The self priming pump 12-12
A centrifugal pump adds velocity to a liquid, but first it must get the liquid.
As the centrifugal pump throws liquid out from the eye of the impeller, the volute design creates a low pressure area where the liquid used to be. At that point, either atmospheric pressure, gravity, or a combination of the two will fill up the low pressure area with either more liquid or additional air.
The problem with centrifugal pumps is that a given impeller diameter and speed will throw all fluids (either a liquid or a gas) to the same height. Since air qualifies as a fluid it will throw air to the same height as water. That height is not enough to overcome atmospheric pressure, so the centrifugal pump has to have all of its air removed before it will pump a liquid, and that is what we mean by priming the pump. In Volume #6 Paper number #6 I discussed this subject in great detail
There are several methods you can use to remove air from a centrifugal pump:
- You can fill the pump and suction piping with liquid and start all over again.
- You can attach a priming pump to the discharge side of the pump to remove any air in the pump and suction piping. Be sure this pump has a mechanical seal. You never want to use packing in a priming pump because air will leak into the stuffing box through the packing.
- Some people install a foot valve at the end of the suction piping to insure that the fluid will not drain from the pump and suction piping. These valves seldom work out because, like all check valves, they leak.
And this brings us to the subject of this discussion, the self priming pump. The concept is simple. Change the pump design so that the pump will retain enough fluid when it stops, to start again without having to worry about re-priming. A toilet or sink trap performs a similar function when it retains liquid to prevent vapors and odors from coming into your house.
There are a couple of ways to do this:
- Change the volute and impeller casing so that it retains the liquid in a built in reservoir that is filled during the initial priming phase and retains this fluid when the pump completes its pumping task and shuts down. An internal recirculation port then connects the discharge of the pump back to the suction cavity allowing a continuous recirculation of liquid during the priming phase.
- Design a suction and discharge cavity above the centerline of the impeller eye insuring that the pump is always full of liquid.
Take a look at the next two drawings. The drawings describe two versions of the same basic idea. You should be able to see how the centrifugal pump is retaining the fluid around the impellers at shut down.
|Be sure to use balanced, O-ring seals in the stuffing box of these self priming pumps.
The hydraulically balanced feature allows you to seal both vacuum and pressure, and the O-ring static and dynamic seals can seal pressure or vacuum in either direction.
Remember also that you cannot vent a running pump. Centrifugal force throws the liquid away from the eye of the impeller so you will only get liquid out through the vent. The air will then remain trapped in the eye of the impeller and eventually collect enough volume to cause the pump to lose its prime.