Centrifugal and positive displacement pumps compared: curves vs flat flow, end suction, split case and multistage pumps, gear, progressing cavity, diaphragm and peristaltic pumps, and a duty-by-duty selection table.
Pumps fall into two families. Centrifugal (dynamic) pumps spin an impeller to add velocity, then turn it into pressure, so their flow changes with the head they work against. Positive displacement pumps trap a fixed volume and push it out, so their flow stays almost constant whatever the pressure. Centrifugal pumps suit large flows of thin liquids; positive displacement pumps suit viscous fluids, high pressures and dosing. Compare centrifugal pump suppliers in the UAE with the positive displacement makers listed below.
This guide is for plant engineers, maintenance teams and buyers who need to know which pump type fits a duty before asking for quotes. Our guide to water pumps in the UAE and Saudi Arabia covers water duties (boosting, boreholes, dewatering) and how to read a pump curve; here we compare the pump types themselves, including the ones water guides skip: gear, screw, progressive cavity, diaphragm and peristaltic pumps.
The first split is how the pump moves liquid.
NFPA's overview of fire pumps makes the same split for fire duty: centrifugal pumps are the most common because they handle large volumes at high pressure, while positive displacement fire pumps give very high pressure at limited flow and are used mainly for water mist and foam-water systems.
This is the difference that drives selection and control.
| Behaviour | Centrifugal | Positive displacement |
|---|---|---|
| Flow vs pressure | Flow falls as head rises, along the pump curve; the operating point is where the pump curve meets the system curve | Theoretically the same flow at a given speed whatever the discharge pressure; slight internal leakage rises with pressure |
| Closed discharge valve | Can run briefly against a closed valve | Must not run against a closed valve; pressure keeps rising until something fails, so a relief valve is needed |
| Flow control | Throttling valve or variable speed | Speed (flow is proportional to speed) or bypass; never by throttling |
| Viscous liquids | Efficiency falls as viscosity rises | Rotary types handle highly viscous fluids well |
| Priming | Needs liquid in the casing; gas in the casing stops pumping unless it is a self-priming design | Many types self-prime (diaphragm pumps have good suction lift) |
| Typical duties | Water, sewage, agriculture, petroleum and petrochemical pumping | Oils, fuels, bitumen, sludge, slurries, chemicals dosing, hydraulics, high pressure |
For a buyer, the practical rule is: if the duty is a large flow of a thin liquid, start with a centrifugal pump; if the liquid is thick, the pressure very high, the flow must be measured in, or the fluid is shear-sensitive, start with positive displacement.
Within the centrifugal family, impellers range from radial through mixed flow to axial designs, as the photo shows, and the maker matches the shape to the duty. The casing and arrangement then define the pump type:
Two meshing gears (external) or a gear running inside a ring gear (internal, often with a crescent seal) carry liquid around the casing; tight clearances stop it leaking back. Flow equals volume per revolution times speed. The rigid design allows very high pressures and highly viscous fluids, and gear pumps are among the most common pumps for hydraulic fluid power. Typical liquids include bitumen, pitch, diesel, crude and lube oil, and in food plants chocolate, vegetable fats and molasses. External precision gear pumps are usually limited to around 210 bar. Browse gear pump suppliers.
A helical rotor turns inside a stator, forming sealed cavities that travel along the pump. Flow is proportional to speed, in either direction, and shear is low, so they suit viscous and shear-sensitive materials, sewage sludge and oil transfer. Slurries are possible if the carrier liquid is viscous enough to protect the stator. Abrasives significantly shorten stator life, and running dry destroys the stator quickly, so specify dry-run protection. Browse progressive cavity pump suppliers.
A rubber, thermoplastic or PTFE diaphragm moves back and forth between check valves. Diaphragm pumps self-prime, have good suction lift, tolerate dry running, and can be designed for abrasive sludges and slurries with high grit and solids content; they handle low to medium viscosity. A wide choice of diaphragm elastomers makes them compatible with very corrosive solutions, and air-operated double diaphragm (AODD) versions typically meet the safety requirements for flammable or explosive atmospheres because no electric motor sits at the pump. Hydraulically driven versions reach discharge pressures up to 1,200 bar. Browse diaphragm pump suppliers.
Rollers squeeze a flexible tube against a curved housing, pushing liquid along; the tube recovers and draws more in behind. The liquid touches only the inside of the tube, so there are no seals or valves in contact with it and cleaning is simple. They handle slurries, viscous, shear-sensitive and aggressive fluids, and uses include chemical dosing, food and beverage dispensing, and concrete and sewage sludge. The trade-offs: the tube wears and needs periodic replacement, flow pulses (especially at low speed), viscosity limits performance and pressure depends on tube strength. Browse peristaltic pump suppliers.
Screw, lobe and vane pumps are further rotary positive displacement types, and rotary pumps as a class handle highly viscous fluids well. Reciprocating piston and plunger pumps are typically used for thick materials such as concrete and heavy oils and for very high pressures. Like all positive displacement pumps, they need overpressure protection on the discharge.
| Duty | Usual first choice | Why |
|---|---|---|
| Clean water, large flow, moderate head | End suction or split case centrifugal | Efficient at high flow; simple control |
| Clean water, high pressure | Multistage centrifugal | Impellers in series build pressure |
| Thick oils, fuels, bitumen, food fats | Gear, screw or lobe pump | Rotary PD handles high viscosity |
| Sludge, viscous or shear-sensitive product | Progressing cavity pump | Low shear, steady flow proportional to speed |
| Slurries, corrosive chemicals, hazardous areas, portable transfer | Air-operated double diaphragm pump | Self-priming, dry-run tolerant, elastomer choice, air drive |
| Chemical dosing, sterile or abrasive liquids | Peristaltic pump | Liquid touches only the tube; flow set by speed |
| Sewage and wastewater with solids | Submersible centrifugal sewage pump | Large free passage impellers; see our sump and sewage pump guide |
| Hydraulic power units | Gear pump | High pressure, compact |
Three questions settle most borderline cases. First, how viscous is the liquid at the coldest operating temperature? Viscous liquids reduce centrifugal pump efficiency. Second, what solids does it carry, and how abrasive are they? Abrasives significantly shorten progressing cavity stator life. Third, does the flow need to be accurate? Positive displacement pumps whose flow is proportional to speed (gear, progressing cavity, peristaltic) can be used for dosing with a speed controller.
Where leakage is unacceptable (acids, solvents, toxic liquids), sealless designs remove the shaft seal. Finish Thompson, for example, makes sealless magnetic drive centrifugal pumps, including self-priming versions, alongside mechanically sealed centrifugal pumps, drum pumps and AODD pumps. For any pump, state the liquid, concentration and temperature in the RFQ and ask the supplier to name the material of every wetted part and the seal or diaphragm elastomer.
Pump type does not change the regulatory route, but the motor does. Surface pump motors fall under the UAE motor efficiency regulation and Saudi Arabia's SASO 2893 IE3 rule, both summarised in the main guide; air-driven AODD pumps avoid the motor question but need compressed air. Imports into Saudi Arabia go through SABER; see our guide to SASO and SABER certificates for industrial equipment. Browse centrifugal pump suppliers in Saudi Arabia and positive displacement pump suppliers in Saudi Arabia.
Finish Thompson lists sealless magnetic drive centrifugal pumps (plastic and ANSI), mechanically sealed centrifugal pumps, vertical pumps, self-priming mag drive pumps, drum and barrel pumps and air-operated double diaphragm pumps under its FTI Air line. It suits chemical transfer and plants handling corrosive liquids.
EDUR, based in Kiel, Germany, makes multistage centrifugal pumps, peripheral pumps and multiphase pumps that can transport liquid-gas mixtures.
Aurora, founded in Aurora, Illinois, in 1919 and owned by Pentair since 1997, lists horizontal split case, vertical turbine and end suction pumps, including fire pump versions.
When the duty is defined, post a pump requirement on ListGCC and let suppliers propose the type with the data behind it.
More in this series: Fire Pumps and Jockey Pumps.
Centrifugal (dynamic) pumps and positive displacement pumps. Centrifugal pumps use a spinning impeller to add velocity and pressure, and their flow changes with head. Positive displacement pumps trap and push a fixed volume each revolution or stroke, so flow stays nearly constant whatever the pressure. Positive displacement pumps divide further into rotary and reciprocating types.
Use one when the liquid is viscous (oils, bitumen, sludge), when you need very high pressure at modest flow, when the flow must be accurate for dosing, when the product is shear-sensitive, or when the pump must self-prime and tolerate dry running. For large flows of thin liquids such as water, a centrifugal pump is usually more efficient and simpler.
Because it keeps displacing the same volume whatever the pressure. If the discharge is closed, pressure rises until a pipe bursts or the pump is damaged. A relief valve or bypass protects the system. A centrifugal pump, by contrast, can run briefly against a closed valve because its head is limited by its curve.
Diaphragm pumps, especially air-operated double diaphragm (AODD) types, transfer slurries, sludges, corrosive chemicals and liquids in hazardous areas. They self-prime, lift well, tolerate dry running and can use different diaphragm materials for chemical compatibility. Air-driven versions typically meet the requirements for flammable or explosive atmospheres. They suit low to medium viscosity liquids.
A gear pump is a positive displacement pump: meshing gears carry a fixed volume per revolution, giving steady flow at high pressure and handling thick liquids such as oils and bitumen. A centrifugal pump uses an impeller, gives high flow of thin liquids, and its flow falls as pressure rises. Gear pumps need relief protection; centrifugal pumps do not in the same way.
Only up to a point. Viscous liquids reduce a centrifugal pump's efficiency, so the curve published for water no longer applies. Ask the maker for a viscosity-corrected selection, and for thick liquids compare a rotary positive displacement pump such as a gear, lobe or progressing cavity pump.
Featured image: Photo: S.J. de Waard, CC BY 2.5, via Wikimedia Commons
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