By George H. Potter
Why are many firefighter turnouts colored black? Mostly to hide the soot and grime accumulated over a period of time from responding to all kinds of fires.
The principal problem with this accumulation of fine particles of unburned or decomposed fuel residues is they can ignite when exposed to the extreme temperatures encountered in imminent flashover conditions and in the midst of the accumulated fire gases that fuel the flashover, further reducing the user’s chances for survival.
Lighter colored gear shows far too plainly whether the users have been in working fire situations. Laundering cleans most gear to generally acceptable levels.
This is just one of the many considerations which firefighters must understand and evaluate when faced with compartment fires in which flashovers are most likely to occur.
Until recent years, from the mid-1970s onwards, fires in family dwellings, offices, hotels, and so on generally burned with a certain consistency, a condition frequently considered a stable burning fire. This condition existed primarily because of the buildings’ construction (wood frame) and finishings (to some extent porous), and most of all the contents such as furniture and decorative items (made from natural materials).
However, today’s buildings are more tightly sealed, fitted with double-pane glass windows and filled with highly combustible furnishings made of an infinite variety of plastics and other synthetic materials. These materials give off highly toxic and readily flammable gases which are retained within the well-sealed, nonpermeable structures.
Firefighters must be prepared for the possibilities that a flashover could occur during intervention in compartment fires. What is a compartment fire? According to the National Fire Protection Association (NFPA), a compartment fire is any fire within a structure that has a roof (ceiling) and at least two enclosing perimeters (side walls) which can be either parallel or perpendicular. The saving factor of a structure meeting the minimum enclosure conditions is that the fire is constantly ventilated, receiving a continual renewal of oxygen while at the same time expelling the smoke and fire gases.
However, in a compartment fire in a completely enclosed space–one with a ceiling and four enclosing walls–the combustion consumes the oxygen while potentially flammable gases remain within the space, normally accumulating from the ceiling downward. When the temperature inside this space reaches the ignition temperature of these gases and the oxygen’s concentration is within the flammable range, a generalized ignition may well happen, enveloping the entire space in a sudden deflagration. More often than not, this will happen within the first few minutes of a well-seated fire.
When flashover occurs, the temperature inside the compartment may well exceed 1,000ºF and will remain around that point for several seconds. Any firefighters inside that area, equipped with the best turnout gear available (even sparkling clean) and top-of-the-line SCBA will face intolerable conditions, quite probably causing very serious burns if not death.
How can firefighters survive flashover? The first answer is preparing for the possibility that the situation could occur, which includes the following:
- reading the fire situation and having an attack plan (with alternatives)
- having adequate resources available and ready before initial attack
- recognizing the tell-tale signs that “announce” the possibility of a forthcoming flashover
- having charged hoselines with adequate pressure and flow capacity
- wearing reasonably clean bunker gear and adequate SCBA
- knowing when, where, and how to dilute the fire gases and reduce temperatures inside the confined space
This preparation can only be achieved with thorough and competent flashover training. The training in recognition and mitigation of potential flashover covers numerous subjects and situations, among them: understanding what a flashover is; the diverse variations which can occur; knowing the differences between flashover and backdraft; and recognizing and addressing the numerous warning signs that indicate a possible flashover before it happens.
We all know, or should know, the hazards present in compartment fires: retention and accumulation of extreme heat and highly volatile gases (some flammable, some not, but all toxic), and varied densities of smoke, plus the decline in the environment’s oxygen concentration. When these conditions are combined in the right proportions, they present the ideal situation for a flashover. These conditions will generally occur in the initial stages of the fire, often within a very few minutes after ignition. Normally, the fire will be confined within the affected compartment, with practically no flames exiting the area, although this is not always necessarily true, as some cases have demonstrated.
What generally leads to the flashover is that the “ideal” mix of fire gases and retained heat receives fresh oxygen by accidental or provoked ventilation, causing a sudden ignition of the flammable gases and involving the entire volume of the compartment. There are several variations of flashovers, depending on the nature and concentrations of the accumulated gases and other numerous factors. There is one trait common to all of the variations, and this is that a flashover is a type of deflagration; nearly instantaneous ignition of the fire gases within a confined space with liberation of very insignificant mechanical pressure. In other words, a flashover is not an explosion.
However, once a compartment fire has consumed the majority of the fuel and oxygen present and the fire’s actual burning phase decreases to a nearly latent or incandescent state, with significant accumulation of unconsumed fire gases, and again with little or no ventilation, the situation is ideal for a potential backdraft, also known as a smoke explosion. The backdraft will indeed produce a significant mechanical overpressure or explosion, which will more than likely cause severe physical damage. A backdraft can happen well into the time development of a fire, depending on many factors such as fuel load, the burning structure’s volume and “breathing” capabilities, the amount of gases retained inside, and more.
Training to combat compartment fires and controlling potential flashover conditions has evolved from “accidental” situations that appeared in live fire drill situatuions into a complex and very exacting science. Many fire training centers around the world have attempted to simulate or create the conditions for flashover in conventional burn buildings built for this purpose, but the Swedish national fire service academy is credited with creating the most adequate scenario.
The principal drawback of buildings of traditional concrete and similar materials is that concrete and even refractory bricks, are porous and “breathe,” permitting the gradual escape of the fire gases and entry of air, which impedes producing the conditions necessary for a flashover. More than 20 years ago, the Swedes experimented with metal shipping containers and encountered the ideal venue for provoking flashover situations under reasonably controlled conditions. Over the years, the modifications incorporated into the containers–controlled ventilation, multiple access/egress points, and baffles for controlling gas concentrations–have contributed to the development of practice sites that incorporate acceptable safety criteria.
Also, over the years, since flashover training course content and training structures have developed such that they comply with the established and obligatory safety procedures, training in these containers can be considered as relatively “low risk.” This training is an essential factor that can contribute to a firefighter’s chances of survival in a flashover. Firefighters should take active and dynamic part in initial flashover training AND ongoing refresher courses.
Many countries have adopted and adapted the Swedish flashover training programs, incorporating innovations and variations as each individual case demands. The latest innovation has been the introduction of gas-fired simulators, environmently friendly but far from simulating solid fuel fire conditions, closely similar to those which would be found in typical structural fires. Interior or confined space fires are increasing in intensity each year. The survival of firefighters faced with a potential flashover will depend on whether they have had adequate practical flashover operation and survival training, coupled with some good luck.
George H. Potter is a practicing fire protection specialist who has lived in Spain for the past 45 years. He served as an Anne Arundel County, Maryland, volunteer firefighter with the Riva Volunteer Fire Department and the Independent Hose Company in Annapolis and as an ambulance driver with the Wheaton (MD) Rescue Squad. He served six years in the United States Air Force as firefighter, apparatus driver/operator, and crew chief. He has been involved in fire protection system installation, mobile fire apparatus design, and construction and fire safety training. He is a Spain certified as a fire service instructor and a hazmat specialist and is a member of the Board of Governors of the Spanish firefighters’ association ASELF.