This chart shows only the effect of viscosity on the pump flow. Remember, 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 pressure changes.
The pumps behave very differently when considering mechanical efficiency as well. By looking at the efficiency chart to the left you can see the impact of pressure changes on the pumps efficiency. Changes in pressure have little effect on the PD pump but a dramatic one on the centrifugal.
Another consideration is NPSHr. In a centrifugal the NPSHr varies as a function of flow, which is determined by pressure. 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.
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 cavatation. 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 because 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) 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 type of performance behavior between the two different principles.
Hopefully, these rules of thumb will make it easier to determine the proper pump principle for your pumping applications.
March 19, 2008
When to use a centrifugal or a Positive Displacement pump (PD) is not always a clear choice. To make a good choice between these pump types it is important to understand that the two types of pumps behave very differently. By looking at the performance chart below you can see just how different they are. The centrifugal has varying flow depending on pressure or head, whereas the PD pump has more or less constant flow regardless of pressure.
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 losses flow as the viscosity goes up but the PD pump actually increases flow. This is because the higher viscosity liquids fill in the clearances of the pump causing a higher volumetric efficiency.