The LWF Blog

Fire Engineering Design and Risk Assessment – Foam System Discharge Devices continued – Part 7

June 14, 2018 10:23 am

In LWF’s fire engineering blog series for architects and others in the building design industry, we have been discussing methods of fire suppression. In part 6, some of the types of foam system discharge devices were outlined and in part 7, that is continued before moving on to look at Foam system designs.


A foam inlet system involves a breeching on the outside of a building being used by the Fire Service to pump foam into foam spreaders inside the building. The receiving area of hazard will be small, for instance, a flammable oil storage area.


A medium-expansion foam generator is typically a mesh-screened outlet with a foam solution nozzle which is mounted axially in an air inlet. When the solution is sprayed onto the mesh, the air blows it into bubbles into the hazard area. The generators are typically mounted above and beside the particular hazards they are designed to protect.


High-expansion foam generators are similar in design to a medium-expansion foam generator, except it may have more than one spray nozzle in the air inlet. Some units may also have a fan driven by a foam solution motor or electric motor. These generators are mounted above the level that the foam is needed to reach and draw fresh air to provide aspiration of the solution.


Foam System Designs


A foam system design is one which must deliver foam at or above a minimum density in mm/min or litre/min per square metre for a minimum duration of time. The parameters vary according to the hazard in question, the discharge device and the foam concentrate in use. Both British Standards and NFPA standards give specifics for use and the application must be undertaken by an experienced engineer of foam systems.


Such systems use the Assumed Maximum Area of Operation (AMAO) as the basis for design, while the duration of discharge is dependent upon the type of system and hazard in question.


Sprinkler, deluge and other spill fire hazards will require a 10-minute supply of foam in order to be effective. ‘Fuel in-depth’ hazards, such as tanks, need a longer discharge time which is likely to be in the region of 30 to 65 minutes and should be judged depending on the volatility of the fuels in use.


In part 8 of this blog series, LWF will look at the necessary components and materials for foam systems and the necessary testing to check it can function correctly. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.


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.


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