Centrifugal vs Positive Displacement Pumps: Differences and Selection

The two families work on different principles, and the difference shows up in how flow responds to pressure, how they cope with viscosity, and what happens if a valve is closed downstream. Understanding that behaviour makes selection much easier than comparing catalogue pages.

How a centrifugal pump works

An impeller spins the liquid, converting rotational energy to velocity and then to pressure in the casing. Flow depends on the system: as the discharge resistance rises, flow falls along the pump curve. Output is smooth and continuous. Centrifugal pumps are simple, compact and work well on thin, clean to moderately dirty liquids. See centrifugal pumps, including multistage pumps for higher head and submersible pumps for wells and sumps.

How a positive displacement pump works

A fixed volume of liquid is trapped and forced out by a piston, plunger, diaphragm, gear, lobe, screw or vane. Flow is nearly proportional to speed and largely independent of discharge pressure, so the pump will keep building pressure against a closed valve until something fails or a relief valve opens. A pressure relief device is therefore mandatory. See positive displacement pumps, diaphragm pumps and rotary lobe pumps.

Key differences

When centrifugal is the natural choice

• Water supply, circulation, cooling and boosting. See hydro-pneumatic booster systems.
• Large flows at moderate pressure.
• Thin, clean or lightly contaminated chemicals compatible with the materials.
• Applications needing steady flow and low maintenance.

When positive displacement is the natural choice

• Viscous products: oils, syrups, paints, pastes, polymers.
• Accurate dosing, such as chemical injection; see dosing and metering pumps.
• High pressure at modest flow, such as cleaning or hydro-testing; see high pressure pumps.
• Shear-sensitive products in food or cosmetics.
• Self-priming duty with variable suction levels.

Data needed for sizing

1. Fluid name, temperature, density and viscosity at operating temperature.
2. Flow rate and the system's total head or discharge pressure.
3. Suction conditions: flooded or lift, and available NPSH.
4. Solids content and particle size.
5. Material compatibility, sealing arrangement and any hazardous-area need.
6. Power supply and duty cycle.

Hybrid considerations

Some systems use both: a centrifugal transfer pump feeding a PD booster, or a PD dosing pump injecting into a line pumped centrifugally. Variable speed drives (see speed controllers) control either type, but in PD pumps they change flow directly, while in centrifugal pumps they move the operating point along the curve.

Explore our full pumps category and the industrial pumps overview; brands we can source include pump makers listed in our pump brands section.

Reading the pump curve

For a centrifugal pump, the manufacturer's curve plots head against flow, along with efficiency and power. The system curve, which depends on static head and friction, crosses the pump curve at the operating point. Selecting a pump whose best efficiency point lies close to that crossing reduces vibration, seal wear and energy cost. Positive displacement pumps are described instead by displacement per revolution, speed and differential pressure, with slip increasing as pressure rises and viscosity falls.

Seals and materials

Both families can use mechanical seals, packing, magnetic drive or diaphragm isolation, depending on the fluid. For hazardous, toxic or expensive liquids, sealless designs reduce leak risk. Pump casings, impellers, rotors and elastomers all need to be compatible with the liquid and temperature. Mention solids, abrasive content and any cleaning-in-place routines in your enquiry.

Installation and operating notes

Keep suction lines short and straight with a generous diameter to protect centrifugal pumps from cavitation. For PD pumps, fit a suction strainer and avoid air leaks, which cause noise and lost prime. Use pulsation dampeners with piston or diaphragm pumps where steady flow matters. Check alignment, coupling guards and motor rating before start-up, and commission against a closed discharge only for centrifugal pumps, never for PD pumps.