Self-priming pump

A self-priming pump can clear air from its suction line and start pumping without manual priming, by recirculating liquid in the casing to entrain and expel the air through the discharge. Engine room bilge, ballast stripping, and tank emptying use it because the suction often starts with air or runs dry as the level falls. Most marine self-priming units are centrifugal pumps with an enlarged casing and a separation chamber; the design suits high-head, low-to-moderate-flow service. Priming time depends on suction lift and line volume. A true centrifugal pump without this feature loses suction once air enters and cannot recover on its own.