Regarding the calculations for build up back pressure,

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In the safety valve data sheet, there is usually an entry for \"build up back pressure\". What does this pressure refer to? API specifies 3.7 for this value. Built-up backpressure is defined as the increase in pressure at the outlet of a pressure-relief device that occurs as a result of fluid flow after the device opens. Literally, built-up back pressure is the increase in pressure resulting from fluid flow; so how should this value be calculated? Is it equal to the pressure drop in the outlet pipe? Some people say it isn’t equal, stating that this value needs to be simulated. Let me give an example: suppose the additional back pressure of the safety valve is 3 KPa, and the resistance at the pipe outlet is 10 KPa. In that case, is the build-up back pressure 10? Is the total back pressure 13? Still, this value needs to be simulated. For example, if the pressure at the pipe outlet is simulated to be 5 KPA, then is the build-up back pressure 15 KPa? Is the total back pressure 15+3=18? Are these concepts really vague? May I ask an expert,