The LWF Blog
Fire Safety Engineering for Design – Sprinkler Protection – Part 221
February 10, 2025 11:44 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 220, LWF discussed sprinklers and drenchers used for windows and external edifices. In part 221, we talk about the extinguishing mechanism of sprinkler systems, which could be summarised as ‘wetting, cooling and inerting’.
When a building’s fire safety design includes a sprinkler system, it’s important to think at the design stage about how one of the main functions of a sprinkler will be applied. The pre-wetting of combustible materials is one of the main tenets of sprinkler fire suppression and the building’s layout and arrangements should consider avoiding obstruction to the sprinkler’s spray pattern and discharge.
The sprinkler system is designed with two other main mechanisms in mind – cooling and ‘inerting’.
Cooling of the fuel source on fire will reduce the rate of heat release and cooling will also take place in the flame, which reduces the concentration of free radicals. A proportion of the energy of the fire will be dissipated in heating the water droplets.
In the case of standard sprinklers which have fairly large droplets, the inerting aspect of sprinkler action is fairly minor, but still plays a part. The production of steam assists with the displacement of oxygen from the flame zone.
When you consider what a fire needs to grow – fuel, oxygen and heat – the simplicity of the sprinkler system in working to negate those conditions is remarkable.
When looking to determine appropriate discharge densities, water has a theoretical cooling capability of 2.6 MW.litre-1 per second which can be used, along with an appropriate safety factor, when the heat release rate of the design fire has been assessed.
Various reputable sources (Babrauskas and Grayson 1992, NFPA 2008, Yii 2000) have presented probable values for fire load in offices of between 224 and circa 1000 MJ.m-2.
It’s obvious from the variation in data range that consideration must be given to the density of water discharge required to control a fire outbreak if a fire engineered solution is to be adopted. A simple but safe solution would be to design to the highest figures, which would allow maximum flexibility of future use in the protected building, but a more conservative and measured approach may result in a better use of capital funds, without the guarantee of future flexibility.
It’s important to be able to illustrate to any insurer that the fire engineered approach decided upon meets and exceeds the prescriptive codes that would otherwise have to be used, if premiums are not to increase.
In part 222 of LWF’s series on fire engineering we will look at the rules and standards for sprinkler installations. 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.