The LWF Blog

Fire Safety Engineering for Design – Fire Growth – Part 87

June 27, 2022 11:38 am

LWF’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 86, LWF looked at sprinklers and smoke detector sensitivity. In part 87, we will discuss the effect of sprinklers on fire size, before considering smoke plumes.

If a real fire test were carried out, which is a rarity, and it showed the fire was suppressed quickly when the sprinkler system activated in a given circumstance, then it would seem fair to surmise that the combustion of the fire ceased at the point the sprinklers became operational.

In a design test scenario, the fire will grow until the rising heat of the plume activates the sprinkler heads. The effect of the sprinklers on the design fire size can be taken into account if it is assumed that the activation of the sprinkler means the fire stops growing. The design fire size is estimated based on the size it had grown to before the sprinklers operated, unless there is additional information that might point to the fire growth continuing.

The sprinklers cool the majority of the smoke layer to below 100 °C which means that flashover will not occur when sprinkler systems are operational. The fire should then have a constant rate of heat release.

An alternative assumption is one where after the sprinkler operates, the heat output of the fire will decrease slowly. In other circumstances, the fire may continue to grow but at a slower rate.

It can be assumed the fire will continue to burn until all the available fuel is consumed.

Smoke Plumes

In order to undertake a smoke ventilation calculation, some information is required.

• The plume type. Examples might be an axisymmetric plume or a plume flow from an opening. The entrainment equations associated with different plume types calculate the mass flow of smoke produced.
• The temperature of the hot gases.
• The volume flow rate of the smoke.

By undertaking smoke ventilation calculations, it is possible to determine the effectiveness or performance of a smoke ventilation system.

The proportion of the total heat release rate in the plume will vary depending upon elements such as the fuel type and the compartment characteristics, in terms of flow out of an opening. In a design scenario, the convective portion can be assumed to be 66% of the total heat output of the fire.

In part 88 of LWF’s series on fire engineering, we will look at smoke entrainment and the types of plume. 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.