![]() ![]() Operators disclosed that they would first close isolation valve B at the outlet and then close isolation valve A at the inlet. No issues were found with either type of backpressure and it was confirmed the relief temperature was not reducing the MAWP by running the properties through a process simulator. The typical next course of action was to evaluate the discharge piping hydraulics and ensure that the built-up or superimposed backpressure was lower than the Bellows’ MAWP. Nothing appeared to be amiss in the system details. We will discuss this further in the summary. Note, at this point, the vessel was no longer protected by any relief valve. In order to perform repairs on PSV 1, operators began the process of switching the inline spare (PSV 2), into service by closing the isolation valve, 1B, at the outlet of PSV 1. The refinery experienced an overpressure event and PSV 1 began to leak. Inlet isolation valve is in position A, and the outlet isolation valve is in position B. In this case, a major oil and gas producer had two spring-loaded pressure safety valves, PSV 1 (in service), and PSV 2 (stand-by spare), protecting a single pressure vessel with an isolation valve installed at both the inlet and outlet of each PSV. A recent analysis of an isolation valve installation reinforces the need for such a procedure and highlighted a previously overlooked source of overpressure. The use of isolation valves requires standard operating procedures (SOP) describing detailed steps for isolation. ![]()
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December 2022
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