The LWF Blog
Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 279
March 23, 2026 9:55 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 278, LWF discussed foam system discharge devices, particularly Rimseal Foam pourers for oil tanks with open top floating roofs. In part 279, we talk about subsurface foam units for fixed roof oil storage tanks.
A fixed roof oil storage tank differs from an open top floating roof oil storage tank in a significant way in terms of fire safety. As a floating roof sits directly on top of the liquid, it means there is no space between the flammable liquid and the roof. A fixed roof oil storage tank has a gap between the surface of the flammable liquid and the roof and this fills with flammable vapours or a vapour/air mixture.
The introduction of an ignition source to a fixed roof oil storage tank – a spark through hot work or a lightning strike, for instance – could result in a vapour explosion, an internal tank fire or an explosion causing the roof to blow off or the tank to have a structural failure.
There are various ways to help mitigate the risk including nitrogen blanketing, pressure valves or vapour recovery systems.
A foam fire suppression system for use in a fixed roof oil storage tank would be designed to be subsurface. Sometimes, a subsurface foam unit is referred to as base injection, due to the foam being forced directly into the fuel body, either via a product line or at a point near the base of the tank (but above any water base layer that may be present). The foam then passes through the fuel to form a vapour-tight blanket over the entire surface.
This type of foam fire suppression system is typically used on large hydrocarbon tanks such as crude oil, diesel or kerosene.
The system works by activating when fire is detected on the fuel surface. The foam solution is pumped through the piping to the tank and enters at the subsurface injection point at the bottom of the flammable liquid layer. The foam rises, due to it having lower density and once it reaches the surface, it spreads along the burning fuel. The foam blanket it creates smothers flames, prevents vapour release and cools the fuel surface.
In part 280 of LWF’s series on fire engineering we will discuss foam water sprinklers. 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.