Fire and Gas Systems (FGS) are mitigation methods, this mere fact has brought an never-ending controversy about applicability of safety systems codes and standards to FGS, given that SIS operates as a protection layer instead, as per LOPA analysis.
NFPA 72 introduced performance-based analysis and design approach, in Annex B, that is similar to safety life-cycle phases in IEC61511 standard. Gas detection is not included in NFPA codes nor IEC61511, but ISA S84 WG 6 F&G committee has worked to develop guidelines to help application of performance-based concepts to a FGS implementation and verification.
A method for calculation was presented by the ISA S84 WG 6 F&G committee, which found as an usable Plant Hazard Analysis the Event Tree Analysis (ETA). Calculation details are shown in figure below, a detector coverage probability of 90% has been selected, system response of 99% and effectiveness of 90% on mitigation.
PFDavg = 1 – 0.8019 = 0.1981 (19.81%) → < SIL 1
It has been proved that even with 99% of detector coverage the SIL number will never higher than 1, thus FGS will only achieve SIL 1 at best.
Detector coverage is the main factor on SIL calculation, however it is influenced by placement, amount of detectors for a single hazard, wind speed and direction, ambient temperature, ambient pressure, alarm levels settings and sensor sensitivity. All those elements affect the ability of the FGS to detect the gas leak or flame and execute the Safety Function, even if the hardware operates properly.
So it doesn’t matter if the logic solver, detectors, signaling and actuating devices are certified for SIL 1,2,3 applications, hardware selection alone does not guarantee achieving a desired performance level.
Overall plant safety cannot underestimates FGS performance level compared with SIS. FGS is a protection layer and has to be designed, installed and maintained following every applicable standard, code and engineering practices.