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Self Priming Pumps
A self-priming pump is a centrifugal pump that removes the need for external priming devices. If a centrifugal pump is located above the suction vessel or sump, it must be primed before it can be started (this is sometimes called a “suction lift”). It involves removing air from the suction line and casing, and filling them with liquid instead. If this isn’t done before starting the pump, the pump won’t work – and the air can damage the bearings. A regular centrifugal pump operating with a suction lift completes a priming operation by pulling a vacuum at the top of the pump casing and using a vacuum pump or the plant vacuum system. However, a self priming pump eliminates the need for this external priming mechanism.
When the pump is shut down, the liquid drains out of the suction line and back into the sump. Due to the unique design of the self-priming pump casing, some liquid remains in the bottom of the casing. Then when the pump starts back up, the liquid that had remained in the pump casing along, with some air, is pushed out the discharge by the impeller. This creates a vacuum at the impeller inlet, drawing liquid up the suction line. The priming cycle continues for several minutes until all of the air is pushed out of the discharge and the water has made its way up the suction line to the impeller. Once the process is complete the pump is fully primed and operates like any other primed centrifugal pump.
Self-priming pumps can be used in the same applications that use suction-lift pumps.. The pumps are more expensive because they eliminate the need for external priming mechanisms or any bearings or motors down in the sump that other pumps might require. Typically, self priming pumps limit suction to about20 feet.
Written by PumpScout.com staff.