Self-priming pumps are self-sufficient pumps as they are always filled with fluids to start the pumping action. Oftentimes, pumps located below the liquid to be pumped are self-priming. This is because gravity and air pressure forces fluid to continuously fill the pump, hence preventing ingress of air in the suction line.
Basically, self-priming pumps are used to eliminate the need for an additional equipment, piping, and process to evacuate air at the suction line of the system, which is a must in non-self-priming pumps.
Both centrifugal and positive displacement pumps can be self-priming. However, not all centrifugal have the capability of self-prime. Since they are more widely used in industries, though, they are being optimized to become self-priming. Whereas positive displacement pumps are always self-priming since they can never run dry.
To be self-priming, there are two phases of operation that need to be covered. These phases are the priming mode and pumping mode.
In its priming mode, centrifugal pumps act as a liquid-ring pump. The rotating impeller generates a vacuum at the center of the impeller which draws air into the pump from the suction line. At the same time, it also creates a cylindrical ring of liquid on the inside of the pump casing. This effectively forms a gas-tight seal, stopping air from returning from the discharge line to the suction line. Air bubbles are trapped in the liquid within the impeller’s vanes and transported to the discharge port. There, the air is expelled and the liquid returns under gravity to the reservoir in the pump housing.
Gradually, the liquid rises up the suction line as it is evacuated. This process continues until liquid replaces all the air in the suction piping and the pump. At this stage, the normal pumping mode commences, and liquid is discharged.
Positive displacement pumps operate with a series of working cycles. Each cycle encloses a certain volume of fluid and moves it mechanically through the pump into the system. Depending on the type of pump and the liquid being handled, this happens with little influence from the back pressure on the pump.
A major advantage of the positive displacement pump is its ability to deliver consistent capacities because the output is solely dependent on the basic design of the pump and the speed of its driving mechanism. This means that if the liquid is not moving through the system at the required flow rate, it can always be corrected by changing one or both of these factors.