When to Use PD Pumps (Copyright Viking Pump, Inc.)

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 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 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.