Differences between Centrifugal Pump and Positive Displacement Pump

Centrifugal and positive displacement pumps are powerful tools that can move fluid through complex industrial and municipal systems. But it’s essential to choose the right equipment for your facility’s needs. Next, we'll talk about the differences between these two pumps.

What is a centrifugal pump?

A centrifugal pump is a mechanical device designed to move fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers. Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller's vane tips.

Centrifugal pumps

What is a positive displacement pump?

A positive displacement (PD) pump moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system. The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms, or vanes. The positive displacement pump is the ability to produce a consistent flow rate. The flow rate of a positive displacement pump will remain constant when there are changes in pressure. However, this is not the case with centrifugal pumps, which are designed to react to changes in pressure.

Positive displacement pump

Centrifugal Pump vs. Positive Displacement Pump

Fluid Transport Mechanism

Both centrifugal and positive displacement pumps move water from an entry point to an exit point with a controlled degree of force and volume. However, the mechanisms they use are different. Positive displacement pumps draw fluid into a cavity or expel fluid, and then force fluid out of the cavity by suction. Centrifugal or pneumatic pumps have a rotating impeller that draws fluid into the pump and forces it away from the outlet point at a higher velocity.

Type of conveyed fluid

Each type of pump works best with a different type of fluid. Positive displacement pumps can handle high-viscosity fluids, and their flow rate increases as the fluid thickens. On the other hand, centrifugal pumps also cannot handle viscous fluids due to frictional losses. Additionally, positive displacement pumps can handle fluids that are shear sensitive—or fluids that change when force, stress, or pressure is applied—while centrifugal pumps cannot; the impeller presents a risk to the fluid. Positive displacement pumps can also handle intermittent dry periods and can be primed without the need for liquid in the system to prime. The liquid is required in KNOLL centrifugal pump units to initiate pressurization control.

Centrifugal pumps and positive displacement pumps applications:

Centrifugal pumps are good at pumping thin liquids with low viscosity. These include water, thin oils and fuels, and chemicals. They are the most commonly used class of pumps in high-volume applications requiring high flow at low pressures.

Some popular applications of centrifugal pumps include:

  • Municipal water and water systems
  • Air conditioners and water circulators
  • Irrigation
  • Petrochemical and light fuel transfer station
  • Cooling Tower
  • Boiler feed

With vortex impellers, centrifugal pumps can even handle some fluids laden with solids. However, they work best when pumping large, consistent volumes of water.

Where positive displacement pumps are used:

Positive displacement pumps, on the other hand, excel in high-pressure and low-flow applications with viscous fluids. These pumps are suitable for the following applications:

  • Municipal sewage system
  • Petroleum Processing Center
  • Manufacturing centers that produce or process thick pastes and other viscous materials

Complex facilities such as food processing plants and other manufacturing facilities benefit from a mix of both types of pumps. For example, a food processing operation will require a centrifugal pump to add water to the batch, but a positive displacement pump to control the movement of thicker compounds. Oil processing centers may need positive displacement pumps to handle crude oil, while they can use centrifugal pumps to handle thinner, lighter by-products.

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