Upon starting the pump, pressure drop occurs at the eye of the impeller which will give rise to pressure differences within the pump system (pump casing & suction pipe) and outside the pump system. As pressure continues to drop in the pump system, a mixture of liquid and air will start to flow into the pump due to higher atmospheric pressure outside the pump system. The mixture of liquid and air will flow through the casing chamber designed like a diffuser through the inlet and then to the discharge port. At the discharge port, air being lighter will be extricated though the discharge piping. The liquid being heavier will fall back into the pump chamber and re-mix with the inlet liquid. The process is repeated till air in the suction pipe and pump casing is fully emptied and filled with liquid. At this point, the pump will then be fully primed and achieve the self-priming effect.