This is the second bulletin in a two-part series on flashover and backdraft. In Part 2: Backdraft, you will learn why situational awareness is critical for recognizing when potential backdraft conditions exist.
Learning to recognize when backdraft conditions exist can save your life.
A backdraft is an air-driven event, unlike a flashover, which is thermally driven. Backdraft is usually defined as a deflagration resulting from the sudden introduction of oxygen into a ventilation-limited space containing unburned fuel and gases. When the air combines with the unburned fuel, rapid ignition can occur with devastating force. The normal oxygen level in air is approximately 21 percent. Below 14 percent, visible flame is reduced. When these fuels mix with air they ignite and burn quickly resulting in overpressure.
Indications of a backdraft
Conditions for backdraft might include:
- Black smoke becoming dense, greyish yellow without visible flames. The smoke color is indicating incomplete combustion. Usually the darker the smoke the more incomplete the combustion.
- A well-sealed building might indicate air confinement and excessive heat buildup.
- High concentrations of flammable carbon monoxide could be present as a result of incomplete combustion.
- Little or no visible flame. If flames are present, they may be blue in color. Another indicator, might be flames in smoke exiting the structure, especially in eaves of the structure.
- Smoke leaving the building in puffs and being drawn back in. Fire is trying to find oxygen, and this is the appearance of smoke pulling in under doors or through cracks.
- Smoke stained windows, brown in color, with visible cracking and/or rattling.
- Sudden, rapid movement of air and smoke inward when an opening is made.
Fireground size-up and how to read smoke
Situational awareness and defensive steps to protect yourself and your crew
- Perform a 360-degree size-up prior to opening-up wearing full personal protective equipment.
- Create vertical ventilation prior to making entry.
- Operate from a position of safety. Always plan an escape route.
- Pencil the ceiling with a straight stream, without disrupting the thermal layer, to cool gases below their ignition temperature.
Shan Raffel, operational station officer in Brisbane, Australia, offers these additional notes:
- Never rely on one indicator.
- Backdraft can be more likely in energy-efficient buildings with good insulation and sealed windows (often double- or triple-glazed).
- A developing fire may consume the available oxygen before it's able to flashover, or it may rapidly decline due to insufficient oxygen and the increasing volume of combustion products.
- Have a tactical ventilation plan to remove, confine or dilute the accumulated fuel.