We’ve heard the term “choking” when it comes to athletic performance in not being able to perform in the moment. But what about choked flow when it comes to control valves?
Emerson’s Katherine Bartels and Adam Harmon address this question in an International Society of Automation Intech article, Choked Flow in Control Valves. Katherine and Adam open the article defining choked flow:
If the inlet pressure (P1) and valve flow area are fixed, the flow through a valve will normally rise as the downstream pressure (P2) is reduced. The “Ideal” line in figure 1 illustrates this point, showing how liquid flow rises linearly when charted against the square root of the differential pressure across the valve divided by specific gravity.
In actuality, the maximum liquid flow through the valve can never exceed a choked flow limit, and at this point flow will increase no further, no matter how low the P2 pressure is reduced.
For liquid flow:
…choking is a result of the reduction in pressure through the control elements.
Since total energy is constant in the flow stream:
…if velocity is increased, pressure must fall. As the fluid passes through the restriction, it speeds up, lowering the pressure at that point. Once the liquid enters the much larger outlet piping, the flow rate slows, and some pressure is recovered.
For gas flow:
…the vapor velocity through the valve will increase until the vapor reaches sonic velocity. At this point, the vapor can go no faster because a standing shock wave forms and limits flow.
Katherine and Adam note that choked conditions in vapor flow:
…are very common in relief valves and control valves with very high flows, but can also occur in high velocity flare headers at piping transitions. Choked flow is also common in vacuum systems, because the low air pressures found in these systems greatly reduce the speed of sound, increasing the likelihood of standing shock waves.
Read the article for common choked flow misconceptions such as creation of noise levels, flashing and cavitation conditions, and damage to the control valve due to choked flow conditions. Also they highlight the role of valve sizing, predicting choked flow conditions, and valve selection.
The product bulletin referenced in the caption is also a great source to learn more about choked flow and the importance of valve sizing and selection considerations.