How is cavitation of centrifugal pump
Because the centrifugal pump impeller inlet pressure is lower than the working water temperature of the saturated pressure, resulting in part of the liquid evaporation (vaporization). When the evaporated bubbles enter the area with high pressure, the bubbles are broken under pressure, and the strong impact hits the impeller or pump shell after the rupture. This continuous local impact load gradually causes the fatigue of the metal surface, until the denudation, and then appears the honeycomb erosion hole of different sizes. This liquid vaporization, bubble generation, bubble rupture process caused by a series of phenomena called "cavitation". , is that the centrifugal pump cavitation happens because of the medium into the pump body there are some local low pressure area near the entrance, the pressure will reduce the saturated vapor pressure of medium, leading to a small number of media by evaporation, thus can reduce the local stress of the entrance of the centrifugal pump to medium evaporation pressure factors are the main cause of the centrifugal pump cavitation. The phenomenon of the centrifugal pump is the main disadvantage of the centrifugal pump, not only reduce the efficiency of the centrifugal pump, but also greatly reduce the purity of the transport medium.
Faced with such disadvantages, in order to solve the cavitation phenomenon of the centrifugal pump, we need to carry out the following three points first, the characteristics of the suction device; Second, the structure of the pump body; Third, the characteristics of the medium itself. Only by fundamentally solving this problem can we reduce the frequency of its occurrence and improve the efficiency of the centrifugal pump.
When the pressure of a liquid is reduced to the vaporization pressure at a certain temperature, the liquid produces bubbles. This formation of bubbles is called cavitation. Cavitation bubbles, when flowing to high pressure, reduce in size and burst. This disappearance of bubbles into the liquid due to pressure rise is called cavitation collapse. Pump in operation, if the flow of local area (usually the impeller inlet somewhere later) for some reason, reduced to the absolute pressure of pumping liquid temperature of liquid pressure, liquid at the begin to boil, to produce a large amount of steam, bubble formation, when the liquid containing a large number of bubbles within the impeller forward high, The high pressure liquid surrounding the bubble causes it to shrink so dramatically that it bursts. At the same time when the bubble condenses and breaks, the liquid particle fills the cavity at a high speed, which instantly produces a very strong water strike and hits 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, and the wall thickness will be broken down seriously.
Centrifugal pump cavitation situation:
· Turbidity water pump, the operating temperature is controlled at 80 ° C, the pump pressure rises to normal after the pump is turned on, and the pump pressure at the outlet is normal after the pump is turned on, but the pump pressure drops rapidly about 10 minutes, accompanied by noise, fluctuation and cavitation phenomenon. After review, it is found that the valve cover of sewage station is closed.
· Material pump: the pump pressure rises to normal when the organic matter which is easy to vaporize at about 90 degrees of the material is turned on. Because the pump outlet with small feeding quantity and opening of pump outlet is small, the pump pressure is normal. The pump pressure drops about 30 minutes, accompanied by noise and vibration, and cavitation phenomenon occurs.
After the above phenomenon occurs, two situations can be found:
· One is that the opening of the outlet valve is not large;
· In addition, the temperature of the inlet and outlet materials of the fire pump is significantly higher than the original
It is found that the reason for cavitation in these two cases is the insufficient opening of the outlet valve or the failure of the vertical fire pump
When the outlet valve is not open or open hours, the energy obtained from the pump is not sent away by the fire pump, which is the kinetic energy obtained by the material is transformed into heat energy, so that the material temperature rises, when reaching a certain temperature, cavitation phenomenon occurs in the pump body.
Add a return line at the pump outlet, and open the return valve properly after opening the pump, and there is no cavitation.