The decision to use a Positive Displacement pump (PD Pump) or a centrifugal pump is not always a crystal-clear choice. To make the right choice between these pump types, it is important to understand that they behave very differently. Today we’re discussing when you should choose a Viking positive displacement pump in GA.
Flow Rate Versus Pressure: The centrifugal pump 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 that viscosity has on the capacity of the pump. 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. When there is a viscosity change, there is also greater line loss in the system.
Efficiency vs. Pressure: The pumps again behave very differently when considering mechanical efficiency. Changes in pressure have little effect on the Viking positive displacement pump in GA, but a dramatic one on the centrifugal pump.
Efficiency vs. Viscosity: Viscosity also plays an important role in 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.
Net Positive Suction Head Requirements: Another consideration is NPSHR. In a centrifugal pump, the NPSHR is determined by pressure and viscosity. In a Viking positive displacement pump in GA, 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 to the 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.
Need help choosing between a centrifugal pump and a Viking positive displacement pump in GA for your application? We specialize in a variety of advisory and installation services, all designed to help you get the most out of your liquid handling equipment. And we’re certified to maintain and repair all the equipment we supply to our clients. Call us today for more information.
