By Alex Langbell
As a training officer, I have to look at what kills firefighters on our most common type fire (the residential fire.) One thing that jumps out at me is flashover. Most of us with any time carrying a bottle of air on our back have witnessed them, so it is a danger that is real. Please take a few minutes to review the phenomenon we call a “flashover.”
A flashover is the near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated, they undergo thermal decomposition and release flammable gases. Flashover occurs when the majority of the exposed surfaces in a space are heated to their auto ignition temperature and emit flammable gases (see also flash point). Flashover normally occurs at 500 °C (930 °F) or 1,100 °F for ordinary combustibles.
A lean flashover or rollover is the ignition of the gas layer under the ceiling, leading to total involvement of the compartment. The fuel/air ratio is at the bottom region of the flammability range.
A rich flashover occurs when the flammable gases are ignited while at the upper region of the flammability range. This can happen in rooms where the fire subsided because of lack of oxygen. The ignition source can be a smoldering object, or the stirring up of embers by the air track. Such an event is known as ‘backdraft’.
A delayed flashover occurs when the colder gray smoke cloud ignites after congregating outside of its room of origin. This results in a volatile situation, and if the ignition occurs at the ideal mixture, the result can be a violent smoke gas explosion. This is referred to as smoke explosion or fire gas ignition depending on the severity of the combustion process.
A hot rich flashover occurs when the hot smoke with flammable gas ratio above the upper limit of flammability range and temperature higher than the ignition temperature leaves the compartment. Upon dilution with air it can spontaneously ignite, and the resultant flame can propagate back into the compartment, resulting in an event similar to a rich flashover. The common definition of this process is known as auto-ignition which is another form of fire gas ignition.
The following are some of the signs that we are looking for when they attempt to determine whether a flashover is likely to occur.
- The fire is in a ventilated compartment, so there is no shortage of oxygen in the room
- The neutral plane is moving down towards the floor. In this situation, a flashover is plausible.
- All directly exposed combustible materials are showing signs of pyrolysis.
- “Rollover” or tongues of fire appear.
- There is a rapid buildup of heat given off by the rapidly burning gases. This is generally the best indication of a flashover
Flashover – Thick dark smoke, high heat, rollover, free burning.
Ventilate the compartment – We all know this. Releasing heat and heated gases out of the structure replacing with cooler air.
Close off the compartment – By closing a door in the room that is experiencing pre-flashover conditions, air cannot enter as readily. This can decrease the rate of burning in the room, delaying the flashover. By closing a door, you are also taking the imminent flashover out of the surrounding area so that other nearby rooms can be searched in a safer manner for a longer time.
Cooling the compartment – By cooling off the compartment by aiming the stream of water into the high heat layer, the gases are cooled. This reduction in temperature slows the process of flashover within the compartment.
Full protective gear — Including hoods and structure gloves
If you are caught in a flashover, stay as low to the floor as possible and use hose streams to cool off compartment at ceiling level and find the nearest exit.
Alex Langbell is a lieutenant with the Yakima (WA) Fire Department.
MORE TRAINING BULLETINS
- Understanding Flashover
- Trapped by Flashover : A Survivor’s Journey
- Flashover Survival
- Training Days: Flashover Chamber
- FLASHOVER RISK MANAGEMENT
- FLASHOVER AND BACKDRAFT: A Primer