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Keywords:

  • process hazard and risk analysis;
  • safety instrumented systems;
  • safety instrumented functions;
  • risk tolerance criteria;
  • SIL determination

The fundamental requirement of the IEC 61511/ISA 84 standard on Safety Instrumented Systems (SIS) is to establish the Safety Integrity Levels (SILs) required for Safety Instrumented Functions (SIFs) by evaluating process risks and comparing them to risk tolerance criteria. This article addresses issues that impact the determination of SILs for SIFs and their implications for use of the standard. The issues largely revolve around the interface of the standard with process safety and risk analysis. They include the justification for relative contributions to risk reduction from SIS and non-SIS protection layers, use of appropriate risk tolerance criteria, difficulties with defining hazardous events consistently, the appropriateness of different SIL determination methods, the importance of fully considering common cause failures, the importance of addressing uncertainties, the need to verify risk reduction for non-SIS as well as SIS protection layers, the role of systematic failures, and the need to ensure quality in supporting process hazard analysis studies. © 2013 American Institute of Chemical Engineers Process Saf Prog 32: 346–350, 2013