Thirumalai Karthik (TK) Arasu, Critical Control Valve Specialist at Emerson, recently published an article in the September 2020 issue of Flow Control. The article describes the problems associated with control valve cavitation and how it can be addressed. The article is titled Understanding and Solving Control Valve Cavitation Problems and is summarized below.
You are standing near a recirculation valve on a high-pressure boiler feedwater pump. You notice the pipe and valve shaking violently, and it sounds like rocks are passing through the valve, but you know the line contains clean water. What is going on?
TK explains what is happening:
This is cavitation - a liquid flow phenomenon that can occur in control valves, pumps, boat propellers, pipes, and any situation where liquids are moving across a huge pressure differential.
Figure 1 shows the instantaneous pressure as liquid moves through a control valve. The internal passages of a valve vary in cross-sectional area, with the inlet and outlet areas generally much larger than the area around the plug and seat. Since the total flow at any location in the valve is the same, the liquid velocity around the plug and seat must be much higher to pass the same flow.
Figure 1: Typical pressure curve of a cavitating liquid passing through a control valve.
By Bernoulli’s law, the total energy at every point is constant, so as liquid velocity increases, pressure must fall, creating the pronounced dip in the pressure curve. The vena contracta is the minimum area of a flow stream, and it is located downstream of the flow constriction. This is the point in flow where the average flow velocity is the highest and the pressure is the lowest.
If the pressure in the vena contracta falls below the vapor pressure, then vapor bubbles will begin to form. However, when the pressure recovers to a level above the vapor pressure downstream, the bubbles will collapse back into the liquid. This bubble formation and collapse is called cavitation.
How Bad Can a Bubble Be?
Bubbles may seem innocuous, but these bubbles eat metal. It may be surprising, but most of the damage associated with cavitation does not occur when the bubbles are formed, but when they collapse (see Figure 2).
Figure 2: Diagrams of a collapsing vapor bubble inside a cavitating valve.
Initially, the bubble is spherical, but a collapsing bubble usually develops a dimple, which ultimately penetrates the bubble as it condenses back to liquid. The implosion of the bubble creates a high-speed and potentially destructive microjet and localized shock waves. Either of these phenomena can cause severe damage to valve components when located near the material surface, as shown in Figure 3.
Figure 3: Samples of cavitation damage. Cavitation can erode and destroy plugs, seats, and the walls of the valve or downstream piping. The damage is usually characterized by a dark, pitted, and rough surface.
One can easily detect cavitation in a valve since the valve and downstream piping sound like they have gravel flowing through them and the valve will be vibrating significantly. A typical valve in this service will not last long. It is far preferable to predict cavitation during the design phase, and ISA’s Recommended Practice 75.23 (Considerations for Evaluating Control Valve Cavitation) can aid in that process.
Cavitation is usually more pronounced on high recovery valves such as butterfly, ball, and plug valves. High recovery valves have a lower vena contracta pressure (see Figure 4), thereby increasing the chances of bubble formation and cavitation.
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. Notice the vena contracta pressure in a high recovery valve is significantly lower, increasing the likelihood of cavitation.
TK describes the design effort:
The key process parameters required to evaluate cavitation damage potential are upstream and downstream pressures, fluid temperature, and the vapor pressure of the liquid. Vapor pressure is usually easy to obtain for a pure fluid (water, ammonia, methanol, etc.), but can be quite difficult to determine for combinations of fluids, hydrocarbon mixtures, or poorly studied intermediate compounds. An accurate estimation of vapor pressure is critical to evaluate the intensity of cavitation.
Cavitation is much more likely if a valve is taking a very high pressure drop or the liquid temperature is elevated, translating to a high vapor pressure. The boiler feedwater pump recirculation valve described at the beginning of this article had both of these conditions.
There are two main ways to handle cavitation.
Cavitation Option #1 - Avoidance
One can address cavitation by avoiding it altogether. Cavitation occurs when the outlet pressure of the valve is fairly close to the vapor pressure of a liquid. Consider the valve shown in Figure 5. As the fluid moves down the pipe, the pressure gradually falls, and if the valve is installed in a location where the liquid is approaching its vapor pressure Pv, the likelihood of cavitation is very high.
Figure 5: Control valve is located far down the pipe where the outlet pressure happens to be close to the vapor pressure. This valve will likely cavitate.
However the valve in Figure 6 has been moved upstream where the pressure is higher. Now the valve outlet pressure is well above the vapor pressure Pv, so cavitation is less likely to occur.
Figure 6: Control valve is moved upstream where the line losses are reduced or the liquid head pressure is higher. In either case, the outlet pressure is well above the vapor pressure and cavitation is avoided.
Cavitation Option #2 – Design for It
If cavitation cannot be avoided, the valve must be designed to handle it and minimize the damage through robustness, isolation, or elimination. Often a combination of two or even all three techniques is employed in valve design.
Robustness utilizes high-strength, hardened materials for critical parts of the valve that will encounter cavitation. Notice the difference in Figure 7 between an Alloy 6 valve plug versus the standard stainless plug subjected to the same service conditions.
Figure 7: The valve plug on the left has a high strength Alloy 6 tip while the valve plug on the right is made of 316 stainless. Both have been subjected to the same cavitation conditions for a similar duration.
Isolation attempts to divert or direct the collapsing bubbles into the middle of the flow stream where microjets and shock waves cannot impinge on valve components. There are many techniques to accomplish this, including the two valve designs shown in Figure 8.
Figure 8: The Fisher Cavitrol III 1-stage trim (shown at left) employs several engineered low recovery holes in the hardened steel trim to take the pressure drop. The bubbles of cavitation are injected into the middle of the flow stream where they can do little damage to the plug, seat, or valve walls. Other trims like the Micro-flat design (shown at right) use a downward angle body with a liner insert to direct the bubbles into a protected area away from the seat.
Elimination strives to minimize cavitation by taking the pressure drop in a series of stages rather than all at once. In this manner the deep pressure dip of a single stage is avoided and the pressure remains above the vapor pressure (see Figure 9).
Figure 9: Rather than taking a single pressure drop as depicted in Figure 1, some anti-cavitation trims use a series of pressure drop stages, resulting in the pressure curve on the left and avoiding cavitation. The Fisher CAV 4 trim reduces pressure through four discrete, independent, unequal, and low-recovery stages with adequate recovery areas
If unanticipated, cavitation can quickly destroy valve internals and even erode downstream pipe walls. However, calculations help designers anticipate cavitation problems, allowing them to either rework the equipment design to eliminate or mitigate cavitation, or partner with a control valve vendor to select the best valve design to handle the challenging process conditions.
Figures all courtesy of Emerson
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