Cavitation pump phenomenon

Pump cavitation In 1893, it was confirmed that the destruction of a destroyer propeller in Britain was the result of cavitation. This was the first discovery of cavitation. After that, a great deal of research has been done on the cavitation problems of hydraulic machinery such as propellers, hydraulic turbines and water pumps. Cavitation has always been a crucial issue in hydronic machinery as machines move toward higher speeds. 1, the process of cavitation occurs when the pressure of liquid vaporization of liquid vaporization pressure (also known as saturated vapor pressure), liquid vaporization pressure and the size of the temperature. The higher the temperature, the greater the pressure of vaporization. 20 ℃ normal temperature water vaporization pressure of 233.8Pa (0.0238kgf / cm2), that is, at room temperature (20 ℃) ​​when the water pressure drops to 233.8Pa, began to vaporize, the liquid will have bubbles. The phenomenon of bubbles is called cavitation. When the pump is in operation, if the local area of ​​the overcurrent portion (usually somewhere after the inlet of the impeller), for some reason, the absolute pressure of the aspirated liquid drops to the vapor pressure at that temperature, At the beginning of vaporization, resulting in bubbles. These bubbles flow forward with the liquid to a high pressure, high pressure liquid around the bubbles, resulting in bubble suddenly reduced to rupture (condensation), at the same time the bubble is condensed, the liquid particles will fill the holes at high speed, collide with each other to form Water hit. This phenomenon occurs in the solid wall will make the flow of components by corrosion damage. The above process of generating bubbles and ruptured bubbles to destroy the overcurrent component is the cavitation in the pump. 2, pump cavitation phenomenon ⑴, resulting in noise and vibration. As the pump cavitation, the bubbles in the high-pressure zone continuous burst of sudden rupture, and accompanied by a strong water hammer, and noise and vibration, you can hear like popcorn like crackling sound. According to the noise can detect cavitation nascent. ⑵, over-current components of the corrosive destruction. Pumps working under cavitation for a long time may corrode parts of the pump overcurrent component, typically at the impeller exit and at the inlet to the pressurized water chamber. This is because when the bubbles condense, the surface of the metal is strongly impacted by a razor-like high-frequency (600-25000 Hz) pressure of 49 MPa, resulting in pitting and perforation of the metal surface. In severe cases, the metal grains loose and exfoliate and appear honeycomb. Cavitation damage in addition to the role of mechanical force is also accompanied by electrolysis, chemical corrosion and many other very complex role. ⑶, performance degradation. When the pump cavitation occurs, the energy exchange of the liquid in the impeller is disturbed and destroyed, which is characterized by the flow-head curve, the flow-shaft power curve and the flow-efficiency curve in the characteristic curve. In severe cases, the flow in the pump is interrupted. 3, to prevent the occurrence of cavitation measures ⑴, reduce the geometric suction height (suction depth). ⑵, to reduce the loss of inhalation, to this end can try to increase the diameter, minimize pipe length, elbows and accessories. ⑶ pump in the cavitation occurs, the flow should be reduced or slow down the operation.

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