Continue on: What is Backpressure, Superimposed Backpressure?

Built-up Backpressure of PRV; 

Built-up back pressure is the pressure that will occur when the pressure relief valve (PRV) opens and the pressure is released. This is because there Head/friction loss (Pressure drop) in the pressure relief system:

Example of PRV Backpressure; 

Example of Backpressure Calculation of Pressure Relief Valves (PRV) in the system