# Flow coefficient Cv for Relief Valves

QUESTION
Is there a way to calculate a Cv (flow coefficient) for a Relief Valve?

I was looking for a rigorous calculation to arrive at a Cv value. Equation Cv = 27.66 x A x Cd would still be an approximation. ANSI/ISA 75.01.01-2002 (Flow equations for sizing Control Valves) contains a flowchart for the routine along with the equations.

Unfortunately the Relief valve vendor doesn’t know “who” a Cv is. He would happily tell me the maximum relieving capacity & the Discharge coefficient. Ofcourse I know the orifice dimensions. I was trying to simulate a network with a PSV (compressible flow) and estimate pressure build-up when the relief valve is cracked open.

The question really is: How big should the valve be? A Cv value would have warned me the pressure built-up upstream more accurately during the relief valve flow…

REPLIES

Morten
Why?

You don’t specify a Cv for a PSV - since its not a control valve! I don’t see the point. Its a useless exercise (maybe in all other cases than when simulating). If you wish to simulate the PSV and your tool doesn’t have a PSH surely it must have an orifice. This would represent your PSV fully open. Use a generic ball valve type valve function in the software to simulate the closing/opening of the PSV (and make this valve very big so that it doesn’t limit the flow. It might be a bit unstable since it will only make sense (simulation wise) to open the “valve” fully in one or two timesteps.

I would make the “block valve” as big as your software will allow. If the simulation gets unstable try making it smaller until the simulation becomes stable.

A PSV orifice is similar. Depending on your actual software the nominal size should be the same as your model orifice. But i would just check that in a simple model with your design conditions upstream downstream and then see if the calculated capacity matches your simulated capacity.

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