Make Inlet Piping and PSV One-System

Abstract

For pressure safety valves and associated inlet piping, the API recommends the non-recoverable pressure loss should not exceed 3 % of the set pressure at rated capacity flow, with some exceptions, e.g., for remote sensing pilot operated pressure safety valves. The API further notes pressure losses above 3 % are allowable if an engineering analysis shows valve performance is not impacted during relief. The API provides little guidance on the recommended engineering analysis. Calculations show the inlet piping and pressure safety valve (PSV) should be considered one system. Analysis of the system improves the basis for judging pressure safety valve performance, especially when compared with treating inlet piping separately from the PSV and then somewhat arbitrarily judging performance adequacy. With increasing inlet piping pressure loss, the energy in the velocity head at the inlet to the PSV grows in significance. Analyzing the inlet piping pressure loss separate from the PSV neglects this energy and makes the experience and knowledge of the judging engineer paramount. Given current computational capability and today’s litigious regulatory environment, an analytical and consistent basis for judging PSV performance may be of interest.