A self-priming pump works by using the laws of buoyancy and compression to clear air out of a pumping system automatically, so you don't have to do it yourself. The pump does not need a series of complicated valve devices to keep air out. While normal centrifugal pumps use an impeller to create pressure differences in the water itself, thus forcing water through the pipes, they can also become air-bound. Air leaks into the system, rises to the top of the pump (being lighter than water), and renders the device inoperable until you prime the pump by releasing the air yourself.
Instead of using a chain of foot valves and ejector pumps to remove air, the self-priming pump creates an air and water mixture that cycles through the pump until all the air is removed. Air can be introduced into the system by accident, and then automatically removed by the self-primer system. To do this, a self-priming pumps needs a couple extra components: an air separation chamber that draws off the air bubbles, and a priming chamber that stores a small amount of water that it injects into the system to get the priming process started.
Expelling Air from the System
When active, a self-priming pump goes through a priming cycle, creating a vacuum atmosphere with a rotating impeller similar to a normal centrifugal pump. If there is no air in the water, and a prime is immediately established, the water goes through the cycle once, and is pushed through the pipes by the spinning of the impeller, in the normal fashion.
If there is air in the system, then the air and water are mixed together until it is difficult to differentiate the two, and pumped into the air separation chamber. Here, because the air is more buoyant it will rise to the surface, while the heavier water will sink back down to the impeller. The air is pumped out through the discharge piping, and the cycle repeats until a vacuum is established and water can be pumped without air bubbles.
The priming chamber stores water so that this process can continue. There must always be water in the pump for the self-priming process to work, since something needs to mix with the air and help push it out. If there is no or only a small amount of water in the main pump, the priming chamber will inject more water into the mixture so that the cycle can begin.
When water returns from the air separation step, it usually flows into the priming chamber, which again injects the now-purified water back into the pump. If there is not enough water in the pump or in the priming chamber, then the pump will not function, so the priming chamber often needs the most careful maintenance in a self-priming pump.
Self Priming Pump Variations
There are many variations on the self-priming pump. Some may have two impellers instead of one, and some may use different types of impellers, or be constructed out of different materials. Self-priming pumps do not necessarily pump water; they can also be used to pump oil, sewage and other liquids.