Cavitation occurs as a result of the formation of vapor-filled bubbles and their sudden collapse. Cavitation problems usually arise when the pressure in the pump drops below the vapor pressure of the pumped fluid. Cavitation damage poses a threat to the pump. To prevent cavitation, a minimum pressure, defined as Net Positive Suction Head (NPSH), must be maintained at the suction to ensure the liquid does not boil or vaporize. This minimum pressure prevents the formation of cavitation. The pressure at the suction must be higher than the vapor pressure of the fluid. If cavitation damage exists in the pump, it may be necessary to throttle the control valve on the discharge side. By throttling the control valve, it is possible to reduce the flow rate and the NPSH required by the pump. The important point here is to ensure that the remaining flow is sufficient to cool the pump. Cavitation leads to the formation of small gas bubbles in the fluid, which gradually reduce the pump’s capacity and shorten its lifespan. It also causes noisy operation, which is one of the most disruptive effects. Cavitation occurs due to the rapid movement of any substance within a fluid and can be considered a type of phase change phenomenon.
Negative Effects of Cavitation
Cavitation applies to all systems with pressure and velocity changes, and it can even occur in the human body, such as in joints. Cavitation is proportional to the ambient temperature. When the boiling temperature decreases, cold boiling occurs with water vapor and air bubbles inside the system, leading to cavitation. In pumps, the lowest pressure occurs at the impeller inlet, so cavitation generally accumulates there. Key effects of cavitation include:
- Cavitation is caused by low pressure but can be prevented by controlling the pressure drop.
- It occurs in the liquid phase, making it rarely observable in solid or gas phases.
- It shortens pump life, causes noise, and creates erosive and abrasive effects.
Causes of Cavitation
As mentioned earlier, pressure and cavitation are closely related. Cavitation damage can result from:
- Low atmospheric pressure at the operating points of pumps or turbines.
- Dissolved air or other gases in the liquid.
- Long suction lines or closed suction valves.
- Flow rates exceeding standard limits.
- High resistance in the suction line.
Problems Caused by Cavitation
Cavitation not only shortens pump life and causes noise but also leads to other issues. Performance loss in the system is a major concern. Noise from cavitation reduces pump efficiency, and vibrations may also occur. Over time, wear may develop, and the properties of the fluid in the system may degrade.
How to Detect Cavitation in a Pump
One way to detect cavitation is by comparing pressures. The pressure at any point in the fluid is measured. If the pressure equals or falls below the vapor pressure, cavitation damage is likely. Regular pressure testing, along with system analysis and preventive measures, can reduce the risk of cavitation and ensure a healthy operating environment.