What foam concentrate is — and what changed with the end of AFFF
Foam concentrate is the liquid that, proportioned into water (typically at 1%, 3% or 6%), forms the foam that smothers flammable liquid fires — fuel tanks, dikes, refineries, hangars. For decades the default was AFFF, a fluorinated concentrate with near-Newtonian behavior: thin, predictable, easy to pump.
The global phase-out of PFAS compounds changed the game: the fluorine-free (SFFF) and alcohol-resistant AR-AFFF concentrates replacing it are shear-thinning — at rest, viscosity climbs to thousands of centipoise, and the fluid only "thins" under shear. Pumping systems sized for legacy AFFF often cannot even prime with the new concentrate. Requalifying the pumping became a mandatory part of the migration.
Why the gear pump dominates concentrate duty
Physics decides. A centrifugal pump generates flow through kinetic energy: with viscous fluid, efficiency collapses and the curve "melts" — with SFFF at rest, many centrifugals cannot even prime. Piston pumps tolerate more viscosity but hit a practical ceiling in the low thousands of centipoise. A gear pump displaces volume trapped between teeth and casing: flow is proportional to speed and nearly indifferent to viscosity — the same principle that made it the standard for asphalt and heavy oils.
In the fire protection industry, tests with gear pump proportioners report stable operation with concentrates around 8,000 cP — territory where piston and centrifugal pumps have already dropped out. For the specifier, the reading is direct: modern concentrate calls for rotary positive displacement, and the gear pump is the configuration with the most field track record.
| Pump type | Legacy AFFF (low viscosity) | AR-AFFF / SFFF (high viscosity, non-Newtonian) |
|---|---|---|
| Centrifugal | Viable | Efficiency collapses; may fail to prime |
| Piston | Viable | Practical ceiling ~4,000–4,500 cP |
| Gear (positive displacement) | Viable | Indicated — stable operation reported at ~8,000 cP |
What NFPA 11 expects from concentrate pumping
NFPA 11 is the standard for low-, medium- and high-expansion foam systems: it defines concentrate types, application rates, proportioning methods and system tests. For pumping, the principle is compatibility: the concentrate pump must suit the actual fluid — type, viscosity range at operating temperatures, and wetted materials compatible with the LGE surfactants and glycols.
One point the specifier cannot sidestep: in fixed proportioning systems, the listed equipment requirement (UL/FM) frequently appears in insurer and authority-having-jurisdiction specifications — and listing belongs to the specific equipment, not the pump type. Concentrate transfer, however — unloading trucks, refilling LGE tanks, moving between reservoirs — is industrial pump duty sized for the fluid, where application engineering rules: that is exactly where a nationally manufactured gear pump comes in with rational lead time and cost.
How to specify the foam concentrate pump in practice
Start with the concentrate technical bulletin: the LGE manufacturer datasheet gives the viscosity-versus-temperature curve — the number that sizes everything. From it, define: required transfer flow, circuit head, minimum operating temperature (maximum viscosity) and wetted materials. Elastomers and metallurgy must be validated against the concentrate: LGE surfactants and solvents attack common rubbers, and cast iron in prolonged contact with water-based concentrate calls for corrosion assessment.
From there, selection follows viscous-service gear pump engineering: reduced speed for high viscosity (protecting suction), available NPSH checked at minimum temperature, discharge pressure relief (positive displacement against a blocked line builds pressure up to the driver limit) and compatible sealing. FB Bombas selects the FBE for the specified concentrate from the LGE manufacturer bulletin — the same application method used for decades on viscous fluids such as asphalt and heavy oils, with documented factory curve and testing.
Where the LGE pump fits in the complete fire system
The foam system does not replace the water system — it adds to it. Hydraulic demand is still met by the NFPA 20 set: main, standby and jockey pumps pressurizing the network; the foam circuit adds the LGE tank, the proportioner and, depending on the architecture, the concentrate pump. In plants with tank farms — refineries, terminals, mills — the pair shows up complete: water set sized by demand and LGE circuit sized by the foam application rate.
For the buyer, the practical consequence is good: a single national manufacturer can answer for both sides of the system — the NFPA 20 / NBR 16704 water pump set and the concentrate circuit gear pump — with integrated application engineering, documented FAT and local support.