An Introduction to ISA TR84.00.07

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Overview:

SIL Ratings for Fire and Gas System Hardware

There are many devices (sensors, logic solvers and final elements) used in safety instrumented systems that are independently certified for use in certain safety integrity levels (SIL). There is considerable debate however whether fire & gas system hardware should have SIL ratings at all. Vendors are naturally interested in promoting independently certified hardware in order to differentiate their products. Considering the differences between safety instrumented systems and fire & gas systems, focusing on the SIL rating or performance of the actual fire & gas hardware alone is considered by some to be a misleading and questionable practice. This paper reviews a) the differences between safety instrumented systems and fire & gas systems, b) how typical voting of fire & gas sensors not only reduces nuisance trips (which is desirable) but also reduces the likelihood of the system actually responding to a true demand (which is not desirable), and c) why concepts and standards that apply to safety instrumented systems (e.g., SIL ratings) may not be appropriate for fire & gas systems.