Analysis of PRV Stability In Relief Systems Part I

Challenges associated with PRV stability issues for existing installations are not unique to any particular segment of the chemical process industry. This is an industry wide problem that has received a lot of attention from both OSHA and industry associations such as API, ACC, and AFPM. A consistent definition of what constitutes an Engineering Analysis is currently being proposed by API/ACC/AFPM for inclusion in the upcoming revision to API 520. API, AFPM, and ACC are diligently working on the development of tools and recommended guidance on how to perform an Engineering Analysis to assess PRV stability for existing installations where the 3 % is exceeded. A consistent methodology is emerging that has a solid mathematical foundation and is installation specific.

We do not currently have sufficient evidence to confirm that existing PRV installations with excessive inlet pressure loss that is more than 3 % and less than the valve blowdown (where the reduced flow capacity is still sufficient) present an increased level of risk that is high enough to warrant physical changes to the installations. Physical changes to existing systems may actually increase the risk while modifications are being implemented and there is no assurance that such costly modifications will actually reduce the risk for all installations where the inlet pressure loss exceeds 3%.


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