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What happens when centrifugal pump cavitation occurs

Update:14-02-2021
Summary:

When the liquid pressure at the inlet of the centrifuga […]

When the liquid pressure at the inlet of the centrifugal pump impeller drops to the saturated vapor pressure at the temperature of the conveying liquid, the liquid vaporizes; at the same time, gas dissolved in the liquid may escape from the liquid, forming a large number of small bubbles. When these small vapor bubbles flow with the liquid to the area where the pressure in the impeller channel is higher than the critical value, because the vaporization pressure inside the vapor bubble, and the outside liquid pressure is higher than the vaporization pressure, the small vapor bubble is under the action of the surrounding liquid pressure, It will condense and collapse again.

In the impeller, when the generated small vapor bubbles condense and collapse again, it seems to form a cavity. At this time, the surrounding liquid rushes toward the cavity at an extremely high speed, and the liquid particles collide with each other to form a local hydraulic impact. The local pressure can reach hundreds of atmospheres. The larger the vapor bubble, the greater the local water hammer pressure caused when it condenses and collapses. If these bubbles collapse near the metal surface of the impeller, the liquid particles are like countless small warheads, continuously hitting the metal surface.

This kind of hydraulic shock has a very fast speed and a frequency of up to 2500 l/s, which will gradually fatigue the surface of the material and cause the erosion of the metal surface, and then the appearance of large and small honeycomb corrosion holes. If the generated vapor bubble contains some kind of active gas (such as oxygen, etc.), they will use the heat released when the vapor bubble is condensed to increase the local temperature up to 200-300℃, which will cause electrochemical corrosion of the metal and accelerate it. The degree of damage to the metal. The combined phenomenon of vaporization, condensation, impact and metal erosion of the above-mentioned gas is called cavitation.

The instability of the cavitation process causes the pump to vibrate and noise. At the same time, the flow rate and head are reduced at this time because the air bubbles block the impeller channel during cavitation, and the efficiency decreases. In severe cases, the pump cannot work normally. Therefore, the pump is in operation. The occurrence of cavitation must be prevented. Once cavitation occurs, stop the pump immediately, close the inlet valve, vent the pump completely, and refill the pump. Make sure the pump is full of liquid before starting.

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