Safety instrumented systems in lieu of pressure relief valves

Abstract

There are two aspects to risk assessments: safety issues and regulatory issues. Usually the two are aligned, but not always. This article explains ASME B&PVC Section VIII (Sec VIII) [American Society of Mechanical Engineers, 2013 ASME Boiler & Pressure Vessel Code: Section VIII, Division I. s.l.] requirements along with other industry guidance for using Safety Instrumented Systems (SIS) in lieu of pressure relief devices (e.g., pressure relief valves or rupture disks). For most pressure vessels in petrochemical installations in the United States, the Sec VIII requirements are regulatory requirements. A designer that is implementing facility changes from a corporate risk assessment (e.g., Layer of Protection Analysis or Hazard and Operability Study (HAZOP)) would do well to also meet the minimum regulatory requirements for overpressure in Sec VIII. Typical risk assessments allow the designer to consider both the likelihood and consequences of an overpressure scenario in determining if additional mitigation is required. Whereas, Sec VIII does not allow any credible overpressure scenarios (solely based on likelihood) unless there is adequate capacity provided by a relief device. This article discusses the detailed typical risk analysis compared to the requirements of Sec VIII overpressure protection by system design. The SIS reliability rating or determination of said rating (e.g., SIL‐3 Systems) is outside the scope of this article. © 2014 American Institute of Chemical Engineers Process Saf Prog 33: 345–349, 2014