Cookbook versus Performance Sis Practices

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 paper 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. Introduction Historically, owner/operators used prescriptive practices to define the safety instrumented system (SIS) requirements. These internal practices were based on experience and industry codes, standards and practices. Internal practices provided the approved equipment technology, architecture, voting, diagnostic expectations, installation details, and maximum proof test interval. Many owner/operators mandated a 6 month to 1 year off-line proof test interval for all SIS devices. These practices were further supported by an approved equipment list, providing the model/version of specific equipment demonstrated to work to the desired reliability in the operating environment. Due to successful mechanical reliability and preventive maintenance programs, as well as increased profit pressures, many owner/operators extended their maintenance (or turnaround) interval of the process unit. These extensions yielded significant economic returns through increased production and enhanced product quality. While extension of the maintenance interval conflicted with the previously selected proof test requirements, evidence mounted that SIS target integrity could still be achieved, using a combination of off-setting factors. Cookbooks often did not allow adjustment for higher integrity equipment,