Comparison

Centrifugal vs Positive-Displacement Pumps

Centrifugal pumps give smooth flow that falls as head rises. Positive-displacement pumps deliver nearly fixed flow against any pressure. Compare viscosity, pressure, efficiency and control.

Mechanical, HVAC & Fluids · Last reviewed

Short answer: use a centrifugal pump for large flows of thin liquids at moderate pressure. Use a positive-displacement pump for viscous liquids, high pressure, metering and constant flow.

CentrifugalPositive-displacement (PD)
How it moves liquidImpeller adds velocity, converted to pressure in the casingTraps a fixed volume and pushes it out (gear, piston, diaphragm, screw, lobe)
Flow vs pressureFlow falls as head rises (pump curve)Flow is nearly constant, almost independent of pressure
ViscosityEfficiency drops quickly as viscosity risesHandles high viscosity well
Pressure rangeModerate per stage (multi-stage for more)Can reach very high pressure
Flow controlThrottle valve, speed, or trim impellerSpeed or stroke. A throttle valve does not control flow
Closed dischargeSafe for short periods. The pump simply builds shutoff headDangerous. Pressure rises until something fails, so a relief valve is required
Typical useWater supply, cooling, HVAC, fire, process waterOils, slurries, chemicals metering, hydraulics, hygienic transfer

How to choose

  1. Look at the liquid. High viscosity, shear-sensitive or abrasive fluids steer toward PD.
  2. Look at the flow and head. High flow at moderate head favors centrifugal. Low flow at high pressure favors PD.
  3. Decide whether the flow must stay constant as pressure changes. PD does that. Centrifugal does not.
  4. Check suction conditions. Centrifugal pumps are sensitive to NPSH. PD pumps are generally better at suction lift and self-priming.
  5. Plan the control and protection. PD pumps need a relief valve and not a discharge throttle.

Common mistakes

  • Running a PD pump against a closed valve without a relief valve.
  • Selecting a centrifugal pump for a viscous liquid using the water curve.
  • Ignoring NPSH margin and pump cavitation on a centrifugal pump.

Two scenarios

A chilled-water loop that needs 300 GPM of thin liquid at moderate head. A centrifugal pump is compact, efficient and cheap, and its flow can be controlled by a VFD. It is the standard choice.

A transfer pump for a thick lubricating oil, or a metering pump that must deliver a fixed dose. A positive-displacement pump keeps its flow as the pressure changes and handles the viscosity, which a centrifugal pump cannot.

Cost and life cycle

  • Purchase price: centrifugal pumps are cheaper at large flows.
  • Efficiency: centrifugal pumps are efficient near their best point and lose efficiency with viscous fluids. PD pumps stay efficient with viscosity.
  • Maintenance: PD pumps have close-fitting parts that wear with abrasive liquids. Centrifugal pumps have seals and bearings that need regular attention.
  • Protection: PD pumps need a relief valve on the discharge. Centrifugal pumps tolerate a closed valve for short periods.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Why does the flow of a centrifugal pump change with the system?

The pump curve and the system curve meet at the operating point. Change the system resistance and the point moves along the pump curve.

Can a VFD control a PD pump?

Yes. Flow is roughly proportional to speed, which makes a VFD a common way to control PD pump flow.

What is NPSH?

Net positive suction head: the margin of pressure at the pump suction above the liquid vapor pressure. If the available NPSH falls below the pump's required NPSH, the pump cavitates.

Can I run a centrifugal pump at low flow?

Only above its minimum continuous flow. Below it, the pump heats the liquid and can suffer damage from recirculation. The pump data gives the minimum.

What is a metering pump?

A positive-displacement pump designed to deliver a precise, adjustable flow, for example for dosing chemicals.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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