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Control Valve Sizing


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#1 rsk

rsk

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Posted 17 July 2008 - 11:02 AM

for sizing of Control valve differential pressure across it is taken to be 25-30% of overall diff pressure
why not 100% it will give linear response . Please enlighten on this.
regards

#2 JoeWong

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Posted 18 July 2008 - 04:38 AM

QUOTE (rsk @ Jul 17 2008, 12:02 PM) <{POST_SNAPBACK}>
for sizing of Control valve differential pressure across it is taken to be 25-30% of overall diff pressure
why not 100% it will give linear response . Please enlighten on this.
regards


25-33% sort of rule of thumb and believe to provide correct control capabilty for maximum and turndwn condition. This has to be confirmed during Detailed Design.

100% would be too much. For example, a pump with long pipeline (pressure drop =~50 bar). If you provide 100% pressure drop on control valve (50 bar), the pump shall provide extra 50 bar and letdown through the control valve. It waste a lot of energy. Apart large pressure drop may cause cavitation and nosie problem. Anti-cavitation trim may required. It increase cost. The only good thing is, you may have smaller control valve. for long run, the life cycle cost may prefer low DP Control valve option.

#3 djack77494

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Posted 18 July 2008 - 08:22 AM

The way I look at this situation is that if my control valve is either 0% or 100% open, I am out-of-control. I only have control when my controller can take effective action. It cannot do so when fully open or closed. Also, since my ability to calculate future conditions is less than perfect, I like to further limit the range my valve will operate to 15% to 85% - and that's only because I consider myself to be a very good estimator ;-).




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