Behavior of Fire
The behavior of fire is affected by many factors. As we already know, the following three elements are present in fires: heat, fuel, and oxygen. The properties of each one affects how the fire burns. Fire spreads naturally following a path of least resistance. It means that the fire will firstly go after materials easiest to burn.
To help understand the behavior of fire, it is important to understand the following concepts:
- thermal balance This refers to the rising movement or pattern of fire, the normal behavior when the pattern is undisturbed.
- thermal imbalance This refers to the abnormal movement of fire due to the interference of foreign matter. This thermal imbalance often confuses investigators as to the exact point of origin of a fire.
Dangerous Behavior of Fire
Fire becomes fatal when the following occurs:
- backdraft
- flashover
- biteback
- flash fire
The sudden and rapid burning of heated gases in a confined area that occurs as an explosion. This generally occurs due to improper ventilation. Highly flammable vapors may accumulate in improperly ventilated rooms that when such a room is opened, oxygen from the outside get violently sucked in which could then result in a combustion explosion.
A video demonstrating backdrafts can be found here: https://www.youtube.com/watch?v=v6CFt74ktBw.
This is the sudden ignition of accumulated radical gases produced when there is incomplete combustion of fuels.
OakRidgeFD | Flashover Demonstration
This is initiated by a spark or flash produced when the temperature rises until flash point is reached. When the accumulated gases suddenly burn, a very intense fire can occur. This fire can create fireballs capable of flying over a hundred yards within seconds.
This occurs when fire resists attempts at extinguishment and becomes stronger instead.
A fire that ignites and spreads very quickly at a high temperature, but only lasts a short while.
Workplace Safety and Health Council | Case Study 2: Flash Fire in Pontoon Tank
It is also sometimes called a dust explosion.
Dan Leuthner | Dust explosion
Stages of Fire
anindustrialsoup | 3 Phases of a Fire
Incipient Phase (Growth Stage)
This is the beginning stage of the fire. The following can be observed when a fire is in this stage:
- normal room temperature
- the temperature at the base of the fire is 400-800°F
- ceiling temperature is about 200°F
- The pyrolysis products are mostly water vapor and carbon dioxide, small quantities of carbon monoxide and maybe sulfides.
Free-Burning Phase (Fully-Developed Stage)
During this stage, materials are burning freely in the presence of adequate oxygen. The following characterize this stage:
- accelerated pyrolysis takes place
- convection currents form
- temperature at base is at 800-1000°F
- pyrolytic decomposition moves upward on the walls leaving burnt patterns (fire fingerprints)
- occurrences of flashovers
Smoldering Phase (Decay Stage)
Fire can be said to have reached this stage when there are no more flames. Instead, dense smoke and heat completely fill a confined room. The following can be observed in this phase:
- heat develops in layers and flames vanish due to oxygen levels dropping to around 13%
- incomplete combustion products increase in volume, especially carbon monoxide
- ceiling temperature is at 1000-1300°F
- heat and pressure builds up
- backdrafts can occur