We’ve heard comments like: “my pump is not sucking water from the pool.” The reality is, pumps do not suck water. It’s a matter of physics – to put it plainly, it takes a combination of vacuum, atmospheric pressure along with an impeller to move the water. Pool pumps on a whole are what we call centrifugal pumps. The impeller that moves the water is round; water enters from the center and is thrown off the outside edges. The question is how to get the water to the pump.
As we stated in the previous paragraph, vacuum and atmospheric pressure are needed. At sea level, the pressure exerted on the surface of the earth is 14.7psi. Now let’s explain how that relates to how a pump works. When we prime a pump to get it to start moving water, we take the lid off the strainer cover and fill the area where the strainer basket is located with water. We then turn on the pump and the water in the pump strainer basket area is pumped out by the impeller. This creates a vacuum. The vacuum will exist as long as all the pipe and connections leading up to the pump are air tight and the water level at the suction points (skimmer, main drain, etc.) are covered with water. One thing about vacuums, they do not like to exist (remember your physics classes). Now we have a vacuum in the pipe and strainer basket area along with the atmospheric pressure pushing on the surface of the water. This pressure will now fill this space since the vacuum does not like to exist. As long as the pipe, connections and strainer lid are air tight, the strainer basket area will continue to be filled with water and the pump will continue moving water.
Loss of Prime
We use the words “ loss of prime” is when this vacuum is opened up to the atmosphere by taking the lid off the strainer basket, water dropping low on one of the suction points along with pipe, fittings or connections being compromised.
A little side line to the atmospheric pressure needed to accomplish this act of physics…as we go up in altitude the atmospheric pressure diminishes. Where the pressure at sea level was 14.7psi, it is only 4.23psi at 5000 ft. As we go up in altitude, a pumps ability to pull prime becomes harder. Where a pump can pull prime from 8 ft at sea level, it may only be able to pull 4 or 5 feet at a 5000 ft altitude.