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Positive Displacement Pump

What You Need To Know About Pumps

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Q. I recently replaced a worn-out gear pump used for pumping fat in our feed mill. Is there a better pump type to use? – R.M.M.

A. A good question. First, let me provide some general information on pumps.

The majority of pumps fall into two major groups: positive displacement and centrifugal. In each of these groups, there are many different types of pumps for different applications. And, in many cases, there are multiple types that can be used successfully for a particular application.

Positive displacement (PD) pumps operate by trapping a fixed volume of fluid and forcing that fluid into the discharge pipe. They theoretically produce the same flow at a given speed no matter the discharge pressure and therefore must be provided with a relief or safety valve to prevent damage to the pump, piping, or downstream equipment from over-pressurization.

Positive displacement pumps can be further classified into two general categories: rotary and reciprocating. As the names imply, rotary PD pumps move fluid using a rotating mechanism, and reciprocating pumps use an oscillating mechanism such as pistons, plungers, or membranes.

A centrifugal pump uses one or more impellers to generate flow and pressure dynamically as the impeller rotates. In contrast to PD pumps, a centrifugal pump has varying flow depending on the pressure, head, and viscosity of the fluid. Centrifugal pumps can be further classified as radial flow, mixed flow, or axial flow based on the type of impeller used.

Generally, PD pumps are used for higher-viscosity fluids and fluids containing solids at relatively lower flow rates and higher pressures. Centrifugal pumps are used for lower-viscosity fluids at higher flow rates and lower pressures.

A gear pump is a type of rotary PD pump. There are two types of gear pumps: internal and external. External gear pumps use two identical gears that rotate against each other with the first gear driven by the motor and the second gear driven by the first. Gears coming out of mesh create expanding volume at the pump inlet. As the gears rotate, liquid is trapped between the casing and gear teeth in the pockets between the gear teeth and the meshing of the gears at the pump outlet forces the fluid through the outlet port.

Internal gear pumps also have two gears, one being a rotor with internal teeth and the other a smaller idler gear inside the rotor with external teeth that mesh with the rotor teeth. As with an external gear pump, trapped liquid is moved in the space between gear teeth and forced through the outlet port by the decreasing volume as the teeth mesh.

After trying several different pump styles through the years, our opinion is that internal gear pumps work the best for fats and oils as they provide a good combination of performance, reliability, and value. We size the pumps to run at reasonably slow RPMs to enhance longevity.

If the fat at your facility is particularly abrasive, it may be wearing out your gear pump at a rate that you feel is unacceptable. In that case, you may want to consider the cost-effectiveness of using a progressive cavity pump. Progressive cavity pumps are PD pumps which are particularly effective for pumping fluids with abrasive solids. The up-front cost of a progressive cavity pump is higher than a gear pump, but you may find that it provides a better long-term value proposition if the fat received at your facility is unusually abrasive.

Answered by Jonathan C. Cowles, P.E., Vice President of Sales of WL Port-Land Systems Inc.