What Is Choked Flow? In What Conditions Choking of Flow Occurs?

Within "ordinary" regimes of pressure difference (ΔP) and flow (Q), Q is proportional to √ΔP. However, at high velocities such as near sonic velocity for the fluid in question, this relationship no longer holds. Standing waves and other phenomena can then occur, producing increased resistance to flow--hence "choking."

This phenomenon is important in relief valve discharge systems, for instance; and may occur in other systems with high ΔP. I don't have the information here at home, and even at work my info is fragmentary. Searching on "Colebrook formula" may give you some further insight. You have raised a good question, and I think some answers are "out there," though not widely known.

Chocked flow occurs when the ratio between pressure in front of the restriction (p1) and after it (p2) is p1/p2> (2/(k+1))^(k/(k-1)) where k is the adiabatic coefficient of considered gas. It has NOTHING to do with the Colebrook equation which is related only to FRICTION.

k = Cp/Cv is the ratio of the specific heat capacities under constant pressure and constant volume. For He k=1.66 at 20°C.

If above mentioned ratio > 2.0488 then you come to the chocked flow limit. It is recommended to stay under this limit and take a safety value. If you obtain a chocked flow or not will depend on the rpm of your valve and the tube length. Consider that in helium the sound velocity is higher than in air.

In fact flow is chocked when gas speed in the restriction arrives at sound speed.

Thank you very much for answer. My problem is we have to run compressor at 22 bar and 8 bar as high and low pressure limit. Now as per the critical pressure ratio, choking flow always occur. So, what should I do to change mass flow rate at the starting of the flow? Do I need to do any change in area variation of flow for rotary valve? or anything else? Please help.