Practical Hazops, Trips and Alarms David Macdonald

345 Pages·2008·4.42 MB·English

Practical Hazops, Trips and Alarms
vi Contents
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vi Contents
Practical Hazops, Trips and Alarms
David Macdonald,
BSc (Hons) Inst. Eng., Senior Engineer,
IDC Technologies, Cape Town, South Africa
Series editor: Steve Mackay
AMSTERDAM • BOSTON • HEIDELBERG • LONDON
NEW YORK • OXFORD • PARIS • SAN DIEGO
SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
Newnes is an imprint of Elsevier
vi Contents
Newnes
An imprint of Elsevier
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First published 2004

Copyright © 2004, IDC Technologies. All rights reserved

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British Library Cataloguing in Publication Data
Macdonald, D. M
Practical hazops, trips and alarms – (Practical Professional)
1. Machinery – Safety appliances 2. Machinery – Monitoring
I. Title
621.3'0289

Library of Congress Cataloguing in Publication Data
A catalogue record for this book is available from the Library of Congress

ISBN 0 7506 6274 3

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vi Contents
Contents
Preface................................................................................................................viii
1 Introduction to hazard studies...............................................................................1
1.1 Scope and objectives of this chapter.......................................................1
1.2 Introduction to hazards and risk management.........................................1
1.3 Risk assessment....................................................................................10
1.4 Concepts of Alarp and tolerable risk.......................................................15
1.5 Regulatory frameworks and examples from EU and USA......................21
1.6 Methods of identifying hazards...............................................................26
2 Hazard studies at levels 1 and 2 .........................................................................31
2.1 Introduction.............................................................................................31
2.2 Methodologies for hazard study 1...........................................................37
2.3 Process hazard study 2..........................................................................40
2.4 Practical example of hazard 2 application ..............................................53
2.5 Case study.............................................................................................62
2.6 Conclusion on hazard studies 1 and 2....................................................62
3. Risk reduction measures using alarms and trips.................................................65
3.1 Risk reduction measures........................................................................65
3.2 Terminologies and standards for safety systems....................................66
3.3 Equipment under control ........................................................................67
3.4 Protection layers.....................................................................................72
3.5 The role of alarms in safety....................................................................78
3.6 Alarm types and do they qualify as safeguards?....................................81
3.7 Identification and design of safety-related alarms...................................82
3.8 Key design principles for alarms.............................................................87
3.9 SIS, principles of separation...................................................................90
3.10 Simple and complex shutdown sequences, examples............................92
3.11 Conclusions: the role of Hazops in defining alarms and trips..................95
4 Hazop method ....................................................................................................97
4.1 Introduction.............................................................................................97
4.2 Introduction to Hazop.............................................................................98
4.3 Overview of Hazop method....................................................................99
4.4 Points to note on the examination procedure ...................................... 116
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4.5 Practical exercise: continuous process example..................................120
4.6 Hazop for batch processes and sequential operations.........................120
4.7 Hazops for other disciplines.................................................................124
4.8 Conclusions.........................................................................................129
5 Planning and leadership of Hazops ................................................................ 130
5.1 Introduction......................................................................................... 130
5.2 Organizing the Hazop......................................................................... 130
5.3 The team leader and the team............................................................ 132
5.4 Practical exercise: hybrid batch process example............................... 141
6 Specifying safety instrumented systems ......................................................... 143
6.1 Introduction..........................................................................................143
6.2 Risk reduction by instrumented protection...........................................144
6.3 What affects the safety integrity of an instrument trip? ........................149
6.4 Overview of IEC 61508........................................................................150
6.5 Determining the safety integrity............................................................156
6.6 Design essentials to meet SIL targets..................................................159
6.7 Specifying the SIS requirements..........................................................168
6.8 Documenting the SRS.........................................................................174
6.9 Conclusions.........................................................................................179
7 Hazard analysis methods .................................................................................180
7.1 Introduction..........................................................................................180
7.2 Outline of methods...............................................................................181
7.3 Fault tree analysis................................................................................185
7.4 Practical exercise in FTA.....................................................................192
7.5 Conclusions.........................................................................................192
8 Factors in the choice of protection system .......................................................193
8.1 Introduction and objectives..................................................................193
8.2 Equipment selection.............................................................................197
8.3 Key points about sensors and actuators..............................................206
8.4 Guidelines for the application of field devices in the SIS......................209
8.5 IEC 61508 requirements for field devices............................................216
8.6 Technology issues...............................................................................216
8.7 Guidelines for final elements................................................................218
8.8 Summary of technology and applications.............................................224
8.9 Summary of SIL vs cost.......................................................................224
9 Exercise in specifying an SIS from the Hazop .................................................226
9.1 Introduction..........................................................................................226
9.2 Process description..............................................................................226
9.3 Safety requirements specifications ......................................................231
9.4 Conclusion...........................................................................................236
Contents vii
Appendix A: References used in the manual................................................................237
Appendix B: Some websites for safety systems information.........................................239
Appendix C: Notes on national regulations relevant to hazard study and safety
management.............................................................................................242
Appendix D: Software tools for hazard studies.............................................................247
Appendix E: EPA case study of phenol resin hazards..................................................248
Appendix F: Expanded guideword table for continuous processes...............................249
Appendix G: Methods of reporting................................................................................251
Appendix H: Design and calibration of a risk graph......................................................253
Appendix I: Data capture sheet ....................................................................................257
Appendix J: Glossary of terms used in hazard studies and safety-related systems......259
Practical exercises........................................................................................................264
Exercise 1..........................................................................................................264
Exercise 2..........................................................................................................266
Exercise 3..........................................................................................................268
Exercise 4..........................................................................................................275
Exercise 5..........................................................................................................280
Exercise 6..........................................................................................................286
Exercise 7..........................................................................................................288
Exercise 8..........................................................................................................294
Exercise 9..........................................................................................................302
Answers to practical exercises .....................................................................................306
Exercise 1..........................................................................................................306
Exercise 2..........................................................................................................308
Exercise 3..........................................................................................................309
Exercise 4..........................................................................................................314
Exercise 5..........................................................................................................320
Exercise 6..........................................................................................................323
Index.............................................................................................................................325
Preface
Introduction to the book
This introduction maps out the reasons why this book has been prepared and introduces the ideas
behind it. This section also provides a guide to the contents and uses of the manual.
What is the book about?
This book is about some of the critical activities involved in making sure that a manufacturing
process is:
• Safe for people to work with
• Safe against damage to the environment
• Secure against failures that could result in major asset losses in the business.
It concentrates on the application of hazard study methods and the actions that follow from them for
providing protection against hazards. The book seeks to provide training in three basic steps that form
part of the overall risk management framework for industries such as chemicals, oil and gas,
pharmaceuticals and food processing. The steps can be seen in Figure P.1.
Information on the process
Identify hazards Hazard studies
Hazard analysis and
Evaluate risks
risk assessment
Specify risk reduction measures Trips and alarms

Figure P.1
Scope of the book
Preface ix
• Hazards of a process plant or an activity are identified through the systematic application of
hazard studies based on the best possible information available.
• Hazards may create risks to people, environments and property. The risks may or may not
be acceptable. This requires evaluation through the techniques of risk assessment and
hazard analysis.
• Whenever risks are found to be unacceptable, solutions have to be found either by
fundamental design changes or by providing protection measures. Protection measures may
be mechanical or organizational or they may be provided by safety-related control systems
employing alarm devices or automatic controls. For simplicity in this book we call them
trips and alarms.
For protection measures to succeed they must be based on three key factors:
1. There must be correct and up-to-date knowledge of the hazardous situations that are to be
controlled including knowledge of the possible causes.
2. There must be a clearly defined course of action to be taken in response to the approach to a
hazardous event.
3. The protection systems must be appropriate for the problem and be correctly designed and
maintained.
Appropriate implies that the protection systems must be practical to use, will carry out the correct
actions to restore safe conditions and will be engineered to a level of reliability sufficient to match the
degree of risk reduction demanded by the hazardous event. Furthermore, the protection systems must
not impede productivity or impact negatively on production volumes through its complexity nor create
unacceptable production losses through unreliable operations.
These points appear to be simple and obvious. However, there is a lot of evidence to show that it is not
unusual for risk reduction measures to be out of touch with the original problem they were intended to
deal with. For example, Figure P.2 shows the results of a survey of control and safety system failures by
the United Kingdom Health and Safety Executive (HSE), the body responsible for administration and
control of occupational and home safety in the UK. The survey classified the causes of 34 accidents
involving failures of control systems, which were supposed to protect against such incidents.
HSE summary of findings
The Summary of the problems of failed safety systems found by HSE included some interesting
paragraphs.
Analysis of incidents showed:
• The majority of incidents could have been anticipated if a systematic risk-based approach
had been used throughout the life of the system.
• Safety principles are independent of the technology.
• Situations often missed through lack of systematic approach.
Quoting from the report:
The analysis of the incidents shows that the majority were not caused by some subtle failure mode
of the control system, but by defects which could have been anticipated if a systematic risk-based
approach had been used throughout the life of the system. It is also clear that despite differences
in the underlying technology of control systems, the safety principles needed to prevent failure
remain the same.