How I can calculate the cv of a ball valve

  1. Hi!
    I am working with ball valves and I need to calculate the cv for several sizes. I only knows the pressure drop expressed in terms of the "equivalent feet". Unfortunally, I do not have a lab to measure.
    thanks a lot!
     
  2. jcsd
  3. FredGarvin

    FredGarvin 5,087
    Science Advisor

    The equivalent length is

    [tex]L_{eq}=\frac{K_L D}{f}[/tex]

    where

    [tex]L_{eq}[/tex] is the equivalent length
    [tex]K_L[/tex] is loss coefficient
    [tex]D[/tex] is the flow diameter
    [tex]f[/tex] is the friction factor

    So it would appear that you would have to make assumptions of not only the friction factor, but also the loss coefficient. I would say that you are not going to get to the Cv from where you are with any accuracy.

    Honestly, you should be able to contact the valve manufacturer and they will give you the Cv. I don't know of a single valve maker that doesn't or won't give you that data. Perhaps if you tell us what brand and type of ball valve you have we can hunt the info down.
     
  4. Q_Goest

    Q_Goest 2,970
    Science Advisor
    Homework Helper
    Gold Member

    Hi gabriel
    Honestly, that's the right answer. "Ask the manufacturer."

    I remember one or two very inexpensive ball valve manufacturers who didn't have that information though. They weren't industry quality valves, they were more like valves for home or garden use that didn't have a Cv rating. If that's the case, you can relate Cv to equivalent length as Fred was starting to discuss. Note that:
    K = 891*d^4 / Cv^2
    Where K = resistance coefficient referenced in Fred's post
    d = inside diam (inches)

    Now you can take Fred's equation and this one and you're left with one additional unknown, which is friction factor, f. Sorry, but you can't get any better than that. You have to make an assumption on f as Fred mentions.

    The other way I'd suggest depends on whether or not this is a reduced port ball valve (ie: one that has a ball with an ID smaller than the ID of the pipe). If the ID of the ball is the same as the ID of the pipe, just neglect the valve altogether and assume it's a straight section of pipe. If the valve is a reduced port, calculate the resistance coefficient, K for a sudden contraction, and another for a sudden expansion, add them together, and relate that to Cv from the equation I gave above.

    Attached is a paper that reviews some of this, and in which you can find sudden expansion and contraction coefficients.
     

    Attached Files:

  5. Many thanks for your answers, they were really helpful and also the pdf.
     
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