We've been pretty slow lately so I scheduled flashover training up at the fire academy for our company. Flashover is defined as the "near simultaneous ignition of all combustibles within a confined area." Basically this is when a room gets so hot due to fire that everything lights up at once. This typically happens when the temperature reaches between 800 and 1300 degrees F. This can happen as soon as a few minutes after a fire starts in a room. It's somewhat predictable by a few telltale signs: 1. thick, black smoke banking down from the ceiling 2. sudden increase in temperature which is felt even through fire gear 3. the rolling over of flame and fire across the ceiling. The problem is that the first indicator usually masks the third. And with fire gear being so protective nowadays, the second indicator usually fails to give firefighters enough time to evacuate.
The flashover course is meant to give firefighters a heads up on what imminent flashover conditions look and feel like. Because survival is impossible for anyone in a room that flashes, the flashover container at the fire academy is built so the firefighters are actually set 3 feet below ground level.
On the raised portion, the walls and ceiling are lined with inch-thick pressboard. A 50-gallon metal drum is placed in the middle and filled with some wood from broken up pallets. In front of the pail is a small pile of shredded paper. Paper ignites at around 300 degrees, so the shredded paper stands as a simple thermometer for us to tell when the floor of the room about to flash reaches that temperature.
As the fire grew, temperatures quickly shot up from 90 degrees to about 300 above the fire and smoke started increasing. The smoke was nearly down to the floor by the time the last of our guys tied the regulators of their air packs into their masks. Now, it was just a matter of time. As temperatures in the upper room got up to about 600 degrees, the back doors of the flashover container were opened to let in some fresh air. Then the fire really took off and think black smoke quickly banked down from the ceiling to about head-level where we were sitting (remember, we were 3 feet below the ground level of the fire-floor.)
The instructor yelled "VENT" which meant the front-most guy on the left side was to pull a lever, opening a small vent in the ceiling. Within seconds, the thick smoke began to clear as the fire grew exponentially. At once, the entire front compartment was ignited in vibrant, bright hot flame. The fire came back towards us right above our heads and danced around a bit before the instructor had the vent closed and the fire died down a bit.
Both sides of guys were told to rotate so the front guys could catch a break in the rear and someone else could have front-row seats. As the back door was opened again, we could see all the standing smoke above our heads rush towards the fire as it sucked in the fresh air. The fire in the barrel grew in intensity again and we could see slow-moving black smoke just being sucked out of the particle board lining the walls. This was mostly products of incomplete combustion and carbom monoxide. In a typical house fire, all the contents (couch, tv, carpet, walls) would give off this gas. Upon the vent being opened again, we all saw for ourselves that that gas was exactly what ignited in the air.
Now that most of the smoke was clear, we had a much better view of each subsequent flashover. Some flames slowly snaked through the air right above our heads to where we could actually reach up and touch them. It was truly amazing. Between flashovers, the thick black smoke slowly came down from the ceiling. It was thick and looked like bubbling soup from our view from our knees. The temperatures our heat guns displayed were between 300 and 400 degrees righit above our heads, and up above the fire-barrel went from 800 to off-the-charts when it flashed.
When there was no more fuel left for the fire, we all crawled out and slowly took off our smoking gear, keeping our gloves on so we wouldn't burn our bare hands on the buckles.