Centrifugal Pump vs Positive Displacement Pump: Which One?

Direct Answer

Picture|Centrifugal Pump VS Gear Pump

Moving clean water or thin liquids at high flow? Choose a centrifugal pump. Moving viscous fluids, slurries, or dosing at high pressure with low flow? Choose a positive displacement (PD) pump. A centrifugal pump adds velocity to the liquid and converts it into pressure; a PD pump traps a fixed volume every cycle and forces it into the discharge pipe.

Two Different Working Principles

Picture|Centrifugal Pump VS Positive Displacement Pump

Centrifugal pumps are dynamic machines: a spinning impeller flings liquid outward and the volute converts that velocity into pressure. Flow changes with system resistance — throttling the discharge cuts delivery, up to a safe shut-off head.

Positive displacement pumps trap a fixed volume of liquid each cycle and displace it into the discharge line. A PD pump is a constant-flow machine — at a given speed it delivers nearly the same flow regardless of discharge pressure. But it must never run against a closed discharge valve: with no shut-off head, pressure climbs until a pipe bursts or the pump is damaged — always fit a relief valve. Common types: gear, screw, lobe and vane (rotary); piston, plunger and diaphragm (reciprocating).

Side-by-Side Comparison

Factor	Centrifugal pump	Positive displacement pump
Working principle	Impeller adds velocity → volute turns it into pressure	Trapped volume forced into discharge each cycle
Flow vs. pressure	Flow falls as head rises (curve)	Nearly constant flow regardless of pressure
Max pressure (head)	Moderate per stage; add stages for more head	Very high possible (plunger, piston)
Viscosity	Best below ~50 cSt; efficiency drops fast	Handles thick fluids well; efficiency rises with viscosity
Solids	OK with non-clog impellers	Lobe/peristaltic types excel on slurries
Priming	Must be filled with liquid first	Most types self-prime
Closed discharge valve	Safe at shut-off head, but energy wasted	Forbidden — needs a relief valve
Maintenance	Few moving parts; simple	More wear parts: gears, valves, seals
Relative cost	Lower per unit of flow	Higher per unit of flow

When to Choose a Centrifugal Pump

Choose centrifugal for clean or lightly contaminated liquids at high flow and low-to-medium head, running continuously:

Municipal water supply and booster systems

Fire protection

Agricultural irrigation

HVAC circulation

Sewage transfer with non-clog impellers

This is why the centrifugal family dominates water duty — and why Purity Pump builds its end-suction, double-impeller, multistage and stainless-steel series around it. Since 2010 our pumps have served 130+ countries, with UL, CE, SASO, CCC and CCCF certifications. See how each series works in our centrifugal pump guide.

Picture|Purity Pump Company Introduction

When to Choose a Positive Displacement Pump

Choose PD when the liquid or the duty defeats a centrifugal:

Viscous fluids — oils, syrups, grease, resin: gear, screw or lobe pumps

High pressure, low flow — hydraulics, boiler testing: plunger or piston pumps

Accurate dosing — chemical metering: diaphragm pumps

Slurries and shear-sensitive liquids — food, pharmaceuticals, sludge: lobe or peristaltic pumps

A useful shorthand: if your liquid pours like water, think centrifugal; if it pours like honey or needs exact volume per stroke, think PD.

A straight answer from the maker: Purity Pump builds centrifugal pumps, not PD pumps. If your duty genuinely belongs to positive displacement, our engineers will say so and recommend the right specialist rather than oversell a pump that underdelivers.

A 3-Question Quick Test

What is the liquid? Clean or slightly dirty water → centrifugal. Viscous, abrasive or slurry → PD.

What flow and pressure? High flow with moderate head → centrifugal. Low flow against very high pressure → PD.

How does it run? Continuous duty → centrifugal; intermittent dosing or metering → PD.

Frequently Asked Questions

Why can’t a PD pump run against a closed valve? A PD pump has no shut-off head — it keeps pushing volume until pressure bursts a pipe or breaks the pump. Fit a relief valve.

Which pump type is more efficient? On thin liquids at its best-efficiency point a centrifugal pump reaches 85%+. PD pumps win on viscous and low-flow, high-pressure duties where centrifugal efficiency collapses.

Do positive displacement pumps need priming? Most PD types can lift liquid from a dry start; a centrifugal pump must be filled with liquid first.

Why are fire pumps always centrifugal? Fire protection needs high flow at moderate pressure — the centrifugal sweet spot — so industry standards assume centrifugal machines.

I need to transfer diesel — centrifugal or PD? Low-viscosity fuels at high flow suit centrifugal pumps; for lubricating oil or very high delivery pressure, use a gear or screw PD pump.

Still Unsure? Ask a Pump Engineer

Send our engineers the liquid, flow and head — the fluid alone usually settles the debate. Contact Purity Pump for a free duty-point check, or browse the centrifugal pump range.