The LWF Blog
Fire Safety Engineering for Design – Fire Growth – Part 85
June 13, 2022 10:56 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 84, LWF discussed fire growth rates for pre-flashover fires. In part 85, we will continue to discuss fire growth, but for post-flashover fires, before beginning to look at the effect of sprinklers on a fire.
When designing fire protection for a building, it can be assumed that the smoke layer at ceiling level must reach 600 °C in order for flashover to occur. In the plume above a fire, the temperature at the tip of intermittent flames is about 350 °C, compared to around 550 °C at the tip of sustained flames.
When a sprinkler system is installed as a part of a fire-engineered solution, it can be assumed when it is operating correctly that it will trigger at a temperature well below that required for flashover and will suppress the fire from that point. Other fire suppression systems will have a similar result.
Instances of flashover have been researched extensively, due to the serious consequences of the event. Details of the different correlations are available in the SFPE Handbook, describing the event in terms of equations to allow the calculation of factors.
It is also possible to calculate effective values via equation for ventilation controlled fires and fuel bed controlled fires.
Sprinkler Systems
When considering the effect of sprinkler systems on fire growth, various factors must be taken into account. The naturally buoyant plume of hot smoky gases from a fire rises to hit the ceiling, where it spreads horizontally along the surface area. It is commonly at this point that the pre-activation sprinkler detectors (where utilised) are activated and the sprinkler heads become operational. The time taken for this stage to be reached can vary, depending upon the fire growth rate, the location of sprinkler detectors and the sensitivity of detection.
As the plume of smoke and gases from a fire rises, it draws in cooler air from the surrounding area, causing the overall temperature of the plume to drop. In essence, this means that the further the plume must travel to be detected by the sprinkler system, the cooler the temperature of smoke being detected. High ceiling heights mean cooler smoke plumes. The smoke layer will continue to cool as it spreads across the ceiling. The hotter the smoke reaching the detectors, the more quickly they will operate.
In part 86 of LWF’s series on fire engineering, we will continue discussing sprinkler systems and their effect on fire growth. 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 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.