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Homework Help: Mechanical enery equation to valve equation

  1. Apr 25, 2006 #1
    Hi

    I’m writing my bachelor thesis about valves, and was wondering if anyone could help me with the following derivation:
    I need to show the algebraic link between the mechanical energy-equation and the ”valve equation” that is Cv=Q * [Square](Sg/[Delta]p). This can also be called the flow coefficient-equation.

    Everything in field units (american units)

    All help will be appreciated a lot!
     
  2. jcsd
  3. Apr 25, 2006 #2

    Andrew Mason

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    Pressure is energy / unit volume so the [itex]\Delta P = Q^2\rho/C_v[/itex] expresses an energy loss per unit volume.

    AM
     
  4. Apr 26, 2006 #3
    Derive it from Bernoulli's. As the length of the valve is generally too small, you can neglect difference in elevation (if any) and also it is a safe assumption to consider constant velocity. The pressure drop across the valve then becomes a function of velocity. Express frictional resistance in terms of velocity and equate it to pressure drop.
     
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