E-Book Content
CHAPTER 1
The Meaning and Context of Safety Integrity Targets Chapter Outline 1.1 Risk and the Need for Safety Targets 4 1.2 Quantitative and Qualitative Safety Targets 1.3 The Life-cycle Approach 10
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Section 7.1 of Part 1 10 Concept and scope [Part 1 e 7.2 and 7.3] 11 Hazard and risk analysis [Part 1 e 7.4] 12 Safety requirements and allocation [Part 1 e 7.5 and 7.6] 12 Plan operations and maintenance [Part 1 e 7.7] 12 Plan the validation [Part 1 e 7.8] 12 Plan installation and commissioning [Part 1 e 7.9] 12 The safety requirements specification [Part 1 e 7.10] 12 Design and build the system [Part 1 e 7.11 and 7.12] 12 Install and commission [Part 1 e 7.13] 12 Validate that the safety-systems meet the requirements [Part 1 e 7.14] Operate, maintain, and repair [Part 1 e 7.15] 13 Control modifications [Part 1 e 7.16] 13 Disposal [Part 1 e 7.17] 13 Verification [Part 1 e 7.18] 13 Functional safety assessments [Part 1 e 8] 13
1.4 Steps in the Assessment Process Step Step Step Step Step Step Step
1. 2. 3. 4. 5. 6. 7.
1.5 Costs
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Establish Functional Safety Capability (i.e. Management) 13 Establish a Risk Target 13 Identify the Safety Related Function(s) 14 Establish SILs for the Safety-related Elements 14 Quantitative Assessment of the Safety-related System 14 Qualitative Assessment Against the Target SILs 14 Establish ALARP 14
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1.5.1 Costs of Applying the Standard 15 1.5.2 Savings From Implementing the Standard 16 1.5.3 Penalty Costs from not Implementing the Standard
1.6 The Seven Parts of IEC 61508
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Safety Critical Systems Handbook. DOI: 10.1016/B978-0-08-096781-3.10001-X Copyright Ó 2011 Dr David J Smith and Kenneth G L Simpson. Published by Elsevier Ltd. All rights of reproduction in any form reserved
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Chapter 1
1.1 Risk and the Need for Safety Targets There is no such thing as zero risk. This is because no physical item has zero failure rate, no human being makes zero errors and no piece of software design can foresee every operational possibility. Nevertheless public perception of risk, particularly in the aftermath of a major incident, often calls for the zero risk ideal. However, in general most people understand that this is not practicable, as can be seen from the following examples of everyday risk of death from various causes: All causes (mid-life including medical) All accidents (per individual) Accident in the home Road traffic accident Natural disasters (per individual)
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Therefore the concept of defining and accepting a tolerable risk for any particular activity prevails. The actual degree of risk considered to be tolerable will vary according to a number of factors such as the degree of control one has over the circumstances, the voluntary or involuntary nature of the risk, the number of persons at risk in any one incident and so on. This partly explains why the home remains one of the highest areas of risk to the individual in everyday life since it is there that we have control over what we choose to do and are therefore prepared to tolerate the risks involved. A safety technology has grown up around the need to set target risk levels and to evaluate whether proposed designs meet these targets, be they process plant, transport systems, medical equipment or any other application. In the early 1970s people in the process industries became aware that, with larger plants involving higher inventories of hazardous material, the practice of learning by mistakes (if indeed we do) was no longer acceptable. Methods were developed for identifying hazards and for quantifying the consequences of failures. They were evolved largely to assist in the decision-making process when deve