How does a Self-Priming Centrifugal Pump Work?

A self-priming centrifugal pump has two phases of operation: priming mode and pumping mode.

In its priming mode, the pump essentially acts as a liquid-ring pump.  The rotating impeller generates a vacuum at the impeller’s ‘eye’ 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 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, 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.

When the pump is shut off, the design of the priming chamber (normally involving a ‘goose-neck’ on the suction piping) ensures that enough liquid is retained so that the pump can self-prime on the next occasion it is used.  If a pump has not been used for a while, it is important to check for losses from the casing due to leaks or evaporation before starting it.

What is a Compressed Air Self-Priming Pump?

It is possible to use compressed air, instead of a liquid charge, to prime a pump. Compressed air is blown through a jet into a tapered tube to create a vacuum.  Air from the pump casing and suction line is drawn in with the compressed air and vented. A check valve seals the discharge line, allowing liquid to enter the pump body.  This method has the advantages that the potential for blockages is reduced (because there is no priming chamber) and the pump can be run dry safely.

What are the Common Problems with Self-Priming Pumps?

The ability of a pump to self-prime can be affected by several factors.  The discharge line must not be pressurised or blocked.  With all types of pump, the suction line must be air-tight.  If air continues to be drawn into the pump, the pressure is never reduced and fluid is not drawn up the suction line.

It is also important that the volume of the suction-side pipework is minimised to reduce the priming time.  With excessive priming times, there is a danger that the liquid charge will evaporate before the pump is primed.  The consequential dry-running may then cause damage to the pump.

In the case of centrifugal pumps, anything that affects the efficiency of the impeller will limit the self-priming ability.  If the liquid contains any solids, debris may collect in the recirculation port, impeding the circulation of fluid and the generation of the liquid ring.  Debris collecting on the impeller itself will reduce its ability to generate a low pressure region at the eye.  Also, as a pump ages and succumbs to wear, the clearances between the impeller and volute case increase and the pump is less able to generate a low pressure zone.  Internal clearances can also be affected by incorrect assembly after maintenance.

When pumping water in cold environments, it’s important, if the temperature may fall below freezing, to drain a pump or to provide some form of heating.  Damage may occur if water freezes in the pump or pipework.