The suction performance of centrifugal pumps
The suction performance of the centrifugal pump includes the allowable suction height and the allowable NPSH. The boiling point of the water under normal pressure is 100. C. when the water is heated to the boiling point, the water will cost a lot, and a large number of bubbles and gasification will appear. At high altitude, 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 of the sea on the water surface. When the atmospheric pressure is reduced to a certain extent, the water can also be gasified at normal temperature. From the working principle of centrifugal pump, it can be seen that the reason why the pump can suck up the liquid at the low position is that the centrifugal force is generated by the rotation of impeller, and the relative vacuum is generated at the pump inlet, while the air pressure is on the water surface of the suction pool, which makes the liquid draw into the center of impeller along the suction pipe. In general, the atmospheric pressure is equivalent to 10.3M. (the height of sea wave is zero). If the center of impeller is absolute vacuum and the head loss of suction pipeline is not included, the external atmospheric pressure can only lift the 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 is from the water surface, the greater the vacuum degree at the pump inlet is, that is, the lower the pressure at the impeller inlet can suck up the water. When the pressure at the inlet of the water pump is reduced to a certain degree, that is, the vaporization pressure at the liquid temperature at that time, 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 together with the liquid flow. Due to the centrifugal force, the pressure of the liquid increases gradually, which makes the steam in the bubbles condense suddenly under the high pressure, and the bubble disappears. Because the bubble breaks the rule very quickly, so the surrounding liquid rushes to the original space occupied by the bubble at a very high speed, producing a strong water impact, that is, water hammer effect. The local instantaneous pressure produced by the water hammer can be as high as 10 / 3 MPa. If the bubble clings to the surface of the flow passage, over time, under the impact of this water hammer pressure, the flow to the surface will cause serious damage. It can be seen from practice that the back of the blade inlet is damaged by honeycomb under the action of water hammer, so the pump is not allowed to work under cavitation.
