Priming Centrifugal Pumps:

Most centrifugal pumps are not self-priming. Alternatively, the pump casing must be filled along with liquid before the pump is begins, or the pump will not be able to function. The pump impeller becomes gas-bound and incapable of pumping if the pump casing becomes filled with vapors or gases. For ensure in which a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are situated below the level of the source from that the pump is to take its suction. The similar effect could be gained through supplying liquid to the pump suction under pressure supplied through another pump placed in the suction line.