Appendix 5
Answers to Examples
Answer to Exercise 1 (Section 2.1.1 (d))
Propagation to fatality is 1:2 times 1:5 = 0.1.
Maximum tolerable failure rate leading to single fatality is 10−5 pa/10−1 = 10−4 pa; however the actual process failure rate is 0.05 pa = 5 × 10−2 pa.
Thus the protection system should have a target probability of failure on demand (PFD) no worse than:
10−4 pa/5 × 10−2 pa = 2 × 10−3.
The target is dimensionless and is thus a PFD. the Low Demand column in Table 1.1 is therefore indicated.
Thus the requirement is SIL 2.
Answer to Exercise 2 (Section 2.1.1 (d))
Answer 2.1
Since there are 10 sources of risk (at the same place) the maximum tolerable fatality rate (per risk) is 10−5/10 = 10−6 pa.
Target toxic spill rate is 10−6 pa/10−1 = 10
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