Positive Displacement Pump Working Principle

Positive displacement pump working principle, is by providing a certain force, in the form of kinetic energy, at a fixed fluid volume from the inlet side to the pump outlet. The working principle is very different from the dynamic pump, which in theory, positive displacement pump will produce a fixed flow rate at a certain RPM even though the pump output pressure is variated.

The positive displacement pump can not operate with the control valve system at the discharge line. This is because the positive displacement pump does not recognize the excess head system as in the centrifugal pump (read this following article). If the pump outlet have a throttled valve, the pump output pressure will continue to increase. This is because the positive displacement pump will continue to produce stable fluid flow, if the rotation of the work remains. The increasing output pressure due to throttling is very dangerous to the pump components, and there is a possibility of breakage so that the flow of the pumped fluid returns stable at its working point.

20121017-095828 AM.jpg
Positive Displacement Pump Symbol

In order to anticipate those phenomenon, in order to overcome excessive restriction that potentially increases pump pressure, a positive displacement pump is required to use a pressure relief valve system mounted on the output side of the pump. The relief valve serves to ensure that there will always be a flow in the positive displacement pump when it operates, despite restrictions at the pump output side which can increase pump output pressure. This relief valve will opens and flows the working fluid out of the system or returns to the pump inlet when the pump output pressure rises at a certain value. Relief valve has adjustable working pressure settings as needed. The value of this working pressure that governs when the relief valve should open.

20121017-102025 AM.jpg
Relief Valve Design

Positive Displacement Pump Characteristic Curve

As I mentioned above that positive displacement pumps have very different characteristics from dynamic pumps. The characteristic of the positive displacement pump is determined by the volume of the pump work to which it is “moved” the working fluid. The design of the working volume of the pump is fixed to a certain size, so that at a stable RPM the pump fluid flow will tend to be at a fixed value even when the pump head changes. For easier description please look at the following positive displacement pump characteristic curves.

20121017-051848 PM.jpg
Positive Displacement Pump Characteristic Curve

Comparison of Centrifugal Pumps With Positive Displacement Pumps

Next let us discuss some basic features between the centrifugal pump and the positive displacement pump. First is the characteristic curve of the two pumps.

20121018-034517 AM.jpg

From the curve above, there are striking differences between the two pumps. Variations of fluid flow discharge at the centrifugal pump will always be accompanied by head variations as well. Whereas in positive displacement pumps, variable pump heads tend to be at relatively stable flow rate.

20121018-035108 AM.jpg

The second is the influence of fluid viscosity on fluid flow discharge. In the fixed RPM, the centrifugal pump tends to decrease the flow rate as the viscosity of the working fluid increases. Unlike positive displacement pumps the flow discharge tends to rise as the fluid viscosity rises. This is because the higher viscosity of the working fluid, the clearence of the pump will be more fulfilled by the fluid and result in increased volumetric efficiency of the pump. On this basis the use of positive displacement pumps is particularly suitable for high viscosity working fluids, such as their use in hydraulic systems.

positive displacement pump working principle

The next parameter is the mechanical efficiency value of the pump when operated at a high head. It appears that positive displacement pumps are better at mechanical efficiency at high pump head, in contrast to centrifugal pumps that have optimum mechanical efficiency points but will decrease as pump head increases.

20121018-031654 PM.jpg

The viscosity of the working fluid also affects the mechanical efficiency of both pumps. Thicker viscosity of a working fluid, will lowering the mechanical efficiency of the centrifugal pump, due to the decrease in frictional losses. In contrast to positive displacement pump which would increase its mechanical efficiency if the viscosity of the fluid gets thicker.