Control valve accuracy

[Waramanga]

Hi all,
does anyone have a good reference for the principals of operation of control valves? I am wondering how the pressure drop accross a modlating contro valve effects flow. If I have a low pressure drop will I have poor control. Higher pressure drop = greater control accuracy?
Thanks

 

[BigInch]

You usually want to keep your operable pressure drops within whatever 30% to 80 percent open will give you.

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[jacsc]

Fisher has a pretty good guide. You may also want to check with other control valve manufacturers technical department such as Spirax Sarco as ask the same question, etc...

My understanding is that the accuracy of control depends on a number of things, such as the charateristics of the valve, positioner, actuation etc. and does not depend on the pressure drop. The smaller the pressure drop the larger the valve, the larger the pressure drop the smaller the valve.

 

[YoBob]

Most control valve manufacturers should have a curve that depicts percent of full flow versus stem lift from the closed position.

 

[dcasto]

For grins, plot the rate of change of a control valves Cv/Cg versus travel.

But the issue won't be what the valve manufacturer publishes, it the hysteresis from friction on the mechanical links that will be precieved as accurracy.