Working Principle and Applications of Self-priming Pumps

A self-priming centrifugal pump is able to deliver water by virtue of centrifugal force. Before the pump starts operating, the pump body and suction pipe must be completely filled with water to create a vacuum state. When the impeller rotates at high speed, the blades drive the water to spin rapidly. Under the effect of centrifugal force, the spinning water is flung out of the impeller. After the water inside the pump is discharged, a vacuum zone is formed at the center of the impeller. The water in the sump is then pushed into the suction pipe through the pipeline network under the action of atmospheric pressure (or water pressure). This cycle repeats continuously, enabling the pump to achieve uninterrupted water extraction.

It is important to note that the self-priming centrifugal pump must be filled with water in the pump casing before startup. Failure to do so will cause the pump body to overheat and vibrate, reduce water output, and even damage the pump due to cavitation, which may lead to equipment accidents.

Performance Characteristics of Self-priming Pumps

Centrifugal self-priming pumps can be categorized into two types based on their working principles: internal-mixing type and external-mixing type.

1. Internal-mixing Type

Internal mixing refers to the gas-liquid mixing process that occurs near the impeller inlet.

Before startup, an appropriate amount of liquid is poured into the pump body (submerging the impeller completely). The return valve is opened, connecting the discharge chamber and the suction chamber.

When the impeller rotates, it delivers the liquid in the suction chamber and impeller to the discharge chamber, creating a negative pressure at the impeller inlet. Since the pressure in the suction pipeline is higher than that in the suction chamber, the suction valve opens, allowing the gas in the suction pipeline to enter the impeller through the suction port.

Meanwhile, the liquid in the discharge chamber flows back to the suction chamber through the return valve orifice and is then drawn to the impeller inlet for gas-liquid mixing. The impeller transports the gas-liquid mixture to the discharge chamber.

The gas-liquid separation chamber has a sufficiently large volume to reduce the flow velocity of the mixture. Gas and liquid are separated according to their different densities: the gas is discharged through the discharge pipeline, while the liquid flows back to the impeller inlet through the return valve orifice to mix with incoming gas again.

This cycle continues until all gas in the suction pipeline is exhausted, at which point the self-priming pump completes the self-priming process and enters normal operation.

2. External-mixing Type

External mixing refers to the gas-liquid mixing process that occurs at the outer edge of the impeller.

Before startup, an appropriate amount of liquid is poured into the pump body. When the impeller rotates, it delivers the liquid in the suction chamber and impeller to the discharge chamber, creating a negative pressure at the impeller inlet. The suction valve opens, allowing the gas in the suction pipeline to enter the impeller through the suction chamber.

The gas mixes with the liquid rotating at high speed with the impeller at the impeller outlet. Driven by the impeller, the gas-liquid mixture is transported from the discharge chamber to the large-volume gas-liquid separation chamber, where the flow velocity decreases significantly.

Gas and liquid are separated based on their density difference: the gas is discharged through the discharge pipeline, while the liquid flows back to the outer edge of the impeller through the external flow channel.

This cycle repeats until all gas in the suction pipeline is purged, after which the pump finishes self-priming and operates normally.

Applications of Self-priming Pumps

Self-priming pumps are widely used in industries such as agriculture, forestry, food processing, and chemical engineering. They are mainly used to convey liquids of different properties and provide the required pressure and flow rate for various production processes.