How a centrifugal pump works
A centrifugal pump converts the velocity an impeller gives the medium into head.
The impeller spins inside the casing and flings the medium outwards. The volute slows that flow on its way to the discharge, and the velocity becomes head. Impeller diameter, speed and casing shape set the performance curve.
Two things follow. Flow falls as head rises, so the pump settles where its curve meets the system. And it needs liquid at the impeller eye to work at all: starve it and you cavitate, which pits the impeller and takes the mechanical seal with it.
When to choose a centrifugal pump
Choose centrifugal when the medium is water or as water, the flow is high and the head is steady. It is cheaper to buy and to run than any positive displacement pump. Once solids or viscosity enter the duty that advantage goes, and a rotary lobe or progressing cavity pump earns its cost.
- Clean water transfer, boosting and circulation in building services
- Thin process liquids, and any medium that behaves as water
- High flow against a moderate, steady total dynamic head
- Screened wastewater and effluent, with a solids-handling wet end
- Hygienic duties on thin product, in polished stainless with a cleaning regime behind it
- Above roughly 3–4% solids, where positive displacement takes over
- Viscous or shear-sensitive product, which wants pumping slowly
- Duties that need a near-constant flow while the pressure varies
- Rag-laden sewage with no screening and no solids-handling impeller
- Suction lifts, unless the build is self-priming or the pump is submersible
What a centrifugal pump is good and bad at
Lowest capital and running cost per m³/h on thin media, and the simplest wet end on site to strip and repair.
End-suction, vertical multistage, submersible and hygienic variants cover most of what a UK site runs on water and thin product.
It cannot make more pressure than its curve allows, so it needs none of the protection a positive displacement pump does.
Head rises, flow drops. If the process needs a fixed rate, a positive displacement pump holds it far closer.
Let NPSHa fall below NPSHr at the duty point and it cavitates, which pits the impeller and puts noise into the pipework.
A mechanical seal needs liquid to lubricate it. A shut valve or an empty suction tank is enough to finish it.
Pumps we supply
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A high-purity hygienic centrifugal pump for biotech and pharmaceutical process duties. EHEDG certificate, full material traceability, eight sizes.
A liquid-ring pump for CIP return and other duties where the liquid arrives with air or gas in it. Three sizes to 80 m³/h.
A standardised hygienic centrifugal pump for basic CIP and transfer duties, in four sizes, with an UltraPure execution for pharmaceutical work.
Calpeda BNM, BNMS, BNM4 and BNMS4 are the bronze versions of the NM, NMS end suction range, on EN 733 dimensions and rated to 10 bar.
The Calpeda MXH is a WRAS approved horizontal multistage stainless steel pump for clean water to 66 m³/h and 97 m. Specs, sizes and UK supply.
The Calpeda NM is a close-coupled EN 733 end suction pump for clean water to 240 m³/h and 93.8 m on 0.55–22 kW motors. Specifications, materials and UK supply.
The Calpeda NM4 is the four-pole close-coupled EN 733 end suction pump for clean water, to 330 m³/h and 36.6 m on 0.37–15 kW motors at 1450 rpm.
The Calpeda NMS is an EN 733 end suction pump on a stub-shaft bracket that takes a standard motor, to 300 m³/h and 95 m on 0.75–75 kW. Specs and UK supply.
The Calpeda NMS4 is the four-pole stub-shaft EN 733 end suction pump for clean water, to 660 m³/h and 61.6 m on 0.75–90 kW standard motors at 1450 rpm.
The Calpeda CT, T and TP are peripheral pumps: 3–165 m of head at 0.06–6 m³/h from one turbine impeller. Clean liquids only. Specs, sizes and UK supply.
The Ebara 3D series is an EN 733 standardised cast iron end suction pump for clean water to 138 m³/h and 70 m. Specs, seal options and UK supply.
The Ebara 3M series is the monobloc build of Ebara's AISI 304 stainless EN 733 end suction pump. Sizes, seals, single-phase options and UK supply.
The Ebara 3S series is the rigid-coupling build of Ebara's AISI 304 stainless EN 733 pump, driven by a standard IEC motor, with an ATEX 3SF version.
Inline vertical multistage centrifugal pump, cast iron and AISI 304 stainless, for non-corrosive liquids at high head.
Vertical multistage pump with an MGE E-motor, so the speed control and the motor protection are inside the terminal box rather than in the panel.
The full stainless build of the CR inline multistage pump, in AISI 316, for industrial liquids and treated water.
Hidrostal Axial Column Pump: 6–1500 l/s at discharge heads up to 20 m, screw centrifugal impeller, for land drainage and flood pumping stations.
Hidrostal Axial Diffuser Pumps in two versions, the Axial Column Pump at 6–1500 l/s and up to 20 m, and the high-head Vertical Turbine Pump.
Hidrostal bearing frame pumps: dry-installed screw centrifugal end suction, 0.5–3000 l/s to 90 m, 32–700 mm, horizontal or vertical. Specs and selection.
Hidrostal compact pumps: portable screw centrifugal field units, 0.8–24.6 l/s to 17.7 m, 50–75 mm free ball passage, from 23 kg. Specs and selection.
How to size and specify a centrifugal pump
Fix the duty point first: flow, and the total dynamic head, which is the static difference plus the losses in your pipework. Then check NPSHa against NPSHr at that point. Suction conditions are where most of these go wrong.
Materials, seal and motor follow from the medium and the temperature. The mistake that lands on our desk most often is margin stacked on margin until the pump is too big for the tank feeding it. Send us the real duty and our engineers return a specified pump.
Frequently asked
What is a centrifugal pump used for?
A centrifugal pump moves thin, clean media at high flow. Water transfer and pressure boosting, circulation in building services, thin process liquids, and screened wastewater where the pump has a solids-handling wet end. It is the default choice on any duty where the medium behaves as water and the head is steady.
What is the difference between a centrifugal pump and a positive displacement pump?
A centrifugal pump adds velocity to the medium, so its flow falls as pressure rises. A positive displacement pump traps a fixed volume and moves it every revolution, so its flow holds to within a few per cent while the pressure changes. The trade rule of thumb for the crossover is around 3–4% solids, and it moves again once the medium stops behaving as water.
Can a centrifugal pump handle viscous liquids?
Badly, and it gets worse quickly. Viscosity drags on the impeller, so head and flow fall while the power draw rises, and the efficiency you sized against has gone. Viscous and shear-sensitive media both want pumping slowly, which a centrifugal cannot do. Specify a rotary lobe or progressing cavity pump instead.
Why does a centrifugal pump cavitate?
Because NPSHa has dropped below the pump's NPSHr at the duty point. The cause is nearly always suction conditions, a suction tank that is too small or too little suction head. An oversized pump makes it worse, because it asks for more flow than the tank can give and pulls in air instead. You get a pitted impeller, a failed mechanical seal, and vibration and noise in the pipework.
Are centrifugal pumps self-priming?
Most are not. A standard end-suction centrifugal has to be flooded or primed before it will pump anything, so a suction lift wants either a self-priming build or a submersible pump. If you have been told a standard centrifugal will lift dry from a sump, it will not.