The flow coefficient (Cv) equation illustrates that the flow rate through a valve (Q) increases with the pressure differential (∆P).
Valve Choked Flow
Simply stated, as the pressure drop across the valve gets larger, more flow will be forced through the restriction due to the higher flow velocities.
In reality, the above relationship only holds true over a limited range. As the pressure drop across the valve is increased, it reaches a point where the increase in flow rate is less than expected.
This continues until no additional flow can be passed through the valve regardless of the increase in pressure drop. This condition is known as choked flow.
Choked flow (otherwise known as critical flow) takes place:
-
When an increase in pressure drop across the valve no longer has any effect on the flow rate through the valve.
-
When the velocity of the gas or vapor reaches sonic velocity (Mach 1) at the vena contracta.
To understand more about what is occurring, it is necessary to return to the basics again. Recall that as a liquid passes through a restriction, the velocity increases to a maximum and the pressure decreases to a minimum.
As the flow exits, velocity is restored to its previous value, while the pressure never completely recovers, thus creating a pressure differential across the valve.
If the pressure differential is sufficiently large, the pressure may, at some point, decrease to less than the vapor pressure of the liquid.
When this occurs, the liquid partially vaporizes and is no longer incompressible. It is necessary to account for choked flow during the sizing process to ensure against undersizing a valve.
In other words, it is necessary to know the maximum flow rate that a valve can handle under a given set of conditions.
When selecting a valve, it is important to check the pressure recovery characteristics of valves for the thermodynamic properties of the fluid.
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.