Positive Displacement Pump or Centrifugal - Which Should You Use?
What's the best way to determine if you should use a positive displacement pump or cetfrifugal pump for your application?
Positive displacement pumps and centrifugal pumps have different hydraulic designs, resulting in varying application considerations that affect how ideal each pump would be for performance.
Centrifugal pumps utilize a rotating impeller to impart kinetic energy to the applicable fluid
The fluid then exits the impeller radially. The performance of a centrifugal pump is such that the flow rate is variable as a function of system differential pressure or total head. This means it can achieve variable flow while operating at a constant rotational speed:
Additionally, when pumping, the liquid is sheared by centrifugal pumps. What this means: centrifugal performance is significantly affected by the viscosity of the liquid.
Another factor to consider is that centrifugal pumps are designed for a single best efficiency point (BEP) flow rate... even though they can operate over a wide flow range. As the pump is operated away from BEP to higher or lower flow rates, the efficiency will decrease and the allowable operating region is limited. One last factor to consider is that if the application contains high differential pressure with low flow rates, the centrifugal pump design to meet this condition will have a relatively low efficiency compared to a centrifugal pump designed for a higher flow rate.
Positive displacement pumps include rotary and reciprocating pump technologies, like Gaso and Wheatley pump
Rotary pumps have meshing parts that trap liquid and convey it from the pump inlet to the outlet. Reciprocating pumps have pistons, plungers or diaphragms, which function by trapping a fixed volume of liquid in the pumping chamber and forcing or displacing it through a discharge pipe. The design of positive displacement pumps results in a nearly constant flow rate as a function of differential pressure:
This is a key difference compared to centrifugal pumps. Additionally, these pumps have less shearing of the liquid and therefore their performance is less affected by the liquid viscosity. Because of this, positive displacement pumps are commonly used in applications that require high differential pressure with constant flow rate, a low flow rate, or when dealing with high viscosity liquids.
One application where they are commonly used is in the precise metering of liquids. Because positive displacement pumps have limited flow variation as a function of pressure, they can be designed to accurately and precisely meter the injection of chemicals or additives into a system. Positive displacement pumps are known as "controlled volume-metering pumps (CVMP) or "dosing pumps" when dealing with chemicals or additives. CVMPs are widely used in dosing applications in food, chemical and pharmaceutical processing.
The original article was written by Hydraulic Institute and can be found here. IEQ and gaso.com has added to the original article.
For more information, or to receive a quote from one of our experts, contact us today.