Help request: SIL verification **based on failure (Beta) values
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When using the exSILentia verification software to calculate the PFD, for loop structures of the 1oo2 or 2oo3 type, a smaller value for common cause failure (Beta) results in a lower PFD value and a higher RRF; whereas for loop structures of the 2oo2 or 3oo3 type, a smaller value for common cause failure (Beta) leads to a higher PFD value and a lower RRF. What is the principle behind this? What does the software use to obtain such results? What is the basis?
This post was last edited by ZhiGE on 2020-6-24 at 16:49. 1. The probability of failure of a PFD (Probability of Failure on Demand) system is inversely related to the RRF (Risk Reduction Factor). Therefore, the smaller the PFD, the larger the RRF, and the higher the SIL level. 2. The PFD calculation method for typical voting mechanisms is as follows; by using this formula, it is possible to see the impact of common failure factors on the PFD calculation results for different voting mechanisms. 3. Taking 2002 as an example: the smaller β is, the smaller the PFD is. It doesn’t match what the original poster said.
It is because 1oo2 and 2oo3 can better eliminate common-cause effects, exerting a synergistic effect in reducing PFD.