Centrifugal vs Positive Displacement Pumps: What’s the Difference?

There are two basic types of pumps: centrifugal and positive displacement. What’s the difference?

A centrifugal pump is a pressure-creating device. As pressure goes up, flow goes down in direct correlation.

A positive displacement pump is a flow-creating device. It moves fluid at the same speed regardless of the pressure on the inlet end.

Let’s take a deeper look.

What are centrifugal pumps and how do they work?

Centrifugal pumps are the most common type of pump in use today. They are typically used to move low viscosity fluids through a piping system.

There are many centrifugal pump designs and configurations, but all work on the same basic principle. They convert velocity to flow and pressure. Fluid enters the pump and is accelerated by an impeller as it moves outwards through the vanes to a volute. There the fluid exits the pump at a higher velocity.

You can classify centrifugal pumps by the type of impeller they use.

Radial flow centrifugal pumps are the most common type. They use a rotating impeller that creates a vacuum to move fluid. This fan-like impeller rotates within the pump housing to reduce pressure at the inlet. This motion then drives the fluid to the outside of the housing, increasing the pressure enough to send it out the discharge.

Axial flow centrifugal pumps use a curved propeller-shaped impeller. They draw fluid into their axis and the impeller sends it out on the other side.

Common uses of centrifugal pumps include water, wastewater, petroleum and petrochemical pumping, but they’re used across all industries. About 63% of all rotating equipment in the world are pumps, and of those, about 90% are centrifugals!

What are positive displacement pumps and how do they work?

Positive displacement pumps work by trapping a fixed amount of fluid, then forcing (displacing) it into the discharge pipe. They use a rotary, reciprocating or diaphragm method to move fluid. These pumps can move many types of fluids but are typically used for pumping viscous liquids and those that contain suspended or fragile solids.

You can classify positive displacement pumps by the mechanism they use to move fluid.

Rotary positive displacement pumps use a rotating mechanism to create a vacuum that draws in and captures the fluid. Common examples include gear, screw, vane, peristaltic, rotary lobe and progressive cavity pumps. Progressive cavity pumps are used in challenging applications to transfer highly viscous fluids or those containing solids, such as dirt, grit or sludge.

Reciprocating positive displacement pumps use one or more oscillating pistons, plungers or diaphragms. Valves restrict the fluid’s motion to the correct direction. Reciprocating positive displacement pumps are typically used for applications that need to maintain low flow rates against high resistance. Some may be used to pump highly viscous and heavy fluids, like sludge and slurry. Diaphragm valves are often used when pumping toxic and hazardous fluids.

Linear positive displacement pumps have been used for centuries all over the world to draw up water. Examples include rope pumps and chain pumps.

Whether you need a centrifugal or positive displacement pump, choosing the right one for your application can be tricky. Let the pros at C&B Equipment help – contact us today!