What Is Choked Flow?

  • Choked flow (otherwise known as critical flow) takes place in a valve when an increase in pressure drop across the valve no longer has any effect on the flow rate through the valve. It occurs when the velocity of the gas or vapour reaches sonic (Mach 1) at the vena contracta.
  • Choked flow is not necessarily a problem in valves but does need to be taken into account in the Cv calculations. For liquids, choked flow indicates the onset of full cavitation, which usually requires special steps to be taken to reduce damage.
  • With clean gases there is no problem with choked flow. Use the choked pressure drop in any equation to calculate Cv or flow rates. High noise levels may be generated.
  • Solid particles in gas flow will cause erosion due to the high velocities involved. With liquids full cavitation will occur when the flow is choked.
  • High recovery valves, such as ball and butterfly, will become choked at lower pressure drops than low recovery valves such as globe which offer a more restricted flow path when fully open.