Flashover

 

 

 

By: Eddie Crombie

 

From the first days in academy we begin to learn about fire behavior, specifically flashover.  We are taught that flashover is an event in every fire when everything combustible in a room ignites instantly creating a non-survivable environment for both victims and rescue personnel.  Unfortunately, this is an extremely simplistic view creates a false sense of security.  Try this exercise.  Write the warning signs of flashover you were taught in academy.  I am sure your answers look something like this:

Heat that drives you to the floor.

Seeing surrounding items “off-gas” around you.

The phenomena of fire “rolling over” through the hot upper gas layer.

 

These are all correct answers; however, there is a problem.  If you are in a position to see these signs then you have likely already over extended yourself and are in a position to be seriously injured or killed.  We must have a better understanding of fire behavior.  When we observe the signs of flashover we must understand and predict what is and what will occur.  This situational awareness allows us to orient to the fire ground and make an informed decision to effectively mitigate the situation.

 

Flashover is a step during the development of a fire when the fire transitions through the growth stage and into the fully-developed stage.  This happens instantly igniting any exposed surface within the compartment, driving temperatures as high as 2000ºF and depleting oxygen to as low as 4%.  After this change the fire continues to sustain combustion, spreading fire through the entire structure.  The best textbook definition can be found in NFPA 921: Guide for Explosion and Fire Investigations:

 

A transitional phase in the development of a compartment fire in which surfaces   exposed to thermal radiation reach ignition temperature more or less   simultaneously and fire spreads rapidly throughout the space resulting in full         room involvement or total involvement of the compartment or enclosed area.

To take this understanding a step further we must realize that a flashover can either be fuel or ventilation controlled.  Both end with a fully involved compartment, but start with different initiating events.

 

A fuel-controlled flashover is what we learn about in academy and gives us the traditional development curve seen below. (Fig. 1)

There is an adequate ventilation profile, enough fuel to drive the room to flashover and additional fuel to sustain combustion after flashover takes place.  The easiest way to understand this type of flashover is to think of a running faucet into bathtub with the drain open. The bathtub is the compartment involved with fire and the running water is the amount of heat radiating off the combusting fuel.  As the tub fills with water some of the water is sent down the drain (much like items absorbing radiant heat), however the flow of water is greater than the rate that it is draining.  Eventually the tub can no longer contain the water and its overflows (flashover).  This progress of a fuel controlled flashover is very predictable.  The fire is able to naturally grow and transition into a fully developed fire.  Unfortunately, this is not a perfect world and this type of flashover is rarely seen in modern constructed buildings.

A ventilation controlled flashover is quickly becoming more common.  Much like a fuel controlled environment there is an adequate amount of fuel to drive the fire.  Unlike a fuel controlled flashover there is an insufficient ventilation profile.  This creates a much different development that places us in a very dangerous situation. (Fig. 2)

As the fire grows, the rate of heat being released and the speed of development is initially controlled by the amount of fuel available.  As this development progresses, the fire consumes more oxygen.  In a ventilation controlled fire, the growth is dictated by the ventilation profile.  Eventually growth of the fire exceeds the amount of oxygen available in the compartment.  The fire then transitions into a decay stage and becomes a ventilation controlled fire.  At this point, a very volatile environment exists.  When a fire becomes starved of oxygen, any change in the ventilation profile will directly affect the conditions within the structure.  When ventilation increases, the fire is given the oxygen it needs to drive the compartment to flashover.  This goes contrary to the accepted notation that ventilation improves conditions by allowing the superheated gases to escape.  Rather an uncoordinated ventilation operation often leads to a rapid increase in the Heat Release Rate and eventual flashover of a compartment.

Perhaps the most common change in ventilation occurs when we open the door and enter the structure.  Underwriters Laboratory (UL) conducted a study entitled Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction.  Performing a series of experiments, UL burned various types of buildings to see how fire behavior has changed over time.  Although so many valuable nuggets of information were discovered through this study, one piece gives us a benchmark to avoid being caught in a ventilation controlled flashover.  (Fig. 3)

 

The chart above shows the conditions present five feet from the floor after the front door is forced in a contemporary residential home.  If you look closely conditions deteriorated and flashover took place on average within about 2 minutes after the door is opened and entry is made.  This time frame puts us well into the structure and in a position to get seriously injured or killed.  A more constructive way to look at this data is we may only have 90 seconds to get water on the fire and begin to cool the atmosphere.  This benchmark shows why we must be proficient with our initial line placement and advancement.

Knowing our enemy is only half the battle.  We need to observe our situation accurately to effectively predict an impending extreme fire event.  Before we commit to the interior attack we need to read our exterior situation.  Smoke gives us this ability.  The first condition we need to identify is what type of fire we are dealing with, ventilated or unventilated.  An unventilated fire will show varied smoke conditions, but will have little to no flame since the fire is oxygen starved.

Next, we need to determine what stage the fire is in.  Remember, a container that is hot and rich in fuel, but lacks adequate ventilation a prime candidate for a ventilation induced flashover.  After we force entry we need to give the fire a moment to react and watch for the following signs prior to entry.

 

-An increasing velocity of dark smoke through a common opening.  This shows the fire is oxygen started and rising in temperature.

 

-Any sudden changes in smoke color, velocity, or density.  When a fire is unstable conditions will change quickly and without warning.

 

-The pulsing of smoke is an indication of an inadequate air track as the fire “breathes”, starving for oxygen.

 

-Vent point ignition is a phenomena when fuel rich smoke exits the structure and auto ignites as it mixes with the outside air.  Again showing the volatile conditions hiding within.

 

-A rapid lowering of the smoke layer shows the room is reaching flashover conditions.  This is best witnesses looking through the doorway of the involved compartment.

 

Once inside the structure our senses are impaired and it becomes much more difficult to accurately read conditions.  Furthermore, the protective clothing we wear is designed to be used as a defensive tool, not an offensive.  The interior conditions change so rapidly in today’s fires you can easily over extend yourself and may not have time to react if you experience any of these signs.

 

-A sudden change in heat that indicates the unstableness of the environment.

 

-Flames visible overhead is the carbon monoxide igniting.  Carbon monoxide has            an autoignition temperature of 1128ºF which shows the ceiling is reaching flashover temperatures.

 

-Brief bursts of flames (as opposed to sustained flaming) show that low oxygen levels are present.

 

-Objects visibly off gassing (pyrolysizing) in the room.  These objects are   moments from igniting.

 

If we are able recognize these signs a decision to act can be taken to mitigate the situation.  Ultimately the superheated condition needs to be cooled.  The safest way to do this is with a coordinated attack that includes proper ventilation in combination with effective hoseline use, such as the technique of “penciling” where short blasts from a smooth bore hoseline are directed at the ceiling, as you advance to the seat of the fire.  This can effectively cool the superheated gas layer at the ceiling a give the attack company more time to safely operate.  It should be noted that efforts aimed at delaying flashover will ultimately fail if the attack team does not rapidly reach the seat of the fire.

 

Flashover is much more dynamic than what many of us were taught in academy.  The fires we fight today are completely different from the fires of yesteryear.  Understanding the danger of ventilation controlled flashover and having the knowledge to identify this deadly situation will allow us to operate safely and effectively on the modern fire ground.