selecting of flow control valve

Charbel

Member
Join Date
Jan 2012
Location
Beirut
Posts
307
Dear,

I am working in selecting a flow control valve located downstream of positive displacement pump.

specs is requiring that CV to be between 30% for minimum flow, 50% for operating flow and 70& for maximum flow,

looking at the pump curve (flow and head) it turns out to be a linear curve. so the valve manufacturer considered in his CV calculation that the normal pressure to be 10bar which is the operator pressure, min and maxi pressure corresponding to max and min flow to be 4bar and 16bar.
first question that crossed my mind, how do we decide on the maximum and min pressure? please see pump curve attached, is there any rule for it.

second, he selected a control valve with linear type, which is logical corresponding to the fact that it is located upstream of a positive displacement pump that has linear characteristic.
and the results that he got for the selected control valve

for the normal flow it is around 50% of valve opening which is as required
for maximum flow (which corresponds to min pressure of 4bar), he go 52.6% of valve opening (required should be 70%)
for min flow (which corresponds to max pressure of 16bar), he got 43% of valve opening (required is 30%)

so I am little confused how we do we usually select min and max pressure for them to select the FCV since they affect the CV.

second I find that the valve has problem in rangeability which I guess might have problem when we control it.

Please refer to attached P&ID, were the FCV in concern is FCV-7041 located in the middle of clouds in FCV1 attachment, FCV0 attachment shows location of the positive displacement pumps upstream of the FCV-7041.
any feedback are really appreciated,

thanks a lot!

Charbel

FCV0.JPG FCV1.jpg pump curve.JPG
 
In my opinion you have a fundamental design problem.

It is never, I repeat never, good practice to use throttling to control flow from a positive displacement pump. Flow should be controlled by varying pump speed. If you throttle the suction side of the pump you will probably just induce cavitation and destroy the pump. If you throttle on the discharge side you will mostly increase power consumption and have minimal effect on flow rate.

Your pump "curve" is essentially showing the slip, i.e. internal leakage, of the pump as a function of pressure. It is not analogous to the performance curve of a conventional centrifugal pump. Note that at a 10:1 pressure range of 1 bar to 10 bar flow only changes from 0.475 l/s to 0.36 l/s, a mere 25% flow change.

The answer is to use pump speed, or if it is available pump stroke, to control flow. You will not get good control trying to throttle this type of pump.
 

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