External gear pumps

An external gear pump has two parallel meshing gears — one driven, one driven by the first — that trap fluid in the cavity between gear teeth and the pump casing, then carry it from suction to discharge. Both gears rotate in opposite directions.

The defining characteristic: flow is positive-displacement, meaning each revolution moves a fixed volume of fluid regardless of discharge pressure (within the pump’s rated envelope). Compare this with a centrifugal pump, where flow falls off rapidly as discharge pressure rises.

When to choose external gear over centrifugal

Condition Choose
Viscosity > 100 cSt Gear
Viscosity > 1.000 cSt Almost certainly gear
Stable flow needed (metering, lube, asphalt loading) Gear
High flow / low head water duty Centrifugal
Shear-sensitive fluid (latex, food gels) Internal gear, never external
Suction lift required Gear (self-priming)

Operating envelope of FB Bombas FBE line

The FBE external gear line covers nominal sizes from 1/8” to 6” with maximum operating parameters per the published catalog:

Performance at any specific duty point should be requested from FB Bombas application engineering — published catalog gives the line envelope, not point-by-point data.

Common applications: asphalt loading, fuel transfer, heavy oils and lubricants, viscous resins, and food-grade applications.

What can go wrong

External gear pumps fail differently than centrifugals. The three classic failure modes:

  1. Dry running — without fluid film, gears chew themselves and the casing in seconds. Always interlock the pump with low-level cutouts on the supply tank.
  2. Particulate ingestion — a single hard particle larger than the gear- to-casing clearance gouges both. Fit a 100 µm strainer on suction.
  3. Pressure relief failure — gear pumps cannot self-throttle. If a downstream valve closes with the pump running, pressure rises until something breaks. Always fit an external relief valve sized at 110-115% of normal discharge pressure.

Further reading

References