Cavitation in a control valve is a phenomenon in which the pressure locally drops to the saturated vapor pressure inside the valve, causing vapor bubbles to form, and these bubbles collapse as the pressure recovers downstream. The repeated collapse of these bubbles pounds the metal surface, leading to erosion damage, abnormal noise, and vibration. The fundamentals of countermeasures come down to three points: a design that does not take the entire differential pressure in a single stage, selection of a trim structure that keeps bubble collapse away from the valve body surface, and the use of materials with high erosion resistance. The starting point is to assess the likelihood of cavitation from your line’s inlet pressure, outlet pressure, and liquid temperature.
Key Points of This Article
- Cavitation progresses in two stages, “bubble formation from the pressure drop inside the valve, then collapse as the pressure recovers downstream,” and the impact at collapse is the main cause of erosion, abnormal noise, and vibration.
- There is no single all-purpose countermeasure; the basic approach is to suppress cavitation through a combination of splitting the differential pressure, selecting the trim structure, and using erosion-resistant materials.
- The likelihood of occurrence can be judged to a certain degree from the inlet pressure, outlet pressure, and liquid temperature (saturated vapor pressure), and assessment at the design stage is effective.
How Cavitation Occurs and How to Identify It
In a control valve, when the flow path is throttled in the gap between the valve body and the seat, the flow velocity increases and the static pressure drops accordingly. When this local pressure falls below the saturated vapor pressure of the liquid, part of the liquid vaporizes and fine bubbles form. Then, when the flow path widens downstream of the valve and the pressure recovers, the bubbles collapse abruptly, and the high local pressure generated at that moment repeatedly pounds the metal surface.
The abnormal noise appears as a sound like flowing gravel or as a continuous high-pitched sound, and it may be accompanied by vibration and piping-system noise. Because it progresses differently from flashing (a phenomenon in which the bubbles do not recover downstream and continue flowing as a gas-liquid two-phase mixture), observing the tendencies of the noise, vibration, and damage location together provides clues for distinguishing between them.
Locations Where Erosion Tends to Concentrate
Bubbles tend to collapse at the location where pressure recovers, namely on the downstream side of the valve body, near the seat, and on the inner wall of the downstream piping, and damage tends to concentrate in those vicinities. When a sponge-like or matte-textured eroded surface, consisting of clusters of scattered small pits, is observed, it serves as one indicator to suspect cavitation. Cross-checking the damage location against the flow direction helps in estimating the cause.
Design and Selection Criteria for Suppressing Occurrence
The likelihood of occurrence can be assessed in advance to a certain degree from the relationship between the difference in inlet and outlet pressures (differential pressure) and the saturated vapor pressure corresponding to the liquid temperature. In general, the closer the outlet pressure is to the saturated vapor pressure, and the larger the differential pressure, the more likely cavitation is to occur. With high-temperature liquids, the saturated vapor pressure rises, so care is needed because cavitation becomes more likely to occur even under the same pressure conditions.
There are broadly three approaches to countermeasures. First, rather than taking the entire differential pressure in a single-stage valve, distribute the pressure drop by using multi-stage pressure reduction or combining orifices, thereby easing the local pressure drop. Second, select a structure that keeps bubble formation and collapse away from the valve body surface, such as a multi-stage trim that divides the flow into multiple paths and reduces pressure in stages. Third, under conditions where collapse is unavoidable, use hard materials with high erosion resistance or Stellite-based overlay welding to suppress the progression of damage.
Mitigation Possible on the Operations Side and Its Limits
Not only for new installations but also for existing lines, the load can sometimes be reduced by reviewing the operating conditions. Examples include checking whether the outlet pressure is being throttled more than necessary, and considering the valve’s installation location (a position where downstream back pressure can be secured). However, the effect of these measures is limited depending on the conditions, and under fundamentally severe differential-pressure conditions, a response through the valve’s structure and materials becomes necessary. Since continuing forced throttled operation causes damage to progress, we recommend organizing the conditions early.
Frequently Asked Questions
Q1. How is cavitation different from flashing?
A1. Both are the same in that bubbles form due to pressure reduction, but with cavitation the pressure recovers downstream and the bubbles collapse, whereas with flashing the pressure does not recover and the flow continues as a gas-liquid two-phase mixture. The way damage occurs also differs.
Q2. Does abnormal noise always mean cavitation?
A2. Not necessarily. Abnormal noise can also arise from other factors such as flashing, excessive flow velocity, or resonance. In addition to the nature of the sound, checking the pressure conditions and damage location together serves as a guide.
Q3. Will hardening the material solve the problem?
A3. Erosion-resistant materials are an effective means of slowing the progression of damage, but they do not stop the occurrence itself. The basic approach is to consider them together with distributing the differential pressure and the trim structure.
Q4. From what level of differential pressure should I be cautious?
A4. No uniform threshold can be given. Because it varies with the closeness of the outlet pressure to the saturated vapor pressure, the liquid temperature, and the valve’s pressure-recovery characteristics, judgment is made through case-by-case calculation. As a guide, the higher the temperature and differential pressure, the more caution is needed.
Q5. Erosion is progressing on an existing valve. What should I do first?
A5. Record the damage location and flow direction, the inlet and outlet pressures during operation, and the liquid temperature, and distinguish whether it is cavitation. Then, consider whether easing the conditions is sufficient or whether a change of structure or material is required.
Q6. What information is needed to calculate the conditions for occurrence?
A6. The inlet pressure, outlet pressure, liquid temperature (saturated vapor pressure), flow rate, and type of liquid are the basics. Once these are available, it becomes easier to gauge the likelihood of occurrence and the direction of the necessary countermeasures with a certain degree of accuracy.
Summary
Countermeasures against cavitation in a control valve involve first assessing the likelihood of occurrence from the inlet pressure, outlet pressure, and liquid temperature, and then combining, according to the conditions, the distribution of differential pressure, a trim structure that keeps bubble collapse away from the surface, and erosion-resistant materials. In every case, the accuracy depends on starting from a “guide” figure and then applying it to the actual conditions of your own line.
If you have concerns such as “there is abnormal noise and vibration, but I cannot determine whether the cause is cavitation,” “I know the inlet and outlet pressures but am unsure which countermeasure valve to select,” or “erosion is progressing on an existing valve and I would like to consult about how to replace it under the same conditions,” please organize the conditions such as pressure, liquid temperature, and flow rate, and make use of Yoshitake’s technical and selection consultation. Based on your operating conditions, we will work with you to consider the selection of a valve with a suitable structure and material.