For a water pump running under negative pressure, since the pressure of the water inlet pipe and the pump inlet is lower than the external atmospheric pressure, air will leak into the pump from some loose places (joint surface or shaft seal). When air leaks into the pump, it greatly affects the performance of the pump.
If the pump runs at the design flow rate, although a part of the air leaks in, due to the large flow rate of the pump, that is, the ratio of the leaked air volume Va to the water volume Vw is small, so the air has little effect on the performance of the pump, and Constantly being carried away by water. When the flow rate decreases, the proportion of air gradually increases. Since the specific gravity of air is only a few percent of water, the centrifugal force of the impeller is very small, which will cause the pump head to decrease and the flow to decrease. The larger the volume ratio of the air volume Va to the water, the more serious this effect is.
The centrifugal pump relies on the high-speed rotation of the impeller to form a low-pressure area in the pump casing to pump water, so one of the key conditions for the normal operation of the centrifugal pump is that the water pump and the water inlet pipe do not leak air, and do not suck in air. The water pump not only affects the working efficiency, but also vibrates violently and produces noise when the water pump is running. When the water output of the pump is insufficient, or the water does not flow out, check whether the suction pipe is leaking.
Therefore, for pumps with air leakage, do not adjust the water volume by closing down or closing the outlet door during operation. Once the pump falls into the water, the water supply cannot be restored even if the outlet door is opened, because the pump is full of air at this time. The pump must be stopped and refilled. For pumps with serious air leakage, it is difficult to pump out water after filling, because a large amount of air leaks in during the process of starting the pump to a constant speed, which will lead to falling water, so air leakage must be eliminated.
