The LWF Blog

Fire Safety for Facilities Management Personnel – Fire Development – Part 63

September 14, 2020 1:24 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 62, LWF looked at fire classification, and discussed the development and spread of fire. In part 63, we continue looking at how fire develops inside buildings.

Heat is transferred from the source of a fire through conduction, convection and radiation to surrounding areas and fuels and so, fire is able to spread from its original source. Conduction can occur through poorly insulating elements of construction, such as metal fire shutters.

The most commonly dangerous and therefore, for our purposes, the most important means of fire spreading are convection and radiation. After a fire in a room or space is ignited, hot gases rise in a plume. The plume of hot gases includes air, so the volume of smoke and gases increases with height. When the smoke reaches a barrier – the ceiling in a room – it spreads out in all directions and forms a deepening layer of hot smoke and gases. Obviously, if this were to collect in an escape route, the loss of visibility would make people reluctant to proceed in that direction and be a significant threat to life.

The temperature of the smoke decreases as it rises and at some point, it will become ambient. At this point, the smoke will cease rising and will level out, looking as if it has met an invisible barrier. This is known as stratification and in some circumstances, has been known to delay the operation of fire detectors or sprinkler heads which are commonly installed at ceiling height. In time, the fire will continue to grow and the heat output will increase, causing the smoke and hot gases to reach the detectors at ceiling height, but the fire itself will be larger by that time, than if stratification had not occurred.

When the fire has grown sufficiently for the flames to reach the ceiling, they are deflected horizontally and radiate downwards over a large area of the enclosure. The thermal radiation will increase in strength until flames spread rapidly over combustible surfaces and items in the area may spontaneously burst into flames.

In a space with a low ceiling, such as an office, this stage may be achieved very quickly. It is known as flashover. After flashover, all possible sources of fuel – therefore all items in the room – will be on fire and the potential for survival of any occupants is nil. Flashover usually occurs when temperature of the layer at ceiling height reaches 550 – 600 ºC.

In part 64, LWF will look at how the progress of a fire in a building can be divided into three distinct phases. 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.