Fires can easily be compared to a living being. They all have a lifecycle that they follow, and they want to continue living, sometimes seemingly with desperation. If you talk with firefighters, they will tell you that fires have personalities too—some mean and aggressive, while others lazy and submissive. And by “fire” we mean a fully developed fire.

In fire-speak, the lifecycle of a fire is described in “stages,” identified and explained below. But from a fire suppression standpoint, it is always preferable to attack a fire in the earliest stage—it could be said while it is still in its infancy. Safety is vastly improved; it requires less agent to extinguish; fire damage is greatly limited; and costly displacement or downtime is reduced.

A full-grown adult fire is a totally different story. Once a fire has fully developed, it gains much more of an authoritative characteristic and is infinitely more dangerous and difficult to extinguish.

Most fires do not burn in a linear fashion, but rather they generally follow a bell-shaped growth pattern. When fires occur in an enclosed area, the growth curve is steeper and better defined because the enclosed environment radiates so much heat to the other fuel surfaces that they can ignite without flames touching them.

Fires in enclosed environments are known as compartment fires. They are a different beast from fires occurring in the outside environment. The confinement acts to seal in great quantities of heat and flammable vapors which speeds up the growth process and adds greater intensity.

All fires begin with some initiating event. While not viewed as a “stage” of fire, per se, this initiating event is referred to as ignition. Something happened that brought all three sides of the fire triangle together and a fire started.

The first stage of fire is called the incipient stage. This is shortly after ignition and the chemical reaction is well underway. The fire is small, confined to the area of ignition, and is not having much or any effect on the temperature in the compartment.

As the fire enters the second stage—the growth stage—it is beginning to spread to adjacent fuels. The larger fire is heating up the room quickly and a layer of smoke and fire gases called the thermal layer is accumulating at the ceiling. As the growth stage continues, the ceiling temperature approaches 1000°F. The super-heated gas layer radiates the heat to every surface in the room, and all of the combustibles ignite at once in an event called a flashover. This series of actions can happen in under three minutes.

After flashover, the fire quickly enters its fully developed stage and will vigorously burn until something causes it to decay. It takes considerably more effort and resources to fight a fully developed fire and also exposes those fighting the fire to substantially more risk.

The National Institute of Standards and Technology (NIST) carries out extensive research on fire behavior in an effort to reduce the number of firefighters killed each year. In doing so, they have compiled numerous videos of fire behavior in a compartment. To fully appreciate how fast a fire can move through the stages to a fully developed fire, watch this short video taken at the NIST testing laboratories.

Next in the fire’s lifecycle is the decay stage. A fully developed fire enters the decay stage for one of three reasons:

  1. It consumes all of the fuel
  2. It consumes oxygen to a level below 16%
  3. Someone begins extinguishing it

Although “fire decay” sounds like a good thing, it actually can create an extremely dangerous situation if it occurs for reason #2 above, i.e., oxygen consumption. If a fire burns in a void space–such as the area above a drop ceiling where there is not good airflow—the fire may consume the oxygen in the space until flaming combustion ceases. However, this smoldering fire continues to produce heat and flammable vapors. All it needs to complete the fire triangle is oxygen. If oxygen is suddenly introduced, a backdraft can occur. Backdrafts are also called “smoke explosions” because literally all of the flammable gases in the smoke ignite at once. As a result of it being confined, it creates an explosion. Here is a video of this exact thing happening to a crew of firefighters at a house fire. Incredibly, no one was hurt.

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Recognizing the stages of fire is important. If you are not an equipped firefighter, you should never attempt to directly attack any fire that has grown beyond the incipient stage. Fire protection efforts should always focus on detecting and extinguishing the fire while it is small and before it gets deep seated. Progression of a fire into the growth and fully developed stages may overwhelm fixed systems and require a multi-unit response from the local fire department or plant fire brigade. Early detection coupled with an “always-on-duty” fire suppression system is the best, surest, and safest response to most fire scenarios.

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