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PT Notes

Risk Tolerance Criteria and Safety Instrumented Systems

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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). This is accomplished 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. Consequently, risk tolerance criteria are critically important for compliance with the standard, but the standard does not provide guidance on their development or use.

Methods described in the IEC 61511 / ISA 84 standard for the determination of SILs for SIFs include risk matrices, risk graphs and Layers of Protection Analysis (LOPA). The use of risk matrices and risk graphs is widespread. However, SIL determination must be placed in the context of the overall standard which focuses on validating SILs and ensuring they are met throughout the SIS lifecycle. The standard also requires confirmation of SILs for SIFs by calculation. A large amount of effort is invested to comply with these requirements. Consequently, simple approaches to SIL determination are inconsistent with the overall objectives of the standard. Indeed, it is difficult to justify the use of simplistic approaches to determine SILs followed by detailed and extensive efforts to ensure the SILs are maintained. Moreover, the theoretical foundation of matrix methods, and by implication similar approaches such as risk graphs is questionable. As SILs are the foundation of the IEC 61511 / ISA 84 standard, every effort should be made to ensure they are determined as accurately as possible. At a minimum, approaches such as LOPA should be used.

The LOPA framework can be used to employ both individual and group risk tolerance criteria for both hazard scenarios and the overall facility to ensure that the total facility risk is tolerable and that it is allocated appropriately across receptors for the facility.

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

Risk Tolerance Criteria and the IEC 61511 / ISA 84 Standard on Safety Instrumented Systems , by Paul Baybutt, Process Safety Progress, Vol. 32, Issue 3, pages 307–310, September 2013.

The article is available at: http://onlinelibrary.wiley.com/doi/10.1002/prs.11554/abstract.

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