When to use a Positive Displacement Pump

January 16, 2018 0 Comments

When to use a centrifugal or a Positive Displacement pump (“PD Pump”) is not always a clear choice. To make a good choice between these pump types it is important to understand that these two types of pumps behave very differently.

 Flow rate versus pressure

By looking at the performance chart to the right you can see just how different these pumps are. The centrifugal has varying flow depending on pressure or head, whereas the PD pump has more or less constant flow regardless of pressure.

Flow rate versus viscosity

Another major difference between the pump types is the effect viscosity has on the capacity of the pump. You will notice in the flow rate chart how the centrifugal pump loses flow as the viscosity goes up but the PD Flow Rate pump’s flow actually increases. This is because the higher viscosity liquids fill the clearances of   the pump causing a higher volumetric efficiency. Remember, this chart shows only the Positive effect of viscosity on the pump flow; when there  is a viscosity change there is also greater line  Loss in the system. This means you will also  have to calculate the change in pump flow from the first chart for this pressure change.                                                                                               

Efficiency versus pressure The pumps behave very differently when

considering mechanical efficiency as well. By looking at the efficiency chart to the right you can see the impact of pressure changes on the pump’s efficiency. Changes in pressure have little effect on the PD pump but a dramatic one on the centrifugal.

Efficiency versus viscosity Viscosity also plays an important role in pump mechanical efficiency. Because the centrifugal pump operates at motor speed, efficiency goes down as viscosity increases due to increased frictional losses within the pump. Efficiency often increases in a PD pump with increasing viscosity. Note how rapidly efficiency drops off for the centrifugal pump as viscosity increases.

Net Positive Suction Head Requirements

Another consideration is NPSHR. In a centrifugal the NPSHR varies as a function of flow, which is determined by pressure and viscosity as discussed above. In a PD pump, NPSHR varies as a function of flow which is determined by speed. The lower the speed of a PD pump, the lower the NPSHR.

Operating at different points on the curve

Another thing to keep in mind when comparing the two types of pumps is that a centrifugal pump does best in the center of the curve. As you move either to the left or right, additional considerations come into play. If you move far enough to the left or right, pump life is reduced due to either shaft deflection or increased cavitation. With a PD pump you can operate the pump on any point of the curve. In fact the volumetric efficiency as a percent actually improves at the high speed part of the curve. This is due to the fact that the volumetric efficiency is affected by slip, which is essentially constant. At low speed the percentage of slip is higher than at high speed.

The data presented in these charts is the actual data for a specific application. The centrifugal was picked at its Best Efficiency Point (BEP) and the PD pump (Internal Gear) was selected to match the flow, viscosity, and pressure. Different applications will have different curves and efficiencies. These curves are presented as an example of the performance behavior differences of the two pump principles.

 Pump Selection Scenarios

Now that you have a clearer understanding of the performance differences between these two pump principles, when would you choose to use a PD pump? The following chart lists several such scenarios.

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