Hi, can somebody give me a clue regarding how to find out the Reaction force for a PSV which is in liquid service.

I know for gas and the two phase flow, the reaction force calculation is based on API RP 520. But I couldn't find any equation for the liquid PSV discharge

Thanks and Regard,
Kumar

No takers on this one?

The only thing I can think of is to treat the forces as a slug flow problem with DLF=2, but then the problem becomes what is the liquid's velocity?

Thanks Dave.

I got some more info about how to calculate it. Its based on the mass momentum theory.
Fh=1.414 x Da x Qa x Ro(((P1-P2)/Ro)+(Qa^2/2 x Ap^2))^0.5

Where Qa=Qr(A2/A1)
Ap=0.785((Do-2t) x 0.0254)^2

Da= Dynamic Load Factor
P1=Inlet Pressure(barg)
P2=Outlet Pressure(barg)
Do=Inlet Pipe diameter(inch)
t=pipe wall thickness(inch)
Ro=Liquid density(Kg/m^3)
A1=Required orifice area(inch^2)
A2=Actual Orifice Area(inch^2)
Qr=Required flow rate(m^3/sec)


Regards,
Kumar

The first question is - what kind of RV? Most liquid relief valves I've seen tend to be thermal relief. I.E. they pop to release liquid because the pressure will rise quickly in a liquid full section of pipe/vessel that is completely blocked in.

If that's your case - Reaction force = 0.

My question is somewhat related to the topic. My fluid is LPG but relief is vapor. Will the API RP 520 code still be applicable? Should I use the Vapor Relief flow rate instead of the LPG inlet flow rate to get the correct values?

API 520 is a standard, not a code. It has guidance on sizing relief devices for both liquid and vapor flow scenarios.

API 521 provides more information on determining the required relief rate for various process scenarios, which is then used in the equations in API 520.

Note, if you have an existing system/relief valve you should be able to contact the manufacturer and obtain a datasheet with the reaction forces.

If you're sizing a relief valve in addition to doing the pipe stress, note that relief valve sizing is not a simple "plug in this number and you're done" type calculation. You have to think through all process scenarios that could generate overpressure, determine the relief rates/relief area required for each, and size your device from those results. A pump deadhead or typical process flow rate will likely not be the worst case scenarios.