Safety Integrity Level Selection -- Systematic Methods Including Layer of Protection Analysis
How to Pick a SIL - Apr 24, 2008
The first half of the book provides an introduction to the subject; both a purpose and a connection to the safety lifecycle. A key to a quantitative assessment process is setting a limit for tolerable risk, where risk is a function of consequence severity and event frequency. This may be set As Low As Reasonably Practicable (ALARP), which may vary from country to country or from company to company. Based on the design, PHA and risk analysis, the correct Safety Integrity Level (SIL) can be selected for each Safety Instrumented Function (SIF). Because many of the techniques are based on probability theory, there is a chapter to explain the assumptions, calculations, and symbology.
The second half of the book covers various methods for the allocation of safety layers and SIL selection, starting with determination of consequences. Event tree analysis, reliability block diagrams, fault tree analysis, and Layer of Protections Analysis (LOPA) are different methods. Event tree analysis is covered in some detail, as is LOPA, a simplified version of event tree analysis. There are other books that go into more detail on these methods. The final chapter shows how qualitative and risk chart methods can be applied to SIL selection.
Marszal and Scharpf summarize several approaches to SIL selection. Their book is a helpful reference since many people are involved with risk assessment. The first edition is worth buying and won the Thomas Fisher award as the best seller from ISA the year it was published.
Best resource in SIL Selection - Feb 07, 2008
Marszal's & Scharpf's text is one of the best ones available on the subject. As the text is regarding SIL selection, it focuses more on Safety Life Cycle (SLC) phases, namely, Hazard & Risk Analysis, Allocation of Safety Requirements to Protection Layers & to some extent Safety Requirement Specification. As with other texts on the subject, it discusses other phases of SLC also.
The aspects that I liked most in the text are coverage of the concept of "Tolerable Risk" (Chapter 3), "Consequence Analysis Overview" (Chapter 6), and "Semi-quantative risk analysis techniques for SIL determination: Event Tree analysis/LOPA". Chapter 3 describes as to how Risk should be expressed (PLL, Individual Risk, Risk Aversion Factor, Societal Risk), meaning of Tolerable Risk, moral & legal issues associated with the subject. It's one of the best and concise explanations that I have ever read on the subject.
Subject of Consequence Analysis is introduced nicely in the text. The chapter is required in order to appreciate the vast field of consequence analysis associated with Hazard. However, I would like to see more material on the subject in next revision of the text.
Lastly, semi-quantitative methods like Event tree analysis, LOPA for SIL determination is nicely covered. One of the important feature that is covered is calibration of Risk matrices/Risk graphs w.r.t definition of Tolerable Risk. Examples are given together with exercises at the end of each chapter to help understand the subject. Probability and some material concerning Reliability Engineering is also covered to explain the meaning of PFD, RRF and perform SIL selection exercises.
All in all the text is a nice package and good companion to IEC 61511-3 (in fact it's indispensible).
I am grateful to the Authors for writing such a nice text.
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