Positive displacement pump vs centrifugal pump

Table of Contents Show

What is the difference between a centrifugal pump and a positive displacement pump?
What are the advantages of a positive displacement pump over a centrifugal pump?
Are positive displacement pumps more efficient than centrifugal pumps?
Is a centrifugal pump a positive displacement pump?
Why centrifugal pump is not called as a positive displacement type of pump?

What is the difference between a centrifugal pump and a positive displacement pump?

There are two main types of pumps available: positive displacement and centrifugal (rotodynamic) pump. It’s important you can distinguish between the two to ensure the pump type selected is appropriate for the application at hand.

Positive displacement pump: the working principle involves an operation that moves fluid by trapping a fixed volume, usually in a cavity, and then forcing that trapped fluid into the discharge pipe.

Centrifugal pump: this involves the transfer of kinetic energy from the motor to the liquid by a spinning impeller. As the impeller rotates it draws in fluid, increasing the velocity which moves the fluid to the discharge point. A centrifugal pump is categorised as a non-positive displacement pump.

Working principles differ for centrifugal pumps and positive displacement pumps

Major differences between the two pump types

Factor	Centrifugal	Positive displacement
Mechanics	Impellers pass on velocity from the motor to the liquid which helps move the fluid to the discharge port	Traps a certain amount of liquid and forces it from the suction to the discharge port.
Performance	The flow rate varies with a change in pressure	Flow rate remains constant as change in pressure.
Viscosity	The flow rate decreases as the viscosity increases, even moderate thickness, due to frictional losses inside the pump	The internal clearances allow higher viscosity handling. Flow rate increases with increasing viscosity.
Efficiency	Pump efficiency peaks at a specific pressure - any variations decrease the efficiency dramatically.	Efficiency is less affected by pressure. Positive displacement pumps can be run at any point on their curve without damage or efficiency loss.
Suction lift	Standard centrifugal pumps cannot create suction lift.	Positive displacement pumps create a vacuum on the inlet side, making them capable of creating suction lift.
Shearing	The high-speed motor can lead to shearing of liquids - centrifugal pumps are not recommended for shear sensitive mediums.	The low internal velocity means little shear is applied to the pumped medium - therefore, positive displacement pumps are appropriate for shear-sensitive fluids.

When should you use a centrifugal pump?

Centrifugal pumps are commonly used to transfer low viscosity fluids in high flow rates, within low pressure installations. They are frequently used when a pump is required to transfer large volumes of media. Centrifugal pumps can be used in a wide variety of applications, including but not limited to:

general water supply
sea water transfer
water circulation
air conditioning
light fuel transfer
petrochemical
irrigation
mining
general industrial and manufacturing

If a vortex impeller is selected, some solids can be handled by a centrifugal pump.

As there are little moving parts in centrifugal pumps, they offer lower maintenance requirements and costs. Therefore, they are suited to applications where the pump is used often, or continuously run. The simple construction ensures there are a wide variety of construction options, including plastics or stainless steel, for more corrosive or hygienic applications.

When should I use a positive displacement pump?

Positive displacement pumps are selected for their ability to handle high viscosity fluids, at high pressure. Although centrifugal pumps are the most common pump installed, positive displacement pumps are a solution that can handle more difficult conditions.

Therefore, two types of positive displacement pumps: rotary and reciprocating.

Rotary positive displacement pumps operate via rotation of a pumping element:

progressing cavity pumps
vane pumps
screw pumps
peristaltic pumps
gear pumps
lobe pumps

Reciprocating positive displacement pumps which operate via a constant back and forth motion:

diaphragm pumps
piston pumps

Generally, positive displacement pumps are designed to transfer high viscosity fluids, such as thick oils, slurries, sewage and pastes. Due to their construction, they are also suitable for handling mediums containing high levels of solids, including wastewater and food particles. Screw and vane pumps are ideal for pumping clean fluids, such as fuels and lubrication oils.

As positive displacement pumps are generally lower speed pumps, those with larger pumping chambers, such as progressive cavity, lobe and peristaltic pumps, are typically low shear pumps and provide a smooth flow. This allows shear sensitive products to be pumped effectively and without any changes in composition.

Positive displacement pumps are also capable of handling variations in pressure, flow and viscosity. They can also effectively dose, ensuring accurate metering in every rotation.

Discover the wide range of centrifugal pumps and positive displacement pumps available through Global Pumps here.

Do you want to find out which type of pump is suitable for your application? Not exactly sure which is best? Get in touch with Global Pumps. With over forty years’ experience working in a wide range of industries, we can advise of best practice and ensure an efficient pumping system is selected.

Tags: Centrifugal slurry pumps, Positive displacement pump

What are the advantages of a positive displacement pump over a centrifugal pump?

Another key advantage to the positive displacement pump design is its ability to produce the same flow speed no matter what level the discharge pressure is at, unlike centrifugal pumps that are designed to react to changes in pressure.

Are positive displacement pumps more efficient than centrifugal pumps?

Key Learning: The centrifugal pump is the more efficient pump at low levels of viscosity, whereas the positive displacement pump is the more efficient pump at high levels of viscosity.

Is a centrifugal pump a positive displacement pump?

A centrifugal pump is categorised as a non-positive displacement pump. Traps a certain amount of liquid and forces it from the suction to the discharge port. Flow rate remains constant as change in pressure. The internal clearances allow higher viscosity handling.

Why centrifugal pump is not called as a positive displacement type of pump?

In short, with centrifugal pumps pressure is created and flow results, whereas with positive displacement pumps flow is created and pressure results. Performance. Due to the fact that the flow is the result of pressure, with centrifugal pumps the flow varies with changing pressure.