SIL RATINGS FOR FIRE GAS SYSTEM HARDWARE-ARE WE BARKING UP THE WRONG TREE?

摘要

Current standards covering fire and gas systems are prescriptive and focus on commercial applications such as buildings. Many end users in the process industry believe there is a need for a performance based standard for fire and gas systems used in industrial applications. Other performance based standards such as IEC 61508~1 and 61511~2 use the term SIL (Safety Integrity Level) to describe system performance. There are many devices used in safety instrumented systems in the process industries that are independently certified for use in certain integrity levels. However, there is considerable debate 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. However, 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 may 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.