density and Compressability Factor for Natural Gas Mistures from Compositional Analysis.” GPA, “Standard 2145-Table of Physical Constants for Hydrocarbons and Other Compounds of Interest to the Natural Gas Industry.” GPA Reference Bulletin, “Heating Value as a Basis for Custody Transfer of Natural Gas,” 1984. Green, D. and Perry, R. H., Perry’s Chemical Engineer’s Handbook, 6th ed., New York: McGraw-Hill, 1984. Hawkins, J. and McGowan, A., “Theoretical Introduction to the Use of a Residual Oxygen Measurement Method for the Analysis of Combustion Air Index (CARI) and the Wobbe Index of Fuels,” ISA Chicago, October 2002. Kizer, P., “Natural Gas Energy Determination Review,” ISA Proceedings, 1991. Kizer, P., “Energy Measurement Using On-Line Chromatography,” in 71st International School of Hydrocarbon Measurement,” 1996. Kizer, P., “Operation of On-Line Gas Chromatographs,” in American School of Gas Measurement, 1998. Lange, N. A. and Forker, G. M., Handbook of Chemistry, 10th ed., New York: McGraw-Hill, 1967, pp. 842–843, and Columbia Gas System Data. Larson, B., “Heating Value Technologies for 2001,” in Winnipeg CGA Gas Measurement School, June 5, 2001. Lide, D. R., Handbook of Chemistry and Physics, 71st ed., 1990–1991. McCoy, R., “BTU Determination by Process Chromatograph,” Applied Automation, Inc., a subsidiary of Phillips Petroleum. Melrose, D. C., “Comparison of Calculated and Measured Heating Value of Natural Gas,” presented at AGA Distribution Conference 72-D-2, American Gas Association, Arlington, VA, 1972. Pannill, W. and Sharples, R. J., “Calculation of Gas Heating Value Is Complicated by the Courts,” Oil & Gas Journal, July 2, 1984.

8.9

Carbon Dioxide R. A. HERRICK

(1974, 1982)

To Receiver

B. G. LIPTÁK

(1995, 2003)

AT co2 Flow Sheet Symbol


Types of Sensors:

a) Nondispersive infrared (NDIR); see Table 8.9a for IR analyzer designs and applications b) Gas filter correlation (GFC) c) Orsat

Sample Pressure:

Up to 15 PSIG (104 kPa), but atmospheric or near atmospheric is normal

Sample Temperature:

Up to approximately 120°F (49°C) not a consideration when freeze-out trap is used

Sample Flow Rate:

Generally less than 0.5 acfm (2.35 × 10 m /sec); typically 1 to 2 l/min

Inaccuracy:

a) Can be as high as ±0.2 ppm; typically ±1% to ±2% of full-scale range b) 1% to 2% of full scale, including drift c) Laboratory procedure

Ranges:

a) 0 to 2000 ppm, 0 to 3000 ppm, 0 to 5000 ppm, 0 to 1%, 0 to 2%, 0 to 5%, 0 to 10%, 0 to 20%, and 0 to 100% (see Table 8.9b for overall capability) b) 0 to 5 ppm, 0 to 10 ppm, 0 to 20 ppm, 0 to 50 ppm, 0 to 100 ppm, 0 to 500 ppm, 0 to 1000 ppm, 0 to 2000 ppm (low detectable limit is 0.1 ppm)

Response:

a) Determined by cell volume and sampling rate; typically less than 30 sec b) 90 sec with 30-sec signal averaging time

Costs:

Portable, battery-operated, diffusion type monitor with two alarm settings, digital display, and 4- to 20-mA output is $1500; permanently installed, explosion-proof NDIR analyzer with recorder is about $10,000.


Advanced Pollution Instruments API (www.teledyne-api.com) AMC (Armstrong Monitoring Corp.) (www.armstrongmonitoring.com) Ametek/Thermox (www.thermox.com) Bran & Luebbe (www.branluebbe.com) CEA Instruments (www.ceainstr.com) Dasibi Environmental Corp. (www.dasibi.com) Delphian Corp. (www.delphian.com) Ecotech (www.ecotech.com.au) E&E Process (www.process-controls.com) EMS (Environmental Monitoring Systems) (www.emssales.com) Enviro Technology (www.et.co.uk) Foxboro-Invensys (www.foxboro.com) Horiba Instrument Inc. (www.nettune.net) Innova Air Tech Instruments (www.inniva.dk) International Sensor Technology (www.intlsensor.com) IT Group (www.theitgroup.com) MSA Instrument Div. (www.msanet.com) Purafil Inc. (www.purafilonguard.com) Sensidyne Inc. (www.sensidyne.com) Servomex Co. (www.servomex.com)

–4

3

8.9 Carbon Dioxide

1243

Sieger Gasalarm; Siemens Energy & Automation (www.sea.siemens.com) Sierra Monitor Corp. (www.sierramonitor.com) Sigrist-Photometer Ltd. (www.photometer.com) Teledyne Analytical Instruments (www.teledyne-ai.com) Thermo Environmental Instruments (www.thermoei.com) Thermo Gas Tech (www.thermo.com) Topac (www.topac.com) Yokogawa Corp. of America (www.yca.com)

INTRODUCTION

AMBIENT AIR MEASUREMENT

The measurement of carbon dioxide in the ambient air is the primary concern of geophysicists, rather than of the air pollution control engineer. The precise measurement of the carbon dioxide content of the atmosphere is of significant concern in determining long-term changes in the composition of the atmosphere. The measurement techniques used by geophysicists are highly precise compared to the techniques used for air quality- or air pollution-related measurements. Air quality-related measurements can be used to monitor the return air quality from occupied spaces and, based on those measurements, to modulate the rate at which fresh air is being introduced. In air pollution-related applications, carbon dioxide is hardly ever measured in the ambient air. It is usually measured at emission points since some combustion equipment regulations are stated in terms of allowable pollutant discharges corrected to 50% excess air.

The precise knowledge of the carbon dioxide concentration in the atmosphere is necessary. An increase in global carbon dioxide concentration of only 1% (or about 3 ppm) has significant consequences on the weather. The instruments used to measure atmospheric carbon dioxide concentrations must, of necessity, be highly precise, such as the gas filter correlation (GFC) units discussed later.

Nondispersive Infrared Type These instruments are discussed in detail in Section 8.27. Since the absorption bands of water and carbon dioxide somewhat overlap, a freeze-out trap (−80°C or −112°F) is often used in the sample preparation system to remove the water prior to the measurement.

TABLE 8.9a IR Analyzer Applications Summary Organic Vapors Analyzer

Carbon Monoxide

Carbon Dioxide

Simple Molecules

NDIR

Mid-IR filter

Complex Molecules


Comments

Single-component analysis: same as above, including ammonia, vinyl chloride, carbon tetrachloride, methyl ethyl ketone, ethylene dichloride, etc.

Single-component analysis: ethylene dichloride, water, phenol, methyl alcohol, etc.; moisture in solids Stack analysis, single-component gas analysis

Multiple-filter near-IR Multiple-filter mid-IR

Solids (Reflection)

Single-component analysis: ethylene, CO, acetylene, methane, etc.

Near-IR filter

Correlation spectrometer

Organic Liquids

Multiple components for cereal, meat, and paper analysis

Automotive exhaust analysis (CO, CO2, –CH); multiple components for mike analysis, multiple components of gases using a programmable circular variable filter

1244


TABLE 8.9b Typical Applications for NDIR Analyzers Gas

Minimum Range (ppm) 0–300

0–10

Butane (C4H10)

0–300

0–100

Carbon dioxide (CO2)

0–10

0–100

Carbon monoxide (CO)

0–50

0–100

Ethane (C2H6)

0–20,000

0–10

Ethylene (C2H4)

0–500

0–100

Hexane (C6H14)

0–200

0–5

Methane (CH4)

0–2000

0–100

Nitrogen oxide (NO)

0–500

0–10

Propane (C3H8)

0–300

0–100

Sulfur dioxide (SO2)

0–500

0–30

Water vapor (H2O)

0–3000

0–5

When using prepared calibration standard mixtures of carbon dioxide in nitrogen, an inaccuracy of ±0.2 ppm is attainable. At normal atmospheric CO2 concentration levels of approximately 314 ppm, this error is equivalent to ±0.06%. Nondispersive infrared (NDIR) type CO2 monitors for heating, ventilation, and air conditioning (HVAC) and industrial applications are available in both portable and permanently installed designs. The portable units are usually battery operated, and their ambient sample is received by a combination of diffusion and convection effects in the sensor head, without any pumps or filtering. These units are usually provided with digital displays, one or two alarm settings, and analog or digital output signals. The more expensive, permanently installed or wall-mounted NDIR units often include data loggers, which can store about 1000 readings along with their times and dates. Some of these units can also detect other gases, such as CO, H2S, or O2. Gas Filter Correlation Type When very accurate low-level measurements are needed, or when background gases that have the potential to interfere with the measurement are present, GFC is used. In these designs, the measuring and reference filters are replaced by gas-filled cuvettes. The reference cuvette is filled with CO2 and the measuring cuvette usually with nitrogen. In addition to being unaffected by the presence of background gases, both the accuracy and the response time of these instruments are better than those using filters. If DFC is used in combination with single-beam, dual-wavelength technology, it is virtually immune to obstruction of the optics. This, in turn, prevents drift and thereby reduces the frequency at which recalibration is needed. SOURCE MEASUREMENT Measurement of the carbon dioxide, carbon monoxide, and oxygen concentration of flue gases from boilers has been


Maximum Range (%)

Ammonia (NH3)

done for many years (see Figure 8.3k). These measurements allow precise setting of boiler operating variables for maximum fuel economy. Before the use of the infrared analyzer became accepted practice, mechanical instruments were used to continuously determine the carbon dioxide content of flue gases. Their operation was based on the reduction in gas volume resulting from the absorption of carbon dioxide in a strong alkaline solution. This is the principle of the Orsat analyzer, still used as the standard method for manual determination of combustion gas composition. For air pollution testing purposes, the carbon dioxide content of the flue gas is determined only during the few hours of the test. The usual procedure is to slowly withdraw a low-volume integrated sample into a plastic bag over the duration of the test. The bag sample is then analyzed manually using an Orsat analyzer or instrumentally using a NDIR analyzer. Bibliography Annual Book of ASTM Standards, West Conshohocken, PA: American Society for Testing and Materials, 2002. Ewing, G., Analytical Instrumentation Handbook, New York: Marcel Dekker, 1990. “Gas Detectors and Analyzers,” Measurement and Control, October 1991. Lipták, B. G., Ed., Environmental Engineers’ Handbook, 2nd ed., Chelsea, MI: Lewis Publishers, 1996. Lipták, B. G., “Saving through CO2 Based Ventilation,” ASHRAE Journal, July 1979. Lodge, J. P., Methods of Air Sampling and Analysis, Chelsea, MI: Lewis Publishers, 1988. Meyers, R. A., Ed., Encyclopedia of Analytical Chemistry: Instrumentation and Applications, New York: John Wiley & Sons, 2000. Sherman, R. E., Process Analyzer Sample-Conditioning System Technology, New York: John Wiley & Sons, 2002. Van den Berg, F. W. J., Hoefsloot, H. C. J. and Smilde, A. K., “Selection of Optimal Process Analyzers for Plant-Wide Monitoring,” Analytical Chemistry, 74(13), 3105–3111, 2002.

8.10

Carbon Monoxide R. J. GORDON

(1974, 1982)

B. G. LIPTÁK

To Receiver

(1995, 2003)

AT co Flow Sheet Symbol

Detector Types:

A. Nondispersive infrared (NDIR); see Table 8.9a for IR analyzer designs and applications B. Mercury vapor C. Gas chromatography D. Electrochemical fuel cell E. Catalytic oxidation F. Others, from color change badges, dosimeter tubes, and radon canisters to mass spectrometers, are discussed in Section 8.60.

Reference Method:

Infrared

Application:

For ambient air monitoring, electrochemical sensors are used most often; for detecting stack gas concentration, infrared sensors are used most often.

Ranges:

A. From 0 to 1, 0 to 5, 0 to 10, 0 to 20, 0 to 50, and 0 to 100 ppm for ambient and 0 to 200, 0 to 500, 0 to 1000, 0 to 2000, 0 to 5000, and 0 to 10,000 ppm for other applications, including stack gas. (See Table 8.9b for overall range capability.) B. 0 to 50 ppm C. 0 to 200 ppm D. 0–50 to 0–500 ppm or more E. 0 to 500 ppm

Sensitivity:

Generally 1 ppm; chromatographs can provide 0.1 ppm and mercury vapor analyzers 0.05 ppm

Response Times:

Infrared units are usually adjustable down to a few seconds, while electrochemical sensors require 30 to 60 sec.

Inaccuracy:

For NDIR sensors, ±1% of full scale for up to 1000 ppm and ±2 to 3% of full scale for higher ranges. For infrared stack gas analyzers, 2 to 4% of reading can be expected, while for electrochemical ambient monitors, 1 to 3% of full scale is usual.

Costs:

Pocket-size, battery-operated personal toxic gas monitor—$400 to $700; continuous industrial electrochemical or infrared monitor/alarm/transmitter—$400 to $2000; portable, battery-operated flue gas analyzer—$2000 to $5000; mercury vapor analyzer— $7500; NDIR with recorder included—about $10,000; gas chromatograph (see Section 8.12)—$25,000 and up


Advanced Pollution Instruments—API (www.teledyne-api.com) AMC (Armstrong Monitoring Corp.) (www.armstrongmonitoring.com) Ametek/Thermox (www.thermox.com) Bran & Luebbe (www.branluebbe.com) CEA Instruments (www.ceainstr.com) Dasibi Environmental Corp. (www.dasibi.com) Delphian Corp. (www.delphian.com) Drager Safety Inc. (www.draeger-usa.com) Ecotech (www.ecotech.com.au) E&E Process (www.process-controls.com)


1246


EMS (Environmental Monitoring Systems) (www.emssales.com) Enviro Technology (www.et.co.uk) Foxboro-Invensys (www.foxboro.com) Horiba Instrument Inc. (www.nettune.net) Innova Air Tech Instruments (www.inniva.dk) International Sensor Technology (www.intlsensor.com) IT Group (www.theitgroup.com) MSA Instrument Div. (www.msanet.com) Purafil Inc. (www.purafilonguard.com) Sensidyne Inc. (www.sensidyne.com) Servomex Co. (www.servomex.com) Sieger Gasalarm; Siemens Energy & Automation (www.sea.siemens.com) Sierra Monitor Corp. (www.sierramonitor.com) Sigrist-Photometer Ltd. (www.photometer.com) Teledyne Analytical Instruments (www.teledyne-ai.com) Thermo Environmental Instruments (www.thermoei.com) Thermo Gas Tech (www.thermo.com) Topac (www.topac.com) Yokogawa Corp. of America (www.yca.com)

INTRODUCTION Carbon monoxide (CO) is a toxic gas and, as such, it is monitored in the ambient air for personal safety reasons. The threshold limit value (TLV) adopted by the American Conference of Governmental Industrial Hygienists for CO is 50 ppm. In industrial applications, lower limits are often used for alarm set points. Electrochemical and catalytic sensors are used in personal toxic gas monitors. Carbon monoxide is also an indicator of incomplete combustion and, therefore, it is measured to optimize boilers and other combustion processes (see Figure 8.3k). For these applications, the most frequently used analyzer is the nondispersive infrared (NDIR) sensor (Section 8.27). In the chemical processing industries, when higher sensitivity (0.1 ppm) measurements of carbon monoxide are required, mercury vapor and gas chromatographic analyzers (Section 8.12) are also used. Each of these techniques will be briefly described in the discussion that follows.

CALIBRATION TECHNIQUES Ambient carbon monoxide analyzers (Table 8.10a) are calibrated with gas mixtures of known concentrations. Such

mixtures may be prepared by volumetric dilution of pure carbon monoxide with nitrogen or helium, which are free of carbon monoxide. If the volumes used in dilution are accurately known, the concentration may be calculated. A known small volume of pure carbon monoxide is placed in an evacuated tank of known volume, and the tank is then filled with the dilutor gas. The pressures of the carbon monoxide volume and the final mixture must be known (or else known to be equal). Smaller samples may be prepared in plastic bags by injecting pure carbon monoxide from a gas syringe into a stream of dilutor gas metered accurately into the bag with a flow device such as a wet test meter. Because cylinder nitrogen commonly has small amounts of carbon monoxide present in it, it must not be assumed to be pure without verification. Pure helium is reliably free of carbon monoxide. A useful procedure is to zero the detector using helium. After that, one can measure the CO content of the available cylinder nitrogen and can make the required correction. This permits using the less expensive nitrogen for most calibrations. A reference gas mixture whose carbon monoxide content is not accurately known can also be analyzed gravimetrically or volumetrically by various methods, but these techniques are usually less convenient and often require large amounts of gas.

TABLE 8.10a Atmospheric Carbon Monoxide Analyzers Detection Method NDIR

Range, ppm

Sensitivity, ppm

Advantages

Disadvantages

0–25, 50, 100

0.5–1

U.S. reference method, accurate, stable, dry gases

Sensitive, water, and CO2 interferences (correctable), zero gas problems

Mercury vapor (hot HgO + CO releasing Hg vapor)

0–50

0.05

Sturdy, accurate, dry gases, high sensitivity

Interferences by water and other gases

Gas chromatography (reduction of CO to CH4, flame ionization detection)

0–200

0.1

Accurate, high sensitivity, also read CH4, dry gases

Complex and expensive


8.10 Carbon Monoxide

NONDISPERSIVE INFRARED ANALYZERS

Amplifier Meter

1

NDIR analysis is the reference method for the U.S. National Air Quality Standard for carbon monoxide and is discussed in detail in Section 8.27. It allows continuous analysis, because carbon monoxide absorbs the infrared radiation at a wavelength of 4.6 microns. Because infrared absorption is a nonlinear measurement, it is necessary for the analyzer to accurately linearize its output signal. A schematic diagram of a typical NDIR analyzer is shown in Figure 8.10b. Infrared radiation from a hot filament is chopped to pass alternately through sample and reference cells, to be absorbed in the detector cell divided by a pressuresensitive diaphragm. If the sample contains carbon monoxide, it will absorb part of the radiation, causing that half of the detector to exert less pressure on the diaphragm, whose distortion is converted to an electrical signal for rectification and amplification. Sample airflow is continuous at or sometimes above atmospheric pressure. The cell is commonly 0.5 m long. The measuring range usually extends from a minimum of 0.5 to 1 ppm up to a full-scale range of 25 to 100 ppm. Response times are in the range of less than 1 to 5 min. Although the NDIR response is nonlinear, it is assumed to be linear over the limited calibration range in use. Some instruments correct for nonlinearity in the output amplifier. These analyzers can be operated by nontechnical personnel. Interferences Carbon dioxide and water vapor in the sample interfere with the measurement. Filter cells filled with these gases, or optical filters when placed in front of the cells, can minimize effects of normal atmospheric levels of these interfering gases. The control of water vapor interference by its removal with desiccants (e.g., silica gel) or by a refrigerator condenser

R Condenser Microphone

Radiation Source Filter Cell M

Sample Cell

Chopper Filter Cell

Reference Cell

Radiation Source

Receiver

FIG. 8.10b NDIR carbon monoxide analyzer.

is preferable in many cases, even at the cost of some increase in response time. Gas Filter Correlation When very accurate low-level measurements are needed, or when background gases that have the potential to interfere with the measurement are present, gas filter correlation (GFC) is used (Figure 8.10c). In these designs the measuring and reference filters are replaced by gas-filled cuvettes. The reference cuvette is filled with CO and the measuring cuvette usually with nitrogen. In addition to being unaffected by the presence of background gases, both the accuracy and the response time of these instruments are better than those using filters. If DFC is used in combination with single-beam, dual-wavelength technology, it is virtually immune to obstruction of the optics. This in turn prevents drift and thereby reduces the frequency at which recalibration is needed.

FIG. 8.10c GFC-type carbon monoxide analyzer. (Courtesy of Teledyne Technologies Co.)


1247

1248


Sample In Micron Diaphragm Pump Filter Sampling System Sample Out

are normally at much lower concentrations in air than carbon monoxide. Water also interferes and should be removed by a dryer. This instrument has found particular use in nonurban measurements where carbon monoxide levels are low.

Charcoal Filter Dryer Reacter (Red Hgo, 200°c)

Sample Optical Path

Iodine Charcoal Filter Analyzer

Flow Regulator

GAS CHROMATOGRAPH Gas chromatographs are discussed in detail in Section 8.12. 3 Figure 8.10e illustrates an automated gas chromatograph that is the heart of a high-precision and specificity system measuring methane and carbon monoxide (Figure 8.10e). A precolumn prevents carbon dioxide, water, and hydrocarbons other than methane from reaching the molecular sieve separation column. After separation, a catalytic nickel reactor converts carbon monoxide to methane, which is detected by flame ionization. The system permits determination of both methane and carbon monoxide about once every 5 min. The output is linear for both components and can be read from 0.1 to 200 ppm. The instrument is relatively complex and expensive, however, and requires technically trained operators.

Preheater Mercury Lamp 2537

Reference Optical Path

FIG. 8.10d Mercury vapor carbon monoxide analyzer.

MERCURY VAPOR ANALYZER 2

Carbon monoxide is oxidized by hot mercuric oxide as follows: ( 210° )

CO + HgO(s) → CO2 + Hg(g)

8.10(1) ELECTROCHEMICAL ANALYZER

The mercury vapor released may be measured photometrically. An analyzer based on this principle (Figure 8.10d) can be operated continuously. It has higher sensitivity than does the NDIR type, but it also suffers from some interference. Detection levels go down to 0.025 ppm, and changes of a tenth of that can be observed. Oxygenated hydrocarbons, olefins, and hydrogen interfere with the measurement, but all

5-Micron Filter

Solenoid Valve

CH4 - CO Sample Loop

1

A galvanic cell for continuous carbon monoxide analysis is based on the reaction of carbon monoxide with iodine pentoxide: (150° C )

5 CO + I 2 O 5 → 5 CO 2 + I 2 Total HydroCarbon Sample Loop

Exhaust Sample Air In

Air Pump Solenoid Valve

Calibration Gas

Exhaust Separation Column

Catalytic Reactor

Inject Valves Backflush Valve

Holding Amplifiers

Recorders CH4

CO

PreColumn Flame Ionization Detector

Total Hydro-Carbon 0-10MV Output

Electrometer

FIG. 8.10e Methane and carbon monoxide analyzer.


8.10(2)

8.10 Carbon Monoxide

1249

The iodine liberated is absorbed by an electrolyte and reaches the cathode of a galvanic cell where it is reduced. The resulting current is measured by a galvanometer. Interference by mercaptan, hydrogen sulfide, hydrogen, olefins, and acetylene may be minimized by sampling through an absorption tube of mercuric sulfate on silica gel. Water vapor interference can be eliminated by the use of a drying column. The same reaction is used in a coulometric method with a modified Hersch-type cell. The iodine is passed into the cell, and the current flow is measured by an electrometer. The interference possibilities are the same as those for the galvanic analyzer. The minimum detectable concentration is 1 ppm with good precision if flow rates and temperature are controlled. Careful column preparation is required, and the response time is relatively slow.

Portable Monitors For purposes of personal protection, battery-operated portable units are available. These units are usually provided with one or two alarm set points and with memory for some thousands of data points, together with their times and dates. Table 8.10f lists the ranges, resolutions, and alarm set points for a number of toxic gases, including carbon monoxide. These pocket-size, battery-operated, portable electrochemical detectors are usually provided with digital displays and audible alarms. They can be configured for one or more monitoring channels. Figure 8.10g illustrates a unit with four individual channels of detection.

TABLE 8.10f Typical Range, Sensitivity, and Alarm Set Points of Portable Personal Protection Monitors Gas

Range

Resolution

Alarm Set Points (low/high)

O2

0–30%

0.1%

19.5/23.5%

CO

0–500 ppm

1 ppm

35/200 ppm

H2S

0–100 ppm

1 ppm

10/20 ppm

SO2

0–20 ppm

1 ppm

2/10 ppm

NO

0–250 ppm

1 ppm

25/50 ppm

NO2

0–20 ppm

0.1 ppm

1/10 ppm

NH3

0–50 ppm

1 ppm

25/50 ppm

PH3

0–5 ppm

0.1 ppm

1/2 ppm

Cl2

0–10 ppm

0.1 ppm

0.5/5 ppm

HCN

0–100 ppm

1 ppm

4.7/50 ppm

Source: Abstracted from Cole-Parmer Catalog 2001/2002, Vernon Hills, IL, 2001.


FIG. 8.10g Typical pocket-size portable electrochemical detector configured for four individual channels of detection. (Courtesy of Sensidyne Inc.)

CATALYTIC ANALYSIS The catalyst Hopcalite will oxidize carbon monoxide to car1 bon dioxide. The resultant temperature rise may be recorded continuously as a measure of carbon monoxide concentration. The catalyst temperature and residence time must be controlled to avoid interference by hydrocarbons. The method is not suitable for most air monitoring applications because of low sensitivity. SPOT SAMPLING OF AMBIENT AIR When only intermittent analyses are required, it is convenient to collect samples in the field for later analysis in the laboratory. Rigid glass bulbs or stainless steel tanks may be evacuated and then simply opened briefly to collect the air sample. Plastic bags may be filled by means of a small air pump. The samples may be analyzed later by various means, including use of a continuous analyzer at some other location. The samples may be analyzed in a central laboratory by an infrared spectrophotometer with a long-path gas cell or by suitable gas chromatographic apparatus.

1250


Some colorimetric methods are also available for carbon monoxide analysis, although in general, their sensitivity and precision are low for atmospheric work. An NBS colorimetric indicating gel, if freshly prepared, will limit errors to 5 to 10%, with detectability down to 0.1 ppm. The technique is simple but time-consuming and tedious, with interference by oxidizing and reducing gases.

CONCLUSIONS As discussed in more detail in Section 8.59, for simple and less accurate measurements of short-term carbon monoxide levels, gel tubes can be used. For continuous and higher precision measurements, nondispersive infrared analysis is the most common. If it is necessary to also measure methane, the combination gas chromatograph with carbon monoxide analyzer is worthy of consideration, but this is an expensive choice. The mercury vapor analyzer is suitable where the carbon monoxide levels are low.

References 1.

2.

“Air Quality Criteria for Carbon Monoxide,” U.S. Department of Health, Education, and Welfare, National Air Pollution Control Administration Publication AP-62, chap. 5. Robbins, R. C., Borg, K. M., and Robinson, E., Journal of the Air Pollution Control Association, Vol. 18, pp. 106–110.


3.

Stevens, R. K., O’ Keefe, A. E., and Ortman, G. C., “A Gas Chromatographic Approach to the Semicontinuous Monitoring of Carbon Monoxide and Methane,” 156th National Meeting of the American Chemical Society, Atlantic City, NJ.

Bibliography ACGIH, Air Sampling Instruments for Evaluation of Atmospheric Contaminants, 9th ed., Cincinnati, OH: American Conference of Governmental Industrial Hygienists, 2001. AIHA, Workplace Environmental Exposure Level Guide Series, Akron, OH: American Industrial Hygiene Association, 1979 to 2002 (full set of 109 individual guides on toxic chemicals). Annual Book of ASTM Standards, West Conshohocken, PA: American Society for Testing and Materials, 2002. ASHRAE Standard 110–1995, “Method of Testing Performance of Fume Hoods.” Bretherick, L., Bretherick’s Handbook of Reactive Chemical Hazards, Stoneham, ME: Butterworth-Heinemann, 1999. Bay, H. W., “Electrochemical Technique for the Measurement of Carbon Monoxide,” Analytical Chemistry, October 1974. Dailey, W. V., “A Novel NDIR Analyzer of NO, SO2 and CO Analysis,” in Analysis Instrumentation, Vol. 15, ISA, 1977. Draeger Gmbh, Draeger Tube and CMS Handbook, Leubeck, Germany: Draeger Gmbh, 2002. NIOSH, NIOSH Manual of Analytical Methods: Supplement, 4th ed., 2 vols., Bpi Information Services, 1996. Ewing, G., Analytical Instrumentation Handbook , New York: Marcel Dekker, 1990. “Gas Detectors and Analyzers,” Measurements and Control, October 1991. Lodge, J. P., Methods of Air Sampling and Analysis, Chelsea, MI: Lewis Publishers, 1988. Proctor, N. H. et al., Chemical Hazards of the Workplace, New York: John Wiley & Sons, 1996. Swartz, N., “Carbon Monoxide Monitoring for Prevention of Fires,” ISA/93 Technical Conference, Chicago, September 19–24, 1993.

8.11

Chlorine G. P. WHITTLE

To Receiver

(1974, 1982)

B. G. LIPTÁK

(1995, 2003)

AT Cl Flow Sheet Symbol

Methods of Detection:

A. Colorimetric: visual (A1), spectrophotometric (A2) B. Amperometric (B1), polarographic (B2) C. Iodometric (For a detailed discussion of wet chemistry analyzers and autotitrators, refer to Section 8.66.)

Sampling:

All three methods can be used both on grab samples and as automatic continuous analyzers.

Sample Pressure:

Generally atmospheric or near atmospheric. For continuous analyzer, water pressure is reduced to atmospheric.

Sample Temperature:

All methods are generally limited to the range of 32 to 120°F (0 to 49°C), with method B employing automatic temperature compensation within this range. Method A may or may not require precise temperature control, depending on the reagents employed.

Sample Size of Flow Rate:

In method A1, grab samples as small as 5 ml are sufficient. For method A2, flow rates of 10 to 75 ml/min are generally specified. In method B1, flow rates of 100 to 750 ml/min are required. B2 design probes are available for in situ installations without sampling.


Enclosures for units are available in fiberglass, styrene, urethane-painted steel, vinylcovered aluminum, and other corrosion-resistant construction, suitable for modular or control panel installation. Wetted parts are constructed of polyvinyl chloride (PVC), Teflon, Lucite, polyethylene, or glass. In method B, gold or platinum measuring and copper reference electrodes generally are employed.

Readout:

All designs have indicating meters or transmitted output signals for remote display, recording, and control. High–low alarm actuation or chlorinator controls are also available.

Specificity:

Methods A and B determine either free or total residual chlorine; method C measures total residual chlorine.

Interferences:

For method A, interfering substances may include other oxidants, e.g., manganese, nitrite, and chlorine dioxide, as well as turbidity and color. In method B, nitrogen trichloride and chlorine dioxide may interfere with free chlorine determinations.

Inaccuracy:

Generally ±2 to 5% of full scale for ranges up to 20 ppm

Ranges:

For method A, available ranges include 0 to 1 ppm free chlorine, and 0 to 3 ppm, 0 to 5 ppm, and 0 to 10 ppm total chlorine. For method B, 0 to 0.1, 0 to 1, 0 to 2, 0 to 5, 0 to 10, and 0 to 20 ppm or higher are available. The three-electrode units can measure chlorine residuals from the parts per billion (ppb) range to as high as 60 mg/l. For method C, the measurement range is from 0.001 to 10 mg/l.

Response to Chlorine Concentration Change:

Three minutes or more for method A2, generally less than 10 sec for method B (continuous analyzer)


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Cost:

Visual test kits are available from $25 to $100. A portable amperometric titrator costs about $1800. A free chlorine electrode in acrylic flow cell and with residual chlorine controller is about $2000. A corrosion-resistant amperometric transmitter system with probe cleaner costs about $8000, with the yearly cost of chemicals and maintenance amounting to $3000 or more. The purchase cost of polarographic transmitters is similar, but the cost of chemicals is much less. Chlorine in gas samples can be detected by UV analysis or by gas chromatography; both are discussed in later sections in this chapter.


Applied Analytics Inc. (www.a-a-inc.com) Capital Controls Co. (www.capitalcontrols.com) Emerson Process Management, Rosemount Analytical (www.raihome.com) Foxboro-Invensys (www.foxboro.com) G.C. Industries Inc.; Hack (www.erwater.com) Hanna Instruments Inc. (www.hannainst.com) Honeywell Industry Solutions (www.iac.honeywell.com) Ionics Inc. (www.ionics.com) Kaiser Optical Systems Inc. (www.kosi.com) K-Patents Inc. (www.Kpatents.com) Kuntze Instruments Inc. (www.kuntze-instr.com) Omega (www.omega.com) Prostar Technologies Inc. (www.prostar-tech.com) Rosemount Analytical Inc. (www.processanalytic.com) Teledyne Analytical Instruments (www.teledyne-ai.com) Thermo Orion/Orion Research Inc. (www.iscpubs.com) Tytronics & Nametre (www.tytronics.com) Uniloc/Delta; Sierra Monitor Corp. (www.sierramonitor.com) Wallace & Tiernan Stranco (www.wanat.com)

INTRODUCTION The concentration of chlorine is of interest in both liquid and gas samples. The discussion in this section concentrates on the measurement of total or free residual chlorine in water, which can be measured by colorimetric, amperometric, polarographic, or iodometric analyzers. The measurement of chlorine concentration in gas samples is not discussed here, except to note that ultraviolet (UV) analyzers (Section 8.61) and chromatographs (Section 8.12) with thermal conductivity or flame ionization detectors are most often used to make this measurement. Chlorine gas-leak detector alarms are also available. These units pull a sample of an airstream through an electrochemical cell, which produces a current. This current is proportional to the chlorine concentration in the air sample. These units are capable of detecting concentrations as low as 0.5 ppm by volume.

RESIDUAL CHLORINE ANALYZERS Chlorine does not exist as Cl2 in water. Therefore, the amount of free available chlorine is defined as the amount of chlorine that exists in the form of HOCl. Total available chlorine, on the other hand, is defined as the chlorine that exists in any of the following forms: HOCl, NH2Cl, NCl3, or NHCl3. The


combined available (residual) chlorine is obtained as the difference between total and free chlorine. Standard laboratory methods for determining aqueous chlorine concentrations involve iodometry, in which free iodine liberated from potassium iodide is titrated with sodium thiosulfate, using starch as an indicator. All active forms of chlorine will liberate free iodine, and therefore, the iodometric methods of analysis are limited to the determination of total active chlorine. For an automatic total residual chlorine analyzer with a 4-20 mADC logarithmic output, refer to Figure 8.11a. Other titrimetric procedures employing indicators other than starch have also been developed for the determination and differentiation of the active chlorine forms. The amperometric titration method is based on polarographic principles and allows determination of the various forms of active chlorine. Various colorimetric procedures are available in which a colorless indicator is oxidized to a colored product, the color intensity of which is proportional to the chlorine concentration. Many of the colorimetric procedures allow the determination of the various forms of chlorine. Colorimetric procedures are adaptable to field determination by the use of kits containing the necessary reagents, color standards for visual estimation, and portable, battery-operated colorimeters. The amperometric titration and colorimetric methods have also been adapted to automated continuous analyzers.

8.11 Chlorine

1253

Reagent Container

Head Regulator Sample Metering Capillary

Overflow

Reagent Metering Capillary Mixing Channel Sample Heater Sample Cell Lamp

Overflow To Indicator or Other Displays Photocell Light Duct

Water Sample In Drain

FIG. 8.11b Chlorine colorimetric analyzer.

FIG. 8.11a Iodometric total residual chlorine analyzer with a range of 0.001 to 10 mg/l and a 4- to 20-mADC logarithmic output. (Courtesy of Thermo Orion/Orion Research Inc.)

COLORIMETRIC ANALYZERS In the Environmental Protection Agency (EPA) recommended DPD 330.5 method of chlorine analysis, a chemical reaction causes the intensity of the color of magenta to change in proportion to the concentration of chlorine. In the automatic analyzer, the color intensity of the sample is measured twice: once before reagents are added, for automatic zeroing and to compensate for any turbidity or natural color in the sample; and again after addition of the reagents and stirring of the sample. The second measured wavelength, around 510 nm, is compared with the reference. The chlorine concentration is calculated on the basis of the difference between the two readings. Figure 8.11b illustrates the basic components of a chlorine colorimetric analyzer. The water sample is introduced into a head regulator where the sample flow rate is regulated by an overflow arrangement and a capillary delivery tube. The sample is passed into a channel and is mixed with the colorimetric reagent metered through another capillary tube


from a storage container. The treated sample flows over a sample heater, if required, and into the sample cell, which fills and overflows at a predetermined frequency. If chlorine is present in the water, a characteristic color develops with an intensity in proportion to the amount of chlorine present. A filtered photocell develops an output signal, which is in proportion to the reduction in intensity of the transmitted light through the sample. Periodic standardization of the analyzer is required and is done by adjustment of the indicator to read 0 ppm chlorine on an untreated water sample and to read on the extreme upper end of the scale when the photocell is completely shielded. Automatic Colorimetric Analyzer Features Automatic free or total chlorine analyzers can control in the on–off mode by stopping and starting a feed pump and sounding alarms, or can provide throttling control by modulating the pumping rate of the chlorine feed pump. The output signals can be to computers or printers through an RS232 interface, or can be analog selected from among the choices of 0 to 10 mV, 0 to 100 mV, 0 to 1 V, and 4 to 20 mA. The measurement range is usually 0 to 5 mg/l with a 0.01 resolution, but other ranges from 0–1 to 0–10 mg/l are also available. These analyzers can perform an analysis every 2.5 min and use about 500 ml of reagents and color indicators per month. Therefore, monthly refilling is necessary. The flow cell should be cleaned at the same time the reagents are refilled. Although requiring about a monthly 1 h of maintenance, these

1254


are relatively inexpensive and stable analyzers that are insensitive to sample pH variations or other interferences.

AMPEROMETRIC ANALYZERS In amperometric analyzers, the composition of the electrodes is so selective that the polarization of the measuring electrode prevents a current flow when no strong oxidizing agent is present in the sample. On the other hand, when a strong oxidizing agent such as chlorine is present, the electrodes are depolarized and a current flow is generated. This current is proportional to the amount of chlorine residual in the sample. Because pH affects the dissociation constants, the water samples are usually conditioned with a buffer solution. Similarly, because of sensitivity to temperature variations, temperature compensation is provided. Types of Amperometric Analyzers Amperometric analyzers are used in three basic forms: as titrators, as free residual chlorine analyzers, and as total residual chlorine analyzers. The amperometric titrator determines the end point of a 200-ml sample in a manually filled sample jar by using a portable, battery-operated instrument.

During titration for free residual chlorine, a buffer solution maintains the sample pH at neutrality and a titrant (phenylarsene oxide) is added until no more current is produced by the electrode cell. This is considered the end point, and the amount of titrant used is an indication of free residual clorine in the sample. When total residual chlorine is measured, the titration process is similar, but the buffer is at a pH of 4 and potassium iodide is also added to the sample. Free Residual Chlorine Analysis For free residual chlorine analysis, the sample pH is held between 5 and 9. Simple flow-through units are available that do not require the use of buffers or reagents (Figure 8.11c). These analyzers are temperature compensated and operate in a range of 0 to 2 ppm of free chlorine, with an inaccuracy of about 0.1 ppm. Figure 8.11d shows the components of a typical amperometric analyzer, which does require buffers or reagents. The water sample is delivered to a diaphragm-type regulator for flow rate control. If required for pH control, a metered amount of buffer solution is also pumped to the regulator and is mixed with the sample prior to its reaching the cell block. In the cell block, contact is made between the electrodes and the sample, and a direct current is generated in proportion to the chlorine in the sample. The cell block usually contains

Flowrate Adjustment Knobs

Outlet Adaptor

Buffer Reagent Container

Indicator or Recorder

High Flow Limit

Ball Float

Flow Regulator

Low Flow Limit Rotameter

Cell Block

Buffer Pump Reference Electrode

Outer Electrode

Flowmeter

Inner Electrode

Measuring Electrode

Receptacle Housing

Tee and Adaptor Inlet Assembly

Thermistor Assembly

Water Sample In

Filter

Cable Assembly Sample Drain

FIG. 8.11c Measurement cell of a free residual chlorine analyzer.


FIG. 8.11d Chlorine amperometric analyzer.

8.11 Chlorine

1255

To Insure Proper Performance this End of Sensor Must Face Up " (1.90 cm)

" NPT (1.00 cm)

1 " (NPT) (3.81 cm) 3 8" (9.84 cm) 7

6 " (16.51 cm)

1 "(NOM) (3.81 cm)

Outlet

2 " (7 cm)

5 " (14.60 cm) Pressure Compensation Gland Protective Cap

" Tubing Connections Inlet

FIG. 8.11e Amperometric chlorine probe for in-line or sample bypass-type installation. (Courtesy of Rosemount Analytical Inc.)

grit, which cleans the electrodes by means of sample velocity agitation. Periodic calibration is required through a separate determination of chlorine and is usually performed on a laboratory amperometric titrator using a standardized phenylarsine oxide solution. Membrane Probes Microprocessor-based chlorine analyzers are available with built-in diagnostics and simulator features. They can automatically detect chlorine and temperature and can also be equipped with a pH detector probe. The amperometric probe consists of a gas-permeable membrane, a gold cathode, and a silver anode (Figure 8.11e). The probe is filled with a salt electrolyte. As chlorine penetrates the membrane, it is reduced at the gold cathode, which causes a current to flow that is proportional to the chlorine concentration in the water. The main advantages of the immersion probe designs include the elimination of the sampling system and the elimination of the continuous need for chemical reagents and buffers. The elimination of sample pumps and other sampling system components not only lowers costs and maintenance, but also reduces transportation lag, which makes the installation better suited for closed-loop operation. The elimination of the need for the continuous feed of chemicals reduces the yearly operating cost by thousands of dollars. The maintenance that is still needed is usually limited to replacing the membrane and refilling the probe with fresh electrolyte once every 2 months. The membrane probe is shown in Figures 8.11e and 8.11f. It can be provided with an in situ nutating sensor or with paddle-oscillation-type cleaner agitators to provide the required apparent sample velocity at the membrane. The probe


Sensor

1" NPT (25 mm)

FIG. 8.11f Immersion probe-type polarographic chlorine detector. (Courtesy of Rosemount Analytical Inc.)

can be inserted in pipes at pressures of up to 150 PSIG (10.6 bars) or can be handrail mounted (Figure 8.11f) in open tanks, with the local chlorine transmitter mounted on or above the handrail.

1256


Sample Inlet

Motor

Screen

Rotating Scrubber Cylindrical Copper Electrode

chemicals that are sometimes used to adjust the pH include acetic acid and sodium hydroxide. The cost of these chemicals can reach several thousand dollars per year, and the scheduled replenishment of these materials also adds to the maintenance cost of operating the plant. Electrode Cleaners

Drain Rotary Valve Buffer Inlet

Overflow to Drain Cleaning Balls

Electric Signal to Monitor

Gold Electrode

Keeping the chlorine electrodes clean also requires attention. In some designs, the measuring electrode rotates at 1550 rpm to maintain ideal electrolysis conditions (relative velocity) between the sample and the electrode surface. In other designs, the space between electrodes contains plastic pellets, which are continuously agitated by the swirling water or by a rotating scrubber (Figure 8.11g).

FIG. 8.11g

Electrode cleaner used in chlorine or ozone analyzers. Buffers and Reagents The main disadvantage of some amperometric chlorine analyzers is the need for the periodic replacement of substantial quantities of buffering and reagent chemicals. When measuring free residual chlorine, the buffer maintains a pH of 7 and the reagent that is added to the buffer is usually potassium bromide. When measuring total residual chlorine, the butter maintains the pH at around 4 and the reagent added to the buffer is usually potassium iodide. Other

CONCLUSIONS Colorimetric and amperometric analyzers may be obtained with diverse accessories serving various functions, including alarm actuation and external control. Automatic temperature compensation is also available for the amperometric analyzer. The three-electrode amperometric designs do not require zero calibration and are capable of detecting chlorine residuals as low as 1 µg/l (1 ppb). The relative merits of the continuous colorimetric and amperometric analyzers are listed in Table 8.11h.

TABLE 8.11h Relative Merits of Colorimetric and Amperometric Analyzers


Consideration

Colorimetric

Amperometric

Type of sample

Better suited for clarified natural or treated waters than for highly turbid or colored waters and wastewaters

Turbidity and color generally not a problem; applicable to both treated water and wastewater

Interference

Intefering ions should be absent; oxidized manganese compounds produce serious interference

Copper and silver ions may interfere by plating out on electrodes

Sample temperature

Temperature control may or may not be required, depending on reagent employed

Manual or automatic temperature compensation required

Speed of response

Generally 3 min or more required to detect a change in chlorine concentration

Chlorine concentration change detected in 10 sec or less

Calibration

Analyzer precalibrated; periodic standardization requires only simple manipulations

Periodic calibration required by separate analytical technique

Reagents required

External reagent solution required

External buffer may be required for varying sample pH

Maintenance stability

Cell staining may require periodic cleaning

Electrodes may require periodic cleaning

Stability

Drift compensated for by relatively simple standardization step

Drift not a problem when electrodes are kept clean

Initial cost

Generally less expensive

Generally more expensive

8.11 Chlorine

Bibliography Adams, V., Water and Wastewater Examination Manual, Boca Raton, FL: Lewis Publishers, 1990. AIHA, Workplace Environmental Exposure Level Guide Series, full set of 109 individual guides on toxic chemicals, Akron OH: American Industrial Hygiene Association, 1979 to 2002. Annual Book of ASTM Standards, Sections 11.01 and 11.02, West Conshohocken, PA: ASTM, published yearly. Buffle, J., Ed., In Situ Monitoring of Aquatic Systems, New York: John Wiley & Sons, 2002. Chlorine Residual Analyzers, Instrumentation Testing Association (http:// www.instrument.org/res.htm), 1990. Clark, K., “Chlorine Analyzers Cut Costs, Improve Performance,” InTech, May 1998. Dubois, R., van Vuuren, P., and Tatera, J., “New Sampling Sensor Initiative:” An Enabling Technology, 47th Annual ISA Analysis Division Symposium, Denver, CO, April 14–18, 2002. Ewing, G., Analytical Instrumentation Handbook, New York: Marcel Dekker, 1990. Grayson, M., Ed., Kirk-Othmer Concise Encyclopedia of Chemical Technology, 4th ed., New York: John Wiley & Sons, 1999.


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Meagher, R. F. and Grinker, J. R., “Sensors for Wastewater Plant Control,” Instruments Technology, Vol. 28, No. 5, May 1981, p. 51. Meyers, R. A., Ed., Encyclopedia of Analytical Chemistry: Instrumentation and Applications, New York: John Wiley & Sons, 2000. Mitchell, M. K. and Stapp, W. B., Field Manual for Water Quality Monitoring, 12th ed., LaMotte Company, Chestertown, MD, 2000. Nassau, K., The Physics and Chemistry of Color, New York: John Wiley & Sons, 1983. Riley, T. et al., Principles of Electroanalytical Methods, New York: John Wiley & Sons, 1987. Shaw, A. et al., The Use of Online Respirometric Monitoring, WEFTEC2001 Conference, Water Environment Federation, 2001. Sherman, R. E., Process Analyzer Sample-Conditioning System Technology, New York: John Wiley & Sons, 2002. Skoog, D. A., Principles of Instrumental Analysis, 5th ed., Florence, KY: Brooks/Cole, 1997. Standard Methods for the Examination of Water and Wastewater, New York: APHA, AWWA, and WPCF, latest edition. Van den Berg, F. W. J., Hoefsloot, H. C. J., and Smilde, A. K., “Selection of Optimal Process Analyzers for Plant-Wide Monitoring,” Anal. Chem., 74(13), 3105–3111, 2002. Water Quality and Treatment, The American Water Works Association, Inc., New York: McGraw-Hill, latest edition.

8.12

Chromatographs: Gas RAYMOND ANNINO

Sampling System

(1995, 2003)

AT

Chromatograph To Receiver

Flow Sheet Symbol


Type of Sample:

Vapor or vaporizable liquid

Sample Pressure:

0 to 100 PSIG (1 to 7 bar)

Ambient Temperature:

–4 to 122°F (–20 to 50°C); however, a sheltered environment is recommended.

Analysis Zone:

140 to 356°F (60 to 180°C) provided that the desired oven temperature is at least 18°F (10°C) above ambient (stable oven temperature ±0.1°F (± 0.05°C) at steady ambient ±1°F (± 0.5°C) for –4 to 122°F (–20 to 50°C) ambient change)

Contacting Material:

Stainless steel or surface-deactivated steel for trace analysis of reactive compounds, Teflon

Auxiliary Utilities:

Instrument air-dry, oil-free, available at 50 PSIG min (3.5 bar) and 2 to 3 scfm (1 to 1.5 sl/sec); carrier gas, zero air, and hydrogen for FID; possible steam tracing or electrical heating for sample lines

Cycle Time:

2 to 20 min, depending on application and packed or capillary column operation

Special Features:

Accepts other inputs (e.g., flow rate and density in the calculation of output, e.g., BTU/h); built-in diagnostics with local and remote indication; multicomponent readout; stand-alone or networked; communication link (fieldbus or Ethernet directly to the plant or local LAN and local control system); local and remote operator interface; single- or multistream analysis

Location:

Class 1, Groups B, C, and D, Division 1 hazardous areas

Analyzer Cost:

Basic analyzer, $35,000 to $45,000 (depending on number purchased)

Installed Cost:

$45,000 to $125,000 (depending on type, number, and application)

Suppliers:

ABB Process Analytics (www.abb.com/analytical) Applied Instrument Technologies (www.orbital-ait.com) Daniel Industries (www.danielind.com) Fluid Data/Thermo Onix (www.fluid-data.com) HNU Process Analyzers (www.hnu.com) Questar Baseline Industries (www.baselineindustries.com) Rosemont/Emerson (www.processanalytic.com) Siemens Applied Automation (www.sea.siemens.com) SRA Instruments (www.sra-instruments.com) Wasson-ECE Instrumentation (www.wasson-ece.com) YEW Yokagawa (www.yokorawa.co.jp) Among these suppliers, about 65% of the market is shared by ABB and Siemens AA. About 30% of the market is shared by AIT (supplying the Foxboro 931D), Thermo Onix, Daniel Industries, Rosemont, and Yokagawa.

8.12 Chromatographs: Gas

INTRODUCTION Gas chromatography (GC) is a method for separating the components of a sample that contains a mixture of volatile compounds. The separations are made in order to determine the quantity of each of the sample components of interest. It has become one of the most often used procedures in analytical chemistry for separation and analysis. The reasons for its popularity can be traced to its ease of use for the separation of complex mixtures, its high sensitivity, and the small sample required for the analysis. In the elution form of GC, the sample mixture to be separated is vaporized and injected into a flowing stream of carrier gas (the mobile phase) that carries it into a column containing a so-called stationary phase. The stationary phase is a high-boiling nonvolatile liquid that is suspended on an inert solid with a large surface area or coated in a thin film on the walls of a small-bore tube (capillary, or wall-coated open tubular (WCOT) column). The support-coated solid can be packed into a fairly large-bore column (packed column) or attached to the wall of a small-bore capillary (support-coated open tubular (SCOT) column). Similarly, a solid stationary phase (such as silica gel, alumina, or charcoal) can be packed into a column or suspended on the walls of a small-bore tube. Separations occur because the sample components have different solubilities in the liquid stationary phase or different

1259

adsorbtivities on a solid stationary phase. Therefore, each sample component is retarded a different amount by the stationary phase and is carried down the column by the mobile phase at a different rate. Provided that a stationary phase has been selected that maximizes the solubility or absorbtivity differences, complete separation will occur with each sample component emerging from the column at a different time. A detector that responds to some property difference between the carrier gas and the sample components is placed at the end of the column. It yields a signal that, when recorded as a function of time, produces the familiar chromatogram such as that shown in Figure 8.12a. The observed peak separation (resolution, R) is a function of the above-mentioned solubility or adsorbtivity differences and the efficiency of the column, that is, its ability to produce narrow peaks. The efficiency (hetp) of a GC column is expressed as height equivalent to a theoretical plate (N). The smaller the hetp, the more efficient the column (that is, there are more theoretical plates per unit length of column). The resolution of peaks 5 and 6 in Figure 8.12a at retention times of tR5 and tR6 is calculated from the chromatogram as follows:

R=

2( t R 6 − t R 5 ) w5 + w

8.12 (1)

FIG. 8.12a A typical packed column chromatogram showing fully and partially resolved peaks. Retention time of an unretained sample component (such as air) measured from the time of injection = tM . Retention time of sample components 1 to 8 = tR1 to tR8. The resolution of peaks 5 and 6 = 1.1. The number of theoretical plates calculated using peak 8 = 4158.


1260


where w5 and w6 are the respective peak widths measured at the base of the peak. In this case, the resolution of the two peaks is 1.1. Two compounds of equal concentration are baseline resolved at a resolution of 1.5. The theoretical plate as calculated from a peak (number 8 in Figure 8.12a) using Equation 8.12(2) is found to be 4158 and was obtained using a packed column. N = 16

t 2R 8 w82

8.12(2)

• •

and the efficiency of a column of length, L, is hetp =

N L

retention times), a precisely regulated temperature environment must be provided for the column. In some cases, to minimize the analysis time for samples that contain a wide boiling range of compounds, the temperature of the oven is varied linearly with time. This procedure is called temperatureprogrammed chromatography. Although they employ the same principles, the process gas chromatograph (PGC) is quite a different instrument than its laboratory counterpart. These differences appear as the result of a number of factors, not the least of which include:

8.12(3)

The time required for each component to emerge is called the retention time, tR, and the magnitude of the detector signal (peak height or peak area) is related to the amount of the compound present in the injected sample. If these individual retention times and detector responses are made to remain constant (by adequate control of carrier gas flow rate, column temperature, amount of stationary phase, and detector variables), the process can be automated so that the instrument will provide this information quite precisely and cyclically on a repetitive basis.

• • •

The need for continuous, reliable operation of the unit The need for the cycle time of the analysis to be shorter than the time required for the process control system to achieve proper control action The nature of the environment in which the PGC is placed The diverse user attitudes regarding maintenance, and their capability regarding the operation of the unit The necessity to interface with modern-day computercontrolled systems

Thus, a premium is placed on simple, reliable design to ensure a low mean failure rate, but straightforward, rapid repair when required, and the strategic placement of appropriate sensors to aid in diagnosing problems and to provide alarms with regard to the operation of the unit. To summarize, the PGC has the following distinguishing attributes:

BASIC CHROMATOGRAPHIC INSTRUMENTATION • The basic instrumentation necessary to accomplish the above-described chromatography is extremely simple (see Figure 8.12b). It consists of a supply of carrier gas (commonly helium, hydrogen, or nitrogen) with appropriate pressure regulation, a sample introduction device (described later on) to inject a fixed volume of sample into the carrier gas stream, a column containing the stationary phase, and a detector. Since the degree of solubility of the sample components in the stationary phase is temperature dependent (and thus so will be the

FIG. 8.12b Basic elements of a chromatograph.


• • • • •

Located in the plant as close as possible to the sample point to minimize sample transport time Dedicated to monitor one or more components in one or more process streams Designed for continuous, unattended operation Designed for operation in hazardous environments Designed to withstand exposure to weather, humidity, dust, and corrosive atmospheres Contains integral hardware and software to allow communication with the process control system as well as remote maintenance stations


1261

FIG. 8.12c Basic elements of a multistream process gas chromatographic system.

•

Contains alarms and diagnostic aids to continuously monitor the health of the instrument and aid in diagnosing problems

A process gas chromatographic system (as shown in Figure 8.12c) consists of both the PGC and a sample handling system (SHS). They will be discussed separately in the following sections.

COMPONENTS OF A PROCESS GAS CHROMATOGRAPH The architecture of a PGC can be broken into two principal parts: the analyzer, containing the hardware that makes up the basic chromatograph; and the programmable controller, containing the electronics for the control and operation of the chromatograph plus a communication package. What follows is a detailed discussion of these various parts and specifications of the PGC. The traditionally designed unit, such as illustrated in Figure 8.12d, is fairly large (typically 40" × 26" × 18" in. HWD) and is mounted in a protected analyzer enclosure. An alternative so-called transmitter design, focused on a narrow market such as natural gas analysis in custody transfer appli1 cations, has been available for some time. However, the development of a full-featured transmitter to compete with the more traditional design in a variety of applications has a 2 checkered history. A totally pneumatic PGC transmitter was introduced in the mid-1970s and was on the market until 1989. More recently, a detailed transmitter design appeared 3 in the literature but was never commercialized, and in the


FIG. 8.12d Typical PGC for mounting in an analyzer shelter. This unit can be obtained as shown with a single large oven or, with this space divided to provide two separate ovens each with its own thermostat. (Photo courtesy of Siemens Applied Automation.)

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Analyzer

FIG. 8.12e A “transmitter” designed PGC; Dimensions 28" × 12" × 15" HWD. The analyzer and programmer/controller are mounted separately in their own explosion proof enclosure (similar to the old dP cell hats). The ubiquitous notebook PC is shown as the remote analyzer maintenance station. (Courtesy of Rosemont Analytical Inc.)

mid-1990s a unit was introduced to the U.S. market (desig4 nated as the SGC 3000 ) that has since disappeared but may be offered in Japan and Europe. Finally, in the late 1990s a transmitter design (illustrated in Figure 8.12e) was marketed that is still current and appears to be quite successful. All of the functions that will be discussed in the following text also pertain to this latter design. The advantages of the transmitter are that it can be mounted close to the sample point (thereby decreasing sample transport time and thus cycle time) and it does not require an analyzer shelter (thus decreasing installed cost). The overall size of the transmitter and its sample handling hardware is much smaller than the shelter-mounted units for ease of field mounting. In previous designs, the oven space necessary to mount the complex valve and column arrangements was sacrificed along with its ability to handle many applications. This severely limited the market for such a design and may account for its rather limited success over the years. However, the newer design appears to be much more flexible, offering parallel chromatography, complex column valving options, many detector options, and a full communication package. In addition, efforts have been made to design the units for easy maintenance. In the absence of a complex sample-handling package, however, the transmitter, with its simple modular sample handling unit close-coupled to it, is basically a single- or, at most, a dual-process stream analyzer. However, to accommodate users who wish to spread the cost of the analyzer by using it for the analysis of several streams, the transmitter equipped with an external samplehandling package can handle up to 16 streams. The only advantage left for this design then is the decreased cost due to the elimination of the analyzer shack required for the traditional PGC.


The analyzer section contains all of the basic elements of the chromatograph, namely, columns, sample and columnswitching valves, and detectors, all enclosed in a precisely thermostated oven. In some designs, the associated pneumatic components are also placed in the oven, and in others they are not. Which option is elected depends on the quality of the components and robustness of the software to tolerate retention time changes that may occur when pressure and flow controllers experience a varying temperature environment. The oven may be designed for isothermal and temperature programming operation. As mentioned previously, temperature programming is a procedure for decreasing the analysis time for a sample consisting of compounds of widely divergent boiling points and polarity. The temperature of the column is usually maintained at some initial value for a fixed period of time while the low boilers elute from the column, and is then raised in a linear fashion (at a suitable rate) during the remainder of the cycle so as to ensure the separation of the remaining sample components in a much shorter period of time than if the analysis had been run isothermally. The oven then must be cooled rapidly to exactly the same starting temperature and programmed at the same rate if one is to obtain reproducible retention times for the various sample constituents—clearly not a trivial design problem, especially if the unit is to be certified for use in Division 1 areas. Oven For economy and design simplicity, a single isothermal temperature zone is most frequently used. However, units with two separated temperature zones are now available. This provides the application engineer with tremendous flexibility, since parallel chromatography can be run on the same sample at the same or different temperatures with columns containing the same or different stationary phases. In this manner, the solution to difficult applications can be considerably simplified and the cycle time shortened. Although isothermal oven operation is usually the choice, temperature-programmed units are offered to extend GC applications to other volatility measurements such as simulated distillation, Reid vapor pressure, etc. The majority of PGCs utilize air-bath ovens for both isothermal and temperature-programmed applications. In this design, each of the analyzer elements, such as the oven heater and detectors, is built to be explosion-proof for mounting in a basically non-explosion-proof air-bath enclosure. A continuous flow of instrument air (at 3 to 4 scfm) is passed through a heater (see Figure 8.12f) and the oven, and then vented inside or outside the analyzer shelter. The temperature of the oven is usually in the range of 50 to 225°C (±0.1°C), but may be limited by the area safety classification regarding the permitted surface temperature (the T rating). The T rating may also limit heat-up rates.


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FIG. 8.12f Basic elements of an explosion-proof isothermal air bath oven heater assembly.

Also available for the analyzer shelter-mounted units is an airless heat-sink oven of about the same size as the air-bath units. The heating elements in this oven are not exposed, but cast into the oven walls, providing conductive and convective heat transfer with heat-up rates not much slower for the same oven volume than that of the air-bath variety. Also, higher temperatures can be achieved with this design within a given area T rating than with the air-bath oven. The advantages of such a design are that operational costs are decreased because: • •

It does not require instrument air for its operation. Power consumption is less than half that of the airbath unit since only conductive heat loss must be compensated, while with the air-bath design, new ambient air must be heated continuously.

In addition, with this design the explosion-proof detectors can be mounted outside the oven (similar to laboratory GC design), providing much more working space within the oven (see Figure 8.12g). The second and oldest design that has evolved through the years is achieved by placing all the elements of the analyzer in an explosion-proof enclosure. This design also does not require purge air for the oven. Historically, this has lead to a very heavy and unwieldy package, but now, using the smaller, modern engineered analyzer and electronic components, compact units such as the PGC transmitter are available. Field maintenance of the analyzer portion of these instruments consists of replacement of the total oven–GC assembly (see Figure 8.12h for details of the transmitter construction). Similarly, as with modern traditional design, the electronics package has been modularized to allow quick replacement of electronic components. Valves Valves are used for the injection of sample and as column switches in the various column configurations that may be required in the application. The most popular of these are the


FIG. 8.12g One half of a dual airless oven assembly showing the placement of the explosion-proof detector outside of the oven. Space for the explosion-proof heater assembly is not required in an airless oven and an application using “valveless” column switching has further reduced the oven space needed for the application. (Photo courtesy of Siemens Applied Automation.)

rotary, sliding plate, diaphragm mechanical, and no-moving5 part valves, and the so-called valveless or Deans switch. The sample valve is designed to deliver a fixed volume of sample to the head of the column. For very small samples (less than a microliter), the sample volume is defined internally by a slot machined into the movable member of the valve (sliding plate or rotor), and for larger volumes, by an external loop made from a length of deactivated tubing attached to the valve. In the past, the smaller volumes were used to inject a liquid volume of sample (which was rapidly vaporized in the carrier gas). However, with the increasing popularity of capillary columns (which require much smaller samples than their packed column analogs), these valves are being used for the injection of gaseous samples. In any case, because liquid samples pose particular problems with regard to sample valve reliability, sample handling, and safety, liquid sample injection is best avoided by vaporizing the sample before presenting it to the sample valve. Rotary Valve The most popular rotary valve (especially for capillary chromatography) contains a conical rotor with machined grooves interconnecting the outer ports that are drilled into the outer body. Two positions of the rotor define different flow paths, as shown in Figure 8.12i.

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FIG. 8.12h An exploded view of the “transmitter” illustrates the modular construction of the unit. Rapid field maintenance of the analyzer is met by replacement of the total oven assembly which includes columns, detectors, valves, actuators, heaters, temperature sensors, and electronic pressure/flow controller components. In the electronics section, the printed circuit board assemblies are “euro-card” slide rail mounted and may be removed/replaced without the use of tools. (Courtesy of Rosemont Analytical Inc.)

FIG. 8.12i Ratary valve.

to form a mating seal with the rotor. The rotor is held against the stator by a factory-calibrated threaded cap that applies the correct force. An exploded view of a manually operated version of such a valve is shown in Figure 8.12j. The PGC version of this valve is actuated either by a compressed air operator or by an electrically driven solenoid. Although these valves meet the stringent demands for capillary chromatography, there are disadvantages in that they require regular maintenance, since the rotors wear and must be replaced after about 200,000 cycles. In addition, they require a much more complicated driver since the simple linear motion commonly available must be converted to a rotary one.

The rotor faces are of filled Teflon or similar polymer that provides chemical resistance as well as sealing and lubricity. The valve body is made of stainless steel, or other corrosion-resistant alloy, with highly polished conical faces

Sliding Plate The sliding plate valve comes in both linear and rotary versions, the former being the most popular by virtue of the simple operator required for lateral rather than rotary movement of the slider.



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FIG. 8.12j Exploded view of a manually actuated rotary valve. (Courtesy of Valco Instrument Co. Inc.)

In a manner similar to that of the rotary valve, the switching is done by a plate with channels machined in its face that is switched over holes that are drilled into the valve body. Again, the slide is made of filled Teflon or similar polymer, for the reasons outlined above. The slider is moved between its two positions either by a diaphragm operator mechanically linked to it (with compressed air, released by an external solenoid, supplying the necessary force) or directly with a solenoid whose shaft is linked to it. Diaphragm The diaphragm valve comes in 10-, 6-, and 4port versions and operates on a different principle than the sliding plate or rotary valves. The flow of gases between adjacent ports arranged in a circular pattern in a machined and polished metal surface is controlled by a Teflon diaphragm. Two versions of the diaphragm valve are available. In one, the force is applied to the diaphragm by a series of plungers that are in turn moved by two pneumatically actuated pistons. This is illustrated in Figure 8.12k for a six-port sample valve. The other version uses pressure-on-diaphragm activation with no moving parts. Both versions are highly reliable (1-million-cycle rating for the piston-operated one, and 16 million for the no-moving-part variety). Although the sample valve example shown in Figure 8.12k utilizes an external sample loop, these valves are also available with small internal-volume loops. Columns Packed columns are still the most commonly used in most PGC applications. However, open tubular (OT) capillary column solutions are making inroads in many cases. WCOT columns are often preferred because they are more efficient than packed columns, and thus it is possible to simplify the column configuration necessary to achieve the desired separations. In addition, they often allow for faster analysis cycles. There is a downside, however, to using capillary columns in PGC applications in that many of the older designed PGCs will not adequately support them. Also, because of the ease of


FIG. 8.12k Schematic view of a six-port diaphragm sample inject valve. Carrier gas enters at port 3 and exits at port 4. Sample in/out ports are 1 and 6. In Position A, the sample loop is being filled with sample. In Position B, sample is swept from the loop by carrier gas. (Courtesy of Valco Instrument Co. Inc.)

plugging the small-diameter columns and connecting plumbing, sample handling to produce extremely clean samples is of paramount importance. Very small sample volumes must be injected to maintain the efficiency of the WCOT columns. Thus, special sample valves and additional hardware to split the sample delivered from the sample valve are necessary. Also, detectors designed with small internal volumes are necessary, as well as critical attention to plumbing details, to avoid unswept volumes that degrade chromatographic peaks by creating long tails on the trailing edges. In addition, in some cases, the small sample volumes may lead to inadequate detectability by some detectors. Packed Columns Packed columns in 1/8-in.-outer diameter (o.d.) (0.079-in., 2-mm-inner diameter (i.d.)) stainless steel tubing are in widest use, although the smaller-diameter 1/16-in.-o.d. (0.039-in., 1-mm-i.d.) micropacked (smallerdiameter particles) are increasingly being used. Occasionally, the larger 3/16-in.-o.d. columns are used in special application. The length of these columns will vary with the difficulty

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TABLE 8.12l Popular Stationary Phases Stationary Phase

Comments

OV-101™ (100% methyl silicone) useful from 0 to 330°C

Most frequently used GC phase. Low polarity, separates homologous series of compounds according to their boiling points.

OV-17™ (50% phenyl−50% methyl silicone) useful from 50–250°C

Moderately polar silicone phase.

OV-25™ (75% phenyl−25% methyl silicone)

Highly polar silicone phase. Selective for aromatics over aliphatic hydrocarbons.

OV-225™ (25% cyanopropyl−25% phenyl−50% methyl silicone useful from 0–250°C

Increased polarity over OV-25. Increased retention of aromatics over aliphatics, alcohols over ethers, ketones over primary alcohols.

Carbowax 20M (polyethylene glycol, mol. wt 15,000–20,000) useful from 60−225°C

Generally useful polar phase.

in making the separations. Longer columns produce more plates and thus have more separating power. However, the maximum column length is ultimately determined by the pneumatic equipment. Large pressure drops are avoided by keeping column lengths to less than 12 ft. The stationary phase is coated on support particles (sieved to 80 to 100 or 100 to 120 mesh) of Chromosorb™ (a diatomaceous earth product that has the large surface area needed to make efficient columns). Some popular stationary phases and the separations for which they are used are listed in Table 8.12l. Additionally, it has been suggested in the literature that most separations can be accomplished by using mixtures of 6,7 a polar and nonpolar phase in the proper proportions. Indeed, computer-aided series-connected capillary column design procedures have been published that produce column systems optimized for the analysis restraints (such as analysis time and detectability) that are operative for that particular 8 application. Active solid supports such as alumina, activated charcoals, and silica gels have been used in the past for the separation of fixed gases and light hydrocarbons. However, their use is now avoided if possible because their activity is difficult to stabilize, leading to changes in retention time and separating power. Similarly, molecular sieves (synthetic alkali metal aluminosilicates) once used primarily for the separation of hydrogen, oxygen, nitrogen, methane, and carbon monoxide must be protected from carbon dioxide (by using a trapping column ahead of it) and from other hydrocarbon gases that are adsorbed and deactivate the column. Many of the applications that once used these solid phases can now be performed with synthetic porous polymer phases such as Porapaks™ and HaySep™.


WCOT (Capillary) Columns In the past, capillary columns were avoided because of their fragility (they were made of glass since metal capillaries had active surfaces that produced less efficient columns due to tailing peaks) and also the stationary phase tended to bleed off over a period of time. However, these problems are no longer present because of two technological breakthroughs. The fragility problem has been solved by coating the outside of the fused silica capillary with a polyamide (similar to the coating used for optical 9 fibers), which turns it into a tough, bendable capillary tube. In addition, the stationary phase stability problem has been eliminated by performing the appropriate chemistry on the stationary phase after it is coated on the walls of the capillary tubing to produce partial cross-linking of the phase to itself 10,11 and to the silica surface. Columns of this type are classified as having stabilized stationary phases and are readily available from all GC column vendors. Although capillaries of 0.25-mm i.d. are commonly used in laboratory GCs, it is much more common to see the socalled megabores (0.32- to 0.50-mm i.d.) used in PGC applications as a direct replacement to the packed column. Minimum hardware modifications are necessary on older PGCs to accommodate these larger capillary columns. They provide comparable or better efficiency than their packed column analogs and, in many cases, better separations, since longer columns can be used (because of their high permeability). The characteristics of the various columns used in process GC are summarized in Table 8.12m. COLUMN AND VALVE CONFIGURATIONS One of the most severe restraints operative in PGC applications is that of timely analysis cycle time. Since the cycle time includes the sample transport time as well as analysis time, this restraint has implications from both a chromatographic and a sample handling point of view. The latter concern will be discussed later on in this text. The solution to analysis time has historically been to use multiple columns in various configurations. Multiple columns and columnswitching systems serve several important functions, namely: •

•

•

Housekeeping: To ensure that all components are removed from the columns during each analysis cycle. The method commonly used to achieve this objective is called backflushing. Discard unmeasured components: In many process control applications, a rapid analysis of only one or two components is required, not the total analysis. Both backflushing and heart-cutting are used. Simplify the separation problem: A single column that will separate all of the measured sample components within the desired cycle time may be difficult to find. However, it may be possible to optimize separate columns for different portions of the analysis and then combine them in such a fashion to achieve the


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TABLE 8.12m Characteristics of Columns Used in PGCs Type mls/min

Length, m

o.d. mm

i.d. mm

Flow

Packed Conventional Micropacked

0.1–5 0.02–1

3.18 (1/8") 1.59 (1/16")

2 1

10–50 1–10

Capillary

1–100

0.4–0.8 (1/64–1/32)

0.1–0.5

0.5–10

Type

Advantages

Disadvantages

Conventional

Ease of fabrication Large Sample capacity affords High detectability Relatively inexpensive

Large carrier gas consumption Modest efficiency

Micropacked

Lower gas consumption Higher effiency Faster analysis

Higher head pressure reqd. Lower sample capacity Difficult to fabricate

Capillary

Lower gas consumption Higher effiency Fast analysis Best separating power

Lowest sample capacity Reduced detectability Expensive

desired analysis. This simplifies the application problem. Trap-and-store methods, as well as parallel and series columns, are frequently used for this purpose. While the solutions to some applications may have changed with the advent of more instrumentation that can support capillary, temperature-programmed, and parallel chromatography, many remain that can only be solved with the use of the various column configurations discussed below. While there are literally hundreds of possible column configurations, they are all built from a few basic ones. Hardware Six- and 10-port valves are often used in multicolumn chromatography. However, an alternative switching procedure, 5 the so-called valveless or Deans switching, is used extensively in process GC, particularly in Europe. Basically, this valve depends on applying a pressure differential to change the flow pattern of carrier gas. The columns are connected in series by a T, the center connection of which is connected to a second source of carrier gas through a solenoid located outside the heated oven. The solenoid controls the supply of carrier gas to the tee to shift the flow pattern (see Figure 8.12n). The main hardware advantages of this arrangement are that solenoids that satisfy the lower temperature specification are much easier to find; it is a less expensive solution to the problem, since a solenoid and a tee are less expensive than a high-temperature rated chromatographic grade-switching valve; and, finally, the assembly takes up much less oven space than a conventional rotary or diaphragm valve. Sample Injection The primary function of the sample inject valve is to place a sharply defined fixed volume of sample


FIG. 8.12n Backflush to vent configuration using a Deans switch. Column 1 is backflushed to vent in the “fail-safe” unenergized position of the solenoid. The auxilary regulator, PR2, supplies carrier gas to both columns in its backflush mode. Except for the sample valve, there are no moving parts within the heated analyzer oven. (From Annino, R. and Villalobos, R., Process Gas Chromatography: Fundamentals and Applications, Research Triangle Park, NC: ISA, copyright 1992 ISA. Used with permission. All rights reserved.)

into the carrier gas stream so that it can be carried into the column for separation into its individual components. The sample volume is defined by an external length of tubing connecting two of the valve ports, as shown in Figure 8.12k,

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FIG. 8.12o Rotary valve with internal sample loop for the injection of small volume liquid or vaporized sample.

or internally by a groove machined into one of the valve faces (see Figure 8.12o for an internal sample loop in a rotary valve). The latter configuration is used for small-volume gaseous or liquid injections. In the case of some liquid samples, a special valve is used that is designed to have two distinct temperature zones: 1) a cool zone near ambient through which the sample circulates, and 2) an internally heated vaporizing zone where the liquid sample is injected into the carrier gas stream, vaporized, and carried into the column. This type of sample valve is called a vaporizing liquid sample valve. An important feature of this design is that the circulating liquid sample does not contact the heated zone. Hence, it can be used for samples that will not tolerate heat, for example: • •

Samples that tend to polymerize (such as styrene) Samples with a high vapor pressure such that heating them to the temperature of the column would increase their vapor pressure above the line pressure and result in partial vaporization (flashing) of the sample.

The internally heated vaporizing valve is also used if local or company safety codes prohibit the introduction of flammable liquids into the heated analyzer zone.

Capillary columns require much smaller injection volumes than do packed columns. For gas samples, the appropriate volumes (0.5 to 10 ml) can be obtained using the external or internal sample loop. Acceptable liquid sample volumes for capillary chromatography, however, are a thousand times smaller than this and are obtained by adding a splitter between the sample inject valve and the column. The splitter is essentially a Y specially designed so as not to produce any discrimination of the vaporized sample according to some function of composition; that is, it provides only a volume split of the sample. Since one leg of the Y may be operating at 200 to 500 times the column flow rate, it potentially constitutes an increased operating cost for the instrument. For this reason, it is normally dead-ended, except when injecting sample. Backflush The column configuration most frequently used in process GC is the backflush precut to vent. Both this and the sample inject operation are shown combined in the 10port valve configuration shown in Figure 8.12p, using a rotary valve, and in Figure 8.12n, using a Deans switch. The backflush procedure performs the all-important housekeeping function of removing all unmeasured heavy components or those that may degrade the analytical column (such as water) from the column system each cycle. The system consists of at least two columns: a precut (or stripper) column, C1, and an analysis column, C2. The precut column is backflushed shortly after the sample is injected, thus allowing only the components that are to be measured to enter the analytical column. Typically, the precut column is approximately one third the length of the analytical column. This allows sufficient time for all components to backflush during the time necessary to chromatograph the rest of the sample on the analytical column. Another version of backflush, called backflush to detector, is illustrated in Figure 8.12q. It is used when it is desired to have a measure of the “heavies” that are backflushed.

FIG. 8.12p A 10-port rotary valve with an external sample loop configured in Position B for the injection of sample into column 1 and column 2 and, in Position A, for backflush to vent of the sample components trapped on column 1 while the rest of the sample is separated on column 2 and the sample loop is refilled with fresh sample. (Courtesy of Valco Instrument Co. Inc.)



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FIG. 8.12q Backflush to measure configuration: an 8-port rotary valve is configured so that the backflushed components can be flushed through the detector and measured as a group. (Courtesy of Valco Instrument Co. Inc.)

FIG. 8.12r Heart-cut configuration: a six-port plate valve is configured so as to divert a large portion of a major component in order to simplify the separation of a small concentration of a sample component riding on its tail.

Heart-cutting Heart-cutting is one of the most useful column configurations. It is frequently used for the analysis of trace components that elute immediately after a large concentration of another sample component that interferes with its determination (for example, trace amounts of acetylene in ethylene). The design consists of two columns, a heart-cut column and an analysis column, as shown in Figure 8.12r, using a six-port sliding-plate valve. In the normal position of the heart-cut valve, the effluent of the heart-cut column is diverted to vent. When the component of interest is about to elute, the heart-cut valve diverts it into the analysis column and then returns to its venting position. A heart-cut of only the compound of interest and a narrow band or “tail” from the major component are thus introduced into the analysis column. This operation may be repeated for each component of interest, although it is usually not practical to measure more than three heart-cut components per analysis. The trap-and-store configuration consists of two analytical columns arranged in series through a column-switching valve, as shown in Figure 8.12s. The switching valve is used


FIG. 8.12s A six-port plate valve configured so that a group of early eluting sample components pass into a storage column which is then isolated from carrier gas flow (the group of components remain stationary on the column except for a samll amount of longitudinal diffusion) and stored until the rest of the sample is separated and measured by the detector. When the valve is again switched, the components on the storage column are developed and measured by the detector.

to direct the effluent from column 1 into column 2 or through a bypass restrictor to the detector for measurement. In a manner similar to that used in heart-cutting, two or more components that are not separated on column 1 are diverted to column 2, as shown in Figure 8.12s, normal position. In the other storage position of the valve, carrier gas is not available for this leg. Column 2 is thus isolated, and the diverted compounds are stored while the rest of the sample is developed in column 1 and passed on through the restrictor to the detector for measurement. The trapped compounds are then passed to the detector for measurement. DETECTORS Of the many detectors that have been proposed for use in PGC over the years, two emerge as the most popular and have been applied to a far-ranging number of applications. They are the

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FIG. 8.12u Simple constant current circuit for a thermistor bead TCD with a gain of 5. (From Annino, R. and Villalobos, R., Process Gas Chromatography: Fundamentals and Applications, Research Triangle Park, NC: ISA, copyright 1992 ISA. Used with permission. All rights reserved.)

FIG. 8.12t Single sensing filament, single reference filament, flow-through TCD with replaceable elements shown in a classical Wheatstone bridge circuit. In a properly designed TCD block assembly, the inclusion of reference filaments imparts greater stability to the signal by balancing out small temperature variations in the cell block.

thermal conductivity detector (TCD) and the flame ionization detector (FID). Their popularity stems from their basic simplicity and ruggedness coupled with their acceptable sensitivity for most applications. There are other detectors used in PGC applications (such as the flame photometric and other ionization detector), but only under special circumstances. Thermal Conductivity Detector The TCD is a concentration-responsive detector of moderate −9 5 * sensitivity (MDQ = 10 g) and dynamic range (10 ).* It is a reliable, simple, easy-to-maintain, and relatively inexpensive detector with a universal response. The unit shown in Figure 8.12t is of classical design and consists of a cavity in a metal block with a filament (made of a metal with a large resistance/temperature coefficient such as tungsten or an alloy such as tungsten-rhenium) suspended in the carrier gas flow stream. In support of capillary chromatography, the newer detector designs strive to minimize detector cavity volume. The filament is heated electrically and reaches some equilibrium temperature (and corresponding resistance) based on the power supplied to it and the thermal conductance of the gas passing over it, and the temperature of the cavity wall. The cavity wall temperature is maintained constant by the oven environment, *

MDQ is the minimum amount of solute that will produce a detector signal twice the peak-to-peak noise of the detector. The dynamic range of the detector is the range of concentration of the test solute over which a change in concentration results in a measurable change in detector signal.


FIG. 8.12v Basic constant resistance/temperature circuit for a TCD. (From Annino, R. and Villalobos, R., Process Gas Chromatography: Fundamentals and Applications, Research Triangle Park, NC: ISA, copyright 1992 ISA. Used with permission. All rights reserved.)

and the carrier gas is selected for its large thermal conductivity (helium or hydrogen), compared to the components of the sample. Thus, as sample components– carrier gas mixtures enter the detector, the consequent thermal conductance change experienced by the filament environment leads to a change in the equilibrium temperature and resistance of the filament. Traditionally, the filament is made to be one arm of a Wheatstone bridge that is supplied with a constant voltage. The change in null voltage experienced by the bridge when the filament changes resistance is recorded as the chromatographic signal. Modern TCD electronics, however, favor constant-current (Figure 8.12u) or constant-resistance designs (Figure 8.12v) for faster response and also for filament protection in the presence of large sample concentrations and cessation of carrier gas flow.


Alternatively, the filaments can be replaced with thermistors (sintered mixtures of manganese, cobalt, and nickel oxides along with trace elements). They are mounted in the form of a bead on platinum wire leads and coated with glass to make them inert. Thermistors have a very large negative temperature coefficient, making them very sensitive sensors. However, they have a limited temperature range and their sensitivity decreases with increasing temperatures Thus, they are most useful in low-temperature applications. Also, their use is commonly restricted to helium carrier gas as the glass covering becomes embrittled and cracks when hydrogen is used.

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TABLE 8.12x FID Effective Carbon Number (ECN) Atom

Type

ECN

Carbon

Aliphatic

1.0

Carbon

Aromatic

1.0

Carbon

Olefin

0.95

Carbon

Acetelenic

Oxygen

Ether

−1.00

Oxygen

Primary Alcohol

−0.60

1.30

Flame Ionization Detector Even though the use of a FID involves increased operating cost and increased analyzer complexity, its very large 7 dynamic range (10 ), high sensitivity (0.015 coulombs/g C), –12 low detection limits (10 gC/sec), and vanishing small dead –2 volume (10 µl) have made it probably one of the most popular GC detectors in use today. With optimized operating parameters, it is possible to determine as low as 20 pg, or about 5 ppb, of a sample. The FID consists of an oxygen-rich hydrogen flame that burns organic molecules, producing ionized fragments. These ions are subjected to an electrical field produced by impressing a potential (usually 150 to 300 V) across a jet and the collecting electrode (the jet is usually made negative, and the collector positive). The essential components of a FID are shown schematically in Figure 8.12w. The hydrogen fuel is mixed with the column effluent and then fed to the jet. An outer sheath of

FIG. 8.12w Schematic illustration of an FID assembly. The capillary column is run up inside the jet to the tip (thus minimizing detector dead volume). The collector is positively polarized usually with +300 volts. The flame is supported by an external sheath of air.


air supports the flame. In some designs, the flame points downward. This ensures removal of water from the flame vicinity together with any solid particles that may be present. The presence of either of these substances will contribute to the background noise of the system and thus reduce minimum detection levels. The response of the detector depends on the flow rate of all three gases—with the hydrogen flow rate being the most critical (the ratio of air to hydrogen is usually around 10). The response of the FID, which varies with the identity of the component in the flame, depends primarily on the number and type of carbon atoms being oxidized. However, the identity and manner in which other atoms are combined with the carbon also influences the response. The concept of 12 effective carbon number (ECN) has been introduced as a means of relating the various responses. The ECN is the sum of the contributions made by the individual carbon atoms modified by the functional group contributions (a sample of these ECNs is given in Table 8.12x). A number of compounds cannot be detected by the FID. These include the fixed gases, oxides of nitrogen, H2S, SO2, COS, CS, CO, CO2, H2O, NH3, and the noble gases. However, if the carrier gas is doped with an ionizable gas (such as methane), even these gases can be detected (yielding negative peaks as they dilute the concentration of ionizing gas during the elution). Also, in the case of carbon dioxide, monoxide, or formaldehyde, chemically reacting the gases as they emerge from the column to form methane makes them FID responsive. Thus, most suppliers of PGCs offer a methanizer, which is placed between the end of the column and the FID. Hydrogen is also teed in at this point. The methanizer consists of a length of 1/8-in.-o.d. tubing filled with a catalyst of 10% (wt/wt) nickel on Chromosorb P, over which the reducing reaction occurs. The reaction is nonselective in that all hydrocarbons are converted to methane. The FID is a very inefficient ionizing source, and it is –14 only its very low noise level (10 A), allowing large amplifications, that makes this detector so sensitive. As shown in Figure 8.12y, high-gain, low-noise, electrometers mounted as close to the detector as possible are required to exploit the full range of this detector. In addition, to maintain this low noise level, highly purified gases should be used.

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FIG. 8.12z Basic flame photometric detector.

FIG. 8.12y Schematic of a simple high impedance current amplifier circuit used for the measurement of the small currents generated by the FID. (From Annino, R. and Villalobos, R., Process Gas Chromatography: Fundamentals and Applications, Research Triangle Park, NC: ISA, copyright 1992 ISA. Used with permission. All rights reserved.)

Flame Photometric Detector The flame photometric detector (FPD), illustrated in Figure 8.12z, is essentially a flame emission spectrometer design optimized for use as a GC detector. It is an elementspecific detector and is used primarily for the determination of sulfur- or phosphorus-bearing compounds (e.g., H2S, CS2, SO2, COS, mercaptans, and alkali sulfides in various pulp milling processes and in petroleum fractions). The column effluent is fed to a hydrogen/oxygen-rich flame where individual atoms contained in the sample or the

reactants of these atoms with hydrogen and oxygen are excited to higher electronic states by the energy of the flame. These excited atoms and molecular fragments subsequently return to the ground state with the emission of characteristic atomic or molecular band spectra. For sulfur, the S2 species at 394 nm is used, while the 526-nm emission from HPO is used for phosphorous. A narrow-band-pass filter is used to isolate the appropriate wavelength, and its intensity is measured with a photomultiplier tube (PMT). This detector has a number of severe limitations. These include response dependency on the O2/H2 ratio, the H2 flow rate, the type of sulfur compound, quenching of the sulfur emission by large concentrations of other organic compounds, and even the length of time that the flame is lit. The quenching of the sulfur emission by other organic compounds and the flame extinguishing problem caused by large concentrations of organics such as the solvent peak are largely overcome by the dual-flame assembly shown in Figure 8.12aa, but at the expense of detection limits as the sample is diluted in passing to the second flame. In addition, the sulfur compound response dependency is eliminated with

FIG. 8.12aa Schematic of a FPD showing details of a dual flame assembly. The dual flame scheme is used to minimize the quenching of the sulfur emission by other organic compounds and also to eliminate the flame quenching problem when large concentrations of organics are present. It also provides the same sulfur response for all compounds since they are all oxidized to SO2 in the lower flame. (Courtesy of Varian Associates.)



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this detector because all the sulfur compounds are converted to SO2 in the lower flame. 4 The FPD response to sulfur and phosphorous is 10 greater than its response to hydrocarbons, and its MDQ is at the nanogram level. However, since the detector signal is due to the emission of the S2 species, it is not a linear function of concentration. Theoretically, it should be proportional to the square of the sulfur concentration (in practice, a factor of 1.8 provides a better fit to the calibration curve), and thus become much more sensitive to changes in sulfur concentration as the concentration increases. The reduced detectability and response characteristic at trace levels of the sulfur sample due to the detector’s quadratic response can be improved by doping the carrier gas with a volatile sulfur compound (methyl mercaptan is a good choice) to bring the signal further up on the calibration curve. Finally, the PFD response is greatly dependent on carrier gas identity and flow rate, as well as oxygen (or air) flow rate to the flame. Clearly, it is not a very robust detector. 13

Pulsed Flame Photometric Detector

In a conventional FPD, the detectivity is limited by light emissions of the continuous flame combustion products. Narrow-band-pass filters used to isolate the wavelength also limit the fraction of the element-specific light that reaches the PMT, and are not completely effective in eliminating the flame background and hydrocarbon interference. In a pulsed FPD (PFPD), the fuel gas (H2) flow is set so low that a continuous flame cannot be sustained. By inserting a constant ignition source into the gas flow path, the fuel gas ignites and propagates back through a quartz combustion tube to a constriction in its path where it is extinguished; then it refills the detector, ignites, and repeats the cycle. Carbon light emissions and the emissions from the hydrogen–oxygen combustion flame are complete in 2 to 3 msec, after which a number of heteroatomic species give delayed emissions that can last up to 20 msec. These delayed emissions are filtered with a wide-band-pass filter, detected by a PMT. By using the leading edge of the flame background emission to trigger a gated amplifier with an adjustable delay, heteroatomic emission can be amplified to the virtual exclusion of the hydrocarbon background emission. The PFPD is thus uniquely characterized by the addition of this time domain information (illustrated in Figure 8.12bb), which allows the detector to selectively detect many other elements (such as As, Sn, Se, Ge, Te, Sb, Br, Ga, In, Cu, etc.) with no hydrocarbon interference. In the PFPD illustrated in Figure 8.12cc, hydrogen and air (3) is continuously fed into the small pulsed flame chamber (6), together with sample molecules eluted from the GC column (14). The combustible gas mixture is also separately flowing into (4), a light-shielded, continuously heated wire igniter (12). The ignited flame is propagated back to the gas source through the pulsed flame chamber (6) and is self terminated in a few milliseconds, since the pulsed flame


FIG. 8.12bb Emission spectra of sulfur, phosphorous, and carbon from a PFPD showing their separation in the time domain. The initial hydrocarbon emission spike serves as an excellent reference from which to time the appearance of the selected emission. This provides infinite selectivity against hydrocarbon emission as well as unique heteroatom identification capability. (From Amirav et al., Pulsed Flame Photometric Detector for Gas Chromatography report Tel Aviv University, Tel Aviv, Israel, 2001.)

cannot propagate through the small hole at the bottom of the combustion chamber. The continuous combustible gas flow creates another ignition after a few hundred milliseconds in a pulsed (∼3 Hz) periodic fashion. The emitted light is transferred with a light pipe (8) through a broad (not narrow)band-pass filter (9) and detected with a PMD (10). The PFPD is much more sensitive than the continuous −13 −14 −12 flame version (2 × 10 gS/sec, 1 × 10 gP/sec, and 2 × 10 gN/sec) and much more selective from unwanted hydrocarbon emission, with total discrimination against hydrocarbon com7 pounds (selectivity greater than 10 ). The PFPD sulfur mode has similar detection limits as that of the sulfur chemiluminesce detector (SCD), but its detection of signal to noise is better at practical detection levels (due to its quadratic response). Quenching is still a problem, however. It can be reduced by injecting smaller samples, but at the expense of sensitivity. The real advantage of this detector is with mass limited samples, such as when using capillary columns rather than packed columns, and for trace analysis.

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FIG. 8.12cc Schematic view of a PFPD. (Reprinted with permission from Amirav, A. and Jing, H., Anal. Chem. 67(18) 3305. Copyright © 1995 American Chemical Society.)

Orifice-Capillary Detector The orifice-capillary detector (OCD) schematically illustrated in Figure 8.12dd was used in a fully functional, commercially available PGC whose operation was based 2,14 Designed as a transmitter totally on pneumatic power. primarily for control applications, this PGC measured one or two components of the sample. It traded sensitivity for increased reliability and lower installed cost. An intrinsically safe product was ensured by eliminating all electrical power for its operation (the oven was heated by steam and the temperature was regulated by a pneumatic temperature regulator). The chromatogram generated from the detector was produced by the variation in differential pressure generated across an orifice by a density change in the carrier gas resulting from the elution of sample components from the column. This differential pressure was amplified by a pneumatic amplifier and fed to a pneumatic computer programmed to detect and measure the peak height of the two desired sample components and transmit these measurements to a controller as updated analog trend signals. 15 There has been a proposal in the literature for increasing the minimum detection levels of such a device. The differential pressure from the OCD is converted directly to a frequency modulated optical signal, and the complete chromatogram is transmitted over optical fiber to a remote station for more sophisticated computer data processing. In addition, the capability of the pneumatic PGC is also expanded to that of a


FIG. 8.12dd Orifice-capillary detector and pneumatic preamplifier. A properly designed capillary compensates for any differential pressure fluctuations across the orifice due to changes in flow rate so that the final differential pressure signal output of the amplifier is the result only of changes in density of the carrier gas due to elution of sample components from the column.

multicomponent analyzer. This design modification changes this PGC to one of a split architecture with the nonelectric analyzer unit in the field and the electronic programmercontroller (PC) remotely positioned. Such an instrument has not yet been commercialized.


Miscellaneous Detectors There are a number of other detectors that have been introduced since the birth of GC, but because of issues such as high maintenance requirements, high customer burden, and unreliable operation over extended periods in the field, they have been limited primarily to laboratory applications. However, there are times when the minimum detection limits of the application cannot be satisfied by the basic detectors normally used in process GC. In those cases, some of these less-than-robust detectors have been used. Photoionization Detector The photoionization detector (PID) functions by irradiating the column effluent with high-energy ultraviolet (UV) light generated by a high-voltage discharge lamp containing a noble gas (e.g., krypton) (Figure 8.12ee). The ions are collected at a polarized electrode, and the resulting current is measured with a FID-type electrometer. This detector is selective in that only compounds whose ionization potential is less than the UV radiation will be ionized. Compounds with aromatic ring structures give the highest sensitivities (as much as 10 times the FID), while compounds like methane and ethane with ionization potentials greater than 12.98 eV give no response with the commonly used PID tubes (9.5, 10.0, 10.2, 10.9, and 11.7 eV). The PID has a 7 dynamic range of 10 , extending from 2 pg to 30 mg. An advantage to this detector is that it does not require auxiliary gases, as does the FID. This advantage is offset by the fact

FIG. 8.12ee Principal elements of a PID showing a 200 V supply for the collecting electrodes and a separate 400 V supply for the UV lamp. (From Driscol, J.N. and Spaziani, F.F., “PID Development Gives New Performance Levels,” Research/Development, May 1976.)


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that it does not respond sensitively to many of the compounds of interest, and it requires much maintenance to maintain quantitative accuracy. Electron Capture Detector The electron capture detector (ECD) (see Figure 8.12ff) is yet another type of ionization detector. The column effluent passes between two electrodes, one of which has been treated with a radioactive source (tritium or nickel-63; the latter is preferred because of its extended detector stability) that emits high-energy electrons. These electrons produce large quantities of low-energy thermal electrons in the carrier gas, which are in turn collected by the other electrode to produce a steady-state current in the presence of pure carrier gas. Compounds eluting from the column that have an affinity for thermal electrons reduce this steady-state current, thereby producing the chromatogram.

FIG. 8.12ff Schematic of an ECD arranged for constant voltage operation. Also shown is the chromatogram produced by the reduction of the standing current when compounds of higher electron affinity than the carrier gas are eluted from the column and pass through the detector. (Courtesy of Varian Associates.)

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The detector is thus highly selective, with halogenated compounds being the most responsive (detection at the picogram level). Other groups exhibiting good selectivity include anhydrides, peroxides, conjugated carbonyls, nitriles and nitrates, and sulfur-containing compounds. Maintenance is of critical importance with this detector, more so than with any other GC detector (except the helium ionization detector, discussed next). It responds exceptionally well to oxygen, necessitating leak-free systems and oxygenfree carrier gases. Also, response to water vapor can cause unstable baselines so that molecular sieve traps (which require periodic maintenance) are required in the carrier gas lines. Finally, care must be taken to use this detector only with columns of very low bleed, as the condensed stationary phase in the detector can easily be polymerized (by radiation and electron bombardment) to a hard insoluble deposit, which is almost impossible to eliminate and which interferes with the proper functioning of the detector. Although a pulsed mode of operation has improved the linear range of the ECD, it has not improved its robustness, and all of the above-outlined concerns still apply. Discharge Ionization Detectors In contrast to the olderdesign ionization detectors that required a radioactive source

of strontium, nickel, or tritium to produce metastable helium atoms by collisions with the high-energy electrons generated by the source, the newer designs require only helium. Two electrodes support a low-current arc through the helium makeup gas flow. The helium molecules between the electrodes are elevated from their ground state to form a helium plasma cloud. As the helium molecules collapse back down to the ground state, they give off a high-energy photon that will ionize all compounds having a ionization potential lower than 17.7 eV. Thus, the ionization detector will respond to volatile inorganics that the FID does not (such as NOx, CO, CO2, O2, N2, H2S, and H2). The ions are collected at polarized electrodes to produce a current, as in any ionization detector. One version of this detector utilizes a steady-state helium plasma cloud, while a more recent design (see Figure 8.12gg) pulses the discharge (pulsed discharge detector (PDD)). Performance of the discharge ionization detectors is equal to or better than that of the older-design ionization detectors that require radioactive sources. In its helium photoionization mode of operation, it is a universal, nondestructive, highsensitivity detector responding to fixed gases in the low parts per billion range. Its response to both inorganic and organic 5 compounds is linear over a wide range (10 ).

FIG. 8.12gg Schematic of a pulsed discharge detector configured as a PHID and as a PECD. A stable low power, pulsed DC discharge, in helium is utilized as the ionization source. Eluents from the GC column flowing counter to the flow of helium from the discharge zone are ionized by photons from the helium discharge and the resulting electrons are focused toward the collector electrode by the two bias electrodes. The principal mode of ionization is photoionization in the range of 13.5 to 17.7 eV. (Courtesy of Valco Instrument Co. Inc.)



The PDD is extremely versatile. If the carrier gas is doped with methane at the column exit (see Figure 8.12gg), the PDD can operate as an electron capture detector with sensitivity and response characteristics similar to those of a conventional radioactive ECD. Its MDQ for halogenated compounds –15 –12 ranges from 10 to 10 g. In addition, when the helium discharge gas is doped with a suitable noble gas such as argon, krypton, or xenon (depending on the desired final ionizing power), the PDD can function as a specific PID for selective determination of aliphatics, aromatics, etc., as discussed in the section devoted to photoionization detectors. The HID is a high-maintenance item. It is the only detector that can measure permanent gases at the 1-ppb level. Therefore, leak-free plumbing is an absolute necessity. It responds to trace impurities, including water, in the carrier gas, thus requiring traps in the carrier gas stream to ensure the removal of these trace impurities. Clearly, this is a disadvantage for users not willing to devote maintenance time to monitor and frequently replace these traps. In addition, the HID is easily fouled by deposits from column bleed, which also contributes to a high noise level. Thus, its lowest detection limit is reached only with active solid supports such as molecular sieves.

CARRIER GAS FLOW CONTROL Controlling carrier gas flow rate is of critical importance if one is to reproduce peak areas and retention times, and thus automate the analysis. Two methods, pressure control and flow control, may be used, but pressure control is by far the favored procedure used in process gas chromatographic instrumentation, for the following reasons: 1. A pressure regulator has an almost immediate response to downstream upsets (such as valve switching). A flow controller responds to flow upset only as fast as its flow set point allows. 2. A pressure regulator has the ability to provide some compensation for variation in retention time caused by changes in temperature. 3. A pressure regulator has the ability to supply the proper carrier gas flow rate to the column even in the presence of downstream leaks between it and the sample valve. All pressure regulators are, to some degree, sensitive to temperature and input pressure. Tank regulators will not provide sufficiently stable output pressures under varying conditions of temperature and tank pressure. Thus, another regulator is mounted in the line close-coupled to the analyzer. Ideally, it should be mounted in the analyzer oven compartment to ensure a stable temperature environment, but few if any can withstand the 398°F (200°C) temperatures © 2003 by Béla Lipták

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that are sometimes used. Therefore, it is important that this regulator have a minimum temperature coefficient (not as important a requirement when the analyzer is mounted in an analyzer shelter that has a fairly stable temperature environment) and that it can tolerate the input pressure changes it will experience from the tank regulator that is likely to be located outside, exposed to the varying temperatures of the environment. For pressure drops across the column of 75 psi (5.3 bars), the relative error in peak area will be 1.7 times the relative error in the pressure drop. Therefore, to limit the error from this source to 0.5%, one must control pressure to within 0.4 PSIG (0.03 bars). In addition, assuming one is using a pressure regulator that is referenced to ambient pressure, the error caused by variations in ambient pressure will be 1.4 times the relative barometric pressure variation. Thus, errors of 0.2 to 0.5% due to changes in weather are difficult to 16 eliminate. Recently, electronically controlled pressure regulators have become available and have been incorporated into PGCs. The heart of such a regulator is a solid-state pressure transducer that produces an electrical output proportional to pressure. This output is used to operate the integral control valve and to produce an output signal. In the case of the transmitter, miniature fluidic thermistors (fluistors) are used as pressure/flow controllers. Such an addition to the PGC is a tremendous advantage for maintenance staff since it allows for control of carrier gas flow rate from the remote analyzer maintenance station (AMS) rather than at the unit.

PROGRAMMER-CONTROLLER The PC section is an extremely important part of a modern PGC, containing all the electronics to power the system, the controller, the data reduction hardware and software, and, of increasing importance, the communications package. Its design has changed radically over the past few years with the advances in electronic technology. The PC unit can in theory be located in the field at the analyzer (stand-alone) or in a remote area such as the control room or instrument room close by (split architecture). The stand-alone design is by far the most popular. Although in this option the analyzer electronics are also usually located within the PC section, the primary purpose of the PC is to control all the functions in the analyzer. These include sample injection and column switching as well as various housekeeping tasks, such as modification of the detector signal, auto-zeroing, peak gating, integration, conversion of area units to concentration, and data transfer to the proper location. The transmitter utilizes what looks like a split-architecture design in that the electronics are packaged separately from the analyzer, but the two are close-coupled to each other in the field to yield a standalone PGC.

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Most PC designs allow for operation in a Division 1 area * when used with X or Y* air purge, and in Division 2 areas without purge. Programmer Programming tasks are managed by an on-board microprocessor-based computer. In older microprocessor-based PGCs, the PC was contained on several plug-in circuit boards located in a separate compartment from the analyzer. Current designs reflect the advances in electronics and focus on completely modular designs that allow for complete replacement of large portions of the electronic package (plug and play) for rapid repair and ease of maintenance. A number of provisions are made in the design to ensure the unit’s rapid return-to-service in case of power failure. Peak Processor The peak processor section of the programmable controller has as its primary functions the detection of chromatographic peaks produced by the sample constituents of interest and the determination of the appropriate peak areas or peak heights. Peak detection is performed by comparing the instantaneous slope of the original (i.e., the rate of change) to some reference value. The objective is to differentiate between noise and the true onset of a peak. The peak detector can be disabled except for distinct time intervals (defined by so-called gates) and integration performed only if a peak is found within these gates, or alternatively, integration can be forced between the gates. Most systems have the capacity of measuring different types of peaks and allocating areas on the basis of some internal logic. This peak allocation is usually determined as follows: 1. Incompletely resolved peaks: Areas are allocated by dropping a perpendicular from the valley minimum to the baseline. 2. Proportional area allocation: Areas are allocated in proportion to the gross areas of the peaks. 3. Tangent skimming: Used for small rider peaks on the tail of major peaks. A tangent is drawn from the valley point to the back of the rider peak. 4. Forced integration: Bypasses slope detector logic and forced integration between start and stop points. 5. Peak height: Value of the peak maximum used as direct measure of concentration. The peak areas are then converted to concentration using information obtained previously from the analysis of calibration standards. Both the peak selection and peak measurement procedures are selected by the user from a menu of choices at the time of setup. *

These are Instrument Society of America (ISA) definitions of purge systems. X purge allows a general-purpose instrument to be placed in a Division 1 area, while Y purge allows for a Division 2-rated instrument to be used in a Division 1 area.


Data Acquisition In modern microprocessor-based PGCs, the amplified detector signal is sampled by an analog-to-digital (A/D) converter and the digital values are stored in the RAM for processing. This is in contrast to the older procedure for acquiring chromatographic data, that is, monitoring the detector signal as a function of time with a potentiometer recorder. Recorder outputs are still present in some currently available PGCs to assist in setup and maintenance purposes. However, the record that is produced may not be as diagnostically useful as in the older designs, since, in many cases, it is not a record of the raw detector signal but an analog signal reconstructed from the RAM-stored filtered digital values. Input–Output The term process control generally implies the direct control of a process variable by an on-line instrument. Commonly, adjustments are made continuously in process conditions in order to reduce to zero the difference between the measured value of this variable and a desired (set point) value. For instruments whose output is a continuous-trend representation of the measured variable (e.g., pH, conductivity, pressure, temperature, or flow), there is no problem in using this signal directly in a control scheme by comparing it to the set point and generating an error signal that is used to drive the process back to the set point. However, if the process variable is the concentration of one or two process stream constituents, the use of a chromatograph for the measurement presents special problems. The chromatographic signal is not continuous, but rather is transient. Furthermore, more than one component is represented in the information contained in the chromatogram, and this information is encoded as a function of time. Therefore, to be useful as a control instrument, the output must be deciphered and presented to the control loop in a form that is useful. For the traditional analog controller, this consists of a continuous analog signal (4 to 20 mA) whose magnitude is proportional to the concentration of the desired process variable. For an intelligent controller, this consists of a digital signal encoding the desired value of the process variable. Thus, a modern chromatograph must be able to deliver either of these outputs. The input–output (I/O) characteristics of a typical PGC are summarized in Figure 8.12hh. Cyclically updated 4- to 20-mA analog trend signals proportional to the concentration of the analyzed peaks are available, as well as digital communication lines, RS-232 or RS-485. Various alarm signals are also generated. Additionally, provision is sometimes made to accept inputs from other devices where these data are required for a calculation of the desired process variable (e.g., flow rate and density measurements for BTU analysis). Communication Communication with the plant’s distributed control system (DCS) is a must for modern PGCs. However, because of the large number of different DCSs in place—all using communication systems proprietary to the manufacturer of that


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FIG. 8.12hh I/O characteristics of a typical microprocessor-based PGC.

system—direct interfacing with all networks is all but impossible for the user to ascertain that a gateway is available to interface the PGC to his or her control system. Over the years there has been much discussion and research into the development of a digital communication system to replace the traditional 4- to 20-mA analog standard. Worldwide adoption of an industry standard is still in the future. However, emerging from the various discussions within standards groups is a consensus that the local PGC will never be connected directly to the main data highway, but rather, communications will take place on a so-called fieldbus. Fieldbus is a generic term that describes a new digital communication network that will be used in industry to replace the existing 4- to 20-ma analog signal standard. The


network is a digital, bidirectional, multidrop, serial-bus communication network used to link isolated field devices. The bidirectional specification allows data to be transmitted in two directions at the same time, multidrop can be interpreted as a single bus with many nodes connected to it, and serial means that the data are transmitted serially according to RS232 or RS-485 protocol. Currently, there are two advanced protocols for all digital communication with field devices for measurement and control of continuous processes. These are the Foundation Fieldbus (FF) and the Profibus-PA (also in the field is a hybrid analog/digital protocol called HART, which uses a frequencybased digital signal superimposed on the 4- to 20-mA analog signal).

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FIG. 8.12ii Illustration of the various hardware and communication configurations that must be supported by a PGC. (Courtesy of The Foxboro Co.)

In the midst of this fast-changing environment, current PGC design must allow for operation in a number of modes—stand-alone or multiple—reporting either to a single process measurement, control computer, and compound computer, or to the DCS (Figure 8.12ii). Thus, many modern designs support the popular local area network (LAN) Ethernet protocol, as well as the new FF and Profibus-PA and older Modbus protocol. Operator Interface The operator interface has undergone many changes during the years following introduction of the gas chromatograph as a process control measurement device. In the original, rather crude chromatograph (from a modern technology viewpoint), the operator interface for control data also served as the maintenance technician’s interface. However, in the modern system, the two functions have been largely separated. GC data, status, and validation inputs necessary to control and evaluate the process are sent to the control room, and the DCS console thus becomes the process or plant operator’s interface. Maintenance and management data required to achieve maximum uptime are sent to a separate I/O interface called an analyzer maintenance station. This station can be positioned at the analyzer or at a remote location close by, with access to the system made with the ubiquitous industrial-hardened personal computer, or through a modem at a remote location some miles away. At the PC-based network workstation, any analyzer can be programmed and monitored. Graphical displays are available as an operator aid for simple operation, maintenance, and diagnostics. In addition, most programmer-controllers have a close-coupled I/O


FIG. 8.12jj Close-coupled full function AMS panel. (Photo courtesy of Siemens Applied Automation.)

interface that includes a keyboard or display panel (illustrated in Figure 8.12jj) providing access to the application and data reduction programs. Real-time chromatograms can be displayed here as well as hours of stored chromatograms complete with voltage and cycle times for future comparisons to simplify ongoing maintenance. Thus, from either of these locations, the maintenance technician can monitor the performance of the analyzer and reprogram the unit if necessary. The full-featured close-coupled I/O interface described above is not available for the transmitter. Instead, a panel of LED indicator lights visible through a window in the electronics enclosure (see Figure 8.12kk) provides the technician


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this type of alarm. Additionally, continuous read and write tests on the RAM memory are performed by the central processing unit (CPU) to detect the failure of a particular cell. Quantitation Since peak area is less sensitive to instrument variables than peak height, it has become the method of choice for translating the chromatographic record into process sample composition. There are four principle methods used to relate the record of detector response to the sample composition. They are: 1. 2. 3. 4.

FIG. 8.12kk Close-up view of the see-through LED panel in the electronic enclosure. (Courtesy of Rosemont Analytical Inc.)

with an overview of the instrument’s operational status. Further information and programming options are available through a PC-based AMS. A communication structure that combines all of the above-discussed options is shown in Figure 8.12ll. The user requirements will vary depending on the size and complexity of the plant or process under consideration. Thus, the PGC is designed to satisfy simple as well as complex requirements. Alarms and Diagnostics The amount of self-checking and diagnostic features that will continuously monitor the health of the instrument and alert personnel to problems and their source will continue to increase in importance as an integral part of the PGC design. Present units have two types of alarms: system alarms and data alarms. System alarms are concerned with the pneumatic, mechanical, and electrical operation of the system. They include low carrier gas pressure, low calibration gas pressure, low sample flow, detector failure, oven temperature high/low, PROM error, and EEPROM error. The data alarms (Hi/Lo) are used to monitor the results of the analysis. In some cases, these alarms alert only the operator; in other cases, action is taken automatically (such as not updating the trend signal with a new value when larger- or smallerthan-expected deviations occur). Retention time drifts and low area counts obtained for a calibration sample are examples of


Reference to calibration standards Relative response factors Internal normalization Internal standard

All of these methods rely on some form of calibration of the detector by comparing its response to a calibration standard containing a known concentration of selected compounds. It is not always possible to prepare a standard of the process components (because of instability of the compounds, dew-point problems, expense, etc.) However, the preparation of stable mixtures of reference compounds for which the relative detector responses to the compounds of interest are known may be possible. Process sample composition is then calculated using this information. The advantage over direct calibration is that all components of the sample do not need to be separated, only the components of interest. This leads to faster and simpler solutions to some application problems. The disadvantage of these direct calibration procedures is the direct effect that instrument variables (such as pressure of the sample loop) have on the accuracy and precision of the analysis. Normalization, on the other hand, eliminates sample volumes and pressure as variables, but requires that all sample components be measured. The normalization procedure involves taking the individual component measurements and adjusting each according to its response factor, and then dividing by the sum of these individual adjusted areas to yield the percent composition of the sample. SAMPLE HANDLING The importance of the sample handling system and sample conditioning system (SHS/SCS) for ensuring reliability to the operation of the PGC and validity to its output cannot be overemphasized. In the author’s experience, over 70% of the problems encountered in maintaining gas chromatographs can be traced to failures in the SHS/SCS. The reader who wishes to read detailed information on this subject should refer to Section 8.2 and the bibliography at the end of this section. Unlike the laboratory chromatograph, when samples are usually held in an appropriate container and hand-carried to the laboratory where the chromatograph is situated, the PGC must receive a sample straight from the sampling point, untouched by human hands. Moreover, for the PGC

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FIG. 8.12ll A typical multi-PGC interface to the DCS. Some variation will exist among the various vendors. In this configuration only PGC data, validation, system alarms, and stream sequence control are available to the DCS operator. All control and data communication is available to the AMS operator. (Courtesy of The Foxboro Co.)

to provide an accurate analysis of the process stream, a sample must be made available that is representative of the process composition. This is no trivial task, since the process sample may be quite hot, may be under considerable pressure, or may contain water vapor, solids, condensed liquids, and so on. Thus, the PGC requires a sample handling front end to obtain, transport, and condition the sample. The sample handling and conditioning system must: 1. Obtain a sample of the process stream that is representative of its bulk composition 2. Transport this sample to the analyzer within a time period such that the transport time plus the analysis time (i.e., the turnaround time) is short enough to satisfy the requirements of the control algorithm 3. Condition the sample (i.e., clean, vaporize, condense, adjust pressure, dilute, etc.) and present it to the analyzer in a condition appropriate to the analyzer specifications 4. Return the unused sample either to the process or to a waste disposal system


Sample Probe Because the low-velocity portion of the sample that is found along the walls of a process pipe is not representative, and also because debris is likely to accumulate along the walls, a simple tap into the process line to obtain a sample is not acceptable. A sample probe is required, which by virtue of its design can also serve quite effectively as a preliminary stage of conditioning. This is accomplished by 1) removing the sampling point from the walls; 2) orienting the probe so that the sample must turn 180° at the point of entry, thus excluding larger particles that cannot make the turn due to their momentum; 3) including a large-porosity filter in the probe; and 4) designing the probe to be large with respect to the sample line and allowing gravity to assist in separating particles from the gas phase. Sample Transport The sampling point and analyzer may be separated by hundreds of feet. It is necessary, therefore, to divert a portion of the process stream (through an appropriate probe), transport


it to the analyzer, and return it either to the process stream at a lower pressure point or to a suitable waste disposal system. The time necessary to transport the sample to the analyzer can, in some cases, constitute the largest share of the system dead time or turnaround time. Mounting the analyzer closer to the sampling point is the most direct way of decreasing this time. Lacking this alternative, the next best solution is to increase the flow sample in the line. An added plus to using a so-called fast loop is that it provides additional mixing due to turbulent flow, and thus further ensures a representative sample. Sample transport time is a function of sample line length, line diameter, the absolute pressure on the line, and the sample flow rate. Neglecting compressibility, the transport time for a gas sample is given as t lag =

VL(p + 14.7)(530) F(14.7)(t + 460)

8.12(4)

where V = volume of sample per unit length, ml/cm L = length of sample line, cm F = volume flow rate (ml/min) of sample under standard conditions (14.7 PSIA, 70 °F) t = temperature of sample, °F p = sample line pressure, PSIG Additional time must be allowed to purge sample conditioning equipment and any unswept volumes that are present due to poor design. Commonly, systems are designed to provide sample flow rates between 0.5 and 15 GPM (2 and 50 l/min). Sample Conditioning Conditioning of the sample either to allow easier handling or to put it into a form acceptable to the analyzer is always required to some degree. Conditioning may be as simple as pressure reduction and filtering or as complex as scrubbing and drying. Pressure reduction is the most effective way to prevent condensation of a condensable gas sample, and maintenance of a minimum pressure is the most effective way to prevent a liquid sample from vaporizing. Thus, pressure regulators for vaporization and pressure reduction are a common element of a conditioning system. In all cases, conditioning must be accomplished without affecting the sample composition. Some common sample conditioning elements are given in Table 8.12mm. The basic technology of sample conditioning has not changed much over the last 40 years. Currently, a SCS is constructed using components (listed in Table 8.12mm) that have evolved from industries such as pneumatic and hydraulic service. These are mounted in a cabinet (which is usually secured to the outside wall of the analyzer shelter) and interconnected with tubing and fittings. Not being designed specifically to fit process analyzer needs, they have large dead © 2003 by Béla Lipták

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volumes that are difficult to purge, thus leading to decreased sample system performance. Recently, there has been some initiative directed toward changing the above-discussed conventional design to fit the 17,18 specific requirements of the process analyzer. The potential of a smart modular miniature process analyzer sampling system integral to the analyzer is being explored with the idea of decreasing the installed cost of the analyzer, increasing reliability of the system, and, at the same time, decreasing cost of manufacture. One problem with sampling systems, however, is that they are application dependent. Thus, the design must allow for the convenient connection of simple modules to form the more complex one required for the particular application. A proposed design (borrowed from the gas management systems used in the semiconductor industry) 19 shown in Figure 8.12nn uses a substrate assembly that provides the flow path for a process sample and consists of a variety of sample conditioning components. In turn, it is attached to a manifold system that will also accept various flow components and provide a flow path between two or 20,21 more substrates. Thus far, implementation of these ideas has been restricted to fairly simple and straightforward applications 22 such as clean, dry, light hydrocarbon streams. The SHS used in this case is diagrammed in Figure 8.12oo and consists of four substrate assemblies manifolded together to produce the product shown in Figure 8.12pp. The PGC transmitter described previously is offered as a single- or dual-process stream analyzer (with two sample calibration streams) complete with a modular sample handling system for close-coupling to the sample tap. Unfortunately, sampling systems usually fail during process upset conditions (just when the analysis is most needed to bring the process back into control). Thus, whenever possible, these systems should be overdesigned to be able to handle the upset condition. A process data sheet containing the information summarized in Table 8.12qq is required to design a proper sample conditioning system. This definition of all the process conditions such as pressure, temperature, phase, particulates, chemical composition, physical properties (viscosity, density, etc.), and any possible process upsets that might occur, such as breakthrough of contaminants and runaway thermal reactions, ensures a design that will guarantee a correct sample presentation to the analyzer or, alternatively, a controlled shutdown or process hold with accompanying alarm signal. Multistream Analysis The PGC must be able to accommodate at least two streams, the process stream and the calibration or standard line. The clean calibration sample can bypass the SHS and be plumbed directly into a stream-switching valve that will allow the process stream to be diverted while the standard sample is flowing through the analyzer sample loop (Figure 8.12rr). The sharing of one PGC among a number of process streams is not recommended but, unfortunately, is common

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TABLE 8.12mm Common SHS/SCS Elements Element

Function/Comments

Filter

Remove particulates from the sample. Filtering below 10 microns requires vigilant preventive maintenance unless some sort of self-cleaning filter can be devised (e.g., cyclone separation with automatic cleanout).

Vaporizing regulator

If possible, at least in the case of a PGC, it is best to present the sample as a vapor. The vaporizing regulator is best located at the sampling point (to provide short transport times for the sample), but it can be placed at the analyzer.

Condensors/separators

Used in applications where condensables are to be removed from the vapor sample

Coalescers

Used to force finely divided liquid droplets to combine into larger droplets so they can be separated by gravity

Knockout pots

Used to collect liquid that has been separated from the sample. Should be provided with automatic drains.

Aspirators, ejectors

This is the preferred pump (by creating a vacuum) if the sample pressure is not sufficient to drive the sampling system. It is inexpensive and quite reliable because of the absence of moving parts

Rotameters

Consists of a ball or float in a tapered tube. Commonly used to measure flow. It is quite unreliable as the float tends to stick when the steam becomes dirty. It is a good indirect indicator of filter integrity.

Pressure regulators

Nonbleeding type, using corrosion resistant stainless steel and Teflon. Used to regulate pressure between sampling point and the analyzer as a means of controlling sample flow rate. Since the quantity of sample is directly proportional to the sample in the loop, in most cases a “block and bleed” configuration is used at the analyzer to equilibrate the sample to atmospheric pressure (this still does not eliminate inaccuracies due to changes in atmospheric pressure).

Pressure gauges

Installed downstream from regulators as an aid in setting regulator pressure and as an aid in maintenance checks. Installing in high speed by-pass lines and downstream of the analyzer eliminates extended turnaround problems that can occur because of the time required to flush sample from the pressure gauge bourdon tubes.

Steam or electrical heat tracing

To avoid condensation in the sample lines.

FIG. 8.12nn Illustration of the architecture of a modular SHS with the various miniature SHS components surface mounted amd interconnected on a so-called substrate. Individual substrates can then be manifolded together to form the comple sample handling system. (Photograph ©2002 Swagelok Company; text ©2002 R. Annino.)



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FIG. 8.12oo Schematic of a simple SHS circuit for one process stream and a calibration sample. (From Goedert, M. and Guiochon, G., Journal of Chromatographic Science, 7, 323, 1969.)

FIG. 8.12pp Hardware implementation of the circuit shown in Figure 8.12oo minus the miniature flow controllers and the pressure indicator (guage). (From Goedert, M. and Guiochon, G., Journal of Chromatographic Science, 7, 323, 1969.)

practice in the industry as a way of justifying the cost of the PGC. It is questionable if the savings are real when one considers the increased complexity of the SHS/SCS (one for each stream) and the maintenance problems that are incurred. Add to this the fact that if the analyzer goes down, the analysis of more than one stream is affected—a result that may have more impact on product cost than purchasing separate analyzers. The availability of a lower installed cost PGC with its simpler modular SHS may affect a shift in this paradigm in the future. In any case, stream switching requires a design that prevents mixing of the process stream samples. The doubleblock-and-bleed-with-bypass purge illustrated in Figure 8.12rr is probably the most efficient circuit for accomplishing this objective. In this circuit, all sample streams are flowing © 2003 by Béla Lipták

continuously, even in the unselected lines, thus ensuring upto-date sample composition when selected. Sample stream 2 is selected in this case and continuously flushes the PGC sample loop and all other sample lines. Sample Disposal The vent of the PGC is almost totally carrier gas and is usually vented to the atmosphere, as is the sample loop vent line, unless the sample is toxic or otherwise environmentally harmful. In such cases, the sample must be vented to a specific waste disposal area. Alternatively, if a low-pressure point in the process stream is available, the sample can be returned to the process. Since the accuracy of the analysis is directly

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TABLE 8.12qq Minimum Process Sample Information Required for Proper SHS Design Liquid

Vapor

Process pressure and temperature

Process pressure and temperature

Bubble point at the process or highest ambient temperature (whichever is highest) and atmospheric pressure

Dew point and lowest ambient temperature

Viscosity

Average molecular weight

Specific gravity

Specific gravity

Sample composition under normal and upset conditions

Sample composition under normal and upset conditions

Sample return point pressure

Sample return point pressure

Distance between sampling point and the analyzer

Distance between sampling point and the analyzer

Required sample turnaround time (lag time of the SHS/SCS)

Required sample turnaround time (lag time of the SHS/SCS)

Maximum pressure for which the PGC sample valve is specified

Maximum pressure for which the PGC sample valve is specified

Whether the PGC valve can accept either liquid or vapor sample

Whether the PGC valve can accept either liquid or vapor sample

Minimum flow rate required through the PGC sample loop

Minimum flow rate required through the PGC sample loop

Corrosiveness of the sample (related to material compatibility)

Corrosiveness of the sample (related to material compatibility)

Sample toxicity

Sample toxicity

INSTALLATION

FIG. 8.12rr Dual solenoid sampling system with block, bleed, and back purge.

affected by the pressure of the sample loop (unless one is using normalization procedures), provision must be made to either measure this pressure or provide hardware to maintain a constant pressure. In summary, the SHS/SCS is a very important part of PGC hardware. Proper design and maintenance of this part of the system will ensure a large uptime for the analyzer. There has been some attempt to utilize the micromachined type of GC that has been used with some success, in the laboratory and as portable GC devices, as the analyzer 23 portion of a PGC. The advantages of adopting such a design are, analytically, to achieve faster analysis and, engineeringwise, to design a smaller analyzer. Such a design puts an even greater burden on the SHS to provide an absolutely particle-free sample. © 2003 by Béla Lipták

Typical utility provisions for PGCs are outlined in Table 8.12ss. Additional support services may be required, such as refrigerated or heated sample runs, an analyzer shelter, etc. As a result, installation costs account for the principal portion of a traditionally designed PGC system. One of the prime advantages offered by the PGC transmitter is a significant reduction in installation and operating costs. However, severe environmental conditions can prevail at many industrial locations that can have an impact not only on the instrument but also maintenance personnel. Thus, mounting the PGC in a walk-in environmentally controlled analyzer shelter may be necessary for the protection of maintenance personnel while they are working on the instrument. A very approximate breakdown of these installation costs is given in Table 8.12tt. The difference between the installed transmitter cost and that of the traditional PGC may be more striking when one considers the cost of running heat-traced sample lines. However, one must not be misled by claims that in all cases the sample lines to a transmitter will be shorter since it will be mounted close to the sample point. Where the transmitter is mounted will always be a compromise between short sample lines and accessibility for maintenance purposes. In addition, provision must be made for space to house the analyzer maintenance station. This unit may be located close by in a general-purpose area or located off-site and connected to the analyzer through a modem. Required gases (such as carrier gas, calibration gas, FID fuel, etc.) are stored as compressed gases in cylinders that should be placed in an easily accessible area outside, but fairly close to the analyzer shack.


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TABLE 8.12ss Typical Utility Requirement Item

Specification

Instrument air

4 scfm per heated zone, 2 scfm purge

Instrument air quality

Clean, dry, −40°C dew point, oil free, particles ≤ 5 ppm microns, ISA grade hydrocarbon free

Carrier and other gases

Compressed gases, application dependent, may include helium, nitrogen, hydrogen, air, etc.

Compressed gas quality

Hydrogen, 99.995% ultra pure grade Burner air, ≤ 1 ppm hydrocarbons and ≤ 5 ppm water Helium, ≤ 99.995% ultra pure grade Nitrogen, 99.995% ultra pure grade

AC power

115 VAC, 50/60 Hz 10 amp service (per oven) or 220/230 VAC, 50/60 Hz

TABLE 8.12tt Breakdown of Installation Costs Item

Traditional PGC

PGC Transmitter

Analyzer with basic SHS

45,000

33,000

Analyzer shelter (1/4 pro rata)

35,000

NA

Installation and startup Total*

20,000

5,000

100,000

38,000

* This is a minimum installation cost as each unit may require specific hardware/software depending on the specific PGC and the application. Sample/Utility Connection Costs Electrical 1 in. conduit 2 in. conduit

$ 28/ft $ 35/ft

Sample Lines Bare Heated

$ 24/ft $ 85/ft

Instrument Air 1 in. carbon steel Steam Line (insulated 1.5 in.)

$ 30/ft

SUMMARY Although the PGC is perhaps the most expensive of the commonly used process analyzers, it provides the compositional information that is required for tight quality control. The introduction of the truly field-mountable PGC with the remote diagnostic and maintenance procedures that involve one technician doing a quick field replacement of the designated module may lead to an even greater increase in the use of PGCs in process control applications. Acknowledgment The author thanks Stephen Staphanos of Rosemont Analytical and Robert Bade of Siemens/Applied Automation for their fruitful discussions concerning PGC design, and Peter


Vanvuren of Exxon/Mobil and the Center for Analytical Process Control for information regarding modular SCS design.

References 1. For example, see ABB’s models 8000/8100 BTU/CV Transmitters described at http://www.totalflow.com/Data%20Sheets/8000.htm. 2. Annino, R., Curren, J., Jr., Karas, E., Lindquist, R., and Prescott, R., Journal of Chromatography, 126, 301, 1976. 3. Annino, R., Journal of Chromatography A, 678, 279–288, 1994. 4. Smart Gas Chromatograph 3000, SL-53-364R (8M) 8/94,  Honeywell Inc. 5. Deans, D. R., Journal of Chromatography, 43, 43, 1984. 6. Maier, H. J. and Karpathy, O. C., Journal of Chromatography, 8, 308, 1962.

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7. Hildebrand, G. P. and Reilly, C. M., Analytical Chemistry, 36 47, 1964. 8. Villalobos, R. and Annino, R., JHRC, 13 764, 1990. 9. Dandeneau, R. and Zerenner, E. H., JHRC, 2 351, 1979. 10. Rigaud, M., Chebroux, P., Durand, J., Maclouf, J., and Madini, C., Tetrahedron, 44, 3935, 1976. 11. Madini, C., Chambaz, E. M., Rignaud, M., Durand, J., and Chebroux, P., Journal of Chromatography, 126 161, 1976. 12. Sternberg, J. C., Gallaway, W. S., and Jones, D. T .L., in Gas Chromatography, Brenner, N., Callen, J. E., and Weiss, M. D., Eds., New York: Academic Press, 1962, p. 231. 13. Amirav, A. and Jing, H., Anal. Chem. 67(18), 3305, 1995. 14. Annino, R. and Voyksner, R., Journal of Chromatography, 142, 131, 1977. 15. Annino, R., Caffert, C., and Lewis, E., Analytical Chemistry, 58, 2516, 1986. 16. Goedert, M. and Guiochon, G., Journal of Chromatographic Science, 7, 323, 1969. 17. Fussell, E., “NeSSI (New Sample/Sensor Initiative) at CPAC Fall 1999 Meeting,” InTech, September 19, 2001. 18. Dubois, R., VanVurreen, P., and Tatera, J., “Searching for Higher Ground: Unveiling NeSSI II,” presented at IFPAC, San Diego, CA, January 23, 2002. 19. Mracek, K., “Fluid Components for Small, Smart, Sampling Systems,” presented at 1999 Fall Meeting of CPAC, University of Washington, Seattle, WA, 1999. 20. Simko, D. M., “Miniature Modular Sample Systems: From Concept to Reality,” Program Session 238, User-Manufacturer Exchange, presented at PittCon 2002, March 20, 2002. 21. Doe, S., “Surface Mount Technology for Sample Conditioning Systems,” Program Session 238, User-Manufacturer Exchange, presented at PittCon 2002, March 20, 2002. 22. Cumbus, J., “Application of Smart Modular Sample Systems at an Olefins Plant,” presented at IFPAC, San Diego, CA, March 17–22, 2002. 23. For example, see the µPGC 100 offered by SRA Instruments, www. sra-instruments.com/ANGLAIS/adt2.htm.


Bibliography Annino, R. and Villalobos, R., Process Gas Chromatography: Fundamentals and Applications, Research Triangle Park, NC: ISA, 1992. Center for Process Analytical Chemistry, http://www.cpac.washington.edu. Clevett, K. J., Process Analyzer Technology, New York: John Wiley & Sons, 1986. Cornish, D. C., Jepson, G., and Smurthwaite, M. J., Sampling Systems for Process Analyzers, London: Butterworth. Dubois, R., van Vuuren, P., and Tatera, J., “New Sampling Sensor Initiative:” An Enabling Technology, 47th Annual ISA Analysis Division Symposium, Denver, CO, April 14–18, 2002. Fieldbus Tutorial, www.ta.eng.com/industry/mforum/tbtut/tbtut1.htm. Foundation Fieldbus-Frequently-Asked-Question, www.fieldbus.org/About/ FAQ/Answers. Fussell, E., “An Open Discussion on the Future of Fieldbus,” InTech, September 10, 2001. Guiochon, G. and Guillemin, C. L., Quantitative Gas Chromatography, Amsterdam: Elsevier, 1988. Houser, E. A., Principles of Sample Handling System Design, Research Triangle Park, NC: ISA, 1977. Huskins, D. J., General Handbook of On-Line Process Analyzers, Chichester, U.K.: Ellis Horwood. McMahon, T. K., “The New Sampling/Sensor Initiative,” Control, August 2001. Meyers, R. A., Ed., Encyclopedia of Analytical Chemistry: Instrumentation and Applications, New York: John Wiley & Sons, 2000. Sherman, R. E., Process Analyzer Sample-Conditioning System Technology, New York: John Wiley & Sons, 2002. Thomson, M., “Interfacing Sample Handling Systems for On-Line Process Analyzers,” www.measurementation.com.au/tp-1.htm, January 14, 2002. Van den Berg, F. W. J., Hoefsloot, H. C. J., and Smilde, A. K., “Selection of Optimal Process Analyzers for Plant-Wide Monitoring,” Analytical Chemistry, 74(3), 3105–3111, 2002. Verhappen, I., “The Basics of Analyzer Sample Systems,” InTech, May 20, 1999. Villalobos, R., “Process Gas Chromatography,” Analytical Chemistry, 47(11), 983A, 1975.

8.13

Chromatographs: Liquid L. P. ROOF

(1982)

B. G. LIPTÁK

Sampling System

(1995, 2003)

AT

Liquid Chromatograph To Receiver

Flow Sheet Symbol

Types:

Laboratory: thin-layer and paper chromatography

Column:

Liquid–solid absorption, liquid–liquid partition, gel permeation (exclusion), and ion exchange

Detectors:

Fiber-optic probes, differential refractive index, fixed- or variable-wavelength ultraviolet, dielectric constant, electrical conductivity

Type of Sample:

Liquid

Sample Pressure:

5 to 1000 PSIG (35 to 7000 kPa)

Sample Temperature:

60 to 300°F (16 to 149°C)


0 to 122°F (−18 to 50°C)

Contacting Materials:

Stainless steel, Teflon standard; all conventional materials available

Utilities Required:

Electrical power, carrier solvent, air at 100 PSI (700 kPa)

Repeatability:

±0.5% for most applications

Cycle Time:

3 to 20 min for most applications

Special Features:

Multicomponent readout, molecular weight readout

Costs:

Laboratory system component costs: HPLC columns cost from $300 to $1800; solvent delivery pumps range from $2000 to $5000; fixed-wavelength ultraviolet–visible (UVVIS) detectors cost $2500; variable-wavelength ones are about $5000; microprocessorbased data acquisition integrator costs about $3000. Complete process HPLC units cost about $50,000, and their installed cost is about $100,000.


Beckman Coulter (www.beckman.com) Gynkotek HPLC Inc. (www.gynkotek.com) Hewlett Packard (www.hp.com) HPLC Technology Co. (www.hplc.co.uk) Regis Technologies Inc. (www.registech.com) Rohm Haas (www.rhcis.com) Siemens Energy & Automation (www.sea.siemens.com) Thomson Instrument (www.hplc1.com) Waters Corp. (www.waters.com)

INTRODUCTION For the types of process samples that cannot be easily vaporized, the liquid chromatograph can be considered. In a liquid chromatograph column, the stationary phase can consist of

a finely powdered solid adsorbent packed into a thin metal column, and the mobile phase can be an eluting solvent that is forced through the column by a high-pressure pump. The mixture to be analyzed is injected into the column inlet and, after separation in the column, it is monitored at the 1289


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exit by a variety of detectors. A wide variety of liquid chromatograph packings, eluting solvents, and detectors are available. Their combinations are selected to obtain the desired resolution.

COMPARISON WITH GAS CHROMATOGRAPHS In many ways the liquid chromatograph is similar to the gas chromatograph. The basic difference is that the carrier is a liquid instead of a gas. All the other changes in the instrumentation used result from this difference. In fact, the only components that are significantly different are the components, that are in direct contact with the carrier solvent. These components will be discussed here. Section 8.12 described the gas chromatograph and should be referred to for a basic understanding of the operation of chromatographs. Carrier Flow A typical carrier flow diagram of the liquid chromatograph is illustrated in Figure 8.13a. The sample valve injects a measured volume of sample into the controlled flow of carrier liquid, which transports it through the columns and into the detector. Interaction between the stationary column-packing material and the flowing liquid carrier causes the sample components of interest to move through the column at different velocities, providing the separation. For each sample component, the detector provides an electrical signal proportional to its concentration in the carrier. The electrical signal is recorded as a chromatogram or otherwise displayed in any of the standard chromatographic forms. Flame Proof Vent

Cover

The Main Components The instrument construction is similar to that of the gas chromatograph. The liquid chromatograph is divided into two parts: the analysis section and the control section. The analysis section is located near the process stream and is often contained in an instrument house. This section includes 1) the chromatographic oven containing the valves, columns, and detector; 2) the carrier supply; 3) an electronics compartment containing the circuitry for the detector, temperature control, valve operators, and local data handling; and 4) the sample preparation system, which may be in a separate temperaturecontrolled oven. The control section is often located in the control room and includes the programmer to control the instrument and process the data, and the data display, which may include a strip-chart recorder, a digital display, or a digital computer for further data processing. The programmer discussed here includes stream selection, which may be a physically separate unit. However, the liquid chromatograph operating conditions are different from those of the gas chromatograph. In particular, the carrier pressure is typically 1000 PSIG (7 MPa) at a flow rate of 1 ml/min, and the chromatographic cycle time is typically 3 to 20 min.

HPLC COLUMN SELECTIVITY AND RESOLUTION As the constituents of interest pass through high-pressure liquid chromatograph (HPLC) columns, they are retarded due to the selective absorption or adsorption effects of the column packing. The more efficient a column, the more able it is to produce narrow peak bands during the elution of the sample.

Relief Valve

In

Solvent Tank

Sample Out

FCV PI Filter

Sample Valve

Pump

Waste Column Detector

PCV PI

Chromatograph Oven

FIG. 8.13a Carrier flow diagram of the liquid chromatograph.


Solvent

8.13 Chromatographs: Liquid

Process Out

B A B

B A B

B

A

A B

A B 1 A B

A B 2

A

Process In Pumps

single-component solvent or a blend of solvents. The selection and uniformity of solvents and solvent blends are critical to the liquid chromatograph, affecting both component separation and detector response. A typical carrier reservoir is a 15-gal (57-1) stainless steel tank equipped with a flame-proof vent and a relief valve for flammable solvents. Some solvents are affected by oxygen or moisture in the air, and an inert gas, such as nitrogen, is either bubbled through or blanketed over the carrier in the tank.

A

Effluent

Supply Pumps

A B

B

Injection Valves

A

B

3 Chromatographic Columns

Detectors

FIG. 8.13b Relative efficiency, selectivity, and resolution of chromatographic 1 columns.

TABLE 8.13c Selectivity Coefficient Names Used in Case of Different Columns

Column Type

Name Used for Selectivity Coefficient

Liquid–solid adsorption

Adsorption

Liquid–liquid partition

Partition

Gel permeation (exclusion)

Permeation

Ion exchange

Distribution

For example, column 1 in Figure 8.13b is inefficient, while columns 2 and 3 are efficient. Although the efficiencies of columns 2 and 3 are similar, their selectivities—the relative affinity of the sample constituents for the packing—are different, and the selectivity of column 3 is better, or higher. The coefficient, which expresses selectivity (Table 8.13c), is called by different names as a function of the type of column used. Selectivity can be improved by increasing column size or by raising the ratio of distribution factors. The combined effect of efficiency and selectivity is called resolution. In Figure 8.13a, the resolution of column 3 is the best. Unfortunately, the steps that will improve resolution (smaller tubing diameter, smaller injection volume, finer packing, larger or multiple columns) also tend to extend analysis time and decrease downtime and maintenance due to plugging.

CARRIER SUPPLY The carrier supply is made up of 1) the liquid carrier, 2) a reservoir to hold it, 3) a carrier pump, 4) pressure and flow controls, and 5) a filter and gauges. The carrier is either a


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There are two types of carrier pumps: air-driven and motordriven. Both are reciprocating pumps containing check valves to divert the liquid through the pump. Air-driven pumps are powered by a large-diameter air piston, which is connected to a small-diameter liquid piston. The pump recycles at the end of each stroke. The air-driven pump provides a pressure, relatively independent of flow, that is nominally some multiple of the air pressure. Motor-driven pumps are powered with an electric motor connected to an eccentric that drives one or more liquid pistons. The motor-driven pump provides a flow, relatively independent of pressure, that is controlled by motor speed or piston stroke length. Pressure and Flow Controls Pressure and flow control depend on the type of pump used. Air-driven pumps provide a pressure that is reduced to the desired value with a diaphragm pressure regulator. Flow control is provided with a diaphragm-type differential pressure regulator that controls a preset pressure difference across a fixed restriction. This system has the advantages of flexibility and of providing a continuous, not pulsating, flow, but requires numerous components. Carrier control with motor-driven pumps is generally limited to flow control. Two flow control methods are in use. The first option is to manually set the motor speed or pump stroke to the desired flow. This has the advantage of simplicity, but the quality of control is often insufficient. If automatic, closed-loop control is provided; the quality of control is improved, but the system becomes more complex and expensive. Valves Liquid chromatographic valves operate on the same principles as gas chromatographic valves. The air-operated diaphragm valves, plug valves, and rotary valves discussed in Section 8.12 are also used. These valves are strengthened and use higher seal forces due to the high operating pressures. They also have smaller internal flow passage to reduce sample–carrier mixing in the valve.

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COLUMNS Liquid columns are packed in a 1 4 - or 3 8 -in. (6- or 9.6-mm) -outer-diameter straight stainless steel tube 2 to 12 in. (50 to 305 mm) long. The total column length in an instrument rarely exceeds 4 ft (1.2 m) and is more often 1 ft (0.3 m) or less. The columns are filled with particulate packing that is typically in the 5- to 10- micron range. There are four types of liquid columns classified by the principle of separation that is utilized in them: liquid–liquid columns, liquid–solid columns, size exclusion columns, and ion exchange columns. Table 8.13d lists a few typical column applications. Liquid–Partition Columns Liquid–liquid columns (liquid–partition columns) are filled with solid support, usually silica, coated with a liquid stationary phase. To prevent loss of the liquid, it is usually chemically bonded to the support. The separation depends on the relative solubility of the sample components in the stationary and mobile, or carrier, phases. Liquid–liquid separations are subdivided into normal and reverse phase separations. In normal phase separation, the stationary phase is more polar (in the chromatographic sense, not dipole moment) than the mobile phase. While in reverse phase separation, the stationary phase is less polar than the mobile phase. When sample components can be separated with either type, the components elute from the column in reverse order when changing from the normal to the reverse phase.

TABLE 8.13d Typical Column Applications Column Type

Applications

Liquid–liquid, normal phase

Phenols Esters Pigments Cooking oils

Liquid–liquid, reverse phase

Pesticides Herbicides Organic aromatics Water pollutants

Liquid–solid

Plasticizers Antioxidants Organic acids Water pollutants

Size exclusion

Polymers Resins Carbohydrates Hydrocarbons

Ion exchange

Inorganic ions Dies Detergents Sugars


To improve separation, the binary mixture of solvents can be changed in the carrier. This is called solvent programming or gradient elution and is comparable to temperature programming in gas chromatography. Its use is limited almost exclusively to laboratory analysis. Liquid–Adsorption Columns In this case, the liquid–solid columns (liquid–adsorption columns) are filled with an adsorbent. Silica is the most common, but alumina and even charcoal are also used occasionally. Separation occurs as the sample components are retained by the adsorption sites on the packing. The mobile phase competes with the sample for adsorption sites and displaces the sample so it can move through the column. As with liquid–liquid columns, the mobile phase is chosen or blended to provide the desired separation of sample components. Solvent programming can also be used. Gel–Permeation Columns In gel-permeation chromatography, sample components are separated on the basis of their molecular size. Large molecules move straight down the column, while small molecules stick in the pores of the porous beads of the gel and are retarded. Size exclusion columns (steric exclusion columns or gelpermeation columns) are filled with a porous solid, such as silica, or a porous polymer, such as cross-linked polystyrene (Figure 8.13e). The pore size in the column varies uniformly over a specific range, for example, 300 to 600 Å. While moving through the column in the mobile phase, a sample component with a chain length (or more correctly, hydrodynamic volume) of 400 Å can diffuse into any of the pores that are 400 Å and larger. On the other hand, a sample component with a chain length of 500 Å can diffuse into only Polymer Molecules

Column Packing

Solvent Flow Chromatographic Column

FIG. 8.13e Three stages in the chromatographic separation of polymers: (left) at sample injection; (center) during separation; (right) at sample elution.

8.13 Chromatographs: Liquid

those pores 500 Å and larger. Since it enters fewer of the stationary pores, the 500 Å component moves through the column faster than the 400 Å component. Thus, long-chain or high-molecular-weight compounds elute from the column first. The mobile phase has no direct effect on the separation process in the columns.

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Refractive Index The refractive index detector measures the difference between the refractive index of the sample compounds and the carrier. With the proper choice of carrier, it is sensitive to all sample compounds, but the sensitivity is generally lower than that of the optical absorbance detector. It is also quite sensitive to temperature and carrier composition variations.

Ion Exchange Columns For process samples containing ions, ion-exchange chromatography can be used. These columns can be packed with ion-exchange resins, which contain exchangeable ions and therefore can separate the ions in the process sample from the neutral or oppositely charged components. Ion exchange columns are filled with a cross-linked polystyrene resin containing charge-bearing functional groups on its surface. The stationary phase is called anion exchange if the functional groups are positively charged, and cation exchange if negatively charged. For the sample ions to be retained, they must be of opposite charge to the functional groups. The mobile phase is generally water containing a fixed quantity of ions of the same charge as the sample and buffered to a specific pH. The sample ions are separated, depending on how strongly they interact with the functional groups on the stationary phase in competition with the ions in the mobile phase. The quantity, the type of ion, and the pH in the mobile phase control the separation. Electrophoresis Mixtures of ions can also be analyzed by using a column of polymeric gel, which is saturated with an electrolyte. In this case, the sample to be analyzed is spotted onto the gel and two electrodes are connected (5000 V), which cause the migration of positive ions toward one and the negative ones toward the other. This method is used in analyzing mixtures of proteins.

Dielectric Constant The dielectric constant detector measures the difference between the dielectric, constants of the sample compounds and the carrier. Because, for compounds with no dipole moment, refractive index and dielectric constant are related, the advantages and disadvantages of this detector and the refractive index detector are similar. However, if, for example, the sample compounds have a dipole moment and the carrier does not, the dielectric constant detector has a higher sensitivity and more uniform response than the refractive index detector.

APPLICATIONS The liquid chromatograph extends the advantages of the chromatograph to nonvolatile and thermally unstable samples, as well as polymers and inorganic salts. The prime advantages are the ability to both qualitatively and quantitatively analyze multicomponent streams with the versatility to analyze a wide range of samples. When other instruments provide a satisfactory analysis, the liquid chromatograph is rarely used because it is more complex, more expensive, and sometimes less sensitive, and it requires a longer analysis time. However, there are many sample streams that can be analyzed satisfactorily only with the liquid chromatograph. Reference

DETECTORS

1.

All detectors used in the liquid chromatograph are simplified, small internal-volume versions of other analyzers. The most common ones are based on the measurement of optical absorbance, refractive index, or the dielectric constant.

Bibliography

Optical Absorbance The optical absorbance detector measures the absorption of a fixed wavelength in the ultraviolet or visible spectrum. It is highly sensitive to many absorbing sample compounds and relatively insensitive to external effects, such as temperature, flow, and carrier composition. It can only measure absorbing compounds in a nonabsorbing carrier.


Miller, T. E., “Process Liquid Chromatography: The Next Step in OnStream Analysis,” InTech, September 1981.

Ahuja, S., Selectivity and Detectability Optimizations in HPLC, New York: John Wiley & Sons, 1989. Covey, T., “Liquid Chromatography/Mass Spectrometry for the Analysis of Protein Digests,” Methods of Molecular Biology, 61: 83–99, 1996. Dolan, J. W. and Snyder, L. R., Troubleshooting Liquid Chromatography Systems, Totwa, NJ: Humana Press, 1989. Dubois, R., van Vuuren, P., and Tatera, J., “New Sampling Sensor Initiative”: An Enabling Technology, 47th Annual ISA Analysis Division Symposium, Denver, CO, April 14–18, 2002. Fussell, E., “Molding the Future of Process Analytical Sampling,” InTech, August 2001, 32. Gjerde, D. et al., Ion Chromatography, Berlin: Springer-Verlag, 1987.

1294


Guillemin, C. L., “Process Liquid Chromatography: Promises and Problems,” InTech, August 1982. Jutila, J. M., “Guide to Selecting Gas and Liquid Chromatographs,” InTech, August 1980. Kenkel, J., Analytical Chemistry for Technicians, Chelsea, MI: Lewis Publishers, 1988. Lindsay, S., High Performance Liquid Chromatography (Analytical Chemistry by Open Learning Series), 1992. Mehl, J. T., Nicola, A. J., et al., “Direct Coupling of Thin-Layer Chromatography with Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry,” American Laboratory, 30–38, 1998. Meyers, R. A., Ed., Encyclopedia of Analytical Chemistry: Instrumentation and Applications, New York, John Wiley & Sons, 2000. Miller, T. E., “Sample System for Automated Liquid Chromatography,” Industrial Research & Development, August 1980.


Mowery, R. A., “Process Liquid Chromatography,” in Automated Stream Analysis for Process Control, New York: Academic Press, 1982. Niessen, W., Liquid Chromatography–Mass Spectrometry (Chromatographic Science), Vol. 79, 1998. Pasch, H. and Trathnigg, B., HPLC of Polymers (Springer Laboratory), 1998. Sherman, R. E., Process Analyzer Sample-Conditioning System Technology, New York, John Wiley & Sons, 2002. Snyder, L. R. et al., Practical HPLC Method Development, New York: John Wiley & Sons, 1988. Thomson, M., “Interfacing Sample Handling Systems for On-Line Process Analyzers,” www.measurementation.com.au/tp-1.htm, January 14, 2002. Uwe, D. N. et al., HPLC Columns: Theory, Technology, and Practice, 1997.

8.14

Coal Analyzers D. H. F. LIU

(1995)

B. G. LIPTÁK

(2003)

Types:

Thermogravimetry (TG), oxygen combustion bomb, total sulfur analysis, x-ray fluorescence (XRF), atomic absorption spectrophotometry (AA), coal slurry analyzers, prompt gamma neutron activation analyzers (PGNAAs), and the pulsed neutron type

Precision of Measurements:

Heating value, ±200 to 300 BTU/lb; ash content, ±0.05 wt%; and moisture content, ±0.05 wt% moisture


Fisher Scientific (www.fisher.co.uk) Gammametrics/Thermo Electron (www.gammametrics.com) Kanawha Scales (www.kanawhascales.com) Leco Corp. (www.leco.com) Parr Instrument Co. (www.parrinst.com) Science Applications Inc. (www.saic.com)

INTRODUCTION Coal is widely used as a source of power and heat by the chemical, paper, cement, and metal industries. In the U.S., there are some 900 coal preparation plants and coal-fired power plants, and some 100 coal analyzers have already been installed in them. One key consideration in operating coalburning facilities is the control of SO2 emissions to the atmosphere from coal-fired power plants. Because the most economical method of reducing the sulfur content of coal is through the blending of various coals, on-line coal analyzers are often needed. The characteristics of coal are monitored for environmental protection, quality assurance, and process control. This section will describe the instrumental methods for: 1. Proximate analysis: Used to establish the rank of coals, to show the ratio of combustible to incombustible constituents, to provide the basis for buying and selling, to evaluate the benefits, or for other purposes 2. Gross calorific value: Provides the basis for buying and selling 3. Sulfur analysis: Used in coal preparation and in the determination of potential sulfur emissions from coal combustion or conversion processes, or in the determination of coal quality against contract specifications 4. Composition analysis of major and minor elements in coal and coke ash: Predicts slagging and fouling characteristics of combusted materials and potential use of ash by-products (see American Society for Testing

Materials (ASTM) Standards D346 and D2013 on methods for collection and preparation of coal samples for laboratory analysis) The various methods for the on-line monitoring of coal streams are described in the paragraphs that follow. THERMOGRAVIMETRY Due to the recent addition of microcomputer control and dedicated data reduction, the thermogravimetry (TG) technique has become popular for routine approximate analysis of coal and coal products. The TG unit performs a multistep analytical sequence automatically and unattended. The sample is loaded into a two-arm furnace tube (Figure 8.14a) and is sequentially dried and burned; the residue is then weighed. The tube allows the active gas (air or oxygen) to enter near the top of the furnace rather than through the balance mechanism. The low-mass furnace provides both rapid heating rates and a short cooldown time. The microcomputer controller provides automatic switching between the purge gas and the active gas. Bituminous Coal Analysis Figure 8.14b shows a typical proximate analysis of a bituminous coal using the automated TG system. The furnace is heated to 230°F (110°C) at a rate of 140°F/min (60°C/min) and is held isothermally for 5 min while water is vaporized off from the coal sample. The furnace is then heated at a rate 1295


1296


Balance Arm

Active Gas Inlet

Antistatic Tube

Anticonvection Shield

Oxygen Bomb

Sample Pan Purge Gas Outlet

Furnace

Bucket

FIG. 8.14c Oxygen combustion bomb and bucket used in the isoperibol calorimetry. (Courtesy of Parr Instrument Co.)

Thermocouple

FIG. 8.14a Two-arm furnace tube for active gas introduction in the microcomputer-controlled thermogravimetry of coal and coal products. (Courtesy of the Perkin-Elmer Corp.)

purge is switched back to nitrogen, and the system automatically cools back to load temperature. The elapsed time of the proximate analysis program and cooling of the tube back to load temperature totals 30 min.

GROSS CALORIFIC VALUE H2O: 0.9%

100

950°C Weight Profile

Volatiles: 23.5%

Weight %

80

Temperature Profile

60 Fixed Carbon 67.0%

40

Cycle Time: 30 Min.

20 110°C Ash: 8.5%

0 0

Time (Min) N2

30 O2

N2

Gross calorific value is determined by burning a weighed sample of coal. A calibrated isoperibol oxygen bomb calorimeter is used for that purpose under controlled conditions (Figure 8.14c). The bucket, which holds the oxygen bomb, provides good circulation and rapid thermal equilibrium for the bomb. Thermal jacketing is provided by a circulating water system, which maintains cooling water flow around the bucket. A microprocessor control system monitors and controls the jacket temperature, fires the bomb, and monitors the temperature in the bucket. The test continues until the controller determines that equilibrium has been reached. A microcomputer uses the sample weight and temperature data—applying correction for acid, sulfur, fuse, and any added combustion aids—to calculate the gross calorific value.

Purge Media Used

Fig. 8.14b A typical proximate analysis of coal by microcomputer-controlled thermogravimetry.

of 176°F/min (80°C/min) to 1742°F (950°C) and held for 7 min while nitrogen is flowing through the TG, until all volatile matter is expelled from the sample. After the nitrogen purge, the purge gas is switched to either air or oxygen and the fixed carbon content of the char is oxidized, leaving the ash content as the residue. The values from these determinations are read directly in weight percent from the chart recorder. The air or oxygen


TOTAL SULFUR ANALYSIS An ASTM method for determining sulfur in coal uses the washings from the oxygen bomb calorimeter. Sulfur is precipitated as barium sulfate from the washings. The precipitate is filtered, ashed, and weighed. An automatic titrimetric system is also available for rapid sulfur determination using the oxygen bomb washings. The washings are titrated with a lead perchlorate solution to obtain a lead precipitate. The titration takes place in a nonaqueous medium to ensure complete precipitation and a sharp end point with a lead ion-selective electrode.

8.14 Coal Analyzers

Resistance Heating - 1350°C

Open Port Combustion Tube Flow Controllers

Purge

F

Sample

ASH ANALYSIS

Collection Moisture Traps Dust Trap

Measurement Unit

within the infrared spectrum. Total sulfur as sulfur dioxide is determined on a continuous basis.

Lance

F

Oxygen

Solid State IR Detector

Pump

Stop Reusable Boat Exhaust

Electronics and Microprocessor

Integral Balance

System Inflow

FIG. 8.14d Apparatus for the determination of sulfur by the infrared detection method.

Other procedures use high-temperature tube furnace combustion methods for rapid determination of sulfur in coal and coke using automated equipment. The instrumental analysis provides a reliable and rapid method for determining sulfur contents of coal or coke. Figure 8.14d illustrates the high-temperature combustion method of sulfur detection using infrared absorption detection procedures. The sample is burned in a tube furnace at a minimum operating temperature of 1350°C (2462°F) in a stream of oxygen to oxidize the sulfur. Moisture and particulate matter are removed from the gas by traps filled with magnesium perchlorate. Sulfur dioxide is measured, using an infrared absorption detector. Sulfur dioxide absorbs infrared energy at a precisely known wavelength

The major or minor elements in coal ash can be determined using x-ray fluorescence (XRF) techniques. The ash is fused with lithium tetraborate or other suitable flux and either ground or pressed into a glass disk. After that, the pellet or disk is irradiated, using an x-ray beam of short wavelength (high energy). The characteristic x-rays of the atom that are emitted or fluoresced upon receiving the primary rays are dispersed, and their intensities are measured at selected wavelengths by sensitive detectors. Detector output is converted into concentration by computerized data-handling equipment. All elements are determined and are reported as oxides. They include Fe, Ca, K, Al, Si, P, Mg, Ti, and Na. These major and minor elements can also be determined by atomic absorption spectrophotometry (AA). See Section 8.22 for a more detailed description of XRF and AA. ON-LINE MONITORS Figure 8.14e shows a schematic of a continuous monitor of moisture, ash, and BTU in coal. In this system, a microwave analyzer measures the moisture content of the coal, without requiring physical contact with the solids. (See Section 8.34 for an explanation of the principles of a microwave absorption hygrometer.)

Photon Detector Photons Photons

Input Hopper Adjustable Gate Microwave Moisture Meter

FE

Neutron Shield Dewar

S

S H

Mass Sensor Tachometer

Neutron Shield

Reflector with Nuclear Source Emission Source (Neutrons)

FIG. 8.14e Coal analyzer. (Courtesy of Science Applications Inc.)


1297

Belt Exit Hopper

Gamma Detector

1298


The fingerprint of a given type of coal is its distinctive gamma spectrogram. This is produced by the detection and counting of photons released from atomic nuclei in the coal as it passes over a small source of neutron emissions. The precision of the measurements of heating value, ash, and moisture content varies with the type of coal, typically 200 to 300 BTU/lb, 0.05 wt% ash, and 0.05 wt% moisture.

References 1. 2. 3.

www.kanawhascales.com/CoalScan/9500/9500.htm www.gammametrics.com/gmm2/GMMcsa.html A prototype elemental coal analyzer based on pulsed neutrons, www.wku.edu/Dept/Academic/Ogden/Phyast/API/research/coal.htm

Bibliography Gamma-Based Analyzers The Harwell spectrometer is used in the prompt gamma neu1 tron activation analysis (PGNAA) in-line coal analyzer. This package also measures coal density and moisture content. This multiple detector package also includes a feed hopper to direct the coal into the analyzer and a discharge conveyor, which takes away the coal under variable speed control. The system also includes an RS422–RS232 interface and remote displays for reporting the readings made by the analyzer. An analyzer is also available for determining the ash and 2 solids contents of coal–water mixtures or coal slurries. This analysis is made by the use of three probes, which are immersed into the coal slurry stream. The ash probe uses a source of lowenergy x-rays to measure not only the ash content, but also the concentration of iron. The density probe used gamma rays to detect the percent solids (slurry density) of the mixture. The low-intensity neutron source in the aeration probe detects the amount of air in the slurry, so that the density measurement can be corrected for aeration effects. Recent Developments Western Kentucky University is developing a prototype coal analyzer operated with microsecond-wide 14-MeV neutron pulses and containing several gamma ray detectors. This analyzer measures the density and sulfur content of coal along with its BTU, moisture, and volatile matter content. This pulsed fast/thermal neutron analyzer is self-calibrating and provides improved accuracy in the determination of elements such as carbon, oxygen, and sodium. If you want to follow this developmental effort, refer to Reference 3.


ASTM standards on coat testing, sampling, and analysis, American Society for Testing and Materials, West Conshohocken, PA, www.astn.rog/. Baur, P. S., “Control Coal Quality through Blending,” Power, March 1981. Brown, D. R., “Coal BTU Measurement Study: Monitoring of Moisture in Coal,” Research Project 983–1, Vol. 5, Electric Power Research Institute, Palo Alto, CA, 1979. Brown, D. R., Gozani, T., and Bozorgmanesh, H., “Nucoalyzer On-Line Nuclear Analysis of Coal,” 1980 Coal Technology, Houston, TX, November 1980. Buckler, R. J., “Nuclear Assay of Coal,” Research Project 983–1, Vol. 8, Electric Power Research Institute, Palo Alto, CA, January 1979. “Coal Analyzer Determines Sulfur Content On-Line,” Power, September 1980. Cooper, H. R., On-Stream X-Ray Analysis, A.M. Gaudin Memorial, Vol. 2, Fuerstenau, M.C., Ed., New York: American Institute of Mining, Metallurgical, and Petroleum Engineers, 1976, pp. 865–894. Cooper, H. R., “Using On-Stream X-Ray Fluorescence,” InTech, July 1981. Gleit, A., Coal Sampling and Analysis, Westwood, NJ: Noyes Publications, 1986. Gozani, T., et al., “Nuclear Assay of Coal,” Research Project 983–1, Vols. 1–8, Electric Power Research Institute, Palo Alto, CA, April 1980. Gozani, T., Hogg, R., and Luckie, P., “Coal Rheology and Its Impact on Nuclear Assay,” Research Report 983–1, Vol. 7, Electric Power Research Institute, Palo Alto, CA, April 1980. Harrison, J. W., “Coal Sampling Systems and Quality Control,” Coal Technology 80, Vol. 4, Houston, TX, November 18–20, 1980. Kiuru, E. S., “A New Instrument for Accurate On-Stream XRF Measurement,” PROMECON ’81: Proceedings of the Process Measurement and Control Conference, Instrument Society of America, Research Triangle Park, NC, 1981. “Overview of Coral Conversion Process Instrumentation,” Report ANL-FE49628-TM01, Argonne National Lab, Argonne, IL, May 1980. “Reading the Composition of Coal,” EPRI Journal, July/August 1980. Subramanian, T. K., “How to Calculate BTU Values of Coal,” in Cool Age Operating Handbook of Preparation, Vol. 3., Merrit, P. C., Ed., New York: McGraw-Hill. Yeager, K., “Nuclear Analysis of Coal,” EPRI Journal, June 1981.

8.15

Colorimeters J. E. BROWN

(1969, 1982)

Color

B. G. LIPTÁK

(1995)

M. W. REED

AT

(2003) To Receiver Flow Sheet Symbol

Types:

Spectrophotometers and colorimeters (one-dimensional and tristimulus)

Types of Samples:

Liquid, gas, or solid

Wavelength Ranges:

200 to 1000 nm is typical

Spectral Resolution:

2 to 20 nm

Source Lamp:

Halogen, Xenon flash, LED, IR, fluorescent, fiber optics

Detectors:

CCD and photodiode linear arrays or photocells


Standard materials, ordinary or quartz glass

Cell Length:

From 0.5 to 1000 mm

Sample Pressure:

Vacuum to 150 PSIG (10.6 bars)

Inaccuracy:

1 nm for array spectrophotometers, 1% for colorimeters

Costs:

$100 for a single-channel color analog channel; $1000 for fiber-optic spectrophotometers; on-line scanning systems cost $20,000 to $60,000


Bayer Diagnostics (www.glucometer.com) Byk-Gardner, Inc. (www.bykgardner.com) Brinkmann Instruments (www.eppendorfsi.com/search.asp) Cole-Parmer (www.coleparmer.com) DataColor International (www.colorite.com) Dolan Jenner Industries (www.dolan-jenner.com) Fisher Scientific Co. (www.fishersci.com) Guided-Wave (www.guided-wave.com) Hach Chemical Co. (www.hach.com) Honeywell Microswitch Div. (www.honeywell.com) HunterLabs (www.hunterlab.com) Mahlo International (www.dfmg.com.tw/dhtml/%BCe%BCw/mahlo.htm) Minolta Inc. (www.dimage.minolta.com) Monitek (www.monitek.com/Metrisa/contactinfo.asp#monitek) Ocean Optics Inc. (www.oceanoptics.com) Omega Engineering (www.omega.com) Rosemount Analytical (www.emersonprocess.com/RAlhome) Starna (www.starna.com/d_color/x_color.html) Thermo Electric Corp. (www.thermo.com/eThermo/CDA/Products/Pr)


1300


TABLE 8.15b Color and Wavelength Association Approximate Wavelength (µm)

FIG. 8.15a Turbidity measurement.

INTRODUCTION Colorimetry involves the transmission, absorption, or reflectance of visible light and can be extended into the ultraviolet (UV) and near-infrared (NIR) spectrum. Gratings used in current spectrophotometers can be specified over a range of 200 to 1200 nm, with the visible range being from 400 to 700 nm. Visible color can be measured as an indication of a concentration or as an indication of pH in titration using dyes. Colorimeters and spectrophotometers associated with autotitrators and wet chemistry analyzers are discussed in a separate section of this chapter. Visual colorimetry involves the matching of the sample color against standards in cuvettes, Nessler tubes, or thin films. Blood glucose meters use glucose oxidase on test strips and measure multiple-wavelength absorbances proportional to glucose concentration. Simple color wheel kits are used in water quality analysis to match depth of color in the liquid sample with a reference color calibrated in the concentration of the specific chemical end group for such measurements as pH or arsenic concentration. These are being replaced with low-cost hand-held colorimeters and spectrophotometers, which can be programmed for field use. This includes methods for turbidity measurement by nephelometry, which uses the ratio of scattered to transmitted light in suspensions, as shown in Figure 8.15a. If colored filters such as RGB (red, green, and blue) are used, the device is a colorimeter. If a grating or prism is used, the device is a spectrophotometer. Due to the current technology of matching of CCD and diode arrays to entire spectral images, an entire spectral curve may be measured continuously. This is usually the case when the color of paper, textiles, films, or solids is being measured. COLOR MEASUREMENT Color measurements involve that part of the electromagnetic spectrum that is sensed by the human eye and brain. This region is approximately 400 to 700 nm. The colors of the rainbow are associated with specific wavelengths of visible light as listed in Table 8.15b.


Associated Color

400–450

Violet

450–500

Blue

500–570

Green

570–590

Yellow

590–610

Orange

610–700

Red

Industrial standards involve color matching in which color standards are compared either by eye or with the assis1 tance of a spectrophotometer. In the world of art, separate color-matching methods have evolved that are based on color wheels that could be correlated to machine vision by taking spectral measurements of the subjective reference colors. Absorbance and Transmittance Colorimetry Absorbance spectra from spectrophotometers are used in the field of colorimetry of liquids where spectral absorbance curves can be compared one-dimensionally. The reason for converting to absorbance is that a logarithmic relation typically exists between transmittance at a certain wavelength and the concentration of colorants in liquid solutions. This 2 is known as the Beer–Lambert law. Absorbance curves are generated from transmittance data with absorbance defined as the negative of the natural logarithm of T/To, where To is the transmittance of the reference standard. This standard might be a white reference tile or a cuvette of distilled water, depending on the sample application. If the spectral curve is from light reflected from a solid, these reflectance curves can also be curve-fit and compared one-dimensionally. This is suitable for quality control of materials. When consumer products are accepted or rejected based on their color perception by humans, tristimulus model parameters such as XYZ, Lch, or Lab are generated from spectral data and tolerances are set. SPECTROPHOTOMETRIC ANALYZERS Interference filters are selected for desired wavelengths as determined from the spectral relationship curves. Detector photoconductors are chosen to give spectral response in the visible region. All known photo detectors are least sensitive in the blue end of the spectrum. This can be dealt with by using prefilters or by using narrow spectral ranges, which are calibrated for more sensitivity. Analyzers have to be built to carry out colorimetric analysis based on existing standards. Hundreds of one-dimensional color standards are used in the process industries. Other organizational databases include the American Public Health Association (APHA), the National Bureau of Standards (NBS), the National Institute of Standards and Technology

8.15 Colorimeters

1301

TABLE 8.15c Saybolt Color Scale

—

1

2

Depth of Oil, in. (mm)

Color Number

20 (508)

+30

16 (406)

+28

12 (305)

+26

20 (508)

+25

16 (406)

+23

10.75 (273.05)

+20

8.25 (209.55)

+18

6.25 (158.75)

+16

10.50 (266.7)

+15

9.00 (228.60)

+13

7.25 (184.15)

+10

5.75 (146.05)

+5

4.50 (114.30)

0

3.50 (88.90)

–5

3.00 (76.20)

–9

2.50 (63.50)

–13

2.125 (53.975)

–16

(NIST), the American Association of Textile Chemists and Colorists (AATCC), and the American Society for Testing and Materials (ASTM). All are accessible on the Internet. The Saybolt color scale in Table 8.15c is similar to ASTM D156, “Test for Color of Petroleum Products.”

FIG. 8.15d Fiber-optic spectrophotometer.

2.0 Tristimulus Value, CIE Standard

Color Standard Number

Z 1.5 Y

1.0

.5

X

X mu

0

Spectrophotometer Design The design of fiber-optic spectrophotometers is shown in Figure 8.15d. These units are small enough to fit in one’s hand and, if purchased in larger quantities, are sold at unit costs of $500. Improvements in spectrophotometers include a flashed xenon light source with dual-beam measurement. This is actually necessary for flashed light sources due to the variability of flashes. Dual-beam machines measure the spectrum of both the light source and the reflected light for each measurement. Spectrophotometers are characterized by the geometry of the light source and by the view of the photosensitive device. A common geometry is the so-called 45/0 design. This is achieved by illuminating the sample with a ring of light or fiber-optic cables at an angle of 45°. The reflected light is viewed at an angle of 0° or normal to the surface. Other geometries are 0/45 and 0/0. The processed reflected light may be captured with an integrating sphere. An integrating sphere may or may not be used. Such a device is needed to compare the diffuse and specular components of the reflected light. The number of spectral measurement points has increased from about 16 in the 1980s to about 1000 currently. Sometimes colorimeters


400

500

600 700 Wave Length

FIG. 8.15e Spectral response of CIE standard values.

are marketed as 0/45 spectrophotometers by using up to 16 different filtered fiber-optic receptors in a ring for a 16-point spectral curve. This is inappropriate for materials having a directional shade. A true spectrophotometer collects the reflected light and then forms a spectrum. The buyer needs to make sure of the basic design inside the machine. TRISTIMULUS METHOD (REFLECTANCE) Reflectance color measurements utilize the reflected light from a sample to monitor the surface color of the sample. This measurement can be done with a one-dimensional color standard such as those discussed above, but in applications requiring better color definition, a tristimulus method must be used. This method uses the Commission International del’Eclairage (CIE) model of human color perception. The method requires that the reflected spectrum from 400 to 700 nm be masked by the three functions shown in Figure 8.15e. The areas

1302


under the resulting curves measured as X, Y, and Z are correlated to human color perception. These values are normalized against standard values for different lighting conditions and types of observers. The algorithm has variations depending on the vision model used. The parameters X0, Y0, and Z0 are defined to be the areas under the white reference reflectance mask curves. Values of X0, Y0, and Z0 will vary depending on the design of the spectrophotometer. To model the human color space, the Lab shade model is used. L is the measurement of lightness and is calibrated to have the values of 0 to 100. Lch (cylindrical coordinates for lightness, chroma, and hue) and Lab (rectangular coordinates) are two three-parameter models.

Y = Y + PY(I)*R(I) Z = Z + PZ(I)*R(I) NEXT I IF X/X0< = 0.008856 OR Y/Y0< = .008856 OR Z/Z0< = .008856 THEN L = 903.3*(Y/Y0) ‘low light condition A = 500*((7.787*(X/X0)+16/116) −(7.787*(Y/Y0)+16/116)) B = 200*((7.787*(Y/Y0)+16/116) −(7.787*(Z/Z0)+16/116)) ELSE

The Lab Algorithm of the Textile Industry Following is a section of the computer code, written in BASIC, by the author that takes a 16-point color spectrum and computes Lab. The PX(I), PY(I), and PZ(I) functions are those shown in Figure 8.15e. For tables of spectral data and more information on computing Lab under specific illuminants and 3 angles of observation, see Judd and Wyszecki. section of BASIC code for on-line L a b determination: X0 = 97.9392:Y0 = 100:Z0 = 117.9648 ‘2DEG ILLUM C values of X, Y, Z for ‘the ‘white reference tile determined at calibration PRINT #1, “m” ‘serial port command to spectrophotometer to send 16 ‘ASCII spectral values from 400 to 700 nm every 20 nm FOR I = 1 TO 16 INPUT #1, T(I) ‘The 16 values make up the raw transmittance or ‘reflectance curve for the sample

L = 116*(Y/Y0)^.33333 – 16 ‘note that if Y = Y0 then L = 100 A = 500*((X/X0)^.333333 −(Y/Y0)^.333333) B = 200*((Y/Y0)^.333333 −(Z/Z0)^.333333) ‘be sure to set all sums to zero for repeating this The Lab algorithm is commonly used in the textile, automotive, and plastics industry. The parameter a ranges from 0 to ± 128 (red to green), and the parameter b ranges from 0 to ± 128 (yellow to blue). A white reference sample is taken by calibration to have the value 100, 0, 0, while the dark reference is taken by calibration to have the value 0, 0, 0. Calibration is a very important issue, and vendors employ many methods. The Lab or Lch measurement is often called the shade of the material. Shade measurements can be made 4 from different angles and areas of view. A recent patent that measures shade from different directions both on-line and off-line is given as a reference. Figure 8.15f illustrates how fiber optics can be combined with robotics.

NEXT I FOR I = 1 TO 16 R(I) = (T(I)-D(I))/((W(I)-D(I)) ‘W(I) and D(I) are the white and dark ‘tile values ‘respectively determined at calibration of the system. Program ‘must ‘read white and dark calibration tile spectral curves and store in ‘arrays W and D. Data files are read for PX, PY, and PZ arrays. X = X + PX(I)*R(I) ‘numerically integrates areas


FIG. 8.15f Fiber optics robotically positioned.

8.15 Colorimeters

1303

On-Line Shade Monitors On-line shade monitors that feature robotics for scanning webs may also use fiber optics to enable scanning while keeping the light source and spectrophotometer stationary. Fiber optics-based spectrophotometers can be interfaced to host computers that also control stepper motors for positioning the fiber optic over a moving web, or the web can 4 be scanned. Driver software is usually available for the spectrophotometer that will run in an application for data storage in large data files for statistical process control and trending. The output information from continuous systems can be used for alarming or for analog data transmitted by 4 to 20 mA. Spectral information is transmitted serially by RS 232, RS 422, USB (IEEE 1394). FIG. 8.15g In-line transmittance cell.

References CONTINUOUS COLOR MONITORS 1.

In-Line Liquid Color Measurement 2.

In-line liquid color measurement is illustrated by Figure 8.15g. Recent advances in fiber optics have allowed the light source and the photosensitive device to be remotely located from the actual sensor windows. Various cells can be installed in flow lines from 0.5 to 6 in. in diameter, or in situ probes can be used for batch vessels. Temperature and pressure must be specified. The width of the light path is a key consideration due to the absorptivity of the liquid. Glass rods of different lengths can be used as cell windows. Pressures may range up to 150 PSIG and temperatures to 250°C. The cost is lower if the sample stream can be reduced in size, pressure, and temperature. Cells can be in housings rated for NEMA 1, 4, 7 or Class 1, Group D, Division 1 explosion-proof.


3. 4.

Pearcy, B., Decorative Painting Color Match Sourcebook, Cincinnati, OH: North Light Books, 1997, see also www.gotcs.com. Weast, R. C., Ed., Handbook of Chemistry and Physics, 54th ed., Cleveland, OH: CRC Press, 1973. Judd, D. B. and Wyszecki, G., Color in Business, Science, and Industry, 2nd ed., New York: John Wiley & Sons, 1963. United States Patent 5,559,605, “Method and Apparatus for Determining the Directional Variation of Shade of Pile and Napped Materials,” M. W. Reed, assigned to Milliken and Co., September 24, 1996.

Bibliography Ewing, G., Analytical Instrumentation Handbook, 1990. Grayson, M., Ed., Kirk-Othmer Concise Encyclopedia of Chemical Technology, 4th ed., New York: John Wiley & Sons, 1999. Nassau, K., The Physics and Chemistry of Color, New York: John Wiley & Sons, 1983.

8.16

Combustibles R. NUSSBAUM

(1969, 1982)

AAH

B. G. LIPTÁK

(1995)

J. F. TATERA

(2003)

AIS Combustibles Sampling System Flow Sheet Symbol


Types:

A. Measurement of filament temperature or resistance in catalytic combustion sensors is most common. Thermal conductivity is used at higher concentrations. Electrochemical and semiconductor sensors can be used when hydrogen and other known gases are to be detected. B. Flame ionization and photoionization with or without a chromatograph can be used for accurate hydrocarbon detection. Response varies and gases of concern need to be known in the design and selection phase of the project. C. Infrared can be used for both point and area (open-path) applications. It cannot detect hydrogen. Note: All three types are available as portable and fixed devices, and Type A is also frequently found in a personal (pocket) device version.


Many choices exist and offer the opportunity to select an appropriate one for a given application. Stainless steel and polymer sensor heads with ceramic and metal sensors are usually offered. Various polymer and metal constructions with the appropriate optical window selections for photoionization and infrared applications are available.

Inaccuracy:

A. 5% of lower explosive limit (LEL); linearity and repeatability from 2 to 3% of LEL B. ppm concentrations can be detected and monitored C. ppm and low % LEL levels achievable, but vary dramatically and usually more a function of the application than the instrument

Drift:

A. 1 to 3% of LEL per month B. No generally accepted drift range per value C. No generally accepted drift range per value

Cost:

A battery-operated portable gas leak detector with sensing probe costs from $300 to $1000; a combined oxygen and combustibles sensor, microprocessor based, portable with diffusion sampling, costs $2500. For a permanently installed single-channel monitor with alarm or for a multichannel system, the cost per channel is $1000 to $2500. With sampled remote head installations, the installation cost of tubing can increase the per-channel cost to $3000 to $5000, and when a flame ionization or photoionization detector is used, the cost is more like $5000 to $10,000. A portable chromatograph with electrochemical detector and 50-ppb sensitivity costs about $15,000 to $20,000. Infrared systems cost about $1200 to $2700 for a point system and $7000 to $20,000 for an open-path system.


ABB (www.abb.com) (C) American Gas and Chemical Co. Ltd. (www.amgas.com) (A, C) Bacharach Inc. (www.bacharach-inc.com) (A) Bascom-Turner Instruments (www.bascomturner.com) (A) B W Technologies (www.bwtechnologies.nl or www.gasmonitors.com) (A) Cole-Parmer Instrument Co. (www.coleparmer.com) (A) Control Instruments Corp. (www.controlinstruments.com) (A, B) Delphian Corp. (www.delphian.com) (A, C) Detector Electronics Corp. (www.detronics.com) (A, C)

8.16 Combustibles

1305

Draeger Safety Inc. (www.draeger.com) (A, C) Enmet Corp. (www.enmet.com) (A) Gastech Inc. (www.gastech.com) (A) General Monitors (www.generalmonitors.com) (A, C) Heath Consultants (www.heathus.com) (B, C) International Sensor Technology (www.intlsensor.com) (A, B, C) Macurco Inc. (www.macurco.com) (A) MSA Instrument Div. (www.msanet.com) (A, C) Sensidyne Inc. (www.sensidyne.com) (A) Sick Maihak Inc. (www.sickmaihak.com) (B) Sierra Monitor Corp. (www.sierramonitor.com) (A) Teledyne Analytical Instruments (www.teledyne-ai.com) (A, B) Zellwegner Analytics Inc. (www.zelana.com) (A, C)

INTRODUCTION The principles of operation and the applications of combustibles analyzers will be discussed in this section. These instruments are designed to detect the presence and measure the concentration of combustible gases and vapors on a continuous basis. The methods of detecting the presence of combustible gases and vapors can utilize the phenomena of catalytic combustion, electrical resistance, luminosity, thermal conductivity, infrared (IR) absorption, or gas ionization. Of the above methods, the most widely used is catalytic combustion, where a change in the resistance or temperature of the sensing filament is caused by the catalytic combustion of the flammable gases, and this change is measured to detect the concentration of combustibles. The second most widely used and a much newer technique is infrared. As will be seen, both techniques have both advantages and disadvantages. Most suppliers offer a variety of designs, so that the user might select the best choice for his application. The selection process usually considers cost, robustness, selectivity, poison resistance, speed of response, etc. Selection Considerations The most commonly used combustibles detectors are the catalytic filament units, which use a self-heated platinum wire as the catalytic surface to initiate combustion. A special portable variation of this unit is one that can be pinpointed at leaks by pointing a sample probe at the seals on manholes, tanks, or other containers that are likely to leak. In some instruments, two filaments are provided: a catalytic combustion filament for low ranges, and a thermal conductivity filament for higher ranges. When the goal of the measurement is the detection of total hydrocarbons, or if the presence of lead, silicone, chlorinated compounds, or sulfur compounds could otherwise poison the catalytic filament, infrared and flame or photoionization analyzers should be considered. Flame ionization instruments are discussed in Sections 8.12, 8.25, and 8.59 and involve the burning of the sample in a hydrogen flame. Since the flame of pure hydrogen contains practically no ions, even traces of organic material can be detected by the drastic rise in the number of ions in the flame.


Measuring circuits for catalytic bead-type sensors usually include the Wheatstone bridge for resistance and nullbalance potentiometers with thermocouples for temperature measurements. In addition to the discussion of the measuring means, complete loops consisting of measuring, readout, and alarm devices and their applicability are covered in the following paragraphs.

TERMINOLOGY, DEFINITIONS, AND BACKGROUND INFORMATION In order to sustain combustion, each combustible gas or vapor requires a particular amount of oxygen. Some combustible gas mixtures ignite more easily than others (Table 8.16a). Additionally, the energy that is required to spark combustion also varies with the composition of mixtures. Lower explosive limit (LEL): The lowest concentration of gas or vapor in air where, once ignition occurs, the gas or vapor will continue to burn after the source of ignition has been removed. Upper explosive limit (UEL): The highest concentration of gas or vapor in air in which a flame will continue to burn after the source of ignition has been removed. Flash point: The lowest temperature at which a flammable liquid gives off enough vapors to form a flammable or ignitable mixture with air near the surface of the liquid or within the container used. Many hazardous liquids have flash points at or below room temperatures. They are normally covered by a layer of flammable vapors that will ignite in the presence of a source of ignition. The vaporization rates of the various liquids are a function of their vapor pressures, and vaporization rate increases with increased temperature. Flammable liquids are therefore more combustible at higher temperatures. As can be seen from Table 8.16a, the ranges of air percentages within which some liquids and gases are flammable

1306


TABLE 8.16a Properties of Some Flammable Liquids and Gases

Material

Chemical Formula

Specific Gravity Air = 1

(°F)

(°C)

Lower

Upper

Methane

CH4

0.55

1193

645

5.3

15.0

Natural gas

Blend

0.65

1163

628

4.5

14.5

Ethane

C 2H6

1.04

993–1101

534–596

3.0

12.5

Propane

C 3H8

1.56

957–1090

514–588

2.2

9.5

Butane

C4H10

2.01

912–1056

489–569

1.9

8.5

Toluene

C 7H8

3.14

1026–1031

552–555

1.3

6.7

Gasoline

A blend

3–4.00

632

333

1.4

7.6

Acetone

C3HO

2.00

1042

561

2.6

12.8

Benzene

C 6H6

2.77

968

520

1.4

6.7

Carbon monoxide

CO

0.97

1191–1216

644–658

12.5

74.0

Hydrogen

H2

0.07

1076–1094

580–590

4.0

75.0

Hydrogen sulfide

H 2S

1.18

655–714

346–379

4.3

45.0

are extremely wide. In detecting the presence of such vapors or gases, their LELs are usually of most interest, and, in order to maintain safety, flammable gas and vapor concentrations must be kept below those limits. Since air is usually the diluent and is almost always present, all concentrations above LEL are usually dangerous. Instruments are commonly calibrated with ranges in LEL units. LEL is selected as a limit on acceptable safety, because in order to reach a buildup of atmospheric concentration of flammables, which is above the UEL, the concentration must have necessarily passed through the full hazardous explosive range. Similarly, bringing the concentration back down to a safe level below the LEL, the concentration must pass again through the full hazardous explosive range.

CATALYTIC COMBUSTION ON A HEATED FILAMENT When mixtures of flammable gases or vapors in air come in contact with a heated and catalytically treated, fine, uniform, homogeneous platinum filament, combustion is induced at a temperature considerably below the normal ignition temperature of the particular gas or vapor. The heat generated by the combustion is measured by sensing the change of temperature of the filament by using thermocouples or by measuring the change of resistance of the filament. Limitations One of the common limitations of catalytic combustion type analyzers is the poisoning of the filament by silicon, sulfur, chlorinated compounds, or lead compounds. When detecting the concentration of leaded gasoline vapors, which contain


Ignition Temperature in Air

Flammability Limits in Air (% vol.)

tetraethyllead, a solid lead combustion product can form (by condensation) on the filament surface, which reduces its catalytic activity. One way to protect the filament against lead condensation is to maintain the filament at a temperature that is high enough to prevent this condensation. Compounds containing silicone can also poison the filaments. These effects impair the life of the sensor to different extents, depending on sensor packaging. Specially packaged diffusion head sensors (to be discussed shortly) are more likely to last longer on such services than do the flowing sample type systems. Filament poisoning by chlorinated or sulfur compounds is also a serious problem. In addition to special catalytic bead protective measures, ionization and infrared detectors should be considered as an alternate means of measurement where sensor poisoning is an issue. A variety of filament protection means have been added to increase the poison resistance of the sensors. Figure 8.16b illustrates one such design, in which the catalyst support consists of a low-density macroporous structure that surrounds the platinum wire deep within the bead assembly. This provides both protection and an increased catalyst surface area. The reported result is a 10-fold or better increase in sensor life expectancy on such services as hexamethyldisiloxane (HMDS), leaded gasoline, Freon-12, ethyl mercaptan, and the like. Life expectancies are usually defined in terms of exposure concentration hours. One high-concentration exposure of a poison has been known to knock out a sensor, and many do not respond in a fail-safe way. For this reason, nonpoisoning techniques should be considered, when poisoning is an issue.

8.16 Combustibles

115 V.A.C. 60 CYCLE

1307

FILAMENT VOLTAGE ADJUST (VARIABLE TRANSFORMER)

FINE ZERO ADJUST

POROUS STRUCTURE TO MAXIMIZE SUPPORT AREA FOR POISON-RESISTANT CATALYST

COARSE ZERO ADJUST

FILAMENT TRANSFORMER SEALED IN AIR

REFERENCE FILAMENT

ACTIVE FILAMENT SAMPLE FLOW

THERMOCOUPLE

FIG. 8.16b Porous bead construction provides poison resistance to catalytic combustion-type sensor.

+

THERMOCOUPLE −

−

+ − MILLIVOLT POTENTIOMETER CIRCUIT

Measuring Circuits Whether the measurement is based on the change of temperature or resistance, it is convenient to use two filaments. One filament is constantly exposed to the sample (detector filament). The other is hermetically sealed in an inert atmosphere (reference filament). The reference filament is not activated with a catalyst, but its temperature resistance characteristics are similar to those of the detector filament. Its inert surface is usually exposed to the sample in a way that simplifies measurement compensation for changes in sample temperature, flow, and other potentially interfering characteristics. The active detector filament and often the inert reference filament are mounted in a measuring chamber that is relatively large with respect to the size of the filaments. This permits a relatively large volume of sample to pass through the instrument, which ensures that the measurement filament is in contact with the sample and is measuring the current sample conditions. This design still only allows a relatively small portion of the sample to come in contact with the sensor, thereby increasing its useful life. Thermocouple Detector In this design two thermocouples are used. One thermocouple is bonded to the reference filament, the other to the detector filament. The two thermocouples are connected in series opposition, so that a differential electromotive force (emf) is developed and applied at the terminals of the potentiometric circuit (see Figure 8.16c). When a combustible gas or vapor is admitted to the measuring chamber, combustion increases the temperature of the detector filament, resulting in an increased emf for the thermocouple bonded to it. The temperature of the reference filament remains the same as the sample temperature, since no combustion occurs on its bead. The potentiometric


+

FIG. 8.16c Thermocouple detector.

transmitter or the indicating, recording, and alarming instruments respond to the resultant differential emf. Wheatstone Bridge Detector A Wheatstone bridge is typically used for resistance measurement. Its operation is based on the comparison of an unknown resistance to a resistor of known value, as shown in Figure 8.16d. In this figure, R1 = R2 = constant R3 = reference R4 = sensor’s measured resistance (compared to R3) For current I to be zero, V1 = V2 V1 =

R3 V R1 + R 3

V2 =

R4 V R2 + R4

R3 R4 = R1 + R 3 R 2 + R 4 R 3R 2 + R 3R 4 = R 4 R1 + R 4 R 3 R 3R 2 = R 4 R1 R3 = R 4

8.16(1)

1308


3/4 N.P.T R1 WHEATSTONE BRIDGE

R2

L

V

V1

CONDUIT

V2 R3

R4

INLET FLASHBACK ARRESTOR FILTER

OUTLET FLASHBACK ARRESTOR

SAMPLE INLET COMPENSATOR FILAMENT

FLOWMETER

SAMPLE COMPRESSOR

SPAN

TO SHUTDOWN OR VENTILATION CIRCUIT

4 mA LEVEL R27 R

ALARM HORN

TB1

ZERO ADJUST

EXHAUST

+

RECTIFIER

POWER-ON LIGHT

DS1

BRIDGE OUTPUT

+ GREEN LED

CURRENT RELAY

17

CALIBRATION WIRE LOOPS

R1 ALARM LIGHT

DETECTOR FILAMENT

+

DS2 S1

ZERO RED LED PUSHBUTTON CALIBRATION SWITCH

ALARM RELAY

− TRANSFORMER

SENSOR

TO NO V.60 ~ 1f

FIG. 8.16d Wheatstone bridge detector with accessories.

In the catalytic bead-type combustibles detectors, R3 is the reference filament and R4 the detector filament. If the sample contains no combustibles, the bridge circuit remains in balance. If, however, there are combustibles in the sample, the combustion will cause heating of the detector filament. The change of resistance of the detector filament due to heating will result in unbalancing the bridge in proportion to the amount of additional heating caused by the combustible material in the sample. The output voltage of the bridge, which is in proportion to the concentration of combustibles in the sample, is detected by a transmitter or is used to operate indicating or recording instruments or to actuate alarms. Diffusion Head Analyzers In contrast to most analyzers, the diffusion head analyzer does not require a sampling pump or a controlled sample flow. Rather, the diffusion head type analyzer generates sample movement by diffusion, density difference, convection, or similar effects. Diffusion-type catalytic bead sensors are available in poison-resistant designs and in intrinsically safe or explosionproof construction. Figure 8.16e illustrates a conduitmounted, diffusion type transmitter with 4- to 20-mADC output. This unit is provided with a stainless steel sensor head and a polyvinyl chloride (PVC)-coated anodized aluminum conduit. Diffusion sensors can be used in still air or provided with plant air aspirators or pumps to draw a sample flow over the sensor.


FIG. 8.16e Diffusion-type sensor in combustible gas transmitter. (Courtesy of Sensidyne Inc.)

Semiconductor sensors are also available in diffusion head designs. Semiconductor sensors respond to a combustible (target) gas that has been absorbed onto the doped surface of a metal oxide semiconductor, by displaying a change in the resistance of the semiconductor surface. By varying the doping layer, the manufacturer can vary the responsiveness of the detector to various materials. As with the catalyst bead surface effect sensors, poisoning is an issue. Diffusion type electrochemical and semiconductor sensors are also available to detect hydrogen, using a sensor that is not responsive to other hydrocarbons. This is desirable in semiconductor manufacturing plants, where it is a continual task to monitor for hydrogen leaks. Sampling System When diffusion-based systems are not adequate, active sampling systems may be required. The sampling systems should be carefully designed. Most importantly, the sample admitted into the analyzing cell should be wholly representative of the combustible components that are present in the monitored area. The sample should also be free of particulate matter and moisture. In applications where the sample is at excessively high or low temperatures, it is advisable to use sample conditioners. This is particularly important if the sample is hot and humid and tends to cool while passing through the sampling line. The reason is that cooling would result in condensation,

8.16 Combustibles

which, in turn, could block the sample line or introduce a time lag in the analyzer response. The sampling system should permit transport of the sample to the analyzer cell at the proper rate and minimum transportation time lag. Since the vapors of all flammable liquids are heavier than air, detection of such vapors requires that the probes be located near the ground. Gases like hydrogen are lighter than air and require elevated probe locations. In dealing with gases, their molecular weight (heavier or lighter than air) will decide whether sampling probes should be near the ground or the ceiling of the monitored area. This may seem trivial, but is still worth mentioning. Accessories

HAZARDOUS AREA

CONTROL ROOM ELECTRICAL SIGNAL

S

SELECTION OF COMPLETE INSTALLATION The selection is usually made from among three basic systems, and the choice is based on the plant layout, the required speed of response, and economic considerations. The three choices are 1) remote head (continuous measurement, continuous readout); 2) multiple head (continuous measurement, sequential readout); and 3) tube sampling system (sequential measurement, continuous readout).


SAMPLE

DETECTOR CELL (AREA 1)

AIS (FOR AREA 1)

AREA 1 FILTER

REFERENCE GAS (AIR) FILTER

ELECTRICAL SIGNAL DETECTOR CELL (AREA 2)

AIS

It is important to make sure to avoid the propagation of flame when the sampled air containing an explosive mixture of gas is ignited on the detector filament. This is not a problem when the concentrations are low or below the LEL, but a leak or spill can result in concentrations exceeding the LEL (within the explosive concentration envelope), and this leak or spill can become a source of ignition in that area of the plant. Flashback arrestors of coiled copper screen or sintered metal are usually provided at the inlets and outlets of filament chambers. These prevent the energy that is liberated by combustion from propagating to the outside. Samples containing hydrogen or acetylene, with concentrations of oxygen in excess of that found in normal air, have high rates of flame propagation. In such mixtures, standard flame arrestors cannot dissipate the energy liberated by combustion and, therefore, special flame arrestors have to be used. To ensure safe operation of the detectors, a variety of alarms are provided. These alarms can signal filament failure, power failure, alarm relay failure, and low sample flow rates (not available for diffusion head type designs). Many alarms do not detect sensor poisoning as a sensor failure. If in a particular application poisoning is likely, one should make sure to thoroughly understand the functioning of the alarms before issuing a purchase order. To ensure that an adequate amount of sample passes through the measuring chamber, flow meters (rotameters) and needle valves can be provided for all except the diffusion head type of units.

1309

(FOR AREA 2) EXHAUST

SAMPLE AREA 2 S

F1

F1 EXPLOSION PROOF PUMP

FIG. 8.16f Remote head system.

Remote Head System The remote head system offers the maximum application flexibility, but it does that at the highest initial cost. As shown in Figure 8.16f, this system typically consists of a number of locally mounted analyzer heads (suitable for hazardous areas) and an equal number of panel-mounted control and readout devices. The maximum number of areas monitored from one central panel is a function of the capacity of the sample pumps (or aspirators) and of the physical size of the panel. Because the analyzer heads are located in the monitored areas, the speed of response is fast. Samples are continuously drawn, and the electrical signal corresponding to the measured combustible concentration is instantaneously transmitted to the control unit. The used sample is continuously withdrawn from the analyzer head through the tubing to the aspirator and is exhausted. Since the analyzer head is in the monitored area, it can be temperature controlled to prevent condensation. The remote head system should be selected where fast response is essential and justifies the cost. Multiple Head System Multiple head systems are used where at least four or more areas are monitored and a cyclic readout with the accompanying time delay can be tolerated. The multiple head system consists of a number of analyzer heads (one in each area to be monitored), one control unit with readout, and one or more

1310


CONTROL ROOM

CONTROL ROOM

HAZARDOUS AREA

READ-OUT WITH PILOT LIGHTS IDENTIFYING MONITORED AREA

S SAMPLE

DETECTOR CELL (AREA 1)

AIS

AREA 1

AREA CENTRALLY LOCATED TO ALL MONITORED AREAS

READ-OUT WITH PILOT LIGHTS IDENTIFYING MONITORED AREA AIS

MONITORED AREAS

SAMPLE SELECTOR

FILTER SEQUENCER

BY-PASS DETECTOR CELL

REFERENCE GAS (AIR)

SAMPLE AREA 1

F1 FILTER F1

FILTER

ELECTRICAL SIGNAL DETECTOR CELL (AREA 2)

EXHAUST

SAMPLE AREA 2

SAMPLE AREA 2 REFERENCE AND PURGE GAS EXHAUST

S

SAMPLE AREA 3

F1 ELECTRICAL SIGNAL

FILTER

F1 EXPLOSION PROOF PUMP

FIG. 8.16g Multiple head system.

sample pumps. The electrical circuit incorporates a single readout device common to all analyzing cells. A separate alarm unit is associated with each detecting unit. The sample is drawn continuously to each sample chamber. The pump continuously withdraws the expended sample. The electrical output of each unit is transmitted to the panel, where sequential readout is provided. The dwell time for each area is typically 10 sec; i.e., if four areas are being monitored, 40 sec elapse between subsequent readings for a given area. This system is less costly than the remote head arrangement, but it can be used only where the combustible concentration buildup is likely to occur at a slow rate (see Figure 8.16g). Tube Sampling System The tube sampling system consists of one analyzer head, one readout device, and a sample pump. This may be the least expensive arrangement, but sometimes the tubing cost (purchase and installation) can exceed the amount saved on the instrumentation. Samples from different areas are sequentially admitted to the common analyzer head. The electrical signal is then transmitted to the readout device. A sample selector unit, consisting of time-sequenced solenoid valves, is arranged to admit one sample to the detector and connect all other sample lines to the sample pump. The sample is drawn continuously through each line. The sample selector is located at the analyzer head; thus lag time between successive analysis and delay due to sample travel is minimized, since a fresh sample is always present at the sample selector.


FIG. 8.16h Tube sampling system.

One possible means of improving the system is to use separate pumps for the sample analyzed and for those that are bypassing the detector. A clean gas purge can be provided after each analysis to prevent an erroneous reading caused by residual carryover in this type of system (see Figure 8.16h). In order to eliminate the problems associated with condensation in the sample tubes, these arrangements should be used only where true gases and vapors with boiling points well below ambient temperatures are to be detected. Tube sampling systems usually have a 30-sec dwell time per hour. Therefore, they should be considered only if such slow response can be tolerated. For additional safety, readout devices can be calibrated with full-scale ranges as low as 0 to 20% LEL. The alarm switches contained in the measuring circuit are used to actuate alarms, start ventilation, shut down sparking devices, and so on. These systems are found in coating ovens, solvent recovery, and soybean extraction plants, just to mention a few typical applications.

CONCLUSIONS FOR CATALYTIC DETECTORS In the diffusion head type analyzer, the use of sample pump or aspirator is eliminated. Dispensing with any moving part increases reliability. Therefore, the use of a diffusion head analyzer is recommended wherever a flowing sample is not needed or where clean, dry samples are to be analyzed. Large amounts of particular matter, moisture, and dust can and will cause plugging, which is difficult to detect in diffusion head analyzers since they cannot be furnished with low-sample-flow alarms.

8.16 Combustibles

In addition to the above, the selection parameters should include the considerations of plant layout, required speed of response, rate of gas buildup, and economy. Comparing the Wheatstone and the thermocouple cells, the following should be considered. Whereas Wheatstone bridge cells use a fine, helical filament, the thermocouple cell uses a heavy, straight filament with a much longer useful life. Further, the evaporation on the exposed filament results in a constant change of base resistance of the filament. In the Wheatstone bridge circuit, this change of base resistance produces a shifting of zero and requires frequent rebalancing of the bridge. The temperature change measured by the thermocouple is independent of filament deterioration. Thus, for the thermocouple detector, the zero drift is reduced to a negligible amount even over long periods of time. Therefore, the thermocouple detector is often superior to the Wheatstone bridge-type detector.

FLAME IONIZATION AND PHOTOIONIZATION DETECTORS The theory and operation of flame ionization detectors (FIDs) and photoionization detectors (PIDs) are described in greater detail in the sections describing chromatography (Section 8.12), hydrocarbon analyzers (Section 8.25), and total carbon analyzers (Section 8.58). Both detectors are commonly used in chromatography and have been utilized for combustibles monitoring in both portable and fixed installation designs. Flame Ionization Detectors The FID actually burns the sample in a hydrogen flame. In a simple combustibles application, no columns or carrier gases are used and the sample is used as an oxygen/air source. The sample is consumed during the combustion process. In a chromatography application, extremely clean air (as the source of oxygen) is introduced into the chromatographic column’s effluent (which contains the sample) and is sent into the flame. In these configurations, the only variable sources for ion formation in the flame are the components of interest in the column effluent or combustible contaminants in the combustibles sample. The appropriate combustible materials in the sample form ions in the flame. An oxygen-rich hydrogen and air flame basically exhausts water, nitrogen, and unconsumed oxygen. None of these are ionic in nature. A charged electrical field is positioned across the flame, and it can conduct a current utilizing available ions in the flame as its conductor. When most combustible materials are introduced into the flame, they produce ions in their combustion products, and these are detected by the increased flow of current across the electric field (flame). This detection method has been called a carbon counter, because of its response profile. It essentially responds to each carbon atom in the sample that has been consumed and used to form an ion in the flame. For example, one molecule of


1311

ethane has nearly twice the response of one molecule of methane. This has both advantages and disadvantages in combustibles monitoring applications. The sensor is very sensitive to larger organic molecules. Its response to a mixture that may vary in composition can be difficult to calibrate, since different components have different LEL concentrations (see Table 8.16.a) and different detector responses. Specific and unique calibrations may be needed for each sample or application. The instrument cannot detect hydrogen (no ions are formed in the flame). These advantages and disadvantages are listed only as examples and are by no means exhaustive. Each application needs to be studied in full detail prior to selecting an appropriate measurement method. Photoionization Detectors The PID utilizes a high-energy light source (normally ultraviolet (UV) radiation) as its source of ionization and measures the resulting flow of a current through the ionized sample, across a charged electric field. This detector also has several advantages and disadvantages. It does not require auxiliary utilities (fuel gases). It can easily be made portable. It does not necessarily respond (depending on the ionization source chosen) to many potential components of interest. It requires frequent calibration and maintenance (as radiation sources deteriorate). Several different lamp strengths are available, and an appropriate one needs to be selected for a given sample. The ionization potential (IP) (eV) of each molecule needs to be matched to the strength of the ionization source being used. For example, acetone has an IP of 9.71 and can be ionized by most common lamps having IPs of 9.8, 10.6, or 11.7 eV. Of course, each different lamp has a different response factor for acetone, while methanol has an IP of 10.85 and therefore responds only to the 11.7-eV lamp. Methane has an IP of 12.51 and would not respond to any of these lamps. These advantages and disadvantages are only given as an example and by no means are exhaustive. As mentioned previously, each application needs to be fully studied prior to selecting an appropriate measurement method.

INFRARED COMBUSTIBLES DETECTORS Infrared combustibles monitors are primarily a simplified and special-purpose version of an infrared filter photometer. In cases of very simple applications, they have even been used as a substitute for an infrared photometer and actually used to monitor a process sample that was introduced to them. For this reason, they have sometimes been called the poor man’s IR analyzer. Infrared photometers and spectrometers, and the technologies that they are based on, are discussed in great detail in Section 8.27, “Infrared and Near-Infrared Analyzers.”

1312


Basically, an infrared beam of radiation that will excite the target gas molecules is used to measure the concentration of combustible gas molecules in the sample. For combustible gas monitoring, the radiation wavelength chosen is usually one that is absorbed by the C H bond of most hydrocarbon molecules. When the beam of radiation excites the molecules, a portion of its energy is absorbed and the amount of energy absorbed (lost to the beam) can be correlated to the amount of the target gas in the sample. Because many other factors could impact the intensity of the selected beam of IR radiation, these instruments usually also monitor a reference (another) wavelength of radiation that is not absorbed by the combustible gas, but is influenced by several of the other factors that could affect the measured beam’s intensity. Infrared combustibles monitoring instruments are available as both point and open-path (area) monitors. Even the point monitors are sometimes called open, because the IR measurement cell is actually open to the atmosphere. They typically rely on atmospheric diffusion to supply the sample and, consequently, the cell must be open to allow the diffusion of sample into the measurement area to take place. Open-path instruments, on the other hand, actually use a large, open atmospheric path as their measurement cell (tens to hundreds of meters). IR combustibles monitors are a relatively new innovation in the field of combustibles monitoring, but they have already gained wide acceptance as a niche technology. They perform well on many samples that other technologies have problems with. This is especially true for many gases that can poison other combustibles sensors and for monitoring requirements where the likely points of leakage are difficult or impossible to predict. Diatomic molecules like hydrogen, oxygen, and nitrogen have no usable IR absorbance and cannot be detected by these IR monitors. Consequently, IR combustibles monitoring systems should not be used for hydrogen or hydrogen-containing combustibles mixtures. The response of each potential gas or mixture to the detection method needs to be considered when selecting a monitor.

Point Infrared Systems Point IR systems monitor the sample at the measuring head, just like the other previously discussed point style combustibles monitors. If it is intended to monitor a sample that is not diffusing into the sensor head and is not located immediately adjacent to it, the sample must be transported to the sample head using a sample transport system. A couple of point IR designs are shown in Figures 8.16i and 8.16j. Figure 8.16i depicts a reflector style point sensor design, where the IR source and detector are both located on the same side of the sample chamber. The measurement and reference IR beams are transmitted, reflected off of a mirrored reflector, and pass through the sample twice during the analysis. With this type of sensor there is no chemical reaction of the gas, and as such, the materials that poisoned catalytic beads cannot poison these sensors. But nothing is perfect or without its own Achilles’ heel. For an IR point monitor to do its job, the IR radiation beam must pass through the sample and be partially absorbed by the sample of interest before reaching the detector. If the sample becomes opaque to the IR measurement beam or if the optical path is otherwise blocked (condensation or dirt on the windows, heavy fog, dust, etc.), the instrument can be rendered inoperable. Typically, the mirror and windowed instrument compartment are purged or maintained at flameproof enclosure IR-sources

sapphire window heated reflector Gas

beam splitter measuring detector

reference detector

FIG. 8.16i IR reflector style point sensor. (Courtesy of Draeger Safety Inc.)

Active Filter

Source

Optical Windows

Gas Detector Reference Filter Beam Splitter

FIG. 8.16j IR one-pass point sensor. (Courtesy of General Monitors Inc.)


Fresnel Lens

8.16 Combustibles

a temperature that is intended to prevent condensation on either the window or mirror. Figure 8.16j depicts a one-pass point IR sensor design, where the IR source and detector are located on opposite sides of the sample chamber. This analyzer is very much analogous to the reflector style sensor head depicted in Figure 8.16i, except that the beam of radiation only passes through the sample once. In the reflector style, the beam has the opportunity to interact with the sample twice, and if all else was equal and good, the instrument should depict twice the sensitivity (or twice the interference to things like fog, dust, etc.). In real life, the sensitivities are usually not that different between the different designs, and the benefits are more often expressed in the form of a smaller sensor head or other geometric benefits. Area (Open-Path) Infrared Systems All of the previously covered combustibles monitoring technologies can be classified as point monitoring systems. They only measure the atmosphere at the points where they have been located (the gas of interest diffuses to a sensor) or

sampled from. To monitor a large area, one would have to locate many monitors (points) and hope that they represent the area’s general atmosphere. Open-path IR combustibles monitors project their IR beams in a path that is typically 10 to 200 m in length and monitor all of the combustibles in that path. This is not really an area (more of a line or path) detector, but the value it determines can be viewed as more representative of an area value, and few instruments could provide a value that more nearly represents an area than would be needed with point monitors. Figure 8.16k depicts some of the conceptual differences between point and open-path applications. The figure shows examples of leak detection applications under both no-wind and mild wind conditions. It can be seen that the leak cloud shape varies as a function of atmospheric conditions. Leak cloud shapes also vary as a function of the composition and the conditions of the sample. Lighter and hotter gases rise faster, and samples under different pressures produce different rates of release and dispersion. Therefore, it is very difficult to locate a sensor (point

Open Path Detector 10% LEL

25% LEL 50% LEL 100% LEL

Leak Source

Point Detector

3 TO 5 MPH WIND 50 PPM Open Path Detector

1% LEL

10% LEL 100% LEL

Leak Source

Point Detector

Fig. 8.16k IR open-path vs. point monitoring concept. (Courtesy of General Monitors Inc.)


1313

Open Path Detector

1314


or open-path) in a way that will always accurately measure and detect a leak, unless it is located almost exactly at the source of the leak. Both leak examples in Figure 8.16k show the use of both a point and an open-path monitor. It is easy to see the benefit one can gain by locating a good point monitor at a potential leak source. By so doing, one would get both an earlier and quicker detection, as the sample concentration is always higher closer to the leak. It is also easy to see the benefits of locating a single openpath monitor along a pipeline that may contain many potential leak sources, as opposed to installing many point sensors, which would be very costly and impractical. Open-path sensors can also better cover a general area, where the positions of potential leak sources may be difficult to predict. Similar benefits can be visualized regarding other applications, like perimeter monitoring, room monitoring, fence line monitoring, or other general-area monitoring tasks. It is important to note that the point and open-path techniques utilize different reporting values. Point techniques utilize parts per million (ppm) or percent LEL (%LEL) values, as described in the definitions at the beginning of this section and as listed in Table 8.16a. Open-path techniques utilize the units of parts per million or LEL meters (ppm.m or LEL.m). These will be discussed in more detail later in this section. It is fair to say that in most applications, open-path monitors are used more to detect leaks than to determine the absolute degree of hazard associated with the leak. This is because of the various leak cloud shapes that could exist and the way the instruments add or average the concentrations along their path. Hydrocarbon Gases in the Atmosphere

LEL-METER PPM-METER

Properly applied open-path monitors can be effective in monitoring combustible hydrocarbon gases in the atmosphere. Figure 8.16l shows an example of a typical open-path monitor. This installation involves installing two field devices. In this example, they are the source and the receiver or detector sections of the monitoring instrument. In other examples,

4-20 MA Alarm Relay 4-20 MA Warning Relay Alarm Relay Fault Relay

they may consist of the instrument (source and detector) and a reflector. In all cases, proper positioning and alignments are crucial to the success of the application. The beam must be positioned in a way that will enable it to detect the leaks of interest. The alignment is typically done with the aid of a vendor-supplied or recommended rifle scope that is mounted on one of the sections of the unit; using it helps in precisely aligning the beam, so that it properly hits the other unit. The instrument needs to be located not only where it can make the best measurement of the leak, but also where it can perform. It must not be located where it can be exposed to shock or vibration, because this could make the alignment unstable or impossible. The monitor also must not be located where people, cars, or other equipment can block the beam. Figure 8.16m depicts how an open-path instrument can compensate for partial blockages of its beam by lightobscuring interference, such as rain, fog, dust, etc. Essentially, it calculates the ratio of the measurement and the reference radiation signal. Most partially obscuring interferences will reduce both signals to the same extent. Therefore, the ratio of the signals is relatively unaffected by the interfering obstruction. On the other hand, the presence of a combustible gas will reduce only the measurement signal and therefore will result in a change in the ratio of the two radiation signals. This naturally is not the case if the signals are totally or nearly totally blocked. In that case, the instrument sensitivity and ability to detect a combustible gas are partially or completely lost. Sunlight, flames, and many other light sources also produce infrared radiation. To reduce or eliminate their effects on the performance of IR instruments, choppers, filters, focusing optics, digital signal processing techniques, and other aids are utilized. In general, these miscellaneous IR sources do not interfere with the performance of today’s IR type combustibles monitors. Yet, under extreme situations, they can still reduce sensitivity, by swamping the detector with too much radiation, and consequently, they should be considered when designing the field installation.

Receiver

Gas Cloud

Source

Slgnal Processing Electronics

24 V dc Supply 24 V dc Supply

.

Digital Display

FIG. 8.16l A single-pass, open-path IR system. (Courtesy of General Monitors Inc.)


8.16 Combustibles

Rain, Fog etc. Gas cloud

sample reference

ratio

FIG. 8.16m Open-path IR signal response. (Courtesy of Zellweger Analytics Inc.) 100% LEL

50% LEL

10% LEL

1315

in the optical path simultaneously, the instrument would report the total presence of combustible gases as 3 LEL ⋅ m. Clearly, the 1 m 100% LEL cloud is potentially explosive and the most dangerous of the three, but this method of monitoring does allow one to distinguishing between them. It cannot distinguish between small clouds of high concentration and large clouds of low concentration. It simply measures the total amount of target gas in its optical path. For this reason, the use and applicability of open-path IR combustibles monitoring is limited. Yet, this sensor still fills a niche market, and it can make some difficult monitoring applications feasible and practical. All of the combustibles monitoring technologies reviewed in this section have strengths and weaknesses. Each has some advantages and disadvantages relative to the others. It is up to the user and the supplier to work together in evaluating these differences and picking the most appropriate technology for a given application.

Bibliography 1m

2m

10 m

FIG. 8.16n Open-path IR measurement units. (Courtesy of Zellweger Analytics Inc.)

Point Measurement The open-path IR systems monitor the concentration over the length of their optical path, but how can they be used to measure the concentration at any one point along this path? Do they operate like radar and actually monitor the concentrations at various points along the optical path? No, they do not. They essentially measure the number (concentration) of combustible gas molecules along the path in an integrated or cumulative fashion and report a number that incorporates the dimensions of both the concentration and distance. Figure 8.16n illustrates how these instruments measure and report their readings. It should be kept in mind that the measurement is along a beam or path and, therefore, it is not detecting an area or a point. Percent or parts per million (ppm) readings are obtained by integrating the product of the concentration of the gas (along the length of the IR beam) by the length of the cloud (along the optical path). In Figure 8.16n, there are three clouds in the optical path of the monitor. Each cloud has a different gas concentration and size, but all three are equal in LEL-meter units. In terms of their open-path reporting dimensions (100% × 1 m = 50% × 2 m = 10% × 10 m = 1 LEL·m each), if all three clouds were


Anderson, G. L. and Hadden, D. M., The Gas Monitoring Handbook, Perl River, NY: Avocet Press, 1999. Baucke, C. G., “Application Considerations for Catalytic Combustible Gas Detectors,” in Analysis Instrumentation, Vol. 12, Research Triangle Park, NC: ISA, 1974. Burgess, D., “The Flammability Limits of Lean Fuel-Air Mixtures,” in Analysis Instrumentation, Vol. 12, Research Triangle Park, NC: ISA, 1974. Callahan, J., “Performance Standards for Combustible Gas Detectors,” Instrumentation Technology, December 1981. Chou, J., Hazardous Gas Monitors: A Practical Guide to Selection Operation and Applications, Raleigh, NC: SciTech Publishing, 2000. Clansky, K. B., The Chemical Guide to the OSHA Hazard Communication Standard, 6th ed., South Yorkshire, U.K.:Roytech, 1991 (revised annually). Dailey, W. V., “Monitoring Toxic and Flammable Hazards,” InTech, February 1973, pp. 23–28. Duncan, J. E., “CSA Standard C22.2 No. 152: Combustible Gas Detection Instruments,” AID Symposium, ISA, Research Triangle Park, NC, May 11, 1976. Jessel, W., “Planning and Designing Gas Detection Systems,” Sensors, January 2002, Vol. 19, No. 1. pp. 34–39. Johanson, K. A., “Gas Detectors by the Acre,” InTech, August 1974, pp. 33–37. Merman, J. M., “Application Considerations for the Installation of Combustible Gas Detectors,” ISA96 Symposium, ISA, Research Triangle Park, NC, 1996. Rayburn, S., The Foundations of Laboratory Safety, New York: SpringerVerlag, 1990. Sherman, R. E., Rhodes, L. J., and Tatera, J. F., “Combustible Gas Analyzers,” in Analytical Instrumentation: Practical Guides for Measurement and Control, Research Triangle Park, NC: ISA, 1996, pp. 291–308. White, L. T., Hazardous Gas Monitoring, Norwich, NY: William Andrew Publishing, 2000.

8.17

Conductivity Analyzers A. BRODGESELL (1969, 1982) J. R. GRAY (2003)

Standard Design Pressure:

CIT

K. S. FLETCHER

(1995), REVIEWED BY

R. R. JAIN

CE Flow Sheet Symbol

To 500 PSIG (3.5 MPa)

Standard Design Temperature: To 390°F (200°C) Element Materials:

Cells: glass, epoxy, and stainless steel. Electrodes: platinum, nickel, titanium, and carbon. Electrodeless: epoxy, Noryl, PFA, PEEK (polyether ether ketone), and polypropylene.

Cost:

For $700 one might obtain an analyzer with these features: panel-mounted monitor, general-purpose electrical class; NEMA 1 environmental protection; ±1% accuracy; RFI/EMI protection; two-electrode contacting sensor with 3/4-in. NPT process connection; single analog output. For $1500 one might obtain an analyzer with these features: pipe or surface-mounted field monitor; Division 2 electrical class; NEMA 4X environmental protection; ±0.5% accuracy; RFI/EMI protection; high-temperature electrodeless sensor capable of measuring hot acid, base, or salt solutions; dual analog outputs.

Range:

0 to 0.05 µ S/cm minimum; 0 to 2 S/cm

Inaccuracy:

Up to ±0.5% of full scale


ABB (www.abb.com) Analytical Technology Inc. (www.analyticaltechnology.com) Electro-Chemical Devices (www.ecdi.com) Endress + Hauser (www.endress.com) The Foxboro Company (www.foxboro.com) +GF+ Signet (www.gfsignet.com) GLI International (www.gliint.com) Honeywell (www.honeywell.com) Horiba Instruments, Inc. (www.horiba.com) Knick (www.knick.de) Mettler-Toledo (www.mt.com) Osmonics Lakewood (www.osmonics.com) Rosemount Analytical (www.rauniloc.com) Sensorex (www.sensorex.com) Thermo Orion (www.thermo.com) Thornton (www.thorntoninc.com) Van London Co. (www.vanlondon.com) Yokogawa (www.yca.com)

Conductivity sensors measure a solution’s ability to conduct electricity, which is a function of all dissolved ionized solids in the solution. These detectors are packaged either as probes (with isolating valves for removal, without opening up the process) or in the flow-through designs. 1316 © 2003 by Béla Lipták

INTRODUCTION Conductivity analyzers measure ionic concentration of electrolyte samples. Cells and instrumentation are designed to measure the electrical resistance (or its reciprocal, the conductance)

8.17 Conductivity Analyzers

1317

TABLE 8.17a a 2 –1 Equivalent Conductivity of Several Ions at Infinite Dilution at 25°C (S⋅cm ⋅mol ) Cations H

Γ, degrees

b

+

349.8

+

K

43.5

Na Ca

+

+2

Mg Cu

+2

+2

(n-Bu)4N

+

Tempco, degrees

Γ, degrees

Tempco

198.6

0.018

76.4

0.0202

–

71.42

0.020

–2

80.0

0.022

69.3

0.02

44.5

—

30.4

0.025

Anions –

0.0139

OH

0.0193

–

C1

50.11

0.0220

SO3

59.50

0.0230

SO4

–2

53.06

0.022

CO3

53.6

0.02

HCO3

19.5

0.02

b

–

Picrate

–

c

a

Data from reference 1. Data are on an equivalent basis. c Tempco = (1/Γ°) (dΓ°/dT). b

in a volume element of the electrolyte and to limit electrode– solution interfacial contributions to this measurement. A variety of sensors have been developed, some using electrodes in contact with the sample, and others not. These sensors can be combined with modern microelectronics, often with integrated software programs, which improves the quality of measurement of concentration of ionic components in process samples.

E

THEORY OF OPERATION The unit of conductance, the reciprocal of the ohm, is the siemens (S). This unit impresses a measurement of the mobility or velocity of ions in an electrolyte under an imposed electric field. The value of this unit depends on the number and hence on the concentration of ions present, which provides its value in analytical measurements. However, since the mobilities of dissimilar ions are different, the measurement response provided by the cell is useful only if the detected component is the sole, or at least the major, contributor to the measured conductivity. Equivalent conductivity is defined as the conductance that is reported for one gramequivalent weight of the conducting ion. The mobility of ions is affected by temperature and by the total concentration of all ions in the solution. Mobility of ions and hence conductance increases with temperature (about 2% per °C) and also with dilution. Table 8.17a shows values of conductance, expressed as equivalent conductivity, of several ions at 25°C corrected to infinite dilution, together with temperature coefficients. Figure 8.17b shows the relationship of cell geometry to the measured conductance of the solution. The electric field applied to the cell is E/d. The current density i/A is the sum of the individual charge carriers in the field, and therefore the conductivity L (which from Ohm’s law is the current divided by the voltage) is given by L = (a/d) Σizi·ci·Γi


8.17(1)

Area = a, cm2

d, cm

FIG. 8.17b Simplified schematic of two-electrode conductivity circuit.

where L = a = d = ci = Γi =

–1

the conductance in ohm or siemens 2 the area of electrodes in cm the distance between electrodes in cm 3 the concentrations of the participating ions in equiv./cm the equivalent conductivity of the participating ion in 2 S·cm /equiv. zi = the charge on the participating ion

THE CELL CONSTANT It is not convenient to measure the ratio d/a geometrically for each cell, but since it is constant for any given cell, it –1 may be assigned a value θ, in cm , termed the cell constant. It is determined experimentally using solutions of accurately

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TABLE 8.17c Specific Conductivity of Potassium Chloride Solutions Used for Determination of Cell Constants Approximate Normality

k µ S/cm

Temperature (°C)

Weight, KCl in g/l of Solution

1.0

72.2460

0 18 25

65,176 97,838 113.342

0.1

7.4265

0 18 25

7138 11,167 12,856

0.01

0.7440

0 18 25

773.6 1220.5 1408.8

Resistivity in ohm-cm

108

107

Conductivity in µS/cm

10 −2

10 −1

106 1

105

104

103

102

10

1

10

102

103

104

105

106

Ultrapure water Demineralized water Condensate Natural waters Cooling tower coolants Percent level of acids, bases, and salts 5% Salinity 2% NaOH 20% HCI Range of contacting cells Range of electrodeless

FIG. 8.17d Resistivity/conductivity spectrum of aqueous electrolytes. (From Light, T. S., Chemtech, August 1990, pp. 4960–4501.)

known concentrations of potassium chloride, for which values of specific conductivity (the conductance of a cube of the solution, 1 cm on each side), denoted by k (in S/cm), have been precisely determined per American Society for Testing Materials (ASTM) Standard D1125–77. A few specific conductivity values are listed in Table 8.17c. The cell constant is readily determined using the expression θ = k/L, where k (the specific conductivity) is known from the tabulated values, and L (the conductance) is measured using the cell being calibrated. Note that conductance and resistance are values read by measuring instruments and have the units of siemens and ohms, respectively. Conductivity and resistivity are intrinsic properties of solutions; they are obtained after application of the cell constant and have the units of siemens-centimeter and ohm-centimeter, respectively. Cell Dimensions As the dimensions of the cell are changed, the cell constant varies as the ration of d to a. For solutions of low conductivity


(about 0.05 to 200 µS/cm), the electrodes can be placed closer − together, giving cell constants in the range of 0.1 to 0.01 cm 1 . Similarly, for more conductive solutions (about 10 to 20,000 µ S/cm), electrode separations can be increased to give −1 cell constants of 1, 10, or sometimes 100 cm . This has the effect of adjusting the actual conductance read by the instrument to a conveniently measured range. Signal-to-noise considerations limit resistance measurements to less than about 2 Mohm. Conductance measurements are limited by signal level magnitudes to less than about 0.2 mS. Composition measurement using conductivity is popular in industrial process measurement and control applications, because of the inherent simplicity and reliability of the technique. Cells are available that cover a resistivity range of 1 8 to 10 in aqueous electrolytes (Figure 8.17d). Three types of cells are used: two-electrode, four-electrode, and electrodeless. The four-electrode and electrodeless cells and their associated instrumentation are shown in Figures 8.17e and 8.17f, respectively.

8.17 Conductivity Analyzers 3

High Impedance Voltage Measuring Amplifier

Sense Electrodes

Drive Electrodes

Drive Electrodes

Cell

1319

Current Source

interfaces. Ideally, the electrodes are made of platinum and are coated with a layer of platinum black. As the conductance of the measured solution decreases, polarization and coating effects become less significant, and metals other than platinum, such as Monel and titanium, are considered as inert electrode materials. Particularly noteworthy is the class of two-electrode conductivity applications, called resistivity measurements, which employ titanium two-electrode cells in monitoring the highpurity water used in semiconductor manufacturing, steam turbine applications, and nuclear reactors. Four-Electrode Measurement

Detector

Oscillator

FIG. 8.17e Four-electrode conductivity measuring circuit.

Cable Primary Toroid

Secondary Toroid

Cell Bore Cell Housing

Induced Electric Current

FIG. 8.17f The electrodeless conductivity cell and instrument. (From Light, T. S. et al. Talanta, 36, 235–241, 1989.)

Two-Electrode Cells Two-electrode cells are best suited for measurement in clean solutions to avoid errors caused by the formation of coatings and films on the electrodes. In these designs, it is desirable to minimize the interfacial impedance of the electrodes with the solution, because the goal is to measure the bulk conductivity of the electrolyte. Derivation of Equation 8.17(1) assumed no iR (potential) loss at the electrodes or in the leads to the cell. The cell and instrument are designed accordingly. In order to avoid significant electrolysis, a small-amplitude (usually sinusoidal) waveform having a frequency in the range of 100 to 1000 Hz is used for excitation. In addition, the electrode materials are selected to reduce polarization or iR (potential) drops at electrode–solution


Four-electrode conductivity is useful for high conductance when coating and fouling of electrodes are a concern. Current is imposed across two drive electrodes, and the potential drop through the electrolyte is detected between two points in the cell using two sense electrodes (Figure 8.17e). The sense electrodes are monitored with a high-input impedance, voltagemeasuring amplifier to minimize the current drawn and electrode polarization. Polarization at the drive electrodes has no effect on the measurement, provided the drive voltage is able to maintain 4 the control current through the cell. This voltage increases with fouling and can be used as a diagnostic tool to signal the user when cleaning is required. Because of geometrical considerations, four-electrode designs are not suited to probe configurations. Precise measurements require flow-through cells that allow linear distribution of current across the sense electrodes. When used in probes, four-electrode measurements are best applied to setpoint control. Typical applications include measurement of salts, acids, and alkalis in chemical processes in the mining, metallurgy, pulp and paper, and aluminum industries, where samples often contain solids, oils, or other materials that form insu5 lating coatings on the electrodes. Electrodeless Cells One way to eliminate electrode polarization effects is to eliminate the electrodes. Techniques to do this are referred to as electrodeless conductivity measurement or, alternately, 6,7 as inductive or toroidal sensing of conductivity. The probe shown in Figure 8.17f consists of two encapsulated toroids. When immersed in the electrolyte, the solution forms a conductive loop shared by both toroids. One toroid radiates an electric field in this loop, and the other detects a small, induced electric current. Practically speaking, the two toroids form a transformer whose coils are interconnected by the resistance of the electrolyte. The radiated field is typically 20 kHz, and the induced current, which is proportional to the conductivity, is amplified, rectified, and displayed. These probes are encapsulated in nonconductive, temperature-stable, and chemically resistant materials such as the fluorocarbon polymers.

1320


MEASUREMENT APPLICATIONS Modern conductivity analyzers, with on-board computers, provide essential features such as temperature correction to reference values, digital display of concentration data from measured values, controller functionality, self-diagnostics, and calculations such as water subtraction and percent rejection. Concentration Measurements Temperature compensation and concentration computation data for common acids, bases, or salts are often imbedded in the instrument’s software. Data for less common materials may often be loaded by the end user. Concentration can be derived from the conductivity of an electrolyte, when there is a significant increase or decrease in conductivity with increasing concentration. While concentration measurements are most often applied to a single electrolyte in solution, they can also be applied to mixtures of electrolytes, when the ratio of the components of the mixture is constant. Concentration measurement is also used in batch reactions, where the progress of the reaction is accompanied by a significant increase or decrease in conductivity. Output signals, either digital or analog, are used in control systems for measurement and control of such processes as boiler feed water or the monitoring of gas scrubber solutions, pickling and plating liquors, etc. High-Purity Water Measurements A special class of temperature compensation has evolved for measuring high-purity water. Monitoring of high-purity water is used in semiconductor processing. In power and pharmaceutical applications, it is necessary to distinguish the temperature coefficient of pure water from that of ionic purity. Today’s “intelligent” instruments can measure both conductivity or resistivity and temperature. They can therefore compute the value of conductivity or resistivity of pure water at the measured temperature. They can also calculate the measurement contribution due to the impurities and output the conductivity or resistivity of the process water, referenced to a standard temperature, or the concentration of impurity after subtraction of the contribution due to the water (for example, as total dissolved solids (TDS) or particles per million (ppm) NaCl). Multiple sensors are used in conjunction with ion exchange columns or reverse osmosis systems to monitor and control inlet and outlet resistivity across the bed and the breakthrough of unwanted ions and, as a predictive tool, to signal the need for bed regeneration. If the sample temperature is not controlled, water temperature compensation is necessary for accurate measurements for conductivity values at or below 1 µS/cm, or resistivity readings at or


above 1 Mohm⋅cm. A variant of this temperature compensation, called cation conductivity temperature compensation, is used for high-purity water samples, such as the effluent of a cation exchange bed, which has acid as the major impurity. Corrosive and Fouling Applications The other extreme, i.e., highly conductive solutions—those that are highly concentrated, corrosive, and contaminated with fouling materials—is best measured with the electrodeless designs. Here, coatings only affect the measurement response to the extent that they alter the geometry, and hence the cell constant, of the probe. The relatively large size of these probes renders this effect small or negligible. Examples of applications include on-line analysis of oleum in H2SO4 SO3 H2O, measurement and control of alkalinity and solution strength in many industries using lime slaking, industrial dishwashing rinse control, gas scrubber solution 6 concentration control, and many others. Pulp processing uses extremely corrosive chemicals, high temperatures and pressures, and samples entrained with solids and particles; such processing provides an example of how conductance analyzers are applied. Figure 8.17g shows how the conductivity sensors are integrated into a continuous 9 Kraft digester commonly used in paper pulp making. Temperature, flow, and alkali concentration data are used by the control systems to control the uniformity of the pulp by manipulating the residual alkali strength in the cooking liquor in response to such changing feed properties as wood chip composition, species, chip moisture, and uniformity of concentration of makeup chemicals. For a summary of conductivity analyzer applications, refer to Table 8.17h. To Control System CE White Liquor Charge H.P. Steam Digestor To Control System CE

Heat Exchange

Recirculation Line

Fig. 8.17g Example of conductivity measurement used in control of batch digestor for paper making.

8.17 Conductivity Analyzers

1321

TABLE 8.17h Conductivity Applications Process

Application (Usage) and Comments

Chemical streams

To measure and control solution strength.

Steam boilers

Blowdown is a method of lowering the amount of dissolved solids in a boiler by dilution. To control buildup of dissolved solids to prevent scaling and corrosion. Condensate return is usually checked for quality before being returned to the boiler. If out-of-limits, it is dumped.

Waste streams

A means of determining the amount of dissolved salts being discharged.

Cooling towers

Bleed control is a method of reducing the total dissolved solids in a tower by dilution (similar to blowdown). To prevent scaling and corrosion. For bleed control, the electrodeless conductivity system works best to minimize maintenance and failure.

Fruit peeling

Strong caustic is used, and its strength can be determined by conductivity.

Rinse water

Plating shop running rinse water is monitored for dissolved salts—a method of reducing water consumption.

Semiconductor rinse water

Requires ultrapure water, usually measured in mega-ohms/centimeter.

Interface determination

Usually used in food processing, e.g., dairy and brewing. Most commonly used in cleaning in place (CIP); interfaces in pipes are easily determined and can be diverted by valves controlled by conductivity.

Demineralizer output

Determination of ion exchange exhaustion.

Reverse osmosis

Efficiency of reverse osmosis (RO) operations is usually monitored by comparing inlet and outlet conductivity or TDS ratio (cell 1/cell 2). The inlet conductivity is installed upstream of the RO feed pump to avoid highpressure requirements. Also, abnormal readings can be used to diagnose membrane fouling, improper flow rate, membrane failure, etc.

Desalination

Similar to reverse osmosis and demineralization process.

Deionization process

Conductivity or resistivity measurement provides capability for monitoring and controlling the acid and caustic dilution. Regeneration of deionizers requires consistent application of acid and caustic to obtain repeatable results. Savings is provided by consistent regeneration, which assures deionized water availability, less frequent regeneration, long resin life, and conservation of costly reagents. More precise control can be obtained by using conductivity ratio measurement. A comparison of inlet and outlet (ratio of cell 1/cell 2) conductivity across the bed can determine the unwanted ions and the need for bed regeneration, which can compensate and control for variations in mineral concentration of feed water.

Ion exchange

Occasionally loses resin. If a resin bead or fines are trapped between the electrodes of a cell, it is shorted and produces a very low resistivity (or high conductivity) reading. This feature is a great help in troubleshooting.

CALIBRATION AND MAINTENANCE Calibration of Conductivity Sensors Conductivity measurements can be calibrated using conductivity standards or on-line, by grab sample analysis. The conductivity sensor should be given sufficient time to reach the temperature of the standard solution, or in the case of online calibration, the sensors should be calibrated only after the process has been at a stable temperature for some time. By so doing, temperature compensation errors are eliminated, because the temperature element in the conductivity sensor will have had time to reach the same temperature as that of the standard or the process. This is especially important when using electrodeless sensors, which typically have a much larger mass than contacting sensors, and therefore require more time to reach thermal equilibrium.


Conductivity or resistivity measurements in high-purity water applications cannot be calibrated by using standards or calibrated on-line by using grab samples. This is because of the extreme sensitivity of the samples to contamination by trace amounts of electrolytes and even to atmospheric CO2. It has been argued that the accuracy of calibrations, when using conductivity standards of less than 100 µS/cm, can be 9 questionable. Therefore, the conductivity sensors on high-purity water applications should be calibrated by calibrating the input to the analyzer with precision resistors (usually done by the manufacturer) and using a conductivity sensor with a predetermined cell constant, which is then entered into the software of the analyzer by the user. Further calibrations are done using a certified reference conductivity system.

1322


Maintenance of Conductivity Cells Conductivity measuring systems may be designed to be troublefree and produce reliable measurements; however, some maintenance is required, especially for the electrodes. In addition, the cell may require periodic cleaning depending on the type of application, the quality of the water passing through it, and the type of cell used. Some types of contaminants may not interfere directly with the measured conductivity—e.g., organic materials, rust, and suspended solids—but may form deposits on the electrode surfaces. In most cases, these surfaces can be cleaned with a bristle brush and a weak detergent solution. Problems may also occur in hard-water applications, where gradual formation of scale will reduce the active area of the electrodes, which over a period of time will result in an apparent decrease in conductivity. For this type of fouling, simple brush cleaning is insufficient, as it will not remove scale from the cell. To remove the scaling, the electrode should be treated with a 10% solution of formic or hydrochloric acid. The presence of bubbles will indicate that the scale is being dissolved. It takes about 2 or 3 min and is complete when the bubble formation ceases. Then the cell should be thoroughly rinsed to remove all traces of acid before it is returned into the process. Cells with stainless steel electrodes are generally used in applications where a low conductivity is combined with low concentrations of organic contamination. For high-purity water applications, titanium electrodes are used, because of their better performance characteristics in very low conductivity samples. In applications where fouling or corrosion is anticipated, the need for cleaning can best be minimized by the use of electrodeless conductivity sensors.

applications up to 20,000 µS/cm, typically contacting conductivity sensors are used. For higher conductivity ranges, or in samples that can foul or corrode the metal electrodes, electrodeless detectors are the best choice. For conductivity measurements at 1 µS/cm and below, contacting conductivity sensors should be used in conjunction with a conductivity analyzer, which is provided with high-purity temperature compensation. References 1.

2.

3.

4.

5.

6. 7. 8. 9.

“The Measurement of Electrolytic Conductance,” in Handbook of Analytical Instrumentation, Ewing, G. W., Ed., New York: Marcel Dekker, 1989. ASTM D1125–82, “Standard Test Methods for Electrical Conductivity and Resistivity of Water,” 1983 Annual Book of ASTM Standards, Vol. 11.01, Philadelphia: American Society for Testing and Materials, 1983, pp. 149–156. Braunstein, J. and Robbins, G. D., “Electrolytic Conductance Measurements and Capacitive Balance,” Journal of Chemical Education, 48, pp. 52–59, 1971. “Conductivity and Conductometry,” in Laboratory Techniques in Electroanalytical Chemistry, Kissinger, P. T. and Heineman, W. R., Eds., New York: Marcel Dekker, 1984. Anderson, F. P., Brookes, H. C., Hotz, M. C. B., and Spong, A. H., “Measurement of Electrolytic Conductance with a Four-Electrode Alternating Current,” Journal of Scientific Instruments (Journal of Physics E), Series 2, 2, pp. 491–502, 1969. Light, T. S., Chemtech, August 1990, pp. 4960–4501. Light, T. S., McHale, E. J., and Fletcher, K. S., “Electrodeless Conductivity,” Talanta, 36, pp. 235–241, 1989. Lavigne, J. R., Instrumentation Applications for the Pulp and Paper Industry, San Francisco, CA: Miller Freeman Publications, 1979. Gingerella, M. and Jacanin, J. A., “Is There an Accurate LowConductivity Standard Solution?” Cal Lab, July–August 2000, pp. 29–36.

Bibliography CONCLUSION Selecting the right conductivity cell includes having information on the cell constant for the analyzer, the conductivity range, the materials of construction selected to resist corrosion, and the appropriate mounting of the sensor. When designing a conductivity measurement system, the first consideration is the conductivity range of the sample. In


Bevilacqua, A. C., “Ultrapure Water: The Standard for Resistivity Measurements of Ultrapure Water,” Semiconductor Pure Water and Chemicals Conference, Santa Clara, March CA, 2–5, 1998. Gray, D. M. and Bevilacqua, A. C., “Cation Conductivity Temperature Compensation,” International Water Conference, Pittsburgh, PA, November 1997. Morash, K. R., Thornton, R. D., Saunders, C. H., Bevilacqua, A. C., and Light, T. S., “Measurement of the Resistivity of Ultrapure Water at Elevated Temperatures,” Ultrapure Water Journal, 11(9), pp. 18–26, December 1994.

8.18

Consistency Analyzers

Consistency AIT

A. BRODGESELL (1969, 1982) B. G. LIPTÁK M. H. WALLER (2003)

To Receiver


Types:

Blade, rotary, probe, optical, microwave, radiological

Element Materials:

Stainless steel and titanium

Normal Design Temperature:

Up to 250°F (120°C)

Normal Design Pressure:

Up to 125 PSIG (8.6 bars)

Range:

0.01 to 15% consistency

Sensitivity:

0.01 to 0.03% consistency

Repeatability:

0.5% of reading

Inaccuracy:

Function of empirical calibration, usually 1% of reading

Cost:

Laboratory units, $3000 to $10,000; continuous industrial units, $6000 to $30,000


ABB (www.abb.com) Berthold Technologies (www.berthold.com) BTG (www.teambtg.com) CyberMetrics (www.cyber-metrics.com) DeZurik/Copes-Vulcan (www.dezurik.com) Electron Machine Corp. (www.electronmachine.com) Honeywell (www.honeywell.com) Metso (www.metsoautomation.com) NDC Infrared Engineering (www.ndc.com) Ronan Engineering (www.ronan.com) Thermo MeasureTech (www.thermo.com) Thompson Equipment Co. (www.teco-inc.com)

INTRODUCTION By definition, consistency is expressed as a percentage by dividing the mass of solid material by the total mass of a wet sample, resulting in units of mass per unit mass. Mechanically, consistency is the resistance to deformation or shear by fibrous materials, and thus is related to apparent viscosity. Such materials include wood pulp, dough, tomato paste, paint, gelatin, or drilling mud. This section will focus on those methods used for the measurement of consistency in the paper industry, involving pulp–water mixtures. In the

laboratory, consistency is measured using a gravimetric 1 method described in TAPPI Test Method T 240 om-88. General industrial methods used for consistency measurement are 2–4 described in a number of sources. In order to have good control over the basis weight of the paper product, it is necessary to maintain the consistency of the feed constant. An increase in temperature or an increasing inorganic materials content will reduce the viscosity, and thus apparent consistency, while increasing freeness (ability of the suspension to release water), increasing fiber length, or increasing pH will increase the apparent consistency. Pipeline 1323


1324


velocity also can influence the consistency reading for mechanical sensors and, therefore, it is advisable to measure consistency in turbulent locations at constant flow rate. Consistency is also related to turbidity and sludge or suspended solids detectors.

WATER

WATER VERTICAL

HORIZONTAL

IN-LINE CONSISTENCY MEASUREMENT Forty years ago, consistency was considered to be a mature measurement, almost totally relying on mechanical devices for shear force measurements in the 2 to 5% range. Today, we still rely on mechanical shear force measurements, but in addition, we have devices utilizing light scattering, light transmission, nuclear radiation, radio waves, and microwaves. Mechanical measurement devices might be categorized as either static (a fixed probe or blade) or moving (blades, rotating disks, or propeller). The development of high-intensity light-emitting diodes 25 years ago allowed further development of optical consistency sensors. These gauges relied on scattered or transmitted light for measure5 ment up to 4% consistency. Several other approaches to consistency measurement have been attempted with varying degrees of success. Gamma attenuation devices measure consistency on the basis of density changes. Recent developments in plastic scintillation 6 detector technology have improved sensitivity and stability. Because the density difference between fibers and water is very small, high sensitivity is a must. Unfortunately, fillers are quite dense, and if present in the pulp, will yield a false high reading. Similarly, the presence of air will yield false low readings. Microwave measurement techniques offer the promise of being independent of pulp type, fiber length, brightness, color, and flow rate. The most prevalent commercial technique is the measurement of propagation velocity, or time of flight through the stock, which is a function of the relative permittivity of the material. Because of the factor of 10 difference in the dielectric constant of water and wood fiber, velocity is a strong function of consistency. These microwave devices measure both fiber and filler, and compensation must be made for the filler amount and type. In addition, this method is sensitive to air, conductivity, and temperature, for which compensation must be made. One commercial device quotes specifications of a measuring range up to 8% consistency (C), with a sensitivity and accuracy of 0.0005% C at a flow velocity in the range of 1 7 to 16 ft/sec (0.3 to 5 m/sec). A similar microwave propagation technique uses the phase difference between an original wave and one that passed through the stock to determine consistency. This device is claimed to be resistant to the effects of contamination and bubbles, with a range of 1 to 8 10% consistency. Ideally, the complete process stream should be exposed to the sensor, but in very large flows this is not practical and, therefore, samples are taken. The sample should be taken


FIG. 8.18a Installation of blade-type consistency transmitter in vertical and horizontal pipelines. (Courtesy of DeZurik/Copes-Vulcan.)

from the center of the pipe, usually from the discharge of a centrifugal pump so that separation or settling of solids is minimized (Figure 8.18a). Mechanical Devices These consistency-measuring instruments detect consistency of the process fluid as shear forces acting on the sensing element for consistencies greater than about 1%. In rotary devices, the shear force is reflected as the torque required to maintain the sensor at constant speed, as the imbalance of a strain-gauge resistance bridge, or as a turning moment. The instruments are calibrated in-line; thus, the output is not in terms of bone-dry consistency, but rather some arbitrary, reproducible value. Stationary sensors depend on the process flow for measurement, and for such instruments, the output is affected by the velocity of the flow. For blade sensors, the sensor contour is designed to minimize flow effects on the output over the operating flow range. On the other hand, rotating sensors do not depend on process flow for a measurement. While these units are also sensitive to flow velocity variations, they generally can be used over wider flow ranges. In addition, the rotary motion of the sensor produces some self-cleaning action, whereas the fixed sensors depend on either a properly designed contour or an upstream deflector to prevent material hang-up. Probe Type This sensor transmitter functions as a resistance bridge strain gauge. The bridge elements are bonded to the inner wall of a hollow cylinder that is inserted into the process. The shear force acting in the cylinder, due to the consistency of the process fluid, causes an imbalance of the resistance bridge. The amount of imbalance is proportional to the shear force and the consistency of the process fluid. The resistance bridge is powered from a recorder that also contains the AC potentiometer electronics. Pipeline velocity must be measured as compensating information, and a deflector in the pipe upstream of the probe prevents accumulation 9 of strings and like material. The sensor is mounted through a threaded bushing furnished with the unit. Flowing velocity must be between 0.5 and 5.0 ft/sec (0.15 and 1.5 m/sec) in order to obtain repeatability, of around 0.1% of bone-dry consistency, up to 16%.

8.18 Consistency Analyzers

AIR BLEED NOZZLE

1325

ROTARY SENSOR

AIR SUPPLY PNEUMATIC RELAY

COARSE ZERO ADJUST

FEED BACK UNIT 3-15 PSIG OUTPUT

FORCE BAR

FINE ZERO ADJUST

TORQUE

POSITIVE FEED BACK UNIT

TORQUE ARM (CONNECTED TO MOTOR HOUSING)

FULCRUM + SENSOR

− SENSITIVITY ADJUSTMENT

FLOW TRANSMITTER SCHEMATIC

F1

FORCE BAR TO PNEUMATIC TRANSMITTER

F3

SEAL FLUSH WITH PIPE WALL (FULCRUM)

F2

SENSOR BLADE

MATERIAL FLOW SHEAR FORCE BLADE DETAILS

AIR NOZZLES

RANGE ADJUSTMENT FIXED RESTRICTIONS

DIFFERENTIAL BOOSTER

20 PSIG AIR SUPPLY OUTPUT 3-15 PSIG

F1

FIG. 8.18b Stationary blade sensor and transmitter schematic. (Courtesy of Invensys Process Systems/Foxboro.)

Blade Types The sensing element of this instrument is a fixed blade, specially shaped to minimize the effects of velocity. As shown in Figure 8.18b, material flowing past the blade, which is positioned along the line of flow, creates a shear force. Velocity of the process produces two drag forces, F1 and F2, whose resultant F3 acts through the fulcrum. The moment arm of F3 is therefore zero, and the effect of velocity on the measurement is negligible over a range of 0.75 to 5 ft/sec (0.23 to 1.5 m/sec). Changes in consistency up to the 12% level are transmitted through the blade to the force bar, causing small changes in the relationship between flapper and nozzle. Therefore, the relay unit output pressure changes until the force due to the feedback unit balances the shear force. The instrument can be mounted on any line 4 in. (100 mm) or larger. Mounting is through a 2-in. (50-mm) flange supplied with the instrument. Most new instruments use electronic systems for force measurement, replacing pneumatic devices. Moving-blade devices stroke the blade, cutting across the flow in the plane of the blade, measuring the time required to complete the stroke. Higher consistencies will require a longer time, and vice versa. Compensation for velocity is affected by a deflector mounted upstream.


NEGATIVE FEED BACK UNIT

FIG. 8.18c Air schematic of rotating sensor.

Rotating Sensors This unit consists of a motor-driven, ribbed disk immersed in the process fluid. The disk is rotated at constant speed, and variations in torque output by the motor are sensed by a torque arm. The motor is suspended by flexure bearings and anchored to the torque arm, which senses motor reaction torque. The tip of the torque arm is located between two air nozzles so that minute movements of the arm, caused by torque variations, are reflected as changes in two air output pressures (Figure 8.18c). The nozzle pressures are fed back to bellows that react to the torque arm movement by exerting an opposing force until equilibrium is reached between increased nozzle pressure and the force exerted by the torque arm. In many cases, electronic systems for torque measurement have replaced pneumatic devices. Although this unit is less sensitive to flow changes than the strain gauge and force balance types, problems are introduced by the shaft seal required for this design. The torque variations must reflect only consistency changes and, therefore, shaft friction variations are detrimental to the measurement. One of the latest mechanical transmitters has the appearance of two blades on a rotating disk. It is claimed to measure torque and consistency from 1 to 14% on an absolute basis by operating on the pulp while in plug flow. The establishment

1326


Light Source

Light Source

Light Source

A

B

C D

Backscattering Depolarization

D

Stock

Laser light

Absorption

Capillary with Sample flow

D2

D D

Detector Filter or Polarizer

D1

FIG. 8.18e Measurement principle of the kajaaniRM-200 C for woodfree pulp. (Courtesy of Metso Automation.)

FIG. 8.18d A variety of optical sensors.

of plug flow requires a calming length (L) determined from 10 the following equation: L = R/[7 D%C]

8.18(1)

where L is in feet R is flow in gallons per minute D is pipe diameter in inches %C is percent consistency Optical Sensors The range of these measurements is generally 1% and below for transmission devices, and up to 4% for reflection sensors. Accordingly, optical devices, either in transmission or scatter mode, are the sensors of choice for low consistencies, relying on the fiber’s interaction with light, as shown in Figure 8.18d for three types of sensors. Sensor A uses linearly polarized light from either a halogen bulb or a semiconductor laser, which is passed through the measurement cell. The transmitted light is split into two beams, one passing through a second transverse-plane polarizing filter, the other passing through a third in-plane polarizing filter. The beams are detected by photodiodes and combined to produce a relative depolarization signal, which is a function of the total fiber and filler. The signal is insensitive to brightness, color, freeness, or soluble additives. Sensor B is based on the transmittance of light as a function of consistency. Unfortunately, this sensor is relatively sensitive to changes in freeness and color, exhibiting nonlinear behavior with changes in filler and dissolved solids. Sensor C uses forward- and backscattered light to produce a signal combined from the several detectors that is proportional to consistency. This type of sensor can be used at much higher consistencies (ca. 4%), and, in general, its sensitivity to variations in the content of nonfibrous substance lies between that of sensors A and B. The exception to this 11 rule is filler, for which this sensor is the most sensitive.


FIG. 8.18f The kajaaniRMi for wood-containing pulp. (Courtesy of Metso Automation.)

Measuring Woodfree Pulp Optical sensors are frequently used to manage retention on a paper machine by measuring the total consistency at the head box and in the machine white water early and late in the forming zone. One such device, the kajaaniRM-200 C, is illustrated in Figure 8.18e. This device is similar to sensor A in Figure 8.18d, in that a polarized light beam is directed through a glass capillary cell, where the sample continuously flows. The transmitted light is directed through a special aperature disk for scattering measurements, and then through a second polarizer, which splits the light into cross-polarized and parallel-polarized components that are detected by photodiodes. The depolarization signal mainly indicates the total consistency of the sample, and the attenuation of light is affected by the total consistency and filler consistency. Attenuation is strongly affected by scattering and light absorption. Since backscattering and attenuation are influenced by small particles, filler 12 consistency is calculated from these signals. Consistency of Pulp Containing Wood For pulp containing a considerable amount of mechanical fibers, and thus a large fraction of lignin, the depolarization scheme loses effectiveness. Another sensor has been developed that uses two light sources and a combination of optical measurement principles, including depolarization, absorption, and scattering at several wavelengths from the ultraviolet (UV) to the near infrared (NIR). An outline of this sensor is shown in Figure 8.18f.

8.18 Consistency Analyzers

3

1

1327

4

2 DC 5

AC

PEAK 6

7

8 9

1 Light source

10

6 Peak detector

2 Suspension

7 AC filter

3 Detector

8 Computer

4 Amplifier

9 Input signals

5 DC filter

10 Output signals

FIG. 8.18g The BTG Wet-end Consistency Analyzer. (Courtesy of BTG Pulp and Paper Technology AB.)

The near-IR semiconductor laser light is polarized, passed through the cell, and then depolarized, as before, in Figure 8.18e. UV light from the xenon lamp is directed through the cell via a filter and polarizing prism. The forwardscattered light is directed through the lens and aperature disk to photodiodes. Backward scattering is also measured for both the UV and IR lights by detection with a photodiode before the cell. Light extinction, as well as backward and forward scattering, is measured at several different wavelengths. The signals are processed to monitor total solids and 13 filler consistencies and flocculation in the sample. Figure 8.18g shows the components of an analyzer with a sensing scheme that is similar to sensor A in Figure 8.18d, while Figure 8.18h describes a typical signal trace from this analyzer. A light beam is directed at the suspension, and a photo detector senses the transmitted light. Three independent filters process the detector signal. The first filter determines the mean value VDC of the transmitted light; the second determines the peak value VP; and the third extracts the AC 14 component VAC of the signal. The peak method used in the analysis assumes that the suspension is substantially characterized by large and small particles. The large particles (fibers) form a relatively transparent network within which the much greater number of smaller particles (fillers and fines) float freely. Observation of a typical suspension over time reveals that the great number of small particles is relatively constant, whereas the number of large particles is few and variable. The average value of the transmitted light determines VDC. Deviations from this mean value are mainly due to the large particles passing through the light beam. The highest light intensity and VP occur when no fibers are passing


Light Transmission

VCW FPC VP LPC VDC AC Signal Time

FIG. 8.18h A sample signal trace from the BTG Wet-end Consistency Analyzer. (From Wold, D., “The Peak Method of Optical Analysis Realizes the Benefits of Low-Consistency Measurement,” in UpTimes, No. 5, Säffle, Sweden: BTG Pulp and Paper Technology, 1999, pp. 24–25.)

through the beam and the light is being dimmed only by the suspended fine particles. Thus, the respective amounts of large and small particles in the suspension can be determined 15 by the mean and peak values. Referring to Figure 8.18h, VCW is the detector signal for clear water and is used as a reference value. The AC signal, VAC, is plotted along with VCW , VP , and VDC. The large particle content (LPC) is the difference between VP and VDC, while the fine particle content (FPC) is the difference between

1328


VCW and VP . The total consistency is obtained by summing LPC and FPC. The consistency measurement at 30% for TMP or CTMP pulp is based on dielectric measurement of the water content and an optical measurement using an NIR technique with reflectance spectroscopy. This technique is based on the resonance vibration of water, which appears as absorption bands in the infrared region of the spectrum. A typical sensor uses four fixed wavelengths of the spectrum—two located in the absorption bands of water and two in a region where the 16 effect of water is minimal.

SUMMARY While convenient from an installation standpoint, mechanical in-line instruments are all sensitive to flow variations. Fixed sensors are more likely to be plagued by material buildup, particularly if the sample contains fibers. Rotating sensors are self-cleaning because sensor motion will tend to spin off any material; however, variations in shaft seal friction can be troublesome. The newer optical sensors have found great utility at low consistencies, and the search for alternative methods for consistency measurement continues.

References 1. Consistency (Concentration) of Pulp Suspensions, T 240 om-88, TAPPI Test Methods, 1, Atlanta, GA: TAPPI Press, 1988. 2. Jansson, I., Ed., Accurate Consistency, Säffle, Sweden: BTG Pulp and Paper Technology AB, 1999. 3. Ostroot, G. F., The Consistency Control Book, Atlanta, GA: TAPPI Press, 1993. 4. Waller, M. H., Measurement and Control of Paper Stock Consistency, Instrument Society of America Monograph 5, Research Triangle Park, NC: ISA, 1983. 5. Jack, J. S., Bentley, R. G., and Barron, R. L., “Optical Pulp Consistency Sensors,” Pulp & Paper Canada, 91(2): T76–80, 1990. 6. Petersen, D. E., “Nuclear Density Consistency Meter Evaluation,” Proceedings, 1994 Process Control Symposium, Atlanta, GA: TAPPI Press, 1994, pp. 9–12. 7. “MIC-2300 Consistency Sensor,” Data Sheet D2009/0en, BTG Pulp and Paper Technology AB, Säffle, Sweden, 2000.


8. “Consistency Sensor,” http://www.cyber-metrics.com/consiste.htm, CyberMetrics, Alpharetta, GA, 1998. 9. Thompson, H. A., “Consistency Control, Medium and High Range,” 1986 Engineering Conference Proceedings, Atlanta, GA: TAPPI Press, 1986, pp. 593–596. 10. Preikschat, E., “ISO-Torq: The Next Generation of Rotating Consistency Transmitters,” TAPPI Journal, 82(7): 133–139, 1999. 11. Reed, H. W. and Corbett, J. O., “Optical Consistency Measurement,” in Instrumentation in the Pulp & Paper Industry, Research Triangle Park, NC: Instrument Society of America, 1985, pp. 21, 25–35. 12. Kaunonen, A., Lehmikangas, K., Nokelainen, J., and Tikkanen, P., “Practical Experiences of How to Control Wet End Operations Using Continuous Retention Measurement,” Proceedings, 1991 Papermakers Conference, Seattle, WA, April 8–10, 1991, Atlanta, GA: TAPPI Press, 1991, pp. 39–45. 13. Kaunonen, A., “Improving Runability and Quality through Consistency Measurement and Control,” Paper Technology, 38(3): 41–48, 1997. 14. Wold, D., “The Peak Method of Optical Analysis Realizes the Benefits of Low-Consistency Measurement,” in UpTimes, No. 5, Säffle, Sweden: BTG Pulp and Paper Technology, 1999, pp. 24–25. 15. Shaw, P. and Fladda, G., “A Modern Approach to Retention Measurement and Control,” Paper Technology, 34(3): 36–40, 1993. 16. “High Consistency Measurement, above 30%,” http://www.consistency. com/templates/main.cfm?id=616.

Bibliography Balls, B. W., “Towards Better Understanding of Consistency Measurements,” Measurement Control, Vol. 1, No. 9, September 1968. Casey, J. P., Ed., Pulp and Paper: Chemistry and Chemical Technology, 3rd ed., Vol. 1, New York: Wiley, 1980. Cooper, H. R., “Using On-Stream X-Ray Fluorescence for Slurry Composition Analysis,” InTech, July 1981. Denny, R. and Sinclair, R., Visible and Ultraviolet Spectroscopy, New York: John Wiley & Sons, 1987. Dykes, J. T., “Consistency Installations and System Design Techniques,” TAPPI, 46(11): 680, 1963. Ewing, G., Analytical Instrumentation Handbook, New York: Marcel Dekker, 1990. McGill, R. J., Measurement and Control in Papermaking, Bristol, England: Adam Hilger, 1980. Nassau, K., The Physics and Chemistry of Color, New York: John Wiley & Sons, 1983. Staff, A Consistency Manual, Process Control Committee, Technical Section, Montreal, Canada, June 1967. Torborg, R. H., “Fine Tuning of a Consistency Control System for Maximum Performance,” Pulp Paper, March 1980, pp. 134–138. Waller, M. H., “Measurement and Control of Paper Stock Consistency,” ISA Conference, Houston, TX, October 1992.

8.19

Corrosion Monitoring D. H. F. LIU

(1995)

B. G. LIPTÁK

To Receiver

AT Corrosion


Type:

A. Electric resistance B. Linear polarization resistance

Design Pressure:

A. Up to 6000 PSIG (422 bars)

Design Temperature:

A. Up to 1000°F (560°C)


Wide range of corrosion-resistant metals or alloys

Cost:

$3000 to $5000


Accurate Corrosion Monitoring Co. (www.acm-corrosion.com) ACM Instruments (www.acminstruments.com) Arizona Instrument (www.ariz.com) CMS Corrosion Monitoring (www.a-bau.co.at/cms-e.htm) Cormon (www.cormon.com) CorrOcean (www.corrocean.com/fsmcorrmon) Cosa Instrument Corp. (www.cosa-instrument.com) Dasibi Environmental Corp. (www.dasibi.com) Endevco (www.endevco.com) InterCorr International Inc. (www.intercorr.com) Purafil Inc. (www.purafil.cpm) Rohrback Cosasco Systems (www.corrpro.com/rcs/whatis.htm) Waltron Ltd.

INTRODUCTION A detailed tabulation of the chemical resistance of materials is given in Appendix 3 of this volume. A more condensed summary of the corrosion resistance of some widely used materials is also given in Table 8.19a. In selecting the materials of construction for instruments, one should also keep in mind that most process fluids are not pure and that the rate of corrosion is also a function of flow velocity and of dissolved oxygen content. Nondestructive testing, such as ultrasonic scanning, is also used in inspecting storage tanks, pressure vessels, and piping. If the plant’s atmosphere contains corrosive gases, it is also desirable to enclose, purge, or otherwise protect the more sensitive instruments or their components. In such installations it is advisable to protect the stems of control valves by boots. Corrosion monitoring can be based on 30- or 60-day coupons, which provide data only on the average rate of

corrosion. Mobile monitoring laboratories can provide spot checks, or in the more critical cases, permanent monitoring equipment can be installed to provide continuous corrosion rate and pitting tendency readings. In cooling systems, where chlorine is used to control biological deposition, corrosion monitoring is needed because if the chlorine residual is too high, corrosion will occur.

CORROSION MONITORING TECHNIQUES A variety of corrosion monitoring techniques are listed in Table 8.19b. These can monitor total corrosion, corrosion rate, and the state of corrosion. Some of them can also determine the composition of products and detect the presence of defects or changes in physical parameters. However, any monitoring technique can provide only limited information, and the techniques should be regarded as complementary rather than competitive. Where more than one technique can 1329


1330


TABLE 8.19a Corrosion and Cavitation Resistance of Various Materials Trim or Valve Body Material

Relative Cavitation Resistance Index

Approximate Rockwell C Hardness Values

Corrosion Resistance

Cost

1 5 12 28 60 70 120 140 150 160 200 200 300 340 350 400 1000

0 Very high 2 30 40 3 25 72 54 35 40 72 32 44 47 55 45

Fair Excellent Poor Fair Fair Fair Poor Good Good Excellent Good Fair Excellent Excellent Excellent Fair Excellent

Low High Low Low Low Low Low High Medium Medium Medium High High Medium Medium High Medium

Excellent Excellent

Medium High

Aluminum Synthetic sapphire Brass Carbon steel, AISI C1213 Carbon steel, WCB Nodular iron Cast iron Tungsten carbide Stellite 1 Stainless steel, type 316 Stainless steel, type 410 Aluminum oxide K-Monel Stainless steel, type 17-4 PH Stellite 12 Stainless steel, type 440C Stainless steel, type 329, annealed Stellite 6 Stellite 6B

3500 3500

TABLE 8.19b Instrumentation for Corrosion Monitoring Method

Measures or Detects

Corrosion coupon

Average corrosion rate over a known exposure period by weight loss

Linear polarization

Corrosion is measured by electrochemical polarization resistance method

Electric resistance

Metal loss is measured by the resistance change of corroding metal element

Analytical

pH of process stream

Radiography

Flaws and cracks by penetration of radiation and detection on film

Ultrasonics

Thickness of metal and presence of cracks, pits, etc., by changes in response to ultrasonic waves

Eddy current testing

Use of a magnetic probe to scan surface

Hydrogen sensing

Hydrogen probe used to measure hydrogen gas liberated by corrosion

Analytical

Concentration of corroded metal ions or concentration of inhibitor; oxygen concentration in process stream

Potential monitoring

Potential change of monitoring metal with respect to a reference electrode

provide the information required, such a cross-check can be very valuable, and their differences can add to their value. This section focuses on coupon monitoring and on two frequently used on-line corrosion monitors: 1) the electric


44 44

resistance monitors, and 2) the linear polarization resistance (LPR) monitors. Coupon monitoring provides long-term performance data on general corrosion and information on localized corrosion. Electric resistance monitors give medium- to longterm data on general corrosion or erosion. LPR monitors provide real-time operational data on general corrosion and enable engineers to study the dynamics of the corroding system and to observe the effects of the addition of various corrosion inhibitors.

CORROSION COUPON MONITORING Coupons are the simplest and most frequently used devices in the monitoring of corrosion. Coupons are small pieces of metals, usually of a rectangular shape, which are inserted in the process stream and are removed after a period for study. Specimens for standard corrosion, stress corrosion, crevice corrosion, and galvanic corrosion tests are shown in Figure 8.19c. The most common use of coupons is to determine average rate of uniform corrosion over the period of exposure. The coupon or specimen is weighed before and after exposure. The average corrosion rate is calculated from the weight loss, the initial surface area, and the time exposed. The rate is usually expressed in mils per year (mpy) (1 mil = 0.001 in.) or millimeters per year (mmpy), since corrosion is generally a long-term effect.

8.19 Corrosion Monitoring

1

2

3

1331

1"

FINE POLISH (NO. 120) 2" ROUGH POLISH (NO. 40)

WELD

DRILL & STAMP

STANDARD TEST PIECES FOR CORROSION TESTING

2

WELD POLISHED

1

POLISH (NO. 120)

CRACKS HERE

STRESS CORROSION

NON METALLIC WASHER CREVICES

BOLT SAME MATERIAL OR INSULATED

CREVICE CONTACT

CATHODE ANODE

GALVANIC CONTACT

FIG. 8.19c Specimens for standard corrosion, stress corrosion, crevice corrosion, and galvanic corrosion tests.

Improper cleaning of coupons or specimens after exposure is the main source of error in determining the corrosion rate. American Society for Testing and Materials (ASTM) Standard 61–81 describes the recommended practice for preparing, cleaning, and evaluating test specimens. In addition, Fontana and Greene (see bibliography) have useful information, particularly on techniques for sample cleaning after exposure. The measurement of the depths even of extremely shallow pits is important. Pit depth should be measured to an accuracy of 1.0 mil using a micrometer.


Time of Exposure Proper selection of the time the sample will be exposed is critical. In batch processes that involve cyclic exposure conditions, the coupon should undergo all of the batch conditions, including periods of shutdown. Exposure should be made for two or more complete cycles of operations to ensure that an equivalent of 2 weeks exposure time is obtained. Usually a 2-week test is acceptable, but a 1-month test is preferable.

1332


Normally, initial corrosion rates are high, because as the corrosion products form, they tend to protect the surface and, as a consequence, the corrosion rate will drop below the initial rate. Therefore, it will be necessary to conduct tests of sufficient duration to compensate for this effect.

CORROSIVE ENVIRONMENT

MEASURING ELEMENT

PROBE

Advantages and Limitations Advantages of the coupon monitoring technique are as follows: 1. The technique is suitable for all environments. 2. Coupons provide information about the type of corrosion present. Coupons can be examined for evidence of pitting and other localized forms of attack. 3. There are a variety of coupons available for specialized analysis.

REFERENCE ELEMENT

PROTECTED FROM CORROSIVE ENVIRONMENT

CABLE

AMPLIFIER

METER

Limitations of the technique are as follows: BRIDGE BALANCE RESISTOR

1. High corrosion rates for short periods of time may be undetectable and cannot be correlated to process upset conditions. 2. The technique requires plant shutdowns for installation or removal. Highly qualified personnel and reasonably sophisticated test procedures are required for the interpretation of the results.

ELECTRICAL RESISTANCE MONITORS Figure 8.19d is a simplified diagram of an electric resistance corrosion monitor. It shows a probe that has both an exposed measuring element and a protected reference element. They are connected to a Wheatstone bridge measuring an output circuit, and a power supply. The measuring element is a loop of wire that is exposed to the corrosive environment. The reference element is encapsulated inside the probe body by a thermally conductive plastic so that it will be at the process temperature. The output is directly proportional to the resistance of the thinwire measuring element, because as corrosion increases, it causes the thickness of the measuring element to decrease (Figure 8.19e). Advantages and Limitations Electrical resistance monitors are basically automatic coupons, and they share many characteristics with coupons when it comes to advantages and limitations. The resistance probe must be allowed to corrode for a period of time before accurate corrosion measurements can be made. Advantages of the electric resistance technique are as follows:


POWER SUPPLY

FIG. 8.19d Basic diagram of electric resistance corrosion monitor with wire loop probe. (Courtesy of Rohrback Instruments.)

1. The technique is suitable for all environments except liquid metals or some conductive molten salts. The process material, which causes the corrosion, need not be an electrolyte (in fact, it need not be a liquid). 2. A corrosion measurement can be made without having to see or remove the test sample. 3. Corrosion measurements can be made quickly—in a few hours or days—or continuously. Sudden increases or decreases in corrosion rate can be detected, so that the user can modify the process to reduce the corrosion. 4. The method can detect low corrosion rates that would take a long time to detect with weight-loss methods. Its accuracy is comparable to the coupon method. Limitations of the technique are as follows: 1. The technique is usually limited to the measurement of uniform corrosion only and is not generally satisfactory for localized corrosion. 2. The probe design includes provisions for temperature variations. This feature is not totally successful. The most reliable results are obtained in constant-temperature systems. 3. The technique does not provide an instantaneous corrosion rate, so any corrective actions must be delayed until an average corrosion rate can be determined.

8.19 Corrosion Monitoring

1333

FLUSH MOUNTING TANK WALL

0.75" (19 mm)

INSERT PROBE

WIRE ELEMENT

STRIP ELEMENT

BEFORE CORROSION BEGINS ...

... ELEMENT PARTIALLY CORRODED ...

... END OF LIFE AS CORROSION PROCEEDS, THE CROSS SECTION OF THE ELEMENT IS REDUCED

FIG. 8.19e Probe configurations of corrosion sensors.

4. The resistance method measures a combination of chemical and physical erosion without distinguishing between the two.

causes a change in potential of the test electrode when compared to the freely corroding reference electrode. The relationship between the change in current flow (dI), the change in polarization voltage (dE), and the corrosion rate (CR) of the test electrode is as follows:

LINEAR POLARIZATION RESISTANCE MONITORS Figure 8.19f shows a three-electrode and a two-electrode linear polarization system. In addition to the electrodes, the three-electrode system includes an ammeter, a voltmeter, and an adjustable current source. The meters have adjustable null features to compensate for normal variations in the surface conditions of the test and reference electrodes. A small electrical current flows between the test and auxiliary electrodes. Because corrosion occurs at the anode, the test electrode is protected when it is the cathode. The direction of current flow is then reversed, and the test electrode becomes the anode and its corrosion rate is accelerated. The change in corrosion rate caused by the reversal in current


CR = K

dI dE

8.19(1)

Since all corrosion measurements are made at a constant polarization potential of 10 mV, the voltage term becomes constant, so the corrosion equation can be reduced to CR = KI

8.18(2)

Under constant polarization voltage, the corrosion rate is directly proportional to the current required to produce that polarization voltage. A value representing the K factor is

1334


V

A

A

AUXILIARY

TEST

REFERENCE

V

3 ELECTRODE SYSTEM POLARIZED BY 10 mv ANODICALLY AND CATHODICALLY COMPARED TO REFERENCE ELECTRODE.

2 ELECTRODE SYSTEMS IMPRESS A 2 mv DIFFERENCE BETWEEN THE TWO ELECTRODES. IT IS ASSUMED THAT ONE ELECTRODE IS SHIFTED 10 mv ANODICALLY AND THE OTHER 10 mv CATHODICALLY. MEASUREMENTS CAN BE MADE IN EITHER DIRECTION.

VITON GASKET

BLOW OUT PREVENTOR

SPECIAL GASKETMETAL SEAL

END VIEW

3-ELECTRODE ASSEMBLY

FIG. 8.19f Linear polarization probes.

designed into the equipment so that the readings are linear and calibrated directly in mils per year. Advantages and Limitations Advantages of the linear polarization resistance technique are as follows: 1. The polarization probes measure corrosion rate almost instantaneously. They measure the instantaneous corrosion rate instead of the average corrosion rate. 2. The probes are useful for comparing relative corrosion rates. Thus, it is possible to use the data to determine the process variables that give the lowest corrosion rates. 3. The commercially available polarization probes supply pitting tendency information. Limitations of the technique are as follows: 1. The probes will not work in nonconductive fluids or fluids containing compounds that coat the electrodes (e.g., crude oil). 2. The absolute accuracy of the corrosion measurement is not as reliable as the one obtained from corrosion coupons.


3. The method measures the combined rate of any electrochemical reactions at the surface of the test sample. If reactions other than corrosion reactions are possible at comparable or greater rates, the measured rate will also include these other reactions.

Bibliography ASTM Standard 61-81, “Recommended Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens,” 3(2), Philadelphia, PA: American Society for Testing and Materials, 1983, p. 87. Clansky, K. B., “The Chemical Guide to OSHA Hazard Communication Standard,” Roytech, revised annually. Corrosion Handbook, Coxmoor Publ. Co., 2000. Fontana, M. G. and Greene, N. D., Corrosion Engineering, New York: McGraw-Hill, latest edition. Moran, G. C., “Corrosion Monitoring in Industrial Plants,” West Conshohocken, PA: American Society for Testing and Materials, 1986. Moreland, P. J. and Hines, J. G., “Corrosion Monitoring: Select the Right System,” Hydrocarbon Processing, 57 (11), 1978, pp. 251–255. Perry, R. H., Ed., Chemical Engineers Handbook, New York: McGraw-Hill, 7th edition, 1997. Rohrback Cosasco Systems, Inc., “Corrosion Monitoring Primer,” Bulletin 901, Santa Fe Springs, CA, 1989. Schweitzer, P., Corrosion Resistance Tables, New York: Marcel Dekker, 1986.

8.20

Differential Vapor Pressure Sensor B. G. LIPTÁK

RECEIVER

PDT

(1995, 2003)

DVP Flow Sheet symbol

Purpose:

Compare vapor pressure of reference fluid to that of process fluid

Wetted Parts:

316 stainless steel

Typical Ranges:

From 10–0–10 in. H2O (254–0–254 mm H2O) up to 425–0–425 in. H2O (10.8–0–10.8 m H2O)

Maximum Working Pressure:

1500 PSIG (105 bars)

Maximum Working Temperature:

250°F (121°C)

Ambient Effect:

1% or less per 100°F (55°C)

Repeatability:

0.1% of span

Dead Band:

0.1% of span

Inaccuracy:

0.5% of span (better with microprocessor-based transmitters)

Cost:

The sum of the cost of a d/p transmitter (about $1500), a temperature bulb, and the cost of attaching and filling the bulb with the desired reference fluid; total cost about $2500


Not manufactured as a complete unit. Any d/p transmitter can be converted to act as a differential vapor pressure transmitter by connecting a thermal bulb filled with a reference fluid to one side of the transmitter. For suppliers of d/p transmitters, refer to Section 5.6.

INTRODUCTION

DESIGN AND OPERATION

Vapor pressure is the pressure that a vapor exerts when it is in equilibrium with its own liquid. At a constant temperature, the vapor pressure is just as unique a characteristic of a liquid as is its boiling point (Table 8.20a). Therefore, by measuring the vapor pressure of a fluid, one can determine the composition of that fluid. If the temperature of the process varies, temperature compensation is needed. With microprocessor-based, smart instruments, such compensation is easily provided, if the relationship between vapor pressure and temperature is entered into the compensation software (Figure 8.20b).

The differential vapor pressure transmitter is a d/p cell with one of its sides connected to a temperature bulb filled with a reference fluid (Figure 8.20c), while the other side is connected to the process. This way the sensor can continuously compare the vapor pressure of the process material with the vapor pressure of the sealed-in reference fluid. If the reference bulb is inserted into the process, the need for temperature compensation can be eliminated. This way the temperature of the reference fluid is always the same as that of the process. Consequently, if the two compositions are the same, their vapor pressures will also be identical. 1335


1336


TABLE 8.20a Refrigerant Characteristics

Chemically Inert and Noncorrosive Yes

For low-temperature service

116

No

Yes

Yes

Low-efficiency refrigerant

132

T, F

No

Yes

175

92

No

(1)

Yes

34

169

555

T, F

No

(2)

High-efficiency refrigerant

26

108

67

No

Yes

Yes

Most recommended

–11

21

95

178

(3)

Yes

(4)

Expansion valve may freeze if water is present

R

+14

12

66

166

T, I

No

(4)

Common to these refrigerants:

CHCl2F

RO

+48

5

31

108

No

Yes

Yes

a. Evaporator under vacuum

Ethyl chloride

C2H5Cl

RO

+54

5

27

175

F, I

No

(5)

b. Low compressor discharge pressure

Freon-11

CCl3F

C

+75

3

18

83

No

Yes

Yes

c. High volume-to-mass ratio across compressor

Dichloro methane

CH2Cl2

C

+105

1

10

155

No

Yes

Yes

Latent Heat in BTU/lbm at 18°F (– 7.8°C)

c

b

No

b

T, F

Refrigerant

a

Mixes and/or Compatible with the Lubricating Oil

Toxic (T), Flammable (F), Irritating (I)

Condenser Pressure in PSIA if Operating Temperature is 86°F (30°C)

Evaporator Pressure in PSIA if Operating Temperature is 5°F (–15°C)

Boiling Point in °F at Atmospheric Pressure

Applicable Compressor (R = Reciprocating, RO = Rotary, C = Centrifugal)

Feature

Ethane

C2H6

R

–127

236

675

148

Carbon dioxide

CO2

R

–108

334

1039

Propane

C3H8

R

–48

42

155

Freon-22

CHCIF2

R

–41

43

Ammonia

NH3

R

–28

Freon-12

CCl2F2

R

–22

Methyl chloride

CH3Cl

R

Sulphur dioxide

SO2

Freon-21

Remarks

For low-temperature service

Notes: (1) Oil floats on it at low temperature; (2) corrosive to copper-bearing alloys; (3) anesthetic; (4) corrosive in the presence of water; (5) attacks rubber compounds. °F − 32 1.8 b PSIA = 6.9 kPa c BTU/lbm = 232.6 J/kg a

°C =

The operating principle of this analyzer makes it ideal for product quality control applications. For example, it can be used in distillation, dryer, evaporator, or similar applications where the product is a binary material and the goal is to continuously check the composition of the product. In case of distillation column control applications, a reference fluid-filled bulb is connected to one side of a standard d/p transmitter (Figure 8.20d), and the bulb is inserted into


the liquid on the control. The other side of the d/p cell is connected to the vapor space above that tray. This way, if the product being made and the reference fluid are the same, their vapor pressures will be identical and the d/p cell will read zero. This is a very convenient analyzer for product composition control on optimized distillation columns, where the operating pressure is not constant but is “minimized” as a

8.20 Differential Vapor Pressure Sensor

PRESSURE (kPa) PSIA

−40 −20

0

20

40

60

80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 300 22

)

(2070) 300

1337

L CO AL

NPR YL OP YL AL AL CO WA CO HO TE HO L IS R OA L M YL AL CO IS HO OB L UT

140 120

R HE ET

-X Y LE N E M

60

40

P

-B UT

-C Y M EN E

50

30

YL

AL

N

CO

100 90 80 70

25

PR OP

HY

L

AL

HO

CO

L

HO

L

OR OE LU IF

ET

OR OT R HL IC TR

YL

CH NE LE

HY (F E

AN TH

ET OR OM

LU

HO

TA N

RI DE LO

OP IS

ET M ) 13 -1

(F NE HA

E( AN TH ON OR OM HL

250 180 160

ISO

20 ATMOSPHERIC PRESSURE

A ME CET O TH YL NE AL CO HO L

DI

(103.5) 15 (69) 10 (55.2) 8

OF

BU TA NE LU OR OM E

OF ON RO M LO

DI C

(138) 20

CH

(207) 30 (172.5) 25

ME TH YL

HL OR OD I

(276) 40

IC

(345) 50

TR

(414) 60

F-

FL UO RO ME CH TH AN LO RI E( DE F1

2)

21

)

-1

1)

MO

NO

100 90 80 70

EN

RI LO CH L HY ET

CH

(828) 120

DE

SU LP HU RD IO XI ISO DE BU TA NE

LO RO D

(966) 140

E

IFL UO RO ME PR OP TH AN AN E E(

(1380) 200 (1242) 180 (1104) 160

(690) (621) (552) (483)

250

F-

(1725) 250

15 10 8

(41.4)

6

(27.6)

4

4

(20.7)

3

3

(13.8)

2

2

(6.9)

1

−40 −20 (−40)

0

20

(−17)

(−29)

40 (4.4)

(−6.7)

6

TEMPERATURE °F (°C)

60

80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 (26.7)

(15.6)

(48.9)

(37.8)

(71) (60)

(93.3) (82)

(115.5)

(104.4)

(137.8)

(126.7)

(160)

(148.9)

(171)

(182)

(204.4)

193.3

(226.7)

(215.6)

(248.9)

(237.8)

(271)

(260)

(293.3) (282)

(215.6)

(304.4)

FIG. 8.20b Relationship between vapor pressure and the temperature of some liquids.

PROCESS CONNECTION DIFFERENTIAL PRESSURE SENSOR

TEMPERATURE BULB

REFERENCE LIQUID

On the other hand, a differential vapor pressure analyzer will correctly measure the binary composition under variable column pressure conditions, because as the column pressure drops, it also lowers the boiling point on the tray. This in turn will lower the reference bulb temperature and with it the vapor pressure inside the reference bulb. Therefore, as long as the tray composition is constant, the pressure differential will also be constant, even if the column pressure varies.

FIG. 8.20c Differential vapor pressure transmitter.

LIMITATIONS

function of the available cooling tower water temperature. In such applications, a temperature measurement will not accurately reflect composition, because the boiling point varies with pressure.

The limitations of this analyzer include that 1) it correctly reflects the composition of only binary materials; 2) it is essential for a successful installation that the reference fluid be stable; and 3) there can be a transient upset if the column pressure changes faster than the pressure in the reference bulb can follow.


1338


FIG. 8.20d A “smart” force balance d/p transmitter can be fitted with a reference bulb to serve as a differential vapor pressure transmitter. (Foxboro instrumentation courtesy of Invensys plc.)

Reference 1.

Foxboro Co., Technical Information Sheet 371-91a and General Specification GS 2B-1d3 B, Foxboro, MA, 2002.

Bibliography Dyer, S. A., Wiley Survey of Instrumentation and Measurement, New York: Wiley, 2001.


Figliola, R. S. and Beasley, D. E., Theory and Design for Mechanical Measurements, 3rd ed., New York: Wiley, 2000. Hughes, T. A., “Pressure Measurement,” EMC Series, Downloadable PDF, ISA, Research Triangle Park, NC, 2002. ISA, Industrial Measurement Series: Pressure (Video VHS, PAL, and NTSC), 2002. Johnson, R., “Pressure Sensing: It’s Everywhere,” Control Engineering, 2001. Marrano, S. J., “How to Choose and Apply Pressure Transmitters,” Control, March 2000. von Beckerath, A. et al., WIKA Handbook on Pressure and Temperature Measurement, Wika Instrument Corp., Lawrenceville, GA, 1998.

8.21

Dioxin Analysis D. H. F. LIU

(1995)

B. G. LIPTÁK

(2003)

Type of Sample:

Gas sample containing particulate


Generally atmospheric or near atmospheric

Standard Design Temperature:

–25 to 1500°F (–32 to 815°C)

Sample Velocity:

7 to 167 ft/sec (2 to 50 m/sec)


316 or 304 stainless steel for pitot tubes; nickel, nickel-plated stainless, quartz, or borosilicate glass for nozzles

Cost:

$8000 to $16,000 for a complete EPA particulate sampling system (EPA Reference Method 23); $75,000 to $100,000 for laboratory-scale GC-MS unit


Bacharach, Inc. (www.bacharach-inc.com) Cosa Instrument Corp. (www.cosa-instrument.com) MPU Gmb. (www.dioxin.de) Sierra Monitor Corp. (www.sierramonitor.com) Teledyne Analytical Instruments (www.teledyne-ai.com) Xenobiotic Detection Systems (www.dioxin.com)

INTRODUCTION This section deals with the sampling and analysis of recalcitrant dioxin compounds. These measurements are required in ecological risk assessment and in the determination of the toxicity of various samples. The U.S. Environmental Protection Agency (EPA) regulates emissions from municipal waste combustors (MWCs) and sets emission limits for polychlorinated dibenzo-p-dioxines (PCDDs) and polychlorinated dibenzofurans (PCDFs). This section is a summary of the proposed Method 23 for measuring the emissions of PCDDs and PCDFs from MWCs. PRINCIPLE OF OPERATION Figure 8.21a is a schematic of the sampling train that is used to measure the emission of PCDDs and PCDFs from MWCs. A sample is withdrawn isokinetically from the stack through a probe, a filter, and a trap packed with Amberlite XAD-2 resin. Figure 8.21b shows the condenser and absorbent trap. The PCDDs and PCDFs are collected in the probe, on the filter, and on the solid adsorbent.

See Section 8.3 for a detailed discussion of the features and operation of stack sampling systems.

Sample Recovery At the end of the sampling period, the cleanup procedure begins as soon as the probe is removed from the stack. The procedure is as follows. First, any particulate matter and fibers that adhere to the filter holder gasket are carefully transferred to a container (container 1). Next, the absorbent module (XAD-2 sorbent trap in Figure 8.21a) is removed form the train. After that, the material that is deposited in the nozzle, probe transfer lines, and the front half of the filter holder is quantitatively recovered first by brushing, while rinsing each with acetone. Then the probe is rinsed with methylene chloride and all the rinses are collected. The back half of the filter and the condenser are also rinsed with acetone, and the connecting line is soaked with methylene chloride. All the above rinses are collected in container 2. After that, the rinsing is repeated with toluene as the rinse solvent and the rinses are collected in container 3. 1339


1340


STACK WALL TEMPERATURE SENSOR CONDENSER TEMPERATURE SENSOR

HEATED GLASS LINER HEATED AREA

XAD-2 TRAP

"S" TYPE PITOT TUBE

GAS FLOW

TEMPERATURE SENSOR

FILTER HOLDER

INCLINED MANOMETER

RECIRCULATION PUMP

100 ml HPLC WATER

EMPTY

SILICA GEL

EMPTY DRY GAS METER

GAS EXIT

CALIBRATED ORIFICE T

FINE

COARSE

VACUUM GAUGE P

T

VACUUM PUMP

INCLINED MANOMETER

FIG. 8.21a Sampling train.

SORBENT TRAP

CONDENSER FLUE GAS FLOW 37 cm

TO STILL

8 mm GLASS COOLING COIL

# 20/18

# 20/18 WATER JACKET

COOLING COIL GLASS WOOL PLUG

WATER JACKET

XAD-2

GLASS SINTERED DISK

FIG. 8.21b Condenser and absorbent trap.

Sample Extraction All the PCDDs and PCDFs are extracted from the particulate matter, the absorbent, and the rinses. The absorbent module (XAD-2) and the particulate cake from the filter are extracted with toluene in the Soxhlet apparatus. The content of container 2 is evaporated at temperatures less that 37°C and is concentrated to dryness. This residue contains the particulate matter removed in the rinse of the train probe and nozzle.


The residue is added to the filter and the XAD-2 resin for extraction in the Soxhlet apparatus. In order to recover all the PCDDs and PCDFs, the material in container 3 goes through a series of multistep treatments. Analysis The extracted PCDDs and PCDFs are separated by high-resolution gas chromatography (GC), and each isomer is identified and measured with high-resolution mass spectrometry (MS).

8.21 Dioxin Analysis

The total PCDDs and PCDFs are the sum of the individual isomers. The gas chromatograph uses an oven that can maintain the separation column at the proper operating temperature and perform programmed increases in temperature at rates of at least 72°F/min (40°C/min). It uses a fused silica column with a length of 183 ft and an inside diameter (ID) of 0.01 in. (60 m × 0.25 mm ID), coated with DB-5, and a fused silica column that is 91 ft × 0.01 in. (30 m × 0.25 mm ID), coated with DB-225. In operation, the oven temperature starts at 302°F (150°C) and is raised by at least 72°F/min (40°C/min) to 374°F (190°C), and then at 5.4°F/min (3°C/min) up to 572°F (300°C). The mass spectrometer is capable of routine operation at a resolution of 1:10,000 with a stability of ±5 ppm.

CONCLUSIONS The complexity of this method is such that to obtain reliable results, testers should be trained and experienced with the test procedures. All glass components of the sampling train upstream of and including the absorbing module should be cleaned. The method is described in Section 3A of the “Manual of Analytical Methods for the Analysis of Pesticides in Human and Environmental Samples.” On ground glass connections of used glassware, any residual silicone grease sealant should be removed by soaking the glassware in chromic acid cleaning solution first.


1341

Bibliography Bretthauer, E. W., Dioxin Technologies, Dianen Publishing, 1991. Enviormental Protection Agency, Method 8290, “The Analysis of Polychlorinated Dibenzo-p-Dioxin and Polychlorinated Dibenzofurans by High Resolution Gas Chromatography/High Resolution Mass Spectrometry in Test Methods for Evaluating Solid Waste,” SW-846, EPA, Washington, D.C., 1992. Enviornmental Protection Agency, “Standard of Performance for New Stationary Source,” Federal Register, 56(30): 5760–5769, 1991. Hasselriis, F., “Minimizing Trace Organic Emissions from Combustion of Municipal Solid Waste by the Use of Carbon Monoxide Monitors,” National Waste Processing Conference, Denver, 1986 (ASME, 1986). Kemp, C. C., “Panel Session on Dioxin,” National Waste Processing Conference, Denver, 1986 (ASME, 1986). Konheim, C. S., “Panel Session on Dioxin,” National Waste Processing Conference, Denver, 1986 (ASME, 1986). Niessen, W. R., “The Role and Importance of Polychlorinated Dibenzo-pDioxins (CDDs) and Polychlorinated Dibenzo Furans (CDFs) from Resource Recovery Facilities,” National Waste Processing Conference, Denver, 1986 (ASME, 1986). Poillon, F., Dioxin Perspectives, New York: Plenum Press, 1992. Stavropoulos, B. and Harrison, R. O., “Advances in Dioxin Measurement Using High Performance Immunoassay Technology,” Leeder Consulting, Fairfield, Vic. Australia, 2002. Stettler, A., “Results of Investigations of the Five Gases of the Refuse Incineration Installation at Zurich-Josephstrasse Regarding Chlorinated Dioxins and Furans,” Proceedings of the 44th Technical Waste Conference, University of Stuttgart, Germany, 1983. Thompson, J. R., Ed., Analysis of Pesticide Residues in Human and Environmental Samples, Research Triangle Park, NC: U.S. Environmental Protection Agency, 1974. World Health Organization Summary Report, “Working Group on Risks to Public Health of Dioxins from Incineration of Sewage Sludge and Municipal Solid Waste,” Naples, FL, March 1986.

8.22

Elemental Monitors D. H. F. LIU

(1995)

B. G. LIPTÁK

To Receiver

AT Elements

(2003)

Flow Sheet Symbol

Type of Instrument:

A. ICP (inductively coupled plasma) analytical atomic spectrometer A1. ICP-OES (optical emission spectrometer) A2. ICP-AES (atomic emission spectrometer) A3. ICP-MS (mass spectrometer) B. XRF (x-ray fluorescence) spectrometer

Element Range:

A. Virtually all elements B. A1 to U

Cost:

A1. $50,000 to $200,000 A3. $140,000 to $350,000 B. $200,000 to $300,000 (on-line); $10,000 to $30,000 (portable)


Agilent Technologies (A) (www.agilent.com) Finnigan MAT GmbH (www.thermoquest.com) Intax GmbH (B) (www.intax-berlin.de) Jobin Yvon, Horiba (B) (www.jyinc.com) Jordan Valley (B) (www.forevision.com) Kontron GmbH (A) (www.kontron.com) Leeman Labs Inc. (A) (www.leemanlabs.com) Oxford Instruments (B) (www.dxcicdd.com) PerkinElmer Instruments (A) (www.instruments.perkinelmer.com) Phototronics Products (A) (www.phototronics.com) Shimadzu Sci. Instruments (A) (www.shimadzu.com) Spectro Analytical Instruments (B) (www.spectro-ai.com) Thermo ARL (B) (www.thermo.com) Thermo Elemental (B) (www.thermoelemental.com) Varian Inc. (A) (www.varianinc.com)

INTRODUCTION Since its appearance in the 1960s, atomic absorption (AA) has become a widely used analytical tool. It is a popular analysis technique for elemental analysis in solids, liquids, and loose powders of either organic or inorganic materials. Typical applications include the detection of sulfur (S) and lead (Pb) in refineries; the measurement of oxides in such raw materials as limestone, sand, bauxite, ceramics, slags, and sinters; and the sensing of major and minor elements (Cu, Fe, Ni, etc.) in food and chemical products. Compared to earlier techniques, AA is faster, more sensitive, and free from interference. The inductively coupled plasma (ICP) atomic emission technique first appeared in the late 1970s and has been recognized as a technique with remarkable selectivity and sensitivity. It has simultaneous or rapid sequential multielement determination capability at the major, minor, trace, and 1342 © 2003 by Béla Lipták

ultratrace concentration levels without change of operating conditions. X-ray fluorescence (XRF) is an analytical technique that is well suited to on-line applications. It has been used to measure the elemental composition of solids, liquids, and slurries. Solids analyzed have been in such diverse forms as powders, granules, sheets, or discrete parts. XRF is fast, reliable, and nondestructive (Figure 8.22a). It is also less expensive than other techniques, such as neutron activation, and introduces few health or safety hazards.

ATOMIC ABSORPTION SPECTROMETER Figure 8.22b illustrates the basic operating principles of flame and furnace AA spectrometers. In flame AA, the sample is aspirated, nebulized, and passed into a flame that is burning

8.22 Elemental Monitors

1343

at about 3000°K. The flame causes the sample compounds to dissociate. The sample atoms then can absorb radiation at wavelengths, called resonance lines, that are specific for the elements. The radiation source is a hollow cathode lamp in which the cathode contains the element being determined, and the monochromator is tuned to the appropriate resonance wavelength. Most commercial AA instruments can determine only one element at a time. In furnace AA, a hollow graphite cylinder replaces the flame. The cylinder is heated electrically to a temperature high enough to atomize the sample. This unit is called a furnace atomizer, graphite furnace, or carbon rod. The device improves detection limits by 100- to 1000-fold, as compared to flame AA, but the furnace exhibits poorer precision and slower analysis speed, experiences more interference, and costs more.

INDUCTIVELY COUPLED PLASMA DETECTOR FIG. 8.22a X-ray fluorescence spectrometer. (Courtesy of Roentec GmbH, Berlin, Germany.)

FLAME ATOMIC ABSORPTION SPECTROMETER DETECTOR FLAME HOLLOW CATHODE LAMP

Figure 8.22c shows a schematic of an ICP type detector. A stream of argon passes through a radio-frequency (RF) induction coil, producing considerable heat. When “seed electrons” are introduced, an argon plasma is formed in the fireball region of the plasma from 8000° to 10,000°K. Argon is used to sustain the plasma because it requires less RF power than other gases. The outer flow supplies the plasma gas and cools the quartz torch. The inner flow carries the sample aerosol into the plasma. An optional middle flow helps to position the plasma and prevents carbon buildup on the tip of the central tube.

GRATING

OBSERVATION ZONE MANOCHROMATOR

BURNER

MAGNETIC FIELD LINES

SAMPLE

COPPER INDUCTION COIL

FURNACE ATOMIC ABSORPTION SPECTROMETER SAMPLE ENTRANCE

HOLLOW GRAPHITE CYLINDER QUARTZ TORCH

TO MONOCHROMATOR

OPTICAL BEAM

COOLANT FLOW INTERMEDIATE OR AUXILIARY FLOW ELECTRICAL HEATING

FIG. 8.22b Atomic absorption spectrometers.


AEROSOL CARRIER FLOW

FIG. 8.22c Inductively coupled plasma.

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SIMULTANEOUS ICP SPECTROMETER

PHOTOMULTIPLIER DETECTORS

EXIT SLIT

2. ICP SOURCE ENTRANCE SLIT DIFFRACTION GRATING

SEQUENTIAL ICP SPECTROMETER DETECTOR EXIT SLIT

3. ICP SOURCE

MOTOR-DRIVER GRATING

ENTRANCE SLIT

4.

FIG. 8.22d Inductively coupled plasma spectrometers.

Operating Principle Figure 8.22d illustrates the basic operating principles of both the simultaneous ICP spectrometer and the sequential ICP spectrometer. In either system, the sample is nebulized and introduced into the center of the plasma. The temperature causes the molecules to dissociate. At the same time, the atoms are excited and the excited atoms emit radiation. Every element emits radiation at its own characteristic wavelength. In simultaneous ICP, the light passes through the entrance slit of a polychromator and is then diffracted by a grating. Exit slits and photomultiplier (PM) detectors are placed to capture the radiation of the wavelengths from the elements of interest. The electrical current produced by the PM detector is proportional to the concentration of the element emitting that light. In sequential ICP, the light output from a plasma torch passes through the entrance slit of a monochromator. The grating angle (and hence the output wavelength) is varied by a stepping motor under computer control. The grating stays at each wavelength long enough to make the measurement. It then is driven rapidly to the next desired wavelength.

5.

in a given sample. This produces a significant advantage in a sequential instrument, and an even greater advantage in a simultaneous instrument. In either case, speed in the two techniques differs by an order of magnitude. Flexibility: The ICP technique gives a highly linear response, which enables simultaneous analysis of elements over a broad range of concentrations. This simplifies sample preparation and calibration. ICP requires no special cathode lamps for specific elements, so the sequential instruments become permanently available for analyzing any element. Also, the ICP technique enables the analysis of elements such as S and P that cannot be handled using the flame AA technique. It also provides increased capability for analyzing large series of elements such as B, W, lanthanides, and refractory in general. Matrix interference: ICP spectroscopy is much more free from chemical and physical interference. An uncommonly high plasma temperature in an inert environment (Ar gas) reduces the problem of matrix interference. Detection limits: The great sensitivity inherent in the technique and its improved signal-to-noise capacity give ICP better detection limits than those available with traditional flame AA. But the detection limits of furnace AA are better than those of a plasma for most elements. The exceptions are rare earth elements (B, U, Zr, etc.) plus nonmetals (S, Cl, I), which the AA furnace cannot measure. Precision: Both systems use a pneumatic transfer system for samples and are very similar with respect to the degree of precision (i.e., repeatability) that can be obtained. There are no limitations to the use of ICP, as it can be used on gases, liquids, and solids (if spark ablation, laser ablation, or slurry introduction accessories are provided), so that vapors can be introduced into the plasma.

In spite of its advantages, the use of ICP has been limited by its relatively high cost, which can be justified only for large-scale analysis tasks or for solving problems that cannot be solved with flame AA. Because of increased productivity and lower prices, many laboratories have replaced AA spectrophotomers with ICP systems and left only the simpler tasks to be performed by flame AA spectrophotomers. X-RAY FLUORESCENCE SPECTROMETER

SELECTING AMONG AA AND ICP When deciding whether an AA instrument or an ICP instrument is the best option, the following should be considered: 1. Speed of analysis: The higher speed of ICP analysis stems from its capability to analyze several elements


XRF spectrometers irradiate the sample with a beam of highenergy x-rays that excite the elements present in the sample, so that they produce characteristic x-rays. This phenomenon is called x-ray fluorescence. XFR provides specific analyses of total elemental concentrations, without regard to chemical combinations.

8.22 Elemental Monitors

The sample is excited by a source of electromagnetic radiation. This radiation excites the elements in the sample, causing them to fluoresce (i.e., give off their characteristic x-rays). The energy of these characteristic x-rays identifies the element, and the intensity is a measure of the element’s concentration in the sample. The resulting signal is integrated over a period to give an average measure of concentration during this time.

DETECTOR SPECIMEN SOURCE WAVELENGTH-DISPERSIV X-RAY SPECTROMETRY

CRYSTAL SPECTROMETER

Instrumentation

SPECIMEN

On-line XRF analyzers can be configured in different ways to excite the process sample or to detect the characteristic fluorescence. The right combination of excitation and detection methods depends on the specific application. The x-ray source may be an x-ray tube or radioactive isotope (iron55, curium-244, cadmium-109, or americium-241). The detection method may be either wavelength-dispersive (WDXRF) or energy-dispersive (EDXRF) XRF. When choosing either the anode used for the x-ray or a specific radioactive isotope, proper consideration should be given to the range of elements to be measured and the corresponding concentrations. Figure 8.22e illustrates the principles of WDXRF and EDXRF instruments. With WDXRF, the x-ray energies are separated according to specific wavelengths by diffracting crystals. The respective intensities are then measured by individual proportional counters. Sample excitation is by x-ray tube. With EDXRF, characteristic fluorescence is detected by a low-temperature, solid-state detector or a gas-filled proportional counter and sorted electronically to produce an XRF spectrum of x-ray-intensity energy. In EDXRF, sample excitation is by a low-level radioisotope source. The WDXRF technique offers the best resolution, shortest analysis time, and highest sensitivity. It is well suited for

SOURCE

ENERGY-DISPERSIVE X-RAY SPECTROMETRY

DETECTOR

FIG. 8.22e

X-ray fluorescence spectrometers.

the most demanding tasks, such as analyzing neighboring elements with large differences in concentrations. An EDXRF can be very compact and economical. It gives excellent sensitivity with good resolution, especially for light element analysis. The EDXRF systems are very useful in quality control, troubleshooting problems, etc. Table 8.22f gives a partial list of proven and potential online XFR applications. A wide choice of sampling systems is now available for continuous monitoring of liquids in pipelines or tanks, granular solids in chutes, parts on conveyors, coatings on moving sheets of metals, or pigments in moving plastics or paper sheets. As on-line XRF technology advances, continued growth in the number of applications and installations can be expected.

TABLE 8.22f List of Proven and Potential On-Line XRF Applications Petroleum Refining • S in crude, refinery streams, gasoline, fuel oil, diesel, and MEA, DEA, and TEA stripper bottoms • V, Ni, and Fe in crude residues, cracking catalysts, coke, and bunker fuels • Ca, Zn, Cu, S, Cl, and Ba in lube-blending operations • S in sour water Chemical and Petrochemical • Ti, Zn, Ca, and other fillers in polyolefins and other polymers • Sn, Br, Sb, P, Cl, or other fire retardants in fiber (nylon, orlon, etc.) manufacture • S, Cl, and Br in elastomers • P in detergents • Monitoring catalysts—typically Co, Mo, Ni, Mn, Br, Ti, Pt, and Rh in processing • Blending additives manufacture (Ca, Zn, and Ba) Inorganic Chemical Processing


1345

1346


TABLE 8.22f Continued List of Proven and Potential On-Line XRF Applications • • • • • • • • •

Phosphoric acid manufacture Metal treatment (plating) chemical manufacturing Inks and paints (Ti, Zn, Pb, etc.); TiO2 manufacture China and other types of clay purification (Ti and Fe) Photographic chemicals (As and Br) Rare earths and rare earth chemical manufacture Extractive hydrometallurgy (Cu, Zn, U, Hf, and Zr) Glass manufacture—monitor Si, Fe, Ti, Ca, and K Cement industry—monitor Fe, Ca, and Si

Pulp, Paper, and Rayon • Ca and Ti additives in pulp • Lignosulfates (S) • Wood preservatives (As, Cu, and Cr) • Viscose manufacture (Zn and S) Catalyst Industry • Precious metal catalysts (Pt, Pd, and Rh) • Auto emission, reforming catalysts • Manufacture of Mo, Co, Ni, and Zn-based catalysts • Catalyst recovery operations Plating and Electroplating • • • •

Zn Ni and Zn Fe alloy coating of steel Metals in pickling baths Zn phosphating bath Zn Pb, Au Ni K, and Ni Br in plating baths for PC boards and electronic components

Metal Refining and Hydrometallurgical Operations • All base metals (Zn, Cu, Pb, Mo, Co, Ni, etc.)

Bibliography Clevett, K. J., Process Analyzer Technology, New York: John Wiley & Sons, 1986, pp. 383–388. Fiorini, C. and Longoni, A., IEEE Transactions Nuclear Science, 46, N6, p. 2011, 1999. Fitzgibbon, P., “X-Ray Fluorescence for On-Line Elemental Analysis,” Control, March 1992. Gatti, F. and Rehak, R., Nuclear Instrumental Methods, A225, p. 608, 1984.


Jobin, Y., Division of Instrument S.A., ICP Technique as a Replacement for Automatic Absorption Coupled Plasma (ICP), Edison, NJ: Jobin Yvon, 1989, pp. 1–22. Kahn, H. L., “AA or ICP? Each Technique Has Its Own Advantages,” Ind. Res. & Dev., February 1982, pp. 156–160. Lechner, P. et al., Nuclear Instrumental Methods, A377, p. 346, 1996. Ramanujam, R. S. and Fitzgibbon, P., “X-Ray Fluorescence for On-Line Elemental Analysis,” Control, March 1990. X-Ray Fluorescence, US EPA, Washington, D.C., 2001. X-Ray Fluorescence, Amptek Inc., Bedford, MA, 2002.

8.23

Fiber-Optic Probes D. H. F. LIU

(1995)

M. W. REED

TO RECEIVER

FOP

(2003) AT

UV-VIS-NIR Flow Sheet Symbol

Type of Process Fluid:

Liquids, solids, gases


Up to 1000 PSIG (70 bar)

Standard Design Temperature:

Up to 900°F (480°C)


Stainless steel (others are available), sapphire, sealing materials

Costs:

From $1000 for wand probe with tip to $2500 for spectra-caliper probe; fiber-optic spectrophotometer systems range from $2000 to $20,000


ABB-Bomem Inc. (www.bomem.com) Banner Engineering Inc. (www.bannerengineering.com) Bran & Luebbe (www.branluebbe.com) Brinkmann Instruments (www.eppendorfsi.com/search.asp) Dolan-Jenner Industries (www.dolan-jenner.com) Edmund Scientific (www.edsci.com) Guided-Wave (www.guided-wave.com) L.T. Industries, Inc. (www.LTIndustries.com) Ocean Optics Inc. (www.oceanoptics.com) Perstorp Group (www.perstorp.se) Thermo Electron Corp. (www.thermo.com)

INTRODUCTION Fiber-optic techniques offer several advantages for chemical and optical analysis, because the probes can be installed in situ to monitor the composition of chemical process streams in real time. This method of analysis does not disturb the process, and if the composition can be determined spectrophotometrically, the readout instruments can be several hundred meters from the probe. The probe can be located in toxic, corrosive, radioactive, explosive, high- or low-temperature or -pressure, and noisy environments. Because the measurement signal is optical, the cables are immune to microwave or electromagnetic interference. The term fiber optic is a misnomer because there are no fibers in the bundle, but rather thousands of glass filaments. Excessive bending of these filaments will cause them to break, which will reduce the light capacity.

obtained for remote measurement of absorption, transmission, fluorescence, and reflection in the ultraviolet (UV), visible (VIS), and near-infrared (NIR) regions. An electrically stabilized light source supplies the polychromatic light. Light travels to the sampling probe via one fiber-optic cable and returns to the instrument via a second. This is referred to as a bifurcated fiber-optic cable. The sample interacts with and transmits or reflects light to the input optics. External lighting may be substituted, and in that case, only a single fiber-optic input cable is needed. Because of the lower cost of today’s diode array spectrophotometers using fiber optics and serial bus communications, it is more economical to use multiple spectrophotometers rather than multiplex fiber-optic inputs to just one spectrophotometer. Current instruments can scan at rates exceeding 10 scans per second without diminishing accuracy. INSTRUMENTATION

PRINCIPLE OF MEASUREMENT Most present-day fiber-optic sensors use linear diode arrays combined with optical gratings, as shown in Figure 8.23a. Depending on the grating and diode array, spectral data are

Laser excitation permits long-distance transmission of excitation radiation to get a useful signal from the sample. Deuteriumarc and tungsten-halogen lamps provide UV or NIR radiation. See Section 8.61 for other energy sources for UV. 1347


1348


DETECTORS

GRATING LIGHT CHOPPER MONOCHROMATOR PORT

RETURN OPTICAL WAVEGUIDE SENSOR

LAMP SOURCE OPTICAL WAVEGUIDE FILTER

SOURCE PORT LENS PAIR

FIG. 8.23a Modular optical train.

Glass Optical Fiber

Probes

The absorption characteristics of silica glass limit it to a spectral range of about 200 to 2200 nm and, therefore, optical quartz fiber optics are preferred. Fluoride glass is used for infrared up to 3200 nm. Two types of silica filaments are available. Certain hydroxyl ( OH) concentrations are needed to pass light in the UV spectrum. High OH filament is needed for the 250- to 800-nm range. Low OH filaments are used for the 400- to 2100-nm range. As the length of the fiber-optic cable increases, the signal is attenuated. To avoid this problem and the cost of ultrahigh-purity glass, it is often now possible to place the spectrophotometers close to the process. This also reduces the effect of cable movement. Long cables allow bundles to shift and bend, which introduces noise. Stainless steel sheathing is normally used. Several thousand filaments can be arranged in random or grouped into different arrangements such as bifurcation. Operating temperatures can range from –140°C to 480°C.

Figure 8.23b shows a variety of probes for UV-VIS-NIR measurements. Wand Probe Wand or dip-type probes are single-sided probes for optical transmission, absorbance, reflectance, and fluorescence measurements. They can be used in laboratory reactors, process lines, or laboratory bench settings. Designed in single-strand and fiber bundle configurations, the wand probe employs a mirrored tip to reflect from the sending fibers to the receiving fibers. The absorbance tips are available in a variety of materials and path lengths (i.e., 1 mm to 4 cm). Without the tips, the fiber bundle probes can be used for reflectance and fluorescence measurements. The mirror tip is made of gold, rhodium on brass, or rhodium on gold, and the body is made of 316 stainless steel (others are available, e.g., Hastelloy and Monel). This probe style can be adapted to laboratory robotics systems for automated processing of a high volume of samples.

Plastic Optical Fiber Acrylic monofilaments of different diameters can be enclosed in polyvinyl chloride (PVC) or stainless steel sheaths and used in temperature ranges from –30°C to 70°C. Most glass fiber-optic cables will have a polished epoxy seal at the ends of the bundles to provide a liquid and airtight seal. Thus, the ends of glass, as well as plastic fiber optics, can be destroyed by acidic and caustic environments. To deal with this, cells are made to accommodate the light path and the sample stream. Banner Engineering Inc. offers a wide selection of glass and plastic fiber-optic cables, accessories, and evaluation samples.


Spectra-Caliper Probe Spectra-caliper probes are used for transmission measurements through solid samples such as plastic films, fibers, biological tissues, and intact plant leaves. This device provides a precise, variable path length from 0.1 to 5 mm and is also useful for measurement of light-absorbing liquids. Transmission Probe Transmission probes are always used in pairs for optical transmission and absorption measurements. They are typically installed in a flow cell for liquid analysis, or directly in a sample or process line. The two probes are arranged in 180° geometry to pass light straight

8.23 Fiber-Optic Probes

1349

MIRRORED TIP

1. WAND PROBE LIGHT SHIELDS

4. LONG PATH FLOW TUBE SAPPHIRE REFLECTOR

2. SPECTRA-CALIPER PROBE

5. GEM PROBE

SECTION 3. TRANSMISSION PROBE SET-UP BUNDLE SECTION

SMA CONNECTOR SECTION

SIX-TO-ONE PROBE (SINGLE-FIBER TO ANNULUS CONFIGURATION

DIFFUSE REFLECTANCE (NO TIP)

ABSORBANCE

FLUORESCENCE 6. SIX-TO-ONE PROBE SET-UP

FIG. 8.23b Sensor probe assemblies for both general and special applications. (Courtesy of Guided Wave Inc.)

through the sample. The probes are constructed of 316 stainless steel (other materials are available, e.g., Hastelloy and Monel). A sapphire window is sealed into the body with a glass frit. The sapphire window is also a lens that collimates light in transmission and refocuses light on receiving. The path length through the sample may be from 0.5 mm to 10 cm or more. The upper temperature limit for the probe is more than 300°C, and the lower limit is less than 15°K. The probes can operate in high vacuum or at pressures up to 5000 PSIG (352 bars). Long-Path-Flow Tube The long-path-length gas probe is used for the measurement of gases or vapors in stacks, in process pipes, or through the atmosphere. A pair of probes is required for transmission-type measurements. One probe sends a highly collimated light across the sample, and the second collects the light and focuses it back onto a single fiber. The optics can be purged with inert gas, and the window can be continuously cleaned by an air wipe. GEM Probe GEM probes use gemstonetips such as saphire. The GEM probe is based on attenuated total reflec-


tance (ATR) concepts and single-strand fibers. In the probe, light is transmitted by a single-strand fiber to the sapphire crystal. The light is internally reflected in the sapphire to a receiving single-strand fiber cable. The spectrum of any strongly absorbing, lower-refractive-index material in contact with the sapphire can be measured. The probe serves as a short-path-length measuring tool. The GEM probe can be used in water and in all common organic solvents. The remote surface reflectance probe is constructed with a sapphire or quartz window to pass light toward the sample surfaces at an incident angle of 0°. Reflected energy is collected at a 45° angle. Both a liquid cooling loop and a nitrogen purge are included in the probe for heat dissipation. This probe is useful for solid and powder materials. Six-to-One Probe The six-to-one probe has been developed for fluorescence, reflectance, and Raman. With this probe, it is possible to illuminate the sample with light from an appropriate source (xenon arc, laser, etc.) and then measure the reflected, scattered, or emitted light using six fibers mounted around the light-conducting central fiber.

1350


There are assemblies for use in constructing probes for measuring transmittance through or reflectance from sheets or other flat surfaces, and for working with cuvettes. Sample Interfaces In batch or nonpressurized processes that are accessible, fiber optics in dip probes can be used. On continuous processes that are hot or under pressure, the probes must be mounted in a sample stream. Sometimes the sample stream is sent to a heat exchanger for precooling. After the analysis is made, the sample stream may be returned to the process. In order to be able to isolate the probe for replacement and cleaning, block valves should also be installed. Some of the installation options are illustrated in Figure 8.23c.

Interchangeable dual-detector modules (Figure 8.23a) give high performance over a wide range. A grating positioning system allows rapid scanning at one detector’s assigned spectral range and then switches to the second detector to complete the scan. Successful on-line instrumentation requires a stable measurement system. This means synchronous detection where the instrument senses and compensates for drift in every cycle. Software Most fiber-optic spectrophotometers are supplied with spectral data analysis software for windowing desired wavelength ranges. Ocean Optics Inc. provides its SpectraScope™ operating software with features such as automatic calibration, external signaling, and real-time color measurements.

Detectors Photomultipliers are commonly used for the UV detectors; silicon photodiodes are used for visible detectors; and germanium photodiodes and photoconductive lead sulfide detectors (both with thermoelectric cooling) are used for NIR detectors. Indium–gallium–arsenide is another material commonly used for NIR detectors. A 512-element silicon diode array with a fixed grating is available for the NIR region (700 to 1100 nm). See Section 8.61 for other types of detectors used in UV analyzers.

A STAINLESS STEEL CELL BODY

O-RING SEAL

FIBER OPTIC

FIBER OPTIC COUPLING

COLLIMATING OPTICS

B

PROCESS LIQUID

C ANALYZER PROBE

ORIFICE

ISOLATION VALVES

RECEIVER

NIR FLOW LINE

FIG. 8.23c Alternative methods of on-line installation of NIR analyzers: (A) measurement of the complete process stream, (B) bypass configuration, (C) probe-type design.


Four approaches can be used for qualitative and quantitative chemical analysis. These techniques include absorption, fluorescence, scattering, and refractive index change. Absorption In the UV region, low concentrations of many unsaturated organic compounds can be measured. Some inorganic compounds such as residual chlorine in chlorinated solvents have been measured at 360 nm, and chlorine dioxide at 350 nm. See Section 8.61 for additional examples. In the visible region, colorimetry of dyes and pigments and metal analysis can be carried out as described in Section 8.15. In the NIR region, concentrations of water and organic compounds with bonds such as CH, OH, NH, and unsaturated bonds can be monitored. The absorbance spectrum can be analyzed for octane numbers of gasolines, polymer properties and cure rates, water in solvents and resins, and food analysis for fat, protein, and sugar. Fluorescence

TEMPERATURE SENSOR

SAPPHIRE WINDOW

APPLICATIONS

Figure 8.23d illustrates the use of remote fiber fluorescence (RFF) for chemical analysis. An RFF system couples a highintensity light into a single, large-core quartz fiber. Upon exiting the fiber, the rays of light impinge upon the sample, which gives off a characteristic fluorescent emission. The emission travels back to a photodetector, which, with the aid of a computer, can provide both qualitative and quantitative information. The fiber in the sample is called an optrode. Using the RFF technique, measurements have been demonstrated with uranyl, chloride, iodide, iron, plutonium, and sulfate ions. RFF techniques can be used for monitoring groundwater near hazardous waste dumps. Groundwater does not normally fluoresce, but contaminated water typically does.

8.23 Fiber-Optic Probes

1351

WALL FIBER OPTICAL CABLES OPTRODE(S) WAVELENGTH SHIFTER/SELECTOR

LASER SOURCE

COUPLER

1 TO 1000 m

FLOW CELL STATIC CELL

FLUORESCENCE DETECTOR

COMPUTER

FIG. 8.23d In RFF, the optical fibers relay the fluorescence signals from a variety of remote sampling sites. CUVETTE OPTRODE

ILLUMINATION — BLUE

FLUORESCENT DYE SENSITIVE TO OXYGEN QUENCHING

SAMPLE SOLUTION OPTICAL FIBERS FLUORESCENCE — GREEN + SCATTERED BLUE

FIG. 8.23f The construction of a fiber-optic fluorescent probe.

TO LASER AND SPECTROMETER SAPPHIRE BALL OPTRODE SAMPLE SOLUTION

TO LASER AND SPECTROMETER

FOCAL LENGTH = 0.5 mm

MEMBRANE OPTRODE SAMPLE SOLUTION INTERACTIVE VOLUME

MEMBRANE TO LASER AND SPECTROMETER

FIG. 8.23e Optrodes are used in RFF to couple the fibers to samples.

Optrodes Figure 8.23e shows the optrodes used in the RFF. The simplest optrode is just the bare end of the optical fiber inserted into the sample or a fluorescence cuvette. If greater sensitivity is required, a small sapphire ball is used to focus laser light at a point about 0.5 mm into the sample. This ensures that weak fluorescence is concentrated near the fiber.


HYDROPHOBIC, GAS PERMEABLE ENVELOPE

Another option is the membrane optrode, in which the sample flows into a small chamber, where it is interrogated by the laser light. This allows a closer control of the sample environment because fluorescence can be environmentally sensitive. A material does not have to be fluorescent to work with the technique. A target of known fluorescence can be used. The sample can react with the target and enhance or diminish its fluorescence. Aluminum and other metals have been analyzed by using a reagent immobilized in the form of a powder and attached to a bifurcated fiber-optic cable. The metal reacts with the reagent, giving a fluorescent signal. The response time was 1 to 2 min with a detection limit of 0.027 ppm. Oxygen Probe Figure 8.23f shows an oxygen probe, based on fluorescence quenching. In this probe, a high-intensity blue light is transmitted to the dye through one leg of a bifurcated fiber-optic probe. The blue light, upon impinging the dye, gives off a characteristic green fluorescence. The level of fluorescence diminishes with the increasing levels of oxygen that pass through the hydrophobic, gas-permeable polypropylene membrane. Oxygen reacts with the dye perylene diburyrate. The partial pressure of oxygen is a function of the ratio of blue light intensity to green light intensity. Scattering The scattering concept has been used to determine the volume fraction of one immiscible liquid in another (see Section 8.39) and to characterize smoke in a stack (see Section 8.46).

1352


MICROLENS

M2

M′1

RECEIVE

TRANSMIT

TRANSMITTING FIBER

RECEIVING FIBER

SHORT-PASS COLOR FILTER

M1

MICROLENS

FIG. 8.23g Multipass reflection sensor used for Raman scattering.

Raman laser scattering has been used to detect low concentrations of various gases. Figure 8.23g shows a multipass sampling probe designed to enhance the system sensitivity. The spherical mirrors labeled M1 and M1′ provide for multiple passes of the light beam. A portion of the scattered light is injected directly into the receiving fiber, while additional scattered light reflects from M2 into the receiving fiber. Refractive Index Figure 8.23h shows a reflective fiber-optic probe for sensing refractive index change. The index of the prism is sufficiently higher than that of air, so that a condition of total internal reflection will exist within the prism. Light transmitted to the prism is reflected into the fiber-optic probe. If the prism is immersed in a liquid with a refractive index higher than that of the prism, the reflected light will decrease significantly. Such a device can provide a measure of concentration, because many components of a solution alter the refractive index of the liquid, as a function of concentration. For example, sulfuric acid has a refractive index of 1.33 in low concentrations and 1.46 in concentrations over 90% by weight. Bibliography 1998 Global Photonics Technology Forecast, Photonics Spectra, January 1998. Control Staff, “Fiber Optics, Spectroscopy Team for Chemical Identification,” Control, February 1993.


LIQUID LEVEL PRISM LIGHT PATH

FIG. 8.23h Reflective fiber-optic probe installed to detect changes in refractive index.

“Fiber Optics Simplify Remote Analysis,” C&EN, September 27, 1982, pp. 28–30. Fitch, P. and Gargus, A. G., “Remote UV-VIS-NIR Spectroscopy Using Fiber Optic Chemical Sensing,” American Laboratory, 17(12), 1985, 54–71. Krohn, D. A., “Chemical Analysis with Fiber Optics,” Analysis Instrumentation, Vol. 22, 1984, 43–50. Landa, I., “Visible (VIS), Near Infrared (NIR) Rapid Spectrometer for Laboratory and On-Line Analysis of Chemical and Physical Properties,” SPIE, Vol. 665, 1986, pp. 286–289. Maugh, T. H., “Remote Spectrometry with Fiber Optics,” Science, November 26, 218, 1982, pp. 875–876. Munsinger, R. A., “Fiber Optic Colorimetry,” Electro-Optical Systems Design, February 1981, pp. 43–47. Schirmer, R. E., “On-Line Fiber-Optic Based Near Infrared Absorption Spectrophotometry for Process Control,” Proceedings of the Instrument Society of America, Ann Arbor, MI, 1986, pp. 1229–1235. Schirmer, R. E. and Gargus, A. G., “Applications of Remote Chemical Sensing Using Fiber Optics and UV-VIS-NIR Spectroscopy,” American Laboratory, 18(12), 1986, pp. 30–39. Seitz, W. R., “Chemical Sensors Based on Fiber Optics,” Analytical Chemistry, 56(1), 16A–34A, 1984. Workman, J., Jr., “Imaging, Chemometrics and New Developments in Sensor Technologies,” Standardization News, October 2002, pp. 23–25.

8.24

Fluoride Analyzers J. S. JACOBSON (1974, 1982) B. G. LIPTÁK (2003)

A. T. BACON

To Receiver

(1995), REVIEWED BY

R. R. JAIN

AT Fluoride Flow Sheet Symbol

Principles of Operation:

A. Detector tubes B. Electrochemical C. Paper tape D. Ion mobility spectrometry E. Infrared spectrometry F. Ion-specific electrodes G. Silicon dioxide sensors H. Ion chromatography I. Titration J. Colorimetric K. Gas chromatography


Surfaces of equipment contacting fluoride should be stainless steel, Teflon, epoxy, polyethylene, or polypropylene

Concentrations Measured:

Monitoring for worker protection is usually in the 0- to 10-ppm range. Leak detection in the surroundings of hydrogen fluoride (HF) handling equipment requires sensitivity in the high parts per million (ppm) range. Monitoring for low-level environmental damage may require sensitivity in the parts per billion (ppb) range.

Sensitivity Range:

A. 0.1 to 20 ppm B. Low ppm C. Low ppm D. 0.05 to 10 ppm E. Low to mid ppm F. Low ppb to high ppm, function of sampling time, volume G. Qualitative response to high ppm levels H–K. Measurement is usually in the ppm range; sensitivity depends on sampling parameters.

Calibration:

Calibration of vapor analyzers usually is performed with permeation-type devices or with calibrated gas standards. Liquid phase analyzers are calibrated using solutions of an appropriate fluoride compound such as sodium fluoride.

Accuracy:

A. ±20% of actual measurement at low humidities, inaccurate at high humidities B–D. ±10% of full scale E. Varies with type of instrument and humidity level F, H–K. These laboratory and automated techniques are capable of providing high accuracy (±3% of actual measurement) under ideal conditions, but this is often compromised by inaccurate sample collection practices. G. Qualitative only

Cost:

A. $3/analysis, $50/sampling kit B. $1000 to $3000 C. $3500/point D. $17,000


1354


E. $9000 (common IR) to $90,000 (FTIR) F. $500 for electrode; $1000 to $3000 for associated electronics (for water/lab analysis); $30,000 to $80,000 for automated system G. $1400/point H. $10,000 to $20,000 (lab method) I. $500 (lab method) J. $1000 to $5000 (lab method); $25,000 (automated system) K. $10,000 to $20,000 (lab method) Partial List of Suppliers:

Alltech Associates Inc. (www.alltechweb.com) Davis Inotek (www.davisontheweb.com) Dionex Corp. (H) (www.dionex.com) Environmental Technologies Group (D, G) (www.envtech.com) Fisher Scientific (I) (www2.fischersci.com) Hanna Instruments Inc. (www.hannainst.com) K-Patents Inc. (www.kpatents.com) Sensidyne (A, B) (www.sensidyne.com) Thermo Gas Tech (www.gastech.com) Thermo Orion (www.thermo.com) Tytronics & Nemetre (www.tytronics.com) Zellweger Analytics (www.zelana.com)

INTRODUCTION

TYPES OF FLUORIDE COMPOUNDS

The number of analytical techniques available for the analysis 1 of fluoride is fairly large. In the past, several of these techniques were only used in the laboratory. Table 8.24a lists a variety of automatic and manual methods for the detection of fluorides in ambient air, stacks, and process streams. In order to select the most appropriate method for a given application, the physical state and properties of the fluoride compound must be taken into consideration.

Fluorides can exist as gases, liquids, and solids. The most common form of fluoride is hydrogen fluoride (HF). This compound is very volatile. Its boiling point is 66.9°F (19.4°C), and therefore it is most often analyzed in the vapor state. All the methods that are described as gas or vapor fluoride analyzers detect this compound. Other compounds, such as UF6, which exist in the vapor phase, hydrolyze quickly in ambient air or solution to form

TABLE 8.24a Orientation Table for Fluoride Measurement Methods

Automatic or Manual Method

Suitable for Water Analysis

Gas or Particulate Measurement

Suitable for Stack/Process Monitoring

Suitable for Organic Fluoride Monitoring

Tubes

M

No

G

No

Electrochemical

A

No

G

Some

c

No

Paper tape

A

No

G

Some

c

No

Ion mobility spectrometry

A

No

G

Ion-specific electrodes

A/M

Silicon dioxide sensor Ion chromatography

a

G/P

A

No

G

M

Yes

G/P

b

Yes Yes

M

Colorimetric

A/M

a

Gas chromatography

A/M

a

Infrared

A

a

Yes b

Yes

Titration

Yes

Some

Yes c

Leak detection


No

Some

c

No

G/P

b

Some

c

No

G/P

b

Some

c

No Yes Yes

Yes

G

Some

c

No

G

Some

c

Manual method can be performed in lab; automatic systems are available. Particulate measurement requires special collection techniques. c Suitability for stack monitoring depends on moisture present, interference, etc. d Required pyrolysis equipment available from manufacturer; other methods may be adapted with custom equipment. b

No

d

d

8.24 Fluoride Analyzers 2

HF, and thus can be analyzed indirectly by these methods. Other liquid and gaseous fluorides are much more stable and do not easily form HF. These compounds must be broken down prior to analysis. A common method of conversion is thermal decomposition, which is followed by analysis of the resulting HF. Many solid fluorides are soluble in water and form fluoride ions. If fluoride is present in the air in the form of fine particles, these compounds can be collected on filters and analyzed by wet laboratory or automated methods. Since the automated wet methods operate by impinging an air sample into a wet collection stream, if total fluoride concentration 3,4 measurement is required, care must be taken to assure that the particles are freely admitted into the collection vessel. Other fluoride-containing solids are not water soluble and require thermal decomposition at very high temperatures prior to analysis as HF.

ANALYZER TYPES In most cases, the goal of the analysis is to determine the quantitative amount of fluoride in a sample. In other cases, only the presence of HF needs to be determined. Qualitative sensors such as silicon dioxide are used to detect HF leakage from valve connections and other joints. These sensors do not provide quantitative information. Most other devices discussed in this section provide some degree of quantitative information. Analyzers are also grouped according to their operation, which can be automatic or manual. Purely manual devices such as detector tubes require an operator to collect and read the sample. Some of the manual laboratory techniques have been automated and depend on colorimetric, ion-specific electrodes and gas chromatography (GC) techniques for their

analysis. Yet other analyzers are fully automatic and do not require operator attention, except for maintenance. Fluoride analyzers are also grouped on the basis of the sample phase into vapor and liquid analyzers. In the case of ion-specific electrodes, titration, and colorimetric methods, fluoride analysis requires an aqueous sample. On the other hand, the concentration of airborne fluoride is determined by using an impinged or absorbed sample. The remaining techniques are used for analysis of the vapor phase. The selection of an analyzer for a particular application will depend on the type of process sample, which is to be monitored, cost, accuracy, speed of response, range, and frequency of maintenance. These considerations are discussed below. Gas and Vapor Analyzers Detector Tubes These devices utilize a hand-operated pump to draw a known volume of air through a calibrated tube. The fluoride concentration in this case is indicated by the resulting color. The initial investment in equipment is very low, but this method is acceptable only if infrequent measurements are sufficient. Detector tubes are commonly used for spot monitoring in industrial hygiene applications. Electrochemical Cells An air sample is presented to the cell, either actively or passively (Figure 8.24b). The sample permeates into the cell, usually through a membrane, where an electrochemical reaction produces a current proportional to the concentration. The initial cost is relatively low. Some types require fairly high maintenance, although this is less of a concern on more modern designs. Cells are most often used in ambient air monitoring and are not easily adapted for process control. Some models are prone to freezing at low temperatures and do not operate well at humidity extremes.

CARRYING HANDLE

COVER RELEASE

ELECTROLYTE RESERVOIR

GAS OUTLET

FLOWMETER

FIG. 8.24b Portable gas monitor with electrochemical sensor. (Courtesy of Sensidyne Inc.)


1355

LCD DIGITAL DISPLAY INDICATOR LIGHTS CONCENTRATION SELECT

FUNCTION SELECT FLOW SET

1356


GAS SAMPLING HEAD

CHEMCASSETTE TAPE LIGHT REFLECTED FROM TAPE SURFACE

OPTICS FILTER

CADMIUM SULFIDE PHOTOCELL

TUNGSTEN LAMP

SIGNAL TO MEASURING CIRCUIT

SIGNAL TO ZERO CIRCUIT

FIG. 8.24c Fluoride concentration can be detected by monitoring the reflected light intensity from a chemically treated tape. (Courtesy of Zellweger Analytics, which MDA Scientific Inc. joined.)

Paper Tape An air sample is drawn through a chemically treated tape, and the colorimetric change is read with a photometer (Figure 8.24c). The tape is automatically advanced as fluoride is detected. The initial cost is moderate, but expendables can be expensive if fluoride is constantly present. Tape has the advantage of not requiring an independent calibration, as each tape is calibrated at the factory. Tape is most often used for ambient air monitoring and is easily adapted for multipoint sampling. Tape is not commonly used for process control or outdoor applications. Ion Mobility Spectrometry Air is drawn into the instrument and the sample permeates through a membrane into a cell, where it is ionized by a small radioactive source. The analysis technique is similar to that of time-of-flight mass spectrometry (MS) but is performed at atmospheric pressure. The initial cost is moderately high, but maintenance is minimal. This technique has found wide acceptance in the petrochemical industry due to its high reliability, good interference rejection, and ability to operate accurately at extremes of ambient temperature and humidity. This technique is most often used for outdoor ambient air monitoring and can be adapted for process control and stack monitoring. The chief disadvantage is the requirement for a constant supply of dry 5,6 instrument air, and purging in flammable environments. Infrared Spectroscopy A gas sample is drawn into an optic cell where a characteristic infrared (IR) absorbance is used to determine concentration (Section 8.27). Several types are available, including nondispersive infrared (NDIR) and Fourier transform infrared (FTIR). Prices range from moderate to expensive, depending on analyzer type and features. Sensitivity is determined mainly by path length. In most cases, water vapor produces a strong interference, masking HF response. The advantage of FTIR is the ability to monitor for several gases with the same analyzer. IR analyzers are typically


used in applications such as stack monitoring, after sample cleanup. IR analyzers may also be used to monitor for certain fluorinated organic compounds. Ion-Specific Electrodes The concentration of fluoride ions in solution is monitored by using an ion-selective electrode (Section 8.28), which provides a millivolt output signal in response to the activity of fluoride ions in the solution. The fluoride-selective ion electrode, in conjunction with a reference electrode, develops its potential across a doped fluoride crystal membrane. When the electrode is placed in a solution, the ions from the solution move to the surface of the membrane and the electrical charge of the ions creates a potential difference across the membrane. At equilibrium, this potential difference is proportional to the activity of the fluoride ions in the solution. These probes are inexpensive for laboratory applications but become expensive when used as automatic on-line monitors in continuous measurement applications. The advantages of selective-ion detection of fluoride include their selectivity, large dynamic range, and good sensitivity. Disadvantages include relatively high maintenance, which includes the need for buffering the solution sample using concentrated acetate or citrate for pH adjustment between 5.3 and 5.8. In addition, CDTA or EDTA complexing agents are also used to prevent interference from metals. The electrodes are very sensitive to temperature effects and must be carefully controlled. This method is most often used for lab analysis and in applications that demand the selectivity and sensitivity provided. Silicon Dioxide Sensors Atmospheric HF causes etching of a thin layer of silicon dioxide. A light source and photo-optic receiver are used to monitor the rate of etching. Initial and operating costs are both low. Little maintenance is required other than periodic replacement of the disposable sensor tip. Because of the highly specific nature of the reaction and the ability to respond quickly to relatively high localized concentrations around flanges, etc., this method is most commonly used to monitor for leaks. The output is essentially qualitative. Laboratory Methods The most common laboratory methods are ion-specific electrodes, ion chromatography, titration, 8 and colorimetric methods. Ion-specific electrodes have been discussed above. Ion chromatography uses modified highperformance liquid chromatography equipment. Although expensive, this equipment can also determine a large number of other ions at the same time. Titration and colorimetric methods rely upon chemical reactions, which produce a color change. Both of these methods rely upon common lab equipment and procedures. All of these lab methods can be used in the analysis of water samples. Automated and semiautomated equipment exist for most of these methods.

8.24 Fluoride Analyzers

Organic Fluoride Analysis The two most common methods of analyzing organic fluorides, such as Freons, are gas chromatography and pyrolysis followed by any of the conventional analysis procedures. GC methods may make use of a universal detector and simply rely on retention time to identify and quantitate the sample, or a halogen-specific detector such as a thermionic bead may 9 be used. Pyrolysis kits are available for use with detector tubes. Other Methods Two other methods have been demonstrated as having potential for the detection of fluoride: mass spectrometry and openpath laser-based systems. Commercial MS analyzers are available for lab and process use, and these instruments have the theoretical capability to identify both organic and inorganic fluorides in the gas phase, although their use would be very limited because of the relative complexity of these devices. Laser-based open-path systems have been demonstrated in the prototype stage but are not yet commercially available.

References 1. 2.

3.

Korolkoff, N. O., “Survey of Toxic Gas Sensors and Monitoring Systems,” Solid State Technology, December 1989. Bostick, K. D. and Angel, E. C., “Evaluation of the Sensidyne Toxic Gas Sensor for HF Vapor,” Report K/PS—5076, prepared by Oak Ridge Gaseous Diffusion Plant, Oak Ridge, TN, September 1987. Pack, M. R. and Hill, A. C., “Further Evaluation of Glass Fiber Filters for Sampling Hydrogen Fluoride,” Journal of Air Pollution Control Association, Vol. 15, 1965, pp. 166–167.


4.

5.

6.

7.

8.

9.

1357

Mandl, R. H., Weinstein, L. H., Weiskopf, G. J., and Major, J. L., “Separation and Collection of Gaseous and Particulate Fluorides,” Paper CD-25A, Second International Clean Air Congress, Washington, D.C., 1970. Bacon, A. T., “Ion Mobility Spectroscopy: A New Method of Monitoring for Hydrogen Fluoride, Ammonia, and Other Industrial Gases,” Paper #258, Pittsburgh Conference, New York, March 1990. Bacon, A. T., Getz, R., and Reategui, J., “Ion Mobility Spectrometry Tackles Tough Process Monitoring,” Chemical Engineering Progress, June 1991. Elfers, L. A. and Decker, C. E., “Determination of Fluoride in Air and Stack Gas Samples by Use of an Ion Specific Electrode,” Analytical Chemistry, Vol. 40, 1968, pp. 1658–1661. Farrah, G. H., “Manual Procedures for the Estimation of Atmospheric Fluorides,” Journal of the Air Pollution Control Associations, Vol. 17, 1967, pp. 738–741. Annino, R. and Villalobos, R., “Application of Process Gas Chromatographic Instrumentation to Environmental Monitoring,” American Laboratory, October 1991, pp. 15–26.

Bibliography Adams, V., Water and Wastewater Examination Manual, Chelsea, MI: Lewis, 1990. Evans, R., Potentiometry and Ion Selective Electrodes, New York: John Wiley & Sons, 1987. Fresenius, W., Water Analysis, Berlin: Springer-Verlag, 1988. Groves, B. and Howard, V., Fluoride: Drinking Ourselves to Death, Newleaf, Miami, FL, 2002. National Institute of Occupational Safety and Health, Manual of Analytical Methods, 2nd ed., Vol. 3, Method S176, 1977–1980. National Institute of Occupational Safety and Health, Manual of Analytical Methods, 3rd ed., Methods 7902 and 7903, 1984. Occupational Safety and Health Administration, Analytical Methods Manual, Method ID110, 1985. Occupational Safety and Health Administration, Chemical Information Manual, Method IMIS1270, 200. Wagner, B. M., Health Effects of Fluoride, Washington, D.C.: National Academy Press, 1993.

8.25

Hydrocarbon Analyzers R. J. GORDON (1974, 1982) B. G. LIPTÁK (1995) I. VERHAPPEN (2003), WITH REVIEW COMMENTS


To Receiver

BY

F. D. MARTIN

AT HC or Combustibles Flow Sheet Symbol

Type of Measurements:

A1. Total hydrocarbon by flame ionization B1. Methane by chromatography B2. Methane by flame ionization B3. Methane by mass spectrometry C1. Hydrocarbon classes by flame ionization C2. Hydrocarbon classes by mass spectrometry C3. Hydrocarbon classes by infrared D1. Individual hydrocarbons by chromatography E. Hydrocarbon dew point in natural gas (chilled mirror) F. Ion mobility spectroscopy G. Laser-induced absorption radar

Reference Method:

Gas chromatography with flame ionization for nonmethanes

Sensitivity:

0.1 ppm (A1, B2, C1); chromatograph followed by electrochemical sensor can detect 50 ppb

Ranges:

From 0–10 ppm to 0–100% lower explosive limit (LEL)

Inaccuracy:

±1%

Costs:

Infrared and flame ionization types cost $10,000 to $20,000; chromatographic units cost $25,000 to $100,000; and mass spectrometers start at $50,000 and can reach $250,000. A portable chromatograph with electrochemical detector costs $15,000.


(See also Sections 8.12, 8.16, 8.39, 8.58, and 8.59) Agar Corp (www.agarcorp.com) AIL Systems Inc. (www.edocorp.com) Air Instruments & Measurements Inc. (www.aimanalysis.com) Boreal (www.boreal-laser.com) Carlo Erba (Italy) (www.carloerbareangenti.com) CEA Instruments Inc. (www.ceainstr.com) Draeger (www.draeger.com) E.G. & G. Chandler Engineering (formerly Perkin Elmer), (www.egginc.com) Forney Systems (formerly Anarad Inc.) (www.anarad.com) General Monitors (www.general-monitors.com) Gow-Mac Instrument Co. (www.gow-mac.com) Heath Consultants Inc. (www.heathus.com) Invensys (formerly Foxboro Co.) (www.invensys.com) Michell Instruments Ltd. (www.michell.co.uk) Mocon Inc. (formerly Baseline Industries Inc.) (www.baselineindustries.com) MSA Instrument Div. (www.msanet.com) OptiGas (www.optigas.com) SAES Group (formerly Molecular Analytics) (www.ionpro.com) SensIR Technologies (www.sensir.com) Siemens Applied Automation (www.sea.siemens.com/ia/) Spectrex (www.spectrex-inc.com) ThermoOnix (formerly Fluid Data/AMSCOR) (www.thermo.com) Zellweger Analytics (www.zelana.com)

8.25 Hydrocarbon Analyzers

INTRODUCTION

ANALYZER TYPES

Hydrocarbon analysis is discussed in several sections in this chapter, including Section 8.12 on chromatographs, Section 8.16 on combustibles detection, Section 8.39 on oil-in-water measurements, and Section 8.58 on total organic carbon analysis. Therefore, it is advisable to also refer to these sections when investigating the features and capabilities of a particular hydrocarbon analyzer type. The hydrocarbons comprise a large class of individual components (Table 8.25a). The most abundant hydrocarbon, methane, is nonreactive in photochemical reactions and is a less hazardous air pollutant. The remaining hydrocarbons vary widely in their complexity and reactivity. For this reason, the type of hydrocarbon analysis selected depends on the purpose for which data are needed. To obtain highly detailed analyses for numerous hydrocarbons requires expensive and sophisticated equipment, whereas a single measurement for total hydrocarbons is simpler and somewhat less expensive. There are also methods of intermediate complexity, which permit determination of properties without identification of the individual stream components.

Flame Ionization Detectors

TABLE 8.25a Atmospheric Hydrocarbon Analyzers Hydrocarbon Type

Method

Limitations and Interferences

Total (as carbon)

FID

Some response to carboncontaining nonhydrocarbons

Methane

GC

Expensive equipment (can also be used for carbon monoxide)

Methaneonly subtractive

Column preparation fussy column and FID

Aromatics, olefins, paraffins

Mass spectrometry

Freeze-out required, expensive

Subtractive columns and FID

Column preparation fussy

Individuals


Flame ionization detectors (FIDs) are used to measure total hydrocarbons at low concentrations such as for pollution, leakage, or safety monitoring. In the FID analyzer, a flame of pure hydrogen, which contains almost no ions, is used. When even traces of organic compounds are introduced by the sample, the hydrogen flame ionizes the carbon atoms, resulting in a large number of ions in the flame. The resulting ionic current is measured as an indicator of hydrocarbon concentration. In dual-flame detectors, the concentration of total hydrocarbons and methane can be identified separately. Flame ionization detection is the most suitable means of analysis for most hydrocarbons at the levels found in polluted air. It may be used alone for the measurement of total hydrocarbons or as a detector after separation by a column device such as a gas chromatograph. In FID analyzers, a sensitive electrometer detects the increase in ion intensity in a hydrogen flame when a sample containing organic compounds is introduced. The response is approximately in proportion to the number of organically bound carbon atoms in the sample, so the detector is basically a carbon atom counter. Carbon atoms bound to oxygen, nitrogen, or halogens, however, give a reduced response. There is no response to nitrogen, carbon monoxide, carbon dioxide, or water vapor. The results are usually expressed in terms of the calibration gas used, e.g., parts per million (ppm) carbon as methane. The response is rapid and can be as sensitive as 0.1 ppm. The response to various hydrocarbons is not perfectly uniform, and such variations should be taken into account in interpreting FID data. The FID analyzer is a reference method for total hydrocarbons in U.S. Air Quality Standards. Basically, it is an air-metering device attached to a FID (Figure 8.25b).

HYDROGEN SOURCE

PRESSURE REGULATOR

MANOMETER

AMPLIFIER

EXHAUST

Mass spectrometry

Freeze-out required, expensive, data reduction requirements large

Infrared spectrometry

Freeze-out required, expensive, not total class coverage

Ion mobility spectrometry

Clean sample required, limited knowledge in industry

Laser-induced absorption

Expensive, specialized support required

Perimeter monitoring

Concentration/unit length rather than point value

GC

Expensive, data reduction requirements large

1359

PARTICLE FILTER AND CONDENSATE TRAP SAMPLE AIR

METER NEEDLE VALVES

EXHAUST IONIZATION DETECTOR FLOW RESTRICTORS

AIR PUMP

PRESSURE COMBUSTION REGULATOR AIR (BOTTLE OR LINE)

PARTICLE FILTER AND CONDENSATE TRAP

NEEDLE VALVE

MANOMETER

FIG. 8.25b Hydrocarbon analyzer and hydrogen FID.

1360


FID BURNER

ELECTRODES TO AMPLIFIER

200 V

FUEL CAPILLARY 20—30 ml/min

300 ml/min

BURNER AIR CAPILLARY

20 ml/min PRESSURE GAUGE

AIR CAPILLARY

SAMPLE CAPILLARY

PRESSURE GAUGES DIAPHRAGM PUMP 5000 ml/min 3-WAY VALVE BACK PRESSURE REGULATOR

FILTER PRESSURE REGULATORS FUEL

BURNER AIR

ZERO SAMPLE SPAN GAS GAS GAS

BYPASS

Fig. 8.25c Sampling system for hydrocarbon analysis by FID. ATMOSPHERIC VENT

SAMPLE VALVE

SAMPLE LOOP

DETECTOR EXHAUST

OVERFLOW

FLOWMETER NITROGEN VENT

WATER SAMPLE

SAMPLE CARRIER GAS

N2

THERMOSTATTED ENCLOSURE

EXTRACTION COLUMN

N2

SEAL LEG

FLAME IONIZATION DETECTOR

RECORDER

FIG. 8.25e Gas chromatograph.

DRAIN

FIG. 8.25d Detection of purgeable hydrocarbon concentration in water samples. (Courtesy of ThermoOnix, formerly Fluid Data/AMSCOR.)

In microprocessor-controlled hydrocarbon analyzers, up to eight channels can be obtained for the simultaneous analysis of eight samples. Manufacturers also provide sampling systems (Figure 8.25c), which make remote calibration and automatic self-checking possible. If discrete component analysis is not required, it is also possible to use liquid samples in conjunction with FID-type hydrocarbon analyzers if the hydrocarbons can be purged by nitrogen from the continuous liquid sample. Such a sampling system is shown in Figure 8.25d, where a nitrogen extraction column is used, serving to remove the hydrocarbon, which can be removed by purging. Here the nitrogen is introduced at the bottom of the extraction column, and as it travels


AMPLIFIER COLUMN

upward, it removes the purgeable hydrocarbons. The overflow level in the head tank, which seals the top of the column as the purge nitrogen leaves it, holds the extraction column pressure constant. Gas Chromatography The only practical method for the measurement of specific hydrocarbons is gas chromatography (GC). In GC, the basic apparatus (Figure 8.25e) is a carefully prepared tubular column of a finely divided solid with provisions for passing a steady flow of a carrier gas (often helium) through it (Section 8.12). The column is temperature controlled and the packing usually supports or is coated by a nonvolatile liquid phase. Preceding the column, there is a means for sample introduction (and sometimes sample splitting), and following the column is a detector. A high-sensitivity column usually employs a FID. There are a great many optional variations in the makeup of a GC instrument, involving sample valves, precolumns, columns, programmed temperature and pressure changes, stream splitters, and detectors.


Traditionally, the output was recorded on a strip chart (possibly with a peak integrator) or converted to digital form for computer handling. In full-scale analyses for scores of hydrocarbons, data reduction from analog strip-chart recordings becomes very tedious and time-consuming. Most gas chromatographs use a computer interface to analyze and report the results of each analytical cycle as well as a wealth of related equipment health conditions. Supplemental techniques increase the utility of GC for atmospheric hydrocarbons. A common method is to pass a sample of air through a small freeze-out trap, sweep out the air with helium, and then warm the trap and introduce the condensables to the GC column in one concentrated slug. This extends the lower limits of sensitivity to the parts per billion range. Subtractive columns may be in parallel or in series with conventional columns to help relate the various recorded peaks to specific hydrocarbon classes. Calibration Complete GC calibration would require hundreds of different pure hydrocarbons and is never fully achieved. There are usually few unknowns left in careful work except in the more complex higher-molecular-weight ranges. Calibration is based on the fact that under carefully reproduced conditions, a given hydrocarbon will always require the same length of time to pass through the column to the detector. Ambiguities arising where components overlap may be resolved by change in column packing, increasing the analytical cycle time and column length, or sometimes by the use of subtractive columns. An elaborate but useful technique is to attach another analytical instrument such as an infrared (IR) or mass spectrometer to the outlet of the detector for peak identification. In Figure 8.25f an instrument is described for combined analysis of methane and carbon monoxide. This is a simple

5-MICRON FILTER

SOLENOID VALVE

CH4CO SAMPLE LOOP

TOTAL HYDROCARBON SAMPLE LOOP

EXHAUST SAMPLE AIR IN

AIR PUMP

INJECT VALVES

SOLENOID VALVE

BACKFLUSH VALVE

HOLDING AMPLIFIERS RECORDERS CH4

CALIBRATION GAS

EXHAUST SEPARATION COLUMN

CO

PRECOLUMN

TOTAL HYDRO CARBON

FLAME IONIZATION DETECTOR

CATALYTIC REACTOR

0-10 MV OUTPUT ELECTROMETER

FIG. 8.25f Methane and carbon monoxide analyzer.


1361

GC, set up to determine methane, with a programmed arrangement of valves and a catalytic carbon monoxide-tomethane converter. As a result, the readout has two peaks, one for methane directly and the other for methane formed by reduction of carbon monoxide. Nonmethane Hydrocarbons A column of adsorptive charcoal may be treated with methane just until breakthrough occurs; that is, no more methane will be adsorbed, but other hydrocarbons are still retained. This column can then be used with a FID as an analyzer for methane only. Used in parallel or in switched alternation with the detector without any column, it allows determination of nonmethane hydrocarbons by difference. Reactive Hydrocarbons Certain specially prepared columns can be used to adsorb specific hydrocarbon classes. These are useful in GC and could be used with a simple FID analyzer alone to give analysis by broad classes. A column of crushed firebrick supporting mercuric sulfate will adsorb olefins and acetylenes, and one of palladium sulfate will then adsorb aromatic hydrocarbons (except benzene). A combination of these subtractive columns has been applied successfully to automobile exhaust analysis and for use in atmospheric work. Spectrometric Methods Either infrared or mass spectrometry may be used with considerable advantage for individual hydrocarbon determination, although the sensitivity of both methods would require concentration such as by a freeze-out trap if used for atmospheric measurements. Both types of instruments are complex and expensive. Since the calibration requirements are about equal to those needed for GC, the data interpretation is generally more complicated and the extent of coverage is less; these methods have been widely supplanted by GC for atmospheric hydrocarbon detection work. These instruments may be useful for analysis of a particular type or class of hydrocarbon and might make it unnecessary to set up a gas chromatographic capability in some cases. Laser-Induced Doppler Absorption Radar Laser-induced Doppler absorption radar (LIDAR) can be used to remotely measure chemical concentrations in the atmosphere. Two different laser wavelengths are selected so that the molecule of interest absorbs one of the wavelengths while the other wavelength is selected to be in a region of minimal interference. The difference in intensity of the two returned signals is then used to determine the concentration of the gas being measured using the Lambert–Beer law. By measuring the time taken for the light to travel and return, it is also possible to determine the distance from the base station to the measurement point. It is therefore possible to use LIDAR to “map” the chemical composition of an emission source in three dimensions, and from a great distance.

1362


Spectroscopy Perimeter Monitoring Perimeter monitoring can be done with both ultraviolet (UV) and IR spectral equipment. The selection between IR and UV is determined by the absorption of the gases to be measured. Typical installations have a transceiver at one end and a retro-reflector at the far end. The light, typically at two wavelengths (reference and measurement), is transmitted to the mirror at the other end of the path of interest and then reflected back to a receiver in the same electronics enclosure. The resultant measurement is an average of the concentration of the gas of interest over the distance between the two units. Newer models have separate transmitter and receiver assemblies, thus making it possible to measure around corners by using the appropriate reflectors. Path lengths of several hundred meters are typical, though distances of greater than 1 km are possible. Ion Mobility Spectroscopy Since this technique is not often used to monitor hydrocarbons, it is mentioned here only for the sake of completeness. This technique is best suited to detect elements in Group VIIA of the periodic table (Cl, Fl, I, etc.), though the range is being expanded and has also been applied to the detection of ammonia as well as other chemical agents, especially those associated with explosives. While in theory this technique can measure multiple gases, the selective membranes, dopants, diluents, and spectrometer configurations are normally optimized to measure a single gas. Hydrocarbon Dew-Point Meter The dew point of natural gas is that temperature at which significant condensation starts to be formed on a chilled surface. A chilled-mirror instrument of the dark-spot design can be used for this measurement (Figure 8.25g). With this

technique, almost invisible films, having sensitivities on the order of 1 ppm, become detectable. In this instrument, the optical surface is provided with a V-shaped depression. When the surface is dry, the reflection from it results in a dark spot (as shown in Figure 8.25g). As hydrocarbon condenses on this surface, the scattered light intensity in the dark-spot region is reduced. Optical fibers are used to detect this reduction of intensity, while miniature thermocouples measure the surface temperature of the mirror.

CALIBRATION METHODS Only dynamic calibration is completely satisfactory. In this procedure, known concentrations of the gases to be measured are passed into the analytical system, preferably in exactly the same way as the unknown samples are collected. This may be difficult to accomplish because of low concentrations or because some components might condense or react with other components. Standard calibration mixtures of one or several of the common hydrocarbons in carrier fluids are available commercially in high-pressure gas cylinders. These should always be checked against a reference standard, or at least against a previously calibrated analyzer. If much calibration is necessary, it may be useful to set up a gas-handling and dilution system to prepare standard mixtures. One fairly simple technique for hydrocarbons, such as butane (which can be liquefied at moderate pressure), uses permeation tubes. These are sealed tubes of a specific plastic, partially filled with liquid hydrocarbon. As long as some liquid remains, the hydrocarbon will effuse through the walls of a given tube at a rate depending only on temperature. To generate a known concentration, carrier gas is passed over the tube at a controlled rate in a temperature-controlled vessel. The tube may be calibrated gravimetrically. In another technique, used to calibrate atmospheric hydrocarbon analyzers, a known amount of hydrocarbon is added by syringe or by crushing an ampoule, either in a large rigid vessel of known air volume or into the metered flow of air passing into a plastic bag. The bag must be of inert plastic; it has the advantage of collapsing as the sample is withdrawn, whereas a rigid vessel must be large in relation to the sample size in order to avoid substantial pressure differentials.

ASSESSMENT

FIG. 8.25g Measurements principle of the dark-spot-type chilled-mirror analyzer, which is used to measure the hydrocarbon dew point in natural gas. (Courtesy of Michell Instruments Ltd.)


For total hydrocarbon measurement, the flame ionization analyzer is the most generally accepted detector. For proper operation, it requires the attention of operators and it also consumes compressed gases, but it is reliable and accurate. Nondispersive infrared (NDIR) sensors can only identify hydrocarbon classes, whereas two-wavelength IR detectors are able to identify individual hydrocarbon species. In this design, sensitivity is traded for specificity.


The methods that are capable of identifying specific classes of hydrocarbons are complex and are less precise or reliable, but they are not much more expensive than the flame ionization analyzer. (The spectrometer methods, unless already in use for other reasons, are not as competitive.) If individual hydrocarbon determination at low concentrations is required, there is no good alternative to GC (frequently augmented with a freeze-out step), even though the data reduction is time-consuming (if manual) or expensive (if computerized), and calibration requirements are also demanding. Ion mobility spectroscopy and the other spectrometric techniques are only used in special applications.

Bibliography Annino, R. and Villalobos, R., Fundamentals of Process Gas Chromatography, Research Triangle Park, NC: ISA, 1991. ASTM Standard E1982-98, “Standard Practice for Open Path Fourier Transform Infrared (OP/FT-IR) Monitoring of Gases and Vapors in Air,” West Conshohocken, PA: American Society for Testing and Materials, 1982. Bacon, A. T., “Ion Mobility Spectroscopy Applications for Continuous Emission Monitoring,” Analysis Division Spring Symposia, Farmingham, MA: ISA, 1996. Burgess, D., “The Flammability Limits of Lean Fuel-Air Mixtures,” in Analysis Instrumentation, Vol. 12, Research Triangle Park, NC: ISA, 1974 (ISA-AID74414). Clansky, K. B., “The Chemical Guide to the OSHA Hazard Communications Standard,” Burlinganac, CA: Roytech, 1991 (revised annually).


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Dundas, M. E., “New Technologies in Infrared Hydrocarbon Detection,” ISA Conference, Houston, TX, October 1992. Gokeler, U. and Maurer, T., “Area Monitoring of Specific ppb Level Constituents Using Process Gas Chromatography,” Analysis Division Spring Symposia, Denver, CO: ISA, 2002. Kagann, R. H. and Kricks, R. J., “Improvements in Open-Path FTIR for Measurement of Ambient Air at Industrial Facilities,” Analysis Division Spring Symposia, Phoenix, AZ: ISA, 1999. Lowell, F., Pevoto, J., Silvers, R., and Converse, J. G., “Static Headspace Technique Applied to Volatile Chemicals Analysis in Dirty Wastewater Stream,” Analysis Division Spring Symposia, Farmingham, MA: ISA, 1996. McKinley, J. J., “Permeation Tubes for Calibration and Validation of Trace Gas Analyzers,” Analysis Division Spring Symposia, New Orleans, LA: ISA, 1997. McKinley, J. J., “Fundamental Considerations for Effective Use of Permeation Tubes for Calibration and Validation of Process Analyzers,” Analysis Division Spring Symposia, Research Triangle Park, NC: ISA, 1998. Kaspersen, P. and Linnerud, I., “Applications of Tunable Diode Laser Spectroscopy in Process Analysis and Control,” Analysis Division Spring Symposia, Charleston, WV: ISA, 2000. Rayburn, S., “The Foundations of Laboratory Safety,” Berlin: SpringerVerlag, 1990. Roczko, A., “Industrial Applications for an Open Path Infrared and Ultraviolet Gas Detector,” Analysis Division Spring Symposia, Farmingham, MA: ISA, 1996. Webber, K. and Bacon, T., “PPB Level Process Monitoring by Ion Mobility Spectroscopy (IMS),” Analysis Division Spring Symposia, New Orleans, LA: ISA, 1997. Wright, R. S., Kong, E., Bahner, M. A., Clayton, C. A., Nunez, C. M., and Ramsey, G. H., “Analytical Techniques for Measuring Hydrocarbon Emissions from Manufacturers of Fiberglass-Reinforced Plastics,” Analysis Division Spring Symposia, Farmingham, MA: ISA, 1998.

8.26

Hydrogen Sulfide D. H. F. LIU

(1995)

B. G. LIPTÁK

TO RECEIVER

AT

(2003)

H 2S Flow Sheet Symbol


Analyzer Types:

Electrochemical gas diffusion, solid-state or gold-film sensors, tape staining, UV photometric, gas chromatography

Ranges:

0 to 50 ppm is typical for ambient air monitors, with maximum range up to 0 to 500 ppm. For process applications, ranges up to 0 to 100% are available.

Inaccuracy:

3 to 5% of full scale for air monitors; 1% of full scale for process analyzers

Costs:

Pocket-size, battery-operated monitor costs $600 to $1000; portable microprocessorbased diffusion-type unit costs $2500; for UV photometric and chromatographic analyzers, prices are in excess of $20,000 (see Sections 8.12 and 8.61).


ABB Inc. (www.abb.com) Alltech Associates Inc. (www.alltechweb.com) Ametek Process Instruments (www.ametekpi.com) Amko Systems Inc. (www.amkosystems.com) Arizona Instrument Co. (www.azic.com) Bacharach Inc. (www.bacharach-inc.com) Barton Inst. Ltd. (www.barton-canada.ca) CD Nova Instruments (cdnova.com) Davis Inotek (www.davisontheweb.com) Dionex Corp. (www.dionex.com) Drexel Western Ltd. (www.drexelwestern.com) Environmental Technologies Group (www.envtech.com) Fisher Scientific (www2.fischersci.com) Gas Analytical (www.gasanalytical.com) General Monitors (www.generalmonitors.com) Hanna Instruments Inc. (www.hannainst.com) Houston Atlas Inc. (www.hobre.com) Key Safety Devices (www.key-safety.com) K-Patents Inc. (www.kpatents.com) Microwatt Control Devices (www.microwattcontrols.com) MSA Instrument Div. (www.msanet.com) Novachem BV (www.novachem.com) Parker Hannifin Corp. (www.parker.com/instrumentation) Peacock Instr. (www.peacock.ca) Sensidyne (www.sensidyne.com) Sierra Monitor Corp. (www.sierramonitor.com) Spartan Controls (www.spartancontrols.com) Teledyne Analytical Instruments (www.teledyne-ai.com) Thermo Gas Tech (www.gastech.com) Thermo Orion (www.thermo.com) Tytronics & Nemetre (www.tytronics.com) Westech Industrial (www.westech-ind.com) Zellweger Analytics (www.zelana.com)

8.26 Hydrogen Sulfide

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INTRODUCTION Hydrogen sulfide (H2S) is a toxic gas found in many industrial environments. It is commonly monitored for personnel safety, environmental protection, and process control. This section will describe the more frequently used commercial instruments for ambient and on-line measurement of H2S in process gases and liquids.

ELECTROCHEMICAL CELLS For portable, battery-operated instrumentation for ambient air monitoring, the most common sensors are the fuel-celltype electrochemical gas diffusion sensors. Fuel cells convert the chemical energy of fuel and oxygen into electrical energy, while the electrode and the electrolyte remain unaltered. Fuel is converted at the anode into hydrogen ions, which travel through the electrolyte to the cathode, and electrons, which travel through an external circuit to the cathode. If oxygen is present at the cathode, it is reduced by these electrons, and the hydrogen and oxygen ions eventually react to form water. The electrochemical sensors are available in pocket-size, battery-operated packaging, with replaceable “pop-out” sensors. The advantage of these units is that they do not require pumps or aspirators to pull in the sample, and they are unaffected by wind or variations in relative humidity. These pocket-size units are usually configured for a number of channels, which might include H2S, CO, O2, and CH4 (Figure 8.26a). In the sensor, a pair of polarized electrodes is isolated from the ambient air by a gas-permeable membrane. As hydrogen sulfide diffuses through the membrane, an oxidation– reduction reaction occurs and the resulting electrons cause a current flow, which is in proportion to the H2S concentration in the air. Some of the limitations include interferences from background gases such as hydrogen, carbon monoxide, ethylene, sulfur dioxide, chlorine, methyl mercaptan, nitric oxide, and nitrogen dioxide. The units are also available as explosionproof transmitters (Figure 8.26b) for near-ambient temperatures and up to 10-PSIG (0.7-bar) services. The electrochemical sensors are preferred in pure oxygen or in benign atmosphere applications or where ruggedness is less important than accuracy.

FIG. 8.26a Pocket-size, battery-operated electrochemical monitor with four channels, including H2S. (Courtesy of Sensidyne Inc.)

GOLD-FILM AND SEMICONDUCTOR SENSORS Gold films absorb hydrogen sulfide and respond to its concentration by a proportional change in their electrical resistance. These analyzers usually include an internal pump that draws in the ambient air. One advantage of the gold-film sensor is that it is available in the parts per billion (ppb) range, and it is not sensitive to interference by SO2, CO2, or CO.


1.375" 35 mm

FIG. 8.26b Diffusion-type electrochemical hydrogen sulfide detector.

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Analytical Instrumentation to 300°C range, a measurable decrease in electrical resistance occurs if exposed to H2S. The resistance decrease is a logarithmic function of H2S concentration. In the case of the solid-state sensor, hydrogen, isopropanol, ethyl, and methyl mercaptan interfere with the measurement. In general, solid-state sensors are preferred to detect high concentrations or to operate under extreme temperatures, vibration, or in corrosive atmospheres.

SUBSTRATE THERMISTOR

HEATER

SEMICONDUCTOR FILM

LEAD ACETATE TAPE STAINING

CONTACT PADS

FIG. 8.26c 1 Schematic of a solid-state H2S sensor.

Solid-State Sensors Solid-state sensors are used in combustible gas detection equipment for the measurements of H2S in ambient air at the parts per million (ppm) level. The advantage of the solidstate approach is that there is no sampling system involved. The solid-state H2S sensors are formed by depositing thin films on silicon chips, as shown in Figure 8.26c. Separate film layers serve as heaters, temperature-monitoring thermistors, and H2S-sensitive metal oxide semiconductors. If the semiconductor is heated to a constant temperature in the 150

This type of analyzer system is used by industry for attaining compliance with the U.S. Environmental Protection Agency (EPA) Fuel Gas to Combustion Devices Regulations. It has a history of on-stream field performance and reliability for measuring H2S in gas streams such as coal gas, natural gas, and mixed propane–butane. It is specific to H2S. The system’s ability to report H2S levels enables timely corrective actions to be taken to protect against upset, contamination, and corrosion buildup. Figure 8.26d shows the instrument and its operating principles. Sample at a constant flow rate enters a humidifier where it bubbles through a 5% acetic acid solution. The sample then flows into the reaction window of the sample chamber, where it passes over an exposed surface of paper sensing tape impregnated with lead acetate; the tape is automatically driven by a motor. Hydrogen sulfide reacts with lead acetate to form lead sulfide, causing a brown stain on the paper. The rate of reaction and resulting rate of color change is proportional to the concentration of H2S in the sample.

BEAM

TUNGSTEN LAMP

HUMIDIFIER MIDGET IMPINGER 25 ml.

5% ACETIC ACID IN WATER

HUMIDIFIED H2S & SAMPLE

REACTION WINDOW

REFERENCE

PRIMARY FOCUSING LENS

FOCUSED BEAM

SAMPLE IN

REFERENCE PHOTOCELL MEASURING PHOTOCELL

EXPOSED SENSING TAPE

SAMPLE CHAMBER

FIG. 8.26d Rate of change of reflectance-type H2S readout system. (Courtesy of Houston Atlas Inc.)


MIRROR PHOTOCELL FINE FOCUS BALANCING LENS

COLORATION RATE OF CHANGE INDICATOR

(SAMPLE EXPOSURE NOT CRITICAL)

8.26 Hydrogen Sulfide

An optical system, photocells, and first-rate derivative electronic processor provide an output voltage proportional to the rate of change of the photocell output voltage. This output is proportional to concentration of H2S in the sample. A reference photocell compensates for light intensity changes. PHOTOMETRIC ANALYSIS The measurement of the ultraviolet (UV) absorption of H2S provides a sensitive and selective technique for monitoring H2S concentrations in gas streams in which no other UVabsorbing compounds are present. Direct photometric analysis and the analysis systems described below for monitoring low levels of H2S are all operating successfully in gas processing plants and oil refineries. Direct Photometric Analyzer Figure 8.26e shows a flow diagram of a direct photometric analyzer system for monitoring low levels of H2S in interferencefree gas streams. (See Section 8.61 for a detailed description

1367

of the split-beam UV analyzer.) The system uses a bypass stream to decrease sample lag and a self-cleaning filter to minimize particulate matter in the sample. The sample pressure is maintained within the sample cell with a back-pressure regulator after the sample cell. The analyzer sample cell is purged with air or nitrogen when automatic zeroing is required for high-sensitivity measurements. Where the level of potentially interfering compounds is relatively low and not rapidly changing, the above system has operated successfully using the sample gas with the H2S selectively removed as the “zero” reference gas. Potentially interfering compounds are those with conjugated double bonds, such as 1,3-butadiene and aromatics and other sulfur compounds. Where background absorbance is excessively high and changing rapidly, a special system has been developed for selective H2S analysis. In this system, H2S is extracted with a dilute ammonium hydroxide solution, and the strong UV absorption of the ammonium sulfide formed in solution is measured and calibrated for H2S concentration in the gas stream.

LAMP HOUSING

PHOTOMETER SAMPLE CELL

GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTOR

FLOW INDICATOR

TO CONTROL STATION

H2S ADSORPTION COLUMN PRESSURE INDICATOR

BACKPRESSURE REGULATOR

SAMPLE IN

Process gas chromatographs have been designed for environmental monitoring of H2S at the ppm level using the sulfurspecific flame photometric detector (FPD) (Figure 8.26f). See Section 8.12 for the basic principles of chromatography and a more detailed description of the FPD.

AIR IN

STEAM TEMPERATURE REGULATOR

TAIL GAS ANALYZER SAMPLE DISCHARGE

TO STEAM TRAP

FIG. 8.26e Direct low-level H2S photometric analyzer. Transparent Thermal Barrier

For Claus sulfur recovery applications, a top of the pipe analyzer is available, which can measure both H2S and SO2. An installed unit is shown in Figure 8.26g. Interference Filter

Chemiluminescence Detection Region

Photomultiplier Tube

To Supply Voltage

Flame Lens Air In H2 In

GC Column

FIG. 8.26f The flame photometric detector.


Signal Out to Amplifier

1368


Reference 1.

Kaminski, C. and Poli, A., “Electrochemical or Solid State H2S Sensors: Which Is Right for You?” InTech, June 1985, pp. 55–61.

Bibliography

FIG. 8.26g Top of the pipe tail gas analyzer detects both H2S and SO2. (Courtesy of Ametek Process Instruments.)


ASTM Standard D 4323–84, “Hydrogen Sulfide in the Atmosphere by Rate of Change of Reflectance,” American Society for Testing and Materials, Philadelphia, 1984. Clansky, K. B., The Chemical Guide to the OSHA Hazard Communication Standard, Burlinganac, CA: Roytech, 1991. Clevett, K. J., Process Analyzer Technology, New York: John Wiley & Sons, 1986, pp. 288–320. Corrosion Resistant Alloys, Test Methods for H2S Services, Leeds, UK: Maney Publishers, 1996. Denny, R. and Sinclair, R., Visible and Ultraviolet Spectroscopy, New York: John Wiley & Sons, 1987. Dundas, M. E., “Infrared Gauges Hydrocarbon Levels,” InTech, August 1993. Ewing, G., Ed., Analytical Instrumentation Handbook, New York: Marcel Dekker, 1990. H2S Corrosion in Oil and Gas Production, National Association of Corrosion Engineers, Houston, TX, 1981. National Institute of Occupational Safety and Health, Manual of Analytical Methods, 3rd ed., Methods 7902 and 7903, 1984. Occupational Safety and Health Administration, Analytical Methods Manual, Method ID110, 1985. Saltzman, R. S. and Dell, C. G., “Photometric Analyzer Systems for Monitoring Low Levels of Hydrogen Sulfide,” ISA Transaction, Vol. 24, No. 1, 1985, pp. 69–74. Skinner, D., Hydrogen Sulfide in Production Operations, University of Texas, Austin, 1996.

8.27

Infrared and Near-Infrared Analyzers J. E. BROWN (1969) A. C. GILBY (1982) B. G. LIPTÁK, T. M. CARDIS (1995) E. H. BAUGHMAN

(2003)

AT IR

To Receiver Flow Sheet Symbol

INFRARED ANALYZERS Process Streams:

Gas or liquid, with surface analysis of solids

Application and Minimum Full-Scale Range:

Maximum range is usually 100%, with path length adjustment; minimum range, assuming 10-m path length. Ammonia—100 ppm; carbon monoxide—25 ppm; carbon dioxide—20 ppm; ethylene— 100 ppm; hexane—100 ppm; methane—10 ppm; moisture (humidity)—50 ppm; nitrous oxide—10 ppm; propane—100 ppm; sulfur dioxide—100 ppm (Notes: 1. Some of these analytes can be done very well in the UV, for example, sulfur dioxide. 2. The minimum range is also a function of the matrix—the minimum for benzene in air is going to be much lower than that for benzene in gasoline. 3. The normal range is a factor of 10, so ammonia could be 10 to 100 ppm or 1 to 10%, but not 100 ppm to 10%. 4. These are examples only, not an exclusive list.)

Operating Pressure:

Standard from atmospheric to 150 PSIG (10 bars); special up to 1000 PSIG (70 bars)

Ambient Operating Temperature:

−40 to 120°F (−40 to 50°C) is standard; probe temperatures can be higher with special arrangements

Humidity Limitations:

Up to 95% relative humidity (normally the instrument is purged, which negates the effect of humidity in the atmosphere.)


Cell bodies are available in all standard materials; windows can be made of sodium chloride, calcium fluoride, barium fluoride, sapphire, or zinc selenide

Cell Lengths:

For liquids, from 0.004 to 4 in. (0.1 to 100 mm); for gases, up to 130 ft (40 m) enclosed and any length for open-path monitoring

Warm-Up Time:

15 to 20 min. (For most stable operation, allow 16 h for warm-up.)

Repeatability:

±1% of full scale

Linearity:

±0.5 of full scale

Inaccuracy:

±2% of span

Drift:

±1% of full scale for zero and the same for span per day

Costs:

Remember that the installation and upkeep costs are normally much larger than the vendor costs given below. Single-beam portable or laboratory units cost $4000 to $5000; an industrial nondispersive infrared analyzer with diaphragm capacitor costs $8,000; a multigas analyzer pulling in up to five gases from 150-ft (50-m) distances costs $25,000 to $27,000; a microprocessor-based portable spectrometer with preprogrammed multicomponent identification capability for ambient air monitoring and with space for 10 user-defined standards for calibration, AC/DC converter, sample probe, and carrying case costs $20,000; an industrial FTIR costs $75,000 to $125,000.


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ABB Process Analytics—Bomem (www.abb.com/analytical) Ametek (www.westernresearch.com) Anarad (www.anarad.com) Bruel & Kjaer ([email protected]) CEA ([email protected]) Foxboro (www.foxboro.com) Hamilton Sundstrand (AIT Division—Analect) ([email protected]) Horbia Instruments (www.horiba.com) LI-Cor (www.licor.com) Midac (www.Midac.com) MKS Instruments ([email protected]) MSA Instruments (www.msanet.com) Ocean Optics (www.oceanoptics.com) Remspec Corp. (www.remspec.com) Rosemont (www.rauniloc.com) Sensidyne (www.sensidyne.com) Servomex (www.servomex.com) Siemens (www.sea.siemens.com) Teledyne (www.teledyne-ai.com) Wilks Enterprise (www.WilksIR.com) Zellweger Analytics, Inc, (www.zelana.com) The best known are ABB, Servomex, and Siemens.

Suppliers of Fiber Optics, Sample Systems, Standards, and Tool Development for Both IR and NIR:

Axiom, samples systems, fibers, both NIR and IR ([email protected]) Custom Sensors and Technology, sampling tools and analyzers (mike@customsensors. com) Dave Mayes, a developer of spectroscopic tools (www.dsquared-dev.com) Equitech International Corp., fiber switches, fiber connections to the process, sampling systems (www.equitech-intl.com) Fiber Tech Optica, fiber optics only (www.fibertech-optica.com) Remspec Corp. Fiber Optics (www.remspec.com) Solutions Plus, Inc., makers of traceable standards, a division of Ricca Chemical ([email protected])

NEAR-INFRARED ANALYZERS


Process Fields:

Gas, liquid, or solid, but mostly liquid and solid

Some Applications:

Octanes of gasoline, 80 to 100 Octanes of components of gasoline, 60 to 120 Benzene in gasoline, 0.2 to 1% Boiling points of gasoline, 50 to 200°C (122 to 392°F) Cetane of diesel fuel Protein content of wheat Molecular weight of small polymers Caustic in water 0.1 to 10% BTU of natural gas (high pressure) Active ingredient in drugs p-Xylene concentration in mixture of aromatics

Operating Pressure:

150 PSI standard (10 bar) 1000 PSI special (70 bar)


−40 to 120°F (−40 to 50°C ) is standard. (Note: Since the ambient temperature changes will affect the spectrometer, it will require temperature stabilization.)

Stream Temperature:

This restricts cell material only; normally one keeps the temperature constant.

Humidity Limitations:

None—NIRs, like IRs, should be purged; this eliminates the humidity problem.


Cell bodies in all standard materials; windows can be quartz (most common), sapphire, and others

8.27 Infrared and Near-Infrared Analyzers

Cell Path Lengths:

For liquids, 0.04 to 4 in. (1 to 100 mm); for gas, long (unless high pressure too long to be practical)

Warm-Up Time:

Manufacturers normally quote minutes—recommend overnight for best stability

Repeatability:

±0.01% of full scale

Linearity:

±0.5% of full scale

Inaccuracy:

±1% of span (depends on how well the “modeling” has been done; can be much better)

Drift:

±0.01% of full scale and the same for span per day

Costs:

$80,000 to $180,000, depending on number of streams, distance between the analyzer and sample, and sample preparation required. (How can these costs be justified by the user? At one installation, the analyzer is determining 25 properties every 45 sec. At another installation, the plant estimated that the analyzer saved $15 million the first year it was in service. At some locations, the instrument is looking at multiple streams; with a 45-sec analysis time, it is possible to look at several streams and still update the control system as often as needed.)


ABB Process Analytics—Bomen (www.abb.com/analytical) Bran and Luebbe (www.branleubbe.com) Brimrose Corporation of America (www.brimrose.com) Foss-NIR Systems ([email protected]) Guided Wave (www.Guided-Wave.com) Hamilton Sundstrand (AIT Division—Analect) ([email protected]) LTI (www.LTIndustries.com)

In the first part of this section, the infrared (IR) analyzers will be discussed, while the near-infrared (NIR) analyzers will be described in the second part of this section. This is not the only section in this chapter where IR and NIR are discussed. As can be seen from the analyzer selection guide provided in Tables 8.1u and 8.1v, IR and NIR analyzers are applicable to a wide range of analytical tasks. It should also be noted that the boundaries between ultraviolet (UV), visible, NIR, and IR are slowly disappearing. Analyzers are evolving that are capable of operating in all of these spectrums, as discussed in Section 8.22, and as the mathematical tools to handle full spectral ranges are becoming available. The addition of microprocessors or tabletop computers has enhanced the performance of these instruments by providing such features as self-calibration, selfdiagnostics, and chemometric tools, for example, partial least squares (PLS) and principal component regression (PCR), to name two, while design modularity has contributed to simplifying maintenance.

INTRODUCTION This section starts with the description of some general principles of infrared radiation, covering both IR and NIR. That is followed by the definition of some of the basic terms and design configurations. Then laboratory and industrial on-line designs are separately described, and the increasing overlap between the fields is shown. The first part of this section,


1371

which deals with IR analysis, concludes with the description of some newer developments in the field of IR analysis, such as the use of tunable crystals and multiple internal reflection configurations. For an overall view of where process analysis is going, see “Process Analytical Systems: A Vision for the Future,” by Jeff Gunnell and Peter Van Vuuren of ExxonMobil, Journal of Process Analytical Chemistry, Vol. 6, No. 1, 2001, pp. 1–5. The International Forum for Process Analytical Chemistry (IFPAC) conference is held annually in January and normally contains sessions on process IR and NIR; for more information, see www.ifpac.com. This conference is run by Infoscience: telephone, 847-548-1800; address, P.O. Box 7100, Grayslake, IL 60030.

PRINCIPLES OF IR AND NIR ANALYSIS IR absorption (or reflection for solids) is a technique that can be used successfully for continuous chemical analysis of a process. The infrared region of the electromagnetic spectrum is generally considered to cover wavelengths from 0.8 to 20,000 µm. NIR normally covers 0.8 to 2500 µm, and classic IR covers the rest. For IR analysis, these limits are normally −1 put in terms of frequency (cm , wave numbers or the number −1 of waves per cm): 4000 to 500 cm , which corresponds to wavelengths of 2500 to 20,000 µm. Except for a small overlap region, sources and detectors that are needed in the NIR will not work in the IR, and vice versa.

1372


O O O

H

H

H

H

H

H

FIG. 8.27a The three fundamental vibrations of the water molecule: symmetric stretch, bend, and antisymmetric stretch; the amplitudes of the vibrations have been exaggerated for clarity. WAVELENGTH (MICRONS) 2.7

3.0

3.5

4.0

4.5

5.0

0.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

400

0.0

9.0

9.5

10

11

12

13

14

15 16 17 18 19 2021 22 24 25

0.0

0.0

0.1

0.1

0.2

0.2

CO 0.1

0.1

ABSORBANCE

CARBON MONOXIDE 0.2

0.2

0.3

0.3

0.3

0.3

0.4

0.4

0.4

0.4

0.5

0.5

0.5

0.5

0.6

0.6

0.6

0.6

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

1.5

CO

1.5

3600

3400

3200

3000

2800

2600

2400

2200

2000

1.5

1.5

1900

1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

−1

700

600

500

400

FREQUENCY (cm )

400 torr of CO out of total of 600. A diatomic molecule has only one fundamental absorption band. Higher resolution shows the band to consist of many sharp lines spaced about 4 cm−1 apart (rotational line structure). WAVELENGTH (MICRONS)

ABSORBANCE

2.7

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

50

9.0

9.5

10

11

12

13

14

15 16 17 18 19 2021 22 24 25

0.0

0.0

0.1

0.1

0.1

0.1

0.2

0.2

0.2

0.3

0.3

0.3

AMMONIA

0.2

0.3

NH3

0.4

0.0

0.4

0.4

0.4

0.5

0.5

0.5

0.5

0.6

0.6

0.6

0.6

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

1.5

1.5

3600

3400

3200

50 Torr of NH3 out of total of 600.

3000

2800

2600

2400

2200

2000

1900

1.5

1800

1700

1600

−1

1500

1400

1300

1200

1100

1.5

1000

900

800

700

600

500

400

FREQUENCY (cm )

FIG. 8.27b Examples of IR spectra recorded using a laboratory double-beam spectrometer. All spectra are gas phase using a 2-in. (5-cm) cell with N2 added to give a total pressure of 600 mmHg (torr). (Courtesy of Dow Chemical Co.)

Some laboratory spectrometers have both sources and detectors so they can work in both areas. For the process, most gas analysis is done in the IR and most solid and liquid analysis is done in the NIR. The choice is based on workable path lengths. Infrared radiation interacts with almost all molecules (except the homonuclear diatomics oxygen, O2; nitrogen, N2; hydrogen, H2; chlorine, Cl2; etc., and monatomics such as helium, He; neon, Ne; etc.) by exciting molecular vibrations and


rotations that affect the dipole of the molecule (Figure 8.27a). The oscillating electric field of the IR wave interacts with the electric dipole of the molecule, and when the IR frequency matches the natural frequency of the molecule, some of the IR power is absorbed. The pattern of wavelengths, or frequencies, absorbed identifies the molecule in the sample. The strength of absorption at particular frequencies is a measure of the concentration of the species. Analytical laboratory IR is


1373

WAVELENGTH (MICRONS) 2.7

3.0

3.5

4.0

4.5

5.0

5.5

0.0

6.5

7.0

7.5

8.0

8.5

30

0.0

0.1

9.5

10

11

12

13

14

15 16 17 18 19 2021 22 24 25

0.0

0.0

0.1

0.1

0.2

0.2

0.2

0.1

0.2

0.3

9.0

C2H6 O

ETHANOL ABSORBANCE

6.0

0.3

0.3

0.3

0.4

0.4

0.4

0.4

0.5

0.5

0.5

0.5

0.6

0.6

0.6

0.6

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

CH3 CH2 OH

1.5

1.5

3600

3400

3200

3000

2800

2600

2400

2200

2000

1900

1.5

1.5

1800

1700

1600

1500

FREQUENCY (cm

30 TORR ETHANOL OUT OF TOTAL OF 600

−1)

1400

1300

1200

1100

1000

900

800

700

600

500

400

WAVELENGTH (MICRONS)

ABSORBANCE

2.7

3.0

3.5

4.0

4.5

5.0

5.5

0.0

0.0

0.1

0.1

6.5

7.0

7.5

8.0

8.5

100

0.3

CH3 CH2 Cl

9.0

9.5

10

11

12

13

14

15 16 17 18 19 2021 22 24 25

0.0

0.0

0.1

0.1

0.2

0.2

0.2

0.3

0.3

0.3

C2H5 Cl

ETHYL CHLORIDE 0.2

0.4

6.0

0.4

0.4

0.4

0.5

0.5

0.5

0.5

0.6

0.6

0.6

0.6

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

0.7 0.8 0.9 1.0

1.5

1.5

3600

3400

3200

3000

2800

2600

100 TORR ETHYL CHLORIDE OUT OF TOTAL OF 600

2400

2200

2000

1900

1.5

1800

1700

1600

1500

FREQUENCY (cm−1)

1400

1300

1200

1100

1.5

1000

900

800

700

600

500

400

FIG. 8.27b Continued

largely concerned with identification, or qualitative analysis, while process IR is concerned with quantitative analysis. Some typical spectra are shown in Figure 8.27b. The NIR consists of overtones and combinations of these IR bands. Particular groups of atoms tend to absorb at the same frequency with very little influence from the rest of the molecule. These group frequencies are a great help in identifying the molecules from the IR spectra (Figure 8.27c). On the other hand, similar molecules, such as a series of homologous hydrocarbons, have very similar IR spectra. Infrared analysis is, therefore, most straightforward when the component molecules of the sample have significantly different atomic groupings. A mixture of aliphatic hydrocarbons would be better analyzed by another technique, such as gas chromatography. The part of the spectrum offering the best discrimination between molecules is between 7 and 15 µm, –1 1430 and 670 cm , the so-called fingerprint region. Given the very large signal-to-noise ratio in the NIR, one can make very fine separations between similar species; for example, o-xylene can be measured in a mixture of xylenes, ethylbenzene, and benzene.


Beer–Lambert Law The starting point for quantitative analysis is the Beer– Lambert law, frequently just called Beer’s law, which relates the amount of light absorbed to the sample’s concentration and path length. A = abc = log10 I 0 /I

8.27(1)

where A = absorbance I = IR power-reaching detector with sample in the beam path I0 = IR power-reaching detector with no sample in the beam path a = absorption coefficient of pure component of interest at analytical wavelength; the units depend on those chosen for b and c; a newer term, ε, extinction coefficient, is the preferred term in the academic literature b = sample path length, sometimes l is used c = concentration of sample component

1374


FREQUENCY (cm−1)

10000 9000

8000

7000

6000

5000 4000 O-H & N-H STRETCHING

C=O

H-O BONDING

WATER H-O

WATER H-O TERMINAL C-H

TERMINAL C-H TERMINAL C-H

TERMINAL C H

TERMINAL C-H

2000

1400

C=N STRETCHING

C-H STRETCHING

1000

800

C-N STRETCHING

C C & C N C=C STRETCHING STRETCHING

C-C STRETCHING N-H ROCKING

N-H BENDING

AMINES-N H

700

C-O STRETCHING

C=O STRETCHING

ALIPHATIC-C-H ALIPHATIC & ALIPHATIC & AROMATIC-C-H AROMATIC-C-H

AMINES-N-H

1200

C-H ROCKING

C-H BENDING

O-H BENDING

AMINES-N-H NITRILES-C H

1.0

1.1

1.2 1.3 1.4 1.5 1.6 1.7

1.8 1.9

2

3

4

NEAR IR (EXPANDED SCALE)

5

6

7

8

9

10

11

12

13

14

15

WAVELENGTH (MICRONS)

FIG. 8.27c Functional group frequency chart. Fundamental vibrations absorb in the mid-IR; overtones and combination bond are 10 to 10,000 weaker and absorb in the NIR.

Definitions of Terms and Configurations Some of the laboratory spectrophotometers are of the dispersive design, meaning that a prism or grating is used to separate the spectral components in the IR radiation of the source. Most modern laboratory units are of the Fourier transform (FT) type, where a moving mirror generating an interference pattern of the wavelengths accomplishes dispersion, and then the FT converts this detected signal into something useful. See the section on Fourier transform spectrometers. Most industrial process analyzers are nondispersive infrared (NDIR) designs. However, there are some very


0.175 0.150

ABSORBANCE

0.125 0.100 CARBON MONOXIDE IN AIR AT 1 Atm FIXED BAND PASS 4.61 µm CENTER WAVELENGTH 0.08 µm FULL WIDTH AT HALF HEIGHT 5.25 m PATHLENGTH

0.075 0.050 0.025 0

1000 1500 CONCENTRATION (ppm) CARBON MONOXIDE SHOWS A LARGE DEPARTURE FROM THE BEER-LAMBERT LAW. 0

500

2000

2–PENTANONE IN AIR AT 1 Atm 5.25 m PATHLENGTH 8.5 µm CENTER WAVELENGTH

0.3

ABSORBANCE

The law states that concentration is directly proportional to absorbance at a given wavelength and path length at specified temperature and pressure. Note, however, the logarithm function—frequently absorbances over 1 show nonlinear behavior, and sometimes problems start as low as 0.7. Sometimes isolated peaks in the IR can be found that correspond only to the item of interest; then calibration plots of A vs. c can be made up using known samples and used to analyze unknown ones (Figure 8.27d). Beer’s law is also helpful in choosing the optimum path length for accurate analysis. (In some cases, this linear relationship is not observed. See the discussion of linearity later on in this section.) There is also the case where overlapping spectral lines eliminates the simple application of Beer’s law. Note: Not only can concentration of molecular species be identified, but also physical properties can frequently be measured. This is done more in the NIR than the IR, but can be done in both. A statistical approached called chemometrics, PLS and PCR, for example, is used to measure physical properties and solve the overlapping spectral lines problem referred to above. These very powerful programs require care in use and are more frequently used in NIR than IR.

0.2

0.1

0 0

100 200 300 CONCENTRATION (ppm) 2–PENTENONE GIVES A LINEAR CALIBRATION PLOT.

FIG. 8.27d Calibration curves for a typical filter analyzer. Top: Carbon monoxide shows a large departure from the Beer–Lambert law. Bottom: 2-Pentenone gives a linear calibration plot.


good FTIRs on the market for process applications where several components need to be measured. Since oxygen and nitrogen do not absorb in the IR spectrum, dry air is frequently used as a zero reference gas; however, one needs to be aware of the very strong absorbance of CO2 when one is doing this. Detectors are mostly solid-state type, with some microphone types still in existence. The solid-state type, the most common today, functions by converting the incoming photons to an electric current. This current is amplified and then sent to the recording device. The microphone-type detectors are filled with an IR-absorbing gas, which is heated by the radiation it receives and expands as a consequence. It is this expansion that is measured. The microphone type usually lacks uniform sensitivity across the spectrum. The solid-state detectors are generally of lower sensitivity than the microphone type at the microphone type’s peak sensitivity, but they have almost uniform sensitivity across the spectrum. IR Instrument Designs Single-Beam Configuration Designs of IR instruments can be separated into single- and dual-beam configurations. Singlebeam analyzers are the main ones used in the process world (Figure 8.27e). They function by alternating filters in the beam path—the reference chosen to not absorb the species of interest, but to offset any other species that would absorb at the measuring wavelength. (The proper choice of the reference wavelength is not a trivial task. The reference wavelength should be as close to the measuring wavelength as possible, and yet let the measurement be made. Drifts in the source or detector will be compensated best if the measuring and reference wavelengths are close together.) These filters are put on a chopper wheel in the light beam, and as the chopper alternatively spins one filter or the other into the optical path, the difference or ratio in the energies received at the detector will be a function of the concentration SAMPLE IN

SAMPLE OUT

1375

of the component of interest. Because filters normally change absorbance at a given wavelength with temperature, this approach must be temperature stabilized. Also note that dirt on the window will not affect the reading because it should affect both the measuring and reference wavelength the same so the ratio stays constant if the monitoring and reference wavelengths are close enough. A less expensive design uses just the measuring wavelength, from above, eliminating the filter wheel and reference filter. Problem: If the source varies in intensity or temperature, so will your measurement. If the detector changes in sensitivity, so will your result. In other words, to save the first costs, one needs to rezero and respan these instruments very frequently. If no lenses or mirrors are used to direct the IR radiation from the source to the receiver, it is necessary to polish the interior walls of the sample cell to make them highly reflective. Sometimes gold foil is used to achieve this goal. The use of reflective walls can be very expensive because it can contribute to drift as contamination of the wall changes reflectivity. Dual-Beam Configuration In the dual-beam configuration, the IR radiation is allowed by the chopper to pass alternately through the sample or the reference tube (Figure 8.27f ). The reference tube can provide a true zero when it is filled with a nonabsorbing gas, or it can act to balance out the gases which are not of interest, if filled with those gases. A narrowband-pass optical filter is placed in front of the detector to limit the IR energy it receives to the wavelength that is characteristic of the component of interest. (Note: This severely limits the amount of energy getting to the detector; the narrower the band pass, the less energy, and the narrower the band pass, the more specific.) If the sample contains the component of interest, this will attenuate the magnitude of the detected signal in the absorption band of the band-pass filter (Figure 8.27g). The use of the reference cell in the dual-beam configuration reduces the drift caused by power supply, detector changes, or some temperature fluctuations. The use of collimating optics SAMPLE IN

CHOPPER

SAMPLE OUT

REF SYNC

LIGHT SOURCE

SAMPLE TUBE

DETECTOR CHOPPER MOTOR

MEASURING WAVEBAND FILTER

SAMPLE CELL

CELL WINDOWS REFERENCE WAVEBAND FILTER

FIG. 8.27e Single-beam IR analyzer provided with two filters, one for the sample measurement and the other for reference.


SOURCE

DETECTORS FILTERS

REFERENCE TUBE

MIRROR #1 SIG SYNC

CHOPPER

MIRROR #2

FIG. 8.27f Dual-beam IR analyzer where the radiation alternatively passes through the sample tube, which contains the component of interest, or the zero-reference tube, which is free of absorbing gas.

1376


LUFT TYPE MICROPHONE DETECTOR

SAMPLE CELL

DIAPHRAGM

IR LIGHT SOURCE

CONDENSER MICROPHONE TERMINALS

SENSITIZING GAS

REFERENCE CELL

FIG. 8.27g Dual-beam analyzer with internally reflective cell surfaces and Lufttype microphone detector.

LIGHT SOURCE

STACK

BLOWER

BEAM ALTERNATOR

NEUTRAL FILTER DETECTOR

GAS-FILTER REFERENCE CELL

ELECTRONICS

FIG. 8.27h 1 Dual-beam IR analyzer for stack gas monitoring.

also eliminates the need for internal reflection from the interior surfaces of the tubes, thus simplifying their construction and eliminating the associated drift due to dirt accumulating on the wall of the tubes. Dual-Beam Design for Stacks When the IR analyzer is used for in situ stack gas analysis, two different approaches can be used: 1) the detectors and

LED PHOTODIODE SYNC

source on the same side of the stack, and 2) the detectors and source on opposite sides of the stack. For installation purposes, the first is much easier, but it requires an internal filter to remove any solids from the stream. In the second system, shown in Figure 8.27h, both the reference and measuring wavelength are affected the same by scattering bodies in the stream; therefore, there is no need for a filter. This system is frequently used in pollution alarm situations. The reference cell is filled with somewhat less than the allowed emission of the gas of interest, and the IR beam leaving the stack is alternated between a neutral filter and the reference cell. When the stack gases are below the level of concern, the reference cell will remove the absorption lines that are characteristic of the gas of interest. Therefore, the total IR energy from the reference cell is at a reduced value. The analyzer is balanced by selecting a neutral filter, which reduces the energy from all the wavelengths to such an extent that the energy levels received by the detector from the neutral and reference gas filters will be the same. When the pollutant of interest is present in the stack, the energy content of the reference path is unaffected (because the absorption is already complete at the selected wavelength). On the other hand, the IR energy reaching the detector through the neutral filter is reduced (due to the absorption of the pollutant gas) and the ratio between the beams reflects the pollutant concentration at the level of concern in the stack. This measurement is unaffected by particulate concentration variations, within reason, as it affects both paths equally and their ratio is unaffected. The first configuration is shown in Figure 8.27i. Note that only one opening is required in the stack, with no critical alignment across the stack required, as in the other option. It is very critical to keep the reflector mirror clean; some people use a ceramic filter for that purpose. This is the only type that this author has used, and it has worked well.

CHOPPER MOTOR IR SOURCE PURGED PROBE BODY

FILTER WHEEL EXIT WINDOW

OPTICAL MEASUREMENT T/C

DETECTOR

OBJECTIVE LENS DYNAMIC CALIBATION PORT

FIELD LENS/ WINDOW

CAVITY/PATHLENGTH

REFLECTOR CERAMIC FILTER

FIG. 8.27i IR analyzer used for in situ stack gas analysis equipped with ceramic diffusion cell, reflector, and calibration port.



INFRARED ANALYZERS FOR THE LABORATORY Grating Spectrophotometers The standard laboratory IR instrument was a double-beam, optical null spectrophotometer. Diffraction gratings have completely replaced prisms for separating the IR beam into its component wavelengths. Both approaches have been replaced by FTIR, discussed below. Most IRs, old and modern, use a heated black-body source that emits at all IR wavelengths, and the thermal detector responds roughly equally at all wavelengths (Figure 8.27j). The complete spectrum is scanned by rotating the grating and measuring the light intensity passing through its exit slit. Sample cells are placed in the sample beam and, as the spectrum is scanned, the instrument drives an attenuator into the reference beam until the detector sees equal energy from both beams. These spectrophotometers are ideally suited for qualitative analysis. There are computer libraries of spectra and search programs that allow the computer to find the major compound in the sample and sometimes some of the minor ones. In many cases, manufacturers have added computer data handling to make possible both qualitative and quantitative analyses. Filter Spectrometers A circular variable filter (CVF) selects the wavelength to be measured. Microcomputer-controlled, single-beam analyzers –1 working from 4000 to 690 cm have been designed for the quantitative analysis of mixtures. Measurements are made at each analytical wavelength in sequence, and the output is presented as a list of concentrations for mixtures of up to 10 components. Narrow-IR-band-pass filters consist of multiple layers of dielectrics of alternating refractive index on a transparent

GRATING DETECTOR

1377

substrate. They pass a band of wavelengths while rejecting all others. The width of the band pass is typically 1 to a few percent of the center wavelength of the item of interest. Spectral resolution is low when compared to that obtained with a grating instrument, but signal-to-noise ratios are higher. A CVF is made of dielectric layers of continuously varying thickness so that the wavelength selected depends on the angular position of the CVF wheel. Fourier Transform Spectrometers A quite different and very high performance approach to analysis makes use of the Michelson interferometer or some modification thereof. Instead of separating the different wavelengths of light with a filter or a grating for measurement, the compete spectrum is encoded as an interferogram in a few seconds of measurement time, and the spectrum is computed by Fourier transform or fast Fourier transform. Advantages include speed, full-spectrum method, and light throughput (one is not throwing away most of the light as one is looking at a different wavelength). The disadvantage is the moving parts. There has been a major effort to stabilize the moving parts, but they still require smooth movement for good spectra; there must be an attached computer to do the transform, but this can act as a storage device for some of the spectra. A schematic of a typical FTIR or FTNIR spectrophotometer is shown in Figure 8.27k. Collimated light from the source is directed through the beam splitter. Approximately 50% of the light passes through the beam splitter to the fixed mirror, M1. The balance of the light is reflected to the moving mirror, M2. When these two beams are reflected off the mirror surfaces, they recombine at the beam splitter to give constructive and destructive interference as a function of the wavelength of light and the distance that M2 has moved. The included helium–neon laser detection system in the FT analyzer monitors the position and velocity of the moving mirror. This results in excellent wavelength accuracy for the FT analyzers. The displacement of the moving mirror induces phase differences that result in an interferogram. In order to produce a spectrum of absorbance vs. wavelength, the interferogram must be transformed digitally from a plot of detector

SLITS

FIXED (M1) MOVING (M2) MIRROR MIRROR BEAM SPLITTER D-1 D-2 D-3

REFERENCE BEAM DETECTOR

SOURCE CHOPPER

SAMPLE CELL

SAMPLE BEAM DETECTOR ENCLOSURE

ZPD 1/4λ 1/2λ

SOURCE

DETECTOR SIGNAL

MOVING MIRROR POSITION

SAMPLE COMPARTMEN

FIG. 8.27j A widely used laboratory double-beam optical null spectrophotometer.


INTERFEROMETER ENCLOSURE

FIG. 8.27k FTIR spectrophotometer.

1378


response vs. optical path difference. This calculation of the spectrum is carried out using Fourier transform or fast Fourier transform on the attached computer. Since the FTIR or FTNIR analyzer measures all the wavelengths simultaneously on one detector, the entire spectrum can be used for quantitative analysis. FT spectrophotometers are used for multicomponent applications where the high optical resolution allows separation of interfering components. (Note: One should be careful with the term optical resolution. Some suppliers will talk about resolution as the number of data points per unit of the spectrum. This, in most cases, is not the same as their optical resolution. Without high optical resolution, you cannot separate some components, regardless of the data point resolution.) FTIR and FTNIR spectrophotometers are being used online where the full spectrum is needed for analysis. One big advantage of the full-spectrum measurement is that the associated computer programs can be set up to detect unusual samples or impurities in the sample. These unusual samples are frequently referred to as outliers, but they are sometimes an early warning of an unwanted change in raw materials or in the process. Tunable Lasers On the face of it, this is the spectroscopist’s dream come true. –1 Laser diodes are available to cover the 4000 to 400 cm –1 region. A given diode may cover 200 cm with small dis–1 continuities about every cm in its tuning. The devices (PbSSe or PbSnTe) have very high spectral resolution and good power output, but require liquid N2 temperature or below, plus other complex support equipment. They are for the research spectroscopist, but they have been used in isolated cases of process analysis that could not be done by more standard methods. Laboratory Instruments in Process Measurement Normally, laboratory instruments cannot be taken from the laboratory and put onto the process line because the laboratory instrument does not meet the fire code for the process area and it is not stable enough to run 7 days a week, 24 h a day without the need for recalibration. Some companies, such as Dow Chemical, have done a good job in moving many laboratory instruments into the process arena, but with a high investment of manpower. The Fourier transform spectrophotometer has successfully made the move from laboratory instrument to an online tool, as have diode array instruments. In many cases, however, the process instrument no longer looks like its laboratory cousin, and the cost is considerably higher by the time it is temperature stabilized and made explosion-proof. (Process instruments should not need a safe, air-conditioned building to survive in the process world, but this can aid its stability and will certainly aid in its maintenance.)


SAMPLE IN OUT

FILTER CELL

FIXED PLATE

SIGNAL PROCESSING MOTOR

MOVABLE DIAPHRAGM REF CELL SOURCE

FILTER CELL

CHOPPER

FIG. 8.27l Typical NDIR analyzer using positive filtering. Both chambers of the detector are filled with a sample of the gas to be analyzed.

INFRARED ANALYZERS FOR PROCESS APPLICATIONS Single-Component Analyzers This is a type of process IR analyzer that has no analog in laboratory instruments. It is a NDIR analyzer and is used almost exclusively for gas analysis. These analyzers were invariably double-beam and used gas-selective Luft detectors filled with the gas to be analyzed (Figures 8.27g and 8.27l). (They currently use a solid-state detector.) In the usual positive filtering mode, light from single or dual sources is chopped and passed in phase through the sample or reference cells to the detector. Sample molecules attenuate certain wavelengths, and the difference in power between sample and reference beams is sensed as a change in capacitance. This is amplified and displayed to give an output corresponding to concentration. Selectivity is improved by adding a narrow-band-pass filter, which selects the wavelengths of interest, and filter cells filled with interfering species, which remove undesirable wavelengths from both beams. The NDIR technique is most sensitive and selective for small molecules whose spectral fine structure is resolved under ambient conditions. The spectral resolution of these instruments is effectively set by the width of the absorptions line and, in many applications, is much better than the typical laboratory grating spectrophotometer. The Luft detector has excellent sensitivity, but it must be temperature controlled and protected from external vibration. These analyzers need routine calibration for span and zero. (This is true for most process analyzers.) Design Variations Some important variants of this basic NDIR method with Luft detectors should be mentioned: 1. A second detector placed behind the first can be sensitized to measure and compensate for an interferant. 2. Detector chambers can be placed one behind the other with the chopper alternately delivering each beam


to both. A different pressure of sensitizing gas in each detector chamber provides a built-in reference wavelength. 3. The combined beams, alternately admitted to the same chamber of the detector, cause heating and gas flow in and out of the chamber. A sensitive flow meter gives an output proportional to the sample concentration. 4. In a technique called cross-flow modulation, the mechanical chopper is replaced by a valve that exchanges sample and reference gas between the two beams at a frequency of about 1 Hz. High sensitivity and zero stability are claimed for this technique, but reference gas must be supplied continuously, flow rates and temperatures must be carefully regulated, and response time is slow. The solid-state detectors do not require the utilities that the Luft detector requires and are fast enough for the filter instruments. (Many of them do not keep up with a scanning grating, however.) For stability and reproducibility, it is hard to beat the filter instrument with a solid-state detector. Gas Filter Correlation Spectrometers Gas filter correlation (GFC) spectrometers (Figure 8.27m) use a nonspecific thermal detector with a specifying reference gas cell. This is an example of negative filtering. The technique is most useful when high specificity is required and it is not practical to have a reference beam. The IR beam passes alternately through a cell containing a fixed quantity of the gas to be analyzed and through a similar cell filled with a zero gas, such as N2. The combined beam passes through the

READOUT AND CONTROL OF BURNER AIR SUPPLY CALIBRATION CELL

1379

sample, for example, across a smoke stack, and on to a thermal detector. This system, which is similar to NDIR, works well for analyzing small gas phase molecules whose spectra have well resolved, rotational fine structure in the presence of strong interference from other species. One example is the analysis of carbon monoxide, CO, at the 100-ppm level in combustion gas-containing large concentrations of carbon dioxide, CO2, and water, H2O. For accurate data, the sample must be removed from the optical path periodically to check for zero drift. A sealed calibration cell can be used to check span. (Note: Both of these require installation considerations.) Another configuration of the stack gas analyzer GFC has already been shown in Figure 8.27h. It is possible to extend the GFC method to several gases with nonoverlapping spectra by placing a gas mixture in the correlation cell and inserting different narrow-band-pass filters sequentially into the beam. Filter Analyzers Narrow-band-pass filters are used with nonspecific thermal detectors and are the most common IR units available today. Various configurations exist: 1. Single-beam, dual-wavelength (analytical and reference) (Figures 8.27e and 8.27n) 2. Single-beam, multiwavelength (one reference and up to 10 measurings), same basic construction as shown in Figure 8.27n 3. Dual-beam, single-wavelength 4. Dual-beam, dual-wavelength The intent of the dual-beam design is to compensate for source output changes or detector or electronic gain changes.

DETECTOR STACK

SAMPLE OUT IN

DETECTOR

SOURCE

BEAM SPLITTER

COMBUSTION GASES

CHOPPER

CHOPPER CORRELATION CELL

FOCUSSING MIRROR

SOURCE CO

N2

NEUTRAL DENSITY FILTER

FIG. 8.27m The gas filter correlation technique is an example of negative-filtering NDIR. The diagram shows how changes can be sensed in stack CO in the 0- to 1000-ppm range and used for burner control.


RECIPROCATING FILTER FLAG WITH ANALYTICAL AND REF FILTERS

FIG. 8.27n Optical schematic of a single-beam, dual-wavelength filter analyzer. In an alternate arrangement, larger filters can be mounted directly on the rotating chopper wheel. The principle has been extended to multiple wavelengths. If a microcomputer-controlled circular variable filter wheel replaces the reciprocating filter flag, the optical schematic becomes that of a programmable multicomponent analyzer.


It cannot account for changes in sample cell transmission, such as dirt accumulation on windows or scattering particles in the sample stream. The dual-wavelength design aims to use a reference wavelength not absorbed by the sample but affected by the above-mentioned sources of error to the same extent as the analytical wavelength. (Therefore, the reference wavelength should be as close to the analytical wavelength as possible.) The single-beam design is optically and mechanically simpler and lends itself easily to interfacing with a wide variety of sample cells.

108

6000°K 4000°K

POWER RADIATED FROM SOURCE

1380

106 2000°K 104

1000°K

102 273°K 1.0 10−2

Multiple-Component Fixed Filter Analyzer When the components to be analyzed have absorption bands that do not overlap in the spectrum, the single-component analyzers can simply be expanded to measure multiple wavelengths. (Note: Having absorption bands that do not overlap does not imply that the absorption peaks are free from interference—only that some part of the band is free from other species.) There are NDIR analyzers that use two detectors filled with two different gases, such as carbon monoxide and a hydrocarbon. The IR passes through first one detector and then the other. Filter analyzers are more easily expandable to multiple wavelengths, enabling a wide variety of process and quality control analyses to be made. Filters can be mounted on a rotating wheel or inserted sequentially into the beam by a cam mechanism. ABB can analyze up to 10 different gasses with their multiwave units. These instruments typically use analog signal processing. It is possible, but awkward, to compensate for spectral interference by cross-coupling between the different wavelength outputs. Digital signal processing simplifies this problem, but it is not widely used in multiple-wavelength fixed-filter analyzers. Note: Digital processing is very common with fullspectrum techniques. Programmed Circular Variable Filter Analyzer These microcomputer-controlled analyzers, derived directly from the CVF laboratory analyzers mentioned above, are simple and rugged in optical design. The same basic analyzer can be programmed for any number of analyses up to a maximum of about 10 components. The programmable CVF offers a major advantage over using fixed filters. It avoids the whole problem of filter selection and filter wavelength manufacturing tolerances and the prohibitive costs of obtaining fixed filters for one-of-a-kind analyses. The built-in microcomputer makes interference compensation relatively straightforward. Instrument calibration coefficients and analytical wavelengths are obtained using known mixtures of the components of interest and placed in readonly memory (ROM). The microcomputer enables self-diagnostic and selfchecking features to give early warning to trouble and simplify the analyzer service.


10−4 0.1

0.5

1.0

5

10

50

100

WAVELENGTH, µm

FIG. 8.27o 2 Radiated power in w./m /str/µm from an ideal black body as a function of wavelength and temperature. Practical sources will have lower output for a given temperature to the extent that their emissivity is less than unity. The sun’s radiation is close to a black body at 6000°K (5727°C) with peak output in the visible. A Nichrome wire source typically runs a little of 1000°K (727°C) with a peak output near 2.5 µm. Increasing the temperature increases output much more in the NIR than at longer wavelengths.

This technology is most developed for workroom air quality monitoring. Infrared Sources With the exception of the tunable diode laser, all the sources used in IR analyzers are of the black-body type (Figure 8.27o). An element is heated to as high a temperature as is consistent with long operating life. The radiation varies as a function of wavelength as the source temperature increases. Sources used in process analyzers in the mid-IR depend on ohmic heating of an element, such as Nichrome wire, either exposed or embedded in a ceramic matrix. The metal oxide surface layer or ceramic have good emissivity in the mid-IR. The normal source temperature is in the 2000°C range. In NIR, the much hotter tungsten halogen lamp with a quartz envelope is an excellent source. (Remember, the shorter the wavelength, the higher the energy; therefore, the hotter source is required in NIR.) At wavelengths longer than 4 µm, the output drops due to envelope absorption, but even if it did not, the emissivity of tungsten decreases at longer wavelengths. (See the section on NIR for more comments on this source.) Infrared Detectors In all cases, IR energy is modulated so that an AC signal is detected and synchronous demodulation can be used to narrow the noise bandwidth. Beam chopping or modulation is best done after the sample. This avoids errors due to the detector sensing emission from hot samples. The error effect can be large, particularly at the longer wavelengths, beyond 5 µm.


NDIR Detectors These are the gas-filled capacitive microphones already described (Figures 8.27g and 8.27h). They use low-modulation frequencies, 19 Hz or below, and are affected by external noise and vibration. They operate in the IR and are very sensitive. One can also use solid-state devices in this application. Thermal Detectors These are used in the mid-IR by filter analyzers. The pyroelectric detector is usually a wafer of LiTaO3 about 20 µm thick, electroded on each side. It behaves electrically like a capacitor, and a current flows out of it if the temperature changes. It must be used in a chopped beam of IR, most commonly near 50 Hz. The minimum detectable –9 signal is about 10 w. in a 1-Hz bandwidth, and it is somewhat less sensitive than a typical NDIR detector. Pyroelectric detectors must be mounted and sealed to avoid microphonic and acoustic interference (Figure 8.27p). The sensitivity of the evaporated thermopile is best at lower-chopping frequencies, below 10 Hz. It is very rugged and less microphonic than the pyroelectric detector, but also less sensitive. Photoconductive Detectors In NIR, photon detectors such as PbS or PbSe can be used. They are two or three orders of magnitude more sensitive than the thermal detectors and operate best at higher chopping frequencies. The PbSe sensitivity extends almost to 5 µm, if thermoelectrically cooled, and can be used for such common analyses as CH/CO/CO2. Response is strongly temperature dependent, in contrast with the above-mentioned thermal devices. Selecting the Cell

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length means a low sample absorbance, which gives a weak signal in comparison to instrument noise. Too long a path length results in very little energy reaching the detector and Beer’s law often fails. Simple theory, based on the detector/ 3 preamplifier as the chief source of noise, unaffected by the IR power reaching the detector, predicts that most accurate concentration measurements can be made when the sample transmits 1/e of the incident beam (e = 2.73). This corresponds to a transmittance (T) of 36%, or an absorbance (A) of 0.43 A. However, any absorbance between 0.1 and 1.0 A, 80 to 10% T, will normally give good results. Measurements at higher absorbance tend to minimize the effect of certain kinds of electronic drift, while at lower absorbances, they minimize nonlinear ties in the analyzer’s response (see “Calibration: Sources of Analyzer Drift,” below). Therefore, the cell path length should be chosen to put the sample absorbance in this desirable range. When analyzing trace gases, the absorbance is often near the lowest detection limit of the analyzer, even when the longest available path length is used, and that is the best that can be done. Gas Cells Figure 8.27q shows some gas cells used for process analysis. The common type transmits the IR beam straight through to the detector. The path lengths range from 0.1 mm to 50.0 cm (0.004 to 19.5 in.). Some of these long cells, especially those used on NDIR analyzers, are internally gold coated to act as a light pipe. The single-beam analyzers can interface with a wider range of cell type, such as multiple reflection cells with path lengths adjustable between 0.75 and 40 m (30.0 in. and 130.0 ft). These long path cells are especially valuable for analyzing trace contaminants in the air.

Path Length Selection There is an optimum cell path length for analyzing a particular sample. Too short a path

FIG. 8.27p Photograph of a pyroelectric thermal detector alongside an inch scale; when in use, an IR-transmitting window, or lens, seals the air space in front of the detector.


FIG. 8.27q Cells for use with IR process analyzers. The longer-path-length cells are not conveniently interfaced with single-beam analyzers. (a) 2-mm (0.08-in.) cell, gas, or liquid; volume, 0.4 ml. (b) 10-cm (3.9-in.) gas cell (or liquid for NIR); volume, 50 ml. (c) 50-cm (19.5-in.) gas cell, two passes; volume, 600 ml. (d) 20-m (66-ft) variable-path gas cell; volume, 5.4 l.

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In choosing a gas cell, one should consider the volume if only small quantities of sample are available or if faster turnover is required. The most efficient cells have a high path length-tovolume ratio. Sample pressure and temperature must be controlled for accurate results because the instrument response depends on the number of sample molecules in the cell.

by the process stream, free of contaminating films; 3) problems with running calibration checks on the systems; and 4) the difficulty of servicing the crystal without shutting down the process. (If the cell becomes coated with a film, all the spectrometer will see is that film and, therefore, the output becomes a straight line while the process wanders.)

Liquid Cells: Transmission Type Liquid cells have much shorter path lengths to compensate for the higher-density samples. Thickness varies from 0.1 mm to 10.0 cm (0.004 to 4 in.), the longer cells being used in the NIR region, where sample absorption coefficients are lower. Sample streams must be carefully filtered to avoid cell plugging, particularly when very short (0.1-mm) IR cells are used.

Solid Samples

Liquid Cells: Reflection Type The multiple internal reflection (MIR) technique, also called attenuated total reflection (ATR), is a way of avoiding the problems of thin transmission cells. The IR beam makes multiple internal reflections at the surface of a high-refractive index crystal wetted by the sample liquid of lower index of refraction (Figure 8.27r). The effective path length of the beam through the sample depends on the angle of incidence, the refractive indices of the sample and crystal, and the number of reflections. The method is being used increasingly in mid-IR, with the sample being brought to the MIR cell via a sample loop from the process stream. Additionally, MIR cells have been put directly into the process stream with an optical connection to each end of the crystal. The big advantages of this approach are that no time is lost in the sample system and there is no need to remove solids, as they will not interfere with the measurement and can be used to follow batch reactions without withdrawing samples. However, inline applications face a number of problems, including 1) mounting an analyzer directly in the process stream; 2) keeping the surface of the crystal, which is wetted

Sample composition can be determined by analyzing the spectra of IR diffusely reflected from the sample surface. (Note: One is only analyzing the surface with the IR.) The most common application is moisture measurement in, for example, paper as it is being made or feed stocks, such as wood chips (see Section 8.34). Filter analyzers utilizing this principle are used for analyzing grain and other food products for components as protein, oil, and moisture. Most of this work is done in the NIR, where one sees not only the surface but also into the solid, because of the lower absorption coefficients (more on this later in the discussion of NIR).

CALIBRATION: SOURCES OF ANALYZER DRIFT All IR analyzers require initial calibration with known samples. In general, the strategies of reference wavelengths and reference beams do not reduce sources of zero drift completely, and once in service, the analyzer zero and span must be checked and reset on a routine basis. The user’s manual should give guidance on the steps involved. With constant temperature and pressure samples, an IR analyzer is inherently span stable and, in principle, requires only that the cell path length be constant. However, depending on the analyzer design and operating environment, the span should be checked on a periodic basis. In general, the frequency of requiring span calibration is a function of

ANALYZER SLIT

SAMPLE MIR PLATE DETECTOR

FIG. 8.27r Optical schematic of an MIR cell suitable for use in a process sample loop interfaced with a single-beam filter analyzer. Materials used for the MIR crystal include sapphire, silicon, germanium, and zinc selenide.



the quality of temperature controls provided for the analyzer and the application. In NDIR and GFC analyzers, the characteristics of the gas-filled cells vary with temperature. Filter analyzers as well as the NDIR and GFC types contain narrow-band-pass filters that change wavelength and absorbance with ambient temperature. For span stability, these filters must be temperature controlled. Long-term stability will be degraded if a high electronic scale expansion of gain is required, because the cell path length is not optimized for the job (as is frequently the case in the more sensitive analysis ranges). Zero and span samples can be plumbed to a calibration port for use when required. Gas mixtures in cylinders are often used but should themselves be checked, as they are subject to change with age. (The gas can react with the cylinder wall and disappear from the mixture, or it can liquefy and be removed. The liquefaction can be compensated for by heating the cylinder to raise the temperature and constantly stirring the sample; however, this also increases the probability of reaction.) For gas analyses, the component of interest can sometimes be removed by filtering or catalytic oxidation to provide the zero gas. The zero check is frequently automated using a timer or is built into the memory of a microprocessorcontrolled analyzer. Some instruments employ an automated span calibration analogous to the auto zero by inserting a sealed calibration cell or a secondary standard in the form of an attenuating filter into the analyzer beam.

resolve the sample’s absorption features, as is the case for most small gas-phased molecules and some liquids, the measured absorbance will fail to increase linearly at the high absorbances. This is the case with carbon monoxide (Figure 8.27d). These problems, and the nonlinear output of the NDIR and GFC analyzers, are frequently corrected by a linearizing circuit board. Packaging There is some uniformity of packaging among manufacturers of NDIR analyzers for panel mounting. At present, there are no intrinsically safe IR analyzers, and they must be packaged in a purged or explosion-proof housing to be acceptable in a hazardous area. Plumbing and sample cells containing flammable samples should be kept outside the enclosure containing the analyzer source and electronics unless flame arrestors are used in the sample lines. It may be necessary to purge the analyzer head to prevent corrosion or to eliminate absorbing ambient gas molecules from the optical path outside the cell. Many analyzers have a remote readout and control panel option so that the analyzer head can be located close to the measurement site. The IR components are usually mounted on a vibration-isolated rigid structure, and the analyzer head is temperature controlled to some extent.

APPLICATIONS AND ADVANCES

Linearity The detector response of an IR analyzer is not linearly related to concentration. The filter analyzers for many samples follow the Beer–Lambert law, and a logarithmic amplifier provides an acceptable linear output. However, if the analyzer cannot

Table 8.27s gives a summary of some common process applications for IR analyzers. The advances in this technology involve several areas of development. The microprocessor has contributed to the

TABLE 8.27s IR Analyzer Applications Summary Organic Vapors Carbon Monoxide

Carbon Dioxide

Simple Molecules

Nondispersive infrared (NDIR)

♥

♥

♥

Mid-IR filter

♥

♥

♥

♥

♥

same as above, including NH3, vinyl chloride, methylethyl ketone, etc.

FTIR

♥

♥

♥

♥

♥

The advantage of the FTIR is that it can look at multiple species

Correlation spectrometer

♥

Analyzer


1383

Complex Molecules

Organic Liquids

Comments Single-component analysis: methane, ethylene, CO, CO2, etc.

Stack analysis, single component gas analysis

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development of self-diagnostic and self-calibrating designs. Autocalibration was also helped by the use of multiple reference cells when stable reference gases are available. Modular design in conjunction with self-diagnostics has simplified maintenance. The growth of fiber-optic technology (Section 8.23) made the probe-type IR analyzer practical and extended the spectrum by allowing the same probe to use UV, visible, NIR, and IR forms of radiation. One should be careful with such extended spectrum applications, because windows that are transparent in one region of the spectrum may be opaque in another portion. The in-line use of MIR or ATR crystal probes (Figure 8.27r) is also promising, although the task of keeping the probe surface clean is not easy to meet. Another area of development is to minimize the number of moving parts, choppers, shutters, or beam alternators and eliminate the need for multiple paths in the IR analyzer. The goal can be met by the use of acousto-optic tunable filters (AOTFs), which are made of thallium–arsenic–selenide (TAS). This crystal can be tuned by a radio-frequency oscillator, 1 ultrasonic transducer, over a spectrum of 2 to 5.5 microns. The TAS AOTF is an electronically controllable narrowband filter that can be tuned to any desired IR frequency (Figure 8.27t). It can provide a series of chopped and tuned

DETECTOR

STACK

TABLE 8.27u Typical Applications for NDIR Analyzers Minimum Range (ppm) (for 10-m cell)

Maximum Range (%)

Ammonia (NH3)

0–100

0–10

Butane (C4H10)

0–300

0–100

Gas

Carbon dioxide (CO2)

0–20

0–100

Carbon monoxide (CO)

0–25

0–100

Ethylene (C2H4)

0–100

0–100

Hexane (C6H12)

0–100

0–5

Methane (CH4)

0–10

0–100

Nitrogen oxide (NO)

0–10

0–10

Propane (C3H8)

0–100

0–30

Sulfur dioxide (SO2)

0–100

0–30

Water vapor (H2O)

0–50

0–5

IR beams to simultaneously measure a variety of stack gases. The required frequencies can be selected in milliseconds, and the solid-state AOTF is small and rugged. Therefore, it is insensitive to vibration, but must be maintained at a constant temperature. Table 8.27u lists the minimum and maximum ranges of some of the gases and vapors that are commonly detected by NDIR analyzers. Note: These numbers assume that there are no interfering compounds in the stream.

IR SOURCE

AOTF

NEAR-INFRARED ANALYZERS R.F. OSCILLATOR

DETECTOR AMPLIFIER

GATE DETECTOR INTEGRATOR FREQUENCY SYNTHESIZER

A/D CONVERTER

MICROCOMPUTER

SYNC

MICROCOMPUTER

STACK MOUNTED ELEMENTS CONTROL ROOM ELEMENTS

LCD DISPLAY CALCULATOR MICROCOMPUTER

DISPLAY MICROCOMPUTER

KEYBOARD STRIP PRINTER

RS232

OPTIONAL IBM COMPUTER FOR LOGGING AND GRAPHIC DISPLAY

PANEL MOUNTED CONTROL ROOM MODULE

FIG. 8.27t The solid-state tunable crystal eliminates most of the moving parts from the IR analyzer and allows simultaneous measurement of many 1 gases in the process stream.


NIR absorptions are the overtones and combination bands of the IR, particularly involving a hydrogen atom. There are two major effects of this from the on-line analyzer point of view: 1) It is very difficult for a human being to say which band represents which compound. 2) The absorbances are much weaker than in the IR region (about 1/10 weaker per overtone). This means that the first overtone is 1/10 weaker than the IR, and the second overtone 1/10 weaker than the first. At first glance, both effects appear to be undesirable. Yet, the second turns into a major advantage for analyzing either liquids or solids—the path length is much longer. For solids, this means that more than just the surface is seen and can be analyzed. With hydrocarbon samples like gasoline, the path length in the IR region would be about 0.1 mm, while in the first overtone of the NIR, it can be about 1 mm, in the second overtone about 1 cm, and in the third overtone about 10 cm. For process applications, the longer path length has three advantages: 1) more uniform sampling, 2) fewer plugging problems, and 3) the window fouling is much less of a problem. Short path lengths in the IR region can be handled with an ATR or MIR cell, but fouling remains to be a major problem. (Fouling is normally just a surface layer, and with a 10-cm cell, it is not seen.)


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Interpreting the Absorption Bands

Fiber Optics

Returning to the first item, difficulty in interpretation of the absorption bands. Normally in the NIR the objective is not to do qualitative analysis, but quantitative analysis with a trained computer. The computer can distinguish quantities of individual chemical species, for example, p-xylene in a mixture of aromatics, including both o-xylene and m-xylene. However, the normal task is to do quantitative analysis of physical properties like hardness of wheat, octane number of gasoline, boiling points of gasoline, cetane number of diesel, etc. How is the computer trained? A set of samples, normally 30 to 50, is prepared covering the range of interest of all the analytes and “I don’t cares” in the system. The spectrum for each sample is determined and paired with the known analyte concentrations. These data are all put into a computer with a chemometric program that uses a system to fit the spectrum to the data. Once that fit has been determined, the generated model can be used for future samples within the range of the initial data. It is important to remember, that:

One of the big advantages of using the NIR range is the availability of optical fibers. Careful, there are optical fibers and there are optical fibers. To be useful for spectroscopic applications, the fiber needs to be insensitive to outside light, temperature changes, and movement (vibrations). The outside light problem can be solved by proper choice of jacket or keeping the fiber encased in an opaque case. (Remember that just because the jacket is opaque in the visible spectrum does not mean it will be opaque in the NIR.) 5 Temperature changes can cause major problems. In Reference 5, Vickers shows that a 50°C temperature change on a 10-m fiber will cause a change in absorbance at over 1.0 A. K. The solution in such a case is to have a buffer that extracts the cladding modes and have the core, cladding, and buffer coiled inside the jacket to allow for thermal expansion and contraction without straining the fiber. This system, in addition to adding expense to the fiber, lowers the total light throughput of the fiber, but makes it usable so that one is analyzing the light from the stream rather than from the fiber. Plastic-clad silica fiber is very inexpensive and therefore attractive, but it is totally unacceptable for hydrocarbon analysis; one sees the plastic cladding, and the depth of penetration is a function of temperature, which makes background correction very difficult.

1. Before using the model on real samples, it should be tested on some new samples that were not used to make the model. Doing this is desirable, even if timeconsuming and inconvenient. Normally one is using 30 to 50 samples, each having 700 to 1000 spectral data points attached; the computer can find a fit, but does it mean anything? The only way to find out is to test it. 2. Extrapolations from that initial data set are done at your risk. For gasoline, we used about 30 samples of each grade and time of year to build complete models to predict the various properties of the gasoline. Note: The 30 samples per grade per time of year are considered as a starting set, and the working models are 4 based on 50 to 100 samples. Sample Temperature Control If the sample contains water and one is measuring the ionic solutes, then temperature control is absolutely required. Liquid water exists in different forms related to the number of hydrogen bonds, as a function of temperature, and the change of forms causes spectral shifts that will overcome any other measurements one wants to make concerning the ions dissolved in the water. In hydrocarbon streams, the jury is still out—some claim that sample temperature control within 1°C is required for good analysis, while others say they can compensate for changing temperatures. Because, in any optical measurement, one is counting molecules in the optical path, temperature will affect the spectrum. Therefore, it is easiest to control the temperature rather than try to compensate for it when doing high-quality work.


Types of NIRs NIRs come in four basic types: 1. The filter instrument uses two or more filters to pick out reference and measuring wavelengths. These are the lowest-cost instruments and generally only measure one to a few analytes in the stream. 2. The dispersive units with single detectors and moving grating. The moving grating causes some wavelength instability that becomes a major problem with very sensitive measurements. 3. FTNIR is very similar to FTIR; however, since the frequency and wavelengths are inverse scales, at the shorter wavelengths in NIR there is a higher reproducibility factor required for the mirror movement. However, these have been successfully applied in some very rigorous applications, such as measuring the octane of gasoline. 4. The diode array instrument with no moving parts—note that the material of construction in the diode array sets the usable range of the instrument. The most common diodes are made from silicon that become transparent at wavelengths longer than 1100 nm; therefore, these diode arrays are only usable in the third overtone. [The diode array instrument has no moving parts but requires extreme temperature stabilization to prevent wavelength changes due to different distances between the grating and detector array as a function of temperature.

1386


One unit on the market controls the spectrograph temperature to ±0.1°C, while the outside temperature varies from −40 to 120°F (−40 to 50°C).] Sources The optical source for any spectrophotometer is important, but it is even more critical with systems based on fiber optics. The only light that is important in a fiber-optic instrument is the light that can be reproducibly launched onto the fiber. Therefore, big bulbs are not needed when trying to put light into a 200-µm-inner-diameter fiber; the key is to have a short, stable filament. Many filaments are long, compared to the fiber diameter, and the hot spot that is giving off the light moves along the filament. Therefore, sometimes the fiber is in focus of the hot spot and sometimes it is not. This is totally unsatisfactory. What one wants is a short filament, where the hot spot stays fixed and the filament does not sag, which requires a largerdiameter filament. This allows the light to be focused onto the fiber and gives a light bulb with a very long life, exactly what is needed for process analysis. Gases

small differences in the materials. When Dr. James Callis, University of Washington, CPAC, used one of these instruments to determine the octane of gasoline, it became evident that a great deal of information was discarded due to the poor optical resolution. The diode array instrument that was developed improved the optical resolution by a factor of 3 and actually also improved the signal-to-noise ratio, which made it a very usable instrument. This resolution is used, for example, to measure MTBE in gasoline by measuring the small shift in the methyl group attached to the oxygen of the MTBE vs. all the other methyl groups in the gasoline. Calibration Transfer One of the earlier problems associated with NIR analyzers was that each instrument was different. If a calibration model was build on instrument A, then it could not be used on B, even if B was from the same manufacturer and had the same model number. Dr. Bruce Kowalski and his group at the University of Washington, CPAC, developed methods to solve this problem that have now been incorporated into some of the commercial analyzers. Since there is a significant amount of time put into developing a model, one should make sure that the model is transferable to a different analyzer or to the same analyzer after maintenance is done on it, for example, when the light bulb 7 is changed.

The only known NIR application on gas samples involves 6 the BTU content of natural gas. In order to make this measurement, the natural gas line would have to be under high pressure, normally 1000 psi or so, in order to provide a sufficient number of molecules in the optical path to make the measurement.

References

Liquids

1. 2.

Under stable temperature conditions, ions in water can be measured. One analyzer measures the hydroxide ion concentration in a neutralization bath using a filter instrument with the reference chosen to eliminate effects of any other ions. Using third overtone diode array instruments, the octane numbers of gasoline can be determined and, at the same time, measure the MTBE concentration, the boiling points of the gasoline, the Reid Vapor Pressure (RVP), etc. Crude oil properties have been measured using the second overtone region. The key is to properly model all these values when starting up the analyzer. (These same measurements have been made in the second overtone of the NIR using the FT approach.)

3. 4. 5.

6.

7.

Nelson, R. L., “Tunable Crystal IR Analyses,” InTech, June 1987. Cardis, T. M., “Process FTIR Analysis of a Chlorofluoromethane Stream,” 1986 ISA Conference, Houston, October 1986. Mark, H. and Griffiths, P., “Analysis of Noise in Fourier Transform Infrared Spectra,” Applied Spectroscopy, 633 ff., vol. 56, 2002. Beebe, K. R. et al., Chemometrics: A Practical Guide, New York: John Wiley & Sons, 1998. Vickers, G. H. et al., “Influence of Ambient Temperature on the NearInfrared Transmission of Optical Fibers,” Paper 178, Eastern Analytical Symposium, Somerset, NJ, 1990. Van Agthoven, M. A., Mullins, O. C., et al., “Near-Infrared Spectral Analysis of Gas Mixtures,” Applied Spectroscopy, 56(5), 593–598, 2002. Kowalski, B., “Recent Developments in Multivariate Calibration,” Journal of Chemometrics, 5, 129–145, 1991; “The Effect of Mean Centering on Prediction in Multivariate Calibration,” Journal of Chemometrics, 6, 103–111, 1992; “The Parsimony Principle Applied to Multivariate Calibration,” Chemometrics in Analytical Chemistry Conference (CAC-92), Montreal, Quebec, Canada, July 14–17, 1992 (Analytica Chimica Acta, 277, 165–177, 1993.)

Solids Modern NIR got its start with Dr. Karl Norris measuring such characteristics of agricultural products as the hardness of wheat, the protein content in wheat, the food value of alfalfa hay, etc. Because in the solids, the natural bandwidth was very wide, the original NIR instruments had poor optical resolution, but very high signal-to-noise ratios to see very


Bibliography for Infrared Analyzers Cook, B. W. and Jones, K., A Programmed Introduction to Infrared Spectroscopy, London: Heyden & Sons, Ltd., 1972. Dundas, M. E., “New Technologies in Infrared Hydrocarbon Detection,” ISA Conference, Houston, TX, October 1992.


Ewing, G., Analytical Instrumentation Handbook, New York: Dekker, 1997. Gunnell, J. and Van Vuuren, P., “Process Analytical Systems: A Vision for the Future,” Journal of Process Analytical Chemistry, 6(1), 1–5, 2001. Harrick, N. G., Internal Reflection Spectroscopy, Ossining, NY: Harrick Scientific Corp., 1979. Jones, C., “Using Near Infrared Analysis for On-Stream Composition Measurements,” InTech, August 1982. Landa, I., “Visible (Vis) Near Infrared (NIR) Rapid Spectrometer for Laboratory and On-line Analysis of Chemical and Physical Properties,” SPIE, 665, 286–289, 1986. Mark, H. and Griffiths, P., “Analysis of Noise in Fourier Transform Infrared Spectra,” Applied Spectroscopy, 633 ff., vol. 56, 2002. Ryan, F. M., “A New Method of Measuring Stack Emissions,” Westinghouse—1985 Electric Utility Engineering Conference, 1985. Schirmier, R. E., “On-Line Fiber-Optic-Based Near Infrared Absorption Spectrophotometry for Process Control,” Proceedings of ISA, Houston, 1986, pp. 1229–1235. Tissis, G. G. and Wolfe, W. L., Eds., The Infrared Handbook, Arlington, VA: Office of Naval Research, Dept. of the Navy, 1978. Van der Maas, J. H., Basic Infrared Spectroscopy, London: Heyden & Sons, Ltd., 1972. Weiss, M. D., “FTIR Moves Out of the Lab,” Control, September 1991. Wilks, P. A., “Sampling Method Makes On-Stream IR Analysis Work,” Industrial R&D, September 1982.


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Willis, H. A., Advances in Infrared and Raman Spectroscopy, Vol. 2, London: Heyden & Sons, Ltd., 1976, chap. 3.

Bibliography for Near-Infrared Analyzers Applied Spectroscopy has frequent articles on NIR. Hardy, L., NIR Spectroscopy: The BioPharma Guide to Bioanalytical Methods, pp. 38–40. Kelly, J. J., Callis, J. B., et al., “Prediction of Gasoline Octane Numbers from Near Infrared Spectral Features in the Range of 600–1215 nm,” Analytical Chemistry, 61, 313–320, 1989. MacRae, M., “Analyzing New Options,” Pharmaceutical Technology, 26(2), 118, 2002. McClure, W. E., “Near Infrared Spectroscopy, the Giant Is Running Strong,” Analytical Chemistry, 66, 422 ff., 1994. Timmermans, J. H., “A Report of the PQRI Workshop on Blend Uniformity,” Phara. Tech. Spe., 25, 76 ff., 2001. Van Agthoven, M. A., Mullins, O. C., et al., “Near-Infrared Spectral Analysis of Gas Mixtures,” Applied Spectroscopy, 56(5), 593–598, 2002. Vickers, G. H. et al., “Influence of Ambient Temperature on the NearInfrared Transmission of Optical Fibers,” Paper 178, Eastern Analytical Symposium, Somerset, NJ, 1990.

8.28

Ion-Selective Electrodes R. T. OLIVER

(1972, 1982)

S. S. LIGHT

(1995)

AR

W. P. DURDEN

(2003)

AIT

AE ION SELECTIVE Flow Sheet Symbol

Types of Electrode:

Glass, solid state, solid matrix, liquid–ion exchanger, gas sensing


Generally dictated by electrode holder; 0 PSIG for solid state and liquid–ion exchanger; 0 to 100 PSIG (0 to 7 bars) for most electrode types; over 100 PSIG (over 7 bars) for solid-state designs

Standard Design Temperature: 32 to 122°F (0 to 50°C) for solid matrix and liquid–ion exchange; 23 to 176°F (–5 to 80°C) for most others, with 212°F (100°C) intermittent exposure being permissible Range:

From fractional parts per million (ppm) to concentrated solutions

Relative Error:

For direct measurements, an absolute error of ±1.0 mV is equivalent to a relative error of ±4% for monovalent ions and ±8% for divalent ions; for end-point detection or batch control, ±0.25% or better is possible; for expanded-scale commercial amplifiers, error is better than ±1% of full scale.

Costs:

Similar to those of pH installations (Section 8.48); electrodes: $120 to $300; systems: $600 to $8000; hand-helds: $50 to $500


(Suppliers who manufacture only pH probes are not listed here; they can be found in Section 8.48.) Advanced Sensor Technologies Inc. (www.astisensor.com) Consort NV (www.consort.be) Corning Science Products (www.corning.com) Denver Instrument (www.denverinstrument.com) Fisher Scientific (www.fisherscientific.com) Horiba Instruments Inc. (www.horiba.com) Honeywell (www.honeywell.com) Istek Inc. (www.istekco.com) Laval Lab (www.lavallab.com) Metrohm AG (www.metrohm.com) Nico 2000 Ltd (www.nico2000.net) Nico Scientific (www.nicosensors.com) OI Corporation (www.oico.com) Omega (www.omega.com) Thermo Orion (www.thermo.com) World Precision Instruments (www.wpi-europe.com)

INTRODUCTION Ion-selective electrodes comprise a class of primary elements used to obtain information related to the chemical composition of a process solution. They are electrochemical transducers that generate a millivolt potential when immersed in a conducting solution containing free or unassociated ions to 1388 © 2003 by Béla Lipták

which the electrodes are responsive. The magnitude of the potential is a function of the logarithm of the activity of the measured ion (not the total concentration of that ion) as expressed by the Nernst equation (Equation 8.28(6)). The familiar pH electrode for measuring hydrogen ion activity is the best known of the ion-selective electrodes and was the first one to be made commercially available

8.28 Ion-Selective Electrodes

1389

TABLE 8.28a Ion-Selective Electrodes Ion/Species

Type of Electrode

Lower Detectable Limit, ppm

Principal Interferences

Ammonia

Gas sensing

0.009

Volatile amines

Bromide

Solid state

0.04

CN , I , S

Cadmium

Solid state

0.01

Ag , Hg , Cu , Fe , Pb

−

−

=

+

++

++

++

++

++

++

++

++

++

++

++

++

Calcium

Solid matrix/liquid membrane

0.2

Zn , Fe , Pb , Cu , Ni , Sr , Mg , Ba

Carbon dioxide

Gas sensing

0.4

Volatile weak acids

Chloride

Solid state

0.2

Br , CN , S , SCN , I

Chloride

Liquid membrane

0.2

ClO4 , Br , I , NO3 , OH , F , OAc , SO4 , HCO3

Copper (II) Cyanide Divalent cation

a −

Fluoroborate (BF4 ), (boron)

−

−

−

=

−

+

−

−

−

++

+++

−

−

Solid state

0.006

Ag , Hg , Fe

Solid state

0.01

S,I


0.001

Liquid membrane

0.11

=

−

−

I , HCO3 , NO3 , F =

=

S , CN

Lead

Solid state

0.2

Ag , Hg , Cd , Fe

+

++

−

− 4

++

−

Cl , ClO , I , Br

++

−

Nitrite

Gas sensing

0.002

CO2, volatile weak acids

Perchlorate

Liquid membrane

0.7

Cl , ClO3, I , Br , HCO3 , NO3 , etc.

Potassium

Liquid membrane

0.04

Cs , NH4 , H , Ag , Tris , Li , Na

Redox (platinum)

Solid state

Varies

All redox systems

Silver/sulfide

Solid state

0.01 Ag 0.003 S

Hg

Sodium

Glass

0.02

Ag , H , Li , Cs , K , Tl

Thiocyanate

Solid state

0.3

OH , Cl , Br , I , NH3, S2O3 , CN , S

Sulfur dioxide

Gas sensing

0.06

CO2, NO2, volatile organic acids

a

−

−

0.006

0.3

−

−

Solid state


−

−

Iodide

Nitrate

−

−

−

−

+

+

−

+

−

+

−

+

+

+

=

−

++

+

−

+

+

−

+

−

−

+

+ =

Water hardness electrode is also known as the divalent cation electrode.

(see Section 8.48 for pH measurement). With few exceptions— notably the silver-billet electrode for halide measurements and the sodium–glass electrode—the pH electrode was the only satisfactory electrode available to the process industry prior to 1966. Currently more than two dozen electrodes are suitable for industrial use. Table 8.28a lists many of the commercially available electrodes for which process applications have been reported. Many other research sensors have been reported.

THE NERNST EQUATION

E = E asy +

The potential developed across an ion-selective membrane is related to the ionic activity as shown by the Nernst equation: E=

a 2.3 RT log 1 nF a int

8.28(1)

where E is the potential developed across the membrane; a1 is the activity of the measured ion in the sample of process; aint is the activity of the same ion in the internal solution; 2.3


RT/nF is the Nernst slope, or slope of the calibration curve, and is a function of the absolute temperature T and the charge on the ion being measured n; and R is the gas law constant. Table 8.28b shows how the Nernst slope changes with temperature and the charge on the ion. When the ratio of the two activities is unity, the potential across the membrane is zero. Equation 8.28(1) assumes that the membrane has identical selectivity properties on both sides. If for some reason this is not true, the equation is written a 2.3 RT log 1 nF a int

8.28(2)

where Easy is the asymmetry potential and amounts to a few millivolts. This equation is simplified by the fact that aint is fixed by the internal structure of the electrode, giving E = E°′ +

2.3 RT log a1 nF

where E°′ is a new constant.

8.28(3)


TABLE 8.28b Nernst Slopes Electrode Temperature

mV per Decade of Activity

°C

°F

n = ±1

n = ±2

0

32

54.19

27.10

10

50

56.17

28.08

20

68

58.17

29.08

25

77

59.16

29.58

30

86

60.15

29.58

40

104

62.15

29.58

50

122

64.12

32.03

60

140

66.10

33.05

70

158

68.09

34.04

80

176

70.07

35.04

90

194

72.05

36.02

100

212

74.04

37.02

a

The slopes are positive for cations and negative for anions.

MILLIVOLT METER 118.3 − EXTERNAL REFERENCE ELECTRODE

+

METER TERMINALS

INTERNAL REFERENCE ELEMENT

INTERNAL REFERENCE SOLUTION PROCESS SOLUTION

ION-SELECTIVE MEMBRANE

FIG. 8.28c Ion-selective electrode measuring system.

E meter = E° +

8.28(6)

The Reference Electrode In actuality, the potential of a single electrode cannot be measured by itself. It can be measured only in conjunction with a reference electrode and a high-input impedance voltmeter (Figure 8.28c). The latter is necessary to prevent current from flowing through the electrode, an action that would tend to cause electrochemical reactions in the solution phase around the membrane.

8.28(7)

where the output is a millivolt signal to a meter, A is a zero adjustment, and B is a span or slope adjustment around a temperature-independent, or isopotential, point. The input to the electrodes is the composition of the solution in terms of activity. Equation 8.28(6) states that the output of an electrode pair is linear with respect to the logarithm of the activity of the ion being measured (Figure 8.28d). The slope of the curve relating Emeter to log a1 is 59.16 mV (at 25°C for n = 1) or 29.58 mV (at 25°C for n = 2).

+40 ELECTRODE POTENTIAL (mV)

8.28(4)

where kij is the selectivity coefficient of interfering ion j with respect to measured ion i, each ion with charges zi and zj.


2.3 RT log a1 nF

Output = A + B log (input)

A more complete equation takes into account electrode interferences and defines the selectivity coefficient. It is known as the Nicolsky equation: E = E°′ + 2.3RT/nF log (a1 + k ija j )zi/zj

8.28(5)

Under normal operating conditions, the reference electrode is assumed to be constant, as is the liquid-junction potential. However, this is not always the case. Substituting Equation 8.28(3) into Equation 8.28(5) and combining constant terms, including Eref and Ej, gives the general form of the Nernst equation:

ION-SELECTIVE ELECTRODE

KCI SALT BRIDGE SOLUTION

LIQUID JUNCTION

E meter = E − E ref + E j

where E° is a constant for a given electrode system at a specific temperature. It depends on the choice of reference electrode and includes the liquid-junction potential. The Nernst equation for the electrode pair can be written as an instrument input–output equation:

a

+

The potential read on the voltmeter is equal to the algebraic sum of the potentials developed within the system. That is, the observed meter potential is the sum of the potentials developed by the measuring electrode, E, the reference electrode, Eref , and a small but important liquid-junction potential, Ej.

IO-FOLD CHANGE

+20

29.58 mV

1390

0 POTENTIAL VS CONCENTRATION (CONSTANT IONIC BACKGROUND)

−20 10−4

POTENTIAL VS ACTIVITY (VARIABLE IONIC BACKGROUND)

10−3

10−2

10−1

MOLES/LITER

FIG. 8.28d Electrode potential for calcium chloride solutions as a function of concentration and activity.


Concentration and Activity

changed twofold (100% change), then

Ignoring the effects of chemical reactions that would tie up ions, the activity of the ions is related to the analytical concentration, C, as follows: a = γC

8.28(8)

where γ is the activity coefficient and is a measure of the interaction among ions in solution. It can be thought of as an empirical factor to explain the difference between the actual behavior of ions in solution and the ideal behavior. At zero ion concentration, that is, no ionic interaction, γ is taken as unity and the activity is equal to concentration. As the concentration increases, γ decreases at first, passes to a minimum value, and then rises, often to values greater than unity 1 in very concentrated solutions. The activity coefficient is constant when the ionic composition of the solution is constant. Substituting Equation 8.28(8) into Equation 8.28(6) gives E meter = E +

2.3 RT log (γ C) nF

8.28(9)

or, at constant total ionic conditions, E meter = E° + constant +

2.3 RT log C nF

8.28(10)

+40 IO-FOLD CHANGE

29.58 mV

ELECTRODE POTENTIAL (mV)

where the constant term is RT/nF log γ. Equation 8.28(10) is linear with respect to the concentration term (Figure 8.28d). However, if the ionic background of a solution varies, as in the preparation of a series of standards by dilution, the activity coefficient is no longer constant and Equation 8.28(9) is nonlinear (Figure 8.28e). Equation 8.28(6) can predict the change in potential to be expected from a given change in activity or concentration (constant activity coefficient). For instance, if the activity

+20

0

pH = 6 T = 25°C POTENTIAL VS CONCENTRATION POTENTIAL VS ACTIVITY PURE CaCl2 SOLUTIONS

−20 10−4

10−3

10−2 MOLES/LITER

FIG. 8.28e Concentration and ion activity vs. electrode potential.


1391

10−1

E meter = E° +

2.3 RT log 2a1 nF

8.28(11)

Subtracting Equation 8.28(6) from Equation 8.28(11) gives ∆E =

2.3 RT log 2 nF

8.28(12)

or 18 mV at 25°C for n = 1. A similar argument would show that for the same sample, an 18-mV decrease would be observed if the initial activity was cut in half (50% change). This change is not dependent on the magnitude of a1 and is the same whether measuring fluoride at the parts per million (ppm) level, chloride in 4% salt solutions, or the pH of a 15% sulfuric acid solution. Table 8.28f lists the changes in potential to be expected for up to a 10-fold change in activity. Column 1 shows the ratio of the original activity a1 to the final activity a2. The last column shows the equivalent pH change if the + [H ] were being measured. The data indicate that for precise measurements of small changes in activity, it is necessary to use an expanded-scale meter. For example, a span of 60 mV would allow a 10-fold change to be measured, using the full scale of the measuring instrument. Ionic Strength Adjustment Buffers Introduction of the concept of the high-ionic-strength medium serves to remove or minimize two of the disadvantages of ion-selective electrodes. The concentration, rather than the activity, may be interpreted directly from the observed

TABLE 8.28f a Changes in Meter Potential for Changes of Activity ∆E to mV (77°F (25°C))

σ1 σ2

n = 1°

0.1

–60

0.25

−36

−18

−0.6

0.5

−18

−9

−0.3

0.79

−6

−3

−0.1

1.00

0

0

0.0

1.26

+6

+3

+0.1

2.00

+18

+9

+0.3

4.00

+36

+18

+0.6

10.00

+60

+30

+1.0

b

n = 2° −30

b

Equivalent pH change (n = 1) −1.0

Note: Data are for positive ions. For negative ions, sign should be reversed. a See Equation 8.28(12). b Values rounded off from 59.16 and 29.58 (see Table 8.28j).

1392


10.0

TABLE 8.28g Composition of TISAB, a High-Ionic-Strength Buffered Com2 plexing Medium for Measuring Fluoride Ion Concentration 1.0 M

Acetic acid

0.25 M

Sodium acetate

0.75 M

Sodium citrate

0.001 M

Ionic strength

1.75 M

pH

5.0

electromotive force (emf) (see Equations 8.28(9) and 8.28(10)); some chemical and electrode interferences are removed. The high-ionic-strength media are frequently designated by the acronym ISAB, which stands for ionic strength adjustment buffer. Table 8.28g illustrates the composition of total 2 ionic strength adjustment buffer (TISAB). TISAB is used for rendering the fluoride ion electrode virtually specific for the measurement of the total concentration of fluoride ion in solution, even if the test solution has a pH outside of the acceptable range and fluoride-complexing ions such as iron or aluminum are present. In spite of small contributions from the test solution, the high concentration of sodium chloride fixes the ionic strength at a virtually constant value. The acetate–acetic acid buffer holds the pH into the optimum range for the fluoride electrode, and the citrate complexes commonly interfering ions such as iron and aluminum more firmly than does the fluoride. ISAB solutions for other ions have been described in the literature and are available commercially. To utilize ISAB solutions for continuous process measurements, reagent addition systems are employed that mix the test and ISAB solutions together, utilizing process pressure or peristaltic (or other types) pumps. 3

Temperature Effects

There are three temperature effects on ion-selective measurements, including the T term in the Nernst equation (see Table 8.28b), the thermal characteristics of the electrodes, and the 3 thermal characteristic of the solution. The T term in Equation 8.28(6) states that the potential produced by the electrode system is a function of temperature as well as of ion activity. This effect can be compensated for, manually or automatically, by manipulating the input signal to the converter to indicate the true activity at the measured temperature. Temperature can also be compensated for by so designing the electrode pair that there is a zero temperature error at a particular ion activity. Figure 8.28h shows this effect for the fluoride electrode used to control the fluoridation of public water supplies. The Isopotential Point The point at which the temperature curves intersect, 1 mg° F/I, is the control level for fluoridation. This point of intersection is called the isopotential point,


0°C

5.0

25°C 3.0

FLUORIDE (MG/LITER)

Sodium chloride

12.5°C

1.5

1.0 ISOPOTENTIAL POINT 0.5

0.3 25°C 12.5°C 0°C

0.1 +60

+40

+20 0 −20 EMF (MILLIVOLTS)

−40

−60

FIG. 8.28h Isopotential point for fluoride electrode.

and temperature effects are negligible on either side for a 18 to 27°F (10 to 15°C) change in temperature. The isopotential point for pH-measuring systems is normally about pH 7 and has a potential value of 0 mV. The activity coordinate of the isopotential points of the solid-state electrodes is fixed during manufacture, whereas the millivolt coordinate is also dependent on the choice of reference electrode. However, due to the construction of the liquid-membrane electrode, the isopotential points can be changed to fit the process. Role of Electrode Internals and Calibration The second effect associated with temperature is created by the different internal thermal characteristics of the measuring and reference electrodes. This effect can be minimized if the internal elements of the measuring electrode are matched to the reference electrode. Most commercial pH systems employ matched internal elements for both electrodes. The temperature effect on the chemistry of solutions is the third factor that can create an apparent error in measurement. This effect is difficult to quantify but can be offset by calibrating the system with preanalyzed process samples at the expected process temperature.

It should be noted that this is not a system error. The electrode indicates the true activity as a function of temperature changes. As long as the status of the process is what is required, the solution temperature effect is not important because the true activity is the quantity desired. This effect is usually not compensated for by the measuring instrumentation. After sudden changes, time is required for a new state of thermal equilibrium to the reached (approximately one-half hour for a 18 to 27°F (10 or 15°C) change). Therefore, it is important to remember that when electrodes are moved from process samples to standard samples, which are at a different temperature, or when sudden and large process temperature changes occur, one should wait until a new thermal equilibrium is reached. During this time, the potential of the electrode system will drift. The duration of the drift depends on the particular electrode system and the magnitude of the temperature change. Therefore, it is important to avoid changes in temperature during calibration or to allow thermal equilibrium to be established.

∆E, ERROR IN ELECTRODE POTENTIAL, IN MILLIVOLTS


30

1393

FOR ELECTRODES OBEYING NERNST EQUATION AT 25°C E = E° + (59.16/n) log C WHICH GIVES THE ERROR ∆E, (mV) = (59.16/n) log [(1 + RE/100)] NOTE: RE = 100 ∆C/C

UNIVALENT ELECTRODES

20

DIVALENT ELECTRODES

10

0

0

50

100

150

200

250

RE, PERCENT RELATIVE ERROR IN CONCENTRATION

FIG. 8.28i Relative error in concentration as a function of theoretical error in potential.

System Accuracy The accuracy of measurements derived from an analytical system is a composite of all contributing variables. These variables for ion-selective measuring systems are the measuring electrode; the reference electrode, including the liquidjunction potential; the selective-ion potential converter; the recorder; and the temperature and solution errors. The relationship between overall emf errors (∆E) and ionic concentration (C) may be derived from the Nernst equation 8.28(6) by substituting the values of the thermodynamic constants R and F and assuming that the activity coefficient (γ ) is unity in Equation 8.28(8) at 25°C. ∆E = (59.16/n)log (1 + (RE)/100)

8.28(13)

where RE represents the percent relative error in the concentration RE = 100∆C/C

8.28(14)

A plot of Equation 8.28(13) is given in Figure 8.28i. The relative error in measuring activity is dependent only on the absolute error in the emf and is independent of the activity range and of the size of the sample being measured. This is similar to an equal percentage valve in which equal incremental changes in valve opening (electrode potential) produce equal percentage changes in flow (RE in activity) for all valve openings—assuming constant differential pressure (constant temperature). Being a logarithmic device, an electrode gives a constant precision throughout its dynamic range. Concentrated solutions can be analyzed with the same accuracy as dilute solutions. Laboratory Devices Laboratory measuring instruments for ion-selective electrode measurements with an uncertainty of


±0.1 mV have become commercially available. It is possible to make laboratory pH measurements within an error of ±0.002 pH units (equivalent to ±0.12 mV). Similarly, under carefully controlled conditions, ion-selective electrodes may be made repeatable within 0.1 mV. The accuracy attained to date in process instruments has been limited by the reference rather than by the ion-selective electrodes. Process Applications Ion-selective measurement systems for process applications are repeatable to ±1 mV. For an electrode responding to univalent ions, an overall error of 1 mV corresponds to a 3.9% relative error in activity; for an electrode responding to divalent ions, the relative error is 7.8% per millivolt. This means that they are roughly 5% (of value in activity) + devices when measuring univalent ions, including H , in acidic or basic solutions or 10% (of value in activity) devices when making divalent ion measurements. These figures apply only to direct electrode measurements. When electrodes are used as end-point detectors in titrations or batch reactions or in differential systems, relative errors of 0.1% are possible.

TYPES OF ELECTRODES Glass Ion-selective electrodes are classified according to the type of sensing membrane employed. Glass electrodes are constructed from specially formulated glass and respond to ions by an ion exchange of mobile ions within the membrane structure. The membrane is fused to a glass body so that the outer surface makes contact with the sample or process stream, while the inner surface makes contact with an internal filling solution

1394


INTERNAL FILLING SOLUTION

INTERNAL REFERENCE ELECTRODE

GLASS MEMBRANE

INTERNAL FILLING SOLUTION

INTERNAL REFERENCE ELECTRODE

SOLID STATE MEMBRANE

FIG. 8.28j Conventional glass pH electrode.

FIG. 8.28l Solid-state membrane electrode.

SCREENED CABLE

CABLE

CAP

CAP

INNER REFERENCE ELEMENT

SALT BRIDGE SOLUTION

INSULATING GLASS STEM

E9 Ag-AgCI REFERENCE ELEMENT

INNER REFERENCE BUFFER E5

E1 pH MEMBRANE

E8 LIQUID JUNCTION

E2 E3

E4

E6

E7

E1 TO E9 REPRESENTS POTENTIALS DEVELOPED BETWEEN GLASS SOLUTION AND REFERENCE ELECTRODE TYPICAL GLASS ELECTRODE

TYPICAL REFERENCE ELECTRODE

FIG. 8.28k pH electrodes.

containing a constant activity of the ion for which the membrane is sensitive (Figure 8.28j). A stable electrical contact is made with the internal solution by a silver wire coated with silver chloride. Other internal contacts have been used (mercury–mercurous chloride or thallium amalgam thallous chloride), but the silver–silver chloride is the most popular. The internal filling solution must contain a constant chloride–ion activity and be saturated with silver chloride so that a stable potential is maintained at the metal salt–solution interface (Figure 8.28k). In the conventional glass electrode, the internal solution is buffered at a pH of 7 and contains a chloride level similar to that in the external reference electrode (for details of reference electrodes, see Section 8.48). Other glass electrodes in process use are the sodium, ammonium, and potassium ion electrodes. In addition to the concentration already described, the sodium electrode can also be prepared by slicing a thin section


TABLE 8.28m Solid-State Electrodes and Their Membrane Composition Electrode

Membrane

Fluoride

LaF3

Form Single crystal

Silver/sulfide

Ag2S

Pressed pellet

Chloride

AgX

a

Single crystal

Bromide or iodide

AgX Ag2S

Pressed pellet

Cyanide

AgI Ag2S

Pressed pellet

a

X = Cl, Br, or I.

from a rod of sodium-sensitive glass and cementing it to an epoxy body (Figure 8.28l). This eliminates the familiar glass body of the pH electrode. Epoxy construction is not yet available for pH measurement due to difficulties inherent in cementing pH glasses to epoxy. A carbon dioxide and an ammonia electrode can be made from a pH electrode by covering the membrane with a permeable-membrane sac filled with pH buffer. The respective gas in solution will selectively diffuse in or out of the permeable membrane, causing a pH change. The latter is dependent on the activity of the gas in the process solution. Solid State Solid-state electrodes are made of crystalline membranes, and there are scrupulous requirements for the size and charge of the mobile ions within the membrane. The composition of the membrane varies as a function of the required measurement. For instance, the fluoride electrode has a single crystal of doped lanthanum fluoride for a sensing membrane. The silver and sulfide membranes are pressed pellets of insoluble silver sulfide. The small solubility of silver sulfide in solution prevents the coexistence of silver and sulfide ions, except in extremely small amounts, and the electrode can be used to measure either of these ions. Like the sodium electrode, these membranes are sealed in epoxy bodies (Figure 8.28l). Table 8.28m lists some of the commercially available solid-state electrodes and the composition of their sensing membranes.


1395

INTERNAL AQUEOUS FILLING SOLUTION Ag-AgCI REFERENCE ELECTRODE

THIN METAL FILM

SOLID INTERNAL

LIQUID ION EXCHANGE LAYER

FIG. 8.28p Divalent cation electrode tip in cross section.

SOLID STATE MEMBRANE

FIG. 8.28n Solid-state membrane electrodes with solid internals.

SILVER WIRE OR BILLET SILVER SALT

FIG. 2.28o Conventional silver–silver electrode.

Some pressed pellets and the single crystalline silver–salt membranes are capable of having a metal deposited on the surface and an electrical lead connected to the metal deposit (Figure 8.28n). A solid connector permits the use of the electrodes in any position without breaking electrical continuity. Also, there are no internal solutions to deteriorate with time or temperature. Figure 8.28o shows a conventional silver wire or silverbillet electrode that behaves identically to its corresponding solid-state electrode. However, small imperfections in the silver–halide coating expose free silver metal to the process solution, thereby developing variable oxidation–reduction potentials, and these electrodes have not found wide use in industrial applications. When placed in clean, controlled environments, such as the inner-filling solution of ion-selective electrodes, they produce stable reference potentials. Liquid–Ion Exchange There are many ions for which no glass or crystalline membrane can be found that is suitable for process measurements. Fortunately, chemistry is a versatile field and by using techniques familiar in ion exchange and solvent extraction technology, electrodes can be built for some of these ions. An inert hydrophobic membrane, such as a treated filter paper, can be made selective to certain ions by saturating it with an organic ion exchange material dissolved in an organic solvent. This feature requires a construction of the electrode, as shown in Figure 8.28p, which is a cross section of the tip of


POROUS MEMBRANE

ION EXCHANGER RESERVOIR

a liquid–ion exchange electrode. This electrode has two filling solutions, an internal aqueous filling solution in which the silver–silver chloride reference electrode is immersed, and an ion exchange reservoir of a nonaqueous water-immiscible solution, which wicks into the porous membrane. The membrane serves only as a support for the ion exchange liquid and separates the internal filling solution from the unknown solution in which the electrode is immersed. In effect, there is a sandwich, with the bottom layer being the unknown process solution, the filling being the nonaqueous liquid–ion exchange solution, and the top layer being the internal aqueous solution. For example, if the liquid–ion exchanger is selective for calcium, a potential across the membrane is created by the difference in calcium activity between the internal filling solution and the process solution. The electrode is designed so that the liquid–ion exchanger, used as a sensing element, has a very small positive flow into the process stream. Therefore, liquid–ion exchange membrane electrodes require recharging with an ion exchanger. Liquid–ion exchange membrane electrodes come in kit form. The kit contains an electrode body and sufficient ion exchanger, internal filling solution, and membranes to recharge the electrode many times. A single recharging should last several months in a properly designed system. Unlike the solidstate or glass electrodes, liquid membrane electrodes cannot be used in nonaqueous solutions because they would dissolve the liquid–ion exchanger. The body of the electrode is a chemicalresistant plastic. Solid-state matrix electrodes are an improvement over liquid–ion exchange electrodes. The ion exchanger is permanently embedded in a plastic matrix, such as polyvinyl chloride, with a nonporous membrane. Rebuilding or solution replenishment is not required, and improved analytical performance and lower limits of detection are obtained. MEASUREMENT RANGE The upper limit of detection for ion-selective electrodes is the saturated solution. However, due to the problems of making measurements with reference electrodes that have large liquid-junction potentials (Section 8.48), the electrodes are specified as having an upper limit of 1 M. If the problems of large liquid junctions are brought under control, measurements can be made in saturated or nearly saturated solutions.

1396


The lower limit of detection is usually determined by the solubility of the solid-state-sensing element or the liquid–ion exchanger. The solution pH sometimes determines the lower limit of detection. Some dilute solutions are unstable, but activity measurements may be made if the solution is buffered with respect to the ion being measured, that is, if the free ion is in equilibrium with a relatively large excess of complexed ion. This is the case when free silver is measured in photographic emulsions or sulfide, cyanide, or fluoride in acid solutions.

INTERFERENCES All ion-selective electrodes are similar in principle of operation and use. They differ only in the details of the process by which the ion to be measured moves across the membrane and by which other ions are kept away. Therefore, a discussion of electrode interferences will have to be in terms of the membrane materials. The glass electrodes and the solid matrix/liquid–ion exchange electrodes both function by an exchange of mobile ions within the membrane, and ion exchange processes are not specific. Reactions will occur among many ions with similar chemical properties, such as the alkali metals, alkaline earths, or transition elements. Thus, a number of ions may produce a potential when a given ion-selective electrode is immersed in a solution. Even the pH glass electrode will respond to sodium ions at a very high pH (low hydrogen ion activity). Fortunately, an empirical relationship can predict electrode interferences, and a list of selectivity ratios for the interfering ions can be obtained by consulting the manufacturers’ specifications or the chemical literature. Selectivity constants will be described in connection with Equation 8.28(3). Solid–state matrix electrodes are made of crystalline materials, and interferences resulting from ions moving into the solid membrane are not to be expected. Interference is usually by a chemical reaction with the membrane. One, which is observed with the silver–halide membranes (for chloride, bromide, iodide, and cyanide activity measurements), involves reaction with an ion in the sample solution, such as sulfide, to form a more insoluble silver salt. As already mentioned, specific details of electrode side reactions can be found in the manufacturers’ specifications and chemical literature. Solution Interference A true interference is one that produces an electrode response that can be interpreted as a measure of the ion of interest. – For example, the hydroxyl ion, OH , causes a response with the fluoride electrode at fluoride levels below 10 ppm. In + addition, the hydrogen ion, H , creates a positive interference with the sodium ion electrode. Often an ion will be regarded as interfering if it reduces the activity of the ion of interest through chemical reaction. It is true that this reaction (complexation, precipitation,


oxidation–reduction, and hydrolysis) results in an activity of the ion that differs from the concentration of the ion by an amount greater than that caused by ionic interactions. However, the electrode is still measuring the true activity of the ion in the solution. An example of solution interference will illustrate this point. Silver ion in the presence of ammonia forms a stable silver–ammonia complex that is not measured by the silver. Only the free, uncombined silver ion is measured. The total silver ion may be obtained from calculations involving the formation constant of the silver–ammonia complex and the fact that the total silver is equal to the free silver plus the combined silver. Alternately, a calibration curve can be drawn relating to the total silver (from analysis or sample preparation) to the measured activity. The ammonia is not an electrode interference. Most of the confusion stems from the fact that analytical measurements have been in terms of concentration without regard to the actual form of the material in solution, and electrode measurements often disagree with the laboratory analyst’s results. However, the electrode reflects what is actually taking place in the solution at the time of measurement. This may be far more important in process applications than the more classic information. With some of the techniques suggested, the two measurements are often reconciled.

CALIBRATION SOLUTIONS Calibration solutions for ion-selective systems are not normally buffered to resist changes, as are the standard solutions for pH systems. They can therefore be affected by dilution, evaporation, oxidation, or contamination by foreign matter in the process fluid. Thus, more care must be taken in preparing and handling these solutions than is generally needed in a typical pH application. Attention should be paid to eliminate carryover from one test solution to another from distilled water rinses. Calibration solutions should be prepared in accordance with accepted principles of analytical chemistry. Many common chemical standards are available as stock solutions from laboratory supply houses. Generally, only solutions at a reasonably high concentration level (greater than 0.01 M or 100 ppm) should be made for storage. Serial dilutions of these stock solutions should be made at the time of use because very dilute solutions are particularly likely to lose some of their ions by absorption of the walls of the storage vessels. Use of high-grade plastic storage bottles is recommended. Table 8.28q lists some solutions frequently used to check the performance of an ion-selective measuring system. When the ionic background is held constant (sulfide, chloride cyanide, and pH), the potential difference between two of these solutions is Nernstian (see Table 8.28b). For others, the potential difference should be normalized to decide changes in activity.


1397

TABLE 8.28q Calibrating Solutions for Ion-Selective Electrodes

Electrode

Chemical Composition

Ionic Concentration

Approximate Ion Activity

Hydrogen (pH)

0.05 M KH phthalate 0.025 M KH2PO4 + 0.025 M + Na2HPO4 0.01 M borax

4.008 pH buffer at 25°C 6.86 pH buffer at 25°C 9.18 pH buffer at 25°C

10 M H /L −6.86 + 10 M H /L −9.18 + 10 M H /L

Fluoride

22.10 mg NaF/L 2.21 mg NaF/L

10.0 mg F /L − 1.0 mg F /L

Chloride

1.00 × 10 M KCl in 1.00 M KNO3 −3 1.00 × 10 M KCl in 1.00 M KNO3

Silver

1.00 × 10 M AgNO3 −3 1.00 × 10 M AgNO3

Sulfide

1.00 × 10 M Na2S in 1.00 M NaOH −3 1.00 × 10 M Na2S in 1.00 M NaOH

Cyanide

1.00 × 10 M NaCN in 1.00 × 10 M NaOH −4 −1 1.00 × 10 M NaCN in 1.00 × 10 M NaOH

Water hardness

−

−2

1.00 × 10 M Ag −3 + 1.00 × 10 M Ag

−1

1 × 10 M S −3 1 × 10 M S

−1

−2

1.00 × 10 M CaCl2 −4

1.00 × 10 M CaCl2

a

+143 (+178) a −35 (0) a −149 (−114)

−

1.00 × 10 M Cl −3 − 1.00 × 10 M Cl

−

−2

+

−59 0.0

9.8 mg F /L − 1.0 mg F /L

−2

−3

−2

−4.008

Approximate emf vs. 1.0 M KCl, AgCl, Ag Reference at 77°F (25°C)

−2

−

0.61 × 10 M Cl −2 − 0.61 × 10 M Cl

+

−1

−2

+

0.90 × 10 M Ag −3 + 0.96 × 10 M Ag −1

0.15 × 10 M S −3 0.15 × 10 M S −

0.76 × 10 M CN

−4

−

0.76 × 10 M CN

1.00 × 10 M CN

++

1.00 × 10 M Ca or 1000 mg/l as CaCO3 −4 ++ 1.00 × 10 M Ca or 10 mg/l as CaCO3

+443 +385 −860 −800

−3

1.00 × 10 M CN

−2

+118 +177

−3

−

−192

−3

−

−133

−2

++

0.55 × 10 M Ca or 550 mg/l as CaCO3 −4 ++ 0.92 × 10 M Ca or 9.2 mg/l as CaCO3

+34 −18

a

Vs. 4 M KCL, AgCl, Ag reference electrode at 25°C.

To achieve the utmost in accuracy and meanin1397 1397gful measurements, the ion-selective measuring system should be standardized, and thus optimized, in a solution carefully chosen to be chemically similar to the process solution at the point of prime interest. This solution should be at a stable temperature near the actual process temperature (±3.6°F (±2°C)). A grab sample of the process solution analyzed in the laboratory may be the best and most convenient standard to use.

ADVANTAGES AND DISADVANTAGES Compared to other composition-measuring techniques, such as photometric, titrimetric, chromotographic, or automatedclassic analysis, the ion-selective electrode measurement has an impressive list of advantages. An electrode measurement is simple, rapid, nondestructive, direct, and continuous, and, therefore, easily applied to closed-loop process control. In this respect, it is similar to using a thermocouple for temperature control. Electrodes can also be used in opaque solutions and viscous slurries. In addition, the electrodes measure the free or active-ionic species in the process, under process conditions, and consequently the status of a process reaction. However, there are several disadvantages. The specificity of ion-selective electrodes is not quite as good as that of the glass pH electrode. Interferences vary from minor to major; the literature and manufacturers’ data on limitations need to


be consulted for each electrode. Also, the electrodes do not measure the total concentration of ions, the parameter that is often requested. The reason is that prior to the introduction of electrodes, concentration information was the only information available from the chemist due to his classic measurement techniques. Control laboratory chemists and process engineers are not accustomed to thinking in terms of activity, even when making pH measurements (Section 8.48). This habit may well disappear as this new ion-selective technique becomes more popular. There are some manufacturers that have begun packing the electrodes separately and installing them in a junction head. This allows the ion-selective electrodes to be independently replaced and stored dry. The reference electrodes are also independently replaced and stored wet. This allows the best of both worlds for storage life and cost of replacement. However, there are times when concentration is a desirable measurement, for example, material balance calculations or pollution control. The knowledge of material balance allows a prediction as to where a process reaction will be in the future. This information is necessary if a process is to be controlled by introducing changes that will nullify those predicted. In pollution control, it is generally accepted that many ions, even in the combined state, are detrimental to life forms. Fluorides, cyanides, and sulfides, to name a few, are harmful to fish and humans in many combined forms. Yet, their sum total is not measured, but they are detected individually by

1398


ion-selective electrodes. Consequently, pollution control agencies usually require concentration information. The electrodes can be used for this purpose if they are calibrated with solutions matching the process or with ISAB solutions (see below). If this is not satisfactory, an electrode can be used for on-line control, and separate grab samples can be analyzed by other procedures to obtain the information needed to comply with regulations.

Precision and Accuracy Another disadvantage derives from a misunderstanding about precision and accuracy. Many classic analytical techniques name a relative error of ±0.1%. Ion-selective electrodes name relative errors of ±4 to 8% (see “System Accuracy” earlier in this section). In terms of pH, this is equivalent to a measurement of ±0.02 pH units—ordinarily regarded as a satisfactory measurement. When used with some degree of understanding, ionselective electrodes can supply satisfactory composition information and afford closed-loop control that was previously unattainable. When in doubt, the user should consult with the electrode manufacturers or his own analytical chemists.

CONCLUSIONS It is evident from the preceding discussion that ion-selective electrode measurements and pH measurements using the glass electrode are not only identical in theory, but also similar in practice. The electrodes are generally the same size and fit the pH holder assemblies. Electrical insulation of the electrodes is as important as with the glass electrode. In addition, the electrodes are subject to the same fouling by oils and slimes as glass electrodes and can, in general, be cleaned using methods already proven for the pH electrodes. Because of these similarities, discussion of the application of ion-selective electrodes is omitted here.

References 1. 2.

3.

Frankenthal, R. P., in Handbook of Analytical Chemistry, Meites, L., Ed., New York: McGraw-Hill, 1963, p. 1, Table 1-8. Frant, M. S. and Ross, J. S., Jr., “Use of a Total Ionic Strength Buffer for Electrode Determination of Fluoride in Water Supplies,” Analytical Chemistry, 40, 1169, 1968. Negus, L. E. and Light, T. S., “Temperature Coefficients and Their Compensation in Ion-Selective Systems,” Instrumentation Technology, 19, 23–26, 1972.


Bibliography Bailey, P. L., Analysis with Ion-Selective Electrodes, 2nd ed., London: Heyden & Son, Ltd., 1980. Bates, R. G., Determination of pH, 2nd ed., New York: John Wiley & Sons, 1973. Bergveld, P. and Sibbald, A., Analytical and Biomedical Applications of IonSelective Field-Effect Transistors (“ISFETS”), Vol. 23, New York: Elsevier, 1988. Berman, H. J. and Hebert, N. C., Eds., Ion-Selective Microelectrodes, New York: Plenum Press, 1974. Cammann, K., Working with Ion-Selective Electrodes: Chemical Laboratory Practice, Berlin: Springer-Verlag, 1979. Control Staff, “How Can pH Probe Fouling Be Reduced,” Control, October 1992. Covington, A. K., Ion-Selective Electrode Methodology, Vols. 1 and 2, Boca Raton, FL: CRC Press, 1978. “Detecting Pollutants with Chemical-Sensing Electrodes,” Environmental Science and Technology, March 1974. Edmonds, T. E., Ed., Chemical Sensors, New York: Chapman & Hall, 1988. Evans, A., Potentiometry and Ion-Selective Electrodes, New York: John Wiley & Sons, 1987. Freiser, H., Ed., Ion-Selective Electrodes in Analytical Chemistry, Vols. 1 and 2, New York: Plenum Press, 1978 and 1980. Gennett, T. and Purdy, W. C., “Electrochemical Sensors, Part 1: A Review of Their Theory,” American Laboratory, 23, 60–64, 1991; “Electrochemical Sensors, Part 2: Recent Advances,” American Laboratory, 60–66, 1991. Janata, J., Principles of Chemical Sensors, New York: Plenum Press, 1989. Janata, J., “Chemical Sensors,” Analytical Chemistry, 62, 33R–44R, 1990 (review). Koryta, J., Ion-Selective Electrodes, London: Cambridge University Press, 1975. Light, T. S., “Industrial Analysis and Control,” in Ion Selective Electrodes, Durst, R. A., Ed., National Bureau of Standards Special Publication 314, Washington, D.C., 1969, chap. 10. Light, T. S., “Potentiometry: PH and Ion-Selective Electrodes,” in Analytical Instrumentation Handbook, Ewing, G., Ed., New York: Marcel Dekker, 1990. Linder, E., Toth, K., and Pungor, E., Dynamic Characteristics of Ion-Selective Electrodes, Boca Raton, FL: CRC Press, 1988. Madou, M. J. and Morrison, W. R., Chemical Sensing and Solid State Devices, New York: Academic Press, 1989. Moody, G. J. and Thomas, J. D. R., Selective Ion-Sensitive Electrodes, Watford, U.K.: Merrow, 1971. Murray, R. W., Dessy, R. E., Heineman, W. R., Janata, J., and Seitz, W. R., Eds., Chemical Sensors and Microinstrumentation, American Chemical Society Symposium Series No. 403, Washington, D.C., 1989. Rundle, C. C., “A Beginners Guide to Ion-Selective Electrode Measurements,” http://www.nico2000.net/Book/Guide1.html. Solsky, R. L., “Ion-Selective Electrodes,” Analytical Chemistry, 62, 21R–33R, 1990 (review). Vesely, J., Weiss, D., and Stulik, K., Analysis with Ion-Selective Electrodes, New York: Halsted Press/John Wiley & Sons, 1978. Weber, S. J., “Specific Ion Electrodes in Pollution Control,” American Laboratory, vol. 2, July 1970.

8.29

Mass Spectrometers R. C. AHLSTROM

(1982, 1995)

Sampling System

R. A. GILBERT

(2003)

AT

Mass Spectrometer To Receiver

Flow Sheet Symbol

Type of Sample:

Vapor


Atmospheric

Sample Temperature:

122 to 424°F (50 to 200°C); sufficient to provide vapor sample with no condensation


68 to 77°F (20 to 25°C)

Contacting Materials:

Inlet materials of construction designed to be compatible with sample

Auxiliary Utilities Required:

Sometimes cooling water

Cost:

$3000 to $200,000, depending on analyzer section, application complexity, and data report and collection accessories Time-of-flight section: $149,000; with specialized data acquisition systems: $175,000 Magnetic section (food processing industry): $120,000 RGA: $3500; multiple purchase: $3000 Quadrupole section: $45,000 to $90,000 Ion-trapping section: $90,000 $150,000

Inaccuracy:


Repeatability:


Cycle Time:

0.5 to 4 sec per stream, depending upon applications

Special Features:

Multicomponent readout, database of mass spectra, programmable temperature control, coupled to gas or liquid chromatography units


Ametek Process Instruments (www.ametekpi.com) Balzers Instruments (www.bi.balzers.com) Finnigan Mat Corp (www.thermo.com) Jeol (www.jeol.com) Leybold Inficon (www.leyboldinficon.com) MKS Instruments (www.mksinst.com) Perkin-Elmer Corp. (www.instruments.perkinelmer.com) Pfeiffer Vacuum (www. pfeiffer-vacuum.de) Waters Corp. (www.waters.com)

INTRODUCTION Closed-loop computer control is a major requirement of many industrial chemical processes. Knowledge of the composition and concentration of a chemical process stream is an

important element if a modern closed-loop computer control scheme is to be developed. The mass spectrometer is widely used as an analytical tool in the process industries. It provides multicomponent analysis with the accuracy, reliability, and cycle time that are necessary for closed-loop computer control 1399


1400


of a process. For example, typical instruments can sample multiple streams for concentrations of 8 to 12 components in less than 5 sec. The increased use of mass spectrometers in the process industry is the result of properly combining the sample stream, the analyzer, and the data acquisition system into a reliable working unit. PRINCIPLE OF OPERATION Mass spectrometers create and then separate ions of a particular atomic or molecular species by their mass-to-charge (m/e) ratios. For example, a diatomic nitrogen molecule with a mass of 28 might become a single charged ion, and it would have an m/e ratio of 28. If that diatomic molecule became a double charged ion, its m/e ratio would be 14. A mass spectrometer employs a vacuum environment in which ions are created, separated, and ultimately detected. This vacuum requirement allows collisions between the ions and molecules to be minimized. This is possible when the mean free path for the ions is very large. Typically, a vacuum in the order –4 –6 of 10 to 10 torr (mmHg) is required. Consequently, a mass spectrometer must include a sampling port into, as well as a vacuum pumping system that maintains the required vacuum envelope within, the spectrometer. In general, a very small gas sample is introduced from atmospheric pressure through the inlet port into the ion generator section of the spectrometer. Sample ions are produced by the collision of rapidly moving electrons with the gas molecules to be analyzed. The electrons ionize the sample by knocking out one or more electrons from an outer orbit of a neutral sample molecule. The positive sample ions are extracted from the ion generator as fast as they are formed by means of electrostatic fields created by sets of electrodes that also accelerate and focus these ions into an ion beam. This ion beam is directed into the ion separation section of the instrument. Finally, these separate segregated ion groups are directed to an ion collector or detector, which provides an electric signal that is proportional to the number of ions in the detected group. Nonionized molecules exit the spectrometer to the atmosphere via the vacuum pumping system. Sample Input The sample system and sample conditioning for a mass spectrometer are similar to those used for gas chromatographs (Section 8.12). In fact, it is very common to connect a gas chromatography system to the sample input port of a mass spectrometer. In any event, vapor samples are drawn by suction into the mass spectrometer through several possible types of inlet leaks. A sintered metal leak device is a commonly used input port because it allows a uniform molecular flow into the instrument based on the differential pressure across the sintered metal plate. Thin diaphragms made from Teflon may be used for corrosive service, and ultrafine metering valves can also be used. Also available are automatic metering valves that maintain a sample flow based on the pressure in


the spectrometer’s ionization chamber. While a variety of inlet leaks are available, one must be chosen that produces a stable flow of sample gas into the spectrometer and is compatible with the desired sample and the system’s analyzer. Fortunately, most mass spectrometer companies provide instruments with appropriate inlet port options and offer help in making the proper input port selection. Sample Ionization Several methods are available for producing ionic species from the sample gas. Electron bombardment is the most popular but requires periodic filament replacement. Spark generation and optical and chemical ionization methods are also used. Atmospheric ionization mass spectrometry is available and has been shown to be applicable to a wide range of compounds. Although this approach promises to open up a wide range of new applications for mass spectrometry, most instruments currently used in process applications use vacuum environmentbased systems with heated filaments. Since filament life and ease of replacement are of primary concern, most process mass spectrometers have dual-filament assemblies that allow switching from one to the other filament without any downtime. Filament replacement is then postponed until occurrence of the next scheduled preventive maintenance event. Figure 8.29a shows the operation of a heated filament ion source. In this example, the molecules are shown entering the spectrometer from the top. After passing through a negatively charged repeller screen, these neutral molecules interact with a focused electron beam to form a variety of products, including an ion cloud that contains positive ions. These ions are pushed out of the ion generator section of the spectrometer by the repeller screen and then accelerated through an exit slit into the ion separation section of the spectrometer. The sample ions are now z-axis aligned and directed into the magnet region of the ion separation section of the instrument. The bulk of the sample molecules and any other electron impact reaction products are removed by the vacuum system. In general, there are at least four options for the sample ionization section of a mass spectrometer: 1) an open ion source for general high-vacuum applications; 2) a closed ion source for process and gas analysis; 3) a differential ion source for running both process pressure and high-vacuum applications simultaneously; and 4) a cross-beam ion source to enhance the minimum detection level for many different gas species. As with the case of the instrument’s sample inlet port, most mass spectrometer companies provide these options and offer help in making the proper ion generator selection. Ion Separation Although the four major sections of a mass spectrometer— the input port, the ion generator, the ion separation section, and the ion detector—are equally important, the behavior of the spectrometer is characterized by its ion separation section. There are several ion separator section options available. For

8.29 Mass Spectrometers

1401

GAS SAMPLE INPUT ELECTRON FOCUS B THERMAL FILAMENT SHIELD

FILAMENT ASSEMBLY # 1

FILAMENT ASSEMBLY # 2

ELECTRON FOCUS A REPELLER SCREEN ELECTRON ACCELERATOR ANODE 1 (ELECTRICALLY TIED TO ANODE 2)

ION CLOUD

ANODE 2 (ELECTRICALLY TIED TO ANODE-1)

ELECTRON BEAM

ION ACCELERATOR

ION FOCUS EXIT SLIT

IONIZED GAS MOLECULES Z-AXIS A

Z-AXIS B

Z-AXIS POTENTIAL

Z-AXIS POTENTIAL

ANALYZER MAGNET

N

S

FIG. 8.29a Ion source operation.

instrumentation and process personnel, the selected instrument design must stress simplicity, automatic operation, and reliability; require a minimum of operator attention; and still meet the analysis needs. Further, it must be serviced and maintained by personnel with minimal mass spectrometer technical expertise. The various separation options are discussed below. Magnetic Sectors There are two types of magnetic sector instruments: fixed magnetic sector and electromagneticfocusing sector. The fixed magnetic sector instrument utilizes a permanent magnet to produce a magnetic field at right angles to the direction of motion of the ion beam that exits the instrument’s ion generation section. This normal interaction forces the ions to bend in a circular trajectory proportional to their m/e ratios. The concept of the fixed magnetic sector ion separation process is shown in Figure 8.29b. The top section of the


illustration is the sample ionization section of the instrument and succinctly summarizes all of the details provided in Figure 8.29a. The center portion of the diagram shows the fixed magnetic sector ion separation portion of the instrument. In this case, multiple collectors are employed to obviate the need for scanning, which vastly simplifies the electronic system and does not require an external programmer to select the operational parameters. For each process application, the collectors are located at a predetermined point in the instrument focal plane. For example, if the situation required the instrument to measure the atmospheric composition of a process stream sample, the ion collectors would be placed along the magnetic sector focal plane to intercept the ion beams for nitrogen (m/e = 28), oxygen (m/e = 32), and argon (m/e = 40). This will require three of the eight possible real-time mass-to-charge ratio measurements that could be made by the instrument.

1402


SAMPLE INLET GAS MOLECULES

ANODE DUAL FILAMENT ION SOURCE OUTPUTS #1

INLET LEAK ELECTRON BEAM FILAMENT

ION SOURCE FILAMENT ELECTRON REPELLER ION CHAMBER ION FOCUS ELECTRON SUPPRESSOR

TOTAL PRESSURE

ION FOCUSING LENSES OBJECT SLIT ION BEAM

#2 #3

MASS SEPARATOR (ANALYZER)

#4 #5

ANALYZER MAGNET −U − Vcoawt

#6 #7

QUADRUPOLE MASS FILTER U + Vcoawt

ANALYZER VACUUM ENVELOPE

#8 ION CURRENT DETECTOR BAFFLE

FARADAY COLLECTOR

ION PUMP MAGNET

DETECTOR

QUADRUPOLE MASS SPECTROMETER

ION PUMP

FIG. 8.29b Fixed magnetic sector mass spectrometer operation.

With these three measurements, the output of the instrument could continuously monitor the atmosphere composition of the process stream and provide data for an appropriate control decision. In addition, there would be five additional spectrometer output channels to monitor other components of the process stream. The concept of the electromagnetic-focusing sector ion separation process is distinctly different from the multiple collector idea shown in Figure 8.29b. The electromagneticfocusing sector utilizes changes in either the accelerating voltage or magnetic field to focus the desired ionic species onto a single collector. When accelerating voltage is used, the ions are focused and accelerated as a beam into a magnetic field. The acceleration voltage is varied to select the ions with a particular mass. For various ionic mass selections, the product of mass and acceleration voltage is a constant. Since the multiple mass ion beam entering the magnetic field contains ions of essentially equal energy, the momentum of each ion depends upon its mass. Because the radius of curvature of the ions in the magnetic field is different for ions of different momentum, only ions corresponding to one particular mass are focused on the collector. This signal is amplified and displayed as a signal proportional to the particular concentration (partial pressure) of the gas type with that mass. Magnetic field variation is an alternate way to make a single mass ion beam exit an electromagnetic-focusing sector in a mass spectrometer. The principal advantage of magnetic field scanning is the wider mass range achieved. Although the mass is not directly proportional to the magnetic field, circuits have been designed to provide linear spacing of masses. When changing the magnetic field, the accelerating voltage is held constant at a value that corresponds to the higher mass of interest. When magnetic field variation is used to sort sample ions, the relative heights of different mass


+

+ + +

+

−

+

+

+ +

+

−

+ +

+

+ ,

NEUTRAL GAS SPECIES SMALL MASSES NEUTRALIZE ON POSITIVE RODS TUNED MASSES PASS THROUGH FILTER LARGE MASSES NEUTRALIZE ON NEGATIVE RODS QUADRUPOLE MASS FILTER

FIG. 8.29c Top: Quadrupole mass spectrometer. Bottom: Quadrupole mass filter.

peaks do not occur in the same proportion as they do when the ion separation is accomplished by changing the accelerating voltage. In the latter case, the low-mass peaks are enhanced in both magnitude and resolution because of the more favorable accelerating voltage at which they appear. When the magnetic field is varied at an acceleration voltage corresponding to the highest mass of interest, this enhancement does not occur. Therefore, both the magnitude and resolution of mass peaks will be less. Quadrupole Filter The use of a quadrupole mass filter to separate the sample ions that leave the ion generation section of a mass spectrometer is also possible. Figure 8.29c illustrates the operation of this type of ion separation section. Again, the top portion of the upper panel of this figure shows


the ion generation section of the instrument. The sample ions exit this ion source through the electron suppressor and then enter the quadrupole filter region of the instrument. A quadrupole m/e filter consists of four high-precision crafted cylindrical rods located precisely in an orthogonal array. The m/e-separated ion groups exit the quadrupole separation section and strike a single detector, which produces a signal proportional to the number of ions with the selected m/e ratio. The lower panel of Figure 8.29c provides a top-down view of the travel path for ions entering the quadrupole ion separation portion of the instrument. As an ion moves at uniform speed among these rods, it undergoes complex oscillatory spiral motion transverse to its travel axis. The actual ion motion is considerably more complex than shown, and the path lines in the figure only represent outer boundaries of particle motion. This motion pattern is created because diametrically opposing rod pairs are electronically connected to also form positive and negative rod pairs. The positive pair has a radio frequency (RF) voltage superimposed on a positive DC voltage, while the negative pair has a negative DC and RF voltage that is 180° out of phase with the positive pair. The varying electromagnetic field that results creates an environment that separates the ions by their mass-to-charge ratio. For a given set of voltages, all ions below a given mass interact with the positive set of rods. These low-mass ions are neutralized and removed from the ion beam, while higher-mass ions remain in the ion beam. In turn, the negative set of rods interacts, neutralizes, and removes ions above a given mass value. The lower-mass ions remain in the ion beam and continue traversing the quadrupole section. By adjusting the RF-to-DC ratio, an ion pass-band can be created so that only ions with a specific narrow m/e ratio can exit the quadrupole ion separation section of the instrument. Under high-resolution conditions, the passed ions barely miss the rod pairs at the extremes of their transverse oscillatory motion, and the mass range that is allowed to exit the quadrupole filter is very narrow. A unique property of the quadrupole mass filter is the fact that the number of m/e ions that are passed is directly proportional to the voltage applied to the rods. Thus, by changing the voltages, various ionic masses of interest are allowed to pass through to the collector where they are measured. Ion-Trapping Section Figure 8.29d illustrates an ion-trap mass spectrometer that uses an interesting twist for its ion separation section. In this case, the geometry of the section plus the RF voltages applied combine to create an environment in which the ions undergo stable oscillations and remain trapped in that environment. Three electrodes, two end-cap electrodes that normally are at ground potential, and between them a ring electrode to which an RF mega-Hertz voltage is applied, generate the quadrupole electric field that can trap the sample ions. The ion separation is accomplished by introducing a change in operating voltage. This alters the quadrupole elec-


1403

INJECTION LENS SYSTEM

DETECTOR PROBE BEARING SAMPLE END-CAP ELECTRODE

RING ELECTRODE

END-CAP ELECTRODE

FIG. 8.29d Ion-trap mass spectrometer.

tric field and allows trapped ions of a particular mass-tocharge ratio to adopt new but unstable trajectories. Thus, if the amplitude of the RF voltage applied to the ring electrode is systematically varied, ions of successively increasing m/e ratios are made to adopt unstable trajectories and to exit the ion trap, where they can be detected using an externally mounted electron multiplier. Time-of-Flight Filter In time-of-flight mass spectrometers, the ionized sample is subjected to a negative-polarity-accelerating field, and then the ion beam is directed into a timeof-flight drift region. This drift region is the ion separation section of the spectrometer. The lighter ions travel faster through this region than the heavier ions, producing mass separation by the amount of time it takes for the ions to traverse this drift tube. To accomplish this type of mass separation, the ionized sample is introduced in discrete pulses of 20,000 to 35,000 pulses per second. On each cycle the sample ions enter the flight tube (drift region) where the separation by mass values is accomplished. Upon exiting, the ions are then converted to electrons and amplified using an electron multiplier. The final amplified signal is detected by the detector’s anode and fed to a suitable data acquisition system. Figure 8.29e illustrates the operational concepts for time-of-flight spectroscopy. Ion Detection It is important to understand that the ion collectors are not selective detectors. They will provide a signal if an ion with the appropriate mass-to-charge ratio emerges from the ion separator and strikes the detector surface. Thus, for this example, the conclusion that the atmospheric composition of the process stream is being monitored depends on the assumption that only ions created from the nitrogen, oxygen, and argon in air contribute to the collector signals at m/e ratios of 28, 32, and 40, respectively. If the process gas stream has other sources for these atoms, the conclusion is not immediately valid.

1404


ANODE (+4.75 KVDC)

CATHODE MAGNET MAGNET CATHODE

CATHODE IONS ACCELERATED TO CATHODE ANODE

Fig. 8.29e Time-of-flight mass spectrometer.

Ion detection errors develop for a variety of reasons. Errors that originate in the spectrometer’s ion generation source or because of changes in conductance of the inlet leak are common. For the process stream atmospheric composition example, the three m/e ion collector outputs would be electronically scaled to create equal sensitivities for each measurement and then summed to create an analog of the sampled atmosphere. This error adjustment scheme is accomplished within the instrument with analog or digital circuitry. In either case, the sum is then compared to a fixed reference, and if it is properly calibrated, no error is created by this comparison. If a hypothetical drift is introduced, an error correction signal is created and is fed back to each channel through the instrument’s gain adjust elements. The gain gives an equal percentage change in each channel and drives the error to zero. The effectiveness of this technique for stabilizing calibration over extended periods has been proven in process applications. Error adjustments are accomplished by amplifier feedback circuits or digital correction techniques. Vacuum Environment To maintain the needed vacuum level within a mass spectrometer, it is common to employ two vacuum pumps in series. With this pump arrangement, the removal of molecules from the instrument’s vacuum environment becomes a two-stage pumping process. At the high-vacuum stage, the molecules enter the −5 −6 pumping system at a pressure of 1 × 10 or 1 × 10 torr and −2 are compressed to a pressure of 1 × 10 torr. At this lowvacuum stage of the pumping process, the molecules enter the −2 second pump and are compressed from 1 × 10 torr to atmospheric pressure. The turbomolecular pump is a common selection for the high-vacuum pumping stage, while a roots blower or a rotary vane pump is a candidate for the low-vacuum-toatmosphere pumping stage. Pump choice depends on required environmental conditions within the process being monitored. The pressure at both vacuum stages is monitored, and the vacuum sensor signals are used for diagnostic and control purposes. Vacuum sensors are discussed in Section 5.14.


CATHODE TRAPPED ELECTRONS

ACTIVE GASES GETTERED AND BURIED BY CLEAN TI AND Ta (TI AND Ta SPUTTERED TO ANODE)

FIG. 8.29f Ion pump operation.

For the spectrometer example presented within this section, an ion pump has been selected for the high-vacuum pumping stage. This choice is representative of an alternative pump type and is essentially a passive device that pumps by a combination of chemical gathering and physical burial. Like the cryopump, it does not require an additional pump connected in series; however, most systems include a parallel pumping turbomolecular pump or roots blower to lower the overall system-pumping burden. Figure 8.28f shows a typical ion pump. An electric field is established between the anode structure and the two cathodes, and a magnetic field is created by a permanent magnet. This creates a trapped electron cloud that continuously ionizes the neutral gas in the pump. The ions are accelerated out of the anode and impinge upon the cathode surface where they sputter cathode material. The tantalum and titanium cathode materials coat the pump structure. These two materials are very active getters and chemically combine with most of the ions present to remove them from the vacuum environment. Inerts are removed by actual burial under the sputtered material or by direct implantation.

Data Reduction and Presentation Today’s mass spectrometers are usually packaged with a complete computer-based data reduction system. In fact, for online industrial applications, there may be too many data manipulation options too easily available to shift operators. In any event, the manufacturers will be able to initially help design


1405

CONCLUSIONS

FIG. 8.29g RGA operational concept.

the spectrometer output capabilities to meet your stated needs. In addition, there are an ample number of third-party venders that can provide compatible software to meet the needs appropriate to any specific and special application.

RESIDUAL GAS ANALYZERS The residual gas analyzer (RGA) represents a major advancement in mass spectroscopy with respect to industrial applications. Over the last 10 years, this technology has been refined and is now a cost-effective way to meet the needs of several process applications. Figure 8.29g provides a conceptual view of the operation of an RGA. The technology is fundamentally a small quadrupole mass spectrometer that provides m/e resolutions that are suitable for certain types of process environments. In this illustration, the outer case to establish a vacuum environment is not shown. The gas sample enters the ionization cage from the right and is ionized by electrons emitted from a hot filament. The ions are accelerated into the quadrupole section and then separated by their mass-to-charge ratio. Figure 8.29c provides more detail about a quadrupole mass filter. Similar m/e charge ratios enter the detector at the same time and are counted and displayed as an intensity signal at the specified m/e. The integrated semiconductor and the thin-film industries are two examples for extensive industrial application for RGAs. In both cases, the RGA serves as a diagnostic tool for monitoring the status of the process equipment vacuum environment and the characterization of the process within that vacuum environment. The RGA is almost always added into sputter tool clusters and other unit operations within these two industries. Although the RGA can also be inserted within control loops for semiconductor and thin-film manufacturing, to date it does not have widespread use in this manner.


Mass spectrometers are typically classified by the technology they use to separate the ionic masses into clusters. Mass spectrometer examples include the quadrupole mass spectrometer, the magnetic sector mass spectrometer, the timeof-flight mass spectrometer, and the ion-trap mass spectrometer. The residual gas analyzer is an example of a quadrupole mass spectrometer. For mass spectrometer application to the process industry, the requirements of a dedicated, on-line analytical instrument might be stated as follows. The instrument is expected to perform a specific analytical task, on a continuous basis, for extended periods. It must exhibit long-term stability and accuracy while operating over its environmental range. An advantage of quadrupole instruments over electromagneticfocusing ones is that the fields required to focus a particular −3 mass can be changed very rapidly (10 mg/l) concentrations, the electrode is rapid and convenient. The electrode’s liquid ion exchanger deteriorates with time, causing a significant error if it is not recalibrated 1– frequently and rejuvenated at least monthly. Although C1 1and HC O3 ions can be removed by precipitation, the procedure is difficult and time consuming. Automated systems have been developed for the several reductions and determinative colorimetric reactions.

TOTAL NITROGEN The increasing use of organic amines and other nitrogen compounds in manufacturing processes has led to increase in regulation in Europe for wastewater to be treated before being released in the environment. Standards such as ISO/TR 11905 Part 1 and 2, DIN 38 409, and DIN 38 406-E 5–1 include methods for the determination of nitrogen. The European Council requires that all wastewater treatment plants in major 10 cities to measure total nitrogen (TN) by instrumental analysis. Kjeldahl Method The total or Kjeldahl nitrogen standard method determines free ammonia and organically bound nitrogen in the –3 valence state but does not determine nitrites, azides, nitro, nitroso, oximes, or nitrates. Organic nitrogen is determined by subtracting the separately determined free ammonia nitrogen from the total nitrogen. The original analysis consists of


several hours of digestion in boiling sulfuric aid, addition of toxic mercury compounds, then ammonia distillation and detection. The automated Kjeldahl has improved the inherent hazard of the test to the operator. Chemiluminescence Analyzer Figure 8.36d shows a flow diagram for a total nitrogen chemiluminescence analyzer. An aliquot of the sample is injected into a high-temperature furnace in an atmosphere of pure oxygen where nitrogen is converted to NO. The carrier gas transports the resulting steam and gases through the moisture control system. Inside the chemiluminescence detector, the NO formed reacts with ozone to produce an excited state of nitrogen dioxide (N O*2 ). A photomultiplier tube adjacent to the reaction cell detects the photons (light) emitted as the N O*2 returns to its ground state. The integrated signal is proportional to the amount of nitrogen in the sample. Concerns about recovery for different types of nitrogen species have been raised. Some manufacturers have employed more efficient catalysts, a NOx converter, or swept the stream of gases through a reducing chamber containing a very strong reducing agent such as 10 vanadium (III) chloride. Simultaneous measurement of carbon (see Section 8.58) can be conducted by allowing the gas stream to pass through an NDIR detector before the CLD detector. A typical range of measurement is 0.05 to 4,000 mg/l N. Speciation of the different nitrogen species can be accomplished by using a gas chromatograph with the chemiluminescence detector. For higher levels of TN measurements, such as 30 mg/l to % levels, the NOx produced during combustion is converted to N2 , which is detected with a thermal conductivity detector. The NO produced from the combustion of a nitrogencontaining sample can also be analyzed by reacting with an electrolyte in an electrochemical cell. This reaction produces a measurable current, which is directly proportional to the amount of nitrogen in the original sample. The analytical range of measurement is 0.1 to 1,000 mg N/l and precision

8.36 Nitrate, Ammonia, and Total Nitrogen

of 1 to 10% can be achieved, depending on the sample matrix. References 1. ASTM, Standard Methods, American Society for Testing and Materials, Philadelphia, PA. 2. Annual Book of ASTM Standards, Part 23, American Society for Testing and Materials, D 992–71; D 1254067; D 1426–71. 3. Li, J. and Dasgupta, P.K., Analytica Chimica Acta, 398, 33–39, 1999. 4. Aoki, T., Fukuda, S., Hosoi, Y., and Mukai, H., Analytica Chimica Acta, 349, 11–16, 1997. 5. Sawicki, E., Stanley, T. W., Pfaff, J., and Amico, A. D., Talanta, 10, 641. 6. Yang, F., Troncy, E., Francoeur, M., Vinet, B., Vinay, P., Czaika, G., and Blaise, G., Clinical Chem., 43, 657–662, 1997. 7. Davi, M. et al., J. Chromatography, 644, 345–348, 1993. 8. Bignami, S., J. Chromatography, 644, 341–344, 1993. 9. Fanning, J., Coordination Chem. Rev., 199, 159–179, 2000. 10. Martin, J., Takahashi, Y., and Datta, M., Am. Laboratory, 49–53, February 1995.

Bibliography Bruno, T. et al., CRC Handbook of Basic Tables for Chemical Analysis, CRC Press, Boca Raton, FL, 1989. Cerda, A. et al., Sequential injection sandwich technique for the simultaneous determination of nitrate and nitrite, Analytica Chimica Acta, 371, 63–71, 1998.


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Cho, S. et al., A fluorescent nitrate sensing system using a reaction cartridge and titanium trichloride, Talanta, 54, 903–911, 2001. Ensafi, A. and Kazemzadeh, A., Simultaneous determination of nitrite and nitrate in various samples using flow injection with spectrophotometric detection, Analytica Chimica Acta, 382, 15–21, 1999. Evans, R. and James, A., Potentiometry and Ion-Selective Electrodes, John Wiley & Sons, New York, 1987. Ewing, G., Analytical Instrumentation Handbook, Marcel Dekker, New York, 1990. Fresenius, W. et. al., Water Analysis, Springer-Verlag, Berlin/New York, 1988. He, Z. et al., Precise and Sensitive Determination of Nitrite by Coulometric Backtitration Under Flow Conditions, Fresenius J. Analytical Chem., 367, 264–269, 2000. Kissinger, P. and Heineman, W., Laboratory Techniques in Electroanalytical Chemistry, Marcel Dekker, New York, 1984. Libby, P. S. and Wheeler, P., Particulate and dissolved organic nitrogen in the central and eastern equatorial pacific, Deep-Sea Research I(44), 345–361, 1997. Masserini, R. and Fanning, K., A sensor package for the simultaneous determination of nanomolar concentrations of nitrite, nitrate, and ammonia in seawater by flourescence detection, Marine Chemistry, 68, 323–333, 2000. Miranda, K. et al., A Rapid, Simple spectrophotometric method for simultaneous detection of nitrate and nitrite, nitric oxide, Biol. Chem., 5, 62–71, 2001. Scully, F. et al., Disinfection interference in wastewaters by natural organic nitrogen compounds, Environ. Sci. Technol., 30, 1465–1471, 1996. Shugar, G. et al., Chemical Technicians’ Ready Reference Handbook, McGraw-Hill, New York, 1990. Skoog, D. A. and Leary, J. L., Principles of Instrumental Analysis, 4th ed., Saunders College Publishing, 1992.

8.37

Nitrogen Oxide Analyzers R. J. GORDON

B. G. LIPTÁK

(1974, 1982)

(1995, 2003)


AT NOX Flow Sheet Symbol

Analysis Methods:

A. B. C. D. E. F. G.

Infrared Ultraviolet Chemiluminescent Colorimetric Electrochemical Coulometric Gas chromatography

Reference Method:

Colorimetric, applied to integrated samples collected in alkaline solution

Ranges:

A. B. C. D. E. G.

Inaccuracy:

Generally 1 to 2% of span, but some microprocessor-based electrochemical designs can be accurate within 2% of reading, and some chemiluminescent units are accurate within 0.5% of span.

Costs:

The installation and the upkeep costs are normally larger than the first costs of purchasing. A. Single-beam portable or laboratory units, $4,000 to $5,000; an industrial NDIR, $8,000; a multi-gas analyzer pulling in up to five gases from 150 ft (50 m) distance, $25,000 to $27,000; FTIR, $75,000 to $125,000 B. Laboratory spectrophotometers, from $2500 to $5000; industrial units, about $20,000 C. About $10,000 to $20,000 D. About $50,000 E. Pocket-sized personal monitors begin at $750; remote sensor heads begin at $1500; on-site monitors with data loggers begin at $5000; stack gas analyzer with printer, auto calibration, and probe costs $6000 to $8000 G. Installed cost in the range of $50,000 to $100,000


Also refer to the Sections 8.4, 8.12, 8.15, 8.27, devoted to electrochemical analyzers, chromatographic analyzers, colorimetric analyzers, infrared analyzers, and ultraviolet analyzers.

0–500 ppm to 0–10% 0–100 ppm to 0–100% 0–50 ppb to 0–10,000 ppm Down to ppb 0–500 ppm to 0–2500 ppm Down to ppb

ABB Process Analytics-Bomem (A) (www.abb.com\analytical) Ametek (A) (www.westernreseach.com) Anarad (A) (www.anarad.com) Antek Instruments (C) (www.antek.com) Bran and Luebbe, Inc. (B, D, G) (www.branluebbe.com) Bruel & Kjaer (A) ([email protected]) CEA (A) ([email protected]) Cole-Parmer Instrument Co. (www.coleparmer.com) Cosa Instrument Corp. (E) (www.cosa-instrument.com) Foxboro (A) (www.foxboro.com) Hamilton Sundstrand (AIT division, Analect) (A) ([email protected]) Horbia Instruments (A, C) (www.horiba.com)

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TO RECEIVER

8.37 Nitrogen Oxide Analyzers

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Ionics (C) (www.ionics.com) Leco Corp. (F) (www.leco.com) LI-Cor (A) (www.licor.com), Midac (A) (www.Midac.com) MKS Instruments (A) ([email protected]) MSA Instruments (A) (www.msanet.com) Ocean Optics (A) (www.oceanoptics.com) Remspec Corp. (A) (www.remspec.com) Rosemont (A) (www.rauniloc.com) Sensidyne (A, E) (www.sensidyne.com) Servomex (A) (www.servomex.com) Shimadzu Corporation (C) (www.shimadzu.com) Siemens (A) (www.sea.siemens.com) Sierra Monitor Inc. (www.sierrainstruments.com) Teledyne (A) (www.teledyne-ai.com) Wilks Enterprise (A) (www.WilksIR.com) Zellweger Analytics (A, C) (www.zelana.com)

INTRODUCTION

Ultraviolet Analyzers

The oxides of nitrogen are measured both in ambient air and in the gases emitted by industry. While the types of analyzers used for these two applications do overlap, here, an attempt will be made to separate them. Therefore, in this section the devices more often used for industrial emission monitoring will be discussed first. These include infrared, ultraviolet, chemiluminescent, gas chromatographic, and electrochemical devices. Colorimetric and coulometric analysis is more often used for ambient air analysis and will be discussed later in this section.

The absorbance of nitrogen dioxide in the ultraviolet range is shown in Figure 8.37a. Because, in most combustion processes, there is an interest in determining the concentrations of the both NOx and SO2, some of the suppliers of ultraviolet analyzers have combined the two tasks into a single analyzer whereby their concentrations are simultaneously measured (Figure 8.37b). Because nitric oxide (NO) is essentially transparent in the visible and ultraviolet regions, it must be converted to NO2 before it can be measured. In Figure 8.37b, this conversion is achieved by contacting the sample gas with oxygen pressurized to five atmospheres. Once the NO is converted, the total NO2 concentration is measured as NOx. Ranges can be as narrow as 0 to 100 ppm or as wide as 0 to 100% NOx, and the measurement error is about 2% of full scale.

INDUSTRIAL EMISSION MONITORING Paramagnetic Analyzers Nitrogen oxide is attracted by the magnetic field and therefore is detectable by a paramagnetic analyzer. This instrument is described in this chapter under Oxygen Analyzers (Section 8.42) and is not widely used for the measurement of nitrogen oxide concentration. Thermal Conductivity Analyzers The thermal conductivity (see Section 8.57) of nitric oxide (NO) is slightly less than that of air—about 90%. Therefore, although thermal conductivity measurements have been attempted for NO analysis, it is neither a sensitive nor a selective means of measurement. Nondispersive Infrared Analyzers Nondispersive infrared (NDIR) analyzers are suitable for the determination of nitric oxide (NO) concentration. These devices are most often used in stack gas analyzer packages, which, in addition to NOx, also detect the concentrations of carbon dioxide, carbon monoxide, sulfur dioxide, and opacity. Section 8.27 described the nondispersive infrared analyzers, and Figures 8.27h and 8.27t show some of their stackmounted variations.


Chemiluminescent Analyzers 1

Another method of nitrogen oxides determination makes use of the fact that nitric oxide (NO) reacts with ozone to form nitrogen dioxide (NO2), and this reaction is accompanied with the release of light (chemiluminescence). NO + O3 → NO2 + hv (light 0.6 − 3µ )

8.37(1)

If excess ozone is present, the light emission is proportional to the amount of nitric oxide present in the sample. Instruments that operate on the basis of this principle are commercially available. For the determination of NO2 (which reacts with ozone rather slowly), some analyzers are provided with a catalytic converter to first reduce NO2 to NO and then determine the sum of NO2 and NO directly. The apparatus is schematically shown in Figure 8.37c. Electrochemical Sensors Portable or permanently installed probe-type electrochemical sensors are available for NO, NO2, and NOx measurement

1476


2.0 1.8 1.6 SO2

ABSORBANCE

1.4

NO2

1.2 1.0 0.8 0.6 0.4 280

436

0.2

578

0.0 300

250

350

400

450

500

550

600

WAVELENGTH (nm)

FIG. 8.37a The absorbance of SO2 and NO2 in the ultraviolet range. RECORDER

CONTROL STATION

CONTROL STATION

TIMER STATION

LIGHT SOURCE

NO2 SAMPLE CELL PHOTOMETER

STACK

FILTER PROBE

BALL VALVES (PNEUMATIC OPERATED)

VACUUM BREAKER

FLOW INDICATOR

HEATED COMPARTMENT SO2 SAMPLE CELL

AIR O2 TANK TO ELEC PRESSURE REGULATOR

PRESSURE REGULATOR

SOLENOIDS

SAMPLE IN ASPIRATOR POWER SUPPLY

HEATED TUBING 115 V AC EXTERNAL

FIG. 8.37b Ultraviolet analyzer used for the simultaneous measurement of NOx and SO2.


TRAP

PHOTOMETER


CALIBRATION GAS

OXYGEN FLOWMETER

SOLENOID VALVE

OZONATOR HIGH-VOLTAGE POWER SUPPLY

CATALYTIC NO2 NO CONVERTERa

REACTOR

PHOTO TUBE OPTICAL FILTER

SAMPLE AIR

OPTICAL VACUUM WINDOW PUMP

TIME PROGRAMMER

READ-OUT

a CONVERTER CAN BE OMITTED IF ONLY NO IS MEASURED

FIG. 8.37c Chemiluminescence nitric oxide analyzer.

in stacks. These units are microprocessor based, are available with 4 to 20 mA transmitter outputs, and include self-diagnostic and self-calibration features. The probe can be exposed to continuous temperatures of 1550°F (850°C), which for short periods can rise to 2200°F (1200°C). Gas Chromatography There have been a limited number of reports of applications for analysis of oxides of nitrogen by gas chromatography. Such chromatographic columns are fairly short and are provided with electron capture detectors. These are the same types of chromatographs that are used for the analysis of peroxyacyl nitrates. For added details on gas chromatography, see Section 8.12. AMBIENT AIR MONITORING Calibration Methods The reliability of dynamic calibration of gas analyzers is always superior to that of static calibration, but it is also much more

difficult. This is because, in high concentrations, nitric oxide (NO) is rapidly oxidized to nitrogen dioxide (NO2) by air, and NO2 condenses and dimerizes at high concentrations. For these reasons, dynamic calibration for NO and NO2 requires great care. In colorimetric or coulometric analysis, the NO in fact is not measured directly at all but is measured only after its oxidation to NO2, so these analyzers are calibrated only for NO2. Dynamic Calibration Dynamic calibration for NO2 requires preparation of a sample of inert gas containing a known concentration of NO2. This sample may be obtained by gasdilution techniques (with special precautions in handling NO2), gravimetrically, electrolytically, or by use of a permeation tube. The permeation tube (as used with hydrocarbons) is described in Section 8.25. However, NO2 permeation tubes are moisture sensitive. Therefore, during storage, these tubes should be protected from moisture, and only dry gases should be used for dilution. If these precautions are observed, the permeation tube is probably the most convenient method for the dynamic calibration of NO2. Static Calibration Static calibration is carried out with standard solutions of nitrite. Because this procedure does not detect the components of the gas in the sampling line, it is not as complete a test as is the dynamic calibration, but it is much simpler. The stoichiometric ratio of nitrite to NO2 under controlled conditions is usually a constant value. In the Griess–Saltzman method, the consensus (although there has been controversy) is that 0.72 mole of nitrite gives the color of 1 mole of NO2. Using the Jocobs–Hochheiser method, this factor has been found to be 0.63. NO–NO2 Combination Analysis In continuous analysis, it is customary to determine both NO and NO2 as NO2 (Table 8.37d). The NO is oxidized to NO2 by means of potassium permanganate or dichromate or by chromium trioxide, in various formulations. The efficiencies of conversion seem to depend on the length of service and, for the chromium oxidizers, they also depend on humidity.

TABLE 8.37d Nitrogen Oxide Analyzers General Method

Type

Advantages

Disadvantages

Griess–Saltzman Jacobs–Hochheiser

Precise, thoroughly tested, widely used, continuous analysis Precise, stable after collection

Short life of collected sample, sensitive reagents, NO oxidation required. Not adapted to continuous analysis, sensitive reagents, NO oxidation required.

Coulometric

Simple apparatus, continuous analysis

Sensitive to other oxidants, NO oxidation required.

Chemiluminescent

Dry gases only, sensitive photometry, continuous analysis

Requires ozone generator NO2 catalytic reduction.

Gas chromatography

Specific, frequent analysis

Not a developed instrument, expensive and complex.

Electrochemical

Simple apparatus, continuous analysis

Sensitivity not high, NO oxidation required.

Colorimetric


1477

1478


SERIES MODE

AIR IN

NO2 ANALYZER

NO2 ANALYZER

NO NO2 OXIDIZER

NO2 CONCENTRATION

SEPARATOR CONTACTOR

NO CONCENTRATION

EXHAUST OPTICAL CELLS

PUMP

SAMPLE AIR IN

NO2 ANALYZER

REF.

NO2 CONCENTRATION

CONTROL VALVE

OXIDIZER

PARALLEL MODE I AIR IN

SEPARATOR CONTACTOR

SAMPLE

REF.

SAMPLE

DIFFERENCE

NO CONCENTRATION AIR IN

NO2 NO OXIDIZER

NO2 ANALYZER NOX CONCENTRATION (= NO + NO2) PARALLEL MODE II

NO NO2 OXIDIZER

NO2 ANALYZER NO CONCENTRATION

FIG. 8.37e Nitrogen oxides analyzer modes.

Aqueous permanganate seems to be the best choice, even though it may not be completely efficient (hence, NO may be underestimated). Series Analysis In series analysis for NO and NO2 (Figure 8.37e), the air passes through an NO2 analyzer for measurement and removal of NO2, then through an oxidizer to convert NO to NO2, and finally through a second NO2 analyzer. The second analyzer gives a measure of NO concentration. In one type of parallel analysis, two equal air streams are analyzed for NO2, one of them after passage through an oxidizer. The latter gives total oxides of nitrogen (NO + NO2) from which NO is found by difference from the other parallel analyzer. In a second type of parallel analysis, one stream is analyzed directly for NO2, and the second is scrubbed free of NO2 by passage through ascarite, followed by oxidation of NO to NO2 and NO2 analysis. Colorimetric Determination 2

There are two important colorimetric methods for NO2 determination. They are the Griess–Saltzman and the Jacobs– Hochheiser methods.


SPENT REAGENT

AIR FLOW

FIG. 8.37f Colorimetric nitrogen oxides analyzer.

NO2 CONCENTRATION NO2 ABSORBER

PUMP SOLUTION FLOW

REAGENT RESERVOIR

NO2 AIR IN ANALYZER

AIR IN

PUMP

The Griess–Saltzman Method The Griess–Saltzman method is used in most continuous colorimetric NO2 analyzers. It is based on the reaction of NO2 with sulfanilic acid to form a diazonium salt that couples with N-(1-napthy1)-ethylenediamine dihydrochloride to form a deeply colored azo dye. Air is passed into the reagent solution for not over 30 min. After that, time is allowed for development, and the color is measured at 550 nm. The measurable range of concentrations is from 0.02 to 0.75 ppm. In manual use, the color is developed for 15 min and should be read within 1 hr (on a colorimeter or spectrophotometer). In a continuous analyzer (Figure 8.37f ), the gas and liquid flow rates are adjusted for optimal response, and the developed color is potentiometrically read in a flow cell using a 550 nm filter. Response times are usually 5 to 15 min. The Jacobs–Hochheiser Method The Jacobs–Hochheiser method is the standard reference method used for U.S. National Air Quality Standards. The reason for this is that the standards are based on the annual average, and this method allows collection of up to 24-hr integrated samples and delays in analysis of at least 2 weeks. In contrast, with the Griess– Saltzman method, samples must be quickly analyzed. In the Jacobs–Hochheiser method, the air is passed through aqueous sodium hydroxide so that the NO2 is concerted to nitrite ion. Sulfur dioxide is removed from the solution by treatment with hydrogen peroxide and is acidified. The rest of the procedure is the same as for the Griess–Saltzman method except that sulfanilamide is used instead of sulfanilic acid. Efficiencies found with this procedure in the U.S. National Air Surveillance Network are approximately 35%.


TABLE 8.37g Typical Range, Sensitivity, and Alarm Setpoints of Portable Personal Protection Monitors* Gas

Range

Resolution

Alarm Set Points (low/high)

O2

0–30%

0.1%

19.5/23.5%

CO

0–500 ppm

1 ppm

35/200 ppm

H 2S

0–100 ppm

1 ppm

10/20 ppm

SO2

0–20 ppm

1 ppm

2/10 ppm

NO

0–250 ppm

1 ppm

25/50 ppm

NO2

0–20 ppm

0.1 ppm

1/10 ppm

NH3

0–50 ppm

1 ppm

25/50 ppm

PH3

0–5 ppm

0.1 ppm

1/2 ppm

Cl2

0–10 ppm

0.1 ppm

0.5/5 ppm

HCN

0–100 ppm

1 ppm

4.7/50 ppm

* Abstracted from Cole-Parmer Catalog 2001/2002, Vernon Hills, IL, 2001.

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various oxidizer columns, the most commonly used is the permanganate one, but none is completely satisfactory. It is important to calibrate it over a range of nitric oxide concentrations and at various humidity levels. The chemiluminescent method utilizes the reaction between ozone and nitric oxide. This is a dry gas method requiring an ozone generator and compressed gas. In the chemiluminescent method, a catalytic reduction of nitrogen dioxide to nitric oxide is required. For industrial applications, the most often used analyzers are the nondispersive infrared, ultraviolet, and electrochemical types. The chemiluminescent and gas chromatographic techniques are less frequently used. References 1. 2.

Fontijn, A., Sabadell, A. J., and Ronco, R. J., Analytical Chem., 42. Air Quality for Nitrogen Oxides, U.S. Environmental Protection Agency, Air Pollution Control Office Publication No. AP-84, Chap. 5.

Bibliography Portable Monitors For purposes of personal protection, battery-operated portable units are available. These units are usually provided with one or two alarm set-points and with memory for some thousands of data points along with their times and dates. Table 8.37g lists the ranges, resolutions, and alarm set points for a number of gases including NO and NO2. These pocket-sized, battery-operated, portable electrochemical detectors are usually provided with digital displays and audible alarms. They can be configured for one or more monitoring channels.

CONCLUSIONS The most widely used conventional method of ambient air analysis is colorimetric. It is most often based on the use of the Griess–Saltzman reagent (a diazotization method). This is a fairly precise and dependable method but requires a great deal of attention. The colorimetric method is specific for nitrogen dioxide. To analyze for nitric oxide, an oxidation step is required. Of


Dailey, W. V., A novel NDIR analyzer for NO, SO2 and CO Analysis, Analysis Instrum., 15, 1977. Ewing, G., Analytical Instrumentation Handbook, Marcel Dekker, New York, 1990. Hommel, C. O. and Sekhar, N., Parameter monitoring for SO2 and NOx emissions, 1992 ISA Conference, Houston, October 1992. Landa, I., Visible (VIS) near infrared (NIR) rapid spectrometer for laboratory and on-line analysis of chemical and physical properties, SPIE, 665, 286–289, 1986. Lodge, J. P., Methods of Air Sampling and Analysis, 3rd ed., Lewis Publishers, Chelsea, Michigan, 1988. MacRae, M., Analyzing new options, Pharm. Tech., 26(2), 2002. Schirmer, R. E., On-line fiber-optic-based near infrared absorption spectrophotometry for process control, Proc. ISA, 1229–1235, 1986. Scully, F. et al., Disinfection interference in wastewaters by natural organic nitrogen compounds, Environ. Sci. Technol., 30, 1465–1471, 1996. Shugar, G. et al., Chemical Technicians’ Ready Reference Handbook, McGraw-Hill, New York, 1990. Skoog, D. A. and Leary, J. L., Principles of Instrumental Analysis, 4th ed., Saunders College Publishing, 1992. Stoeppelwerth, P. B., Utility boiler control system upgrade, 1992 ISA Conference, Houston, October 1992. Turner, G. S., Design and Performance of an Ambient Level NO/NO2/NOx Monitor, Analysis Instrum., 12, 1974. Van Agthoven, M. A., Mullins, O. C., et. al, Near Infrared Spectral Analysis of Gas Mixtures, Appl. Spectroscopy, 56, 2002. Weiss, M. D., Analyzing stack emissions, Control, July 1990.

8.38

Odor Detection A. TURK


(1974, 1982)

TO RECEIVER

B. G. LIPTÁK

(1995)

W. P. DURDEN

(2003)

AT ODOR Flow Sheet Symbol


A. Organoleptic B. Instruments, such as chromatographic, mass spectrographic, thermal conductivity, catalytic combustion, semiconductor, flame ionization, photoionization. C. Electronic nose

Sensitivity of Detection:

A. About 0.2 ppb B. About 10 to 200 ppb C. ppt to ppb, depending on chemical

Costs:

Chromatograph (See Section 8.12 for details), about $100,000 installed with accessories; portable chromatograph with electrochemical sensor, $15,000; mass spectrograph, about $100,000; photoionization, portable, $3,400 to $6,800; solid state gas hydrocarbon portable, about $1200; polymer, MOV, fiber optic, calorimetric, amperometric, gravimetric, $700 to $100,000


Agilent Technologies (www.agilent.com) Alpha Mos (www.alpha-mos.com) Baseline–MOCON Inc. (www.baselineindustries.com) Bloodhound Sensors Ltd. (www.leeds.ac.uk/ulis/Bloodhound/) Brechbüehler AG (www.brechbuehler.ch/) Cyrano Sciences Inc. (www.cyranosciences.com) E2V Technologies (www.e2vtechnologies.com) EEV Chemical Sensor Systems (www.eevinc.com) Electronic Sensor Technology (www.estcal.com) Environics Industry; Gastech Co. LTD. (www.portadetector.com) Gow-Mac Instrument Co. (www.gow-mac.com) GSG Analytical Instruments (www.gsganalytical.com) Honeywell (www.honeywell.com) HNU Technology (www.hnu.com) HNU Systems UK Ltd. (www.hnu.co.uk) Lennartz Electronics (www.lennartz-electronic.de) Mastiff Electronics (www.mastiff.co.uk) Mitsubishi Electric (www.mitsubishielectric.com) Motech (www.motechind.com) MSA Instrument Div. (www.msanet.com) Nordic Sensors (www.nordicsensors.com) Osmetech plc (www.osmetech.plc.uk) Perkin-Elmer Corp. (www.perkin-elmer.com) Phoenix Electrode Co. (www.phoenixelectrode.com) Schott Company (www.schott.com) LDZ Laboratoire Dr. Zesiger (www.smartnose.com) Thermo Finnigan Austin (www.tmqaustin.com)

8.38 Odor Detection

1481

INTRODUCTION

Flexibility

Odor is a sensation associated with smell, which can be hard to quantify. The same quantities of different materials cause different odor intensities. The unit of odor intensity is based on the odor of tertiary butyl mercaptan (TBM; W = 1.0). Using that reference, H2S, for example, has an odor intensity of 0.08 or 8% of TBMs. Most odorant substances contain sulfur. Table 8.38a lists a number of odorant substances and their relative odor intensities (W).

The human olfactory system is capable of detecting and identifying a wide variety of chemical structures and giving different responses to different materials. Commercially available instrumentation and chemical methods are generally restricted to particular chemical structures and give a similar response to all compounds with that structure.

THE MEASUREMENT OF ODOR This section compares organoleptic and chemical/instrumental methods for odor measurement. The organoleptic methods, which utilize the human olfactory system, are completely subjective. However, techniques are available that can convert subjective measurements into useful objective results. Recent improvements in technology and increased research in the area of instrumentation have made dramatic improvements in the creation of instruments that are capable of surpassing the human olfactory system. In fact, the instruments of today are approaching the sensitivity of the canine olfactory system, which is thought to be as much as a million times more sensitive than that of humans. These developments have overcome the two major shortcomings generally suffered by chemical/instrumental methods: sensitivity and flexibility. Sensitivity The human olfactory system is generally three orders of magnitude more sensitive than currently available chemical/ instrumental methods. Humans can detect and identify odors present in quantities to which commercially available instrumentation and chemical methods are completely insensitive.

The Gas Chromatograph The most successful instrumental method for the measurement of odor has been the gas chromatograph. Gas chromatographs, which monitor the Kraft paper mills and use flame photometric detectors, can determine the concentrations of sulfur dioxide, hydrogen sulfide, and other odorous gases 2 down to about 0.01 ppm. Success in detecting these sulfurcontaining compounds using a coulometric cell with platinum 3 electrodes has also been reported. Levels as low as 0.1 ppm were detected by this method. 4 The threshold levels detected by the human nose are as low as 0.00021 ppm (trimethylamine). Thus, it was apparent that, except for special situations, the human nose, with its attendant sociological, psychological, and physiological complications, was the only odor sensor available for general odor measurement. However, the detection of odor using chemical or chromatographic methods for determining measurable levels of individual chemicals in a vapor phase is not really odor detection but gas or chemical detection. The detected chemical(s) have an odor and, based on experience, the output of the chromatograph can be interpreted into an odor associated with, for example, the smell of a peach. True odor analysis will not only determine that the odor is a peach, but whether it is a green, ripe, or rotten peach. Today’s instruments can now determine the quality of the odor in near real time. These odor detectors and analyzers are trained to recognize the odors and the levels of distinction by

TABLE 8.38a 1 The Relative Odor Intensity of Different Chemicals Odor Intensity Abbreviation


Name

Formula

(W)

EM

Ethyl mercaptan ethanethiol

CH3H

1.08

DMS

Dimethyl sulfide methylsulfide

(CH3)2S

1.0

IPM

Iso propyl mercaptan 20-propanethiol

CH3CHSHCH3

0.88

MES

Methyl ethyl sulfide methyl thioethane

(CH3)2CH2S

0.66

NPM

Normal propyl mercaptan 1-propanethiol

(CH3)CH2CH2SH

0.85

TBM

Tertiary butyl mercaptan 2-methyl 2-propanethiol

(CH3)3CSH

1.00 (ref)

SBM

Secondary butyl mercaptan 1-methyl 1-propanethiol

(CH3)2CH2CHSH

1.99

DES

Diethyl sulfide ethyl sulfide

(C2H5)2S

0.22

Thiophane

Thiophane tetrahydrothiophene

C4H8S

EIS

Ethyl isopropyl sulfide

1.63 0.07

1482


BRAIN OLFACTORY BULB OLFACTORY AREA TURBINATE BONES

odorous materials, these receptors send signals to the olfactory bulb, where they are relayed to higher centers of the brain. In these higher centers, the signals are integrated and interpreted in terms of the character and intensity of the odor. Sample Preparation

TONGUE

FIG. 8.38b The human olfactory system. TRIGEMINAL NERVE

TO OLFACTORY BULB

OLFACTORY NERVE FIBERS

OLFACTORY CELL

Because of the extreme sensitivity of the human nose, the concentrations of odorants in air can be extremely low yet can produce a strong response. These low concentrations present a problem with regard to the manner by which samples are presented to the human sensor. Improper handling of an ambient air sample or improper preparation of an odorant standard can result in erroneous results because of adsorption of the odorants on the walls, incomplete mixing of an odorant with dilution air, or impure dilution air. Errors can also result from the type of system used to bring the sample in contact with the olfactory sensors. The use of syringes or other means by which the nose is not immersed in the sample yield lower apparent odor intensities than do 5 methods whereby the nose is fully exposed to the sample. The use of an odor room can also yield misleading results because of natural odors generated by the body and adsorption by clothing. The best approach appears to be the sniff box shown in Figure 8.38d. In this case, the human sensor is kept in an air-conditioned room into which charcoal-filtered air is blown to maintain a slight positive pressure. This prevents the sample in the sniff box and other contaminated air from entering the room. For measurement, the whole face is placed in the sample stream within the sniff box.

SUPPORTING CELL

ODOR PANELS TRIGEMINAL ENDING MUCUS OLFACTORY HAIRS

FIG. 8.38c Section of an olfactory epithelium.

The use of humans as measurement instruments introduces a variability that is difficult to control. This variability is due to the moods, biases, and other vagaries associated with the human sensor. Each human being is a unique creation, with differences in intelligence, persistence, sensitivity, experience, EXHAUST FAN

exposure to standards of odor. Once trained, these devices will provide a proportional reading of the odor, much the same as a human olfactory system but without the subjectivity.

CONDITIONED AIR SNIFF BOX

THE HUMAN OLFACTORY SYSTEM The human olfactory system actually involves more than the nose. As a stream of air is drawn in through the nostrils, it is warmed and filtered by passing over the three baffle-shaped turbinate bones in the upper part of the nose (Figure 8.38b). Some of the air swirls past the olfactory receptors located high up in the nasal passages just below the brain. These odor receptors consist of hairlike filaments attached to the end of the fibers of the olfactory nerve and the trigeminal nerve endings (Figure 8.38c). Upon being stimulated by


ODOR SAMPLE

FIG. 8.38d Diagram of sniff odor test system.

MINIMUM ODOR ROOM

8.38 Odor Detection

and interests. These differences make the response of an individual an unreliable indicator in itself. The use of a panel of individuals for the measurement of odor levels statistically eliminates the unreliability of the individual. An odor panel generally consists of 5 to 10 people. The people who compose the odor panel need not have unusual olfactory abilities. However, they should be able to distinguish among odors of different intensities, discriminate among different odor qualities, and communicate the perceived sensations in terms of reference standards. They should also be emotionally receptive to making quantitative and discriminatory judgments without expressing their preference. Training of an Odor Panel

1483

Sensors S1

S2

S3

Sn

SIGNAL PREPROCESSOR AND PATTERN CLASSIFIER

ANALYSIS ENGINE AND DISPLAY MODULE

The primary tasks of an odor panel are the following: 1. Judge the relative intensity of an odor at different dilutions. 2. Discriminate among the different odor qualities. 3. Combine the intensity and quality of an odor to give a composite profile that can be communicated to the scientist in charge of the test. The panelists are expected to follow instructions and also to render independent judgment reflecting their own sensations. Tests In addition to exposing the panelists to various levels of odors similar to those to be measured, three tests can be used for training purposes. These are the triangle test, the intensity rating test, and the multicomponent identification test. In the triangle test, three samples are presented to the panelist at the same time. Two are identical, and the third is different in either intensity or quality. The panelist must select the odd sample. The intensity rating test consists of a series of perhaps 20 dilutions of an odorant in an odorless medium. One sample is removed from the series, and the panelist is asked to determine the sample’s proper place in the series according to its odor intensity. The last test is the multicomponent odor identification test, in which three mixtures are presented to the panelist. These mixtures contain, in sequence, two, three, and four odors out of a possible total of eight known standards. The panelist is told how many components to look for and is asked to identify each of them. This group of three tests develops a panelists’ ability to distinguish between different odor intensities, discriminate among odor qualities, and communicate their sensations in terms of predetermined standards. After the group has some experience working together under a competent test director in actual measurement situations, highly accurate measurements of odor levels can be made.


FIG. 8.38e Typical schematic of an electronic nose.

THE ELECTRONIC NOSE Advances in technology have led to significant improvements, both in analysis and in size, in the electronic nose. The basic technology relies on the use of solid state sensors—either chemoresitors, chemodiodes, or electrodes. All electronic noses use the same basic method of sensing and analysis (Figure 8.38e). Primary odors are composed of polar molecules, organic vapors, and phthalocyanines. New sensors such as the chemosensors are devices that convert chemical composition into a quantifiable electrical format. Other new types include conductometric, optical, gravimetric, amperometric, calorimetric, potentiometric, and chemocapacitor sensors. Polymeric Film Sensors Conductometric sensors are made up of conductive polymers and metal oxide semiconductors. The first are composed of a polymeric film composite. This technology was developed concurrently in the U.S. and the U.K. The work in the U.S. occurred at the California Institute of Technology in a joint effort with Jet Propulsion Laboratory (JPL). The developments in the U.K. occurred at the University of Manchester Institute of Science and Technology. In both cases, the developed sensors consist of an array of pairs of electrical contacts that are electrically connected by a composite film. The composite film is composed of a special nonconductive polymer, which is mixed with carbon black in a homogeneous blend. This forms a conductive bridge between the electrical contacts. The composite film is selected to absorb specific vapor analytes. As it does so, it swells, thereby increasing the distance between the carbon black particles. This first changes its resistance characteristics and then breaks the electrically conductive path in the film. The variance in the resistance

1484


between the electrical contacts is used as the output of the sensor. Because of the selected polymer, each sensor in the array is sensitive to a different level of concentration of a chemical. This allows relative concentrations to be determined.

logic, genetic algorithms, cluster analysis, discriminate function analysis, and adaptive models.

Metal Oxide Sensors

This technology allows the analyzers to be small—at least desktop size and, in some cases, easily handheld. There are indications that credit card size devices will eventually be developed. A sensor can be designed to be as specific or as broad in application as desired. There are many possible applications in industrial and commercial safety, drug interdiction and enforcement, explosive detection, pharmaceuticals, medical, food and beverage, perfumes, breweries, wineries, etc. Anywhere an odor can be used to detect the presence or even absence of a substance, these electronic noses can be used. The cycle time is typically a few seconds but can be longer, depending on the chemical and the vapor state. Temperature dramatically affects the vaporization of most chemicals, thereby reducing the amount of chemicals to cause an odor. When a sensor/analyzer is used for batch applications, there is an additional time required to purge and clear the sensor array, allowing it to return to a baseline state. One manufacturer has recently introduced a sensor and analyzer to detect the odor of hydrocarbons in water.

The second type of sensor, metal oxide, employs a similar method of odor detection but uses metal oxide sensors in an array, reacting to the chemicals, changing the semiconductors’ conductance, and producing the reading. The use of metal oxide sensors falls into two categories, thin and thick film. The thin film targets NOx, H2, and NH3. The thick film is used for the detection of odors of alcohols, ketones, and combustible materials. Other Sensors Potentiometric sensors, using MOSFET semiconductors, react to the chemicals in an odor, which change the resistance of the sensor to generate the reading. Chemocapacitors perform the same function by reacting to the chemicals in the odor by generating a change in the sensor capacitance. Gravimetric sensors use gas chromatography (GC) coupled with surface acoustic wave (SAW) and quartz crystal microbalance (QCM) measurements. The GC separates the gas and deposits the chemical compound on a vibrating quartz crystal. This causes a change in the acoustic or sound propagation characteristics as a result of the mass change caused by the chemical absorbed. Response time is approximately ten seconds. Amperometric sensors use the catalytic oxidation of a chemical analyte to generate heat proportional to the amount of chemical present. Optical sensors use specially coated optical fibers. The coating consists of specifically formulated fluorescent dyes with unique characteristics that cause the fibers to react to very specific chemicals, changing the optical characteristics in proportion to the amount of chemical present. Calorimetric sensors are thermal sensors that measure the heat generated by the absorption of an analyte into the sensor coating. The more chemical present, the greater the heat generated.

Applications

References 1. 2.

3.

4.

5. 6.

7.

Training All electronic noses must be trained to recognize the odors as specific smells. Known samples are used to generate defined patterns, odor fingerprints, or (as they have been called) smell prints. These standards are used to define the parameters by which the training occurs. A major challenge is to interpret the odor signals into a specific reading of a specific odor with no false readings. The intelligence in the analyzer must emulate the human equivalent of subjective comparison. This is done through prediction algorithms and sophisticated pattern analysis such as multivariate, fuzzy


8.

Kutzleb, R. E., Odotron: a better way to measure gas odorants, Pipe Line Ind., May 1973. Stevens, R. K., O’Keefe, A. E., Mulick, J. D., and Krost, K. J., Gas Chromatography of Reactive Sulfur Gases in Air at the Parts Per Billion Level, 1, Direct Chromatographic Analysis, National Air Pollution Control Administration, Cincinnati, OH. Applebury, T. E. and Schaur, M. J., Analysis of Kraft Pulp Mill Gases by Process Gas Chromatography, Department of Chemical Engineering, Montana State University, Bozeman, MT. Leonardos, G., The profile approach to odor measurement, Proc. MidAtlantic States Section, Air Pollution Control Association Semiannual Technical Conference on Odors: Their Detection, Measurement and Control, Rutgers University, New Brunswick, NJ. Reckner, L. R. and Squires, R. E., Diesel Exhaust Odor Measurement Using Human Panels, SAE Paper 680444. Defranco, L., The nose knows: Cyrano Sciences’ electronic nose, The Scientist web site, www.the-scientist.com/yr1999/july/tools1_990705. html. Staples, E. J., Real time characterization of food & beverages using an electronic nose with 500 orthogonal sensors and VaporPrint imaging, Sensors Expo Convention, Lake Tahoe, May 2000. NOSE web site, www.nose-network.org.

Bibliography Altpeter, T., Portable Natural Gas Odorant Analyzed, Gas Research Institute publication, undated. Amoore, J. E., Johnston, J. W., Jr., and Rubin, M., The stereochemical theory of odor, Sci. Am., 210(2), 42. Gardner, J. W., and Bartlett, P. N., Electronic Noses, OUP Press, Oxford, 1999.

8.38 Odor Detection

Lee, C., Odor control plan for hyperion treatment, ISA/93 Technical Conference, Chicago, September 19–24, 1993. Novak, J., Quantitative Analysis of Gas Chromatography, Marcel Dekker, New York, 1989. Pearce, T. C., Schiffman, S. S., Nagle, H. T., and Gardner, J. W., Eds., Handbook of Machine Olfaction, Vch Verlagsgesellschaft Mbh, 2003. Rafson, H. J., Ed., Odor and VOC Control Handbook, McGraw-Hill, New York, 1999.


1485

Reid, R. et al., Properties of Gases and Liquids, McGraw-Hill, New York, 1987. Shugar, G. and Dean, J., The Chemist’s Ready Reference Handbook, McGraw-Hill, New York, 1990. Sullivan, R. J., Preliminary Air Pollution Survey of Odorous Compounds, a literature review prepared under contract no. PH 22–68–25, National Air Pollution Control Administration Publication No. APTD 69–42. Waarvick, C., Automatic odor control systems, InTech, August 1993.

8.39

Oil in or on Water C. P. BLAKELEY

(1974, 1982)

TO RECEIVER

B. G. LIPTÁK

(1995)

I. VERHAPPEN

(2003) AT

OIL

Flow Sheet Symbol

Types of Designs:

A. Reflected light oil slick detector (on-off) B. Capacitance: available in probe form for interface detection or in a flow-through design or in a floating plate configuration for measuring the thickness of oil C. Ultraviolet (UV) D. Microwave (radio frequency): available as an interface probe, as a tape-operated tank profiler, or as an oil in water content detector E. Conductivity probes for interface detection F. Nuclear for interface detection

Range:

A. Generally from 0–50 ppm to 0–100% B. Flow-through dual-concentric detector from 5 to 15% water in oil C. 0–10 ppm to 0–150 ppm oil in water D. Oil content detectable from 0 to 100%

Inaccuracy:

A. Generally from 1 to 5% of full scale B. The flow-through dual-concentric detector has a sensitivity of about 0.05 to 0.1% water C. 0.1 ppm for a 0 to 10 ppm range D. Interface is detected to 5%; tank profile, to 1% or 3 cm, and water concentration, to 0.1%

Costs:

For capacitance, conductivity, and ultrasonic level probes, see Section 3.3, 3.4, and 3.20; for conductivity analyzers, see Section 8.17. C. $12,000 to $20,000 for dual-wavelength unit with auto-zero and 0–10 ppm to 0–150 ppm range


For capacitance, conductivity, and ultrasonic level probe suppliers, see Sections 3.3, 3.4, and 3.20; for conductivity analyzer suppliers, see section 8.17. Agar Corp. (D) (www.agarcorp.com) Bailey Controls Div. (www.abb.com) Caldon (D) (www.caldon.net) Delta C Technologies (B) (www.delta-c.com) Endress+Hauser Instruments (B) (www.endress.com) Invensys (Foxboro Co.) (B) (www.invensys.com) FMC Invalco (C) (www.fmcinvalco.com) Ohmart/Vega (F) (www.ohmart.com) Phase Dynamics (D) (www.phasedynamics.com) SeCap (C) (www.sentech.no) Synetix (F) (www.synetic.com) Teledyne Analytical Instruments (C) (www.teledyne-ai.com)

INTRODUCTION

PROCESS INDUSTRY MEASUREMENTS

The measurement of oil in or on water is an important requirement in both industrial and environmental pollution protection related applications. Both applications are discussed in this section, starting with the industrial process applications.

The most common applications of these sensors are for interface measurements between the layers of oil and water in tanks and pipelines. Conductivity, capacitance, and ultrasonic level probes (Sections 3.3, 3.4, 3.20) and probe or flow-through


8.39 Oil in or on Water

INNER ELECTRODE

POTTING COMPOUND

1487

OUTER ELECTRODE

OUTER ELECTRODE

BODY WELDMENT

MIDDLE INSULATING SLEEVE

OUTER INSULATING SLEEVE

INNER ELECTRODE

FIG. 8.39a Dual-concentric capacitance probe for the detection of water in oil.

conductivity analyzers (Section 8.17) are widely used for detecting such interfaces. To avoid redundancy, these devices will not be discussed in this section, but other capacitance and radio-frequency (microwave) types will be covered in the paragraphs that follow. In other applications, it is desirable to detect the amount of oil that is dispersed in a water stream. Ultraviolet analyzers are well suited for the measurement of oil in water and are discussed in Section 8.61. When the dispersed oil content increases, float- and displacement-type level sensors (Sections 3.7 and 3.8) and density detectors (Chapter 6) can also be considered. A radio-frequency (RF) type oil concentration detector will also be described below.

FIG. 8.39b Flow-through water-in-oil detector utilizing two concentric capacitance electrodes. (Courtesy of Endress+Hauser.)

In a radio-wave detector, the transmitter produces waves that are of fixed frequency and contain a constant amount of energy. The more of this energy that is absorbed by the process fluid (the more water is in the mixture), the lower will be the voltage at the detector. The advantages of this design (relative to capacitance systems) include wider range (0 to 100%), lower sensitivity to buildup, insensitivity to temperature and salinity variations, and suitability for highertemperature operations (up to 450°F, or 232°C).

Capacitance-Type Water-in-Oil Detectors The capacitance of water is much higher (its dielectric constant is about 80) than that of oil (about 2), so measuring the dielectric constant is a convenient way to tell them apart. In addition to conventional capacitance probes, special dualconcentric designs (Figure 8.39a) are also available to detect the interface between water and oil in tanks. In addition, flowthrough sensors are also available for in-pipeline applications. The flow-through version of the dual-concentric electrode design is illustrated in Figure 8.39b. Here, the electrodes consist of two concentric pipes that are insulated from each other, thereby forming the capacitor through which the process stream flows. The flanged spool piece is available in sizes from 2 to 8 in. (50 to 200 mm) and is designed for operation up to 150 PSIG (10.6 bar) and 212°F (100°C). The unit is available with switching or transmitting (4 to 20 mA) electronics. The water-in-oil sensor is most often applied for the purpose of setting the maximum amount of water that is allowed to be present in the oil. When that concentration is reached, the flow is diverted or other corrective action is taken. Radio-Frequency (Microwave) Sensors When a cup containing water and oil is placed in a microwave oven, the water will heat up, but the oil will not. This is because shortwave RF energy is absorbed much more efficiently by water than by oil.


Rag Layer and Tank Profiler Sensors The radio-wave oilin-water sensors are available as probe-type sensors for water–oil interface control. A typical application is free water knockout (Figure 8.39c), in which the probe is installed horizontally at an elevation of one-third of the diameter from the bottom and is set to open the water dump valve when the emulsion concentration drops below 20% oil (80% water). This way, the emulsion (rag layer) will build up above the probe. These instruments can also provide a 4 to 20 mA transmitted output signal and can detect water concentration within an error of about 5%. A portable tank profiler is also available that uses the same principle of operation. Here, the tape-supported radiowave element is gradually lowered into a tank, which can be up to 100 ft (30 m) tall. As the sensor is lowered, it measures both the location of the interface (within an error of 0.12 in., or 3 mm) and the emulsion concentration throughout the tank height (from 0 to 100% within an error of 1%) (Figure 8.39d). Water-in-Oil Probes A water-in-oil monitoring probe is also available that can detect the water concentration over a range of 0 to 100% in tanks or pipelines (Figure 8.39e) within an error of 0.1%. All of these devices are available in explosion-proof construction and can be provided with digital displays.

1488


OUTPUT VOLTAGE

OIL OUTLET

DETECTOR

OIL ANTENNA EMULSION

D

SAMPLES 20% OIL 115 v AIR

TRANSMITTER

80% WATER

D/3

WATER

SAMPLES

OIL/WATER INLET

WATER DUMP 9-18v DIRECT CURRENT

FIG. 8.39c Radio-wave oil–water interface detector probe. (Courtesy of Agar Corp.)

USE FLEXIBLE CABLE TO ALLOW FOR MOVEMENT

ELECTRONICS MODULE FRONT THUMBSCREWS REEL MODULE

4.20 mA O/P

CRANK 1/2" VALVE (12 mm) SEAL HOUSING

READ TAPE

POWER SUPPLY RELAY

AIR OIL SURFACE INTERMITTENT SOUND SIGNAL BEGINS

115/230 VAC OR 2" BALL VALVE 12-24 VDC (50 mm)

TANK OR PIPE WALL

% WATER IN OIL INDICATED

ANTENNA GUARD

OIL/WATER INTERFACE SOUND SIGNAL BECOMES CONSTANT

FIG. 8.39e Water-in-oil concentration can be detected within a 0.1% error. (Courtesy of Agar Corp.) WATER

FIG. 8.39d Tape-type tank profiler using radio-wave oil–water sensor. (Courtesy of Agar Corp.)

When applying this technology to measure oil in water, one must remember that there is a crossover point, at around 80% water concentration, at which the solution changes from being water continuous to oil continuous. This crossover results in a discontinuity in the analyzer output. Therefore, these devices suffer a significant loss in accuracy around this measurement point. In addition, many of these sensors are also sensitive to the salinity of the water phase. This can be corrected through the use of strapping tables, which should be selected by the user during calibration.


Conductivity and Capacitance Sensors There is a substantial difference between the conductivities of water and hydrocarbons. This difference is often used as the basis for detecting the interface between these two fluids. Vessel profiles can also be estimated by using an array of self-contained capacitance cells on a common mounting frame. If the appropriate software is available, the resultant capacitance profile of the vessel can be used to signal the multiple interfaces in that vessel to a resolution as good as 1 cm. Ultrasonic Sensors As was illustrated in Figure 6.7b, a flow-through ultrasonic densitometer is available for mounting between a pair of pipe flanges. It measures and transmits an analog signal


1489

FIG. 8.39f Wafer-type ultrasonic pipeline interface detector. (Courtesy of Caldon, Inc.)

proportional to the density and, hence, the amount of hydrocarbon in the flowing stream. By using a number of pipeline interface detectors (Figure 8.39f), it is possible to use this meter to estimate fluid viscosity, thus making it useful as an interface detector between batches of product in a pipeline. The time of passage of the ultrasonic pulse and the attenuation of the signal are used to calculate both the density and the viscosity of the flowing fluid. The viscosity is calculated from the slight rarefaction and compaction of molecules in the liquid, which cause viscous shearing of the fluid that absorbs acoustic energy. Kinematic fluid viscosity is determined from the energy loss of the meters’ acoustic pulses as they pass through the fluid. The analog outputs, which correspond to relative density and kinematic viscosity, can be used to detect the passing of an interface between two liquids in a pipeline. This information can be used to properly divert the flow into the correct tank. Relative density is determined by the change in sound velocity as a function of density and, because the object of the measurement is to compare the density of water against that of oil, an estimate of water or oil content can be made.

AMPLIFIER (LOCAL OR REMOTE)

STRIP SOURCE

20 FT MAX.

CELLS

FIG. 8.39g Strip source and cell receivers can be used for tank profiling.

As was the case for the conductivity array, software is then used to determine the actual interfaces between the phases. This interface is normally at the inflection point between two layers of differing densities. Ultraviolet Oil-in-Water Analyzer

Nuclear Sensors Several companies use the attenuation of gamma radiation to measure the density of the fluid between a source and detector, thus allowing a complete vessel profile and hence an estimation of the composition inside. One manufacturer of this technology uses an array of fixed sources and detectors, and others use a matched source and detector moving up and down in a well to provide a density profile (Figures 8.39g and 8.39h).


Figure 8.39i illustrates the sampling system of a continuous oil-in-water analyzer used for the monitoring of steam condensate, recycled cooling water, and refinery or offshore drilling effluents. The UV analyzer used in this system is a single-beam, dual-wavelength analyzer. This is superior to the singlewavelength designs, because it is able to compensate for variations in sample sediment content, turbidity, and algae concentration, and for window coatings. The cell operates

1490


the sample is sent through a high-speed, high-shear homogenizer, which disperses all suspended oil droplets and the oil, which is adsorbed onto foreign matter so that the sample sent to the analyzer becomes a uniform and true solution. Once per hour, the analyzer is automatically rezeroed. In this mode, the sample water is sent through a filter that removes all the oil, and, after sparging, it is sent to the analyzer. This oil-free zero-reference sample still contains all the other compounds as contained in the measurement. Therefore, it can be used to zero out this background.

DETECTOR SOURCE CAN INSERT AN ISOLATING GATE SHUT-OFF VALVE HERE

ENVIRONMENTAL POLLUTION SENSORS Oil floating on water forms a mechanical barrier between the air and the water. It prevents oxygenation and kills oxygenproducing vegetation on the banks of streams. By coating the gills of fish, these materials also prevent breathing and cause the fish to suffocate. Outfalls from ships and municipal or industrial waste treatment plants must therefore be monitored for oil, and oil must be removed to prevent oil-bearing wastes from entering the receiving waters. Continuous monitors are available to detect any hydrocarbons that are floating on the surface of the water. Oil in the water is equally undesirable, because it contributes to the biochemical oxygen demand (BOD) and can also be toxic to aquatic biota, to the fish food in water, and possibly to the fish themselves. Optical methods of detection, when used for both types of contamination, require regular, conscientious maintenance for continuous and reliable performance. The capacitance approach used for the monitoring of oil film thickness on the water appears to require less maintenance but is limited to detection of floating oil. Each application must be evaluated separately, because presently available sensors have limited capabilities.

FIG. 8.39h Backscatter-type traversing detector can detect interface levels while also drawing a specific gravity profile for the tank. (Courtesy of Ohmart Corp.)

according to Beer’s law [Equation 8.27(1)], which relates oil concentrations to UV energy absorption by the fixed length cell. The UV measuring band is centered at 254 nm, and the readings are sensitive to 0.1 ppm with a range of 0 to 10 ppm and provide a 90% response in 1 sec. The instrument is automatically zeroed by the sample water being sent to both the measurement and the zeroing sides of the conditioning system. When in the measurement mode,

DEAERATOR UTILITY WATER

SV 4

M

OVERFLOW VALVE

SAMPLE WATER HOMOGENIZER

SV 5 20PSIG AIR

PURGE VALVE

DRAIN SV 3

SAMPLE VALVE

UV SAMPLE CELL

PUMP

20PSIG AIR

3 MICRON OIL REMOVAL FILTER

SV 2

SV

FILTER VALVE

ZEROING VALVE

SPARGER

FIG. 8.39i Ultraviolet oil-in-water analyzer with automatic-zero feature. (Courtesy of Teledyne Analytical Instruments.)


DRAIN


On-Off Oil-on-Water Detector The nephelometer is intended to detect visible oil (hydrocarbon) slicks that are floating on fresh or salt water. It consists of two parts: a sensing head and a controller. The sensing head is inside an explosion-proof housing, which is supported on pontoons or floats in the body of water. An S-shaped baffle directs the flowing water past the sensing head. A beam of light is focused through a lens onto the surface of the water. A second lens refocuses the reflected light onto a photocell. When there is no oil on the water, only a minimum amount of reflection occurs. When floating oil is present, the reflected light intensity increases substantially. The measurement is based on the differential between the outputs of the reflected light photocell and a reference photocell, which is measuring the output of the light source itself. This instrument can provide an output signal that is proportional to the intensity of the reflected light, and it can also perform alarm functions. Care must be taken when using such reflected light measurements to block out sunlight and/or other stray light sources because, if they are allowed to be also reflected, they will introduce an error (typically on the high side). Continuous Oil-on-Water Detector Sensors similar to that deployed in the on-off detectors are also available to estimate the amount of oil floating on the water. The difference between the two applications is that, in this case, the Lambert–Beer law [Equation 8.27(1)] is used to determine the concentration from the detected light intensity. Oil-Thickness-on-Water Detector The previously described devices detect the presence or absence of oil floating on the water. This device permits the measurement of the thickness of the oil layer. The detector consists of a floating sensing head that is connected by a shielded cable to a remote controller. The sensor measures the thickness of the oil layer on the water by capacitance measurement (Figure 8.39j). The operating principle is that of a series capacitor. The formula given is in Equation 8.39(1). t t 1 t = = oil + water C εA ε oil A ε water A

where A = effective area of one capacitor plate t = thickness ε = dielectric constant Because εoil = 1.9 to 2.1, whereas εwater = 80, Equation 8.39(1) can be simplified by eliminating the second term, as shown in Equation 8.39(2). t 1 = oil C ε oil A

WATER

t OIL

t WATER

INVERSE CAPACITANCE MEASURING CIRCUIT

Oil-in-Water Detector When UV radiation is sent through an oil-contaminated water sample stream at a peak intensity of 365 nm, visible radiation is emitted. The intensity of this radiation can be measured by a photocell. The intensity of this radiation increases as the concentration of the fluorescent substance rises. At low –6 concentrations (below 15 × 10 ), the relationship between concentration and the visible radiation is essentially linear. In higher concentrations, some nonlinearity is experienced as a result of a saturation effect. The most common configuration is to pass the process sample through the sensing head in an upflow direction (Figure 8.39k). The head is equipped with two windows that are set at right angles to each other so as to minimize the intensity of direct radiation from the source striking the photocell and also to reduce the effect of multiple scattering of the visible radiation. Optical filters at the incident and at the SAMPLE OUT

8.39(1)

OUTPUT SIGNAL PROPORTIONAL TO OIL THICKNESS

∋ ∋

FIG. 8.39j Parallel-plate capacitor detecting the thickness of an oil layer on water.

WINDOW

PHOTOCELL

UV SOURCE

SAMPLE IN


8.39(2)

Thus, the inverse of capacitance is detected as a value that is proportional to the oil thickness. The circuit generates a DC signal in proportion to the inverse of capacitance and is used for remote transmission. The sensor takes advantage of the large differential in dielectric constants between oil and water. It is claimed that it is not confused by the presence of emulsified sludge (having a large dielectric constant) or by oily froth, which cannot pass under the float.

WINDOW

OIL

1491

FIG. 8.39k Oil-in-water detector.

1492


SAMPLE IN

SHAPING ORIFICE UV SOURCE

PHOTOCELL

TO WASTE

FIG 8.39l Falling-stream oil-in-water detector.

emergent windows are used (not shown) to reduce this effect to a negligible level. For the detection of oil concentration in water, a fallingstream type of detector is also available. In this device, the sample stream is shaped into a rectangle (Figure 8.39l) as it falls through the viewing field of the ultraviolet beam and the photocell. Efficient optical filtration is important to overcome the unavoidable effects of direct reflection of incident radiation from the surface of the shaped stream.

CONCLUSIONS The on-off oil-on-water detector is capable of measuring as little as a few drops of petroleum floating on the surface of water, thus making it possible to detect those oil pollution levels that are visible to the human eye. It therefore serves a useful purpose as an alarm device downstream of plant outfalls and especially during and immediately after oil loading and unloading operations from tankers and tank trucks. This device presents the maintenance problems usually associated with optical measurements, which is that windows must be kept clean. The air column between the water surface and the window does reduce fouling due to splashing, but window cleanliness must be maintained for maximum sensitivity.


The oil-thickness device (Figure 8.39j), being nonoptical, requires less maintenance. Because both devices can detect the absence or presence of oil slicks, they might also find application as oil spill monitors after oil transfer operations. Floating on the surface of wastewater storage sumps or lagoons, the output of the oil thickness monitor can be used to start and stop oil reclamation equipment. These devices cannot be calibrated for a specific oil fraction, but just respond to any floating hydrocarbon. The oil-in-water devices are optical, and even the fallingstream types require clean windows, although they are less subject to fouling than the sample chamber types. They must be calibrated for a specific type of oil, and other oil fractions will introduce errors. They were originally developed to monitor engine oil contamination in boiler feed water and condensate, which can be introduced by steam-driven feed water pumps. These devices detect the presence of a specific hydrocarbon fraction in well segregated waste streams. Where particle size is expected to exceed 5 µm, sample preparation prior to the UV analysis is necessary. Use of a high-shear mixer such as a blender has been found to produce a well dispersed suspension suitable for measurement.

Reference Zanker, K. J., Radio-frequency interface detector, Oil & Gas J., January 30, 1984.

Bibliography Adams, V., Water and Wastewater Examination Manual, Lewis, Chelsea, MI, 1990. Arashmid, M. et al., Analysis of the Phase Inversion Characteristics of Liquid-Liquid Dispersions, AIChE J., 26(1), 51–55, January 1980. Arnold, K. E., Design concepts for offshore produced water treating and disposal systems, J. Petroleum Technol., 276–283, February 1983. Basrawi, Y., Oil-water sampling & monitoring devices, their application to custody transfer, 47th Analysis Division Spring Symposium 2002, April 2002, ISA. Casamata, G. et al., Hydrocarbon separation through a liquid water membrane: modeling of permeation in a emulsion drop, AIChE J., 24(6), 945–949, November 1978. Crawford, H. M., Monitors detect oil in water, API Division of Refining Conference, Houston, May 1966. Jones, K. W., An optical sensor for the food/water interface in a process pipeline, 1992 ISA Conference, Houston, October 1992. Lucas, R. N., Performance of heavy oil: dehydrators, J. Petroleum Technol., 1285–1291, October 1969. Maples, R. R. et al., Interface detector on oil treater cuts oil losses and maintenance, Chem. Process., 88–89, July 1983.

8.40

Open Path Spectrophotometry (UV, IR, FT-IR)

ASAH

AST

J. M. JARVIS

RECEIVER

(2003)

TRANSMITTER AST

Flow Sheet Symbol

Applications:

Ambient air or fence-line monitoring for the detection of toxic or hazardous vapors for emissions monitoring as well as combustible vapors in personnel safety applications.

Types of Devices:

A. FTIR B. Combustibles detection C. Tunable diode laser D. UV

Costs:

A. $86,000 for minimal instrument configuration; $150,000 typical installed cost B. $5,000 to $7,000 C. $35,000 to 75,000, depending on configuration D. $ 25,000

Partial List of Vendors:

Air Instruments and Measurements (A) (www.aimanalysis.com) AIL Systems Inc. (A) (www.ail.com) Boreal Laser Inc. (C) (www.boreal-laser.com) Detector Electronics (B) (www.detronics.com) Draeger Safety Inc. (B, D) (www.draeger.com) General Monitors (B) (www.generalmonitors.com) Industrial Monitor and Control Corp. (A) (www.imacc-instruments.com) Midac Corporation (A) (www.midac.com) Norsk Elektro Optikk (C) (www.neo.no) OPSIS (C, D) (www.opsis.se) Sieger div. Zellweger Analytics (B) (www.zelana.com) Siemens Environmental Systems (D) (www.siemens.co.uk) Simrad Optronics ASA (B) (www.simrad-optronics.com) Spectrex Inc. (B, D) (www.spectrex-inc.com) Thermo-Environmental Instruments (D) (www.thermoei.com) Unisearch Associates (A, C) (www.unisearch-associates.com)

INTRODUCTION The information in this section is used to augment the sections on air quality monitoring (8.5), carbon monoxide (8.10), hydrogen sulfide (8.26), nitrogen oxide analyzers (8.37), ozone (8.44), sulfur oxide analyzers (8.56), and toxic gas monitoring (8.59). For combustible monitoring, this section augments information in sections on combustibles (8.16) and hydrocarbons (8.25).

From the standpoint of instrument design, there is also some overlap of information with sections covering infrared analyzers (8.27) and ultraviolet and visible analyzers (8.61). This section confines itself to the discussion of instruments based on absorption. It omits discussion of LIDAR (acronym for LIght Detection And Ranging, an abbreviation formed in analogy with RADAR) techniques because of their very specialized nature. Also omitted is the discussion of gas cloud imaging technology, which is based on backscatter 1493


1494


absorption. This technology is very new and has not yet been widely adopted in practical industrial situations. Applications Open path monitoring is used for analytical measurements in remote and inaccessible locations. The technique is also used for perimeter monitoring of structures and facilities. Such monitoring is either to detect the release of low levels of toxic and hazardous vapors, or it is to signal the releases of combustible hydrocarbons at relatively much higher levels. The use of open path monitoring is driven by either the need for probing an area without physical intrusion or by the need to monitor an area that is larger than one that can be cost effectively monitored with a requisite number of point detectors. Open path monitoring is a subset of the techniques collectively called long path monitoring. Long path monitoring includes the use of multipass reflection cells to cover distances as great as hundreds of meters and achieve high detection sensitivity. The two general uses of open path vapor detection, toxic and combustible detection, have resulted in instruments that have evolved specifically for each application. Open path toxic gas detection is generally used for very low-level detection— often in fence-line monitoring roles for estimation of emissions from a facility. There are many more open path combustible hydrocarbon (OP-HC) detectors in use than there are toxic detectors. In a petrochemical plant, there can be as many as 150 OPHC detectors, but only about half a dozen or fewer open path toxic detectors. Combustible detection requires much less instrument sensitivity, but instruments are highly engineered to provide much more protection against false alarms and to deliver high availability in excess of 99.9%. Immunity to false alarms is very important, because, when an alarm is actuated, executive action is frequently taken. Nondispersive infrared (NDIR) spectroscopy is most often used in OP-HC detectors. Toxic Sensor Types The types of open path toxic gas detector designs include open path Fourier-transform infrared (OP-FTIR) spectroscopy. This is a very commonly used technique that utilizes

IR Source

a number of very specific absorption bands in the infrared spectrum. A single instrument can be configured for the measurement of a wide variety of multiple components, and these sensors are highly adaptable for survey work. In applications in which FTIR does not have sufficient sensitivity, open path ultraviolet (OP-UV) spectroscopy is frequently employed. This methodology can be used for applications involving the detection of homonuclear diatomic molecules (chlorine, bromine, etc.), which have no infrared absorption, or of molecules that absorb only weakly in the IR region, such as benzene, sulfur dioxide, and nitrogen oxides. Because low-cost, highly reliable, solid state diode lasers have been developed and became available for high-volume telecommunications applications, a new class of open path detectors has been developed and applied to a subset of toxic measurement applications. Instruments in this category utilize the ability of the diode laser to scan over very short wavelength intervals. This method of measurement is referred to as open path tunable diode-laser absorption spectroscopy (OP-TDLAS). INSTRUMENT DESIGNS The four measurement technologies, OP-FTIR, OP-UV, OPTDLAS, and OP-HC, all basically employ spectrophotometers designed for operation in a particular part of the spectrum. All spectrophotometers contain a light source, some transfer optics, a spectral dispersion element or optical filters, and optical detectors. Figure 8.40a shows the primary components or all four classes of sensors. All these designs have a light source that illuminates the gas in the open path. The light source is collimated with transfer optics and is directed through the open path length to the receiving optics. The light is transferred through dispersing and filtering elements and is finally received by the optical detector. Table 8.40b shows how these elements differ in the four designs. Note in Figure 8.40a that the dispersion elements can be utilized to predisperse the light prior to its passing through the open path sample, or it can be dispersed after passing through the sample. In OP-FTIR, the interferometer dispersing element is most often configured to modulate and predisperse the light prior to passing through the sample. This configuration

Transfer Optics

IR Detector Open Path Sample Region

Dispersing/Filtering Element and Transfer Optics in Two Possible Locations

FIG. 8.40a A schematic of a generalized open path sensor.


8.40 Open Path Spectrophotometry (UV, IR, FT-IR)

1495

TABLE 8.40b Common Components of Open Path Spectrophotometers OP-FTIR

OP-HC

Wavelength range (µm)

Atmospheric transmission windows between 2.94 to 4.16 and 8.0 to 14.3

2.3 plus an adjacent reference wavelength band

Single wavelengths between 0.8 and 1.7

OP-TDLAS

Atmospheric transmission window starting at 0.260 extending to visible

OP-UV

Light source

Heated globar

Xenon flash lamp or tungsten filament

Solid-state diode-laser

High pressure xenon arc

Dispersion element

Michelson interferometer*

Narrow-band optical filters*

None required except for electronics to scan laser wavelength

Grating spectrometer

Transfer optics

Gold coated mirrors

Glass lenses or mirrors

Glass lenses or mirrors

Silver mirrors

Optical detector(s)

Liquid nitrogen cooled mercury cadmium telluride photodiode

Indium galium arsenide photodiode

Indium galium arsenide photodiode

Linear CCD array

*These items are not dispersion elements in the classical sense but nevertheless serve the eventual purpose of isolating wavelengths for subsequent radiometric analysis by the optical detectors. Wavenumber (cm−1) 100,000

10,000

1,000

100

HYDROCARBON COMBUSTIBLES SENSORS 300K Blackbody Background Emission

UV SENSORS

TDLAS SENSORS FTIR SENSOR Atmospheric Transmission Windows

0.1

1

10

100

Wavelength (micron)

FIG. 8.40c The regimes of operation in the electromagnetic spectrum for various path sensors. A 300 K (27°C) blackbody emission spectrum is superimposed on the diagram. Note that atmospheric absorption limits the range of FTIR measurements to two atmospheric transmission windows.

prevents thermal (blackbody) radiation from being added to the spectrum of the IR source. Blackbody Radiation Interference As shown in Figure 8.40c, there is significant IR emission in the ambient (27°C) blackbody curve that lies within the usable wavelength range of OP-FTIR systems. (The principles behind FTIR are discussed in more detail in Section 8.27.) The effect of uncompensated thermal background radiation is to introduce a bias and/or variability in the measured gas concentrations that have absorption bands that lie in this region.


The amount of ambient blackbody radiation is generally insignificant at the shorter wavelengths and has little affect on OP-HC, TDLAS, and OP-UV measurements. For these instruments, the dispersing element generally resides with the detector. This architecture provides a convenient, compact, and simple instrument implementation. There can be extenuating circumstances, however, if the detector occasionally is pointed in a direction that has reflected sunlight. Sunlight approximates a 6000 K blackbody and subsequently has significant radiation through the visible portion of the spectrum into the UV. If proper consideration is not given to this potential interference in the

1496


design of the instrument, incorrect concentration measurements will be made. Interferometry Before the 1980s, infrared measurements were made using dispersive spectrometers employing diffraction gratings. Today, as a result of low-cost computing, improved optical designs, and optical detectors, Fourier transform interferometers have almost totally replaced dispersive spectrometers for this task. Interferometers have much increased optical efficiency and, as a result, yield readings faster and with higher signalto-noise ratios, and offer higher spectral resolution than corresponding dispersive instruments. Additionally, as a fundamental consequence of interferometry, wavelength measurement precision is vastly improved over dispersive spectrometers. However, as advantageous as interferometry is in the infrared portion of the spectrum, the advantages quickly disappear at shorter wavelengths. At visible and UV wavelengths, the optical efficiency advantage is nullified by vastly more efficient optical detectors and light sources. (The principles behind UV spectrometry are discussed in more detail in Section 8.61.) The mechanical tolerances for interferometery at short wavelengths become prohibitive as well. Therefore, dispersive spectrometry is still widely used for short wavelengths. Beer’s Law and Path Integrated Concentrations Open path absorption spectrophotometry takes advantage of the fact that a beam of photons can interact with molecules in the atmosphere, depending on the photons’ wavelength. As the photons interact with and are absorbed by the molecules, they are removed from the beam. The reduction in the number of photons in the beam is a measure of the number (or concentration) of molecules in their path. Beer’s law describes the light absorption phenomenon. For light at a given wavelength, I(λ), and a gas concentration profile that varies as a function of position, c(l), over the optical path, L, this phenomenon can be expressed in the differential form in Equation 8.40(1). dI (λ ) = ε (λ ) ⋅ c(l ) ⋅ dl I (λ )

∫

Io

L

 I (λ )  dI (λ ) = ε (λ ) ⋅ c(l ) ⋅ dl ⇒ − ln   = ε (λ ) ⋅ c ⋅ L I (λ )  Io(λ ) 

∫ 0

8.40(2)


Path Integrated Concentration The concentration profile integrated over the path length is termed path integrated concentration (PIC). It is simply the PAC multiplied by the measurement path length. PIC has units of mass per unit 3 volume (often µg/m ) times pathlength in meters. Using the ideal gas law, the temperature and pressure at which the measurements were made and the molecular weight of the species being measured, these units can be converted to the more familiar volumetric units of ppm*m or ppb*m in the case of toxic vapor determination. For combustible gas determination, the more appropriate units are in terms of the lower explosive limit, LEL*m. An open path instrument natively measures PIC. This is frequently misunderstood by newcomers to the field. An open path instrument operating over a 100-m path length will produce the same PIC reading for a narrow, concentrated 1-m thick plume having a vapor concentration of 300 ppm as it will for a 3-ppm vapor cloud dispersed across the entire 100-m path length. In each of these situations, the PAC would be 3 ppm. This is illustrated in Figure 8.40d. OPEN PATH FTIR SPECTROMETRY Open path FTIR (OP-FTIR) instrumentation consists of five major components: an IR light source, interferometer, transmitter and receiver transfer optics, IR detector, and data system/controller. The IR light source is generally a glow bar with high emissivity across the IR spectrum. 300ppm*1m Gas Plume

8.40(1)

The term ε (λ) is called the absorptivity coefficient. This coefficient is determined by the characteristics of the absorbing molecule and depends strongly on the wavelength and more weakly on the temperature and pressure. Generally, the concentration profile, c(l), is not known. If the concentration is assumed to be a constant and uniform average value over the optical path, c , refered to as the path average concentration (PAC), then the equation can be integrated to a more familiar form of the Beer’s law in Equation 8.40(2). I

In this equation, Io(λ) is the original intensity of the optical beam at a given wavelength, and I(λ) is the intensity of the beam after absorption by the molecules in the atmosphere. The fraction I(λ)/Io(λ) is termed the transmittance, as it denotes the fraction of transmitted light through the atmosphere. The negative logarithm of the transmittance is termed absorbance.

3ppm*100m Gas Cloud

Separation distance = 100m Path Integrated Conc. = 300ppm*1m = 3ppm*100m = 300ppm*m Path Average Concentration = 300ppm*m/100m = 3ppm

FIG. 8.40d Illustration of path integrated concentration and relationship to path average concetration for narrow plumes and widely dispersed vapor clouds.


Interferometer The interferometer is generally one of the Michelson variants and is used to modulate the light as a function of wavelength. The interferometer utilizes a moving mirror to generate an optical path difference between two beams of light that are allowed to interfere with each other. The position of the moving mirror must be measured to a tolerance of a fraction of the wavelength of light. This is frequently accomplished by using a highly stable tracking laser and counting interference fringes. The moving mirror and the requirement for high-precision measurement of the optical path difference are two of the vulnerabilities of the interferometer mechanism. They limit reliability over time, operating temperature, and vibration. Much effort has been put into the design of interferometer systems to increase their reliability. Transfer Optics and the Detector Transfer optics are generally telescopes of the Newtonian or Cassegrainian configuration. Reflective optics are required, because inexpensive lenses that can cover the full useful range of wavelengths are not available. The optical detector receives the modulated light. A cooled mercury cadmium telluride (MCT) detector is most often used to obtain the required sensitivity for the measurement. The detectors are cooled using liquid nitrogen (LN2) or by a Sterling cycle heat pump. The LN2 dewar must be refilled every few days, which adds to operating maintenance costs. Sterling cycle coolers reduce the daily maintenance burden but are relatively expensive and have relatively short lifespans. In some applications, in which a relatively low-resolution measurement can be tolerated, deuterated triglycine sulfate (DTGS) detectors operating at room temperature can be used to address this maintenance issue. Data System/Controller Finally, the data system/controller performs a myriad of functions. It monitors the operation of the interferometer and synchronizes the collection of spectral data from the IR detector. It demodulates the spectral data using a fast Fourier transform algorithm and then does spectral manipulation to ratio the background and to compensate for background interferences and variation. As a final step, the corrected spectrum is processed by a pattern recognition and/or quantification algorithm that calculates path average concentration (e.g., average ppm) or path integrated concentrations (e.g., ppm*m). Configurations There are a number of instrumental configurations for OPFTIR instruments. The simplest OP-FTIR systems are the socalled bistatic configurations. In these configurations, either a light source illuminates the open path region and is postmodulated with the interferometer, or the light source is premodulated before illuminating the open path region. The


1497

arrangement of the components of this design is shown in Figure 8.40e. This configuration derives its name from the fact that both the transmitter and receiver must be fixed in a static position and precisely aimed at each other. The first real-time OP-FTIR systems had a direct source, postmodulated bistatic configuration. This configuration has the advantage of simplicity and hence is the least expensive design, but it also has a major disadvantage. This a result of the varying thermal emission of near field objects that produce a varying spectral background at frequencies below –1 2000 cm . If uncompensated, this background will produce a negative bias on the measured concentration. Most of this varying background emission and its effects can be compensated. This is done by turning off the source and by subtracting the spectrum while it is off. The direct source configuration has the advantage of being easily adaptable to a passive monitoring configuration where the thermal emission from the background or hot gas plumes act as the infrared source. The premodulated bistatic configuration solves the background emission problem, because the source modulation occurs much faster than do changes in background emissions. However, since the detector and interferometer are separated by some distance, an additional cable is required to communicate between them. Bistatic configurations in general have the requirement of supplying power at both the receiver and transmitter, which can be a disadvantage in some locations. Additionally, there is a requirement for alignment at both receiver and transmitter, which can be time consuming for mobile systems. It may be less of an issue for permanent fixed systems. The so-called monostatic configurations were developed to address issues raised with bistatic designs. In a monostatic configuration, all of the optical components of the transmitter and receiver are in the same location, and a retroreflector is used to return the light from the transmitter to the receiver. This configuration derives its name from the fact that only the transceiver portion of the instrument needs to be precisely pointed, as the retroreflector returns light to its source regardless of orientation. A diagram of two monostatic configurations is shown in Figure 8.40f. The single-telescope monostatic configuration is relatively simple to implement and is inexpensive. The use of a single telescope makes alignment simpler and lowers costs. The corner-cube array returns the light to the direction from which it came. This property reduces the divergence of the beam on its return path back to the detector compared to the divergence that would result from a flat mirror. Also, the retroreflector array can be very large so as to capture and return essentially all of the divergent signal from the telescope. However, this design requires a beam splitter in the optical path that removes 50% of the light from the outgoing beam and 50% of the light from the return beam for an overall loss of 75% of the total light intensity. The dual-telescope monostatic configuration has greater optical efficiency, as it does not utilize a beam splitter in the

1498


IR Detector

Transfer Optics

Receiver Telescope

Open Atmosphere

Transmitter Telescope

Interferometer

IR Source

Controller/Data System

IR Source Transmitter Telescope

Open Atmosphere

Receiver Telescope

Interferometer

IR Detector


FIG. 8.40e Bistatic configurations for OP-FTIR. Post-modulated configuration (top) uses interferometer to modulate the light after it passes through the open path. The premodulated configuration (bottom) uses the interferometer to modulate the light prior to passing through the open path.

optical path. It utilizes a translating retroreflector, which is essentially a portion of a very large cube. This single, large retroreflector does not have the divergence reversal properties of the corner cube array. The second telescope adds cost and complexity to the system. OP-FTIR sensitivities for various molecular species are listed in Table 8.40g.

OPEN PATH ULTRAVIOLET SPECTROMETRY Open path ultraviolet (OP-UV) spectrometry can be used to measure vapors or gases that have weak absorption characteristics and therefore low sensitivities in the IR spectrum. These include such compounds as nitrogen oxides, sulfur dioxide, benzene, and also homonuclear diatomic molecules, such as chlorine, that have no IR absorption spectra but acceptably strong UV absorption. The UV spectra are much less specific than the IR spectra and do not have well-defined and separated absorption features. The number of compounds that can be determined by


UV are much fewer than ones that are absorbing in the IR spectra. Still, the above listed and environmentally very important compounds make OP-UV an important complementary methodology. The OP-UV Spectrometer A schematic of an OP-UV spectrometer with a monostatic configuration is shown in Figure 8.40h. Bistatic configurations are available as well and are generally provided with fixed source. Sources are generally high-pressure xenon arc lamps that emit a continuum from the UV to the visible spectra. The spatial extent of the arc can be very small, on the order of 250 µm, which lends itself to collimation with very low divergence. As a result, UV instruments can operate with path lengths up to several kilometers. The long distances are facilitated by the low optical divergence and by the fact that water vapor and carbon dioxide do not absorb in the UV spectra as they do in the infrared. On the other hand, the short UV wavelengths can be strongly


1499

IR Source Transmitter Telescope

Open Atmosphere

Interferometer

Transfer Optics


Retroreflector

IR Detector IR Source Transmitter Telescope

Open Atmosphere

Translating Retroreflector

Interferometer


IR Detector

Receiver Telescope

FIG. 8.40f Two monostatic configurations for OP-FTIR both using premodulated configurations. The single telescope configuration (top) uses a cornercube retroreflector array and is less optically efficient. The dual telescope configuration (bottom) is more optically efficient, but requires more telescope complexity.

scattered by aerosols such as fog, snow, smoke, and dust, which greatly degrade their performance. The short wavelength limit is determined by interference of molecular oxygen at wavelengths shorter than 260 nm. The detector depicted in Figure 8.40h is a solid state array detector such as a CCD. The array detector permits the acquisition of intensities from multiple wavelengths simultaneously and without mechanically moving parts. The spectra of many molecules in the UV, notably SO2 and NO, in most cases have periodic absorption patterns resulting from the interaction of the vibrational energy levels with the electronics absorption. The overall absorption band can be quite wide and broadly overlapping. However, the periodicity can be used to identify the absorbing species. A Fourier transform of the UV absorption spectrum is often used to pretreat the data to isolate the absorbing species based on the spatial periodicity. The peaks thus produced can be further treated with spectral quantification algorithms such as the classical least squares method.


OP-UV has been used to detect and quantify the following gases: NO, NO2, NO3, formaldehyde, ozone, SO2, benzene, toluene, and o, m, p-xylenes. Table 8.40i lists selected gases and detection limits available with OP-UV.

OPEN PATH TUNABLE DIODE LASER SPECTROMETRY Open path tunable diode laser spectrometry (OP-TDLAS) is a relatively recent technology that has been applied commercially only to air monitoring within the past decade. Recently, these instruments have been made sufficiently rugged and reasonably simple to use in other applications. Previously, lead salt diodes were used, emitting laser radiation in the 3–20 µm mid-IR portion of the IR spectrum. Instruments based on these lasers were capable of making very sensitive measurements, sometimes in the parts-pertrillion range. They were also very fast, having measurement times as low as 1/10 sec.

1500


TABLE 8.40g Typical Detection Limits for Open Path FTIR* PI-MDC Field Data** (ppm*m)

PI-MDC Noise Limited (ppm*m)

acetaldehyde

6

0.081

acetic acid

1.5

acetone

9 15

0.13

hydrogen sulfide

0.006

isobutane

Species

acetonitrile acetylene

0.6

PI-MDC Field Data (ppm*m)


hydrocarbon continuum

3

0.06

0.088

hydrogen chloride

0.6

0.071

1.3

hydrogen cyanide

1.5

0.016

Species

90 0.6

51.2 0.027

acrolein

1.5

0.15

isobutanol

1.2

0.071

acrylic acid

3

0.088

isobutyl acetate

1.5

0.024

acrylonitrile

1.8

0.179

isobutylene

1.2

0.064

ammonia

0.6

0.057

isoprene

1.2

0.06

benzene

7.5

0.6

isopropanol

3

0.084

1,3-butadiene

0.6

0.054

isopropyl ether

3

0.038

butanol

4.5

0.114

methanol

1.2

0.077

1-butene

3

0.145

methylamine

6

0.137

cis-2-butene

7.5

0.276

methyl benzoate

6

0.038

trans-2-butene

3

0.145

methyl chloride

18

0.289

butyl acetate

1.5

0.027

methylene chloride

1.5

0.068

carbon monoxide

0.3

0.086

methyl ether

3

0.089

carbon tetrachloride

0.6

0.01

methyl ethyl ketone

carbonyl sulfide

0.6

0.014

methyl isobutyl ketone

12

chlorobenzene

3

0.159

methyl mercaptan

chloroethane

3

0.16

methyl methacrylate

1.5

0.06

chloroform

0.6

0.021

2-methyl propene

0.6

0.03

m-cresol

6

0.132

morpholine

0.6

0.016

o-cresol

1.2

0.063

nitric acid

0.3

0.037

p-cresol

3

0.077

nitric oxide

7.5

0.26

4.5 12

15

0.197 0.072 0.462

cyclohexane

0.9

0.01

nitrogen dioxide

1,ibromoethane

1.5

0.101

nitrous acid

m-dichlorobenzene

0.9

0.027

ozone

0.9

0.104

o-dichlorobenzene

0.9

0.025

phosgene

0.3

0.015

p-dichlorobenzene

0.6

0.05

phosphine

0.6

0.109

1,1-dichloroethane

3

0.104

propane

3

0.057

1.5

0.045 0.013

1,2-dichloroethane

9

0.095

propanol

6

0.137

1,1-dichloroehtylene

0.6

0.03

propionaldehyde

3

0.186

dimethylamine

6

1.72

propylene

1.2

0.107

dimethyl disulfide

3

0.101

propylene dichloride

3

0.247

1,4 dimethyl piperazine

0.9

0.034

propylene oxide

3

0.157

1,4 dioxane

0.6

0.037

pyridine

6

0.057

ethane

3

0.094

silane

0.3

etanol

3

0.158

styrene

0.3

0.138

ethyl acetate

1.2

0.025

sulfur dioxide

9

0.046

ethylamine

6

0.131

1,1,1,2-tetrachloroethane

1.2

0.059

ethylbenzene

6

0.119

1,1,2,2-tetrachloroethane

6

0.174

ethylene

0.3

0.041

tetrachloroethylene

0.6

0.059



1501

TABLE 8.40g Continued Typical Detection Limits for Open Path FTIR* PI-MDC Field Data** (ppm*m)


ethylene oxide

3

0.025

toluene

7.5

0.073

ethyl mercaptan

15

Species

Species

PI-MDC Field Data (ppm*m)


0.14

1,1,1-trichloroethane

1.2

0.031

formaldehyde

1.5

0.139

1,1,2-trichloroethane

3

0.067

formic acid

0.6

0.034

trichloroethylene

0.6

0.06

furan

0.9

0.02

trimethylamine

3

0.038

halocarb-11 (CCl3F)

0.3

0.012

1,2,4-trimethylbenzene

1.5

0.098

halocarb-12 (CCl2F2)

0.3

0.016

vinyl chloride

1.2

0.097

halocarb-22 (CHClF2)

0.3

0.017

m-xylene

3

0.111

halocarb-113 (CFCl2CF2Cl)

0.6

0.033

o-xylene

6

0.044

hexafluoropropene

0.3

0.019

p-xylene

6

0.102

*Courtesy of Industrial Monitoring and Control Corporation. **PI-MDC = Path Integrated-Minimum Detectable Concentration. Field data typical for atmospheric path with 20,000 ppm H2O, 360 ppm CO2, and other atmospheric gases present. −5 Noise-limited levels assume an RMS noise of 5 × 10 using reasonably accessible bands.

Beamsplitter UV Source

Transmitter Telescope

Open Atmosphere


Retroreflector

Array Detector

Slit Reflection Grating

Grating Spectrograph

FIG. 8.40h Schematic representation of an OP-UV spectrometer.

TABLE 8.40i Minimum Detection Limits for Selected Gases by OP-UV* Species Nitrogen Oxide

Detection Limit (ppb*m)** 100–225

Benzene

42–150

Ammonia

310–5870

*From Reference 1. **Integration times vary from 1 to 5 min. Pathlength varied from 100 to 250 m.


On the other hand, complexity and cost limited their commercial acceptance. The lead salt lasers required liquid nitrogen cooling for operation, and IR absorptions accessible to the lasers were subject to pressure broadening effects, which limited their use to point monitoring applications using multipass cells operating at reduced pressure. Diode Lasers Diode lasers originally developed and manufactured in large volumes for telecommunications applications have been adopted to OP-TDLAS applications.

1502


Open Atmosphere

Telescope Detector

Reference Gas Cell

Reference Detector

Retroreflector Thermostated and Collimated Laser Diode


FIG. 8.40j Schematic representation of an OP-TDLAS spectrometer in monostatic configuration.

Various diodes operate at or near room temperature and emit radiation between 0.6 and 2.5 µm in the near-IR portion of the spectrum. Wavelength tuning over relatively short wavelength ranges is achieved by temperature tuning as well as by varying the injection current into the diode itself. Temperature tuning is most often performed using a thermostat, which controls a thermoelectric cooler. Current tuning is performed using a programmable current source and is done at relatively high frequencies in the kilohertz range. The absorption ranges that are accessible with these nearIR wavelengths are less sensitive than those in the mid-IR, but they are less susceptible to pressure-broadening effects. This permits their use in the open path monitoring applications. The reduction in detection sensitivity is partially recovered by the use of high-frequency wavelength modulation and by improved signal-detection techniques. Detection limits in the range of low ppm*m of target molecules are generally achievable. Because these diode lasers are used in large volumes for telecommunications applications, their costs are an order of magnitude or two lower than those of lead salt diodes. Additionally, with near-IR wavelengths, traditional and low-cost glass optical materials can be used to fabricate the supporting optical assemblies instead of exotic and expensive mid-IR optical materials. Applications Molecules having absorption spectra that are accessible with diode lasers include NO, NO2, HF, HCl, HCN, HI, NH3, C2H2, CO, CO2, H2S, and CH4. In some fortuitous instances, diode lasers have wide enough tunability to simultaneously measure multiple gases that have closely spaced absorption features. This is the case with HF and CH4 and with CO and CO2. Generally, however, a separate laser is required for each gas analyte of interest. Some commercial devices permit the


fitting of more than one laser into a device so that multiple gases can be measured. Principle of Operation A schematic diagram illustrating the principle of operation of an OP-TDLAS is shown in Figure 8.40j. The monostatic configuration using a retroreflector is representative of commercial architectures. In this configuration, laser light from a diode laser is directed to a beam splitter and then onto a steering mirror in the telescope, which directs the light out into the open atmosphere. The light interacts with the target gas molecules over a one-way optical path of up to 1 km and eventually falls onto a retroreflector array. The retroreflector array returns the light back to the telescope, which serves to focus it onto the optical detector. The small portion of the light intercepted by the beam splitter is directed through a reference cell where a sample of the target analyte gas is present. The absorption of light by the gas in the reference cell is used to generate a feedback signal to the diode laser thermostat to keep the laser wavelength accurately tuned to the center of a gas absorption line. Wavelength Modulation Spectrometry All commercial OPTDLAS devices use some form of wavelength modulation to perform the measurement. The principle of wavelength modulation spectrometry is shown in Figure 8.40k. With this methodology, the laser, which is tuned to the center of an absorption line, is modulated at high frequency about the absorption maximum. Temperature tuning is used to adjust the diode laser output to the center of the absorption line. Current tuning using a programmable current source is used to modulate the laser wavelength at kilohertz frequencies. As the laser wavelength


1503

Spectral Absorption Line Transmission

1 1−α

δδ

Wavelength Modulation Extent

λo Wavelength

Wavelength Modulation

λo + δ

λ(t) = λo + δsin (2π ft)

λo λo − δ cos (2π 2ft)

Detected Laser Power

lo Low Pass Filter

lo(1 − α)

Concentration Signal

Lock-In Detection

lo(t) = lo(1 − α/2) − α/2cos (2π 2ft) Time

FIG. 8.40k Principle of wavelength modulation spectrometry—The laser wavelength is tuned to an absorption line and modulated at frequency f. The laser intensity as it passes through the sample is modulated at frequency 2f. The concentration signal is recovered using lock-in detection techniques.

passes through the peak absorption wavelength, the detected laser intensity drops and then increases as the laser moves to one side of the line or the other. A laser wavelength modulation frequency of f produces an intensity modulation of the laser at frequency 2f because of absorption of the analyte gas. The 2f intensity modulation is detected using lock-in detection. Lock-in detection at high frequency provides signal-to-noise enhancement that somewhat negates the general low absorptivity of the absorption lines. Measurements are typically made at a rate of one sample per second. Table 8.40l lists some representative gases of interest for air monitoring that can be measured using TDLAS along with their approximate detection limits. For industrial applications, HF monitoring by OP-TDLAS has proven to be very useful because of the low detection limits achievable and because of the lack of interference.

OPEN PATH DETECTION OF COMBUSTIBLES A very important use of open path gas detection is to measure the concentration of combustible vapors. These vapors typically are hydrocarbons such as methane, ethylene, and so on, so the abbreviation OP-HC is frequently used. Detection is not strictly limited to pure hydrocarbons but can also be used


TABLE 8.40l* Some Representative Gases and Approximate PIC Detection Limits for OP-TDLAS Assuming Ability 5 to Measure Absorbance to 1 Part in 10 Species HF

Detection Limit (ppm*m) 0.2

H2S

20

NH3

5.0

CH4

1.0

HCl

0.15

HCN

1.0

CO

40

NO

30

NO2

0.2

*From Frish, M. B., White, M. A., and Allen, M. G., SPIE Paper No. 4199–05, 2000.

to detect a number of organic vapors having a near-IR absorption spectrum due to CH bonds. In terms of numbers of detectors sold, this application far dominates the market. OP-HC detectors can be found in the hundreds in oil and gas production facilities and, in lesser

1504


numbers, in downstream transportation and distribution facilities as well as petrochemical facilities. In recent years, OP-HC detectors have seen increased usage to augment detection by traditional point hydrocarbon detectors, especially in situations where the probability of detecting a leak with a point detector is low. OP-HC detection is especially useful for perimeter monitoring of tank farms, process areas, and other places where combustible vapor leaks can happen over a widely dispersed area. Because the emphasis is on safety applications, these detectors are optimized for low maintenance, avoidance of false alarms, and low costs. These detectors typically are housed in flameproof enclosures and are suitable for deployment into Class 1, Division 1, or CENELEC Zone 1 hazardous areas. These devices typically have on-board heaters to melt snow and ice so that they can operate unattended and uninterrupted in inclement weather. OP-HC Detector Design Figure 8.40m shows a simplified schematic representation of an OP-HC detector. The bistatic configuration is most commonly used. Monostatic configurations using a retroreflector require electrical power for heaters and subsequent flameproofed enclosure, which somewhat reduces the advantage of that configuration for this application. The detection principle relies on a two-channel nondispersive photometer. The active channel is equipped with an optical filter that limits light to the detector to the hydrocarbon absorption region of the spectrum. The reference channel observes the intensity of light in an adjacent portion of the spectrum that is free from hydrocarbon absorption. The optical filters are carefully specified to minimize false gas signals from differential absorption resulting from moisture as well as changes in the spectral output characteristics of the source over time.

to mitigate against solar interference, either directly as the sun enters the field of view of the detector at sunrise or sunset or indirectly as reflections from waves in offshore installations or other background objects. Solar radiation that falls within the field of view of the receiver and that has intensity modulated frequency components within the acceptance bandwidth of the receiver electronics can result in a false gas signal. Source modulation is performed at frequencies where there is minimal solar modulation. The short pulse duration of the xenon lamp permits the utilization of a very narrow acceptance bandwidth filter in the receiver electronics. Transmitter–Receiver Separation Distances between the transmitter and receiver vary typically between 10 and >120 m. Most vendors supply at least two models: a short-range unit for operation from 10 to 60 m, and a longer-range unit operating to the >120-m distance. The primary difference between the units is the efficiency of the collimation and condensing optics. In most cases, the worst-case visibility, resulting from fog and other atmospheric phenomena, presents the practical limit to transmitter–receiver separation. Table 8.40n illustrates this point by showing how the maximum acceptable transmitter–receiver separation varies

TABLE 8.40n* Typical Maximum Distance Between Transmitter and Receiver Over Which Proper OP-HC Operation can be Maintained at a Worst-case Fog Visibility Worst-Case Visibility in Fog** (m)

Sources and Interference These devices utilize bright, modulated sources with receiver detection electronics tuned to the modulation frequency. Typically, a xenon flash lamp is used; less typically, a modulated tungsten filament or microfabricated IR source is used. The source is modulated

Control Electronics

Receiver Active Channel IR Optical Filter Detector

Reference Channel

Signal Output to Safety System

FIG. 8.40m Schematic representation of an NDIR-based OP-HC detector.


Transmitter/Receiver Separation Distance (m)

6

10

15

25

18

30

24

40

30

50

36

60

*Courtesy of Detector Electronics. **As measured by Meteorological Optical Range.

Open Atmosphere

Transmitter IR Source


with visibility in fog. Additionally, for very short-range applications 75 µm but, with careful design, down to a few microns G. 1 to 100 µm H. 25 to 600 µm For continuous airborne particle counters: 0 to 10 million or 0 to 1 billion particles/ cubic foot; ranges are selectable to count particles that are larger than 0.3, 0.5, 1, 3, 3 5, or 10 µm. Sensitivity can be as high as 1 mg/m , and particles down to microns are detected. For on-stream particle-size monitors (dry powders or slurries), particle-size distribution data can be measured between 2 to 300 µm.

Inaccuracy:

5% (particle size distribution)

Costs:

A, E, G. $6000 to $18,000 B. $25,000 to $100,000 C. $40,000 to $80,000 D. $15,000 to $40,000 F, I. $1000 to $8000 H. $6000 for regular sludge densitometer, up to $50,000 for size distribution detection with referee method


Boreal Laser Inc. (D) (www.boreal-laser.com) California Measurements Inc. (E) (www.californiameasurements.com) Cilas Laser Particle Size Analyzers (D) (www.cilasus.com) Climet Instruments Co. (D) (www.drugdiscoveryline.com) Fisher Scientific (F, I) (www1.fishersci.com) Horiba Instruments (D) (www.horiba-particle.co.uk) Joyce Loebl (F) (www.joyce-loebl.co.uk) Malvern Instruments (G, I) (www.malvern.co.uk) Markland Specialty Engineering Ltd. (H) (www.sludgecontrols.com) Norsk Elektro Optikk (D) (www.neo.no)


1560


OPSIS (D) (www.opsis.se) TSI Products (J) (www.tsi.com) Unisearch Associates (D) (www.unisearch-associates.com)

INTRODUCTION As is the case with many other sections in this handbook, the coverage of this section also overlaps with some of the others. Particle-size-based particulate separation has been discussed in Section 8.46, whereas optical and laser-type suspended solids detection is also discussed in Section 8.60. This section starts with a discussion of the subject of particle size distribution. After that, the designs of the various laboratory-type particle monitors are described. These units are classified according to sample size into small (a few 0.01 g in the sample), intermediate (a few grams), and large (a few 100 g) units. The last topic in this section is a discussion of continuous, on-line particle-size analyzers.

PARTICLE SIZE AND DISTRIBUTION When the purpose of measurement is to monitor clean rooms, semiconductor production facilities, and other atmospheres that are kept clean by HEPA filters, the analyzers must be able to detect particles down to 0.3 µm in size and must be 3 sensitive down to the concentration of 1 mg/m . When the purpose of the measurement is product quality control in dry-powder or slurry processes (e.g., pigments, catalysts, clays, cements, and plastic powders), the analyzers provide data on both particle size distribution and cumulative (percent smaller than) data in the range of 3 to 200 µm (Figure 8.47a). Application Objectives Accurate determination of particle size and distribution is critical in many industrial processes, such as grinding, agglomeration, crystallization, and emulsification. Objectives may include improving final product quality, as in ceramics, paints, and pigments; improving final product performance,

as in abrasives and catalysts; improving process efficiency, as in crystallization and wastewater treatment; or minimizing energy consumption, as in grinding of ore for subsequent processing. In many processes, more than one of these objectives may be involved. This section discusses the range of particle size measurement from approximately 0.01 to about 1000 µm. The goal of these measurements is determining the quality of material in a process rather than the detection of contaminants in an otherwise particle-free stream. The performance characteristics noted in this section are attainable from standard commercial instruments. Detectors and Sampling Systems Particle size detectors differ in measurement principles and in size range, resolution, and concentration capabilities. Choosing the right measurement principle for the parameter of interest is important, but, irrespective of the method used, the reported particle size can also be affected by other factors such as shape or physical properties. Therefore, it is important to understand the critical particle characteristics for a particular process before the method of measurement is selected. Accurate size measurement also depends on obtaining a representative sample of the bulk material. Bias toward smaller or larger sizes can occur if sampling is done at the wrong place or time in the process, or if a sample is extracted from a settled heap. The size measurement will be of value only if the sample represents the process. The amount of material required for the analysis depends on the measurement technique. In some processes, it can be difficult to obtain a representative sample when only a small fraction of the collected material is inspected. One of the criteria by which particle size measuring techniques are classified is the amount of material inspected. µ

PARTICLE SIZE

RELATIVE VOLUME

%

mV = MEAN VOLUME (AVERAGE SIZE)

mV

µ PARTICLE SIZE (LOG SCALE)

0.1

1

10

50

90

PERCENT PASSING

FIG. 8.47a Typical displays of particle size distribution.


98

99.9

%

8.47 Particle Size and Distribution Monitors

LABORATORY MONITORS From the perspective of sample size, the measurement technique can use small (a few hundredths of a gram), intermediate (a few grams), or large (hundreds of grams) amounts of sample material. These three categories will be separately discussed in the following paragraphs. Sensing Small Samples Optical Microscopy Optical microscopy is often used as a standard method for particle size analysis to calibrate other particle-sizing instruments. Size determination is based on the operator’s criteria and judgment of parameters such as the longest dimension, the diameter of a sphere of equivalent cross section, and so on. When there is a large range of particle sizes, many particles must be counted to convert a number distribution to a weight distribution. Size resolution is limited by the wavelength of light, which restricts the practical use to particles that are larger than about 1 µm. The depth of the field, or the region in which an object is in sharp focus, decreases as the magnification is increased. Size errors can arise from out-of-focus imagery, especially with spherical particles. Electron Microscopy In this case, a fine electron beam is scanned across a sample, producing secondary electrons and other emissions from the material. The emissions are detected and displayed on a cathode ray tube to form an image, such as in a TV picture. Operator interaction and judgment are required to determine the size. The size resolution is limited by the energy of the electron beam, and 0.006 to 0.01 µm resolution is typical for modestly priced equipment. Depth of field of the image is about 300 times greater than in the case of optical microscopy. However, a common source of error is in the evaluation of too few fields of particles. Image Analyzers Automating the analysis of the image of a particle field reduces much of the tedium of counting large numbers of particles. Sophisticated electronic routines, augmented by an editing pointer, provide for rapid and error-free analysis of complex shapes. The input to an image analyzer is a high-resolution TV camera. Therefore, this technique must be combined with another device, such as an optical or electron microscope. Electrical Sensing (Coulter Principle) When this method is used, the sample is suspended in an electrolyte, and the particles are drawn through a small orifice that is immersed between two electrodes. Each particle, as it passes through this orifice, displaces its own volume of the electrolyte and produces a momentary resistance change in proportion to the volume displaced. The pulses are then amplified, scaled, and separated into counting bins. Some level of training and skill is required to obtain good reproducibility, but, with such skill, high reso-


1561

lution can be achieved for narrow distributions. The size range is determined by the orifice diameter. For very broad distributions, blockage of the orifice can be troublesome, and different orifice sizes must be used to cover the full range. The practical lower particle size limit is around 0.5 µm. Optical Scattering (Single Particle) When a light beam interacts with a particle, light is scattered or diffracted. The angular distribution and amount of scattered light depend on the particle size. The passage of individual particles generates scattered-light pulses, which are detected as they pass a probe beam. These pulses are assigned to counting bins according to their sizes. The dynamic range of the electronics used will limit the size range, and the measurement accuracy is influenced by the composition of the particles, which are less than a few micrometers in size. The lowest size commonly measured is about 1 µm for a water-suspended material and a few tenths of 1 µm for an air-suspended material. Low signals for small particles and coincidence at high sample concentrations are the principal limitations. Sensing Intermediate Samples Light Scattering (Multiple Particle) When many particles are viewed by a light beam at the same time, a composite scattering pattern is generated. Several algorithms have been developed to extract the size distribution information from this scattering pattern. By using low-angle scattering and optical analog filtering, the Microtrac approach generates a signal in proportion to the volume of material present in the light beam. The volume size histogram is calculated through a microprocessor program. By adding signals collected at high angles, which are used for measuring particle sizes under 1 µm, the range obtained by this technique can cover from 0.1 to 1000 µm. Resolution is lower than for single-particle analysis. Inaccuracy is a function of composition effects for particles below a few micrometers, but no special training or skill is needed to make the measurement. Material can be suspended in either air or liquid. Other instruments also exist that utilize intermediate-size samples and operate on principles of forward scattering, but they are limited to particles larger than 1 µm. Sedimentation (Photo and X-Ray) Photo sedimentation combines gravitational settling with optical attenuation. The sorting of the particles is based on their hydrodynamic properties, and sizing is based on the cumulative particulate cross section as a function of settling time. Care must be taken to avoid too rapid settling, convection currents, and improper or incomplete correction for optical scattering variations as a function of size and composition. Photo sedimentation is limited to the size range of 1 to 100 µm. By replacing the optical beam with a well collimated x-ray beam, this range can be extended to cover particles of a few tenths of 1 µm in size. Centrifugal sedimentation is also useful in this submicrometer size range.

1562


Sensing Large Samples Sieving This method is best suited to particle sizes that are predominantly coarser than 75 µm. In the sieving operation, the sample is shaken through a series of containers whose bases have regularly spaced, uniform-sized openings. The primary measure is the mass of material retained in each sieve, and the distribution is most often plotted as cumulative fraction vs. the nominal sieve aperture. Several sieve series exist, including Tyler and the American Society for Testing and Materials (ASTM) in the United States, and British and German standards in Europe. Fixed ratios of sizes of square root or fourth root of 2 are common. For particles smaller than about 75 µm, sieving becomes difficult. Problems include the increased cost of producing sieves with uniform apertures and the greater sophistication required of the mechanics of the sieving process. Wet, sonic, and air-jet sieving have all been used for this range, and, with care, particles down to a few micrometers can be handled. Optical Methods Laboratories can also use transmissometry (light absorption across a sample), turbidity detection (light scattered at 90° to the light beam), and backscattering techniques, but because all three of these methods are also sensitive to particle size and composition changes, they are best suited for on-line monitoring of process streams.

Ultrasonic Attenuation When ultrasonic energy is transmitted through a slurry (Figure 6.7b), the amount of energy absorbed is dependent on the particle size, concentration, and ultrasonic frequency. In one approach, two pairs of sensors, each consisting of a transducer and a receiver, operate at two different frequencies. For a given material, a correlation can be established between the size distribution as measured by a referee method and the attenuation of the higher-frequency beam. Corrections resulting from loading changes are based on the lowerfrequency signal. This technique is restricted to particles between 25 to 600 µm, but high concentrations can be accommodated, which makes this approach attractive for on-line control. Entrained air bubbles can cause errors, so they must be avoided or eliminated. ON-LINE MEASUREMENT Particle size analysis for industrial process control is especially valuable if the measurement can be rapid and continuous. The fully automated system can be complex, even if the size measurement technique is simple. Optical Multiple-Particle Analyzer As an example of a complete system, Figure 8.47b illustrates the a package that includes components designed to extract MONITOR SECTION

VOLTAGE OUTPUTS CURRENT OUTPUTS

E/I CONVERTER

DIGITAL OUTPUTS

D/A CONVERTER

STATUS AND ALARM

DIGITAL DISPLAY AND OPERATOR CONTROLS

MICRO-COMPUTER A/D CONVERTER

PREAMP

SLURRY FEED OPTICAL BENCH ASSEMBLY

SAMPLE CELL

SAMPLE EXTRACTOR

SAMPLING PROGRAMMER TIMER AND LEVEL CONTROLLER

DEMAGNETIZER

DILUTION AND AGITATION TANK

WATER SUPPLY DILUTION WATER CONDITIONER

DRAIN PUMP

SAMPLE PUMP

SAMPLE CONDITIONER SECTION SLURRY RETURN

FIG. 8.47b Block diagram of complete system for outline process control using an optical particle analyzer.


8.47 Particle Size and Distribution Monitors

1563

SAMPLE INLET VIEW VOLUME SENSOR CHAMBER CONE MIRROR LAMP

PHOTOMULTIPLIER DETECTOR SAMPLE INLET

CALIBRATOR

OUTLET PURGE AIR VALVE

SAMPLE OUTLET SENSOR CHAMBER

PUMP

FILTER

BYPASS VALVE

FLOW METER

FIG. 8.47c The elliptical-mirror-type airborne-particulate counter is used in clean rooms and is available with automatic sampling. (Courtesy of Climet Instrument Co.)

a sample, measure the particle size through an optical multiple-particle analyzer, and report a specific measurement of the distribution for control. As shown in Figure 8.47b, in the first step of an analysis cycle, water is charged into a mixing chamber through a debubbling circuit that prevents the air bubbles from the entering water. A circulating pump then agitates the water in the mixing chamber and circulates it through the sample cell, where the laser beam detector of the monitoring section is located. When the slurry extraction by the sample extractor is actuated, a particulate sample is drawn and deposited into the mixing chamber. The circulating water carries the particles of the slurry through the sample cell. At the end of the measurement cycle, the mixing chamber is drained, rinsed, and refilled with water in preparation for the next sample. The analyzer is capable of operating with wet slurry or with dry powder samples, and successive samples can be analyzed every few (2 to 15) min. The sample size is between 3 and 10 g, and the results are printed or displayed on particle size distribution curves (Figure 8.47a). The analyzer consumes about 1.5 gal (6 li) of water per cycle and 4.2 SCFH (120 l/h) of air. The range of the analyzer is 0 to 200 µm, with 2.8 µm being the minimum reading. Airborne-Particulate Counter Figure 8.47c illustrates the elliptical-mirror-type airborneparticle counter used in sizing and classifying powdered products or in inspecting the performance of HEPA filters, which are widely used in clean rooms.


The unit operates with a 0.25 SCFM (1 lpm) sample, which is directed to the focal point of its elliptical mirror, where the focused incident light is scattered by the particles in the sample. The scattered (Tydall) light is collected by the elliptical mirror and is focused onto the photomultiplier detector. Because the mirror completely surrounds the particles in the sample, it collects more light energy and therefore is capable of detecting smaller and fewer particles. The minimum particle size it is capable of detecting is about 0.3 µm, and the unit can operate at particle densities of up to 10 million 3 3 particles/ft (353 million particles/m ). The air sampling system includes a positive-displacement pump and a pressure regulator (Figure 8.47c). The return air from the sensing chamber is filtered by an absolute filter and is reused as purge air. This purge air fills the sensor chamber and also surrounds (sheathes) the sample airstream to prevent the particles in the sample from entering the sensor chamber.

Bibliography Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocken, PA, 2002. Allen, T., Particle Size Measurement, Chapman and Hall, London, UK. Brassington, D. J., Tunable diode laser absorption spectroscopy for the measurement of atmospheric trace species, Adv. Spectroscopy, 24, 83– 148, 1995. Clarke, A. G., Industrial Air Pollution Monitoring—Gaseous and Particulate Emissions, Kluwer Academic, New York, 1997.

1564


EPA’s National Ambient Air Quality Standards, U.S. Government Printing Office, Washington, D.C., 1998. Herdan, G. and Smith, M. L., Small Particle Statistics, Elsevier, New York, 1953. Implementation of Clean Air Act, U.S. Government Printing Office, Washington, D.C., 1997. Lodge, J. P., Methods of Air Sampling and Analysis, Lewis Publishers, Boca Raton, FL, 1988. Particle Size Distribution in the exhaust of Diesel and Gasoline Engines, Society of Automotive Engineers, Warrendale, PA, 2000. Particulate Continuous Emission Monitoring (CD ROM), American Institute of Chemical Engineers, New York, 1999.


Prouder, T., Particle Size Distribution III, American Chemical Society, Washington, D.C., 1998. Smith, D., et al., EPA’s Sampling and Analysis Methods Database, Lewis Publishers, Boca Raton, FL, 1990. Tate, J. D., Chauval, P., Taylor, K., Industrial applications of optical sensing, Proc. 89th Annual Meeting and Exhibition of Air and Waste Management Assoc., Nashville, TN, June 23, 1996. Weaver, R., Continuous emission monitoring, Meas. Control, June 1992. Weiss, M. D., Particles size analysis goes on line, Control, August 1990. Wertheimer, A. L., Frock, H. N., and Muly, E. C., Light scattering instrumentation for particulate measurements in processes, SPIE, 129, 1977.

8.48

pH Measurement A. C. BLAKE (1969) T. S. LIGHT (1972) D. L. HOYLE, T. J. MYRON (1974, 1982) J. R. GRAY (2003)

AIT

AE

G. K. McMILLAN

(1995)

pH

Flow Sheet Symbol

Standard Design Pressures:

Vacuum to 100 PSIG (7 bars); special assemblies to 500 PSIG (35 bars)

Standard Design Temperatures:

Generally, 23 to 212°F (−5 to 100°C); sterilizable, −22 to 266°F (−30 to 130°C); Glasteel, (> R(AgCl)

R(PROCESS SOLN.)

R(LIQUID JCT) >> R(FILL SOLN.)

FIG. 8.48gg pH sensor impedance diagnostic measurements.

8.48 pH Measurement

1583

As a result, the impedance reading must be temperature compensated to avoid false low-impedance alarms. In some cases with low-impedance glass electrodes at high temperatures, the impedance drops below the measurable range of the diagnostic circuitry, and the analyzer shuts off the lowimpedance alarm. The glass impedance measurement is most useful in detecting glass electrode cracking or breakage, which results in a sudden drop in impedance. Low-impedance alarms are also useful in detecting shorts in the sensor wiring, whereas high-impedance alarms can detect an open circuit or severe coating of the pH electrode.

exercised to ensure that the reference electrolyte does not contaminate the buffer. For separate electrodes, polarization should be avoided. High-ionic-strength buffers should be employed for highionic-strength process applications. A process calibration should be performed 1 to 8 hr after the electrode is commissioned. (The wait time depends on the degree of solidification of the reference fill.) A zero or standardization adjustment is used to make the pH sensor agree with a sample that was measured with fast and accurate laboratory electrodes immediately after sample withdrawal.

Reference Electrode Impedance Reference electrode impedance is necessarily measured between the reference electrode and a solution ground and is typically in the low kilohm range. The major source of reference electrode impedance is the liquid junction and, thus, an increase in reference impedance can indicate that the liquid junction is plugged or coated or that the reference electrode is not in the process solution. The reference electrode impedance is a much more sensitive indication of coating than glass electrode impedance. It should be noted that poisoning of the reference electrode by process components does not result in an increase in reference electrode impedance unless it is accompanied by a plugging of the liquid junction. A temperature sensor, either internal or external to the pH sensor, is required for temperature compensation and can also be a source of sensor failure. pH sensor diagnostic features can also include detection of opens, shorts, and sense line errors.

Microprocessor-based pH analyzers often have features that help the user to avoid buffer calibration errors. Buffer values as a function of temperature are stored in memory for a twopoint calibration, which allows the buffer value to be corrected for temperature during calibration. There is also a stabilization feature that prevents that analyzer from accepting a millivolt reading that has not fully responded to the buffer or millivolt changes due to warming or cooling of the sensor. Important diagnostic information is also provided after a buffer calibration in the form of the electrode slope, which is a measure of the sensitivity of the electrode and the zero offset, which indicates the reference electrode offset from an ideal value or asymmetry within the glass electrode. Warnings are usually provided to prevent acceptance of a calibration that results in a slope and zero offset indicative of a bad sensor or a user error. Analyzers can also perform fully automatic calibrations if provided with the software and relays for controlling withdrawal of the sensor from the process through an insertion valve, rinsing, and introduction of the buffers. pH electrodes should be stored in either a 4-pH buffer solution to help condition the glass surface or a 7-pH buffer solution to help prevent the loss of ions from the reference fill. Electrodes stored in distilled water for long periods of time will deteriorate as a result of the loss of ions from the measurement glass reference fill.

Sensor Fault Signaling Sensor diagnostic errors are indicated on the analyzer display, and there is almost always a fault relay contact and fault current output value provided for remote indication of sensor faults. With the advent of pH analyzers with digital communications, the nature of the sensor fault can be identified remotely, enabling maintenance personnel to be better prepared to address the problem before visiting the installation.

Buffer Calibration Errors

CONCLUSION CALIBRATION The electrode should be washed with distilled water between buffer immersions. One should also remember that the actual pH of the buffer of process solutions can change with 20 temperature or with carbon dioxide absorption, and the reference junction can take a long time to equilibrate. A 10-pH buffer can drop 0.1 pH per day as a result of carbon dioxide absorption, and, in general, all buffers should be discarded after use and the stock buffer containers tightly sealed. The pH sensor should be allowed to come to the same temperature as the buffer solution. All buffers should be checked with an accurate laboratory electrode and for expiration date before use. For flowing junctions, care must be


The pH sensor should be chosen to meet the pH range of the application. The mounting method should be chosen to make the sensor easily accessible for maintenance and calibration. Most pH applications that involve water and dilute solutions at near ambient temperature are straightforward and pose few problems. Special care should be taken with applications involving components that can poison the reference or foul the sensor, nonaqueous solvents, low conductivity, and extremes of temperature and pressure. In applications where the nominal pH is expected to be at the high or low limits of the 0 to 14 pH range, a conductivity measurement should be considered as an alternative measurement of acid and base concentration.

1584


It should be remembered that the pH of solutions can change with temperature. This is particularly the case with solutions that have a neutral or basic pH. It never hurts to consult several sensor suppliers as to the suitability of their various designs to a particular process application.

17. Guisti, A. L. and Hougen, J. O., Dynamics of pH electrodes, Control Eng., 8(4), 136–140, April 1961. 18. Hershkovitch, H. Z. and McAvoy, T. J., Dynamic Modeling of pH Electrodes, Can. J. Chem. Eng., 56, 346–353, June 1978. 19. McMillan, G. K., Tuning and Control Loop Performance, 2nd ed., Instrument Society of America, Research Triangle Park, NC, 1990. 20. Ingold, W., Calibration of pH electrodes, Ingold technical publication E-TH-5–2-CH, 9–10.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

McMillan, G. K., Understand some basic truths of pH measurement, Chem. Eng. Prog., 87(10), October 1991. McMillan, G. K., pH Control, Instrument Society of America, Research Triangle Park, NC, 1984. Shinskey, F. G, pH and pION Control in Process and Waste Streams, John Wiley & Sons, New York, 1973. McMillan, G. K., Biochemical Measurement and Control, Instrument Society of America, Research Triangle Park, NC, 1987. McMillan, G. K., Woody’s performance review—what’s inline next for pH control, InTech, 35(1), January 1988. Bates, R. G., Determination of pH Theory and Practice, John Wiley & Sons, New York, 1964, 11–12. McMillan, G. K., pH control—a magical mystery tour, In Tech, 37(9), September 1984. Kalis, G., How accurate is your on-line pH analyzer, InTech, 37(6), June 1990. Lichtenstein, S. New concepts in the state-of-the art of pH measurement, ISA 1983 Proc., 87664–741, 461–471. Ingold, W., pH electrodes storage, ageing, testing and regeneration, Ingold technical publication E-TH 7–2-CH. Ingold, W., Principles and problems of pH measurement, Ingold technical publication E-TH 1–2-CH, 17–18. Ingold, W., Principles and problems of pH measurement, Ingold technical publication E-TH 1–2-CH, 19–20. Bates, R. G., Determination of pH Theory and Practice, John Wiley & Sons, New York, 1964, 201–288. Moore, R. L., Good pH measurement in bad process streams, Instrum. Control Sys., 63(12), 39–43, 1990. Rushton, C. and Bottom, A., Measuring pH in low conductivity waters, Kent Tech. Rev. (UK), 24, March 1979. Disteche, A. and Dubuisson, M., Transient response of the glass electrode to pH step variations, Rev. Sci. Instrum., 25(9), 869–875, September 1954.


Bibliography Butler, J. L., Ionic Equilibrium—A Mathematical Approach, AddisonWesley, Boston, MA, 1964. How can pH probe fouling be reduced? Control, October 1992. Gray, J. R., Glass pH electrode aging characteristics, Proc. ISA/93 Tech. Conf., Chicago, IL, September, 1993. Hoyle, D. L., The effect of process design on pH and pION control, Proc. ISA-AID Symp., San Francisco, May 3, 1972. Galster, H., pH Measurement: Fundamentals, Methods, Applications, Instrumentation, Wiley-VCH, New York, 1991. Ingold, W., pH measurement and temperature compensation, Ingold technical publication E-TH-8–2-CH. Kardos, P., Improving pH measurement and control, Instrum. Control Sys., April 1981. McMillan, G. K., A Funny Thing Happened on the Way to the Control Room, Instrument Society of America, Research Triangle Park, NC, 1989, 55–64. Merriman, D. C., Junction potential variations in pH, Proc. ISA/93 Technical Conference, Chicago, IL, September 19–24, 1993. Mooney, E. F., On-line photometric titrations for process control, Proc. 1992 ISA Conference, Houston, TX, October 1992. Moore, R. L., Neutralization of Waste by pH Control, Instrument Society of America, Research Triangle Park, NC, 1978. Pfannenstiel, E. Process pH measurement, InTech, October 2002. Piovoso, M. J. and Williams, J. M., Self tuning pH control, InTech, May 1985. Proudfoot, C. G. et al., Self-tuning PI control of a pH neutralization process, Proc. Inst. Elec. Eng., 130(5), 1983. Skoog, D. A. and West, D. M., Principles of Instrument Analysis, 2nd ed., Saunders College, 1980. Weiss, M. D., Teaching old electrodes new tricks, Control, July 1991. Wescott, C. C., pH Measurement, Academic Press, New York, 1978.

8.49

Phosphorus Analyzer W. H. PARTH

(1974, 1982)

B. G. LIPTÁK

TO RECEIVER

(1995, 2003)

AT PHOSPHORUS Flow Sheet Symbol


A. Colorimetric B. Flame photometric C. Chromatographic

Operating Pressure:

Atmospheric


Most analyzers can be obtained with wetted parts made out of stainless steel, glass,  or Teflon

Inaccuracy:

2 to 3% of full scale

Analysis Time:

From a fraction of a minute to 15 min

Ranges:

A. From 0−5 ppm to 0−20 ppm B. From 1 to 100 ppm The overall capability of measurement for all types is from 0−10 ppb to 0−100 ppm.

Costs:

Laboratory units cost $10,000 to $25,000; industrial installations with sampling system included cost from $25,000 to $100,000. Vane-type filters for sewage applications cost about $2500. (Also refer to Sections 8.2, 8.12, 8.13, 8.15, 8.65, and 8.66 for sampling system, filter, and analyzer costs.)


Also refer to Sections 8.2, 8.12, 8.13, 8.15, 8.65, and 8.66 for sampling system, filter, and analyzer suppliers. Bran+Luebbe (www.branluebbe.com) Hach Co. (www.hach.com) Ionics Inc. (www.ionics.com) Metorex (www.metorex.fi) Rosemount Analytical Inc. (www.processanalytic.com) Rosemount Inc. Div. of Emerson (www.rosemount.com) Thermo Orion (www.thermo.com) Zellweger Analytics (www.zelana.com/)

INTRODUCTION Analyzers used in measuring the concentration of phosphorus have already been discussed under chromatographic (Sections 8.12 and 8.13) and colorimetric (Section 8.15) analyzers and will also be discussed under water quality and wet-chemistry analyzers (Sections 8.65 and 8.66). For this reason, the detailed designs of these analyzers are not repeated in this section. Phosphorus in Wastewater The principal application of phosphorus analyzers is in the control of phosphate removal in sewage treatment plants. By knowing the flow rate and the phosphorus content of raw

sewage, the required optimal quantities of chemical additives can be determined. In addition, a measurement of the phosphorus remaining after treatment may be desired. Phosphorus occurs in wastewater almost entirely in the form of phosphates, including orthophosphates, condensed phosphates (pyrophosphate, metaphosphate, and polyphosphate), and as organically bound phosphates. The various methods of phosphorus detection do not all respond to the total phosphorus present. Other applications are those specific to the control of phosphate addition to high-pressure boiler water as a corrosion inhibitor, and to the measurement of elemental phosphorus in the effluent from a plant that extracts phosphorus from ore. 1585


1586


CALIBRATION SOLUTION

SAMPLE

HEATER

CONSTANT HEAD TANK

S

DRAIN REFERENCE COIL P COLORIMETER REFESAMPLE RENCE FLOW FLOW CELL CELL

BUBBLE TRAP

DELAY COIL

MIXING COIL 2

MIXING COIL 1

DRAIN

U M P

AMMONIUM MOLYBDATE AMINONAPHTHOL-SULFONIC ACID DRAIN

DRAIN

FIG. 8.49a Orthophosphate analyzer.

COLORIMETRIC ANALYSIS

Colorimetric procedures are available to determine the concentration of soluble orthophosphate. In the commonly used 1 aminonaphtholsulfonic acid method of analysis, ammonium molybdate reacts with a dilute phosphorus solution to produce molybdophosphoric acid. This acid is than reduced to the intensely colored complex, molybdenum blue, by the combination of aminonaphtholsulfonic acid and sulfite reducing agents. 1 The stannous chloride method, although slightly more sensitive, is similar to the method just described except for the substitution of stannous chloride for aminonaphtholsulfonic acid as the reducing agent.

as the addition of ammonium molybdate and aminonaphtholsulfonic acid solutions. The reaction and reference samples pass through separate delay coils. In one coil, which gives about a five-minute delay, the reaction sample and the reagents complete the color reaction; in the other delay coil, the reference sample is experiencing the same delay, in addition to the time that the reaction sample spends in the mixing coils. The sample and reference streams from the delay coils are fed into the dual flow cells of a dual-beam colorimeter. A bubble trap ahead of the colorimeter removes any bubbles formed in the analyzer. The photodetectors sense the difference in color intensity between the reaction and reference samples. In this case, the molybdenum blue complex is measured at a wavelength of 6900 Å. The electronic section amplifies the colorimeter output and, if necessary, linearizes it. The use of a dual-beam colorimeter automatically compensates for the variations in inherent color or turbidity of the sample.

Continuous Analyzer

Total Phosphates

A continuous phosphorus analyzer consists of a sample temperature controller, a multiple peristaltic pump for reagent and sample metering, a mixing and time-delay section, reagent storage, a colorimeter, and an electronic readout section. Referring to Figure 8.49a, the water sample is brought to a constant temperature to ensure uniform sample reaction and rapid response time. The sample stream is degassed in the constant head tank and divided into two paths, one for the reference sample and the other for the reaction sample. A multiple peristaltic pump meters the sample streams as well

Total inorganic phosphate can be measured by first hydrolyzing the sample with sulfuric acid at 95°C (203°F). This converts the phosphates in the meta-, pyro-, and polyforms to orthophosphates. Total phosphates (inorganic plus organic) can be determined by an additional step consisting of oxidizing organic compounds (e.g., by boiling in potassium persulfate) to split off the phosphate moiety, which is then available for reaction. Instruments that use a single-beam colorimeter in either a continuous or batch-type analyzer are also available.

For a detailed discussion of colorimetric analyzers, refer to Section 8.15. Laboratory Methods


8.49 Phosphorus Analyzer

1587

HEATED EXHAUST

PULSED FLAME EMISSION

BAND FILTER MIRROR

PHOTOMULTIPLIER TUBE OUTPUT SIGNAL TO AMPLIFIER AND DISPLAYS

S HYDROGEN

C 5

10

15

20

F1

FILTERED WATER SAMPLE

F1

NEBULIZER

FIG. 8.49c Flame photometric detector of phosphorus in water.

P

0

CLEAN AIR

25

5260 Å. The output from the photomultiplier tube is linear over several decades. The detector can measure phosphorus in water by the addition of a nebulizer, which injects a mist of sample water into a clean air stream at a constant rate. The output is linear in the 1 to 100 ppm range.

TIME (msec)

FIG. 8.49b The separation in the time domain of phosphorus, sulfur, and carbon spectra. This PFPD analyzer provides infinite selectivity against haydrocarbon emission as well as unique heteroaton identification 3 capability.

FLAME PHOTOMETRIC ANALYSIS This analyzer detects the photometric flame emission of phosphorous compounds in a hydrogen–air flame. This method was developed by Draegerwerk in Germany and was first applied to the detection of phosphorus compounds in air. As discussed in connection with Figure 8.12y, the method 2 later was used as a detector for gas chromatography. Still later, the pulsed flame photometric detector (Figure 8.12cc) was developed, which uses the pulsed flame characteristics 3 of phosphorus shown in Figure 8.49b. Detector Operation The detector in Figure 8.49c contains a burner with separate delivery tubes for hydrogen and the air sample. If phosphorus is present in the hydrogen-rich (reducing) flame, it will produce a strong luminescent emission between the wavelengths of 4850 and 5650 Å. This emission is at its maximum at 5260 Å and is isolated by a narrow bandpass interference filter. The hydrogen and air are burned in a hollow tip that shields the flame from direct view of the mirror and photomultiplier tube. When phosphorus is present, the emission occurs above the shielded flame, and the light is transmitted directly and by way of the mirror through the filter to the photomultiplier tube. The shield offers specificity from carbon dioxide and hydrocarbons with flame emission at


GAS AND LIQUID CHROMATOGRAPHY Where elemental phosphorus discharges into water, it has been found that a concentration of a few parts per billion is 4 lethal to fish. Laboratory techniques possessing this sensitivity have been developed. The phosphorus is partially isolated by extraction in a suitable solvent such as benzene or isooctane. A portion of the extract is injected into a chromatograph utilizing a flame photometric detector. Mud and tissue samples can also be analyzed rapidly by this method. SAMPLE-HANDLING SYSTEMS The successful application of the phosphorus analyzers just described depends on reliable delivery of a well filtered sample. Such systems are discussed in Sections 8.2 and 8.65. The system shown in Figures 8.49d and 8.49e is capable of handling raw sewage. The primary filter is of the motordriven vane type with alternate stationary and rotating disks, and the clearance between the plates determines the degree of filtration. The second stage of filtering consists of two disposable cartridge filters. A checkvalve diverts the flow to the second filter when the pressure drop across the first indicates that a change is necessary. The regulating pump between the filter stages ensures a constant flow rate in difficult determinations. Provision for backflushing the system is also advisable in some applications. The selection of a good sample-handling system from the options discussed in Section 8.2 is just as critical as the selection of the analyzer itself. Provisions for convenient

1588


calibration are also an essential part of a complete and successful installation.

MOTOR

INFLUENT (2−4 GPM @ 40−50 PSIG)

REGULATING PUMP VANE-TYPE FILTER

References 15 PSIG BYPASS CHECK VALVE

PCV

DISPOSABLE CARTRIDGE FILTERS

1. 2. 3. 4.

TO ANALYZER

RECYCLED EFFLUENT

Bibliography

FIG. 8.49d Raw sewage sample-handling system. FILTERED FLUID

FILTERED FLUID PARTICLES NO CAKE BUILDUP

FIG. 8.49e The rotary disc filter.


Standard Methods for the Examination of Water and Wastewater, American Public Health Association, New York. Brody, S. S. and Chaney, J. E., J. Gas Chromatography, 6, 42, 1966. Amirav, A. et al., Pulsed Flame Photometric Detector for Gas Chromatography, Tel Aviv University, 2001. Addison, R. F. and Ackman, R. G., J. Chromatography, 47, 421, 1970.

Addison, R. F., Chromatography, 14, 421, 1979. Amirav, A. et al., Pulsed Flame Photometric Detector for Gas Chromatography, Tel Aviv University, 2001. Converse, J. G., Calibration and maintenance are part of reliable sample preparation system design, Proc. 1992 ISA Conf., Houston, October 1992. Dawson, R., Data for Biochemical Research, Oxford University Press, New York, 1990. Ewing, G., Analytical Instrumentation Handbook, Marcel Dekker, New York, 1990. Instrumentation and Automation Experiences in Wastewater-Treatment Facilities, EPA-600/2–76–198, Environmental Protection Agency, Washington, D.C. Jutila, J. M., Multicomponent on-stream analyzers for process monitoring and control, InTech, July 1979. Standard Methods for the Examination of Water and Wastewater, American Public Health Association, New York. Treiber, H., Digital minilabs, Digital Imager, November 2000. Wastewater Sampling for Process and Quality Control, Water Environment Federation, 1996. Zimmermann, C. F. et al., Determination of orthophosphate in waters, Environmental Protection Agency, Washington, D.C., 1997.

8.50

Physical Properties Analyzers—ASTM Methods N. S. WANER (1972) I. VERHAPPEN

A. ALSTON

(1982)

D. E. PODKULSKI

(1995)

TO RECEIVER

AT

FLASH POINT OR BOILING POINT, ETC.

Flow Sheet Symbol

(2003)

Analyzer Type:

A. Distillation analyzer B. Vacuum distillation analyzer C. Horizontal still distillation analyzer D. Simulated distillation by gas chromatography E. Air-saturated vapor pressure analyzer—continuous F. Air-saturated vapor pressure analyzer—cyclic G. Dynamic vapor pressure analyzer H. Continuous vapor—liquid ratio analyzer I. Differential-pressure pour pointer J. Viscous-drag pour pointer K. Optical-cloud point analyzer L. Freeze-point analyzer M. Low-temperature flash-pointer analyzer N. High-temperature flash-point analyzer O. Octane engine comparator analyzer P. Reactor-tube continuous octane analyzer Q. Near-infrared inferential measurements

Potential Applications:

Crude fractions (A, B, C, D, I, J, K, L, M); gasoline components/product (A, B, C, D, E, F, G, H, O, P); diesel (K, M); jet or kerosene (L, M); lube oils (I, J, N)

Reference Methods:

ASTM D86 (A, C); ASTM D1160; (B); ASTM D2887/D3710 (D); ASTM D4953–90 (E, F, G); ASTM D1267 (G); ASTM D2533 (H); ASTM D97 (I, J); ASTM D2500 (K); ASTM D2386 (L); ASTM D56/D93 (M, N); ASTM D2699/D2700 (O, P)

Costs:

A. $40,000 B. $56,000 C. $31,000 D. $40,000 E. $38,000 F. $28,000 G. $26,000 to $35,000 H. $85,000 I. $37,000 J. $41,000 K. $28,000 L. $33,000 M. $44,000 N. $41,000 O. $220,000 P. $86,000 Q. $100,000


ABB Process Analytics (D, F) (www.abb.com) Benke (A, B, E, J, K, L, M, Q) (www.benke.de)


1590


Core Labs/Waukesha (H,O) (www.dresser.com) Ocean Optics (UOP) (P) (www.oceanoptics.com) Precision Scientific (A, B, C, E, G, I, K, L, M, N) (www.precisionsci.com) Rotork Ltd. (C, D, G, J) (www.rotork.com) Siemens Applied Automation (D) (www.sea.siemens.com.ia/)

INTRODUCTION This section deals with on-stream analyzers, which measure a physical property of a process stream. More specifically, onstream analyzers are related to ASTM Standard Test Methods for refinery processes and products. Table 8.50a provides an overall orientation of the various physical property analyzers that are available. Prior to the introduction of on-stream analyzers, analyses were done in the laboratory on periodic grab samples, and the results were reported to the process unit operator at some later time, permitting set point adjustments of parameters such as flow, temperature, pressure, and level. Continuous on-stream plant analyzers offer many advantages over laboratory analyses, including the characteristics enumerated below.

is the currently accepted laboratory standard for determining the boiling characteristics of petroleum products distilled at atmospheric pressure. The method employs a batch technique and approaches a single plate distillation process without reflux. The petroleum products analyzed are complex mixtures of components, and a low level of fractionation is achieved. True boiling-point distillation, in columns with 15 to 100 theoretical plates and at reflux rations of 5:1 or more, produce greater separation of components. The apparatus and procedures for true boiling-point determination are not standard, are complex, take longer to perform, and are not as widely used. Distillation curves for a few hydrocarbons are shown in Figure 8.50b along with a comparison of curves generated by ASTM Method D86-IP-123 and by true boiling-point determinations for kerosene.

Advantages of Continuous Analyzers 1. Continuous measurement of the stream, eliminating long time lags 2. Reduction of errors caused by unrepresentative samples or by changes in sample composition caused by sample handling 3. Elimination of human errors characteristic of nonautomated laboratory procedures 4. Ability to recognize process trends, thus permitting the automatic control of a given process variable by closed-loop control 5. Cost reductions resulting from minimization of laboratory analyses 6. Closer control resulting in smaller tolerances in final product specifications and reduction in quality “giveaway” 7. Feasibility of implementing in-line blending systems, which result in economic benefits resulting from the elimination of tankage, and from increased system flexibility and better quality control 8. Ability to provide continuous inputs to computerized process control systems for plant optimization 9. Direct measurement of process variables rather than detection of properties by inference DISTILLATION ANALYZERS Laboratory Measurements Distillation analyzers were introduced to provide data on the volatility characteristics of process streams and separation efficiency of distillation units. ASTM Method D 86-IP-123


ASTM Method D 86-IP-123 A sample is heated in an Engler flask at a prescribed rate. Packing is not used, and some refluxing occurs as a result of condensation (Figure 8.50c). The vapors that are produced flow through a condenser immersed in an ice-water bath, and the distillate is collected in a graduated cylinder. The initial boiling-point temperature is defined as the reading of the thermometer at the instant the first drop of condensate falls from the lip of the condenser tube. As the higher boiling fractions vaporize, condense, and collect in the graduate, corresponding temperature readings are recorded to permit the plot of a curve of temperature vs. percent of sample recovered. The end point or final boiling point is described as the maximum thermometer reading observed during the test; it usually occurs when all the liquid has been boiled off from the bottom of the flask. Usually, the percentage recovered does not equal the 100ml sample charge, partly because of the inability of the apparatus to condense the lightest fractions. A curve of temperature vs. percent evaporation is determined by adding the percent of light ends lost to each of the recorded percentages recovered. The precision of this method is a function of the temperature change vs. boiling rate. Repeatability ranges from ±2 to 9°F (±1 to 5°C), and the reproducibility ranges from ±5 to 20°F (±3 to 11°C). ASTM Method D 1160 This method provides for measurements under vacuums, ranging from 1 mmHg (133 Pa) absolute to atmospheric, to a maximum liquid temperature of 750°F (399°C). Results are not comparable with other ASTM distillation tests, although they may be converted to corresponding vapor temperature at 760 mmHg by reference to Maxwell and Bonnel vapor pressure charts.

TABLE 8.50a Orientation Table for Physical Properties Analyzers Repeatability (+/−)

Flow Rate GPH (LPH)

Pressure PSIG (kPa)

Cycle Time (minutes)

Cost (in $1000s)

30°F (17°C) below IBP

5

40

Precision Scientific

50–250 345–1724

180°F (82°C) max

16

56


0.4 (1.5)

5–100 (35–700)

40°F (22°C) lower than IBP

2

31

Rotork

3–12°F (2–7°C)

4(15) (sample inject)

5–150 (35–1035)

Below expected IBP

10–30

40

Asea Brown Boveri Applied Automation Rotork

0.1 psia

1.6 (6)

10–100 (69–690)

50–110°F (10–43°C)

2

38


0.75

Analyzer

Type

Range

Distillation

Distillation

5–95% 100–650°F (38–343°C)

1% sample boiling range

1.2 (4.5)

20–150 (138–1035)

Vacuum

650–1000°F (343–538°C)

1% sample boiling range

2.3 (8.7)

Horizontal Still

5–95% 150–650°F (65–343°C)

Equal or better than ASTM

Simulated Distillation

2–98% 0–1000°F (–17–538°C)

Air-saturated Continuous

2–19 psia

Vapor Pressure

Temperature F (C)

0–20 psia

0.05 psia

Bypass flow

Dynamic

0–20 psia 0–200 psia

Equal to or better than ASTM

10–50 (38–190)

75–500 (520–3450)

70–120°F (21–49°C)

Vapor/Liquid

Continuous

10–30 V/L to 150°F (66°C)

0.5 V/L

2–4 (7.6–15.1)

50–150 (350–1050)

Normal Blending Range

Pour Point

Differential Pressure

–75−+50°F (–59−+10°C)

5°F (2°C)

2 (7.6)

Instrument Engineers' Handbook, Volume 1, Fourth Edition: Process Measurement and Analysis

Instrument Engineers' Handbook, Volume 1, Fourth Edition: Process Measurement and Analysis

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Process Engineers Pocket Handbook

Project and Cost Engineers' Handbook, Fourth Edition (Cost Engineering)

Materials for Engineers and Technicians Fourth Edition

Real Analysis, Fourth Edition

Instrument Flying Handbook

Measurement Science for Engineers

Handbook for Process Plant Project Engineers

Handbook for Process Plant Project Engineers

Control Valve Handbook Fourth Edition (Fisher, Emerson Process Management)

Particle Size Measurement Volume 1

Measurement Science for Engineers

Environmental Engineers' Handbook, Second Edition

AIAA Aerospace Design Engineers Guide, Fourth Edition

Psychoneuroimmunology, Two-Volume Set, Volume 1-2, Fourth Edition

Video Production Handbook, Fourth Edition

Plastics Technology Handbook, Fourth Edition

Exploring Chemical Analysis , Fourth Edition

Reservoir Engineering Handbook, Fourth Edition

Reservoir Engineering Handbook, Fourth Edition

Finance Director's Handbook, Fourth Edition

Facilities Management Handbook, Fourth Edition

Environmental Encyclopedia, Fourth Edition volume 1 and 2

Productivity Measurement and Analysis

Mechanical engineers' handbook, book 3: Manufacturing and measurement

Administrative Assistant's and Secretary's Handbook , Fourth Edition

Structural Analysis, Fourth Edition , SI Edition

Measurement in Nursing and Health Research, Fourth Edition

Introduction to Mechatronics and Measurement Systems, Fourth Edition

Instrument Engineers' Handbook, Volume 1, Fourth Edition: Process Measurement and Analysis