How a Self-Priming Pump Works
The word "self-priming pump" describes a centrifugal pump that may use an air-water mixture to attain a fully-primed pumping condition.
First, let's define a centrifugal pump:
A centrifugal pump is any pump that uses centrifugal force to make a pressure differential in a substance, thus resulting in pumping action.
The simplest way to visualize this action is to assume the result of a car tire flicking water off a wet road. The pumping action is not from a "scooping" action by the vanes (the blade-like wings) on the impeller, but instead from the centrifugal force.
The movement of water via a centrifugal pump
Standard (non-self-priming) centrifugal pumps come in many types. When they are powered by flooded suction lines or in submersible applications, the impeller is surrounded by enough drinking water to generate the pressure differential and therefore to pump normal water.
Air is the main enemy of a typical (non-self-priming) centrifugal pump. When the typical centrifugal pump encounters air, it can become air-bound. It's much harder to pump air than to pump normal water, so when mid-air "binds" the pump, the pump can't force water out.
When everything's working right, a typical (non-self-priming) centrifugal pump will work like this:
A standard centrifugal pump
When air gets into a standard (non-self-priming) centrifugal pump, the pump becomes air-bound, like this:
An air-bound pump
When this air-binding happens, the pump is jammed. It will not operate before air can be removed in some way.
So, you might ask, why aren't these pumps made to pump air as well as water? The answer is that air and normal water have different properties. You wouldn't use a roof fan instead of a onboard motor in a vessel, like everyone else wouldn't use a sump pump to run your air conditioning equipment. Water is a lot more thick than air, therefore the blades used to go air can be much flimsier but have to go much faster. To go normal water, the blades have to be much sturdier, nonetheless they can move much more slowly. The propeller blades over a cruise ship change at around 100 RPM, while a jet turbofan engine unit turns at 10,000 RPM or even more.
What's Different In regards to a Self-Priming Pump?
A "self priming pump" centrifugal pump overcomes the problem of air binding by mixing air with water to make a substance with pumping properties much like those of regular water. The pump then eliminates mid-air and moves drinking water only, as being a standard centrifugal pump.
It's important to comprehend that self-priming pumps cannot operate without normal water in the casing.
Here's how it operates:
During the priming pattern, air gets into the pump and mixes with drinking water at the impeller. Normal water and air are discharged mutually by centrifugal action of the impeller in to the water reservoir. The air naturally will rise, as the normal water will sink.
A self-priming pump combining water and air
Air-free water, now bulkier than air-laden water, flows by gravity back off in to the impeller chamber, prepared to mix with an increase of air coming in the suction line. Once all air has been evacuated and vacuum pressure created in the suction collection, atmospheric pressure pushes water up into the suction lines into the impeller, and pumping starts.
A self-priming pump after the air has been evacuated from the pump
Recirculation of normal water within the pump ceases when pumping begins. Next time the pump is started, it'll "self-prime" -- that is, it'll be able to once more mix the water and air in the casing to make a pumpable fluid before pump is totally primed again.
A self-priming pump mixing normal water and air
This type of pump differs from a typical centrifugal pump in that it has a water reservoir included in the machine which enables it to rid pump and suction type of air by recirculating water within the pump on priming cycle. This normal water tank may be above the impeller or in front of the impeller. In either case, the "self-priming" capacity for the pump originates from the pump's ability to retain normal water after the initial prime.