SIL study of F&G system

Hi everybody here!
I have problem for SIL verification of an exiting f&g (fire & gas) detection system,
the problem is evaluating the amount of PFD (probability of failure on demand).
In my opinion PFD evaluation for f&g system is different from common SIF in process system,
because in the case of exiting a disturbance in process all redundant sensors sense the disturbance and the probability of failure for all of them is equal. because they are a system of for example 2oo3 and we evaluate SIF's PFD for them.
But in analogous f&g system by 2oo3 voting system, there is no equal probability of sensation of gas concentration by three sensors. one or two or three sensor detect the gas cloud and when we evaluate PFD we are not sure that they sense the cloud really or no!???? so we dont know there is any demand or no?! and if we have not any demand the calculation of PFD will be meaningless.
therefore How we can evaluate the amount of SIL?

i appreciate the answer to this subject because i will go initiate a SIL study of the HRSG BMS.

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

thank you for your information. Have you a SIL study example that you can share with me?
Thanks [email]jose_manuel_freitas@yahoo.com[/email]

[QUOTE=matajaldin123;205263]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[/QUOTE]

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.