Centrifugal pumps are not self priming. If initially there is no liquid a the eye, there will be no pumping action for a centrifugal pump. In absence of liquid, air (sometimes vapour) will be present at the eye, and owing to its light density air could be thrown out under centrifugal force only if the speed of the impeller is very very high (like in a Turbocharger Blower). In such a case, where normally a the start of the pump the level of the liquid is below the eye of the impeller, we can make use of a self priming unit.
Figure above shows an automatic arrangement for pumping out bilges, using a centrifugal pump, wherein the air (vane) pump will get engaged automatically and draw out any air at the start or during running. Once the air is drawn out it will get disengaged automatically.
Discharge side of the pump is connected with one side of the piston (engage / disengage mechanism) as shown in the figure. Consider the pump is started with no liquid at the eye of the impeller. Now the impeller will be rotating but the absence of liquid at the discharge (means no discharge pressure) makes the piston to move forward due to spring pressure and thus the bevel connected to the air pump rotor shaft engages with the rotating shaft of the centrifugal pump. This drives the air pump to remove any vapour or air present inside the pump suction and the liquid rises to prime the pump. Once the pump is primed discharge commences, discharge pressure rises which acts on the piston thereby pushing the piston against the spring pressure. Thus the air pump gets disengaged. Hence whenever there is any ingress of air or vapour in the pump suction, discharge pressure reduces and air pump engages to remove the same.
“Basic Marine Engineering” by J.K. DHAR; Principles of Marine Engineering – Series (PMES)
“Marine Auxiliary Machinery” by H. D. McGeorge