Foam Fire Extinguisher (AFFF)

Overview

A foam fire extinguisher is a portable pressure vessel that discharges a blanket of aqueous film-forming foam (AFFF) to extinguish fires in ordinary combustibles and, above all, flammable liquids. It is the standard choice for Class A and Class B hazards such as petrol, oil and solvent fires, where its foam does two jobs at once: it floats a thin aqueous film across the fuel surface to seal vapours, and it forms a cooling, smothering blanket that excludes oxygen. Unlike a dry-powder unit, foam leaves a wet barrier that resists re-ignition, which makes it well suited to fuel stores and vehicle workshops.

The pressure boundary is the lined Cylinder, charged with a pre-mixed Foam Charge of AFFF concentrate and water. The charge is held under gas pressure by the Pressurization System system, whose gauge, a Pressure Sensor, confirms the cylinder is in the green band. Pressure is sealed by the Valve Head, whose spring-loaded stem stays shut until the lever is squeezed, locked meanwhile by a pull pin and tamper seal. On discharge, gas drives solution up the Siphon / Pickup Tube and out through the Discharge Assembly assembly, whose aspirating nozzle whips air into the stream to expand it into foam. The Mounting Bracket keeps the unit ready at the hazard.

How it works

In standby the cylinder holds the foam solution under about 15 bar of stored nitrogen, with the gauge needle in the operating band. To use it the operator pulls the safety pin, breaking the tamper seal, aims the nozzle, and squeezes the lever. Squeezing depresses the valve stem off its seat, and the trapped gas pressure forces the AFFF solution up the internal siphon tube, through the valve and hose, and into the aspirating branch pipe. The branch pipe is a venturi: the fast solution stream draws in atmospheric air and beats it into the liquid, expanding a few litres of solution into a much larger volume of soft foam. Applied gently onto a burning liquid, the foam spreads and a layer of surfactant water films out ahead of it, sealing flammable vapours while the blanket cools and smothers the fire. On Class A fires the foam soaks and cools the fuel. The operator works around the fire until the blanket fully covers the surface, then keeps it intact to prevent burn-back.