The suction performance of centrifugal pump
The suction performance of centrifugal pump includes the allowable vacuum height and NPSH. The boiling point of water under normal pressure is 100. C. if the water is heated to the boiling point, the water will cost a lot of bubble and gasification. At high altitudes, the air is thin, the air pressure is low, and the water is less than 100. C will boil. Therefore, the vaporization of water is not only related to the temperature, but also to the atmospheric pressure on the water surface. When the atmospheric pressure is reduced to a certain extent, water can also be gasified at room temperature. According to the working principle of the centrifugal pump, the reason why the pump can suck up the liquid at the lower part is that the centrifugal force is generated by the rotation of the impeller, and the relative vacuum is generated at the pump inlet, and the atmospheric pressure on the water surface of the suction tank makes the liquid suck into the center of the impeller along the suction pipe. In general, the atmospheric pressure is equivalent to 10.3M. If the center of impeller is absolute vacuum and the head loss of suction pipe is not considered, the external atmospheric pressure can only lift water by 10.3M. It can be seen that the height of the pump is limited.
In the range of pump suction height, the higher the pump installation position from the water surface, the greater the vacuum degree at the pump inlet, that is, the lower the pressure at the impeller inlet, can water be sucked up. When the pressure at the inlet of the water pump is reduced to a certain extent, that is, equal to the vaporization pressure of the liquid at the current temperature, the liquid begins to boil and vaporize, forming bubbles in the liquid flow, which are filled with steam and gases separated from the liquid. These bubbles enter the impeller along with the liquid flow. Due to the centrifugal force, the pressure of the liquid gradually increase, which makes the steam in the bubble condense suddenly under high pressure, and the type bubble disappears. Because the bubble is very fast, the surrounding liquid rushes to the original space occupied by the bubble at a very high speed, resulting in a strong water hammer. The local instantaneous pressure produced by this water hammer can be as high as 10/3MPa. If the bubble clings to the surface of the channel, over time, under the impact of this water hammer pressure, it will cause serious damage to the surface. The practice shows that the back of blade inlet is damaged by water hammer, so the pump is not allowed to work under cavitation.
