A self-priming pump is a pump which will clear its passages of air if it becomes air bound and resume delivery of the pumpage without outside attention. To accomplish this, a charge of liquid sufficient to prime the pump must be retained in the casing or in an accessory priming chamber.
Working of Self Priming Pumps
During the priming cycle, air enters the pump and mixes with water at the impeller. Water and air are discharged together by centrifugal action of the impeller into the water reservoir. The air naturally tends to rise, while the water tends to sink.
Fig 1: Air sucked inside the pump when stopped.
Air-free water, now heavier than air-laden water, flows by gravity back down into the impeller chamber, ready to mix with more air coming in the suction line. Once all air has been evacuated and a vacuum created in the suction line, atmospheric pressure forces water up into the suction line towards the impeller, and pumping begins.
Fig 2: Water refilled after priming process.
Air-free water, now heavier than air-laden water, flows by gravity back down into the impeller chamber, ready to mix with more air coming in the suction line. Once all air has been evacuated and a vacuum created in the suction line, atmospheric pressure forces water up into the suction line towards the impeller, and pumping begins.
Fig 3: Pump when at rest.
Recirculation of water within the pump stops when pumping begins. The next time the pump is started, it will "self-prime" -- that is, it will be able to once again mix the water and air in the casing to create a pumpable fluid until the pump is fully primed again.
This type of pump differs from a standard centrifugal pump in that it has a water reservoir built into the unit which enables it to rid pump and suction line of air by recirculating water within the pump on priming cycle. This water reservoir may be above the impeller or in front of the impeller. In either case, the "self-priming" capability of the pump comes from the pump's ability to retain water after the very first prime.
Uses of Self Priming Pumps
Self-primers are commonly used in sewage lift stations, where raw sewage is pumped into a treatment facility. They are also commonly mounted on portable skids and used to pump stormwater and for dewatering of construction and mining operations. When used to dewater construction sites, self-priming pumps are often referred to as trash pumps – a reference to the fact that they are capable of pumping the debris that is typically found at construction sites.
Advantages of Self Priming Pumps
Solids handling is one of its most important advantages, as some self-priming pumps can handle solids up to 3 inches. Another advantage, when compared to a submersible pump, is its ability to continue pumping fluid while pump is still situated out of the pit.
Disadvantages of Self Priming Pumps
The disadvantage is its inefficiency, as clearances often need to be larger to allow for better solids handling, also a larger volute to assist with self priming.
