Hi,

Is a PHA/HAZOP necessary to evaluate SIL levels? is it within ISA 84.01 to tabulate SIL levels directly from the PFD (Ave) value calculated from the MTTF figures and the test interval? How should the test interval be selected?

Any recommendations for authoritative and easy to follow reads?

Rgds
B. Wills

 

I believe that you may have a basic misunderstanding of safety integrity level(SIL). There are two SILs, target and realized.

The target SIL comes from a risk analysis which determines the necessary risk reduction (or safety availability) that a safety instrumented function (SIF) has to provide for a specific hazard (based on company risk standards, other available protective functions, consequence, frequency, etc), of which a PHA typically plays a part (PHA identifies the process hazards and consequences and therefore provides valuable information but not typically all the necessary information). The risk analysis is commonly done through a SIL determination or selection study. Most commonly these are done separate from a PHA but some companies combine them. There are various methods used. A common one is the Layer of Protection analysis (LOPA). See ANSI/ISA-84.01 Appendix A and IEC 61511-3 for a description of some of the qualitative methods. There are quantitative methods such as fault tree analysis that are also used.

The realized SIL comes from quantitative verification of the designed system based on the design, the SIF equipment failure rates (MTTFs), the repair rates (MTTR), diagnostics, test frequencies, test coverages, etc. See ISA TR84.00.02, Parts 1-5 for methods to do this.

The test frequency(s) can determined by a number of things such as the required target SIL, system availability for test, on-line testing capability, system architecture, manufacturer recommendations, industry practice, experience, etc.

There is a book available on the subject:

Safety Integrity Level Selection, Systematic Methods Including Layer of Protection Analysis, Ed Marszal P.E. C.F.S.E & Dr. Eric W. Scharpf, MIPENZ, ISA, ISBN 1-55617-777-1, 2002.

Bill Mostia
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