Cookbook versus performance SIS practices

Abstract

A safety instrumented system (SIS) is designed to achieve or maintain a safe state of the process when unacceptable process conditions are detected. An SIS is an independent protection layer that is covered by the performance-based standard ANSI/ISA 84.00.01-2004. The risk reduction allocated to the SIS determines its target safety integrity level (SIL). ANSI/ISA 84.00.01-2004 allows a combination of factors to be considered in the verification of the SIL of the SIS. Performance-based practices provide flexibility to users, yet add complexity to the design process, encouraging project teams to reinvent the wheel for even widely used process equipment. For many engineering applications, prescriptive approaches are favored due to simplicity. These so-called cookbook practices were very common in the process industry when ANSI/ISA 84.01-1996 was issued. They are also the backbone of many application standards and recommended practices. The cookbook typically specifies the SIS and maximum proof test interval based on analysis and accepted practice. The user must ensure that the cookbook assumptions are met by the existing equipment and mechanical integrity program. Otherwise, the installed risk reduction may not achieve the expected performance. This article provides an example of a “cookbook” approach for a simple SIS and illustrates the effect of extending the proof test interval from 1 year to 5 years on its probability of failure on demand. © 2008 American Institute of Chemical Engineers Process Saf Prog 2008