CyberCertPrepOT Security FundamentalsOT Fundamentals: Safety Systems & Physical SecurityMEDIUMFree question
A single channel valve has a dangerous undetected failure rate of 2e-6 per hour and is proof tested yearly. Using PFDavg = lambda DU x TI / 2 with 8760 hours per year, where does the valve land?
- A.SIL 2 at about 0.0175 average PFD
- B.SIL 3 at about 0.0009 average PFD value
- C.SIL 1 at about 0.088 average PFD
- D.SIL 2 at about 0.0088 average PFD
Why D is correct
PFDavg = 2e-6 x 8760 / 2 = 0.00876 which is inside the SIL 2 band of 0.001 to 0.01. Forgetting the divide by two gives 0.0175 and mis-scaling by ten gives the SIL 1 or SIL 3 figures.
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