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The Safety Critical Systems Handbook, 4th Edition by Kenneth L. Simpson, David J. Smith

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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 105 pa/101 = 104 pa; however the actual process failure rate is 0.05 pa = 5 × 102 pa.
Thus the protection system should have a target probability of failure on demand (PFD) no worse than:
104 pa/5 × 102 pa = 2 × 103.
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 105/10 = 106 pa.
Target toxic spill rate is 106 pa/101 = 10

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