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What is cavitation in a centrifugal pump?

2021-10-13

What is cavitation in a centrifugal pump? Since the pressure at the inlet of the centrifugal pump impeller is lower than the saturation pressure at the working water temperature, part of the liquid evaporates (vaporizes). When the vaporized vapor bubble enters the highest pressure area, the vapor bubble ruptures under pressure. After rupture, it will have a strong impact on the impeller or pump casing. This continuous local impact load gradually fatigues the metal surface until it is corroded, and then honeycomb pits of different sizes appear. This series of phenomena caused by the process of liquid vaporization, bubble generation and bubble explosion is called "cavitation". Cavitation occurs in centrifugal pumps because there are some local low pressure areas near the inlet of the medium entering the pump body. The pressure here will reduce the saturated vapor pressure of the medium and cause a small part of the medium to vaporize. Therefore, the factor that the centrifugal pump can make the inlet partial pressure lower than the vaporization pressure of the medium is the main reason for the cavitation in the centrifugal pump. This phenomenon of the centrifugal pump can be regarded as the main disadvantage of the centrifugal pump. It not only reduces the working efficiency of the centrifugal pump, but also greatly reduces the purity of the medium during transportation.



Faced with such drawbacks, if you want to solve the cavitation phenomenon of centrifugal pumps, you should start with the following three points: first, the characteristics of the suction device; second, the structure of the pump body; third, the characteristics of the conveyed medium. Only by fundamentally solving this problem can the frequency of its occurrence be reduced and the efficiency of the centrifugal pump can be improved.

When the liquid is at a certain temperature, when the pressure drops to the vaporization pressure at that temperature, the liquid will produce vapor bubbles. This phenomenon of bubbles is called cavitation. The air bubbles generated in the cavitation process decrease in volume and collapse when flowing under high pressure. This phenomenon in which the bubbles in the liquid disappear due to the increase in pressure is called cavitation collapse. When the pump is running, if the absolute pressure of the pumped liquid drops to the vaporization pressure of the liquid at the current temperature in a local area of the overflow part (usually somewhere behind the impeller blade inlet) for some reason, the liquid will be there. When the liquid starts to vaporize, a large amount of steam is generated and bubbles are formed. When the liquid containing a large number of bubbles passes through the high-pressure zone on the impeller, the high-pressure liquid around the bubbles causes the bubbles to shrink sharply or even burst. As the bubbles condense and burst, the liquid particles fill the cavity at a very high rate. At this time, they will produce a very powerful water hammer and hit the metal surface with a high impact frequency. The impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second. In severe cases, the wall thickness will break.

Centrifugal pump: cavitation situation

The muddy water pump is controlled by a working temperature of 80 degrees. After the pump is turned on, the pump pressure rises to normal, and the pump outlet pressure is normal. The pump pressure dropped rapidly in about 10 minutes, accompanied by noise, fluctuations and cavitation. Inspection found that the valve cover of the Sheung Shui Station.

For the material pump, the pump pressure rises to normal after the material is easily vaporized at about 90 degrees. Due to the small feed volume, the pump output is small and the pump pressure is normal. The pump pressure drops within about 30 minutes, and cavitation occurs along with noise and vibration.

After the above phenomenon occurs, two situations will be found:

One is that the opening of the outlet valve is not large;

In addition, the temperature of the material at the entrance and exit of the fire pump was significantly higher than before.

It is found that the cause of cavitation in these two cases is insufficient opening of the outlet valve or failure of the vertical fire pump.

When the outlet valve does not open or the opening is small, the energy obtained from the pump is not sent out by the fire pump, that is, the kinetic energy obtained by the material is converted into heat energy, which causes the temperature to rise. When the material rises, when the temperature reaches a certain temperature, cavitation will occur in the pump body.

Add the oil return line at the pump outlet, and open the oil return valve appropriately after the pump is turned on and there is no cavitation.