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PT Notes

Determination of Safety Integrity Levels (SILs) Using Layers of Protection Analysis (LOPA)

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

The industry standard for Safety Instrumented Systems (SISs), IEC 61511 / ISA 84, requires that Safety Integrity Levels (SILs) be established for Safety Instrumented Functions (SIFs). The standard describes their determination by comparing the risk of hazardous events that SIFs protect against with risk tolerance criteria to determine if there is a risk gap, i.e. the need to reduce the existing level of risk to meet the risk tolerance criteria. Significant problems are encountered when putting this approach into practice because there is ambiguity in the meaning of the term hazardous event. Risk tolerance criteria for hazardous events are ill-defined and, therefore, they are not the most appropriate criteria to use.

Furthermore, an informed decision on the tolerability of risk for a facility cannot be made by determining only the tolerability of risk for individual hazardous events. Rather, the tolerability of the cumulative risk from all hazardous events for a facility must be determined. Such facility risk tolerance criteria are the type used by regulators.

A risk model can be constructed within the framework of Layers of Protection Analysis (LOPA) that overcomes these problems. The approach allows the allocation of risk to receptors across companies, facilities, processes, process units, process modes, etc.

For more information, you can contact Primatech or consult the article:

Using Risk Tolerance Criteria to Determine Safety Integrity Levels for Safety Instrumented Functions, by Paul Baybutt, Journal of Loss Prevention in the Process Industries, Volume 25, Issue 6, Pages 1000-1009, November 2012. 

http://www.sciencedirect.com/science/article/pii/S0950423012000897

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