×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Flow Control Valve - how to size it?

Flow Control Valve - how to size it?

Flow Control Valve - how to size it?

(OP)
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

RE: Flow Control Valve - how to size it?

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

RE: Flow Control Valve - how to size it?

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

RE: Flow Control Valve - how to size it?

(OP)
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 -

RE: Flow Control Valve - how to size it?

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.

RE: Flow Control Valve - how to size it?

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources