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. 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 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 edge 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 edge. 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 process repeats until the air is fully evacuated and water is successfully drawn up.
