The LWF Blog

Fire Engineering for Healthcare Premises – Fire Development – Part 11

December 2, 2020 11:32 am

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 10 of Fire Engineering for Healthcare Premises, LWF looked at the elements considered in calculating fire growth. In part 11, we discuss terms such as vitiated fires, flashover and backdraft.

A vitiated fire is one which is ventilation-controlled. The amount of oxygen needed for complete combustion depends on the stoichiometric ratio of the fuel. Stoichiometric relates to the quantity of reactant in a substance. In simple terms, this means that the amount of oxygen needed in the atmosphere for complete combustion depends upon the level of fuel reactant.

The specification of the correct heat release (and the rate of release) is crucial to the fire safety design and is dependent upon the availability of air, as well as the properties of the fuel.

Flashover is a natural and inevitable part of fire growth in a compartment where there is sufficient fuel and ventilation. Flashover cannot be prevented by design features, only by limiting the potential for fuel or by using a suppression system to slow the rate of fire growth.

The potential for flashover can be delayed by increasing the distance between potential sources of fuel and by limiting the supply of air to the compartment. Limiting the air to the compartment may cause a vitiated fire and conditions that could lead to a backdraught.

A backdraught is a potentially very dangerous situation for a firefighter to encounter. Where a part of a healthcare building is well-sealed, i.e. the area is a basement or doesn’t have windows or sources of natural ventilation, a backdraught can develop due to the lack of oxygen available to the fire.

The sudden influx of oxygen caused by the compartment being opened, for (most-commonly) a firefighter to enter, causes the fire to explode. Ventilation techniques used by firefighters such as positive pressure ventilation (PPV) can reduce the risk of a backdraught situation.

Suppression techniques such as an offensive fog attack can also be effective. Instead of the fire hose being set to a jet of water, it is set to fog and this is particularly useful in a closed compartment.

A fully developed compartment fire has the most significance for the impact of fire on the structure of the building and its tenability conditions.

In Part 12 of LWF’s blog series, LWF will look at the effects of suppression, compartment fire modelling and cone calorimetry. 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 LWF on freephone 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.