MORE ON FIRE PHENOMENA
RANDOM THOUGHTS
Last month 1 tried to explain my frustration with fire texts that tell us that every rapid oxidation reaction (explosion) on the fireground is a backdraft, or that backdraft is a common occurrence.
If that’s true, what is all this we hear about fire extension in remote areas of structures while firefighters are within “doing their thing”? Construction trends have contributed greatly to smoke explosion (and to “cold smoke” explosion in particular). Explosive gases, the by-product of incomplete combustion, are trapped longer than ever by structures that are tighter due to energy-conservation efforts—in particular, the doubleand triple-pane windows and the virtually airtight aluminum and plastic siding, which covers the original asphalt, wood, and asbestos siding.
While smoke explosions in remote enclosures within the fire building are not as detectable as the conditions forecasting backdraft, some size-up indicators are available. The detection of thermal pane windows, space-age siding, and fire in the top floor, where explosive products can be spread more easily by cockloft or attic space to areas remote from fire extension, are all possible indicators of smoke explosion conditions. Another is new construction techniques: The lightweight wood truss loft between floors creates a “cockloft” on each floor that allows for spread of explosive gases to areas not possible if 16-inch on-center, full-dimensional wood joists had been used. Finally, there’s the time that the fire has been burning prior to our getting a “handle on it” and the fire behavior conditions as we arrive. Is the building still tight or is the flaming visible outside the structure through fire-vented windows?
One of the keys to reversing the possibility of smoke explosion is ventilation. In many cases it is limited to the area actually involved in fire — especially with today’s mandatory use of SCBA.
Ventilation at the rear of the fire should be continuous. The nozzles should have a place to “push the stuff’ safely—safely for the interior team and to prevent the buildup of conditions that will cause secondary fire problems, such as overhead envelopment of the nozzle team, steam burns, and, in this case, smoke explosions.
Another question to think about: “Why are so many firefighters getting burned today? Why, when we artbetter protected than at any other time in the history of fighting fire since Nero’s fiddle?” The answer kind of pushes the threat of true backdraft to the back burner.
FLASHOVER
Flashover is the relatively instantaneous active flaming of the combustible contents of a room or space. As the heat of the fire continues to increase, the exposed materials reach and surpass their ignition temperatures and (at a certain stratum within the room) begin and continue to generate the flammable vapors that can fuel the “flashover” combustion. Without ventilation, the rising products of combustion no longer cool the heat source but synergistically feed heat back into the process. The ceiling temperatures are reaching the “text energy” of 1,110°F or more. The combustibles are past their ignition temperatures, so why does it wait so long to go?
From my firefighting experience, the process needs a couple of things. It needs additional ventilation and it needs “kick-off energy.” The fuel is in the room, ignition energy has dwindled into smoldering flame, and the optimum air mix is not present. As the firefighter moves through the space, additional air enters with him. As he crawls (hopefully) through the occupancy, the vented flaming gets more luminous and licks higher into the stratum of the combustible mixture given off by overheated combustible furnishings. And — bang—everything ignites. W hat can we do? At this time, nothing but escape. You do not survive this condition; you escape it only if you know it’s coming and you know where you are going!
Before this point, however, there is a lot you can do. For one, recognize the increased “routine” possibility of flashover in today’s buildings. Plan for it! Proper ventilation techniques — ongoing—in conjunction with good nozzle-handling techniques can go a long way toward reversing the condition that may continue to flashover. Tile ability of the nozzle team to move depends on ventilation; the ability of the room and contents to build to flashover depends on poor or ineffective or inadequate ventilation. The movement of cool air makes flaming more luminous, but at the same time it carries the heat buildup away.
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Continued from page 128.
If the engine company can recognize the probability of flashover and cool the combustible contents below their ignition temperatures as they move toward the seat of fire, in conjunction with ongoing supportive ventilation, it effectively will shut off the fuel supply to flashover.
The old adage “If you can’t see, crawl!” should become the first commandment of structural firefighting. The walking firefighter will be hit from the waist up with 700 to 1,000° F of heat and flame during flashover; the crawling firefighter will have more time.
It’s true. We are inside the building deeper and longer than ever before. Because of this we must be able to measure the only indicator of impending flashover —the rapid, almost instantaneous, heat buildup that forecasts it.
Some firefighters believe that we are overprotected — a real oxymoron! There is no such thing. But to abandon basic survival behavior in lieu of protection is foolish. I’ve been told by firefighters, “I don’t wear a hood because my ears are my indicators of how hot it is getting.” Another myth. Ears tell you nothing. Jewelers punch holes in the ear lobe while you wait in line for new earrings. Tell that same jeweler to punch a hole in your neck, cheek, back of hand, or underside of your chin and see how much pain there is. The ear is cartilage. It is sometimes irreparably burned before you feel it. Each of us needs some measurement tactic with which we will be able to feel the rapid heat buildup before flashover. I know what to do in those situations; do you?
ROLLOVER
Rollover is the burning of gaseous products of incomplete combustion. It is slower and less dangerous than flashover. It also can be controlled.
Rollover is the ignition of flammable vapor overhead at fire scenes. Did you ever enter an occupancy heavily charged and in a few moments watch flame roll toward you from around corners? Out you go and shut the door. Open it again —no flame. Again enter, and again the devil makes its appearance. What happened?
Again, these products of combustion need extra “stuff”: some ventilation to lean out the too-rich mixture and some additional kick-off energy — a lick of rising flame. We feed the process during our ventilation and forcible entry tactics. The flaming process is at the mercy of internal air currents. Out the rear windows one minute and back through the occupancy to the front door the next. It is controllable with good, aggressive nozzle-handling techniques —even more so than flashover. Again, proper and ongoing ventilation will assure it a place to go and good nozzle technique will push it there *
