During the priming cycle, air enters the pump and mixes with water at the impeller. Water and air are discharged together by centrifugal action of the impeller into the water reservoir. The air naturally tends to rise, while the water tends to sink.
If a pump is located beneath the level of the liquid to be pumped, gravity and air pressure ensure that it is constantly filled with the liquid and there can be no ingress of air into the pump or suction line.
This type of pump differs from a standard centrifugal pump in that it has a water reservoir built into the unit which enables it to rid pump and suction line of air by recirculating water within the pump on priming cycle. In either case, the “self-priming” capability of the pump comes from the pump’s ability to retain water after the very first prime.
There are different types of pump inlet and outlet, such as 90°, Parallel, and Universal.