Safety Assessment—A Method for Determining the Performance of Alarm and Shutdown Systems for Chemical Plants

Safeguards, for costly and hazardous chemical plants, are often provided by means of alarm or automatic shutdown systems. The adequacy of this protection—which is a measure of the capability of the newly installed system in shutting down the plant on the occurrence of a hazardous condition, and its effectiveness over the long term (its reliability)—may be judged by an independent safety assessment. Both qualitative and quantitative aspects of safety analysis are involved in an assessment, the quantitative approach being of particular use where there is a need to compare the anticipated reliability of the proposed system with a numerically specified, reliability requirement. Some of the techniques used in the assessment procedure are discussed in the paper and, by way of example, the performance of a typical shutdown system is assessed and its adequacy judged against a hypothetical plant fault condition.