Delta P Through Valve Calculations

[ham131]

I have a hydraulic operated diaphragm valve and as all valves the delta P at a specific flow rate is a constant. To change the Delta P accross the valve at the same flow rate ( I need to decrease the flow rate ) the only way is to change the effective seat area. The question is:
What is the relation between the pressure drop over the valve and the effective seat area?

I have increased the seat area by means of spacers, and have done another test with new delta P. I am looking for formula or model to get to the optimum seat area.

PS. I know there is more involved, but need to get some sort of relation between seat area and delta P.

please help

 

[BHMech]

Flow coefficient Cv changes with shape and design of valve. Generally the relationship between area and Cv
varies from 18 and 32 times the seat area in square inches. The lower numbers (18 to 26) apply when the seat area
is large in comparison with the valve inlet and outlet areas.

A simplified way of calculating this area factor Af = 32 - 14*Ar where Ar is As/A1, As = seat area & A1 = the
inlet or restricting restricting area.

Once the Cv is established the following simplified equation can be applied for liquids:

Q = Cv (P1-P2/Gw)^0.5

Q = Volumetric flow rate, gpm
Gw = Specific Gravity

B Hall

 

[ham131]

Thank you for the reply. I'll do some calculations based on your suggestions.