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Control valve

Control valve

Control valve

(OP)
Dear experts,

Need your help..
I,m specifying a centrifugal pump and control valve pressure drop to instrument engineer for control valve sizing.
The question is, I have a normal flow and rated flow. The control valve will be fully open during rated flow, the control valve will be throttled during normal flow to ride the pump back on the curve. For instance I had start with a 5 psi pressure drop in rated flow across a control valve. Then I adjusted my discharge pressure in normal and rated flow and made it equal. In doing so, I,ve calculated the pressure drop during normal flow which is of course more than the pressure drop in rated flow, this is to account for less dynamic losses which I incurred in normal flow. Now I get some 10 psid delta p across control v in normal flow and the diff head is same in normal and rated flow.

Is this a valid assumption to make the discharge pressure same in normal and rated flow. Since the control valve will close more in normal flow and will give the same pressure as if when the pump would be running in rated low with less resistance with control valve fully open.

Any suggestion And comments are greatly appreciated.

ProcessEngCAN

RE: Control valve

Dear ProcessEngCAN

To sizing of control valves usually involves a trade-off between pump head and control valve size: the larger the valve, the smaller the pressure drop across the valve and the lower pump head.

By assuming the friction of total flow area is the same in rated flow and normal flow case to calculate the control valve pressure drop, the pressure drop in normal flow case is more than that in the maximum flow rate case. And assuming the pump curve is flat, you can use two flow equation the maximum/minimum flow condition to get the control valve Cv.

Please refer to the paper
A Rational Approach to Control Valve Sizing
Ind. Eng. Chem. Res. 1990, 29, 700-703

Please correct me if I am wrong.

Thank you.

RE: Control valve

Will the differential head from your pump be the same for normal and rated flow? Depends on the pump, if the rated flow is say 10% more normal flow that's not a bad estimate.

" the larger the valve, the smaller the pressure drop across the valve"

I disagree with this statement. Unless you are talking about the valve running wide open, a larger control valve will simply run at a lower % opening. The system curve and the pump curve sets the pressure drop across the control valve.

10 psi dP to me doesn't allow for much variation in process conditions. 10% more flow increases dynamic pressure drops through piping and equipment by about 20%. Small increases in downstream pressures could eat up a 10 psi margin pretty quick. What about fouling in your piping or equipment?

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