GUIDELINES FOR ENABLING CONDITIONS AND CONDITIONAL MODIFIERS IN LAYER OF PROTECTION ANALYSIS

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GUIDELINES FOR ENABLING CONDITIONS AND CONDITIONAL MODIFIERS IN LAYER OF PROTECTION ANALYSIS

Center for Chemical Process Safety New York, NY

^T ^ ^ An AlChE Technology Alliance

<ePS Center for Chemical Process Safety

W I L E Y

Copyright © 2014 by American Institute of Chemical Engineers, Inc.

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Library of Congress Cataloging-in-Publication Data:

Guidelines for enabling conditions and conditional modifiers in layer of protection analysis / Center for Chemical Process Safety, New York, NY.

pages cm. Includes bibliographical references and index. ISBN 978-1-118-77793-0 (hardback) 1. Chemical processes—Safety measures. 2. Chemical processes—Safety standards. I. American

Institute of Chemical Engineers. Center for Chemical Process Safety. TP150.S24G855 2014 660—dc23 2013020445

Printed in the United States of America.

10 9 8 7 6 5 4 3 2 1

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, the American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Commit­tee and Subcommittee members, their employers, their employers' officers and di­rectors, nor Unwin Company and its employees and subcontractors 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 Unwin Company and its employees and subcontractors, and (2) the user of this doc­ument, the user accepts any legal liability or responsibility whatsoever for the con­sequence of its use or misuse.

Contents

List of Tables ix List of Figures xi Abbreviations and Acronyms xiii Glossary xv Acknowledgements xxiii Preface xxv

CONTEXT 1 1.1 LOPA Overview 1 1.2 Pertinent LOPA Variations 10 1.3 When to Use Enabling Conditions and Conditional Modifiers 13 1.4 Risk Criteria Endpoints 16

LOPA ENABLING CONDITIONS 23 2.1 Definition and Defining Characteristics 23 2.2 Interrelationship with Initiating Event 23 2.3 Time-At-Risk Enabling Conditions 23 2.4 Campaign Enabling Conditions 30 2.5 Other Possible Enabling Conditions 35 2.6 Documenting and Validating Enabling Conditions 35

LOPA CONDITIONAL MODIFIERS 37 3.1 Definition and Defining Characteristics 37 3.2 Probability of a Hazardous Atmosphere 42 3.3 Probability of Ignition or Initiation 44 3.4 Probability of Explosion 49 3.5 Pobability of Personnel Presence 5 5 3.6 Probability of Injury or Fatality 60 3.7 Probability of Equipment Damage or Other Financial Impact 65 3.8 Documenting, Managing and Validating Conditional Modifiers 69

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viii Guidelines for Enabling Conditions and Conditional Modifiers

4 APPLICATION TO OTHER METHODS 71 4.1 Quantitative Risk Analysis 71 4.2 Use of Enabling Conditions and Conditional Modifiers with

Scenario Identification Methods 75 4.3 Barrier Analysis and Diagrams 80

APPENDICES 81 A Simultaneous Failures and "Double Jeopardy" 83 B Peak Risk Concepts 91 C Example Rule Set for LOPA Enabling Conditions 95

REFERENCES 99

INDEX 103

List of Tables

1.1 Example LOPA worksheet 9

1.2 Range of possible endpoints for example toxic/flammable 18 release event

2.1 Time-at-risk enabling condition example 25

2.2 Campaign enabling condition example 31

2.3 Partial-year-operation enabling condition example 33

2.4 Other enabling condition example 36

3.1 Probability of hazardous atmosphere conditional modifier 43 example

3.2 Imperial Sugar dust explosion incident causes 50

3.3 Probability of explosion conditional modifier example 52

3.4 Conditional modifier probability: One person present 59

3.5 Antiterrorism building protection level vs. potential injury 63

3.6 Probability of financial impact conditional modifier example 66

3.7 Explosion damage to equipment and structures 66

3.8 Explosion effects on processing equipment 67

4.1 Example HAZOP/LOPA scenarios 78

B. 1 Campaign enabling condition example with a one-week 93 campaign

B.2 Campaign enabling condition example with a one-week 93 campaign, limiting the time-at-risk probability to a minimum of 0.1

C.l Example rule set for use of enabling conditions in LOPAs 96

List of Figures

1.1 Event tree to illustrate different LOPA endpoints 17

2.1 Diagrams for time-at-risk enabling condition examples 29

2.2 Diagram for campaign enabling condition example 33

3.1 Probability of ignition conditional modifiers for external 48 release

3.2 Example correlation for probability of ethylene DDT 54

3.3 Example effect area for determining probability of personnel 60 presence

4.1 Illustration of enabling condition usage in a Fault Tree 72 Analysis

4.2 Illustration of conditional modifiers usage in a Fault Tree 73 Analysis

4.3 Event tree with conditional modifiers 73

4.4 Bow-tie diagram concept 79

A.l "Swiss cheese" model 85

A.2 Simplified example process 86

A.3 Abbreviated fault tree for concurrent initiating events example 87

A.4 Quantified fault tree for concurrent initiating events example 88

B. 1 Illustration of the comparison between annual-average and 92 peak risks

B.2 Concept of placing a limit on how much peak risk can exceed 94 annualized risk

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