Growth Stage Fires:
Key Fire Behavior Indicators

The last post in this series, Incipient Fires: Key Fire Behavior Indicators reviewed stages of fire development (i.e., incipient, growth, fully developed, and decay), burning regimes (i.e., fuel and ventilation controlled) and identified key indicators used to recognize incipient stage fires. This post examines key indicators to identify growth stage fires and their burning regime.

Growth Stage & Burning Regime

Like many concepts in fire dynamics there is a bit of ambiguity between where the incipient stage ends and the growth stage begins. For firefighters, this distinction is important as growth stage fires are deemed to present an Immediately Dangerous to Life and Health (IDLH) threat based on the increasing speed of fire development, toxicity and thermal environment. This triggers Occupational Safety and Health Administration (OSHA) respiratory protection regulations requirements for “two-in/two-out”. 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. Recognizing 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.

A Single Compartment

While most buildings have multiple, interconnected rooms, providing a complex environment for fire development, it is useful to begin by examining fire development in a single compartment (see Figure 1)

Figure 1. Fire Development in a Single Compartment.

neutral_plane_burning_regime

Note: Photos adapted from National Institute of Standards and Technology (NIST) ISO-Room/Living Room Flashover [Digital Video Disk].

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, 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 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.

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.

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.

If the compartment is sealed (e.g., door closed and windows intact), the fire may become ventilation controlled, slowing the increase in HRR and temperature, and eventually moving the fire into the decay stage (defined by decreasing HRR). However, if the compartment is not sealed (e.g., open door), the fire may become ventilation controlled, but HRR can continue to increase as smoke flows out of the involved compartment and air from the remainder of the building flows in at floor level, providing the oxygen necessary for continued combustion.

In growth stage fires, fire behavior indicators are often visible from the exterior of the building. However, depending on fire location and building factors (e.g., energy efficiency, ventilation profile) these indicators may be fairly obvious or quite subtle. Growth stage indicators are listed in Figure 2

Figure 2. FBI: Growth Stage

growth_indicators

In Incipient Fires: Key Fire Behavior Indicators you were presented with a residential fire scenario as an opportunity to give some thought to how key fire behavior indicators may present. Consider

Use the B-SAHF model to help you frame your answers.

You have responded to a fire in a one-story single family dwelling of wood frame construction. A growth stage fire is burning a bedroom on the Alpha Bravo corner of the structure. The fire involves a plastic trash can, the bed, and night stand.

  • What conditions would you expect to see from the exterior of the structure?
  • What indicators may be visible from the front door as you make entry?
  • What indicators would you anticipate observing as you traveled through the living room and down the hallway to the bedroom where the fire is located?
  • What conditions would you find in the bedroom?

As the fire moves through the growth stage, the speed at which conditions change increases rapidly. After making entry, consider if conditions are different than you anticipated?

  • Why might this be the case?
  • What differences in conditions would be cause for concern?

Master Your Craft

More to Follow

The next post in this series will continue examination of the relationship between the B-SAHF indicators, fire development, and burning regime by connecting to the parallel series of posts on flashover and examining fully developed fires.

Ed Hartin, MS, EFO, MIFireE, CFO

References

National Institute of Standards and Technology. (2005). ISO-room/living room flashover [digital video disk]. Gaithersburg, MD: Author.

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Key Fire Behavior Indicators”

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