On the one hand, there is some air in water inlet pipe or pump body. Some customers have not filled with enough water before starting pump. It just looks full. In fact, little air still leave in the pipe or pump body. Pump packing has been worn or pressed too loosen. It has an influence on getting water finally.
On the other hand, the pump speed is too slow. At first, you must install the pump correctly. Two wheel center distance can not too small and the two axis should be parallel. When the drive belt has been worn for a long time, it will easy to slip, and even reduce the pump speed. Sometimes pump itself in mechanical fault may lead to pump speed too slow. For example, fasten nut between impeller and pump shaft is loosening. And friction will be caused by the impeller and pump body directly. So the pump speed will become slow.
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