Stages in Fire Behaviour II

 

Reference: Adapted from CFBT–US (Ed Hartin) and Village Handbook (2005)

Incipient or Beginning Stage

Going back to the basics of fire behavior, 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 increase in volume, the fire has moved beyond its incipient phase and (given adequate oxygen) will continue to grow more quickly (growth stage).

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. Incipient stage indicators are listed below.

Incipient Stage indicators

  Fuel Controlled Burning Regime Ventilation Controlled Burning Regime
Building Size, contents, ventilation profile, and fire protection systems all have a significant influence on potential fire.
Construction, configuration and compartmentation will also influence SAHF indicators.

In typical size compartments, there is generally sufficient air to support incipient stage combustion simply based on compartment size.

Incipient stage fires are generally fuel controlled.

Smoke Limited volume, light colour, limited thickness and buoyancy. Smoke may or may not be visible from the exterior. Lack of a clearly defined hot gas layer in the interior.  
Air Track Air track indicators may be limited to slight smoke discharge from openings in the compartment of origin. Air track indicators may or may not be visible from the exterior.  
Heat Temperature near ambient (even within the compartment of origin).Some condensation may be visible on windows in the fire compartment, particularly as the fire nears the growth stage.  
Flame Combustion is limited to objects near the point of origin. Flame heights are limited (lower than ceiling height).  

Growth or Flaming Stage

In this stage, the fire is producing enough heat to be self–sustaining and will continue to grow so long as fuel and air are available. Key characteristics of a growth stage fire include increasing heat release rate (HRR), significantly increasing temperature within the compartment.

The speed of fire development in the growth stage may be limited by fuel characteristics and configuration or ventilation. Typically compartment fires in the early growth stage are fuel controlled. However, if the compartment is small and/or has limited ventilation, continued combustion will result in slowing fire development as the fire enters the ventilation controlled burning regime. Recognising the ventilation controlled burning regime is critical as increases in ventilation will result in increased HRR. This is not necessarily a major problem unless it is unanticipated or firefighters do not have the capacity to control this additional HRR.

As a compartment fire develops, hot products of combustion and entrained air rise in a plume from the burning fuel package. When the plume reaches the ceiling, hot gases begin to move horizontally (rolling), forming a ceiling jet. As the fire progresses through the incipient stage and into growth, additional fuel will become involved and the heat release rate from the fire will increase. While thermal conditions can be considerably more complex, gas temperatures within the compartment may be described as existing in two layers: A hot layer extending down from the ceiling (over–pressure layer) 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.

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.

As gases within the compartment are heated they expand and when confined by the compartment increase in pressure. Higher pressure in this layer (hence the term over–pressure 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.

Growth Stage indicators

  Fuel Controlled Burning Regime Ventilation Controlled Burning Regime
Building Fuel controlled conditions are more likely if the building is open (e.g. windows and doors).

Ventilation controlled conditions are more likely if the building is closed (e.g. windows and doors).

Smoke Smoke is likely to be visible from the exterior. Inside the building, a well defined upper hot gas layer is likely with smoke spreading to adjacent compartments. Smoke may be visible from the exterior but may be limited due to the buildings normal ventilation profile. Inside the building, upper hot gas layer will be well defined and deepening with smoke spreading to adjacent compartments
Air Track Air track is dependent on the ventilation profile. If openings are at the same level, a bi-directional air track will be observed and velocity of smoke discharge will be increased. Velocity will be greatest closer to the fire. If there is significant ventilation openings, a strong, bi-directional air track is likely with a lowering of the neutral plane. Velocity of smoke discharge will increase but more slowly than if the fire is fuel controlled. Velocity will be greatest closer to the fire.
Heat Temperature in the fire compartment will be significantly above ambient and rising quickly. Condensation disappears from windows in the area of the fire and cracking of window glazing close to the fire is likely. Temperature in the fire compartment will be significantly above ambient and rising. Brown staining (condensed pyrolised product) is likely on windows and window glazing close to the fire.
Flame Flames may be visible from the exterior. Flaming combustion has extended beyond the object or area of origin. Flames reaching the ceiling are bending and travelling horizontally through the upper hot gas layer. Flame indicators are similar to those in the fuel controlled burning regime. However, flaming combustion progresses more slowly and isolated flames may be observed in the upper hot gas layer

 

 

At this 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 700 °C – 1200 °C.

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.

Fully Developed Stage indicators

  Fuel Controlled Burning Regime Ventilation Controlled Burning Regime
Building Fuel developed compartment fires are generally ventilation controlled.

HRR is largely influenced by the ventilation profile. Increases in ventilation will result in increased HRR.

Smoke   Smoke will darken to darker grey, brown or black. Smoke colour is influenced by both what is burning and ventilation. Height of the hot gas layer is dependent on ventilation, but is likely to drop close to floor level.
Air Track   Air track is dependent on the ventilation profile. However a fully developed fire will generally produce a strong, well defined air track. Velocity of smoke and air movement will be high and smoke discharge will generally be turbulent.
Heat   Temperature is at its highest. Intact windows are likely to be blackened and cracked. High temperatures will be evident inside the structure, even a considerable distance from the involved compartment.
Flame   Flames will be extending from compartment openings. Flames observed from the exterior indicate the extent of involvement. Flaming combustion extending out of the compartment may be masked by smoke.

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. While temperature follows heat release rate, the temperature in decay stage fire may remain high for some time (particularly in well insulated, energy efficient buildings). This presents a significant threat as solid fuel packages continue to pyrolise and the involved compartment(s) may contain a high concentration of hot, pyrolised fuel, and flammable gaseous products of incomplete combustion.

Decay Stage indicators

  Fuel Controlled Burning Regime Ventilation Controlled Burning Regime
Building Fires that enter decay based on fuel limitations often involve contents with limited mass, low heat of combustion and /or low heat release rate.

High fire load and fuels with a high heat of combustion increase the potential for vent induced flashover or backdraft.

Smoke Limited volume, light colour, limited thickness and buoyancy. Smoke may or may not be visible from the exterior. Lack of a clearly defined hot gas layer in the interior. Inefficient combustion results in darkening smoke colour and increased thickness with the appearance of texture like velvet. Colour alone is not a reliable indicator.
Air Track May be limited to slight smoke discharge from openings in the compartment of origin. Air track indicators may not be visible from the exterior. Smoke discharge may be turbulent in the early stages and less so as temperature decreases (this is really important). A puffing / pulsing (in and out) of smoke from small openings (i.e. under door) is a strong indicator of potential for vent induced flashover or backdraft.
Heat Temperature near ambient Initially temperature is high, but will drop following decrease in HRR.
Flame Combustion is limited to objects near the point of origin. Limited flaming and surface combustion (smouldering). Flame combustion is reduced. However, early in this stage, flaming combustion can still occur despite low oxygen.

Be Wary

Decay stage indicators can sometimes be subtle and conditions may not look too bad (maybe like an incipient or early growth stage fire if you are not paying close attention and consider the possibilities). It is often assumed (incorrectly) that ventilation induced extreme fire behavior (flashover or backdraft) will occur immediately after an increase in ventilation. Depending on fire conditions and building configuration there may be a significant time lag between ventilation and resulting changes in fire behavior. When ventilation controlled decay conditions are indicated (or suspected), firefighters should move cautiously and take action to change conditions inside the building or compartment (e.g., gas cooling, ventilation).