by Paul Gruhn, P.E., CFSE
There are many devices (sensors, logic solvers and final elements) used in safety instrumented systems that are independently certified for use in safety applications to different safety integrity levels (SIL). There is considerable debate however whether fire and 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 and gas systems, focusing on the SIL rating or performance of the actual fire and 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 and gas systems, b) how typical voting of fire and 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 and gas systems.
Keywords: FGS, F&G, Fire and gas systems, SIL, Safety integrity level, ISA 61511, certification