Flashover! It is the most dangerous time of a fire. When the room bursts into flame, flashover has occurred. The scientific definition of flashover states it is caused by the radiation feedback of heat. Heat from the growing fire is absorbed into the upper walls and contents of the room, heating up the combustible gases and furnishings to their auto-ignition temperature. This build up of heat in the room triggers flashover.
Flashover signals several major changes in a fire: It is the end of an effective search and rescue in a room; it means the death of any person trapped in the blazing room - either civilians or firefighters. It signals the end of using a portable extinguisher to extinguish the fire; an attack hoseline is required after flashover occurs. It signals the end of the growth stage and that the fire is in the second stage of combustion - the fully developed stage. Finally, flashover signals the change from content to a structure fire. This is the beginning of the collapse danger.
When operating at a fire, firefighters want to delay flashover inside a burning room. By delaying flashover you can "buy" several minutes which may be critical. For example, you may want to delay flashover to make a search and rescue of the burning room or allow a firefighter to go above a fire to make a rescue of a trapped victim. Or, you may want to delay flashover to gain several minutes when there is a delay in the placement of the first attack hose-line.
There are three way to delay a flashover:
- Ventilation: By venting windows of a burning room you release the build up of heat in the room. This slows down flashover in addition toimproving visibility in a smoke-filled room.
- No ventilation: By not venting and instead closing the door to the burning room, you can also delay flashover. By not venting, you starve the fire of oxygen, which slows down the combustion rate, which slows down the build up of heat in the room. This may be done when there is a delay in stretching a hoseline and all persons are out of the burning room.
- Portable extinguisher: The discharge of a portable extinguisher can cool the heat down in a burning room temporarily and delay flashover.
For more information go to www.vincentdunn.com and use the Google search feature with keyword: flashover.
Vincent Dunn, a Firehouse Magazine contributing editor, is a 42-year veteran of the FDNY and a deputy chief (ret.), serving as division commander for midtown Manhattan. A nationally renowned lecturer, he is the author of the best-selling text and video series Collapse of Burning Buildings and the textbooks Safety and Survival on the Fireground and Command and Control of Fires and Emergencies. A new book, Strategy of Firefighting - How to Extinguish Fires will be published in April. Dunn has a master's degree in urban studies, a bachelor's degree in sociology and an associate's degree in fire administration from Queens College, City University of New York. He can be reached at 1-800-231-3388 or via through his website at www.vincentdunn.com.