Safe Design and Operation of Process Vents and Emission Control Systems
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Safe Design and Operation of Process Vents and Emission Control Systems
Safe Design and Operation of Process Vents and Emission Control Systems
Center for Chemical Process Safety of the American Institute of Chemical Engineers
CCPS
CENTER FOR CHEMICAL PROCESS SAFETY
An AlChE industry Technology Alliance
A JOHN WILEY & SONS, INC., PUBLICATION
Copyright 0 2006 by John Wiley &. Sons, Inc. All rights reserved. A joint publication of the Center for Chemical Process Safety of the American Institute of Chemical Engineers and John Wiley & Sons, Inc. Published by John Wiley &. Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted i n any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 ofthe 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 1 I 1 River Street, Hoboken, NJ 07030, (201) 748-601 1, fax (201) 748-6008, or online at http://www.wiley.comlgo/permisc;ion. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neilher the publisher nor author shall be liable for any loss of profit or any other commercial damages,, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic format. For information about Wiley products, visit our web'site at www.wiley.com. Library of Congress Cataloging-in-Publication Data:
Safe design and operation of process vents and emission control systems i Center for Chemical Process Safety, p. cm. Includes index. ISBN-13: 978-0-471-79296-3 (cloth) ISBN-10: 0-471-79296-9 (cloth) 1. Chemical industry-Fume control. 2. Chemical plants-Heating and ventilation-Safety measures. 3. Chemical industry-Fires and fire prevention. 4. Air-Pollution. 5. Chemical plants-Piping. 6. Air ducts-Design and construction. 1. American Institute of Chemical Engineers Center for Chemical Process Safety. TH7684.C44.S24 2006 660'.2804-&22 2005033607 Printed in the United States of America. I 0 9 8 7 6 5 4 3 2 1
DISCLAIMER It is sincerely hoped that the information presented in this document will lead to an even more impressive safety record for the entire industry; however, neither the American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Committee and Subcommittee members, their employers, their employers' officers and directors, nor [Contractor name] and its employees warrant or represent, expressly or by implication, the correctness or accuracy of the content of the information presented in this document. As between (1) American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Committee and Subcommittee members, their employers, their employers' officers and directors, and Risk, Reliability and Safety Engineering, LLC, and its employees, and (2) the user of this document, the user accepts any legal liability or responsibility whatsoever for the consequence of its use or misuse.
V
CONTENTS Preface Acknowledgment
XIV
xv
Introduction 1.1 1.2 1.3 1.4 1.5 1.6
Objective Relationship to Other CCPS Publications Industnes and Operations Covered Intended Audience How to Use this Book References
Management Overview 2.1.
Impact on Vent Header Systems
Normal Process and Emergency Systems 3.1
3.2
Types of Vent Header Systems 3.1.1 Normal Process Vent Header Systems 3.1.2 Emergency Vent Header Systems 3.1.3 Combined Vent Header Systems 3.1.4 Considerations Design Philosophy 3.2.1 Design Sequence 3.2.2 Hazards Associated with Combining Vent Streams 3.2.3 Inherent Safety 3.2.4 Flammability and Combustibility
13 14 15 16 16 19 19 21 21 23
Contents
3.3
3.2.5 Toxicity 3.2.6 Reactivity 3.2.7 Regulatory Issues Reference
25 27 29 32
Combustion and Flammability 4.1
4.2 4.3 4.4 4.5
4.6
Flammable Limits 4.1.1 Mixture Stoichiometry 4.1.2 Factors Influencing Flammable Limits 4.1.3 Flammable Limit Variability 4.1.4 Effects of Temperature on Flammable Limits 4.1.5 Effects of Pressure on Flammable Limits 4.1.6 Flammable Limits of Combined Gas Streams 4.1.7 Cool Flame 4.1.8 Hybrid Mixtures Limiting Oxidant Concentration Deflagrations Pressure Piling Detonation Phenomena 4.5.1 Deflagration to Detonation Transition (DDT) and Run-Up Distance 4.5.2 Overdnven and Stable Detonations 4.5.3 Detonation Cell Size References
36 36 38 39 39 40 42 42 43 44 45 46 47 48 50 51 56
UNDERSTANDING REQUIREMENTS
5.1
viii
Understanding the Sources 5.1.1 Identify Vent Sources 5.1.2 Identify Normal Process Vent Streams 5.1.3 Normal Process Vent System, Design Case Scenario 5.1.4 Define Interface Requirements 5.1.5 Identify Hazard Scenarios That Could Result in Emergency Venting 5.1.6 Vent Gas Characteristicsfor Emergency Venting 5.1.7 Emergency Venting Design Case Scenario 5.1.8 Liquid Entrainment or Condensation in Normal Process Vent Headers 5.1.9 Two-Phase Venting
60 60 60 60 60 60 61 61 62 63
Contents
5.2
5.3 5.4 5.5 5.6 5.7
5.1.10 Flammable Gases and Vapors 5.1 .11 Toxic and Noxious Materials 5.1.12 Reactive Systems Regulatory Issues 5.2.1 Historical Background 5.2.2 Brief Review of Laws and Regulations 5.2.3 Improved Air Quality At-Source Treatment Options Combining Vent Streams End-of-Line Treatment Systems Specify Design Requirements References
64 66 68 74 74 76 78 79 80 82 82 82
DESIGNAPPROACH 6.1 6.2
6.3
6.4
6.5 6.6
ix
Design Basis Merging Vent Streams 6.2.1 Features Favorable for Merging Steams 6.2.2 Features that do not Favor Merging Streams Vent Header Systems Handling Flammable Materials 6.3.1 Explosion Prevention 6.3.2 Operating Fuel Lean 6.3.3 Operating Inerted 6.3.4 Operating Fuel Rich 6.3.5 Oxidizers Other Than Oxygen 6.3.6 Explosion Protection 6.3.7 Ignition Sources Vent Header Systems Handling Toxic Gases 6.4.1 Operating Pnnciples for Header Systems Handling Toxic Gases 6.4.2 Piping Design 6.4.3 Combined Relief Valve and Rupture Disk Devices Reactive Systems 6.5.1 Reactive Systems Design Considerations Mechanical Design Considerations 6.6.1 Vent Header Pipe Specifications 6.6.2 Vent Header Supports 6.6.3 Stresses on Vent Header Piping 6.6.4 Shock Waves Downstream of Rupture Disks 6.6.5 Corrosion 6.6.6 Header Operating Pressure and Pressure Drop
85 86 87 87 88 89 90 97 104 107 108 117 118
1 19 120 121 121 121 123 123 123 124 125 125 125
Contents
6.7
6.6.7 Thermal Stresses and Low Temperature Embrittlement 6.6.8 Liquid Knock-Out and Drainage 6.6.9 Expansion Joints and Flexible Connections 6.6.10 Valves in the Vent Header System References
126 126 128 129 130
Treatment and Disposal Systems Selection of Treatment and Disposal Methods Collection 7.2.1 Containment 7.2.2 Collection with Venting 7.2.3 Dump and Catch Tanks 7.2.4 Blowdown Drums and Tanks 7.2.5 Quench Drums 7.2.6 Quench Pools 7.2.7 Advantages and Disadvantages - Collection Systems Physical Separation 7.3.1 Vapor-Liquid Gravity Separators 7.3.2 Knock-Out Tanks and Drums 7.3.3 Mist Eliminators 7.3.4 Cyclones 7.3.5 Advantages and Disadvantages - Physical Separators Absorption 7.4.1 Spray Towers 7.4.2 Tray Towers 7.4.3 Packed-Bed Scrubber 7.4.4 Venturi Scrubbers 7.4.5 Advantages and Disadvantages - Absorption Systems Adsorption 7.5.1 Advantages and Disadvantages - Carbon Adsorption Recovery 7.6.1 Condensing Systems 7.6.2 Gas Recovery Systems 7.6.3 Advantages and Disadvantages - Recovery Systems Thermal Destruction 7.7.1 Flares 7.7.2 Thermal and Catalyhc Oxidizers 7.7.3 Process Heaters Used for Thermal Destruction 7.7.4 Advantages and Disadvantages -Thermal Destruction Systems
133 137 137 139 139 141 142 145 147 147 147 149 151 151 152 153 154 154 154 155 155 156 158 158 159 160 165 165 165 172 174 175 X
Contents
7.8
7.9
Dispersion of Vent Gas 7.8.1 Design and Safety Considerations 7.8.2 Atmospheric Dispersion Design 7.8.3 Advantages and Disadvantages- Dispersion to Atmosphere References
176 176 177 178 179
HAZARD ANALYSIS AND CONSEQUENCE ASSESSMENT 8.1 8.2
8.3 8.4
Hazard Analysis Methods Hazard Analysis Process 8.2.1 Identification of Causes 8.2.2 Development of Consequences 8.2.3 Estimation of Hazard Scenario Risk Consequence Assessment Techniques References
184 185 186 188 189 189 192
Operations and Maintenance 9.1 9.2
9.3 9.4
Daily Inspections Scheduled Inspections and Maintenance 9.2.1 Materials Build-Up 9.2.2 Pressure Relief Valves and Rupture Disks 9.2.3 Conservation Vents 9.2.4 Explosion Prevention Systems 9.2.5 Fast Acting Valves and Chemical Isolation Systems 9.2.6 Explosion Relief Panels 9.2.7 Inemng Systems 9.2.8 Instrument and Controls 9.2.9 Low Point Drains 9.2.10 Corrosion and Erosion 9.2.1 1 Structural Supports for Vent Headers 9.2.12 Insulation and Heat Tracing Management of Change References
196 196 196 199 200 200 200 20 1 20 1 20 1 20 1 20 1 202 202 202 202
Contents
Acronyms and Abbreviations
203
Glossary
207
Selected US Environmental Air Pollution Control Regulations
21 5
Vent Header Design Checklist
225
Normal Vent Header Source Control and Configuration Examples
233
PHA HAZOP Deviation Table
243
Worked Examples G1.
Inerted Flammable Liguid Storage
247
G2.
Flamable Liquid Process Operating Fuel Lean
254
G3.
Flamable Liquid Process Operating Fuel Rich
259
G4.
Flamable Liquid Process Operating Fuel Rich
263
G5.
Refinery Example: Crude and Vacuum Units
267
G6.
Refinery Example: Coker Unit and Gas Processing Plant
27 1
G7.
Reactive System
275
Past Incidents H1.
Combustion Incidents
283
H2.
Reactive Chemical Incidnets
290
H3.
Vacuum Failures
294
H4.
References
295
Contents
Historical Perspective on Air Pollution Control 11. Historical Background on Air Pollution
12. Brief Review of Laws and Regulations 13. Improved Air Quality 14. References
297 299 301 307
PREFACE The American Institute of Chemical Engineers (AIChE) has helped chemical plants, petrochemical plants, and refineries address the issues of process safety and loss control for over 50 years. Through its ties with process designers, plant constructors, facility operators, safety professionals, and academia, AIChE has enhanced communication and fostered improvement in the high safety standards of the industry. AIChE's publications and symposia have become an information resource for the chemical engineering profession on the causes of incidents and means of prevention. The Center for Chemical Process Safety (CCPS), an Industry Technology Alliance of AIChE, was established in 1985 to develop and disseminate technical information for use in the prevention of major chemical accidents. CCPS is supported by a diverse group of industrial sponsors in the chemical industry and related industries who provide the necessary funding and professional guidance for its projects. The CCPS Technical Steering Committee and the technical subcommittees oversee individual projects selected by CCPS. Professional representatives from sponsoring companies staff the subcommittees and a member of the CCPS staff coordinates their activities. Since its founding, CCPS has published many volumes in its "Guidelines" series and in smaller "Concept" series texts. Although most CCPS books are written for engineers in plant design and operations and address scientific techniques and engineering practices, several guidelines cover subjects related to chemical process safety management. A successful process safety program relies upon committed managers at all levels of a company who view process safety as an integral part of overall business management and act accordingly. A team of experts from the chemical industry drafted the chapters for this concept book and provided real world exarr.ples to illustrate some of the tools and methods used in their profession. The subcommittee members reviewed the content extensively and industry peers evaluated this book to help ensure it represents a factual accounting of industry best practices. xiv
ACKNOWLEDGEMENTS The American Institute of Chemical Engineers wishes to thank the Center for Chemical Process Safety (CCPS) and those involved in its operation, including its many sponsors whose funding made this project possible; the members of its Technical Steering Committee who conceived of and supported this concept book project, and the members of its Process Vent and Emission Control Systems Subcommittee. The members of the CCPS Process Vents and Emissions Control Subcommittee were: Christopher Lowe, Chair, Syngenta Crop Protection, lnc. Danny Bice, The Dow Chemical Company James Case, Air Products and Chemicals,Inc. David Kirby, Baker Engineering and Risk Consul tants Peter Lodal, Eastman Chemical Company Ray Mendelsohn, DuPont Edward Zamejc, BP John Davenport was the CCPS staff liaison and was responsible for overall administration of the project. Risk, Reliability and Safety Engineering (RRS), of League City, Texas was contracted to write this concept book. The principal RRS authors of this guideline were: John Birtwistle Tim McNamara Christy Franklyn Additional RRS staff that supported this project includes Donna Hamilton and Cathy Malek. CCPS also gratefully acknowledges the comments and suggestions received from the following peer reviewers; their insights, comments, and suggestions helped ensure a balanced perspective to this concept book: John Alderman, Risk, Reliability and Safety Engineering James Case, Air Products and Chemicals,lnc. Stan Grossel, Process Safety and Design, Inc. xv
Acknowledgments
Neil McNaughton, Innovene William Olsen, Merck and Company, Inc. Tony Thompson, Monsanto The members of the CCPS Process Vent and Emission Control Systems and the peer reviewers wish to thank their employers for allowing them to participate in this project.
xv i
Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins
by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.
INTRODUCTION The American Institute of Chemical Engineers (AIChE) has long been involved with process safety and loss control for the chemical, petrochemical, and hydrocarbon processing industries. The institute has developed strong ties with process designers, equipment builders, constructors, operators, safety professionals, and the academic community. AIChE has enhanced communications and improved safety standards for industry. Its publications are important information resources for the process industries. In 1985, AIChE established the Center for Chemical Process Safety (CCPS) to serve as the focal point for a continuing program to support and advance process safety. Since that time, CCPS has sponsored and published a number of documents, including proceedings of technical conferences and a series of books to improve process safety. This concept book, Safe Design and Operation of Process Vents and Emission Control Systems, is one of that series.
The simplest process vent system is one that consists of one vent device with minimal piping discharging directly to atmosphere at the nearest safe location. Numerous such simple systems exist in industry and satisfy the appropriate safety, health, and environmental requirements; particularly, where the quantities are small and the materials are less hazardous or non-hazardous. In practice, a number of factors have encouraged or required the collection of individual process vents into often complicated systems to collect these streams and treat, disperse, or dispose of them in order to meet increasingly demanding safety, health, environmental, and property protection requirements.
1
Safe Design and Operation of Procem Vents and Emission Control System
1.1
Objective
The objective of this book is to provide guidance for the design, evaluation, and operation of systems to collect and handle effluent gases and vapors vented from processes. These systems may consist of headers and manifolds of piping or ductwork and include other components that route or treat the effluent gases and vapors from their origin in process vessels, equipment, and storage tanks to the ultimate disposal or destruction system. Names for these systems include vent manifolds, vent collection systems, emission control systems, blowdown systems, vapor control systems, or vent header collection systems, as well as other descriptions and names. In this book, these systems are collectively referred to as vent header systems. This book addresses the concepts associated with the design and operation of vent header systems and provides guidance on: Designing vent header systems Preventing fires and explosions Controlling releases of toxics Maintaining safe operations Normal process operations, such as intentional routine controlled venting Emergency operations, for example, overpressure relief End-of-line treatment devices and their effects on the vent header system, including devices such as scrubbers, flares, thermal oxidizers, etc. This book focuses on vent header systems that handle gases, vapors, and entrained liquids that are vented from process tanks, vessels, and equipment. This book does not provide guidance on liquid-full systems, systems primarily intended for the removal, extraction, and collection of dust from otherwise innocuous air streams, or systems intended primarily to exhaust air from or ventilate working spaces. This book does not address the details of selection or computational aspects of sizing vent header piping systems or individual venting devices either for emergency overpressure relief venting or for normal process venting.
2
Chapter 1 -Introduction
A S M E B31.3 - Process Piping [Ref. 1-11contains details for design of piping systems for vent headers. For details on venting devices for emergency overpressure relief, refer to Guideline for Pressure Relief and Efluent Handling Systems [Ref. 1-21, Additional detailed design and sizing guidance for devices that may handle multi-phase flow is available from the Design Institute for Emergency Relief Systems (DIERS) [Ref. 1-31. DIERS operates under the auspices of AIChE as a users group currently comprised of representatives from 210 companies that cooperatively assimilate, implement, maintain, and upgrade the DIERS methodology. The group's purpose is to reduce the frequency, severity, and consequences of overpressure incidents and develop new techniques to improve the design of emergency relief systems. The venting devices for normal process venting are part of each specific process design and are typically standard process control valves and other components. 1.2
Relationship to Other CCPS Publications
Guidelines for Vapor Release Mitigation [Ref. 1-41 contains practices for controlling accidental releases of hazardous vapors and preventing their escape to the atmosphere. Its focus is primarily on pre-release factors. The 1988 guideline remains useful since it focuses on practical engineering design of mitigation systems and post-release mitigation methods. Since the 1988 guideline was published, substantial progress and improvements were made in many areas of mitigation design. To collect and update this progress, CCPS published Guidelines for Post-Release Mitigation Technology in the Chemical Process Industry [Ref. 1-51, The primary focus of the 1997 guideline is the mitigation of accidental releases of toxic or flammable materials and, in particular, countermeasures following a release. These guidelines make limited mention of collecting releases )+om process vent devices into vent header systems. In later chapters, this book discusses prevention of the propagation of fire and explosion within vent header systems. The following two books by CCPS provide useful background information. DefZagration and Detonation Flame Arresters [Ref. 1-61 provides guidance on the selection and proper application of fire and explosion arresting devices used within vent header system lines or at end-of-pipe locations. The book, Understanding Explosions [Ref. 1-71, published in 2003 provides a concise treatise on fires and explosions. 3
Safe Design and Operation of Process Vents and Emission Control Systems
Pertinent to the topic of vent header systems, the book also covers deflagration and detonation basics within closed equipment and purging and inerting of systems. Following extensive research into emergency venting, including large-scale tests involving reactive materials and two-phase venting, The Design Institute for Emergency Relief Systems PIERS) of AIChE published Emergency Relief System Design Using DIERS Technology [Ref. 1-31. It provides essential methodology for the design and sizing of emergency relief devices, but does not provide guidance on vent header systems.
Guidelines for Engineering Design for Process Safety [Ref. 1-81 includes information on flame arresters, pressure relief systems, effluent disposal systems, and provides some information on vent header systems. Guidelines for Pressure Relief and Effluent Handling Systems [Ref. 1-21 contains guidance and information on widely used codes and standards and their application in the detailed design of emergency overpressure relief devices and systems. It also includes the selection and design of systems and equipment to handle vent gases. These previous books and guidelines focused primarily on preventing releases, the detail design of overpressure relief devices, and the mitigation of the effects of releases to the atmosphere. They were primarily involved with emergency overpressure relief scenarios. The previous books and guidelines were not intended to provide guidance for the design and operation of vent header systems intended to collect vent gases from multiple sources during normal process operations, as well as during emergency overpressure conditions. This current book incorporates and consolidates information specific to vent header systems from these and other existing sources, as well as provides new information and learnings where possible. 13
Industries and OperationsCovered
Vent header systems are employed in one form or another in many facilities across numerous industry sectors. The industry sectors most commonly using vent header systems are: Oil and Gas Production and Processing Petroleum Refining Petrochemicals Synthetic Organic Chemicals 4
Chapter 1 - introduction
Agricultural Chemicals Specialty Chemicals Inorganic Chemicals Pharmaceuticals Polymers and Plastics Resins, Coatings and Adhesives Paints Synthetic Fibers The processes employed in these industry sectors vary greatly in complexity and scale. They may be continuous processes from raw materials to finished products, operate in a batch mode, or be a combination of batch and continuous processes. The vent header systems associated with these processes are similarly diverse in complexity and scale. Some of these vent header systems are simple, involving only one vent gas stream routed to a treatment device. Others may collect vent gas streams from multiple sources within a process unit or from several process units. Most vent header systems only handle the normal routine release of gases and vapors from the process. Some are intended to only handle emergency overpressure relief. A limited number are combined vent header systems that handle both normal process vent streams and provide the critical emergency function of safely venting effluent from overpressure relief devices. Many of these vent header systems are environmentally required to treat the vent gases before their release to the atmosphere. 1.4
Intended Audience
This book should be of interest to persons responsible for: Design of new or modification of existing processes that may require the use of a vent header system, including project managers and process design engineers Process safety or hazard analysis of processes with vent header systems Operation of process units or facilities with vent header systems, including operating management and staff and unit process or manufacturing engineers Maintenance, inspection, or testing for process units or facilities with vent header systems 5
Safe Design and Operation ofprocess Vents and Emission Control Systems
This book also provides useful reference for anyone interested in the subject of vent header systems used in the process industries.
How to Use this Book
1.5
This book is organized to meet the needs of those readers new to the issues associated with vent header systems, as well providing experienced readers specific references and design considerations. The organization and content is illustrated in Figure 1-1.
I I
I
Chapter 1 introduction
Chapter 2 Management Overview
Chapter 3 Normal Process and Emeraencv Svstems Chapter 4 Combustion and
Understanding Reauirements
I
introduction and objective Relationship to other CCPS publications Industries and operations covered Intended audience
I
Environmental and societal stewardship concerns History and impact of US environmental air regulations The vanety of purposes and applications for vent header systems Cost implications and business interruptions issues
I
Types of vent header systems Considerations for normal and emergency vent header systems Design Philosophy General Design Flammable limits Hybnd mixtures Deflagrations, pressure piling and detonation phenomena
Underjtandlng the process conditions Vent stream charactedstics Fiammable gases and vapors, toxic and noxious materials, reactive systems Objectives and design concepts for normal, emergency and combined sys!ems
Chapter 6 Design Approach
Combining vent header systems Systems handling flammables and toxics Reactive systems Mechanical design considerations
Chapter 7 Treatment and Disposal Systems
Selection of treatment and disposal methods Coiiection, separation, absorption, adsorption and recovery Thermal destruction Dispersion
Chapter 8 Hazard Analysis and Consequencr
Hazard analysis Hazard Identification anaiysis method Consequence assessment techniques
Chapter 9 Operations and Maintenance
Potential failure modes and concerns Pressure reiief devices
Figure 1-1. Guideline Organization and Content 6
Chapter 1 - Introduction
1.6
References
1-1
American Society of Mechanical Engineers. 2002. B32.3 - Process Piping. New York, New York.
1-2
Center for. Chemical Process Safety (CCPS). 1998. Guidelines for Pressure Relief and Efluent Handling Systems. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Engineers.
1-3
The Design Institute for Emergency Relief Systems PIERS). 1992. Emergency Relief System Design Using DIERS Technology. New York, New York. American Institute of Chemical Engineers.
1-4
Center for Chemical Process Safety (CCPS). 1988. Guidelines for Vapor Release Mitigation. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.
1-5
Center for Chemical Process Safety (CCPS). 1997. Guidelines for Post-Release Mitigation Technology in the Chemical Process Industry. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Engineers.
1-6
Grossel, Stanley S. 2002. Deflagration and Detonation Flame Arresters. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.
1-7
Crowl, D. A. 2003. Understanding Explosions. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Engineers.
1-8
Center for Chemical Process Safety (CCPS). 1993. Guidelines for Engineering Design for Process Safety. New York, New Y ork: Center for Chemical Process Safety of the American Institute of Chemical Engineers.
Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins
by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.
2 MANAGEMENT OVERVIEW A progression of societal drivers for air pollution control have prompted the process industries to control process effluents and emissions. Laws and regulations regarding protection of the natural environment and public health have exerted an increasing impact on the refining, chemical, and other process and related industries. In particular, the requirements regarding air pollution controls have resulted in an increase in the use of vent header systems. The development of regulations in the United States and, in particular, the requirements of the Clean Air Act amendments and related air pollution control regulations have greatly influenced the need for vent header systems and their design and operation. Further discussion of regulatory issues may be found in Chapter 3 of this book; an historical perspective on air pollution control laws and regulations may be found in Appendix I. The importance of safe design and operation of vent header systems has also been emphasized by recent incidents in the process industries. Some of these incidents are described in Appendix H. Impact on Vent Header Systems Environmental air pollution control regulations over the past several decades have arguably reduced the number of discrete emission points and increased the number of emission vent collection systems. The economics of emission reduction and treatment clearly encourage facilities to collect similar vent streams into vent headers for processing in common treatment or disposal systems before the final release to atmosphere. Current air pollution control regulations have extended coverage to more industry sectors and in many cases further restricted allowable end-ofpipe post treatment releases in terms of either or both quantity or concentration. Future regulations should be anticipated to continue this general trend. 2.1.
9
Safe Design and Operation of Process Vents and Emission Control Systems
So, what is the impact of these air pollution controls? Vent header systems of some type have become commonplace in many industry sectors for a wide range of processes. For the most part, they have been added on to processes to meet the environmental requirements. Possibly because of being viewed as an add-on, vent header systems have often been treated similar to a utility service rather than as an extension of the process operation. More often than not, utility systems are not accorded the same level of safety technical review as would be given to a section of the process. An objective of this book is to encourage the design and operation of vent header systems and their treatment/disposal components with an equivalent level of safety review as would be given the process itself. Increasingly, regulations and operating permits have made the availability of a functioning vent collection and treatment/disposal system a requisite for the process to continue to operate within its approved legal limits. This presents another reason to treat the vent header system as a part of the process. The regulations have had an impact on process economics as the number of emission sources that require treatment have increased. As a result, vent header collection systems have increased the number of connected vents. Clearly, it is more cost-effective to treat a larger number of vent streams in a common vent header system than to do so individually. An increasing trend is the collection of vent streams from different processes. However, the connection of multiple processes to a common vent header system increases the probability of unsafe conditions due to differing process start-up and shutdown schedules, vent header ownership issues affecting maintenance or monitoring, and other factors such as: Incompatible vent streams that could result in pluggage, violent reaction, fire, or explosion in the header or treatment equipment Unwanted flow of materials via the vent header from one process vessel to another Vent header and treatment system flow capacity or restriction issues resulting from simultaneous multiple vent streams Increasing difficulty of identifying hazards as the system becomes more complex
10
Chapter 2 - Management Overview
Current and future regulations may also have an impact on the complexity of the treatment systems used. To meet certain requirements, it may be necessary to add additional intermediate or end-of-pipe treatment and disposal systems, such as a scrubber on the combustion products stream from a thermal oxidizer. The addition of multi-step treatment and disposal systems can increase the complexity of the overall vent header system and may increase the probability of creating unsafe conditions. Certainly there are other factors that impact decisions by a facility regarding the use of vent header collection and treatment systems for control of their emissions, including their public image and preservation of the rightto-operate in their communities, but the compelling impact has been from the air pollution control regulations.
11
Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins
by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.
3 NORMAL PROCESS AND EMERGENCY SYSTEMS This chapter describes the functions of normal process, emergency, and combined vent header systems, and provides an overview of their overall design philosophy. The interface between major processing vessels and sources, and the vent header itself, is often unclear or ill defined. Precise definitions of both the physical design and administrative control boundaries between the main processing vessels and the vent collection system are necessary. This should include such issues as the impact of vent header system pressure fluctuations on process equipment, especially distillation systems. 3.1
Types of Vent Header Systems
As a general rule, process equipment, tanks, and other vessels have two complementary venting requirements: Normal Process - to provide normal venting while the process is functioningas intended in one of the normal operating phases. Emergency - to protect equipment and personnel from the effects of excessive pressure or vacuum caused by an abnormal condition that cannot be controlled by the basic process controls or the safety instrumented system (interlocks). These situations include events such as fires or runaway reactions. The regulatory and economic implications for normal and emergency vent header systems can differ significantly. Normal practice is to provide two separate vent systems; however, there are some situations when a single combined system can effectively satisfy both needs.
13
Safe Design and Operation of Process Vents and Emission Control Systems
3.1.1
Normal Process Vent Header Svstems
Normal process vent header systems handle vent gas produced during normal operating phases, including start-up, shutdown, and certain maintenance activities. The sources of these vent gases include, but are not limited to: Vessel breathing (either in or out) due to changes in liquid level, temperature, or variations in the atmospheric pressure Off-gases associated with the process chemistry Inert gas purging or other intentionally introduced gas flows that are not consumed in the process In some instances, vent gases have a commercial or economic value such as refinery fuel gas. In these cases, it may be cost-effective to recover or recycle the flows. When evaluating off-gas streams for recycling it is important to ensure the gases will be compatible with one another and any other process streams they could come in contact with. For example, they should not form flammable mixtures (fuel and oxidizer), build-up solids that could restrict flow in the vent header, or react with each other. It is also important to recognize the potential for trace components to build-up in the system where recycling does not provide an outlet. Normal vent header systems should be sized to handle maximum predicted flows from the equipment while the facility is operating as intended in one of the normal operating phases (start-up, shutdown, etc.). These maximum predicted flows must be achieved without the pressure increasing to a level that could restrict venting from other vessels connected to the vent header system or otherwise creating the potential for undesirable reverse flow to source vessels. Failures due to low temperature brittle fracture and vessel collapse due to vacuum are not uncommon, demonstrating the need to address both the minimum and maximum values of temperature and pressure, as well as other process variables. In addition, normal vent header systems should be able to withstand the worst-case conditions of temperature, pressure, composition, etc., they could be exposed to during an emergency venting incident.
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Chapter 3 -Normal Process and Emergency Systems
3.1.2
Emereencv Vent Header Svstems
Emergency vent header systems are safety systems that safely dispose of vent gases resulting from unintended events outside the range of normal operations. These can include external fire, runaway reactions, human error, equipment, and instrumentation failures. The design of an emergency vent header should be conservative, taking into account the worst-credible overpressure scenario for the equipment it is protecting. This typically results in flow rates that are considerably higher than during normal venting and may involve a greater range of temperatures, pressures, and compositions. Selection of the ultimate disposal route for the emergency relief vent discharge depends on several factors, such as the: Physical properties of the vent gases, such as density, pressure, temperature, etc. Maximum flow rate and quantity of vent gases that could be discharged Toxicity and combustibility properties of the vent gases Historical weather information and topographical features affecting dispersion Proximity of the local community Nuisance issues, such as the odor and noise caused when venting occurs In some cases, non-toxic emergency vent gases can be discharged directly to atmosphere. Other materials may need treatment to address state or city permitting requirements and other regulatory issues. For further details on the treatment of vent gases, see Chapter 7. Most countries have developed or adopted codes defining the requirements for pressure relief systems. Within the United States, many states and local city authorities have adopted Section VIII of the ASME Boiler and Pressure Vessel Code (BPVC) [Ref. 3-11 for tanks and vessels with design pressures above 15 psig. Vessels with operating pressures between 2.5 and 15 psi are typically designed in accordance with API 620 [Ref. 3-21, Tanks that operate at, or close to atmospheric pressure, can be designed using API 650, which also includes an appendix identifying additional design requirements for tanks operating up to 2.5 psig [Ref. 3-31. Vacuum ratings for API 620 and 650 tanks generally do not exceed 1 oz of vacuum. Chapter 6 of this book discusses several requirements for pressure and vacuum relieving devices on vessels that have implications for vent headers systems [Ref.3-11, 15
Safe Design and Operation of Process Vents and Emission Control Systems
3.1.3
Combined Vent Header Svstems
Normal and emergency vent header systems have different purposes. If they are to be combined, the requirements of both must be satisfied without compromising either one. Emergency relief vents are typically the final safety devices protecting equipment from overpressure after the basic process controls and safety instrumented systems have been unable to provide the necessary protection. Although they are required to operate infrequently, they must be designed with a high reliability and meet code requirements. The ASME BPVC, Parts UG 125 through 137 [Ref. 3-11! identifies requirements for pressure relief devices. Generally, it requires any pressure relief device isolation valve to be locked or sealed open to assure there will always be an open vent path for emergency venting, except for maintenance or inspection as outlined by Appendix M of the code, (see Chapter 6 for further details).
In contrast, normal process vent header systems are primarily provided for environmental, health, and process reasons, and allow for the routine (sometimes continuous) emissions of off-gas from equipment. Tanks and vessels generally have separate systems for normal process and emergency vents; however, on occasion it may be cost-effective to combine them. Examples of separate and combined systems are illustrated in Figures 3-1 and 3-2. Figure 3-1 illustrates separate normal process and emergency vent systems, where the normal process vent system serves multiple sources. 3.1.4
Considerations
Vent header systems often collect off-gases generated from multiple equipment items, which inevitably means that gases leaving one vessel can mix with any combination of gases from other vessels connected to the same header. The chemical and physical properties of all streams must be mutually compatible and not form mixtures that are ignitable (i.e., mixtures where both fuel and air are present) or react when combined together. These gases range from being relatively innocuous to flammable, reactive, toxic, and/or corrosive. A process facility may contain multiple vent headers routed to a common treatment device, completely separate systems, or mixed systems where headers are joined after an intermediate treatment step. Several equipment items may also share a common vapor space representing a single vent source to the vent header system.
16
Chapter 3 - Normal Process and Emergency Systems
Figure 3-1.
Equipment with Separate Normal and Emergency Vent Header Systems
Safe Design and Operation of Process Vents and Emission Control Systems
Pilot Flame Detector
+--
Pilot Fuel Gas
Fuel Gas Header
Flare
Combined Normal and Emergency Vent Header
i Combined Vents From Other Process Vessels
Figure 3-2.
18