Katherine Bartels and Adam Harmon, both Design Engineers at Emerson, recently published an article in the August 2021 of InTech. The article describes the poorly understood phenomenon of choked flow, and shows how it can affect control valve sizing and trim material selection. The article is titled Choked Flow in Control Valves and is summarized below.
Choked flow in control valves is often a serious concern for industrial users. Most associate the term with destructive process conditions that can damage valve internals and generate very high noise levels, but choked flow does not always cause these conditions.
This article describes the phenomenon of choked flow and explains why it occurs. It also explains when choked flow conditions are damaging, and shows how this damage can be reduced or avoided.
What is 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 versus the square root of the valve differential pressure divided by specific gravity.
Figure 1: In an ideal world, flow rate through a valve rises as the pressure drop across the valve increases. In reality, the maximum flow will be limited due to choked flow conditions.
However real-world flow does not match ideal flow, as the authors explain:
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.
A similar phenomenon occurs with valves in gas service. If the P1 pressure and flow area remain fixed, flow through the valve will rise as P2 is reduced, but at some point, choking will occur and the flow remain constant, regardless of the value of P2.
Why does choked flow occur?
Choked flow occurs for different reasons in liquid applications versus gas/vapor applications. In liquid applications, choking is a result of the reduction in pressure through the restricted port area as shown in Figure 2. As the liquid moves from the larger inlet area to the reduced plug/ seat (vena contracta) area, it must accelerate significantly to pass the flow.
Figure 2: This graph shows a typical pressure curve of a cavitating liquid passing through a control valve. If P2 is reduced still further, the expanding vapor will create an increasing pressure drop and eventually limit flow.
Bernoulli’s law states that the total energy at every point in the flow stream is constant, so if velocity is increased, pressure must fall. This pronounced pressure dip in the vena contracta becomes more pronounced as flow increases.
If the instantaneous pressure in the vena contracta falls below the vapor pressure, then vapor bubbles will begin to form as the liquid begins to boil. The conversion to vapor increases the volume of the fluid and begins to restrict flow. If the downstream pressure is lowered still further, the vapor volume will increase to the point that flow throughput can increase no further, and the valve flow becomes choked.
Choked flow in gas/vapor applications occur for different reasons, as Kathrine and Adam explain:
In gas applications, 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. Further reduction of the downstream pressure will have no effect on flow through the valve.
Choked flow misconceptions and issues
Choked flow by itself does not directly damage a valve, but there are flow conditions commonly associated with choked flow that can create problems, including:
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 3, 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).
Figure 3: This graph shows a typical pressure curve of a flashing liquid passing through a control valve. Fluid enters the valve as a liquid and exits as a vapor.
Either flashing or cavitation may result in choked flow, but not always. However, if flow is choked in liquid service, a significant level of cavitation and/or flashing will probably be present.
Noise levels: Choked flow does not directly create noise, but high noise will often result during choked flow conditions. In liquid applications, cavitation or flashing creates noise, with the noise level increasing as flow and/or pressure drop are increased
With 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 handling a high pressure drop can generate sound levels greater than 100 dB.
Valve damage due to choking: Users often assume choked flow conditions will damage the valve. However, there are times when a valve is choked and the damage is minimal, and there are times when the valve is not choked and the rate of damage is significant.
Sustained cavitation will almost always damage the valve. Flashing will eventually damage the valve as well, but the effect is less dramatic and immediate. Excessive noise can also damage the valve due to high vibration and metal fatigue.
Fortunately the authors explained there are options to address these issues:
Cavitation, flashing, and noise damage can be alleviated and even eliminated by specifying appropriate valve body designs, special valve trims, and materials of construction (Figures 4 and 5).
Figure 4: This graph compares the vena contracta pressures of a high recovery (ball, butterfly) valve versus a low recovery (globe) valve for the same process conditions. A high recovery valve creates significantly lower internal pressures, increasing the likelihood of cavitation.
Low recovery valves (such as globe valves) or special anti-cavitation trims (Figure 5) can either reduce cavitation, or focus it away from valve internals and walls.
Figure 5: Special valve trims, such as the Emerson Fisher Whisper III trim shown, can be employed to attenuate noise, reduce cavitation, or direct cavitating liquids away from valve components—all of which minimize damage to the valve.
Hardened alloys can be used for critical valve internal components to extend valve life. Noise can be significantly reduced by using low noise trims, inlet and outlet noise attenuators, or downstream modal noise attenuators.
The authors conclude with this advice:
When faced with the possibility of choked flow, or if there are concerns or questions on 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.
Figures all courtesy of Emerson.
About the Authors
Adam Harmon is a Senior Design Engineer at Emerson Automation Solutions, with a focus on valves in Steam Conditioning applications. He graduated with a BS in Mechanical Engineering from Iowa State University and has been with Emerson for 11 years.
Katherine Bartels is a Design Engineer at Emerson Automation Solutions, with a focus on custom anti-cavitation valves. She graduated with a BS in Mechanical Engineering from Iowa State University and has been with Emerson for 6 years.