Overpressure Protection by System Design - Use of High Integrity Pressure Protection Systems
Use of a safety instrumented system (SIS) in lieu of a traditional pressure relief device is commonly referred to as a high integrity pressure protection system (HIPPS). With the approval of ASME code case 2211 in 1996, ASME Section VIII has been updated to allow for overpressure protection by system design in the BPV Code Part UG-140. A detailed analysis and thorough documentation is required to ensure that the vessel is protected by “system design”, or in other words, the pressure is self-limiting, or there are no credible overpressure scenarios that result in the vessel pressure exceeding 116% of the maximum allowable working pressure (MAWP). API 521 Pressure-relieving and Depressuring Systems Appendix E allows replacement of a pressure relief device with an SIS that has a reliability consistent with the risk of an overpressure event. The API approach is similar to the approach outlined by CCPS, i.e. they are risk based accounting for both the probability of the overpressure event and the consequences of such an event. In contrast, the ASME guidance is based only on reducing the likelihood of an overpressure event.
The use of HIPPS to replace conventional pressure relief protection is growing. Some of the main drivers include the need to reduce flare system loads on large green field projects, limit and control the release of green house gases and also to allow for increased capacity in brownfield expansions.
This paper will review the differences in the ASME, API and CCPS approaches for overpressure protection using safety instrumented systems, the “credibility” criteria in ASME and the design of high integrity instrumented systems required to meet the code requirements. The paper will also include a discussion on how different users have implemented HIPPS in the offshore and onshore processes.