The LWF Blog
Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 273
February 9, 2026 10:34 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 272, LWF talked about foam proportioning for foam fire suppression systems. In part 273, we continue to discuss proportioners, and in particular, inductors and bladder tanks.
In order for a line proportioner to work correctly, it is necessary for a pressure drop of around 35% to occur over the unit. In cases where water pressure is low, this can be an issue and result in insufficient pressure at the final discharge outlet. Additionally, the maximum allowable back pressure on the unit is approximately 65%. Back pressure is affected by elevation difference and friction losses between the proportioner and discharge outlet. A high back pressure stops the device from picking up the foam concentrate.
Line proportioners are, in essence, a fixed flow device and the flow/pressure characteristics of the proportioner should be matched with those of the foam discharge device. Neglecting to ensure these are matched is one of the most common reasons for system failure.
Bladder Tanks
A bladder tank works by mixing foam concentrate with water by using incoming water pressure to collapse the internal rubber bladder, which forces the concentrate into a proportioner. The system itself comprises a pressure vessel with a rubber bladder of foam concentrate inside. Water is fed from the foam system inlet water supply under pressure, into the vessel to pressurise the space between the vessel wall and the bladder. The water pressure squeezes the bladder and the foam concentrate is released through a delivery pipe to the proportioner.
The proportioner mixes the water flowing through and foam concentrate at the required ratio.
A bladder tank is used commonly in fixed systems where variable flow is required, water pressure is limited and no power supplies are available to drive the pumps.
It is imperative that care is taken when refilling the system in order to protect the diaphragm inside the foam tank. Breaking the diaphragm would mean the system would be inoperable. Routine care and maintenance of systems using a bladder tank can be more complex than other systems.
Wide range proportioners are available with an operational range from 80 1.min-1 to 5000 1.min-1. These were developed for use in foam enhancement of sprinkler systems.
In part 274 of LWF’s series on fire engineering we will continue looking at the different elements of foam proportioning by discussing balanced pressure proportioning and water-driven foam metering pumps. 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.