The term “self priming pump” describes a centrifugal pump that can use an air-water mixture to reach a fully-primed pumping condition. 

A centrifugal pump is any pump that uses centrifugal force to create a pressure differential in a fluid, thus resulting in pumping action.

The easiest way to visualize this action is to imagine the effect of a car tire flicking water off a wet road. The pumping action is not from a “scooping” action by the vanes (the blade-like wings) on the impeller, but rather from the centrifugal force. 

Standard (non-self-priming) centrifugal pumps come in many types. When they operate on flooded suction lines or in submersible applications, the impeller is surrounded by enough water to create the pressure differential and thus to pump water. 

Air is the main enemy of a standard (non-self-priming) centrifugal pump. When the standard centrifugal pump encounters air, it can become air-bound. It’s much harder to pump air than to pump water, so when the air “binds” the pump, the pump can no longer force the water out.

This water reservoir may be above the impeller or in front of the impeller. In either case, the “self-priming” capability of the pump comes from the pump’s ability to retain water after the very first prime. 

A self-priming centrifugal pump must have water in the casing in order to operate. You cannot pull any self-priming pump right out of the box, turn it on, and expect it to pump. If it’s full of air, it won’t prime. “Self-priming” refers to the pump’s ability to repeatedly turn an air/water mixture into a pumpable fluid — NOT the ability to create a vacuum (literally) out of thin air. In fact, you should never try to run a self-priming pump without water in the casing. It’s dangerous and will often lead to seal failure.