What is Cavitation in Pump?
Pump cavitation refers to the situation where when the pressure on the liquid in the pump decreases, the liquid will vaporize, thereby generating many bubbles. The bubbles will gradually grow larger. The moment the bubble collapse, the high velocity fluid in the pump impacts inside the bubble. This impact force will damage the pump and reduce its service life.
Why Cavitation Occurs?
To understand why cavitation occurs, we must first understand the physical properties of water. As we all know, water and vapor can be converted into each other. When the temperature of water remains unchanged and the pressure drops to a critical value, water will vaporize. This pressure is called vapor pressure.
When vaporization occurs, water will emit vapor and bubbles. This process is called water vaporization, and the vaporization process is the cause of cavitation.
Process Of Cavitation
When the pressure on the water in the pump decreases, the water vaporizes. During this process, a large amount of vapor is produced and the gas dissolved in the water forms bubbles.
When the bubbles flow from the low-pressure area to the high-pressure area in the pump with the water, the bubbles will quickly condense and collapse under the action of the high pressure. At this time, the high-pressure water will quickly flow to the space previously occupied by the bubbles.
When the gas in the bubble has not yet condensed, the high-speed water rushes towards the bubble, impacting the bubble into more small bubbles. The small bubbles will continue to condense and collapse under high pressure.
When the vaporization reaction is repeated, the bubbles on the flow channel surface and near the impeller will have a strong impact on the material, and in severe cases, honeycomb cavitation or even fracture of the wear parts will occur. The active gas in the bubbles will contact the metal for a long time, which will corrode the metal.
What Causes Pump Cavitation?
Pump Cavitation Animation
Pump cavitation is a common problem of pumps. Many improper operations can lead to cavitation.
Cavitation is determined by the pump’s own anti- cavitation ability and the suction end device.
The following are several important reasons for pump cavitation.
Motor speed is too high
When the pump motor speed is too high, the flow rate at the impeller suction port increases. According to Bernoulli’s principle, when the speed increases, the pressure will decrease. When the pressure is lower than the critical value of the vapor pressure, the liquid will vaporize, eventually leading to cavitation.
The suction pipe resistance is too large
When the pump suction pipe is too long, has too many elbows, or has blockages in the pipe, the pressure of the liquid will decrease when flowing through the pipe, which may cause cavitation in the pump.
Pump installation height is too high
Every pump has an important parameter, NPSH. When the pump is installed too high relative to the liquid level, the pump’s NPSHa (available net positive suction pressure head) will be less than NPSHr (required net positive suction pressure head), which will cause the suction pressure to decrease and cavitation to occur.
Liquid temperature is too high
When the temperature of the liquid we transport is too high, a small amount of liquid may vaporize before it reaches the suction end, which makes cavitation more likely to occur.
Multiple pumps in parallel
When our working conditions are more complicated and we use multiple pumps in parallel to transport liquids, if the parallel pumps are unevenly configured, the flow rate of one pump will be too small, the pressure at the suction end will decrease, and cavitation will occur.
How to Identify Cavitation? Pump Cavitation Effects
Damage to The Wear Parts
Cavitation will impact the wear parts, causing honeycomb cavitation on the wear parts, and even breaking or corroding the wear parts.
Vibration
When the pulsation frequency of cavitation is close to the natural frequency of the pump during operation, it will also cause the pump to vibrate violently.
When vibration occurs, the pressure gauge will fluctuate.
Bearing Temperature Increase
Cavitation causes the pump to vibrate violently, increases bearing friction, and raises bearing temperature. Use a thermometer to measure the bearing temperature.
If the bearing temperature is higher than usual, cavitation may have occurred.
Noise
When cavitation occurs, a large number of bubbles collapse and the high-speed water flow impacts, causing severe noise.
The sound of cavitation is similar to the popping sound of stone hitting metal, and the sound will increase or decrease with the flow rate.
Efficiency Decrease
The large number of bubbles produced by cavitation will hinder the flow of water, resulting in a decrease in flow rate, unstable pump head, and reduced pump efficiency.
When the flow meter value is found to be 10% lower than the expected value, it can be inferred that cavitation has occurred.
How to Avoid Pump Cavitation?
Reduce Pump Motor Speed
Adjusting the motor speed to the appropriate speed according to the pump parameter table or pump performance curve will effectively avoid cavitation.
Pipeline Design
Improve the suction pipe design, reduce the number of elbows, allow the water to always flow at a higher efficiency, and reduce the friction loss of the water in the pipe.
Install The Pump Correctly
Each pump has a certain cavitation margin NPSH. Before installing the pump, it must be installed according to the predetermined parameters to ensure that NPSHr is greater than NPSHa.
Regular Maintenance
After the pump is installed, regular maintenance should be performed according to the operating manual provided by the pump manufacturer, such as checking the bearing temperature, regular inspection of the impeller wear, pump vibration, etc.
Choosing The Right Pump
Before starting your project, you should ask the pump manufacturer to design your project to ensure that the pump you choose meets the expected design of the project. This is the best way to prevent cavitation and minimize the probability of cavitation.
Types of Cavitation
| Cavitation Types | Cause | Common Cavitation Sites |
| Vaporous Cavitation | The liquid pressure decreases, vaporizing to form bubbles | Impeller inlet |
| Turbulence Cavitation | The flow rate in the volute or pump is too high, resulting in a decrease in pressure and the formation of bubbles. | Surface of volute |
| Inertial Cavitation | The flow rate is too high, causing impact on the pump and forming bubbles. | impeller |
| Vortex Cavitation | The water flow is vortex-shaped and the gas is drawn into the pump | Pump Inlet |
| Gaseous Cavitation | The liquid pressure decreases and the gas dissolved in the liquid forms bubbles | Chemical pumps |
