Start with viscosity and duty, not with price: a gear pump is a positive displacement machine that pushes a fixed volume per revolution, so it suits thick liquids, high pressure at low flow and metering, while a centrifugal pump accelerates thin liquid and suits high flow at low head. Check NPSH before you commit to either.
Two ways to move a liquid
The split between the families is mechanical, and once you see it, most selection questions answer themselves.
A gear pump traps a fixed volume of liquid between the teeth and the casing and pushes it into the discharge. Flow per revolution is set by geometry, so the pump delivers close to the same volume whether the discharge pressure is 2 bar or 20 bar. That property is what makes it useful for thick fluids and for metering, and it is also what makes it dangerous: if the discharge is closed, the pressure keeps climbing until something fails.
A centrifugal pump spins an impeller that accelerates the liquid outward and converts that velocity into pressure in the volute. Flow is not fixed. It depends on the head the system presents, so the pump runs out on its curve when the head falls and backs up when the head rises. There is a best efficiency point on that curve, and the further the duty sits from it, the worse the pump looks.
Where the differences land
The table summarises the practical consequences. The figures are typical ranges for general industrial service, not a specification for a particular model.
Two rows deserve emphasis because they cause the most failures in the field. A gear pump without a relief valve protecting the discharge is a pressure vessel waiting for a closed valve, and a gear pump run dry will score its bearings and gears within minutes because the pumped liquid is what lubricates them.
Choosing by duty
Four duty types cover most industrial enquiries, and each points in one direction.
Thick liquid. Above a few hundred cSt a centrifugal pump loses efficiency quickly and suction becomes difficult, so positive displacement is the answer. Gear pumps handle oils, resins, syrups, paints and similar fluids at viscosities where a centrifugal impeller would simply churn the liquid.
High pressure at low flow. A gear pump reaches high discharge pressure from a small, slow machine because the pressure is generated by resisting the flow, not by spinning faster. A centrifugal pump of the same flow would need several stages to reach comparable head.
Metering and dosing. Because flow per revolution is fixed, a gear pump with a controlled drive gives a repeatable volume. A centrifugal pump cannot be used this way, since its flow moves with the system head.
High flow of thin liquid. Water, light solvents and similar fluids at high volume and modest head belong to centrifugal pumps. They are smaller, cheaper to overhaul, smoother in flow and better able to handle fine solids. Our own Alphapompe water pump range sits on this side of the line, covering self-priming jet pumps, multistage centrifugal units and motor-driven sets in families such as JET SERIES, CR, CS-CSB, ORA/ORA-C, DRK-ARK and MOTOPOMPE.
NPSH and cavitation
Centrifugal pumps fail at the suction side more often than at the discharge side, and the mechanism is worth understanding because it is measurable before you buy.
Cavitation happens when the absolute pressure at the impeller eye falls below the vapour pressure of the liquid. The liquid boils locally, bubbles form and then collapse against the impeller surface, eroding it and producing the characteristic gravel-like noise, vibration and a drop in flow and head. The installation provides a certain amount of suction energy, called NPSH available, and the pump requires a minimum, called NPSH required. The rule is simply that available must exceed required, with a working margin that is commonly at least half a metre.
NPSH available falls when you raise the pump above the liquid, raise the liquid temperature, lengthen or narrow the suction line, or add fittings. That is why a pump that worked on cold water can cavitate on the same duty at 80 °C without anything on the pump having changed. Gear pumps are less exposed to NPSH in the centrifugal sense but are worse at handling entrained air, and they are not self-priming when dry.
Spares and maintenance
A centrifugal pump has few wear parts: a mechanical seal, bearings and sometimes a wear ring. Overhauls are straightforward and the hydraulic parts are usually inexpensive relative to the pump. Wear shows as a slow drop in head rather than a sudden stop.
A gear pump has more to wear, because the clearances between the gears and the casing are the metering element. As they open up, liquid slips back from discharge to suction, and the symptom is a pump that still runs at the right speed but delivers less flow at pressure. Replacing gears, bearings and seals costs more than a centrifugal seal change, and the pump is less tolerant of contaminated liquid.
What the Alphapompe range covers
Alphapompe is an Italian manufacturer specialising in water pumps, and the families on this site are centrifugal and self-priming designs for water service, not positive displacement machines. If your duty is thick liquid, high pressure at low flow or metering, the specification has to change family rather than change model, and that is a different selection exercise. If your duty is water at moderate head, the water pump range is the right starting point and the selection work is about flow, head, suction conditions and the motor.
Frequently asked questions
What pump should I use for a high viscosity liquid?
A positive displacement pump, and in most industrial cases a gear pump, an internal gear pump or a screw pump. Centrifugal pumps lose efficiency rapidly as viscosity rises and develop poor suction, so above a few hundred cSt they stop being the economic choice. Confirm the viscosity at the actual pumping temperature rather than at ambient, because the difference decides the pump size.
Can a gear pump run dry?
No. The pumped liquid lubricates the bearings and cools the clearances, so dry running scores the gear faces and seizes the pump within a short time. Brief dry suction during priming is a different matter and some designs tolerate it, but continuous dry running destroys the pump. If the process can run the pump empty, fit a dry-run protection device rather than relying on operator attention.
How do I know if a centrifugal pump is cavitating?
The usual signs are a rattling or gravel-like noise from the casing, vibration, a fall in flow and discharge pressure, and a pump that will not reach its rated duty. On the dismantled pump, cavitation shows as pitting on the impeller vanes, typically near the inlet edge. The cure is to raise NPSH available by lowering the pump, cooling the liquid, or shortening and widening the suction line, not to change the impeller.
Why can a centrifugal pump run against a closed valve and a gear pump cannot?
A centrifugal pump is not a positive displacement machine, so when the discharge closes the flow simply falls to zero at the shut-off head and the pump idles. A gear pump keeps trying to push a fixed volume into a closed pipe, so the pressure rises until a joint, hose or seal fails. Every positive displacement pump installation needs a relief valve or bypass on the discharge side.
Which pump type is cheaper to maintain?
On clean, thin liquid duty the centrifugal pump is usually cheaper to keep running, because it has fewer wear parts and overhaul kits are inexpensive. On thick liquid duty the comparison disappears, since a centrifugal pump cannot do the job at all. Compare total cost over the service interval rather than purchase cost, and include the cost of downtime, because a gear pump failure usually stops the process while a worn centrifugal pump often degrades gradually.
Matching a pump to your duty
Send the liquid, the viscosity at pumping temperature, the required flow, the discharge pressure and the suction arrangement, and we will tell you which family fits before any model is quoted. We supply the Alphapompe water pump range and can confirm flow and head against the manufacturer curves for each family. Start from the Alphapompe brand page, browse the water pump range, or look through the full product catalogue and the brand index. Contact our team with the duty point, and read the insights index for more selection guides, including the O-ring material selection guide for the seals that sit between these choices.
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