How a Self-Priming Pump Works

A centrifugal pump which can use an air-water mixture to reach a fully-primed pumping condition, it’s called that the self-priming pump.

Now, we are living in the industrial world which has too many types of machines that can help us work better and more efficiently, and the pump is one of them. The water pumps are now used not only in the industrial field, but also in our house, we can see them working and they do help us a lot. Today we are talking about the self-priming pump which is now popularly used in the industrial field. This kind of pump is much better for people to use, and it does make the work more efficient, so how does a self-priming pump work is today we are going to talk about, and I believe that this world which is developing fast will meet a day that all work will be done by computers and machines, if that day comes, we will be free to enjoy our lives.



Definition of a Centrifugal Pump

First of all, let’s see the definition of a centrifugal pump: a centrifugal pump is any pump that creates a pressure differential in a fluid by using centrifugal force, thus resulting in pumping action.

To imagine the effect of a car tire flicking water off a wet road is the easiest way to visualize this action. The centrifugal force is the source of the pumping action, but not the "scooping" action by the vanes (the blade-like wings) on the impeller.

There are so many types of standard (non-self-priming) centrifugal pumps. The impeller will be surrounded by enough water to create the pressure differential when they operate on flooded suction lines or in submersible applications, thus they can pump water.

A standard (non-self-priming) centrifugal pump has a big enemy which is the air. The standard centrifugal pump will become air-bound when it encounters air. The pump can no longer force the water out when the air "binds" the pump, due to the air is much harder to be pump than water.

A standard (non-self-priming) centrifugal pump will work like this when everything is okay:

The pump becomes air-bound when air gets into a standard (non-self-priming) centrifugal pump:

The pump will be stuck when this air-binding happens. It will continue to operate when the air can be removed in some way.

So, why aren't these pumps designed to pump air as well as water? That is a good question, and the answer is that air and water have different properties. You wouldn't use a sump pump to run your air conditioner, just like that you wouldn't use a ceiling fan in place of a on board motor in a fishing boat, which comes the same theory. The blades have to move air much faster, so these can be much flimsier, because water is much denser than air. The blades have to be much sturdier so that they can move water, so they will move much more slowly. On a cruise ship, the propeller blades turn at around 100 RPM, while a jet turbofan engine turns at 10,000 RPM or more.

What's Different About a Self-Priming Pump?

To create a fluid with pumping properties much like those of regular water, it needs mix air with water, that’s how a self-priming pump overcomes the problem of air binding, and then, it will get rid of the air and moves water only, just like a standard centrifugal pump.

To understand that self-priming pumps cannot operate without water in the casing is much more important. Here’s how it works.

Air will enter the pump during the priming cycle and mix with water at the impeller. Through centrifugal action of the impeller, water and air will be discharged together into the water reservoir, and just as we all know, the air naturally tends to rise, while the water tends to sink.

Air-free water which is heavier than air-laden water will flow back down into the impeller chamber by gravity, ready to mix with more air coming in the suction line. How dose pumping begin? Once all air has been evacuated and in the suction line, there is a vacuum created, atmospheric pressure will force water up into the suction line towards the impeller.

Re-circulation of water within the pump will stop while pumping is beginning, and it will be self-priming pump when the pump is started again. Before the pump is fully primed again, it will be able to once again mix the water and air in the casing to create a pumpable fluid.

What this type of pump is different from a standard centrifugal pump is 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, and it may be above the impeller pump or in front of the impeller. In any condition, after the very first prime, the "self-priming" capability of the pump comes from the pump's ability to retain water.

How About Start a Self-Priming Pump Any Time, Even If It's Dry Inside?

The answer is NO, because there must be some water in the casing, a self-priming pump can operate. It is impossible for it to pump, so you cannot pull any self-priming pump right out of the box and turn it on. It won’t prime until there is no air in it. "Self-priming" doesn’t refer to the ability to literally create a vacuum out of thin air, it is the ability to repeatedly turn an air/water mixture into a pumpable fluid. As a matter of fact, if there is no water in the casing, you should never try to run a self-priming pump. It may cause seal failure, and it is much more dangerous than you have thought.