(This content has been adapted from the Pump Application Manual
Definition of a Centrifugal Pump
The term "self-priming pump" describes a centrifugal pump that can use an air-water mixture to reach a fully-primed pumping condition.
First, let's define a centrifugal pump:
A centrifugal pump is any pump that uses centrifugal force to create a pressure differential in a fluid, thus resulting in pumping action.
The easiest way to visualize this action is to imagine the effect 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 rather from the centrifugal force.
Standard (non-self-priming) centrifugal pumps come in many types. When they operate on flooded suction lines or in submersible applications, the impeller is surrounded by enough water to create the pressure differential and thus to pump water.
Air is the main enemy of a standard (non-self-priming) centrifugal pump. When the standard centrifugal pump encounters air, it can become air-bound. It's much harder to pump air than to pump water, so when the air "binds" the pump, the pump can no longer force the water out.
When everything's working right, a standard (non-self-priming) centrifugal pump will work like this:
When air gets into a standard (non-self-priming) centrifugal pump, the pump becomes air-bound, like this:
When this air-binding happens, the pump is stuck. It won't operate until the air can be removed in some way.
So, you might ask, why aren't these pumps designed to pump air as well as water? The answer is that air and water have different properties. You wouldn't use a ceiling fan in place of a onboard motor in a fishing boat, just like you wouldn't use a sump pump to run your air conditioner. Water is much more dense than air, so the blades used to move air can be much flimsier but have to move much faster. To move water, the blades have to be much sturdier, but they can move much more slowly. The propeller blades on a cruise ship
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?
A "self-priming" centrifugal pump overcomes the problem of air binding by mixing air with water to create a fluid with pumping properties much like those of regular water. The pump then gets rid of the air and moves water only, just like a standard centrifugal pump.
It is important to understand that self-priming pumps cannot operate without water in the casing.
Here's how it works:
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.
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.
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.
So Can I Just Start a Self-Priming Pump Any Time, Even If It's Dry Inside?
No. A self-priming centrifugal pump must have water in the casing
in order to operate. You cannot pull any self-priming pump right out of the box, turn it on, and expect it to pump. If it's full of air, it won't prime. "Self-priming" refers to the pump's ability to repeatedly turn an air/water mixture into a pumpable fluid -- NOT the ability to create a vacuum (literally) out of thin air. In fact, you should never try to run a self-priming pump without water in the casing
. It's dangerous and will often lead to seal failure.
Nothing here is intended to replace or stand-in for proper training and/or the use of your pump's owner's and safety manuals. ALWAYS READ THE SAFETY AND OPERATION MANUALS BEFORE OPERATING ANY PIECE OF MACHINERY, INCLUDING PUMPS.
last revised 11.21.2005