Tech Talk 12
Centrifugal pumps are not self-priming, unless specifically stated. Water must flow in by gravity, otherwise they will lose their prime, pumping will stop and the shaft seal will probably burn out. The best way to mount a centrifugal pump is to locate it lower than the surface of the water from which it is drawing. This is referred to as "flooded suction" (when the power is off, water will fill the pump). An on/off valve may be needed on the suction side of the pump for use when the pump is removed.
A non-self-priming centrifugal pump can be located above the water surface by doing the following, but the performance will be less than the pumps's standard curve shows for flooded suction.
- Install a foot valve (also known as a one-way or check valve) below the water surface. It is typically installed at the end, or the foot, of the suction pipe along with an inlet strainer. The larger the strainer, the better. Never restrict the inlet.
- Install a priming pot (or some other sealable access) to the highest point in the suction pipe, allowing the pipe to be manually filled (primed). When the pump is sealed at the top and has a foot at the bottom, it will not lose its prime. If the suction pipe has a leak, it can either lose water when the pump is off or allow air in when the pump is running. If air gets in, it can kill fish quickly (see Supersaturation Tech Talk 9).
When the inlet of a pump is restricted, its performance is reduced. Lifting water on the suction side restricts flow. Do not lift water higher than necessary and always use a non-restrictive foot valve, suction pipe and inlet strainer.