I have a few queries about relief valve loads (assuming discharge to atmosphere).

The 'thrust at vent pipe exit' which Caesar can calculate is taken from API520, correct?

This load acts in the opposite direction of the vent piping. What about the vertical force which should be acting at the relief valve? I can understand that the horizontal forces will be balanced for steady state flow, but the vertical force from the momentum change should still act upwards on the valve shouldn't it?

API520 seems to suggest there is single force on the vent pipe only when the PRV is connected to a vessel (in which case my 'missing' vertical force would be balanced by the tension in the pipe and vessel). For my situation however, the PRV is branching off a steam line, so this does not seem valid (as the upwards force could lift the pipe).

Also, should a PRV 'pop' case be run? That is, the moment when the PRV first opens and there is only a vertical force and a horizontal force acting at the valve.

I hope I'm making sense. I'd appreciate if someone could shed some light.

Peter