How to Select Control Valves

http://en.wikipedia.org/wiki/Control_valves

Control Valve charachtristic is to be selected by Instrumentation engg. with the help of Process Engg. Since selction of control valve charachteristic u must be aware of the Process Pressure drop Which is occuring across control valve & what is the percentatge of this pressure drop as compare to total system pressure drop.

If most of Press. drop is across CV then U go for "Linear charachteristic" & If majority of Pressure drop is across the other system componet & very littile pressure drop is acrooss Control valve then u need to go For 'Equal Percentage" charchteristic. Normally as a thumb rule we normally go for Linear charchteristic in vcase of level control system while in case of Pressure & flow we need to go for Equal % type. But before selecting any type we must discuu with Process Engg.

In mostly while preparing the data sheet we write as Equal% because if u see that Graph of Stem opening Vs Flow rate i.e Inherent charchteristic of control valve where we assume the Press. Drop is const. As per graph u see that below curve is for Eq% Above to this there is Linear & above to linear there is On-OFF. Now in this grapgh we assume Press. Drop as Const. but when valve is actually installed in the site the press. drop across valve doesn't remain constant it keep on varies which is called as installed charch. of valve. So if u select Equal percentage charchteristic while referring Inherench Charch Graph the same valve will behave like or work like Linear type in Installed condition. So in this way u can control the process.

Now if u select Linear char, as per Inherent grapgh then that valve will work like On-OFF type in Installed condition due to variation of pressure drop. Which is not acceptable as per control valve is concern as we are selecting control valve for regulatory service & not for On-Off service.

Hence bydefault we always go for Equal Percentage type of charchteristic while preparing data sheet.

I hope above is clear to u.

Hi there,

So you meant that pressure drop across the linear CV is always constant and it is variable with equal percent CV?

Thanks

characteristics of the control valve while preparing the datasheet is selected based on the process and the mechanical conditions like

what process, whats are the endconnections

max , min norm flow,pressure, sound, cv, dp, and others

also what type of actuator you need & also depending on line size the size of the actuator, type of positioner etc.

You have a standard datsheet which specifies this. Take a template and fill it.

An instrumentation enginner usually does it.

This here is the Fisher Control Valve Handbook from Emerson. It's a good read:

http://www.documentation.frco.com/groups/public/documents/book/cvh99.pdf

As per ISA

DP at Min flow

----------------- < 1.5 Linear Charcteristics is selected ,

DP at Max flow

when the ratio exceeds 1.5 =% has to be selected

also as thumb rule for the following application v have to use linear characteristics

Spill back/ Antisurge /Pump Minimum bypass

I am still figuring how end connection /actuator /positioner dictates the valve characteristics

Regards

Jose John

More information on control valves. Cheers