Why is it Important to Recognize Flashover in a Room
by John D. DeHaan
California Institute of Criminalistics
In the growth of a compartment (room) fire, flashover is defined as the
transition between the fire growing by igniting one item and then another
(by direct flame contact, radiant heat, convective heating, or immersion
in the got gas layer) to the point where all fuel packages in the compartment
are on fire (full involvement) and are burning as fast as air (oxygen) can
be made available through window and door openings. This transition may
occur throughout a room in just a few seconds in a small, heavily fueled
room, over a period of minutes in large rooms, or may never take place due
to insufficient size (heat release rate) of the fire. Because it is a transition,
defining the "exact moment" is often a problem. It can be likened
to defining the moment when one has crossed a bridge: is it when one sets
foot on the bridge, one gets halfway, or sets foot on the other side? It
is all part of the process, and the time it takes for completion can vary.
Up to flashover, the highest average temperatures are found in the upper
or "hot gas" layer. Temperatures in the lower or "normal"
zone are at or close to ambient. This produces a distinctive area of thermal
damage on the upper walls and ceiling of the room due to radiant and convective
transfer of heat from the fire gases. Extensions from lower portions of
the room (V-patterns) into this layer may then give clues as to the location
and involvement of fuel packages in the room.
If the average temperature of the hot gas layer exceeds 600 degrees Celsius
(with or without the flaming ignition of the hot smoke called flameover
or rollover) the radiant heat from the layer exceeds the minimum igntion
radiant heat flux for exposed fuels and those fuels char and then ignite.
This is the transition to flashover. If flashover occurs, temperatures throughout
the room go to maximum (1000 degrees Celsius is not uncommon) as the two-layer
environment of the room breaks down and the entire room becomes a turbulently
mixed combusyion zone--floor to ceiling. The active mixing promotes very
effective combustion with oxygen concentrations dropping below 3% and very
high temperatures. This environment in turn produces radiant heat fluxes
of 120 kW/m2 or higher, ensuring rapid ignition
and sustained burning of all exposed fuel surfaces. This includes carpets,
flooring, and low-lying fuels like baseboards. Ignition of carpets produces
floor-level flames that sweep under chairs, tables, and other surfaces that
were protected from the downward radiant heat from the hot gas layer alone.
Burning will then proceed throughout the room until the fuel supply is exhausted
or some attempt is made at extinguishment.
Unfortunately, post-flashover burning in a room produces numerous effects
that were once thought to be produced only by accelerated (arson) fires
involving flammable liquids such as gasoline. Wall scorched or charred from
floor to ceiling can result from post-flashover burning no matter how the
fire actually began as well as from the fireball of gasoline vapors. The
combustion of carpets and underlying pads once considered fairly unusual
in accidental fires, is quite common in rooms filled with today's high heat
release materials--polyurethane foam, synthetic (thermoplastic) fabrics
in upholstery, draperies, and carpets. This problem is exacerbated now with
the increasing use of very combustible fibers like polypropylene for the
face yarns of economical carpets as well as in the backings of nearly all
wall-to-wall carpets. These carpets now melt, shrink and ignite under radiant
heat flux exposing the combustible urethane foam pad underneath to the flames.
Fire damage under tables and chairs (once thought to be the result of flammable
liquids burning at floor level) can be produced by the flaming fire of the
carpet as it notes during flashover. High temperatures and total room involvement
once linked to flammable liquid accelerants are produced during post-flashover
burning. Localized patterns of smoke- and fire-damage that can help locate
fuel packages and characterize their flame heights and reveal direction
of flame spread can be obliterated with prolonged post-flashoever burning,
making the reconstruction of the fire very difficult.
The fire investigator, then, should be aware of the possibility of flashover
in modern room fires and what effects post-flashover burning can have. Interviews
of first-in fire fighters may reveal if a room was fully inolved floor-to-ceiling
when entry was first made. Investigators must be aware of what conditions
can lead to flashover and how to recognize when it has and has not occurred.
A thorough understanding of heat release rates and fire spread characteristics
of modern furniture will be of great assistance.
Reprinted with permission from the author.