The air (yellow arrows) is drawn into the pump due to the negative pressure created by the moving impeller and is emulsified with the liquid (blue arrows) contained in the pump casing.
The air-liquid emulsion is forced into the priming chamber where the lighter air is separated and leaves through the discharge pipe; the heavier liquid drops back down into circulation. Once all of the air has been expelled from the suction pipe, the pump is primed and works like a normal centrifugal pump. The pump can also work with an air-liquid mixture.
The non-return valve has a dual function: it prevents the suction pipe from emptying when the pump is off; in the event of accidental emptying of the suction pipe, this holds a sufficient amount of liquid in the pump casing to prime the pump.
The discharge pipe must be free to expel the air coming from the suction pipe.