Structure Of A Self-priming Pump

There are many structural types of self-priming pumps. In the "external mixing" type, fluid flows in the direction of rotation and merges with water returning from the right-side return port, subsequently flowing along the volute casing. As the liquid continuously impacts the impeller vanes and is broken up by the impeller, it mixes vigorously with air to form an air-water mixture; the continuous flow prevents the air and water from separating. At the volute outlet, the mixture is stripped off by the cutwater (tongue) and enters the separation chamber via a short pipe. Inside the chamber, the air separates and is discharged through the outlet pipe, while the water flows back to the impeller's outer periphery via the left and right return ports to mix with air from the suction pipe. This cycle repeats, gradually evacuating air from the suction line and drawing water into the pump to complete the self-priming process. The operation is similar for sewage pumps, oil pumps, diaphragm pumps, screw pumps, and gear oil pumps, except that the returning water flows to the impeller inlet rather than the outer periphery. For "internal mixing" self-priming pumps, the recirculation valve located below and in front of the impeller must be opened upon startup, allowing liquid inside the pump to flow back to the impeller inlet. Driven by the high-speed rotation of the impeller, the water mixes with air from the suction pipe to form an air-water mixture that is discharged into the separation chamber; the air is vented, while the water returns to the impeller inlet through the recirculation valve. This cycle continues until the air is fully evacuated and water is successfully drawn up.

The self-priming lift of the pump depends on factors such as the sealing clearance in front of the impeller, the pump's rotational speed, and the liquid level in the separation chamber. A smaller sealing clearance results in a greater self-priming lift; the clearance is typically set between 0.3 and 0.5 mm. If the clearance increases, not only does the self-priming lift decrease, but the pump's head and efficiency also drop. The self-priming lift of the pump increases with the impeller's peripheral speed ($u_2$); however, once the maximum self-priming lift is reached, further increases in rotational speed do not raise the lift further-instead, they merely shorten the time required for self-priming. Conversely, if the rotational speed drops, the self-priming lift decreases accordingly. Provided other conditions remain constant, the self-priming lift also increases with the water storage level (though this level must not exceed the optimal depth for the separation chamber). To facilitate effective air-water mixing within the pump, the impeller should have fewer vanes-thereby increasing the pitch between them-and preferably utilize a semi-open design (or one with wide flow channels); this configuration allows the recirculating water to penetrate more deeply into the impeller's vane passages.

Most self-priming pumps are paired with internal combustion engines and mounted on portable carts, making them well-suited for field operations.