Cv correction for pipe size

[joeswoes]

I cannot find any reference for the effect that the inlet/outlet pipe diameter may influence the Cv of a throttling valve. We have a 4" butterfly valve bolted to 6" piping and are having control problems throttling the valves to maintain liquid level. We're thinking the Cv through the valve is not per manufacturers data and they didn't know of a correction. Can anyone guide me on this one ?

 

[BobPE]

what are the flows in your system? Cv is a function of the valve and is most likely not the problem.

BobPE

 

[joeswoes]

We're at 300 GPM, full open Cv of valve is 570. Theoretically, the valve should be at 50% open.

 

[TD2K]

How are you determing/estimating the dP for the valve to say it's 50% open?

 

[katmar]

The correction is described in the Fisher Control Valve Handbook. I have a hard copy of this manual, but I do not know if the reps still hand them out. It is a free download from the Emerson site at

http://www.documentation.frco.com/

via the Process Management Documentation Library

 

[FredGarvin]

TD2K is thinking what I am thinking. Why don't you measure the dP across the valve and calculate an effective Cv? At least then you would have hard evidence regarding the printed values.

 

[krb]

This is from a pdf I have about sizing butterfly valves. The same would apply for an control valve.

Piping Geometry Factor: Reducing pipe sizes for
installation of a smaller than pipe size valves will reduce
the effective Cv of the valve. The greater the pipe
reduction, the greater loss of Cv.

Determing the actual Cv would give you something to compare against.