Reading
the Fireground If you have been reading some of the recent “Flashover” articles in Fire Engineering magazine, it is obvious that flashover has numerous definitions, which is interesting from the perspective that flashover is currently a leading cause of fireground injuries and deaths. With these thoughts in mind, let’s review the terms backdraft and flashover as both of these events are related from the perspective that they are capable of injury or death to fireground personnel, yet are unrelated from the perspective of the conditions necessary for their origins: BACKDRAFT Definition: “The introduction of oxygen to
a confined area that is pressurized with heated flammable gases that
are deficient in oxygen that results in an explosive force of
significant intensity.” To understand the background of this definition, let’s review some basic theory. Any fire must have heat, fuel, and oxygen in order to sustain combustion. In air, oxygen is normally 21 percent by volume. When heat, fuel, and oxygen are present in a suitable ratio, a free burning fire will result. If we transfer a free-burning fire to a confined room with no ventilation, the growth of the fire will advance through three phases of progression. In the first phase of a fire, the oxygen content is 21 percent. This fire will produce heat, some smoke, and numerous fire gases, some of which are flammable. At this stage of the fire, the temperatures in the room are beginning to increase over the ambient temperature of the room. As the fire continues to burn, the oxygen content of the room will be reduced to about 17 percent. This is the second phase of the fire, where it burns at a slower rate and the level of smoke and fire gases increase in addition to the interior temperature, which can exceed 1,300-degrees F. If this process is allowed to continue, the oxygen content will be reduced to about 15 percent which will result in the fire entering a smoldering phase which will result in increasing levels of smoke and fire gases and interior temperatures slowly being reduced, although they can easily exceed 1,000-degrees F. This is often referred to as the third phase of a fire and/or the smoldering phase. At this point, another condition is rapidly becoming significant. As smoke and fire gases are heated, they will expand. In fact, the volumetric expansion of gases can increase up to a factor of three, and are capable of creating a significant pressure (up to three times more than the ambient atmospheric pressure of 14.7 psi) inside the room. This internal pressure will force smoke and fire gases from all available openings. At this stage of this process, the room is pressurized, flammable fire gases are high, and oxygen levels are low. If oxygen is suddenly introduced to this scenario, either by breaking a window or opening a door, the fire triangle (heat, fuel, and oxygen) will be completed and an explosion can be expected to occur, often with destructive force to personnel and the room. Trademarks A potential backdraft is relatively easy to recognize by four distinct trademarks:
Once a backdraft has been identified, how can it be eliminated? Although the question and answer are relatively simple, a practical fireground application may prove difficult. A backdraft can be eliminated by vertical ventilation over the involved area. This will reduce the internal temperature (primarily from the top portion of the room which is the location of the most heat) and vertically exhaust the hot smoke and fire gases to the exterior of the structure without allowing the sudden introduction of oxygen. If horizontal ventilation is the only practical method of ventilation, ventilation personnel should ventilate windows and/or doors from the corner of the structure as this is a safe area for ventilation personnel. However, a practical fireground application may be difficult as ventilation must precede initial entry into the structure. The success of this operation is based on the following factors:
Definition Before we go further, it is important to state that there are abundant definitions without one definition being the accepted standard. As an example, a popular definition of a flashover is as follows: “The sudden ignition of exposed combustible
surfaces and/or combustible gases in an involved area that results in
a sudden and intense rise in temperature.” Although this definition is applicable to some fireground flashovers, there is another definition that is not as popular but is potentially more accurate in its description of a fireground flashover: “Flashover is the ignition of the layer of
combustible gases produced by a fire which accumulate close to the
ceiling and can quickly spread to the rest of a room, and in some
cases, igniting the room contents.” Although the two preceding definitions share some similarities, there is a primary difference. The first definition follows the theory that most flashovers are the result of radiant heat from the ceiling that raises the temperature of the room contents to their ignition temperature, resulting in all contents simultaneously bursting into flame. The second definition, although less popular, is based on the principle that the ceiling area is where smoke (fire gases and particulates) begins to accumulate and then continues to collect along with increasing heat. As a result, the area with the most favorable conditions (heat and fuel) for a flashover is the ceiling area within a room. Therefore, if the smoke and fire gases at the upper portion of a room flashover, the flashover can be confined to this area, or can simultaneously raise the heat of the contents in the room and simultaneously expand the flashover to the rest of the room. This concept has received strong support from the Swedish fire service, which has done significant research on flashovers, and noted American fire authorities Walter Haessler, “Fire Fundamentals and Control,” and William E. Clark, “Firefighting Principles and Practices.” Let’s look at this definition by considering a few basic principles. As a fire burns in a room with the correct ratio of heat, fuel, and oxygen, the heat, fire gases, and other products of combustion will rise to the highest point in the room (ceiling area) and then begin to spread horizontally under the ceiling area. This is referred to as mushrooming. As the fire continues to burn, another chain of events is simultaneously taking place. As the heat, fire gases, and products of combustion collect at the ceiling area, their temperature will increase (sometimes dramatically). Although this rising temperature will also increase the temperature of adjacent materials (room contents), the highest temperature in the room will be at the ceiling area. Heat will also bank down in the room and in a short period of time can create three distinct temperature zones as follows:
These three temperature zones can be easily detected by interior personnel who are making entry into a room that is being heated from a fire. In some incidents, personnel close to the floor will encounter a temperature that can be hot but bearable. If the personnel rise up just several feet, the temperature can quickly become unbearable. This is one reason why firefighters are taught to stay low when entering a heated atmosphere as this is the area of least heat (and the best visibility). As a fire continues to burn and the heat within the room continues to increase, there will be a point where the smoke and fire gases at the upper portion of the room have reached their ignition temperature. As a result, the upper portion of the room can flashover and create the following conditions:
Obviously, any of the preceding conditions are immediately untenable for personnel within a room and can result in injury or death. Trademarks Although the warning signs for a backdraft are relatively easy to recognize, the warning signs for a flashover are often subtle and elusive. As an example, some personnel who have experienced a flashover have noted that visibility was acceptable, the level of heat was not objectionable, and in a few cases, SCBA was not necessary. Yet, the environment flashed over! With these thoughts in mind, look for the following conditions:
Note: Rollover is visible small amounts of flame rolling along under the top most portion of a room/ceiling. This normally is a precursor to a flashover. Elimination If the conditions for a flashover have been identified or seems optimum, how can a flashover be minimized or eliminated? There are three primary considerations as follows:
If personnel are caught in a flashover, the current theology is that visibility drops to zero, your sense of direction is likely to be compromised, and there is about 3-5 seconds to escape. PRIMARY DISTINCTIONS There are four primary distinctions between backdrafts and flashovers encountered by fireground personnel:
In our next article, we will look at another reason why it is important to read the modern fireground environment. Editors Note: Chief Mittendorf is the author of Truck Company Operations and Facing the Promotional Interview published by PennWell/Fire Engineering Books. To purchase, return to the Main Page and scroll down to Fire Engineering Books. © Copyright Firenuggets.com 2008 • Click here for Terms and Conditions of Use
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