How do the self priming centrifugal pumps work?

Regular centrifugal pumps rely on a full liquid chamber to function. They can’t pump air and won’t work if there’s air in the system. This is where self-priming centrifugal pumps come in. They tackle this limitation with a two-stage operation: priming mode and pumping mode.

Priming Mode:

1. Acting like a Liquid Ring Vacuum Pump: When the pump starts, the impeller spins. This creates a low-pressure zone at its centre, similar to a liquid ring pump.
2. Air Evacuation: This low pressure sucks air out of the suction line and into the pump housing.
3. Liquid Ring Formation: As the air moves, some of the liquid present in the pump casing gets flung outwards by the centrifugal force. This forms a ring of liquid around the inside of the casing.
   1. Continuous Liquid Supply: There must be some residual liquid in the pump casing before it starts. This can be leftover liquid from a previous operation, or a small amount purposely added for priming. To achieve this, the centrifugal pump’s suction line is installed at an offset to the impeller centre.
   2. Topping Up: In some cases, self-priming pumps may have a reservoir built into the casing. This reservoir holds a small amount of extra liquid that can continuously replenish the liquid ring as needed during priming.
4. Air Trapping: This liquid ring acts as a seal, preventing air from the discharge side from entering the suction side. Any air bubbles that were present get trapped within the liquid moving through the impeller.

Transition to Pumping Mode:

1. Air Removal: With the air removed and the liquid ring established, the pump has achieved its priming objective.
2. Normal Pumping Operation: The pump seamlessly switches to regular centrifugal pump mode. The impeller now efficiently transfers the liquid from the suction to the discharge.