Control valve with flashing across it

[bouk]

Hi everyone,

I have an existing control valve and I wish to check it is adequate for some different flowrates. When I model the valve in Hysys with a pressure drop, I notice that the liquid phase upstream of the valve becomes liquid with a bit of vapour downstream. Do I need to use any different equations to the liquid control valve sizing equation to determine my CV for these new given flowrates?

Has anyone a template I could use for checking?

Regards

 

[casflo]

You must use different equations to obtain the maximum allowable pressure drop in the valve to calculate the valve Cv.
For example if the fluid is water that at the valve inlet begins to flash, the maximum valve pressure drop used to calculate the valve Cv is: 0.04FL**2Ps(1+7(Ps/3206)**0.5
FL is the valve recovery factor. It depends of the valve type and manufacturer. A normal value could be 0.9
Ps is the saturation pressure in psia and the pressure drop is in psi.
If the valve inlet pressure is P1 and is grater than Ps, add to the former valve pressure drop the value (P1-Ps)FL**2 and if P1 is less than Ps you must subtruct (Ps-P1)FL**2
Regards.
casflo