# Choked flow thru a valve

1. Jan 22, 2017

### cjm181

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?

Kr
Craig

2. Jan 22, 2017

### cjm181

qvf = qvr (SQRT 1/d)
=10 * (SQRT 1/1.45)
=10 * 0.83
=8.03litres/s

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)
=6.70litres/s.

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?

Thanks