How to Select the Best Pump for a Pressure Distribution System

Performance requirements need to be taken into account when selecting pumps and controls

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Pumps and controls for a pressure distribution system should be of the appropriate materials and sizes based on the pump requirements. This includes being able to deliver the required amount of discharge at the total dynamic head as calculated through determination of the flow requirements which are based on orifice and pipe sizes, friction loss in the system, and the elevation distance between the pressure laterals and the pump.

In short, selecting the pump is based on the configuration of the pipes the pump is connected to, the elevation that must be overcome including the friction loss in the piping and fittings, and the flow requirements of the system. Think of the pump in terms of these two quantities: the total dynamic head in feet the pump needs to overcome and the amount of discharge that it needs to deliver based on the number of orifices in the laterals.

The gallons per minute required is determined by the configuration of the pipes and the orifice sizing. The total dynamic head required is the sum of three parts. First is the difference from the pump elevation in the tank and the highest elevation of the distribution pipes. Second is the distribution head loss, the pressure required for the distribution laterals to operate correctly. To make sure that this is accounted for, 5 feet of head is used for design purposes. And third, the friction loss in the pipes and fittings. This accounts for the reduction in pressure as a result of the contact between the effluent and the pipe walls. Different pipe materials have different friction loss values. So PVC is different from galvanized pipe.

Once the total dynamic head and gallons per minute delivered have been determined a pump can be selected. Pump manufacturers develop pump curves for each of their models that describe the amount of total dynamic head the pump can overcome at different flows. The point describing the discharge required for the system and the total dynamic head requirement for the system needs to fall under the curve. If it doesn’t, the pump will not do the job and another pump should be used.

The pump discharge assembly, which includes the pipe from the pump and the controls for the pump, should be easily accessible from the surface for maintenance and replacement.

Pump controls and sensors that will provide for delivery to the soil treatment part of the system should be specified from a performance requirement perspective. Mechanical switches are most often used to control the pump. There may be as many as three floats in common systems: one float turns on the pump, one float turns off the pump and a third float on a separate circuit provides for a high-water alarm to warn of a pump problem.

Pump controls should be placed outside the tank, preferably in a nearby building. If a building is not available then a waterproof box on a post near the entrance to the dosing tank will work. Any holes to accommodate electrical wires through the tank or riser wall need to be sealed and made watertight.

About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

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