In its startup mode, the self priming trash pump is essentially a liquid ring pump. Rotating the impeller creates a vacuum in the "eye" of the impeller, drawing air from the suction pipe into the pump. Also, it also forms a cylindrical liquid ring on the inside of the self primer pump shell. This effectively forms a gas-tight seal that prevents air from returning from the discharge tube to the suction tube. The bubbles are trapped in the liquid inside the impeller blade and are transported to the discharge port. There, the air is discharged and the liquid is returned by gravity to the reservoir inside the pump shell.
After vacuuming, the liquid gradually rises. This process continued until the liquid replaced all the air in the suction pipe and pump. At this stage, the normal pumping mode begins, and the fluid is discharged.