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Fire Safety for Facilities Management Personnel – Putting out fires – Part 60

August 24, 2020 1:15 pm

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 59, LWF discussed the combustion of flammable gases. In part 60, we look at how fire can be extinguished.

When considering the ways of extinguishing a fire, it is useful to recall how fire is started and continues to burn.

There must be a source of heat, the fuel must be heated and release vapours, the vapours are ignited and flames appear as a gas-phase reaction between fuel vapour and oxygen. Once alight, the ignited vapours release heat, which continues to heat the fuel, vapours are released and ignited and so the cycle continues.

Extinguishing a fire is simply breaking the cycle. Most commonly, water is used to put out solid material fires and it achieves the aim by cooling the fuel which ensures vapour release is drastically reduced and combustion cannot continue.

A foam applied to a burning liquid forms a physical barrier, preventing the release of vapour into the combustion zone above the surface. In addition, the barrier works to reduce radiation from the flame onto the fuel surface. Water draining from the foam can also assist in cooling the surface of the hot liquid.

Carbon dioxide and other inert gases used on a fire act in a simple but effective way. The use of inert gases reduces the percentage of oxygen in the atmosphere sufficiently to stop the combustion process, by directly removing one of the three necessary components of fire. In order for this method to be effective, concentrations must be in the region of 25 to 30 per cent.

Halons were a particularly effective fire-extinguishing agent before their use was drastically restricted due to links with ozone depletion. The substance is still used in some very limited and controlled environments, such as the Channel Tunnel. Halon can extinguish fires at low concentrations, typically only a few per cent concentration, because they extinguish a fire through chemical interaction. The halocarbons which have replaced halons extinguish fire in a more complex manner and is a more physical effect, with only minor actions on the chemical reactions in the flames.

Dry Powders can be used to extinguish fires and, while complex, it is likely the action is mainly attributable to chemical inhibition.
In part 61, LWF will consider how fires are classified in order to look at the suitability of extinguishing agents. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

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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