STAGES OF FIRE DEVELOPMENT

STAGES OF FIRE DEVELOPMENT

Fire Development – Building Factors, Smoke, Air Track, Heat, and Flame (B-SAHF) are critical fire behavior indicators. Understanding the indicators is important, but more important is the ability to integrate these factors in the process of reading the fire as part of size-up and dynamic risk assessment www.firehouse.com (2014).

The following are the various fire development stages:

1. Incipient Stage

Going back to the basics of fire behaviour, ignition requires heat, fuel, and oxygen. Once combustion begins, development of an incipient fire is largely dependent on the characteristics and configuration of the fuel involved (fuel controlled fire). Air in the compartment provides adequate oxygen to continue fire development. During this initial phase of fire development, radiant heat warms adjacent fuel and continues the process of pyrolysis.

A plume of hot gases and flame rises from the fire and mixes with the cooler air within the room. This transfer of energy begins to increase the overall temperature in the room. As this plume reaches the ceiling, hot gases begin to spread horizontally across the ceiling. Transition beyond the incipient stage is difficult to define in precise terms. However, as flames near the ceiling, the layer of hot gases becomes more clearly defined and increases in volume, the fire has moved beyond its incipient phase and (given adequate oxygen) will continue to grow more quickly.

Depending on the size of the compartment and ventilation profile, there may only be a limited indication (or no indication at all) from the exterior of the building that an incipient stage fire is burning within.

2. Growth Stage

If there is adequate oxygen within the compartment additional fuel will become involved and the heat release rate from the fire will increase. While considerably more complex, gas temperatures within the compartment may be described as existing in two layers: A hot layer extending down from the ceiling and a cooler layer down towards the floor. Convection resulting from plume and ceiling jet along with radiant heat from the fire and hot particulates in the smoke increases the temperature of the compartment linings and other items in the compartment.

As gases within the compartment are heated they expand and when confined by the compartment increase in pressure. Higher pressure in this layer causes it to push down within the compartment and out through openings. The pressure of the cool gas layer is lower, resulting in inward movement of air from outside the compartment. At the point where these two layers meet, as the hot gases exit through an opening, the pressure is neutral. The interface of the hot and cool gas layers at an opening is commonly referred to as the neutral plane.

The fire can continue to grow through flame spread or by ignition of other fuel within the compartment. As flames in the plume reach the ceiling they will bend and begin to extend horizontally. Pyrolysis products and flammable byproducts of incomplete combustion in the hot gas layer will ignite and continue this horizontal extension across the ceiling. As the fire moves further into the growth stage, the dominant heat transfer mechanism within the fire compartment shifts from convection to radiation. Radiant heat transfer increases heat flux (transfer of thermal energy) at floor level.

3. Flashover-Transition to a Fully Developed Fire

Flashover is the sudden transition from a growth stage to fully developed fire. When flashover occurs, there is a rapid transition to a state of total surface involvement of all combustible material within the compartment. Conditions for flashover are defined in a variety of different ways. In general, ceiling temperature in the compartment must reach 500o-600o C (932o-1112o F) or the heat flux (a measure of heat transfer) to the floor of the compartment must reach 15-20 kW/m2 (79.25 Btu/min/ft2)-105.67 Btu/min/ft2). When flashover occurs, burning gases will push out openings in the compartment (such as a door leading to another room) at a substantial velocity.

Recognizing flashover and understanding the mechanisms that cause this extreme fire behaviour phenomenon is important. However, the ability to recognize key indicators and predict the probability of flashover is even more important.

It is important to remember that flashover does not always occur. There must be sufficient fuel and oxygen for the fire to reach flashover. If the initial object that is ignited does not contain sufficient energy (heat of combustion) and does not release it quickly enough (heat release rate), flashover will not occur (e.g., small trash can burn in the middle of a large room).

Likewise, if the fire sufficiently depletes the available oxygen, heat release rate will drop and the fire in the compartment will not reach flashover (e.g., small room with sealed windows and the door closed).

4. Fully Developed Stage

At this post-flashover stage, energy release is at its greatest, but is generally limited by ventilation (more on this in a bit). Unburned gases accumulate at the ceiling level and frequently burn as they leave the compartment, resulting in flames showing from doors or windows. The average gas temperature within a compartment during a fully developed fire ranges from 700o – 1200o C (1292o – 2192o F) Remember that the compartment where the fire started may reach the fully developed stage while other compartments have not yet become involved. Hot gases and flames extending from the involved compartment transfer heat to other fuel packages (e.g., contents, compartment linings, and structural materials) resulting in fire spread. Conditions can vary widely with a fully developed fire in one compartment, a growth stage fire in another, and an incipient fire in yet another. It is important to note that while a fire in an adjacent compartment may be incipient, conditions within the structure are immediately dangerous to life and health.

5. Decay Stage

A compartment fire may enter the decay stage as the available fuel is consumed or due to limited oxygen. As discussed in relation to flashover, a fuel package that does not contain sufficient energy or does not have a sufficient heat release rate to bring a compartment to flashover, will pass through each of the stages of fire development (but may not extend to other fuel packages).

On a larger scale, without intervention an entire structure may reach full involvement and as fuel is consumed move into the decay stage. However, there is another, more problematic way for the fire to move into the decay stage. When the ventilation profile of the compartment or building does not provide sufficient oxygen, the fire may move into the decay stage. Heat release rate decreases as oxygen concentration drops, however, temperature may continue to rise for some time. This presents a significant threat as the involved compartment(s) may contain a high concentration of hot, pyrolized fuel, and flammable gaseous products of combustion.

REFERENCE

Abdullah, J. (2001),”Fire in Tall Buildings: Occupant’s Safety and Owner’s Liability”, Kuala Lumpur : International Law Book Services.

 

Ayuba P, Olagunju, R.E. and Akande O.K (2012).Failure and Collapse of Buildings in  Nigeria: The Role of Professionals and other Participants in the Building Industry.

Interdisciplinary Journal of Contemporary Research in Business.Vol 4, No. 6.

Benjamin, J. A., (1984), “The Challenge of Smoke”, Fire Safety J. vol. 7, 3 – 9.

Bryan, J. L., (1986), “Human Behaviour and Fire”, Fire Protection Handbook, 16th Ed. Quincy, MA :NFPA

Cote, A. E., (1986), “Assessing the Magnitude of the Fire Problem”, Fire Protection Handbook, 16^th Ed., Quincy, MA : NFPA.

 

Craighead, G. (1995), “High-Rise Security and Fire Life Safety”, Boston :Butterworth-Heinemann.