NEW RELIABILITY MEASURE FOR SAFETY INSTRUMENTED SYSTEMS

Abstract

Safety instrumented systems (SISs) are installed to provide risk reduction and the performance of a SIS can be assessed by its ability to reduce risk. This article introduces a new quantitative measure for the risk reduction, denoted PFD*. Compared with the current reliability measures, the new measure takes into account the demand rate, and therefore can be used for SISs operating in both low-demand and high-demand mode. For a SIS operating in low-demand mode, the PFD* is approximately equal to the standard probability of failure on demand (PFD) used in IEC 61508 and related standards. PFD* can therefore be considered as an extension and improvement of the standard PFD. Successful handling of a demand verifies the functional status of a SIS in a way similar to a functional test, and the PFD* will therefore depend on the demand rate. The PFD* can be used to select the functional test interval according to the risk reduction allocated to the specific SIS. The properties of the new measure are analyzed through a case study of a 1-out-of-2 system of pressure transmitters.