FB Bombas Engineering Team7 min read
Gear pumps displace liquid each revolution; centrifugal pumps transfer energy through an impeller, with duty set by pump and system curves. Assess viscosity, flow, pressure, temperature, solids and suction. FBE and FBEI are gear-pump ranges; FBCN is a process centrifugal range and FBOT serves thermal oil. Compare configuration-specific data without a universal viscosity cutoff between technologies.
At a glance
- FBE and FBEI displace liquid each revolution; actual flow depends on slip and inlet conditions.
- FBCN centrifugal pump is the choice for light fluids and high flows, reaching up to 2,200 m³/h and 135 m head.
- There is no universal viscosity cutoff: compare performance and power for the same duty.
- FBOT serves thermal-oil circulation; the catalogue publishes up to 350 °C, subject to configuration.
- Confirm selection against supply curves and conditions; range maxima are not a simultaneous duty point.
Selection combines fluid, duty point, suction and configuration limits.
1. The fundamental difference
Gear pumps are positive displacement. Each rotation transfers a fixed volume of fluid. Flow is proportional to speed and practically independent of pressure. They are ideal for viscous fluids because volumetric efficiency increases with viscosity.
Centrifugal pumps transfer kinetic energy to the fluid through an impeller. Flow depends on the pressure differential and viscosity. With viscous fluids, efficiency drops sharply. They are ideal for high flows of light fluids.
Engineering note
Viscosity affects performance and power but does not decide technology alone. Compare the corrected centrifugal curve with gear-pump actual flow, power and inlet data for the same duty.
2. Side-by-side comparison — data from technical manuals
The table gives published range-level references. Maxima do not form a simultaneous duty point. Casing pressure, differential pressure and head must be distinguished, and each configuration needs confirmation against its curve, material and temperature.
| Parameter | FBE (Gear) | FBCN (Centrifugal) |
|---|---|---|
| Type | Positive displacement | Kinetic (impeller) |
| Max viscosity | Up to 100,000 SSU | Low-viscosity fluids |
| Max flow | Manual MTEC-01/01 (p.1): up to 390 m³/h, 1/8″ to 6″; the 2026 FBE catalogue (p.6) also lists the FBE 10″; larger models made to order | Up to 2,200 m³/h |
| Pressure — confirm for configuration | Catalogue: up to 22 kgf/cm²; validate inlet and differential | Per material and pressure–temperature curve |
| Head | Up to 220 m | Up to 135 m |
| Max temperature | 2026 FBE catalogue (p.3): 10 to 500 °C; confirm configuration | -50 °C to 260 °C |
| Speed | Up to 1,750 rpm (direct) | Up to 3,500 rpm |
| Sizes | MTEC-01/01: 1/8″–6″; current catalogue also includes 10″ | DN 25 to 300 mm |
| Construction | Helical gears | Back-pull-out, radial impeller |
| Maintenance | Front access to seal/packing | Remove bearing without disconnecting piping |
3. When to use a gear pump (FBE/FBEI)
A gear pump is the correct technical choice when the process involves:
- Viscous liquids: assess FBE/FBEI using actual viscosity, flow, pressure and suction data.
- Heated duty: confirm materials, sealing, heating and configuration limits; temperature alone does not select technology.
- Flow proportional to speed (dosing, metering, controlled transfer)
- Fluids that solidify at room temperature (CAP asphalt, paraffins) — CA variant with heating jacket
- Constant flow is needed (a feature the 2026 FBEI catalogue, p.3, lists for the FBEI internal gear pump)
4. When to use a centrifugal pump (FBCN/FBOT)
A centrifugal pump is the correct technical choice when the process involves:
- Low-viscosity liquids: compare the centrifugal curve with demand and suction conditions.
- High flow rates — the FBCN handles up to 2,200 m³/h; the FBE manual line goes up to 390 m³/h, with larger models made to order
- High speeds (3,500 rpm) with direct drive from standard electric motor
- Simplified maintenance is a priority — back-pull-out design allows removing the rotating assembly without disconnecting piping
- Thermal-oil circulation: assess FBOT, whose catalogue publishes up to 350 °C for the range, subject to configuration limits.
5. When both technologies can be considered
When both suit the fluid and required duty, compare these requirements before deciding:
- Energy: compare power at the same duty point, with the same fluid and operating schedule.
- Flow control: define stability, adjustment range and required accuracy; positive displacement is not a measurement certification.
- Solids: provide concentration, size, hardness and abrasiveness; compatibility depends on construction.
- Temperature: check the entire assembly, startup and continuous duty against configuration documents.
Key point
To compare gear and centrifugal pumps, send FB Bombas the liquid, viscosity with temperature, flow and circuit conditions. Confirm configuration and included documentation in the technical proposal.
6. What the curve doesn't show: deadhead, priming and start-up
Closing centrifugal-pump discharge reduces flow to shut-off; energy still enters the liquid and can cause rapid heating. Zero flow must not be treated as normal operation. With positive displacement, blocked discharge raises pressure and requires relief protection sized for the installation.
Startup preparation depends on construction and liquid. Confirm priming, lubrication and sealing in the manual. A conventional centrifugal pump must be primed, but placement above or below the tank depends on installation and suction conditions; self-priming does not authorise any pump to run dry.
Warning
Never operate a positive displacement pump against a closed valve without a sized relief valve — integral to the pump or installed on the discharge line. It is the number one protection requirement in any gear pump installation.
7. Viscosity and energy: the invisible cost of the wrong choice
Putting a centrifugal pump on viscous fluid is not forbidden — it is expensive. The ANSI/HI 9.6.7 correction method (Hydraulic Institute) quantifies the effect: as viscosity rises, correction factors reduce delivered flow and head and pull efficiency down relative to the water-tested curve.
The practical result is a pump that draws more power to deliver less, every day, for its whole service life — a cost that never shows on the purchase proposal but always shows on the energy bill.
In gear pumps, higher viscosity can reduce slip while increasing friction and inlet requirements. Poorly lubricating liquids require compatible construction; do not rule out every rotary pump for water or solvents, or approve FBE/FBEI without checking its specific manual limits.
8. Five selection mistakes engineering sees repeatedly
Check these five points when reviewing the specification:
- Specifying by the 40 °C catalog viscosity instead of the real pumping temperature — an oil that flows easily in the heated line can be 10× more viscous at cold start
- Oversizing the centrifugal "to be safe" — the pump runs far from BEP, with internal recirculation, high radial load and shortened seal and bearing life
- Applying a gear pump to a fluid with no lubricity (water, light solvents) — high slip and premature bushing wear
- Omitting the relief valve in a positive displacement installation — the first line blockage becomes unplanned downtime
- Ignoring available NPSH with hot or volatile fluid — the margin left with cold water vanishes near the vaporization point
9. Summary: data for comparing technologies
Organise the data before requesting selection:
- Fluid and viscosity at minimum, duty and maximum temperatures.
- Operating flows and required adjustment range.
- Inlet and outlet pressures and calculated installation losses.
- Thermal oil: brand, fluid datasheet and circuit for FBOT assessment.
- Viscous products: rheology and heating requirements for FBE/FBEI assessment.
- Process liquids: composition and solids for FBCN assessment.
- Accuracy, pulsation and materials: requirements to confirm in the proposal.