1 April 2022

Centrifugal pump Vs Positive displacement pump

Both centrifugal and positive displacement pumps are effective in moving fluid through complicated industrial systems. As both pumps displace fluid, it is important to identify when each pump type should be used to fulfil the purpose. In this article, you can check out the Centrifugal pump Vs Positive displacement pump. Also, you can learn how they differ from each other, their different applications and more details. 

What is a centrifugal pump?

A spinning impeller in a centrifugal pump transmits the kinetic energy of the motor to the liquid. As the impeller rotates it draws in the fluid causing increased velocity that moves the fluid to the discharge point. 

What is a positive displacement pump?

Positive displacement pumps are characterized by an operation that moves fluid by trapping a fixed amount of volume, usually in the cavity. The trapped fluid is then forced into the discharge pipe.

Centrifugal pump Vs Positive displacement pump: 

  • The clear difference between them is their operation. In the Centrifugal pump impellers imparts velocity from the motor to the fluid, resulting in pressure at the discharge point. Positive displacement pumps trap a certain amount of liquid and force it from the suction to the discharge point. 
  • In a centrifugal pump, the flow varies with change in pressure. Whereas flow rate remains consistent with changing pressure in the positive displacement pump.
  • For a centrifugal pump, the flow rate decreases when the viscosity increases due to frictional losses inside the pump. That is the reason why these pumps are not generally used on viscosities over 850 centistokes (CST). In contrast, the flow rate increases with increasing viscosity in the positive displacement pump. This is because the internal clearances allow higher viscosity handling. 
  • Centrifugal pump efficiency peaks at a specific pressure and any variations in the pressure level decrease the efficiency dramatically. Efficiency is less affected by the pressure in the positive displacement pump. 
  • Standard centrifugal pumps can't create suction lift. Whereas positive displacement pumps can create a vacuum on the inlet side, making them capable of creating suction lift. 
  • Centrifugal pumps are not recommended for shear sensitive mediums because the high-speed motor can lead to shearing of the fluids. Because of the low internal velocity, the pumped medium receives less shear. Thus, positive displacement pumps are appropriate for shear-sensitive fluids. 

The best condition to use a centrifugal pump:

Centrifugal pumps are commonly used to transfer low viscous fluids at a high flow rate, within low-pressure installations. They are used when a pump is required a large volume of media to transfer. 

The best condition to use a positive displacement pump:

Positive displacement pumps are used for their ability to handle high viscous fluids, at high pressure. Thus, they are used for a solution that can handle more difficult conditions. 

As we see above detailed discussion. We come to the point that, with increasing velocity and varying pressure, positive displacement pumps become a necessity. 

Applications of centrifugal pump:

  • Irrigation
  • Firefighting
  • Air conditioners
  • Municipal water supply system

Applications of positive displacement pump:

  • Oil processing centres
  • Municipal sewage system
  • Manufacturing centres in industries

Centrifugal pumps offer lower maintenance costs as there are few moving parts. Therefore, they are suitable for applications where the pump is used often, or continuously. The simple construction ensures there are a wide variety of construction options, including stainless steel or more corrosive applications.

Positive displacement pumps are suitable for handing mediums containing high levels of solids, including wastewater and food particles. As they are generally lower speed pumps, those with larger pumping chambers such as progressive cavities are low shear pumps and provide smooth flow. 

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