1. What sizing a gear pump actually means
Sizing a gear pump means finding the combination of volumetric displacement, speed and motor that delivers the flow the process requires against the pressure the circuit imposes — with the real fluid, at the real operating viscosity and temperature. The calculation order matters: first the fluid sets the limits (allowable speed, maximum pressure), then flow sets the size, and finally pressure sets the power.
This walkthrough follows the rotary positive displacement pump terminology of ANSI/HI 3.1-3.5, from the Hydraulic Institute, and the application data of the FBE series technical manual. It complements — not replaces — selection by viscosity: here the focus is the calculation; there, the model tables by viscosity range.
| Process data | Typical unit | What it defines in the calculation |
|---|---|---|
| Fluid and viscosity at operating temperature | SSU or cSt | Allowable speed, drive type, friction power |
| Flow required by the process | L/min or m³/h | Volumetric displacement (pump size) |
| Discharge pressure + line friction loss | bar or kgf/cm² | Differential pressure Δp and shaft power |
| Suction condition (lift, length, temperature) | m, mca | Available NPSH and cavitation risk |
| Duty cycle (continuous/intermittent) | — | Power reserve and motor class |
2. Step 1 — Flow: from volumetric displacement to actual flow
A gear pump displaces a fixed volume per revolution — the volumetric displacement Vg, expressed in cm³/rev in the datasheet. Theoretical flow is therefore proportional to speed: double the speed, double the flow. This is the fundamental difference from a centrifugal pump, where flow and pressure are coupled on the same curve.
Actual flow is lower than theoretical because of internal slip: fluid backflow through the clearances between gears, casing and covers, from discharge back to suction. Slip grows with differential pressure and decreases with viscosity — with viscous fluids, the clearances seal themselves. The ratio of actual to theoretical flow is the volumetric efficiency ηv, and each model’s value at each condition comes from the manufacturer performance curve.
Q_teorica = (Vg × n) / 1000
Q_real = Q_teorica × ηvTheoretical and actual flow (Q in L/min; Vg in cm³/rev; n in rpm)
3. Step 2 — Differential pressure: what the circuit actually imposes
A gear pump does not "have" pressure: it delivers flow against the pressure the system imposes. Differential pressure Δp is discharge pressure minus suction pressure — and the dominant share, in viscous fluid lines, is usually pipe friction loss, which grows linearly with viscosity in laminar flow. Calculating friction loss at the wrong viscosity (for example, with the fluid cold instead of at pumping temperature) is the error that most often oversizes motors.
With Δp calculated, check it against the model’s maximum allowable pressure at the operating viscosity — in the FBE series, this limit decreases as viscosity rises, and the ranges are tabulated in the technical manual. Every positive displacement pump installation also requires a relief valve (internal, or external in the line): against a blocked discharge, pressure rises until the drive limit or the rupture of the weakest component.
Δp = p_descarga − p_succao
p_descarga = p_destino + Δp_atrito(linha, viscosidade) + Δp_estatico(elevacao)Differential pressure from the circuit
4. Step 3 — Power: hydraulic, shaft and motor reserve
Hydraulic power is the product of flow × differential pressure. In everyday units — Q in L/min and Δp in bar — the conversion constant is 600: P(kW) = Q × Δp ÷ 600. That is the power delivered to the fluid; shaft power is higher, because part of the work is lost to internal friction (viscous and mechanical) and slip. The ratio between the two is the overall efficiency η.
With viscous fluids, the viscous friction share grows and dominates: the same pump, at the same flow and pressure point, demands more power with asphalt than with diesel. That is why the final power does not come from a generic formula — it comes from the model performance curve at the operating viscosity, and the FBE manual tabulates the recommended power reserve over the calculated value, larger for small motors (which have less thermal margin) and smaller for large ones.
P_hidraulica (kW) = (Q × Δp) / 600
P_eixo = P_hidraulica / η
T (N·m) = (Vg × Δp) / (20 × π × η_mec)Hydraulic power, shaft power and torque (Q in L/min; Δp in bar; Vg in cm³/rev)
5. Worked numerical example: lube oil at 60 L/min and 8 bar
Process: transfer lube oil at 40 °C (viscosity in the 1,000 SSU range), required flow of 60 L/min, line differential pressure calculated at 8 bar. Per the FBE manual viscosity × speed table, 1,000 SSU allows direct drive at 1150 rpm.
Adopting, for the example, a volumetric efficiency of 0.90 at this condition (the real value comes from the model curve): the required theoretical flow is 60 ÷ 0.90 = 66.7 L/min. The minimum volumetric displacement is Vg = 66.7 × 1000 ÷ 1150 = 58 cm³/rev — the chosen model is the catalog one with Vg immediately above.
Hydraulic power is 60 × 8 ÷ 600 = 0.80 kW; with an overall efficiency of 0.60 in the example, shaft power is 1.33 kW. With the manual’s reserve for motors in this range, the next commercial motor size closes the selection — subject to confirmation with the model’s certified curve.
- Speed from viscosity: 1,000 SSU → 1150 rpm (direct drive, 6 poles)
- Theoretical flow: 60 ÷ 0.90 = 66.7 L/min → minimum Vg = 58 cm³/rev
- Power: P_hyd = 0.80 kW → P_shaft = 1.33 kW → next commercial motor size, with manual reserve
- Final checks: model maximum pressure at the viscosity, available NPSH at suction, relief valve
6. The four errors that most often compromise sizing
The field rework FB Bombas engineering most often encounters originates in four calculation decisions — all avoidable at the sizing stage.
- Datasheet viscosity instead of operating viscosity: the cold fluid at start-up can be several times more viscous than at steady-state temperature — start-up power and friction loss must be checked at the worst condition, not the nominal one.
- Speed above the allowable for the viscosity: catalog flow at the wrong speed causes suction cavitation and accelerated wear of bushings and gears.
- Motor without reserve over the calculated power: viscosity variations, pressure variations and start-up itself consume the margin — the reserve tabulated in the manual is not conservatism, it is real transient behavior.
- Suction treated as a detail: a long, narrow inlet line with a viscous fluid destroys available NPSH — suction sizing is as decisive as discharge sizing.
7. Checklist: the 7 data points that close the sizing
With the seven data points below, FB Bombas engineering sizes the pump, defines speed and drive and returns the model with its certified performance curve — free of charge and with no obligation. Send them via WhatsApp or the quote form.
- Pumped fluid (trade name and concentration, if a solution)
- Viscosity at operating temperature (SSU or cSt)
- Working temperature — minimum and maximum
- Flow required by the process (L/min or m³/h)
- Discharge pressure — or line layout (diameter, length, elevation, fittings)
- Suction condition: lift or flooded suction, line length
- Duty cycle: continuous or intermittent, starts per day