Self-priming pumps come in many structural types, including molten salt pumps, vacuum pumps, submersible pumps, metering pumps, gear pumps, corrosion-resistant pumps, acid-resistant pumps, and fire pumps that flow in the direction of rotation. Then it merges with the water coming from the right return water hole, flowing along the volute. Due to the continuous impact of the liquid on the impeller blades in the volute, and the continuous shattering by the impeller, it is strongly mixed with air, generating a gas-water mixture, and continuous flow prevents gas-water separation. The mixture is separated by the tongue at the volute outlet and enters the separation chamber along the short pipe. In the separation chamber, the air is separated and discharged through the outlet pipe, while the water flows back to the outer edge of the impeller through the left and right return water holes, and mixes with the air in the suction pipe. This cycle repeats, gradually exhausting the air in the suction pipe, allowing water to enter the pump, completing the self-priming process. Sewage pumps, self-priming pumps, oil pumps, diaphragm pumps, screw pumps, gear oil pumps, and pumps are similar, the only difference being that the return water does not flow to the outer edge of the impeller, but to the impeller inlet. When starting an internal mixing self-priming pump, the reflux valve below the front of the impeller must be opened to allow the liquid in the pump to flow back to the impeller inlet. Under the action of the high-speed rotation of the impeller, the water mixes with the air from the suction pipe to form a gas-water mixture, which is discharged to the separation chamber. Here, the air is discharged, and the water returns to the impeller inlet from the reflux valve. This self-priming process is repeated until the air is exhausted and water is drawn in. The working principle of the internal mixing self-priming pump is similar to that of the external mixing type.
The self-priming height of a self-priming pump is related to factors such as the sealing gap in front of the impeller, the pump speed, and the liquid level in the separation chamber. The smaller the sealing gap in front of the impeller, the greater the self-priming height, generally 0.3~0.5 mm; when the gap increases, in addition to the decrease in self-priming height, the pump head and efficiency also decrease. The self-priming height of the pump increases with the increase of the peripheral speed u2 of the impeller, but when the maximum self-priming height is reached, the increase in speed will not increase the self-priming height, only shortening the self-priming time; when the speed decreases, the self-priming height decreases accordingly. Under other unchanged conditions, the self-priming height also increases with the increase of the water storage height (but it cannot exceed the optimal water storage height of the separation chamber). In order to better mix gas and water in the self-priming pump, the impeller blades should be fewer, so that the pitch of the impeller blades increases; and a semi-open impeller (or an impeller with wider impeller channels) should be used, which is more convenient for the return water to shoot deep into the impeller blades.
Most self-priming pumps are matched with internal combustion engines and mounted on mobile carts, suitable for field operations.