The LWF Blog
Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 284
April 27, 2026 9:12 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 283, LWF talked about foam inlet (semi-fixed) systems. In part 284, we discuss medium-expansion foam generators.
A medium-expansion foam generator is a fire suppression device designed to mix foam solution with air to create voluminous foam. Medium expansion refers to the ratio at which the liquid foam expands when aerated – between 20 and 200 times its original volume. The resulting foam is able to cover surfaces quickly to suppress the fire.
The foam solution and water are pumped into the generator, which contains a nozzle mounted along the air inlet axis. The nozzle sprays the solution as a fine mist. As a part of this process, airflow is induced. At the outlet, there is a mesh screen with air blowing through it. As the solution passes through the mesh screen, bubbles are formed and the liquid is turned into foam.
The medium-expansion foam generator is normally a fixed position device and is placed either above or alongside the hazard it is employed to protect. When placed above, the foam falls into and blankets the area. When it is placed alongside, the foam flows across the surface of the fuel.
The resultant foam is thick and stable with good surface coverage. It resists heat and breakdown and suppresses flammable vapours from escaping. This method also works to prevent re-ignition of the fire.
The system relies on gravity and flow and is not suitable for jetting any distance, unlike low-expansion foam from a branch pipe.
The generators are available in various different sizes to allow protection of different hazard areas. For an especially large hazard area, multiple generators may be used. Common uses include bund protection, flammable liquid pool fires, vehicle fires and small rooms such as pump rooms, generator rooms and fuel storage areas.
In part 285 of LWF’s series on fire engineering we will look at high-expansion foam generators, how they work and the uses to which they are put. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.