Flow coefficient Cv for Relief Valves?

[kvbalu2129]

thread1203-174188

Hi all

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

 

[MortenA]

Why?

 

[Pmatherne]

If you know who made the valve it is easier to just call the manufacturer.

It is possible to calculate it, I think the Fisher 119 manual shows you how or I am sure Anderson Greenwood has something as well

Future PE Engineer
Pet project I am working on to help other engineers, not much yet hoping to get it grow as I learn more

http://www.peexamquestions.com

 

[casflo]

If you know the nozzle diameter of the valve do(in) and the nozzle flow coefficient C, it´s possible to calculate the valve Cv by the equation: Cv = 29.9C(do)**2.
ecasflo

 

[kvbalu2129]

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 alongwith the equations.

 

[kvbalu2129]

Pmatherne /MortenA

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.

 

[MortenA]

Excactle - you dont specify a Cv for a PSV - since its not a control valve! I dont 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 dosnt 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 cloosing/opening of the PSV (and make this valve very big so that it dosnt limit the flow. It might be a bit unstable since it will only make sence - simulation wise - to open the "valve" fully in one ot two timesteps.

Best regards, Morten

 

[kvbalu2129]

Yes, you are correct, MortenA. That's what I am doing. But the question 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.

 

[MortenA]

The "block valve": I would make this as big as your software will allow. If the simulation gets unstable tryk making it smaller until the simulation becomes stable.

The orifice: A PSV orifice is a similar are. 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 matcg your simulated capacity.

 

[kvbalu2129]

I have been able to do both. Consistent results Thanks MortenA.