In one case you are claiming precision that risk analysis can never really have, in the other you are setting a boundary that you are confident will not be violated. A subtle, but very important distinction. As risk analyst, you don’t say that I KNOW that the frequency of an accident is precisely 1.53E-3 per year, instead you say that I am CONFIDENT that the frequency is less than 1.53E-3.
When performing a risk calculation, you give up on the concept of knowing something precisely, and instead, set boundaries with a degree of confidence. As a risk analyst, you must have a different and more humble approach. While this works well for things that can be known precisely, such as temperatures, pressures and flow rates, it is not realistic for risk. Engineers, in general, are taught to perform rigorous calculations to obtain precise numbers. Additionally, this approach violates the spirit and philosophy of how we have performed SIL verification calculations since the advent of IEC 61508. We’ve already larded up the SIL verification process with so many safety factors that adding another one here is going to cross from very conservative over to comical.
#Honeywell sil verification calculation tool full
So, for instance, if you determine that your SIL verification calculation has an error of +/- 5, then a calculation of an RRF of 102 is really an RRF of between 97 and 107, since the 97 does not achieve the SIL 2 target you should modify the design until the full range, including worst case error, is within the SIL band. For instance, if a SIL 2 function is desired and the calculation shows a risk reduction factor of 102 was achieved, is that good enough? The theory being proposed is that you should establish a limit on what RRF value is acceptable based on the amount of error that is present. The objective of this effort was to determine how much of a margin of error should be placed in the acceptance of a SIL verification calculation. During the recent ISA 84 committee meetings related to the ISA TR84.00.02 which discusses SIL verification calculations I was made aware of an effort to attempt to quantify the amount of error in SIL calculations.
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