# Gases at Sonic Velocity - Choked Flow

Greetings to all in this fine community,

If you are familiar with Crane Tech Paper 410, there's an equation to calculate gas flow rate in SCFH. It takes into account pipe losses, pipe diameter, and inlet and outlet pressures.

q'h = 40,700*Y*(d^2)*((DP*P'1)/(K*T1*Sg))^0.5

q'h = SCFH
d = pipe diameter in inches
T1=absolute temp in degrees Rankine
K= total resistance coefficient in the pipeline
Sg=Specific gravity of gas
Y=net expansion factor (tabulated and given)

The flow is going from a larger diameter pipe upstream to a smaller diameter downstream.

Question is, this equation and model is used for short pipe length (around 40 ft and smaller). What happens to the model if your pipe length is longer by say a factor 15?

The inlet pressure is 200 psig. The outlet pressure is atmosphere. The flow has to be choked since k=1.4, and the ratio of P1 to P2 is much higher than 1.9

Would the flow reach sonic velocity inside the pipe or at discharge?

Regards,

Last edited:

SteamKing
Staff Emeritus