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Self-Priming Pumps

What is a self-priming pump?

A unique type of centrifugal pump which eliminates the need for external priming devices. If a centrifugal pump is located above the suction vessel or sump (the pump is said to have a “suction lift”), it must be primed before it can be started, to remove air from the suction line and casing, and to fill the suction line and casing with liquid. This must be done before starting the pump, otherwise the pump will not work, and the air can damage the bearings.

The priming operation on a regular centrifugal pump that is operating with a suction lift is done by pulling a vacuum at the top of the pump casing, using a vacuum pump or the plant vacuum system. A self-priming pump eliminates the need for this external priming mechanism.

How do they work?

When the pump is shut down, the liquid drains out of the suction line and back into the sump. However, due to the special design of a self-priming pump casing, a quantity of liquid remains in the bottom of the casing. When the pump is started again, the equipment goes through a priming cycle, which typically takes several minutes. During the priming cycle, the liquid that had remained in the pump casing is pushed by the impeller out the discharge, along with some of the air that is in the casing. This creates a vacuum at the impeller inlet, which draws liquid up the suction line. The priming cycle continues until all of the air is pushed out of the discharge and the water has worked its way up the suction line to the impeller. Then, the pump is fully primed and begins working like any primed centrifugal pump.

Where are they used?

Self-priming pumps may be used anywhere that a pump is running with a suction lift. The pump may be more expensive than a regular centrifugal pump, but it eliminates the external priming mechanisms, and eliminates any bearings or motors down in the sump that other pump types might require. Normally, suction lifts are limited to about 20 feet with self-priming pumps.

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