November-December 2009

	November-December 2009

Reading Smoke, Part 7

Reading Exterior Smoke

By John Mittendorf

PRESSURE

The pressure of smoke exiting a structure can be an excellent indicator of the amount of fire within the structure in combination with the amount of confinement. If there is a lack of openings to the exterior of the structure, then expect smoke to display visible pressure as it forces its way to the exterior. The key to reading pressure is threefold :

Fire makes smoke. So, the greater the pressure, the more fire within the building.
Always consider the size of the building. A larger building (square footage) can contain more smoke than smaller buildings, thereby potentially concealing the real size of a fire. Conversely, fires in small to moderate buildings can be fairly accurate in indicating the amount of fire within a building.
The rate of curl or expansion of escaping smoke and fire gases is an indicator of a hot fire. A hot fire will increase the pressurization of the smoke and fire gases which will be driven out of all available openings, expanding as it escapes.

(Note: Smoke color is a nice helper, but pressure trumps color any day of the week.)

Let’s apply these considerations to a simple scenario. Assume a 1,700-square-foot single-family dwelling (SFD) with an obvious fire and initial attack personnel are confronted with a large amount of pressurized hot, roiling smoke from the doorway. What would be your initial considerations regarding the safety of entering this structure without proper ventilation as opposed to moderate colored smoke with moderate pressure?

DYNAMICS

Finally, the dynamics of reading smoke can be easily evaluated prior to entering a structure. As an example, assume a commercial building displayed a working fire upon arrival of initial companies. After forcing an opening in an overhead door, the following conditions were observed ̶ high heat, dark smoke down to the floor, and smoke being forcefully drawn inwards. Based on the preceding conditions, it should come as no surprise if the environment suddenly flashed over! This example should be a reminder that after forcing entry and prior to entering a structure, the smoke (or environment) can often give a clear indication of the fire and/or interior environment. Let’s look at the following three examples of the dynamics of smoke that can be based on what is initially often observed prior to entering a structure and/or after the observation of initial conditions:

Static

Assume you are the initial company about to enter a structure and you observe smoke that is static (another term that also describes this condition is residual). Webster’s Dictionary defines static as “at rest or in equilibrium,” which aptly describes this scenario. If you are about to enter the area of the fire, you can assume that either there is a minimal amount of fire or the fire no longer exists as it takes the combustion process to make smoke. If this is the environment you are about to enter, it obviously presents the least amount of danger of the three dynamics as the environment has stabilized and is not being rapidly degraded by fire.

Pressurized

Again, assume that you are the initial company and are about to enter a structure, and upon forcing the door, you observe smoke being forced from the interior to the exterior of the building. At this point, you should evaluate the reading smoke factors of color, amount, pressure, and density. These four factors will be an initial indicator of the status of the interior environment and the fire. As an example, if you encounter a slight pressurization of relatively cool smoke of moderate density, the fire is probably not well advanced and the chance of a flashover is low. However, if you encounter a large amount of roiling dark hot smoke, you can assume you are about to enter a well advanced fire and an environment that may hold the conditions (or increasing conditions) necessary for a flashover. Pressurization is the next most dangerous dynamic after the dynamic of static smoke.

Negative

Finally, assume you are the initial company that has forced a door to a structure and you observe that exterior air, along with interior smoke at the door, are being drawn into the structure. At this point, you should evaluate the following four conditions: (1) as you opened the door, you likely gave the fire what it was looking for prior to your arrival – oxygen, (2) the sudden introduction of oxygen will accelerate the fire, (3) the interior environment can be anticipated to rapidly degenerate and the conditions necessary for a flashover can be expected to increase, and (4) in this case, operations designed to improve the interior environment (i.e., ventilation) should be initiated prior to entering the structure. Obviously, this scenario presents the greatest hazard of the three dynamics we have discussed. Interior visibility will be poor to non-existent.

In our next article, we will discuss reading interior smoke.

Chief Mittendorf is the author of Truck Company Operations and Facing the Promotional Interview. To purchase, return to the Main Page and scroll down to Fire Nuggets Books and Videos.

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