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Choked flow thru a valve

  1. Jan 22, 2017 #1
    1. The problem statement, all variables and given/known data

    Calculate the equivalent choked flow of propane through a particular valve if the choked flow of natural gas through it is 10 litres s–1.

    The relative density* of propane is 1.45 and that natural gas is 0.65.

    2. Relevant equations
    Kvf = qv (SQRT d/DeltaP) ...............1

    d is fluids relative density to water / air
    P is pressure

    Kvf = (SQRTd) x Kvr ........................2

    where Kvf is valve coefficient of actual gas,
    Kvr reference gas

    qvf = qvr (SQRT Pr/Pf) .......................3

    q is volumetric flow rate
    Pr is density of ref gas
    Pf density of actual gas

    qvf = qvr (SQRT 1/d)

    3. The attempt at a solution

    Not really sure where to start with this. The confusion is because I am comparing a gas (propane) to another gas other than air.

    I know I need to find qvf. I am a bit lost with this. Can anyone point me in the right direction or give me a hint please?

  2. jcsd
  3. Jan 22, 2017 #2
    I think the answer is:
    qvf = qvr (SQRT 1/d)
    =10 * (SQRT 1/1.45)
    =10 * 0.83

    Is this correct? I think this works because the equation uses densities relative to air.

    I am worried because I have an example of a similar question that uses the above, only it states the chocked flow is with air. What the concern is the equation does not use the density of natural gas. So I am thinking this formula is tied to air as the reference gas. so should I use this formula?

    qvf = qvr (SQRT Pr/Pf)
    =10 (SQRT 0.65/1.45)

    The answer of 6.7 looks better to me, given the density of NG is 0.65 and Propane 1.45.

    Are any of these correct?

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