Hello, I am trying to calculate the choked flow through a globe valve in order to determine a max flow case. The flow is definitely sonic and I found an equation online that will calculate the choked flow in lbm/s:

w = Cd * A * P * SQRT(g*k*M/(Z*R*T))*(2/(k+1))^((k+1)/(2k-2))

at this website:

http://www.air-dispersion.com/usource.html

I found a units error where they are using the gravitational constant instead of the correction factor, but that doesn't change the result since they are both 32.2.

Perry's eqn. 6.122 is similar albeit a little less worked out:

G = P * SQRT((2/(k+1))^((k+1)/(k-1))*k*g*M/(R*T))

G is the mass flux rate and then I can multiply by the orifice area to get the flow rate in lbm/s.

The big difference between the equations is the C term in the first equation, the coefficient of discharge which I would expect to see in a flow calculation. The Perry's equation assumes a frictionless nozzle and obviously that is a squirrely assumption but is the C term in the first equation there to account for frictional losses?

1## Re: Discharge Coefficient in Choked Flow