Flow Control Valve - how to size it? 1

[acr2003]

Hi,
I am trying to choose a flow control valve but I do not understand the meaning of "Cv" and what that stands for. Is this a characteristic of the valve? How do I read the flow range of a specific Cv valve. I am asking that because I need to choose a valve with a specific Cv but do not know how to do it. I am familiar with flow meters that come with a flow chart for each particular model, but I am getting lost with the flow control valve.
Could anyone please clarify me?
Any help is very much appreciated.
Thanks

 

[hydrae]

arc2003

Cv factor is flow in (Q)gpm at which the pressure drops (dp) by 1 psi through the valve when the valve is full open.
Cv metric is the same in liters/sec to drop 1 kPa they are not equal

With Cv you can use a basic rule of thumb

Q = Cv * dp^0.5

Hydrae

 

[quark]

Good for a start

http://www.cheresources.com/valvezz.shtml

 

[jsummerfield]

What is the fluid?

There formula is different for gas.

Search the Emerson Process or Fisher-Rosemount website for the Fisher control valve sizing program.


John

 

[acr2003]

Hi Guys,
Thanks for the replies and for the help. Quark, I had a look on the link you gave me and answering jsummerfield , my fluid is steam. Anyway, with the information you provided me I was alreday able to figure out what would be the equation for steam and how to use it.
Thanks a lot for the help and orientation on that question.
- acr -

 

[Multistagedisk]

Just be carefull with liquid if the static presure of the process fluid drop to vapor pressure piont of the fluid it will start bioling and increase the CV required. Make sure your selection of valve is correct.Even better if you let your valve supplier select the valve for you.

 

[MortenA]

This link has been reported several times at these pages:

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

Its the Fisher control valve handbook.

Best regards

Morten

 

[davefitz]

If you are just beginning to learn about valve sizing, you will find that many valve vendors have their own correlations and equations, so some confusion may result.

I would recommend you use the ISA handbook of Control Valves, as it may be an industry standard, and also has conversion formulas for the different valve mfr'r correlations.

First you have 4 choices of flow processes: single phase incompressible, single phase compressible, 2 phase cavitaitnig or flashing, and 2 phase inlet and outlet.

For the simplest incompressible case, you have:
W= 63.3* Cv * SQRT ( DP/ sv,i) where
W= flow, lb/hr, DP= pressure drop, psi; sv,i= inlet spec volume ft3/lbm

For compressible gases ,, it is a little more complicated:
W= 63.3*Y*Cv*SQRT ( Pi*X/sv,i)
Pi= inlet pressure , psia
X= max {DP/Pi, 1.4DP/(Pi*R*Xt)}
R= ratio of spec heats
Xt= valve body oblique shock factor( tabulated vlaues depending on valve style)
Y=1-0.33*1.4*X/(R*Xt), not to exceed Y=0.67
choked flow occurs when Y=0.67