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Last month, we looked at rapid fire developments (RFD) in terms of flashover. This term encompasses occurrences such as flashover, backdrafts, and/or smoke explosions, but our focus is on flashover. We are going to continue our look at such phenomena with how we can aggressively cool the environment and the contents.
As a reminder, a flashover is the simultaneous ignition of everything within a room. It occurs when hot gases rise to the ceiling and spread out across the walls. These hot gases are what we call black smoke – unburned particles of combustion. As it spreads out across the ceiling and the walls, it starts to heat up the items found within the room such as the paint on the walls, the furnishings, clothing, mattress, flooring material, etc., until they all reach their ignition temperature. Once they are all at their ignition temperature, a rapid fire development occurs, with everything becoming a big ball of fire. We are looking at temperature ranges between 1,000-1,500°F, which requires a good amount of water to effectively cool it.
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There are signs or indicators that are visible to us with our sight and with our tactile senses to warn us that flashover is impending. When these signs and indicators reveal themselves, firefighters must heed them and take aggressive action. These indicators are as follows:
- Heat buildup: Based on the temperature range of flashover, there will be a period when excessive heat buildup will occur. This heat buildup will descend towards the lower levels of the room, pushing the firefighter down to the floor. This will be the result of radiant heat being produced by the unburnt particles of combustion spreading out from ceiling to walls and then down to the floor area. When this quick increase in heat occurs, and it is an intensive heat, flashover is impending.
- Rollover: This is a visible indicator as it reveals itself in the smoke. A rollover or flame-over is when small gaps open in the smoke layer, allowing air to mix with the unburned particles of combustion and ignite. Small flames can be seen amid the smoke for momentary periods of time and then disappear. This is a warning sign that flashover is coming.
- Thick, dark smoke: This is also a visible indicator, as it will present itself outside the building as well as inside the building. On the inside, you will not see anything except total blackness. On the outside, you will notice this: whenever you have thick dark, acrid black smoke pushing out from a structure under high pressure and with high velocity, flashover is impending. This is an outside warning of what is waiting for us on the inside, or an outside warning for exterior crews and the incident commander to take action to assist the interior crews.
Taking aggressive action against this particular RFD means to act in a smart way so that we gain the upper hand. There are two ways to effectively overcome and eliminate flashover: tactical ventilation and aggressive cooling with water.
Tactical ventilation will aid in the rapid removal of the unburned particles of combustion, the hot gases, the smoke and the heat. This can be accomplished in various methods with the most effective being vertical ventilation. As heat and smoke want to rise naturally, a vertical opening will allow them to rise and escape out of the structure. The key with vertical ventilation is placing the opening directly above the fire to create a direct chimney or travel path to the outside for the smoke and hot gases.
This must be coupled with aggressive cooling and application of water. This is where a handline, minimum 1 ¾ inches flowing a minimum of 150 gpm, should be used to effectively cool down the ceiling, walls, floors, and contents all at once. Having the proper nozzle is going to help with this–this is where we could consider looking at smooth bore vs. automatic nozzles, but that’s a topic for another time.
Aggressive cooling involves using a straight or solid stream of water from the nozzle. We do not want to use a fog pattern of any type when facing preflashover conditions. A straight or solid stream of water is going to allow us to effectively knock down the fire while at the same time reducing the heat inside from the extreme high temperatures to a much lower temperature. An example of this is with the UL FSRI tests conducted in February and March, where temperatures were recorded to have been reduced by about 1,300°F with an aggressive attack with a smooth bore nozzle.
So how do we aggressively cool? We need to move the nozzle in a pattern that will hit the ceiling, the walls, and the floors all in a sequential and consistent fashion. Sweeping across the ceiling then straight down to the floor is one method, as well as sweeping the ceiling and also sweeping the floor. No matter what style chosen, it will involve movement, not a static application of water.
It also involves applying water to cool off the ceilings, walls, floors, and contents as we advance into the structure. This will require us to apply water to “black fire,” the unburned particles of combustion. Remember, they are contributing to the radiant heat against the ceiling, walls, floors, and contents. The black fire needs to be cooled a as well as the container that it is influencing. This will sound overly aggressive and will go against everything taught for the last number of years – we never spray water at smoke. However, with the change in technology and with science, we need to spray water at the smoke to effectively cool it and render it weak in terms of contributing to a flashover.
Mark van der Feyst has been in the fire service since 1999 and is a firefighter with the Fort Gratiot (MI) Fire Department. He is an international instructor teaching in Canada, the United States, and India, and at FDIC. He is also the lead author of Residential Fire Rescue (Fire Engineering Books & Video). He can be contacted at Mark@FireStarTraining.com.
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