What is a Self-Priming Pump?
A self-priming centrifugal pump is a solution to pumps becoming air-bound when air gets into the pump. It does this by mixing air with water to create a fluid that can be pumped and has similar properties to regular water. The pump get rid of the air and pumps the water, just like a standard centrifugal pump.
When the pump is priming, the air enters the pump and mixes with the water in the impeller. The air and water are transferred together by centrifugal action of the impeller into the water reservoir. Consequently the air naturally rises and the water sinks.
Now that the water is free of air it is heavier that air-laden water. It flows by gravity back down to the impeller chamber which is then mixed with more air being transferred through the suction line. Once all of the air has been removed and vacuum created in the suction line, the atmospheric pressure forces water into the suction line towards the impeller and begins pumping.
The water within the pump stops recirculating and it begins pumping. The next time the pumps starts it will 'self-prime'. It will be able to mix the air and water in the casing to create fluid that is pump-able. The pumps prime will be complete and will begin pumping again.
This pumps is different than a standard centrifugal pump as it has a built in water reservoir into the unit which allows it to rid pump and suction line of air by recirculating the water in the pump on the priming cycle. The water reservoir can be located above or in front of the impeller. Either way the self priming ability of the pump comes from the pumps ability to keep the water after it first prime.
Is it possible to start the pump at any time, even if its dry?
It is not possible to start the self -priming pump any time. It must have water in the casing before it can operate. The pump needs air and water to be able to prime and will not work by just turning it on and start pumping. If it is full of air it will not prime. The term self priming means the ability of the pump to continuously turn an air/water into a pump-able fluid. It is advised to never try and run a self priming pump without water in the casing. It can be dangerous and can often lead to failure and breakages.