“Rapid fire spread” is an umbrella term for a number of relatively unusual events — including backdraughts and flashovers — where normal assumptions about life safety, fire spread and containment times do not apply. Alan Field considers whether this is an issue with which only firefighters need be concerned, or if it is one that should be considered as part of a fire risk assessment process.

The overview

Building design, along with fire detection and suppression systems, is typically geared to the kind of fire situations that may arise, which will depend on the structure and intended usage of the premises. The resources devoted to these systems will depend on the risk and severity of potential outcomes as well as legal requirements. Fire evacuation procedures are similarly designed. Deciding whether to consider any extraordinary scenarios is a complex matter that should be based on professional advice. However, it is helpful to be aware of the relevant issues and the questions that need to be asked.

This article focuses on situations where assumptions about speed and severity of fire spread are compromised. This, in turn, can compromise fire alarm and fire evacuation strategies It is especially pertinent where fire alarms are staged, ie building occupants are alerted to a fire in stages, and the evacuation is phased. These tactics are based on how long it is estimated a fire will be contained within its compartment, ie where the building has defined areas or sectors of physical fire resistance or containment to prevent both fire and smoke progressing to other parts of the building.

Understanding the risk

Before considering rapid fire progress, a fire risk assessment should look at other sources of explosion. For example, the location of maintenance chemicals and cylinders, eg propane gas, should always be known, their storage should be strictly minimised and their movement controlled so that, in the event of fire, the risk of explosion is minimised. Equally, the process for isolating gas supplies coming into a building needs to be fully understood and tested; liquefied natural gas, for example, is highly explosive and can seriously compromise any building structure.

Another consideration that should be discussed with fire risk assessment professionals is whether adjoining properties could compromise assumptions about fire spread. The risk of lateral fire spread can be an issue in heritage areas, eg town centres with medieval buildings and some older industrial estates. In both cases, the construction standards and physical separation between units may not meet modern fire safety standards.

The impact of a fire or explosion on neighbouring properties should never be discounted so it might be beneficial to speak to neighbouring building managers. If there are major industrial installations in the area then some may be subject to COMAH (the Control of Major Accident Hazards Regulations 1999, as amended). Those responsible for premises should take note of information about potential off-site emergencies provided by their neighbours. They must be mindful of what could impact on their premises, even if the possibility of a major accident seems remote.

What causes rapid fire progress?

“Rapid fire progress” is a term covering at least five different scientific phenomena. However, in practical terms, what matters is to understand the likelihood of any one them arising and how it will impact on outcomes. These can be summarised under two generic headings: backdraughts and flashovers.

Backdrafts

Backdraughts (or backdrafts) are, essentially, smoke explosions. The toxic gases produced by the fire, chiefly carbon monoxide, become part of the fuel of the fire and this then creates an explosion. The sudden, violent gas ignition will produce shock waves and, depending on the quantity of gas involved, could cause structural damage, accelerate fire spread and compromise fire-resisting compartments. If there are phased evacuation plans for a building, these can then have serious consequences because part of a fire-resisting compartment’s role is to prevent the spread of smoke. It is smoke — with its extreme temperature and the toxic gases within it — that typically kills people in fires. The fire itself rarely kills other than in specific circumstances; rapid fire progress can be one of them.

Flashover

Flashover is a generic term covering a number of scenarios. The International Standard ISO 13943 defines it in terms of a stage of fire transition where the total surface involvement of all combustible materials takes place. In other words, all combustible materials within a compartment ignite simultaneously. The fire will then start to move rapidly. This sudden and total conflagration within a compartment will create a serious and potentially uncontainable fire.

This explains the basic fire protection advice not to open a door if there may be a fire behind it. Once a source of oxygen is suddenly available to the fire, then rapid progress can occur with potentially fatal consequences.

Controls and mitigation

The article has focused on compartments. However, in most circumstances the fire has to take hold before any risk of rapid fire spread can arise and, in turn, before there is any risk of that fire-resisting compartment becoming compromised. In other words, early detection and fast, effective fire suppression are key to minimising the risk of any fire spread — rapid or otherwise. This is particularly true where there is a risk of a fire starting while the premises are unoccupied, or on a complex site where fire could remain undetected for a period of time. Examples of detection measures include heat, smoke and/or carbon monoxide detectors connected to fire-alarm systems; fire suppression systems would include sprinklers.

The other strategy that can be used to prevent rapid fire spread is effective ventilation. For example, since rapid fire spread can create life threatening risks to firefighters, the fire compartment could be ventilated to release smoke and reduce the temperature. This might be by simply cutting a hole in a roof space or using fans to blow into the compartment. This tactic requires a skilled practitioner as ventilation can also increase the oxygen supply to the blaze and, in some circumstances, create a rapid fire spread.

A building can be designed with built-in smoke and heat exhaust ventilation systems (SHEVs). SHEVs are often to be found in modern shopping centres and can protect evacuation routes as well as venting away smoke and heat from a fire. Another example of smoke management would be a pressurised staircase in a high-rise building. Building managers, of course, need to fully understand how these smoke management systems operate. This includes knowing whether there are any limitations or exceptions to their use. The matter should be discussed with the fire risk assessment professionals along with other life safety advisers for the building concerned.

Conclusion

The likelihood of rapid fire progress should always be considered even though, depending on the design and usage of the premises concerned, the risk may be very low. However, this should be clarified on a building-by-building basis, using advice from fire safety professionals.

Last reviewed 26 February 2013