Cavitation and its hazards

Cavitation is a physical property of water and steam changes. When the micro pump is operating normally, the liquid pressure decreases from the pump inlet to the impeller inlet. Near the inlet of the impeller blades, the liquid pressure is low. When the impeller rotates, the high-speed rotating impeller works on the liquid, increasing the liquid energy and the pressure. After the impeller cavitation occurs, the cavitation process is unstable, which will cause the pump to vibrate and make noise. At the same time, because bubbles block the impeller channel during cavitation, the flow rate and head of the pump are reduced, and the efficiency is reduced. Therefore, cavitation is not desired in the water pump during normal operation.

When the pressure near the impeller blade inlet is less than the saturated vapor pressure at the liquid delivery temperature, the liquid begins to vaporize and generate bubbles. The bubbles burst when they enter the high-pressure area in the pump body with the liquid flow, and the surrounding liquid quickly fills the original bubble cavity, producing a hydraulic impact. This phenomenon of bubble generation, development and rupture is called cavitation.

When cavitation occurs, the flow-through parts are eroded and corroded, and the performance of the pump suddenly decreases. In severe cases, the noise of the water pump and the vibration of the unit are generated. The mutual excitation between the two causes the pump to produce strong cavitation resonance, leading to major accidents of equipment damage.

The parts where cavitation is prone to occur in centrifugal pumps are:

1. The front cover plate with large impeller curvature, the low-pressure side close to the inlet edge of the blade;

2. The low-pressure side of the volute baffle and guide vane in the extrusion chamber close to the inlet edge;

3. The sealing gap between the outer circle of the blade tip and the shell of the high-speed impeller without a front cover

And the low-pressure side of the blade tip.

Causes of cavitation

During normal operation, the liquid level of the water pool is stable at a high liquid level and does not change much. Although the vacuum height and other issues are considered during the design and installation of the circulating water pump, the large suction resistance of the pump under actual operating conditions and the intake of air are one of the main causes of impeller cavitation. Although various methods are adopted to prevent the occurrence of cavitation in the circulating water pump during manufacturing, installation and operation, in actual operation, the circulating water pump using traditional packing seal has obvious wear on the pump sleeve. As the running time increases, the radial clearance between the sleeve and the packing continues to increase. If the packing is not tightened in time, the sealing effect will inevitably deteriorate. The air enters the low-pressure area of ​​the impeller inlet from the pump shaft under the action of atmospheric pressure, and enters the high-pressure area of ​​the pump with the water flow, impacting the impeller to cause erosion and corrosion damage of the flow-through parts. This is another important reason for the cavitation of the pump impeller and pump casing.

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