Analysis of cavitation phenomenon of Industrial centrifugal pump

At a certain temperature, when the pressure of the liquid is reduced to the vaporization pressure at that temperature, the liquid will produce bubbles, which is called cavitation. Cavitation can produce vibration and noise, reduce the performance of the pump and destroy the overflowing parts. Cavitation of centrifugal pump in use will bring great problems to pump performance and production.
A. the process of cavitation.
1. When the flow rate of the pump is greater than the designed flow rate, the liquid strikes the back of the blade, and the low pressure part is on the front of the blade near the front cover plate at the blade inlet.
2. When the flow rate of the pump is less than the designed flow rate, the liquid hits the front of the blade at the inlet, and the low pressure is on the back of the blade near the front cover plate at the inlet of the blade.
3. Gas generation.
If the liquid pressure is reduced to the vaporization pressure or lower, the liquid will vaporize and produce bubbles, and the gas originally dissolved in the liquid reflects the escape due to the decrease of pressure.
4. Bubble burst.
Flow to the high-pressure area, rapid condensation, the gas re-dissolves the liquid to cause a local vacuum, and the surrounding liquid particles rush at great speed and collide with each other, resulting in local pressure as high as dozens of MPa, causing noise and vibration.

B.the harm of cavitation.
1. Make the pump produce vibration and noise.
When the bubbles burst, the liquid collides with each other, and at the same time, it also hits the metal surface, which produces noise of various frequencies. In serious cases, the sound of "cracking" in the pump can be heard. At the same time, it causes the unit vibration, and the unit vibration frequency is an integral multiple of the impact frequency, resulting in a stronger cavitation resonance, resulting in the unit being forced to stop.
Cavitation is not only a great threat to the normal operation of hydraulic machinery, but also an important obstacle to the high-speed development of hydraulic machinery. Because the higher the flow rate of the fluid is, the easier it is to cause a lower local pressure and the easier it is for liquid vaporization to cause cavitation.
2. Cause the overcurrent parts to be peeled off and destroyed.
The metal in the bubble burst zone is subjected to fatigue damage caused by high frequency and high pressure liquid shock. Oxygen has a chemical corrosion effect on the metal with the help of the heat release during bubble condensation. Under the double action mentioned above, the blade and cover plate, volute or guide wheel at the outer edge of the impeller will produce pockmarked and honeycomb damage.
The sites damaged by cavitation are usually near the outlet of the impeller and the entrance of the discharge chamber. at the initial stage of cavitation, there are pockmarks on the metal surface, and then the surface shows grooves, honeycombs and fish scales. in serious cases, it can cause blade perforation or even impeller rupture, resulting in a serious accident, seriously affecting the service life of the pump.
3. Reduce the performance of the pump.
Cavitation seriously interferes with the energy transfer between impeller and liquid. The existence of a large number of bubbles clogs the flow channel, destroys the continuous flow of liquid in the pump, and reduces the flow rate, head and efficiency of the pump obviously.