Reading Smoke – Part 5: Flashover

FLASHOVER Elimination: If the conditions for a flashover have been identified or seems optimum, how can a flashover be minimized or eliminated? There are three primary considerations as follows: First […]

FLASHOVER
Elimination: If the conditions for a flashover have been identified or seems optimum, how can a flashover be minimized or eliminated? There are three primary considerations as follows:

  • First and potentially most importantly, do not let your PPE give you a false sense of security and allow you to over commit yourself. Stated from another perspective, do not use your PPE as an offensive weapon when it was designed as a defensive weapon.
  • Any ventilation that is capable of cooling the interior temperature can be effective. As an example, vertical ventilation will cool the upper portion of a room, reducing the temperature that is necessary for a flashover. Although ventilation has always been a top fireground priority, the ability to minimize a potential flashover has increased the importance of ventilation on the modern fireground.
  • Penciling is a term that describes using a specific type of hose stream to cool the area under a ceiling. By using a nozzle (if available) to direct a short stream (straight stream, not a fog stream) at the ceiling, the water can be turned to steam, cooling the upper environment. If this tactic is used, water that does return to personnel is an indicator that the ceiling is not that hot. However, if water does not return to the personnel, the ceiling was that hot, but two objectives have been accomplished.First, it is obvious that unless interior conditions change, there is probable cause to anticipate the chance of a flashover.

    Secondly, the stream of water likely turned to steam which is an effective cooling-extinguishing agent and capable of cooling the ceiling area above the personnel (thereby reducing the chance of a flashover). In this case, personnel should back-up to a position of safety and communicate conditions to appropriate person(s).

  • Remember that if personnel are caught in a flashover, visibility will drop to zero, their sense of direction is likely to be compromised, and there is about 3 to 5-seconds to escape.

PRIMARY DISTINCTIONS
There are four primary distinctions between backdrafts and flashovers encountered by fireground personnel:

  • In the modern fireground environment, why are backdrafts rare, and flashovers are not? The answer is the smoke.First, remember that smoke from conventional materials can normally have a flash point in the range of 1,123-degrees Fahrenheit while the flashpoint of smoke from synthetic materials has a flashpoint of around 800 to 900-degrees Fahrenheit, or less than the flashpoint of conventional materials. Therefore, when the interior temperature reaches 800 to 900-degrees Fahrenheit, the conditions for a flashover are optimal. As a result, a room can easily flashover before reaching optimal conditions for a backdraft.Secondly, a condition known as ladder fuels has changed the way smoke burns on the modern fireground. This term primarily refers to the way ignition temperatures of common gases accelerate the flammability of other gases that are present in a compartment fire. As an example, acrolein self-ignites at 450-degrees, benzene self ignites at 928-degrees, hydrogen cyanide self ignites at 1,000-degrees, carbon monoxide self ignites at 1,123-degrees, and so on. So, let’s assume these gases are present in a structure fire and the temperature is sufficient for acrolein to self ignite. In turn, the subsequent rise in temperature will cause benzene to self ignite, which will accelerate the ignition of the bigger gases and cause hydrogen cyanide to self ignite and so on. This ladder affect is why a room can flashover so quickly today compared to yesterday. The wider range of fire gases that are present on the modern fireground can accelerate the likelihood of a flashover!
  • A backdraft is a true explosion and a flashover is not. When oxygen is suddenly introduced to an oxygen deficient environment, a sudden widespread pressure increase that is capable of destroying structural elements can occur. Conversely, when smoke and fire gases reach their ignition temperature, they will readily burn, but at a speed that is slower than an explosion, and will not generate enough pressure to destroy structural elements. However, there will be a sudden and significant rise in temperature that is capable of instantly involving a room/area with fire and potentially fatal to anyone in the room/area.
  • A backdraft is an air driven event, and a flashover is a temperature driven event. Remember that a backdraft is deficient in oxygen and a flashover is deficient in the proper amount of heat.
  • Timing. A backdraft normally occurs during the decay phase of a fire, while a flashover normally occurs between the growth and fully developed phase of a fire.

In our next article, we will look at the importance of reading smoke.

John Mittendorf

About John Mittendorf

John Mittendorf was a 30 year veteran of the Los Angeles City Fire Department and held the rank of Battalion Chief until his retirement in 1993. He has been a member of the National Fire Protection Research Foundation on Engineered Lightweight Construction Technical Advisory Committee. He has provided training programs for the National Fire Academy, UCLA, and the British Fire Academy in England. He has acted in an advisory capacity for five college fire science advisory boards and is the author of numerous fireground articles for magazines in the United States and Europe. He is the author of the books Ventilation Methods and Techniques, Truck Company Operations, and Facing the Promotional Interview. He currently lectures in the United States and the United Kingdom on strategy and tactics, truck company operations, fireground operations, ventilation operations, and the complete fire officer. He is a member of the editorial advisory board of Fire Engineering magazine.