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Centrifugal vs positive displacement pump: what's the difference?
By Parth Parmar, mechanical design engineer · Updated
A centrifugal pump uses a spinning impeller to give the liquid speed and then turns that speed into pressure, so its flow changes with the pressure in the system. A positive displacement pump traps a fixed volume of liquid and pushes it out every cycle, so it delivers almost the same flow whatever the pressure. Centrifugal pumps suit large flows of thin liquids; positive displacement pumps suit thick liquids, high pressures and accurate dosing.
Side-by-side comparison
| Centrifugal pump | Positive displacement pump | |
|---|---|---|
| Working principle | Impeller adds velocity; volute turns it into pressure | Traps a fixed volume and pushes it out each cycle |
| Flow vs pressure | Flow falls as system pressure rises | Flow stays nearly constant |
| Thick liquids | Efficiency drops quickly as viscosity rises | Handles thick liquids well |
| Pressure | Moderate per stage | Can reach very high pressure |
| Closed outlet | Can run briefly, but the liquid heats up | Dangerous: pressure builds, needs a relief valve |
| Priming | Needs priming unless self-priming type | Usually self-priming |
| Flow control | Throttle the discharge valve or vary speed | Vary speed or stroke; never throttle the outlet |
| Examples | Water supply, cooling water, HVAC, fire pumps | Gear, screw, piston, diaphragm and lobe pumps |
Which one should you choose?
- Choose a centrifugal pump for high flow of water or other thin liquids at moderate pressure, where some flow change is fine.
- Choose a positive displacement pump for viscous liquids like heavy oil, high pressure, or when you need a precise, steady flow such as dosing.
How a centrifugal pump creates pressure
Liquid enters at the centre of the spinning impeller, called the eye. The vanes throw it outward at high speed, and the spiral volute casing around the impeller slows that fast flow down, turning speed into pressure. The pressure a centrifugal pump can make depends mainly on the impeller's diameter and speed, so a bigger or faster impeller makes more head.
Every centrifugal pump has a pump curve: as the pressure it has to push against rises, its flow falls. The pump settles where its curve meets the system curve of the pipes, valves and height it has to lift to. That is why closing a discharge valve partly reduces the flow of a centrifugal pump instead of damaging it.
How positive displacement pumps work
A positive displacement pump traps a fixed volume of liquid and forces it out, again and again. Each revolution or stroke moves almost the same amount, whatever the pressure. The main types are:
- Gear pump — two meshing gears carry liquid around the casing; used for lube oil and hydraulic oil.
- Screw pump — meshing screws move liquid smoothly along their length; common for fuel oil and lube oil, including on ships.
- Piston or plunger pump — a piston pushes liquid out of a cylinder through valves; reaches very high pressure, for example in hydrostatic test pumps.
- Diaphragm pump — a flexing diaphragm does the pumping with no seal on the liquid side; used for chemical dosing.
- Lobe pump — gentle rotating lobes for food and thick products.
Which pumps you find on a ship
A ship's engine room is a good example of both types side by side. Sea water and fresh water cooling pumps, ballast pumps and fire pumps are usually centrifugal, because they move a lot of thin water at moderate pressure. Fuel oil and lube oil transfer pumps are usually screw or gear pumps, because heavy, viscous oil is exactly what positive displacement pumps handle best. Explore a centrifugal pump in 3D, part by part.
Common mistakes to avoid
- Running a centrifugal pump dry. Without liquid the mechanical seal overheats within seconds. Prime the pump first.
- Closing the outlet of a positive displacement pump. Pressure keeps rising until something bursts, so a relief valve must always be fitted and set.
- Ignoring suction conditions. Too little pressure at the pump inlet makes the liquid boil and collapse in bubbles, called cavitation, which eats the impeller. See pump cavitation and NPSH explained.
See how it works inside, part by part, in 3D.
Explore the 3D centrifugal pump →Common questions
Which type of pump is more common?
Centrifugal pumps are the most common pumps in industry because they are simple, compact and handle large flows of water and similar liquids efficiently.
Why does a positive displacement pump need a relief valve?
It keeps pushing the same volume every cycle. If the outlet is blocked, pressure rises until something bursts or the motor stalls, so a relief valve is fitted to open and send flow back safely.
Can a centrifugal pump pump oil?
Yes, for light oils. As the oil gets thicker the efficiency and head fall quickly, so heavy or viscous oils are usually moved with a gear or screw pump instead.
What are the main types of positive displacement pump?
Rotary types such as gear, screw, lobe and vane pumps, and reciprocating types such as piston, plunger and diaphragm pumps.
What is NPSH?
NPSH (net positive suction head) is how much pressure is left at the pump inlet above the liquid's boiling pressure. The system must give more NPSH than the pump needs, or the liquid starts to boil inside the pump and it cavitates.
Which pump is best for transferring diesel or fuel oil?
A screw or gear pump is the usual choice. These positive displacement pumps handle viscous oil well and give a steady flow, while a centrifugal pump loses efficiency quickly as the oil gets thicker.
Sources
References for the facts on this page, and further reading:
- Hydraulic Institute via Pumps & Systems — Differences between positive displacement and centrifugal pumps
- U.S. Department of Energy — DOE Fundamentals Handbook: Mechanical Science (DOE-HDBK-1018/1-93), Pumps
Related guides
How does a centrifugal pump work? · How does a reciprocating air compressor work? (a positive displacement machine for air)
How does a turbocharger work? · How does a centrifugal pump work? · How does a V-belt pulley drive work? · How does a gear reducer work? · How does a planetary gearbox work? · How does a reciprocating air compressor work? · How does a plate heat exchanger work? · Turbocharger vs supercharger: what's the difference? · Belt vs chain vs gear drive: which is better? · How does a marine diesel engine work?
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