TIME TEMPERATURE CURVE Time Temperature Curve The demarcations between the three phases can be identified by a “time temperature curve”. During the incipient (or growth) phase of a fire, shown below as the upward curve, the time can vary depending on the type of fuel, the size of the room, and the amount of oxygen supplying the fire. Flashover occurs at the end of the incipient (or growth) phase and start of the free burning (or fully developed) stage. Backdraft can occur in the smoldering (or decay) phase. FLASHOVER Flashover occurs when a room or other area becomes heated to the point where flames flash over the entire surface or area. Originally, it was believed that flashover was caused by combustible gases released during the early stages of fire. It was thought that these gases collected at the ceiling level and mixed with air until they reached their flammable range, then suddenly ignited causing flashover. It is now believed that while this may occur, it precedes flashover. The cause of flashover is not attributed to the excessive build-up of heat from the fire itself. As the fire continues to burn, all the contents of the fire area are gradually heated to their ignition temperatures, through “thermal radiation feedback”. When they reach this point, simultaneous ignition occurs and the area becomes fully involved in fire. BACKDRAFT Firefighters responding to a confined fire that is late in the free-burning phase or in the smoldering phase risk causing a backdraft or smoke explosion if the science of fire is not considered in opening the structure. In the smoldering phase of a fire, burning is incomplete because not enough oxygen is available to sustain the fire. However, the heat from the free-burning phase remains, and the unburned carbon particles and other flammable products of combustion are just waiting to burst into rapid, almost instantaneous combustion when more oxygen is supplied. Proper ventilation releases smoke and the hot unburned gases from the upper areas of the room or structure. Improper ventilation at this time supplies the dangerous missing link -- oxygen. As soon as the needed oxygen rises in, the stalled combustion resumes, and it can be devastating in its speed, truly qualifying as an explosion. Combustion is related to oxidation, and oxidation is a chemical reaction in which oxygen combines with other elements. Carbon is a naturally abundant element present in wood, among other things. When wood burns, carbon combines with oxygen to form carbon dioxide, or carbon monoxide, depending on the availability of oxygen. When oxygen is no longer available, free carbon is released in the smoke. A warning sign of possible backdraft is dense, black (carbon-filled) smoke. The following characteristics may indicate a backdraft or smoke explosion condition: 1. Smoke under pressure 2. Black smoke becoming dense gray yellow 3. Confinement and excessive heat 4. Little or no visible flame 5. Smoke leaves the building in puffs or at intervals 6. Smoke-stained windows 7. Muffled sounds 8. Sudden rapid movement of air inward when opening is made This type of condition can be made less dangerous by proper ventilation. If the building is opened at the highest point involved, the heated gases and smoke will be released, reducing the possibility of an explosion.
Posted on: Wed, 08 Oct 2014 07:18:58 +0000
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