PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

The international standard on safety instrumented systems (SISs), IEC 61511 / ISA 84, requires the determination of safety integrity levels (SILs) for safety instrumented functions (SIFs) in SISs. Risk / safety matrix, risk graph, layers of protection analysis (LOPA), and other more quantitative methods are identified in the standard as acceptable approaches.

Risk matrices and risk graphs are the simplest of the methods and some companies favor them for that reason. Unfortunately, these simple methods have proven to be seriously flawed. Fortunately, the difficulties can be overcome by using LOPA. However, LOPA is susceptible to errors and considerable care must be taken. Issues that affect the use of LOPA include the nature of hazardous events and hazard scenarios, and the significance, allocation, and use of overall facility risk tolerance criteria.

A procedure has been developed to address these issues. The procedure entails the calculation of overall facility risk and an examination of risk contributions to individuals and groups of people from sources of risk such as different types of hazards, different modes of process operation, and different process units and processes. The procedure is illustrated with an example.

A step-by-step approach for performing the analysis is described and illustrated with an example in the article:

Overcoming challenges in using LOPA to determine SILs, Journal of Loss Prevention in the Process Industries, Volume 48, pages 32 - 40, July 2017.

The article can be accessed for free until May 28, 2017 by clicking on the link below. No sign up or registration is needed.

https://authors.elsevier.com/a/1UrQx3O6UQLIh3

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