When the liquid ring vacuum pump is working, at a certain temperature, when the liquid pressure at the low pressure is lower than the vaporization pressure (i.e. saturated steam pressure) at that temperature, the liquid starts to vaporize and generate bubbles. When the liquid enters the high pressure area, the bubbles break, and the surrounding liquid quickly fills the original bubble holes, generating hydraulic impact. The generation, development and rupture of bubbles are called cavitation.
In the working process of the liquid ring vacuum pump, the absolute pressure of the suction area is consistent with the absolute pressure of the pumped system. In other words, the higher the vacuum degree of the system being pumped, the higher the vacuum degree of the suction area.
Liquid ring vacuum pump needs liquid as working medium. Each liquid has a corresponding saturated vapor pressure at a certain temperature. When the absolute pressure in the suction area is closer to the saturated vapor pressure of the liquid, the liquid is closer to boiling state. At this time, there will be a large number of bubbles on the surface of the working fluid in the suction area. Because the working fluid steam generated by the liquid in the working cavity will occupy part of the working cavity space, the external suction capacity of the pump will be reduced. When the pressure in the suction area reaches the saturated steam pressure of the working liquid, it can be considered that the working cavity is full of working liquid steam. At this time, the suction capacity of the pump is close to 0, and the cavitation phenomenon of the pump is the most serious.
If the working fluid is water, the higher the water temperature is, the more the pumping performance of the pump will decline. The principle of cavitation damage of liquid ring pump is the same as that of centrifugal pump. It is at the place where bubbles are produced and broken. Pitting occurs on the metal surface and honeycomb damage will occur seriously. If there is a large residual stress in the cavitation part of the vacuum pump impeller, it will cause stress release and crack.
Because of the pump cavitation, the bubble in the high-pressure area of continuous sudden rupture, and accompanied by a strong water hammer, resulting in noise and vibration. You can hear the crackling like peas. The experimental results show that the vibration frequency range is 600-25000hz and the pressure is 49MPa.
If the above bubbles break on the metal surface, the metal surface will be subjected to continuous and strong water hammer, and there will be pockmarks, the metal grains will loosen and peel off to form honeycomb, or even perforation. In addition to mechanical action, cavitation damage is accompanied by many complex actions such as electrolysis and chemical corrosion. The actual damage shows that the location of cavitation damage of pump flow passage parts is the place where bubbles disappear.
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