![]() For balanced bellows and pilot operated PSV's in the cases in which the build-up back pressure might increase the total back pressure such that exceeded the acceptable back pressure limits, then back pressure correction should be included build-up back pressure. Then, because in these valves in any case the build-up back pressure should be lower than allowable overpressure, no need to consider build-up back pressure in back pressure correction factor. Superimposed backpressure Excess pressure which is already present in the outlet pipe of the safety valve before the blow-off phase begins. For conventional PSV it is just needed to consider correction for constant (or maximum variable) superimposed back pressure. Excess pressure in the outlet pipe of the safety valve which is built up during the blow-off phase (opening) and is therefore variable. In terms of backpressure correction, which backpressure should be considered? It further goes on to say in Sections 5.3.2.2 & 5.3.2.3, 'Balanced spring-loaded or pilot-operated PRVs should be considered if the superimposed backpressure is variable.
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May 2023
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