What exactly is valve cavitation? How to solve it?

Thread Content

During the operation of valves, many people encounter situations where there is a loud noise and severe vibration accompanying their operation; upon opening them, defects such as pitting and corrosion are found inside, and in some cases there is even perforation leading to leakage. This is known as valve cavitation – many people mistake it for corrosion or wear, and as a result they replace the valves without addressing the root cause of the problem. This leads to repeated failures, which not only increases costs but also affects production stability. In fact, cavitation is not simply a form of “damage”; it is a chain reaction resulting from physical changes in the fluid inside the valve. By identifying the root cause, it is possible to eliminate or significantly reduce this phenomenon, thereby solving the problem at its source. First, understand the problem: What exactly is valve cavitation? In simple terms, cavitation occurs when a fluid flows through the throttling section of a valve, causing the flow velocity to increase sharply and the pressure to drop rapidly. When the pressure falls below the saturated vapor pressure of the fluid at that temperature, the liquid component in the fluid vaporizes, resulting in the formation of numerous bubbles ； These bubbles move with the flow of fluid; when they reach areas with higher pressure, they burst rapidly, generating extremely high local shock forces (theoretical values can reach several thousand atmospheres). These forces continuously impact the inner surface of the valve, eventually causing pitting and erosion. Vibration and noise also occur as a result, and in severe cases, this can lead to the failure of the valve. Many people tend to confuse cavitation with corrosion; it’s important to note that corrosion is a chemical process (such as a reaction between the medium and the material), while cavitation is a physical impact phenomenon. Their causes are different, and so are their solutions—if cavitation is mistaken for corrosion, replacing the material with one that is more resistant to corrosion will not solve the problem and will only result in unnecessary cost waste. It should be noted that in real media, cavitation damage may expose fresh metal surfaces, thereby accelerating chemical corrosion; the two effects can sometimes act together, and a comprehensive consideration is required when making judgments. To determine whether cavitation is occurring, three characteristics are sufficient to consider: sharp noises and vibrations during operation, pitting or honeycomb-like erosion inside the valve (rather than uniform rusting), and failures that often occur in the throttling parts of the valve (such as the valve core and seat). The occurrence of cavitation requires two conditions: first, an excessive pressure difference before and after the valve; second, a excessively high fluid velocity. The combination of these two factors results in a local pressure lower than the saturation vapor pressure. Drawing on hands-on operation experience, there are mainly four common causes: first, improper valve selection – valves with too small a throttling gap (such as ordinary globe valves) are used, which results in a sudden increase in flow velocity as the fluid passes through them ； Secondly, the valve opening degree is not appropriate; for ordinary single-seat valves, globe valves, etc., operating at a low opening degree for an extended period will intensify the throttling effect, resulting in a sharp increase in pressure difference ； Third, the operating parameters are not matched: the fluid temperature is too high and the flow rate is too large, which leads to an increase in the saturated vapor pressure and makes it easier to reach the conditions for vaporization ； Fourth, the internal structure of the valve is unreasonable; there is no design for guiding flow or reducing pressure, which results in vortices forming within the valve and leading to excessively low local pressures. To address cavitation, in most cases no complex equipment modifications are required; by focusing on reducing the pressure difference, slowing down the flow rate, and optimizing the structure, cavitation can be significantly suppressed or reduced. The following are practical methods that can be implemented right away. First, optimize valve selection to prevent cavitation at the source. Abandon the misconception that universal valves are suitable for all situations; select valves based on the specific operating conditions. In scenarios with large pressure differences (such as pressure reduction in high-pressure pipelines), it is preferable to use valves with multi-stage pressure reduction or anti-cavitation designs (such as porous sleeve valves, labyrinth valves, etc.). These valves feature specialized structures for pressure reduction and flow guidance, which help distribute pressure and prevent excessively high local flow speeds ； Avoid using valves with small throttling gaps such as globe valves, as these types of valves are prone to severe throttling, which can lead to cavitation. At the same time, it is recommended to calculate the cavitation coefficient of the valve during selection, to ensure that it is suitable for the pressure difference and flow rate under actual operating conditions. Secondly, adjust the operating conditions to reduce the conditions that lead to cavitation. For ordinary single-seat valves, globe valves, etc., avoid operating them at a low opening degree for extended periods; adjust the opening degree according to production requirements, trying to keep it within the 30%-80% range in order to reduce the throttling effect ； If the operating conditions permit, it is possible to appropriately lower the fluid temperature and reduce the flow rate, thereby decreasing the saturated vapor pressure of the fluid and reducing bubble formation ； If the pressure difference before and after the valve is too large, a pressure reducing valve can be installed in front of the valve to reduce the pressure gradually, thereby preventing a sudden drop in pressure. Finally, optimize the valve internals and daily maintenance to delay cavitation damage. For valves that have already suffered mild cavitation, anti-cavitation components such as cemented carbide valve cores and seats can be replaced. These components have high hardness and strong impact resistance, which can help to delay cavitation-induced wear; however, they cannot completely prevent the formation of bubbles ； Regularly inspect the valve internals; repair or replace them promptly if pitting or wear is detected, to prevent further damage from occurring ； At the same time, regularly clean the impurities inside the valves to prevent them from blocking the throttling areas, which could increase flow rate and pressure fluctuations and lead to cavitation. In general, valve cavitation is not an insurmountable problem; it is a physical phenomenon that can be prevented and treated, provided that the pressure difference and flow rate are controlled to avoid fluid vaporization and the formation of bubbles.