Fire breaks out when heat, fuel, and oxygen meet. Without intervention, a fire will only come to an end under two conditions: when the fuel has run out, or when the oxygen supply is exhausted. In an enclosed space, when a fire can deplete most oxygen, the flames will die down while the fuel continues to burn in a smouldering state under pyrolysis. Pyrolysis does not require oxygen, so it can take place without fire at or above 500C and turn organic compounds into charcoal, tar, or non-condensable gases. These gases have extremely high thermal values ready to burst into fireballs as soon as fresh oxygen re-emerges from any new opening. This is called "backdraft", and is what happened in the Mountain B Pub fire.
Mountain B was an enclosed building with a lot of flammable material, including the ceiling and wall insulation. The building was officially permitted for use as a restaurant. However, it was operated as a pub with live music. Post-fire investigation by the Building Inspectors Association found that aside from the flammable ceiling and sound-absorbing ovary sponge walls, flammable materials were added to fill the gaps in the walls to prevent the sound escaping.
It is believed that a short circuit started the initial blaze, which spread rapidly due to the massive amounts of flammable material. The oxygen in the enclosed space was quickly drawn down to a low level, and the fire almost faded out over time. Unburnt fuel was then turned into gases by pyrolysis. The roof of Mountain B was not heat-resistant and most likely collapsed, which allowed fresh oxygen to ignite the trapped non-condensable gases. This caused a backdraft, sending exploding fireballs rapidly and violently from the roof of the building accompanied by brown and black mushroom smoke from the incomplete combustion. This backdraft should not have occurred if a smoke diffusion control system had been installed to extract smoke from the enclosure during the fire in compliance with regulations. The law also requires that the electrical supply be uninterrupted by the fire for at least one hour.
This is a classic case of a building that failed to comply with the standard fire safety protocol in all phases:
Fire Prevention Phase: In the case of an entertainment service building in which it is difficult to control the heat source, building regulations require that "all interior decoration and soundproof walls must not use flammable materials". In addition, the venue must be equipped with an electrical safety cutoff system in the event of a short circuit.
Professor Worsak Kanok-Nukulchai is a Fellow of the Royal Society of Thailand and Executive Director, Chulalongkorn School of Integrated Innovation and Senior Advisor, Building Inspectors Association.
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