TABLE OF CONTENTS This chapter is a case study, based on a typical gas detection system. It provides an example of the safety-integrity targeting described in Chapter 22. The topic is fully covered in Functional Safety: A Straightforward Guide to IEC 61508, 2nd edition, 2004, Smith D. J. and Simpson K. G. L., Butterworth-Heinemann, ISBN 07506 6269 7.
Assume that our Gas Detection System has the primary function of providing an executive action input to some other Emergency Shutdown system in order to protect a chemical process plant. This ESD system will close valves in order to isolate the flammable gases so as to prevent subsequent explosion or fire. It is assumed that successful operation of the Gas Detection and ESD systems will prevent fatality.
In order to establish a safety-integrity target, it is necessary to establish a maximum tolerable failure rate (or probability of failure on demand) for the safety-related system in question. The table of SIL targets (from IEC 61508) was shown and explained in Chapter 22.
If a maximum tolerable failure rate is to be established then, in turn, a maximum tolerable risk of fatality must first be stated. There are no hard and fast rules but a typical guide, from the Institution of Gas Engineers and Managers Recommendations SR/15, suggests:
| Scenario | Maximum Individual Risk of Fatality |
|---|---|
| VOLUNTARY (EMPLOYEE) | 10 ?4pa |
| INVOLUNTARY (PUBLIC) | 10 ?5 pa |
It might be argued that the operatives in this plant are accepting a voluntary risk and that...
