GUIDELINES FOR Consequence Analysis of Chemical...

GUIDELINES FOR

Consequence Analysis of Chemical Releases

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS

CENTER FOR CHEMICAL PROCESS SAFETY

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 3 Park Avenue, New York, New York 10016-5991

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GUIDELINES FOR


This book is one of a series of publications available from the Center for Chemical Process Safety. A complete list of titles appears at the end of this book.

GUIDELINES FOR


AMERICAN INSTITUTE OF CHEMICAL ENGINEERS

CENTER FOR CHEMICAL PROCESS SAFETY

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 3 Park Avenue, New York, New York 10016-5991

of the

Copyright 0 1999 American Institute of Chemical Engineers 3 Park Avenue New York, New York 10016-5991

ALL rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopy- ing, recording, or otherwise without the prior permission of the copyright owner.

Library of Congress Cataloging-in Publication Data Guidelines for consequence analysis of chemical releases.

P. un. Includes bibliography and index.

1. Chemical plants-Accidents-Evaluation ISBN 0-8169-0786-2

2. Risk assessment. ~ ~ 1 5 5 . ~ 8 7 2 1999 660 . 2 8 0 A c 2 1

99-10049 CIP

This book is available at a special discount when ordered in bulk quantities. For information, contact the Center for Chemical Process Safety at the address shown above.

It is sincerely hoped that the information presented in this volume will lead to an even more impressive safetyrecord for the entire industry: however, the American Institute of Chemical Engineers, its consultants, CCPS Subcommittee members, their employers, and their employers officers and directors disclaim making or giving any warranties or representations, express or implied, including with respect to fitness, intended purpose, use or merchantability and/or correctness or accuracy of the content of the information presented in this document and accompanying software. As between (1) American Institute of Chemical Engineers, its consultants, CCPS Subcommittee members, their employers, their employers officers and directors and (2) the user of this document and accompanying software, the user accepts any legal liability or responsibility whatsoever for the consequences of its use or misuse.

Contents

Preface Acknowledgments 1989 CPQRA Guidelines Acknowledgments Acronyms

1

lx wi

xlli xv

lntroductlon

1.1 CPaRA Definttlons 1.2. Consequence Analysis

1

6 8

Source Models

2.1. Discharge Rate Models 2.1.1. BACKGROUND

2.1.2. DESCRIPTION

2.1.3. EXAMPLE PROBLEMS

2.1.4. DISCUSSION

2.2. Flash and Evaporation 2.2.1. BACKGROUND

2.2.2. DESCRIPTION


2.2.4. DISCUSSION

2.3. Dlsperslon Models 2.3.1. NEUTRAL AND POSITIVELY BUOYANT PLUME AND PUFF MODELS 2.3.2. DENSE GAS DISPERSION

15

15 15 18

40

54

57 57 59 69 75 76 85

1 1 1

V

vi Contents

Exploslons and Flres 3.1. Vapor Cloud Explosions (VCE)

3.1.1 . BACKGROUND 3.1.2. DESCRIPTION

3.1.3. DISCUSSION


3.2. Flash Fires

3.3. Physical Explosion 3.3.1. BACKGROUND

3.3.2. DESCRIPTION


3.3.4. DISCUSSION

3.4. BLEVE and Fireball 3.4.1. BACKGROUND

3.4.2. DESCRIPTION


3.4.4. DISCUSSION

3.5. Conflned Explosions 3.5.1. BACKGROUND

3.5.2. DESCRIPTION

3.5.3. EXAMPLE PROBLEM

3.5.4. DISCUSSION

3.6. Pool Fires 3.6.1. BACKGROUND

3.6.2. DESCRlPnON

3.6.3. %AMPLE PROBLEM 3.6.4. DISCUSSION

3.7. Jet Fires 3.7.1. BACKGROUND

3.7.2. DESCRlPnON


3.7.4. DISCUSSION

4

~

127

131 131

134

151

152

158

158 158

160

174

184

185 185

186

194

199

201 201

202

207

208

210 210

21 1

220

223

225 225

226

229

23 1

Effect Models

4.1. Dosc-Response and Probit Functions 4.1.1. DOSE-RESPONSE FUNCTIONS

4.1.2. PROBIT FUNCTIONS

235

236 236

238

Contents vii


4.2. Toxic Gas Effects 4.2.1. BACKGROUND

4.2.2. DESCRIPTION


4.2.4. DISCUSSION

4.3. Thermal Effects 4.3.1. BACKGROUND

4.3.2. DESCRIPTION


4.2.4. DISCUSSION

4.4. Explosion Effects 4.4.1. BACKGROUND

4.4.2. DESCRIPTION


4.4.4. DISCUSSION

5

240

24 1 241

253

257

26 1 262 262

263

267

269

270 270

271

273

273

Evasive Actions

5.1. Background 5.2. Descrlption 5.3. Example Problem 5.4. Discussion

6

275 275 278 280 280

Modellng Systems

References Appendix: CD ROM Glossary Index

283

285 301 305

31 9

Preface

The original CCPS book Guidelines fir Quantitative Rid Analysis (1989) ( C P Q M Guidelines) contained a long chapter on consequence analysis. When CCPS decided to update the CPQM Guidelines in 1995, preparing a second edition for publication, there were major revisions and additions to the material on consequence analysis. These revisions included more detail on many of the consequence models, additional and updated models which reflect the current state of the art, a more complete presentation of the fundamentals of many of the models, and more worked examples. Spreadsheet solutions for all of the worked examples have also been provided.

Because the revised Chapter 2, Consequence Analysis, of the CPQM Guide- lines, 2nd Edition represents an important topic in process safety, CCPS decided to publish the material as a separate book. This will make the material on consequence analysis more readily and economically available to a broader audience, which uses incident consequence analysis evaluation tools, but does not use quantitative risk analyses. This book includes all of the material in Chapter 2, Conse- quence Analysis, of the C P Q M Guidelines, 2nd Edition, re-formatted as a stand alone book. All worked examples and spreadsheet problem solutions are included. All of this material will also be published in the C P Q M Guidelines, 2nd Edition.

Modeling and understanding the consequences of chemical process incidents (unplanned releases of material and/or energy) are important components of a process safety management program. This activity can be generally classified as incident consequence analysis, and it includes models for:

quantity and rate of material release-for example, flow through a hole in a

atmospheric dispersion of released materials pipe, evaporation from a pool

ix

X Preface

thermal radiation models for fires of various types-for example, pool

overpressure and other models for Merent kinds of explosions-for exam-

impact models which estimate the effect of toxic materials, fires, and explo-

Consequence analysis is an important part of many corporate process risk management programs. It is also required for specified materials by many process safety regulations promulgated by national and local regulatory bodies. For example, the United States Environmental Protection Agency requires a consequence analysis of specified material incident release scenarios for materials cov- ered by the Risk Management Plan (W) regulations required by the Clean Air Act of 1990.

Consequence analysis is also an important tool for Chemical Process Quanti- tative Risk Analysis (CPQRA). CPQRA requires identification of potential incidents (what can go wrong?), estimation of consequences of those incidents (what is the impact if it goes wrong?), estimation of the frequency of the incidents (how likely is it to happen?), and combination of this information into a quantitative measure of risk.

If you are involved in process safety management activities which require modeling of the estimated consequences of chemical process incidents, you will find an introduction to the appropriate models in this book. You will also find references to more detailed information and models published in other books by CCPS and others. This book focuses on the immediate impact of unplanned releases of material and energy-fires, explosions, toxic material releases. It does not cover long term consequences of a single exposure from an incident, or the consequences of extended, lower level exposure to continuing discharges of material to the environment. If your process safety management program requires (or evolves to require) quantitative risk analysis of the risks associated with these types of incidents, the additional material you will need to understand quantitative risk analysis can be found in the Guidelinesfi Chemical ProcessQuan- titatbe Risk Analysk, 2nd Edician.

fires, jet fires, flash fires

ple, physical explosions, vapor cloud explosions

sions on people, the environment, and property

Acknowledgments

The Guidelinesfir Consequence Analysis of Chemical Releases is a stand alone publication of material contained in Chapter 2, Consequence Analysis, of the CCPS Guidelines@ Chemical Process Quantitative Risk Analysis, 2nd Edition. This material has been updated from the 1989 edition of the Guidelinn3r Chemical Process Quantitative Risb Analysis (CPQRA Guzdelines) under the guidance of the Center for Chemical Process Safety (CCPS) Risk Assessment Subcommittee (RASC). Most of the material from the C P Q M Guidelines, which was written by the 1989 RASC members, Technica, Inc. (now Det Norske Veritas [DNV]), and several other contributors, remains in this edition. The contributions of the original edition authors are listed in the 1989 C P Q M Guidelines Acknowledgments.

The material in this book was updated and revised from the original Chapter 2 of the 1989 edition of the C P Q M Guidelines by Dr. Daniel A. Crowl ofMichi- gan Technological University. Dr. Crowl also provided a significant amount of new and updated material, a number of new worked example problems, and spreadsheet solutions for all of the worked examples.

The RASC was chaired by Dennis C. Hendershot (Rohm and Haas Com- pany), and the RASC members include Brian R. Dunbobbin and Walter Silowka (Air Products and Chemicals, Inc.), Arthur G. Mundt (Dow Chemical), William Tilton (DuPont), Scott Ostrowski (Exxon Chemicals), Donald L. Winter (Mobil), Raymond A. Freeman (Monsanto), Arthur Woltman (Shell), Thomas Janicik (Solvay Polymers), Richard M. Gustafson (Texaco), William K. Lutz (Union Carbide), Felix Freiheiter and Thomas Gibson(Center for Chemical Pro- cess Safety).

The RASC also thanks the CCPS management and staff for their support of this project, including Mr. Bob Perry, Dr. Jack Weaver, and Mr. Les Wittenberg. The RASC also thanks the following for their peer review:

xi

xii Acknowledgments

Chuck Fryman FMC Corporation

William Geckler PLG (EQE International)

Doan Hanson United States Department of Energy, Brookhaven National Laboratory

Robert Linney

Ken Murphy

Jerry Schroy Solutia, Inc.

Walt Silowka

Air Products and Chemicals, Inc.

United States Department of Energy

Air Products and Chemicals, Inc.

The RASC dedicates this book to two of our friends and colleagues, Mr. Donald L. Winter of Mobil Oil Corporation, and Mr. Felix Freiheiter of the Center for Chemical Process Staff. Both were significant contributors to this book, to the CPQRA Guidelines, 2nd Edition, and to other activities of the CCPS Risk Assessment Subcommittee for many years. Mr. Winter unfortunately passed away due to a sudden illness during the later stages of the writing of the book. Mr. Freiheiter also passed away as the book was being prepared for publication. Their influence can be found throughout the book.

1989 CPQRA Guidelines Acknowledgments

This volume was written jointly by the CCPS Risk Assessment Subcommittee and Technica, Inc. The CCPS Subcommittee was chaired by R. W. Ormsby (Air Products and Chemicals), and included (in alphabetical order); R. E. DeHan, I1 (Union Carbide), H . H . Feng (ICI Americas, formerly of Stauffer Chemical), R. A. Freeman (Monsanto), S . B. Gibson (du Pont), D. C. Hendershot (Rohm and Haas), C. A. Master (Fluor Daniel), R. F. Schwab (Abed-Signal), and J. C. Sweeney (ARC0 Chemical). T. W. Carmody, F. Freiheiter, R. G. Hill, and L. H. Wittenberg of CCPS provided staff support. The Technica team included B. Morgan, A. Shafaghi, L. G. Bacon, M. A. Seaman, L. J. Bellamy, S. R. Harris, P. Baybutt, D. M. Boult, and N. C. Harris. F. P Lees (University of Lough- borough) reviewed and early draft of the document and his comments are gratefully acknowledged. The substantial contributions of the employer organizations (both in time and resources) of the Subcommittee and of Technica are gratefully acknowledged.

An acknowledgment is also made to JBF Associates, Inc. (J. S . Arendt, D. F. Montague, H. M. Paula, L. E. Palko) for their preparation of the subsection on common cause failure analysis (Section 3.3.1) and inclusion of additional material in the section on fault tree analysis (Section 3.2.1), and the Meridian Corpo- ration (C. 0. Schultz and W. S. Perry) for the preparation of the section on toxic gas effects (Section 2.3.1).

Two specific individuals should also be acknowledged for significant contributions:C. W. Thurston of Union Carbide for assistance in the preparation of the subsection on programmable electronic systems (Section 6.3) and G. K. Lee of h r Products and Chemicals who assisted in the preparation of the subsections

xiii

xiv 1989 CPORA Guidelines Acknowledgments

addressing discharge rates, flash and evaporation, and dispersion (Sections 2.1.1., 2.1.2, and 2.1.3).

Finally, the CCPS Risk Assessment Subcommittee wishes to express its sin- cere gratitude to Dr. Elisabeth M. Drake for reviewing the final manuscript and her many helpful comments and suggestions.

Acronyms

AAR ACGIH ACh4H AEC AGA AICHE/CCPS

AICHE-DIERS

AICHE-DIPPR

AIHA Arr API ARC ASME ATC BLEVE CAER CCPS CEP CFD CMA CPI CPU CPQRA CRC

American Association of Railroads American Conference of Governmental Industrial Hygienists Advisory Commission on Major Hazards Atomic Energy Commission American Gas Association American Institute of Chemical Engineers-Center for Chemical Process Safety American Institute of Chemical Engineers-Design Institute for Emergency Relief Systems American Institute of Chemical Engineers-Design Institute for Physical Property Data American Industrial Hygiene Association Auto-Ignition Temperature American Petroleum Institute Accelerating Rate Calorimeter American Society of Mechanical Engineers Acute Toxic Concentration Boiling Liquid Expanding Vapor Explosion Community Awareness and Emergency Responses Center for Chemical Process Safety Chemical Engineering Progress Computational Fluid Dynamics Chemical Manufacturers Association Chemical Process Industry Computer Processing Unit Chemical Process Quantitative Risk Analysis Chemical Rubber Company

xvi Acronyms

CSTR DOE DOT DSC EEC EEGL EFCE EPA EPRI ERPG ERV ESD ESV FAR FDT FEMA FMEA FTA FR HAZOP HEP HFA HMSO HRA HSE IChemE ICI IDLH IEEE IHI INPO ISBN LC LCL LD LFL LNG LOC LPG MAW

Continuous Stirred Tank Reactor Department of Energy Department of Transportation Differential Scanning Calorimeter European Economic Community Emergency Exposure Guidance Level European Federation of Chemical Engineers Environmental Protection Agency Electric Power Research Institute Emergency Response Planning Guidelines Emergency Response Value Emergency Shutdown Device Emergency Shutdown Valve Fatal Accident Rate Fractional Dead Time Federal Emergency Management Agency Failure Modes and Effects Analysis Fault Tree Analysis Failure Rate Hazard and Operability Hazard Evaluation Procedures Human Failure Analysis Her Majestfs Stationery Office Human Reliability Analysis Health and Safety Executive Institution of Chemical Engineers (Great Britain) Imperial Chemical Industries Immediately Dangerous to Life and Health Institute of Electrical and Electronic Engineers Indvidual Hazard Index Institute of Nuclear Power Operations International Standard Book Number Lethal Concentration Lower Confidence Limit Lethal Dose Lower Flammable Limit Liquified Natural Gas Level of Concern Liquefied Petroleum Gas Maximum Allowable Working Pressure

Acronyms xvi i

MIL-HDBK MORT MSDS MTBF NAS NASA NFPA NIOSH

NOAA NRC NSC NTIS NTSB NUREG OREDA ORC OSHA PE PEL PERD PES PFD PHA P&ID PV PLC PLG PR4 PSM R&D RLG RMP ROD ROF RSST RTECS SHTM SPEGL

NJ-DEP

Department of Defense Military Handbook Management Oversight and Risk Tree Analysis Material Safety Data Sheets Mean Time Between Failure National Academy of Science National Aeronautical and Space Administration National Fire Protection Association National Institute for Occupational Safety and Health New Jersey Department of Environmental Protection National Oceanic and Atmospheric Administration National Research Council National Safety Council National Technical Information Service National Transportation Safety Board Nuclear Regulatory Commission Offshore Reliability Data Handbook Organization Resources Counselors, Inc. (Washington, D.C.) Occupational Safety and Health Administration Process Engineer Permissible Exposure Limits Process Equipment Reliability Data Programmable Electronic System Process Flow Diagram Preliminary Hazard Analysis Piping and Instrumentation Diagram Pressure Volume Programmable Logic Controller Pressurized Liquified Gas Probabilistic Risk Assessment Process Safety Management Research and Development Refrigerated Liquified Gas Risk Management Plan (EPA) Average Rate of Death Average Rate of Failure Reactive Systems Screening Tool Registry of Toxic Effects of Chemical Substances Storage and Handling of Highly Toxic Hazard Materials Short-Term Public Emergency Guidance Levels

m'ii Acronyms

SRD

STEL TCPA TNT TLV TNO TXDS UCL UFL UCSIP UNDO UVCE VCDM VCE V D I VRM VSP

Safety and Reliability Directorate (U.K. Atomic Energy Authority, Warrington, England) Short-Term Exposure Limits Toxic Catastrophe Prevention Act Trinitrotoluene Threshold Limit Values Netherlands Organization for Applied Scientific Research Toxicity Dispersion Upper Confidence Limit Upper Flammable Limit Union des Chambres Syndicales de Llndustrie de Petrole United Nations Industrial Development Organization Unconfined Vapor Vloud Explosion Vapor Cloud Dispersion Modeling Vapor Cloud Explosion Verein Deutscher Inghieure Vapor Release Mitigation Vent Sizing Package

GUIDELINES FOR


Consequence analysis plays an important part in Chemical Process Quantitative Risk Analysis (CPQRA). CPQRA is a methodology designed to provide management with a tool to help evaluate overall process safety in the chemical process industry (CPI). Management systems such as engineering codes, checklists and process safety management (PSM) provide layers of protection against accidents. However, the potential for serious incidents cannot be totally eliminated. CPQRA provides a quantitative method to evaluate risk and to identify areas for cost-effective risk reduction.

A complete and detailed discussion of the entire CPQRA procedure is provided by AICHE/CCPS ( 1999).

The C P Q M methodology has evolved since the early 1980s from its roots in the nuclear, aerospace and electronics industries. The most extensive use of probabilistic risk analysis (PRA) has been in the nuclear industry. Procedures for PRA have been defined in the P M Procedures Guide (NUREG, 1983) and the Probabilistic Safety Analysis Procedures Guide (NUREG, 1985).

CPQRA is a probabilistic methodology that is based on the NUREG procedures. The term chemical process quantitative risk analysis (CPQRA) is used throughout this book to emphasize the features of this methodology as practiced in the chemical, petrochemical, and oil processing industries. Some examples of these features are

Chemical reactions may be involved Processes are generally not standardized Many different chemicals are used Material properties may be subject to greater uncertainty Parameters, such as plant type, plant age, location of surrounding popula- tion, degree of automation and equipment type, vary widely

1

2 1 . Introduction

Multiple impacts, such as fire, explosion, toxicity, and environmental con- tamination, are common.

Consequence analysis is also useful for many other purposes than CPQRA. For example, consequence analysis is used for the following purposes:

Determining the acceptability of a site, or an optimum location on plant

Determining equipment design parameters, i.e. stack height, water deluge

Comparative analysis, such as in equipment design option selection. Identification of potential impacts on adjacent facilities, communities and

Assistance in emergency response, such as evacuation vs. take cover deci-

Compliance with regulations, particularly the EPA Risk Management Plan

property.

requirements, etc.

populations.

sion making.

(RMP).

Acute, rather than chronic, hazards are the principal concern of CPQRA. This places the emphasis on rare but potentially catastrophic events. Chronic effects such as cancer or other latent health problems are not normally considered in CPQRA.

Many hazards may be identifkd and controlled or eliminated through use of qualitative hazard analysis as defined in Guidelines@ Hazard Evaluation Proce- drtres, Second Edition (AICHE/CCPS, 1992). Qualitative studies typically iden- tLfy potentially hazardous events and their causes. In some cases, where the risks are clearly excessive and the existing safeguards are inadequate, corrective actions can be adequately identified with qualitative methods. CPQRA is used to help evaluate potential risks when qualitative methods cannot provide adequate understanding of the risks and more information is needed for risk management. It can also be used to evaluate alternative risk reduction strategies.

The basis of CPQRA is to identlfy incident scenarios and evaluate the risk by defining the probability of failure, the probability of various consequences and the potential impact of those consequences. The risk is defined in CPQRA as a function of probability or frequency and consequence of a particular accident scenario:

Risk = F(s, c,f)

s = hypothetical scenario c = estimated consequence(s) f = estimated frequency

1. Introduction 3

This ccfunction can be extremely complex and there can be many numerically different risk measures (using different risk functions) calculated from a given set ofs, c,f:

The major steps in CPQRA, as illustrated in Figure 1.1, are

Risk Analysis: 1. Define the potential event sequences and potential incidents. This may be

based on qualitative hazard analysis for simple or screening level analysis. Complete or complex analysis is normally based on a full range of possible incidents for all sources.

2. Evaluate the incident outcomes (consequences). Incident outcomes might include the total quantity of material released, a downwind vapor concentration, radiant heat flux, or an explosion overpressure. Source models (Chapter 2) and fire and explosion models (Chapter 3) are the major methods used to determine these outcomes.

3. Estimate the incident impacts on people, environment and property. The effect models take the incident outcomes of step 2 and determine the direct impacts-number of individuals affected, property damage, etc. Effect models are discussed in Chapter 4.

4. Estimate the potential incident frequencies. Fault trees or generic data- bases may be used for the initial event sequences. Event trees may be used to account for mitigation and postrelease events.

5. Estimate the risk. This is done by combining the potential consequence for each event with the event frequency, and summing over all events.

Risk Assessment: 6. Evaluate the risk. Identlfy the major sources of risk and determine ifthere

are cost-effective process or plant modifications which can be imple- mented to reduce risk. Often this can be done without extensive analysis. Small and inexpensive system changes sometimes have a major impact on risk. The evaluation may be done against legally required risk criteria, internal corporate guidelines, comparison with other processes or more subjective criteria.

7. If the risk is considered to be excessive, identrfy and prioritize potential risk reduction measures.

Risk Management: Chemical process quantitative risk analysis is part of a larger management scheme. Risk management methods are described in the CCPS Guidelinesfbr Implementing Process Safity Management Systems (AIChE/CCPS, 1994a), Guide- lines fm Technical Management of Chemical Process Safety (AIChE/CCPS, 1989c),

4 1 . Introduction

/ /

/

/--

Define the potential accident scenarios

Consequence \ ) Analysis

/

Estimate the

frequencies

a- / /

I \ consequences

\ I 1 1

\

Estimate the risk

I

Evaluate the risk

prioritize potential risk reduction

measures

FIGURE 1.1. Chemical process quantitative risk analysis (CPORA) flowchart. The dashed line indicates the steps identified as consequence analysis.

and Plant Guidelines fm Technical Management of Chemical Process Safty (AIChE/CCPS, 1995d).

The seven steps in Figure 1.1 are typical of CPQRA. However, it is important to remember that other risks, such as stakeholder concerns, financial loss, chronic health risks and bad publicity, may also be significant. These potential risks can also be estimated qualitatively or quantitatively and are an important part of the management process.

CPQRA provides a tool for the engineer or manager to quanttfy risk and analyze potential risk reduction strategies. The value of quantification was well described by Lord Kelvin.

1 . Introduction 5

I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thought, advanced to the stage of science, whatever the matter may be.

Joschek (1983) provided a similar definition:

a quantitative approach to safety. . . is not foreign to the chemical industry. For every process, the kinetics of the chemical reaction, the heat and mass transfers, the corrosion rates, the fluid dynamics, the structural strength of vessels, pipes and other equipment as well as other similar items are determined quantitatively by experiment or calculation, drawing on a vast body of experience.

CPQRA enables the engineer to evaluate risk. Individual contributions to the overall risk from a process can be identified and prioritized. A range of risk reduction measures can be applied to the major hazard contributors and assessed using cost-benefit methods.

Comparison of risk reduction strategies is a relathe application of C P Q M . Pikaar (1995) has related relative or comparative CPQRA to climbing a moun- tain. At each stage of increasing safety (decreasing risk), the associated changes may be evaluated to see ifthey are worthwhile and cost-effective. Some organizations also use CPQRA in an absolute sense to confirm that specific risk targets are achieved. Further risk reduction, beyond such targets, may stdl be appropriate where it can be accomplished in a cost-effective manner. Hendershot (1996) has discussed the role of absolute risk guidelines as a risk management tool.

Application of the full array of CPQRA techniques allows a quantitative review of a facilitys risks, ranging from frequent, low-consequence incidents to rare, major events, using a uniform and consistent methodology. Having identified process risks, CPQRA techniques can help focus risk control studes. The largest risk contributors can be identified, and recommendations and decisions can be made for remedial measures on a consistent and objective basis.

Utilization of the CPQM results is much more controversial than the methodology. Watson (1994) has suggested that CPQRA should be considered as an argument, rather than a declaration of truth. In his view, it is not practical or nec- essary to provide absolute scientific rigor in the models or the analysis. Rather, the focus should be on the overall balance of the QRA and whether it reflects a useful measure of the risk. However, Yellman and Murray (1995) contend that the analysis should be, insofar as possible, t rue -o r at least a search for truth. It is important for the analyst to understand clearly how the results will be used in order to choose appropriately rigorous models and techniques for the study.

6 1 . Introduction

1 1 CP QRA Definftfons

Table 1.1 and the Glossary define the terms used in this volume. Other tabula- tions of terms have been compiled (e.g., IChemE, 1985) and may need to be con- sulted because, as discussed below, there currently is no single, authoritative source of accepted nomenclature and definitions.

CPQRA is an emerging technology in the CPI and there are terminology variations in the published literature that can lead to confusion. For example, while risk is defined in Table 1.1 as:

a measure of human injury, environmental damage or economic loss in terms of both the incident likelihood and the magnitude of the loss or injury,

readers should be aware that other definitions are often used. For instance, Kaplan and Garrick (1981) have discussed a number of alternative definitions of risk. These include:

Risk is a combination of uncertainty and damage. Risk is a ratio of hazards to safeguards. Risk is a triplet combination of event, probability, and consequences.

Readers should also recognize the interrelationship that exists between an incident, an incident outcome, and an incident outcome case as these terms are used throughout this book. An incident is defined in Table 1.1 as the loss of containment of material or energy, while an incident outcome is the physical manifestation of an incident. A single incident may have several outcomes. For example, a leak of flammable and toxic gas could result in

a jet fire (immediate ignition) a vapor cloud explosion (delayed ignition) a vapor cloud fire (delayed ignition) a toxic cloud (no ignition).

A list of possible incident outcomes has been included in Table 1.2. The third and often confusing term used in describing incidents is the inci-

dent outcome case. As indicated by its definition in Table 1.1, the incident outcome case specifies values for all of the parameters needed to uniquely distinguish one incident outcome from all others. For example, since certain incident outcomes are dependent on weather conditions (wind direction, speed, and atmospheric stability class), more than one incident outcome case could be developed to describe the dispersion of a dense gas.

1.1 CPORA Definitions 7

TABLE 1 . 1 . Selected Definitions for CPORA

Frequency: Number of occurrences of an event per unit of time. Hazard A chemical or physical condition that has the potential for causing damage to people, property, or the environment (e.g., a pressurized tank containing 500 tons of ammonia) Incident: The loss of containment of material or energy (eg., a leak of 10 Ib/s of ammonia from a connecting pipeline to the ammonia tank, producing a toxic vapor cloud) ; not all events propagate into incidents. Event sequence: A specific unplanned sequence of events composed of initiating events and intermediate events that may lead to an incident. Initiating event: The first event in an event sequence (e.g., stress corrosion resulting in leak/rupture of the connecting pipeline to the ammonia tank) Intermediate event: An event that propagates or mitigates the initiating event during an event sequence (e.g., improper operator action fails to stop the initial ammonia leak and causes propagation of the intermediate event to an incident; in this case the intermediate event could be a continuous release of the ammonia) Incident outcome: The physical manifestation of the incident; for toxic materials, the incident outcome is a toxic release, while for flammable materials, the incident outcome could be a Boiling Liquid Expanding Vapor Explosion (BLEVE), flash fire, unconfined vapor cloud explosion, toxic release, etc. (e.g., for a 10 Ib/s leak of ammonia, the incident outcome is a toxic release) Incident outcome case: The quantitative definition of a single result of an incident outcome through specification of sufficient parameters to allow distinction of this case from all others for the same incident outcomes. For ewnple,a release of 10 Ib/s of ammonia with D atmospheric stability class and 1.4 mph wind speed gives a particular downwind concentration profile, resulting, for example, in a 3000 ppm concentration at a distance of 2000 feet. Consequence: A measure of the expected effects of an incident outcome case (eg., an ammonia cloud from a 10 Ib/s leak under Stability Class D weather conditions, and a 1.4-mph wind traveling in a northerly direction will injure 50 people) Effect zone: For an incident that produces an incident outcome of toxic release, the area over which the airborne concentration equals or exceeds some level of concern. The area of the effect

1 zone will be different for each incident outcome case [e.g., given an IDLH for ammonia of 500 , ppm (v), an effkct zone of 4.6 square miles is estimated for a 10 Ib/s ammonia leak]. For a

flammable vapor release, the area over which a particular incident outcome case produces an effect based on a specified overpressure criterion (e.g., an effcct zone From an unconfined vapor cloud explosion of 28,000 kg of hexane assuming 1% yield is 0.18 km2 if an overpressure criterion of 3 psig is established). For a loss of containment incident producing thermal radiation effects, the area over which a particular incident outcome case produces an effect based on a specified thermal damage criterion [e.g., a circular effect zone surrounding a pool fire resulting from a flammable liquid spill, whose boundary is defined by the radial distance at which the radiative heat flux from the pool fire has decreased to 5 kW/m* (approximately 1600 Btu/hr-@)] Likelihood: A measure of the expected probability or Frequency of Occurrence of an event. This may be expressed as a frequency (e.g., events/year), a probability of occurrence during some time interval, or a conditional probability (i.e., probability of occurrence given that a precursor event has Occurred, e.g., the frequency of a stress corrosion hole in a pipeline of size sufficient to cause a 10 Ib/s ammonia leak might be 1 x per year; the probability that ammonia will be flowing in the pipeline over a period of 1 year might be estimated to be 0.1; and the conditional probability that the wind blows toward a populated area following the ammonia release might be 0.1) (continued on nextpage)

8

TABLE 1.1. Selected Definitions for CPORA (cont.)

1 . Introduction

Probability: The expression for the likelihood of occurrence of an event or an event sequence during an interval of time or the likelihood of Occurrence of the success or failure of an event on test or demand. By definition, probability must be expressed as a number ranging from 0 to 1. Risk: A measure of human injury, environmental damage or economic loss in terms of both the incident likelihood and the magnitude of the loss or injury Risk analysis: The development of a quantitative estimate of risk based on engineering evaluation and mathematical techniques for combining estimates of incident consequences and frequencies (e.g., an ammonia cloud from a 10 Ib/s leak might extend 2000 fi downwind and injure 50 people. For this example, using the data presented above for likelihood, the frequency of injuring 50 people is given as 1 x 1 0-3 X 0.1 X 0.1 = 1 X lo- events per year) Risk assessment: The process by which the results of a risk analysis are used to make decisions, either through a relative ranking of risk reduction strategies or through comparison with risk targets (e.g., the risk of injuring 50 people at a frequency of 1 x events per year from the ammonia incident is judged higher than acceptable, and remedial design measures are required)

The event tree in Figure 1.2 has been provided to illustrate the relationship between an incident, incident outcomes, and incident outcome cases.

1.2. Consequence Analysis

This book provides an overview of consequence and effect models commonly used in CPQRA (as shown in Figure 1.3). Accidents begin with an incident, which usually results in the loss of containment of material from the process. The material has hazardous properties, which might include toxic properties and energy content. Typical incidents might include the rupture or break of a pipeline, a hole in a tank or pipe, runaway reaction, fire external to the vessel, etc. Once the incident is known, source models are selected to describe how materials are discharged from the process. The source model provides a description of the rate of discharge, the total quantity discharged (or total time of discharge), and the state of the discharge-solid, liquid, vapor, or a combination. A dispersion model is subsequently used to describe how the material is transported downwind and dispersed to some concentration levels. For flammable releases, fire and explosion models convert the source model information on the release into energy hazard potentials such as thermal radiation and explosion overpressures. Effect models convert these incident-specific results into effects on people (injury or death) and structures. Environmental impacts could also be considered (Paustenbach, 1989), but are not considered here. Additional refinement is provided by mitigation factors, such as water sprays, foam systems, and sheltering or

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