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Environmental, Health & Safety :: Plant & Personnel Safety :: Safety Instrumentation

September 1, 2007

Tolerable Risk

While determined risk is generally well understood, tolerable risk can be the missing link to complete risk assessments

Mike Schmidt, Emerson Process Management

Safety instrumented systems (SIS) are in the spotlight. More companies have come to recognize the importance of recently published consensus standards, and recent fatal accidents tied to the failure of instrumented safeguards have brought further attention. The result is that facilities all over the world are looking at their safety interlocks and concluding that what they really need is a full-blown SIS.
As some work to upgrade their SISs, they find plenty of guidance available for the early steps — process hazard analysis (PHA), consequence analysis, and layers of protection analysis (LOPA) — and also guidance for the later steps — component selection, failure probability and SIL (safety integrity level) verification calculations, and system-design guidelines. A disconnect comes when end users have developed their lists of safety instrumented functions (SIFs) and are asked to assign an SIL to each one (Table 1)....

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