Positive Displacement Pump or Centrifugal Pump?

When pumping fluids from one location to another, most of the time it is hard to determine a centrifugal pump or a positive displacement pump to be used. Each has its own unique features about flow rate, pressure, viscosity and efficiency. We think how the pumps operate first, then go further the differences between the two pumps and give advice on when to use which pump.

Pumping Styles

Centrifugal pumps produce flow by creating pressure. The pump transfers energy from the motor or engine to the fluid by a impeller. When the impeller is rotating, fluid velocity is increased and therefore pressure. A centrifugal pump thus creates a pressure differential from pump inlet to outlet that results in the creation of flow. The positive displacement pump (PD pump) transfer fluids by trapping a fixed amount of liquid and forcing it into discharge pipe, pumping and thus displacing the liquid. A PD pump creates flow based on displacing volume. However, a PD pump is not 100% volumetrically efficient due to clearances and gaps in the rotors and liner.

Flow Rate Versus Pressure

One of the main differences between the two pump styles is the flow rate at different pressures. a centrifugal pump will decrease flow rate as the pressure increases. However, a PD pump has almost a constant flow even when pressure increases.

Flow Rate Versus Viscosity

Another main difference is the effect of viscosity on the flow rate. the centrifugal pump’s flow rate will decrease as the viscosity increases. Conversely, PD pumps deliver a higher flow as the viscosity increases. There is less slip in the pump. This is due to the fact that fluids with higher viscosity require more energy to move. The PD pump actually performs better because the fluid has a harder time slipping through the gaps and clearances near the rotors.

Pump Efficiency

Centrifugal pumps are designed to have the best efficiency at BEP (best efficiency point), within a range of 80-110% of this BEP, this pump is adequate. However, if you operate the pump outside the designed range, the efficiency of the pump will suffer. For the positive displacement pumps, it applies that efficiency increases with increasing pressure.

So, which pump to use?

This question is not as simple to answer as it seems. The positive displacement pumps are generally necessary with increasing viscosity. The same goes for working with varying pressure. Finally, to use which pump, please contact us to tell your application, we can decide on the best pump for your operation.