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Working principle of a self-priming pump: The liquid level at which the self-priming pump draws in liquid is below the impeller of the pump. Water must be filled in beforehand before starting the pump. To retain the water inside the pump, a check valve needs to be installed at the inlet of the suction pipe; however, this check valve causes certain hydraulic losses while the pump is in operation. However, no further watering is required before subsequent startups (watering is still needed for the first startup). After operating for a short period of time, the pump itself expels the air, allowing water to be drawn in and normal operation to resume. Suction pumps can be classified into the following categories based on their working principle: 1. Gas-liquid mixed type (including internal mixing and external mixing types). The working process of gas-liquid mixed suction pumps is as follows: Due to the special structure of these pumps, when they stop operating, a certain amount of water along with some air remains inside the pump. When the pump starts again, the air and water are drawn into the pipeline by the impeller and then sent to the gas-water separation chamber. In this chamber, the air is discharged from the top, while the water continues to be drawn into the impeller’s inlet and, together with any remaining air, is discharged again by the rotating impeller. This process repeats until all the air in the pipeline has been removed; at that point, the pump has completed its self-priming process and can start pumping water normally. 2. Water ring type: A water ring type self-priming pump combines a water ring wheel with a pump impeller within one casing; the water ring wheel is used to expel gas, thereby enabling self-priming. Once the pump is operating normally, the passage between the water ring wheel and the pump impeller can be blocked using a valve, and the liquid inside the water ring wheel can be drained. 3. Jet type (including liquid jets and gas jets): Jet self-priming pumps are composed of a centrifugal pump and a jet pump (or ejector); they rely on a jetting device to create a vacuum at the nozzle in order to achieve suction.