The LWF Blog

Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 272

February 2, 2026 9:39 am

LWF’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 271, talked about fluorine-free foams and C6 foams. In part 272, we will discuss foam proportioning for foam fire suppression systems.

As a part of a foam fire suppression system, a proportioner exists in order to dispense the concentrate into the water supply, forming a foam solution. The efficacy of the foam solution relies on the correct proportion of concentrate to water ratio, usually described as a percentage.

The proportioner must be accurate in order to function as intended and ensure that good-quality foam is produced by the system as a whole.

A wrongly proportioned mixture can lead to a solution where the concentrate is at too low a percentage and in this case, the foam solution will be too weak and the resulting bubbles may not be stable. Conversely, if the concentrate percentage is too high, the resulting foam mixture will be too stiff and may not be able to flow across the fuel surface to extinguish the fire. The majority of foam concentrate is mixed at 1% or 3%, although some foams can be proportioned at 6%.

Different types of proportioner are produced for varying applications. Firefighters need a proportioner that is mobile and a fire protection engineer would source a type for fixed foam fire suppression systems.

Inductors

Inductors are also known as line proportioners. This type of proportioner adds foam concentrate into the water line by means of the Venturi action – a principle where a fluid’s velocity increases as it flows through a constricted section of a pipe, causing a drop in pressure, which creates a partial vacuum that can “suck” other fluids or objects in.

In practical terms, water at high pressure is fed into the inductor inlet, passes through a nozzle into a small chamber inside the device. Water enters the device with velocity and pressure and as it enters the nozzle, the velocity increases considerably and so the pressure must drop. A foam concentrate container is connected to the induction chamber which allows the foam liquid to be ‘driven’ into the device by atmospheric pressure.

In part 273 of LWF’s series on fire engineering we will continue talking about proportioners and, in particular, inductors. 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.