Choked Flow in Control Valves

Choked flow is a poorly understood phenomenon that can affect control valve sizing, along with trim and material specification. This article was originally published in InTech's July/August issue.

Choked flow in control valves is a subject of serious concern for industrial users. The term is usually associated with destructive process conditions that can damage valve internals or expose operators to noise levels well above OSHA limits. Although choked flow is not always the cause of these conditions, it may indicate when they occur.This article describes the phenomenon of choked flow and shows why it occurs and how it can be predicted. It also explains when choked flow conditions are damaging and how this damage can be reduced or avoided.

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 liquid applications, choking is a result of the reduction in pressure through the control elements. Figure 2 shows the instantaneous pressure as liquid moves through a control valve. The inlet and outlet cross-sectional areas of a valve are much larger than the control area, such as the cage or the area around the plug and seat. Because the total flow at any location in the valve is the same, the liquid velocity in the reduced area (vena contracta) must be much higher to pass the same flow.

Choked flow by itself does not generally damage a valve, but there are flow conditions commonly associated with choked flow that can create problems, including:Noise levels: Choked flow does not directly create noise, but high noise can result from process phenomena normally associated with choked flow. In liquid systems, cavitation can be present during choked flow, which creates noise and can ultimately damage the valve. As downstream pressure is reduced, cavitation transitions to flashing. While cavitation can have a high sound pressure level due to the implosion of the collapsing vapor bubbles from micro-jets and shock waves, flashing will have reduced noise due to the resulting two-phase flow.In vapor flow, noise will rise significantly as the velocity turns sonic. As the downstream pressure is reduced, the extra energy is converted to sound energy. Valves with excessive pressure drop can generate sound levels greater than 100 dB. With either liquid or vapor flows, the overall level of noise is usually related to the differential pressure across the valve. When choking first appears, noise will be present but may not be excessive. As the downstream pressure falls, noise will increase dramatically and can damage valve internals and subject operators to unsafe sound levels.Flashing and cavitation: A common misconception is that choked flow conditions require flashing conditions, but choked flow can occur under cavitating conditions as well. As shown in Figure 2, cavitation will result when the P2 pressure rises above the vapor pressure of the liquid. When this occurs, the bubbles collapse and turn back into liquid. If the P2 pressure remains below the vapor pressure, the liquid will boil and flash to vapor as it passes through the valve and remain a vapor as it exits (Figure 3).

Many valve vendors have control valve sizing programs that can predict choked flow conditions and help users size the valve correctly. However, these programs are only as accurate as the input data, so the correct process and valve information must be entered.The presence and extent of flow choking depends on many process conditions, including the physical properties of the fluid involved, flow rates, upstream and downstream pressures, process temperature and inlet and outlet piping configurations—as well as a number of details associated with the control valve itself. Special parameters, such as pressure drop ratio, pressure recovery factor, and cavitation index, help predict exactly when cavitation or choking will occur, and how much flow a valve will pass. Because the parameters for each body style and trim are different, each option must be evaluated individually to determine the actual flow that can be safely passed under a specific set of process conditions.Such sizing calculations can become complicated, especially when several trim options are available, so it is wise to consult your valve vendor to help evaluate options and determine the best solution for your application.

Choked flow, in and of itself, is not a cause for concern. The confusion stems from the association of choked flow with many negative phenomena that can affect and damage control valves. When faced with the possibility of choked flow, or if there are concerns or questions about how to proceed with valve sizing or selection, contact valve vendors for technical support. They can usually provide valve sizing programs that predict when choking will occur and its impact on valve sizing and selection. They can also help users choose the best combination of materials and trim designs to alleviate damaging conditions.This article was originally published in InTech's July/August issue.

Katherine Bartels is a design engineer at Emerson Automation Solutions, with a focus on custom anticavitation valves. She graduated with a BS in mechanical engineering from Iowa State University and has been with Emerson for six years.Adam Harmon is a senior design engineer at Emerson Automation Solutions, with a focus on valves in steam conditioning applications. He has a BS in mechanical engineering from Iowa State University and has been with Emerson for 11 years.

Check out our free e-newsletters to read more great articles..