SIL study of F&G system

[safetyuser]

Dear Sir,
Actually SIL classification may not applied to F&G system same as other safety functions. This is because of the fact that F&G systems are post event system and they will reduce the consequence of event not likelihood of the events as ESD safety function do.
In the other hand the coverage percent of the F&G system is another problem and in the best contrition the figure is around 50%~60%.
The third reason is F&G system is not low demand system and as you know one F&G sensor may initiate in every day by little gas leakage of false alarm so you may not use PFD formula as ESD safety function.
With regard above matters generally you can define target SIL of F&G system only as a figure that is not valuable for plant safety design.
Regards
Taj

It is my opinion that some protective systems (such as inergen discharge total saturation in manned areas or closing of HVAC dampers when toxic is detected in intakes) do are parts of safety functions and a SIL rating should be properly investigated since they have large parts in reduction of injuries and/or fatalities.
The figure of 60% may be properly enhanced with suitable detailed design and proper selection of F%G field devices.

False alarms are not part of a SIL assessment but it is recognized a spurious trip assessment (i.e. investigation of the robustness needed to false trips avoidance) is the complementary part of the analysis.