API 520 Relief Valve Reaction Force Calculation

[tjpengineer]

For the API 520 All Vapor Flow case reaction force calculation (API 520, PArt II, 4.4.1.1), a component P is referenced as the static pressure in discharge pipe at point of discharge. What value is this? Is this 0 psig for all discharges to atmosphere? Is it the built up back pressure due to the discharge at the time of discharge minus the pressure drop due to friction along the discharge pipe?

 

[hacksaw]

the pipe end is an abrupt change with a discharge coefficient so you can estimate the pressure

the problem is that with compressible flow, you have expansion in the tail pipe and further expansion at the discharge and potential shock formation

seems like the problem has been discussed previously so you might try a search