SIF VERIFICATION PROCESS

It seems logical to list the following equipment for this safety instrumented function pressure transmitter, logic solver, inlet feed valve, pump control relay, and outlet isolation valve. However, for each piece of equipment ask the question, "Is this piece of equipment needed to protect against the specific hazardous event " In this case, the pump is turned off just to protect the pump from backpressure burnout. (NOTE - This may be part of another safety instrumented function.) The outlet isolation valve is closed in order to avoid process disruptions in the remainder of the plant. Neither is required to protect against the hazard and should not be included in the SIF verification calculation. The pump control relay may be part of another SIF intended to protect the pump. However, it is likely that this SIF may have a lower safety integrity requirement. 

Fortunately for the safety instrumented function designer, many valve manufacturers are getting their products and their design processes assessed to lEC 61508 (Ref. 1). These manufacturers supply a safety manual and all failure rate data (typically assuming a well designed final element) needed for SIF probabilistic verification. Some manufacturers are... 

Industrial standards often define, in a prescriptive manner, the SIFs required to safely manage the process or equipment. For existing SIFs, omission of the SIL verification would only apply when the standard is prescriptive and clearly defines the required architectures and operation/maintenance practices necessary to implement the SIF(s). The scope and coverage of the standard should result in an SIS that provides safe process operation. 

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