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how we fight fires

Published Oct. 22, 2009
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The traditional role of the Fire Service is to fight fires. A structure is burning, the fire department gets the call, fire engines are dispatched, water everywhere, and the fire is out. It is never that simple. Today’s complex technological society, with its associated hazards and regulatory demands, requires an approach to firefighting that is simultaneous, methodical, and disciplined. The importance of understanding the dynamics of fire growth and the tactical objectives of fire suppression cannot be understated.

 

The Dynamics of Fire Growth
The first stage of any fire is the smoldering stage. When heat is applied to a combustible material, the heat oxidizes the material’s surface into combustible gases. The oxidation process is exothermic, meaning that the oxidation process itself produces heat. The heat from oxidation raises the temperature of surrounding materials, which increases the rate of oxidation and begins a chemical chain reaction of heat release and burning. A fire can progress from the smoldering phase immediately or slowly, depending upon the fuel, nearby combustibles, and the availability of oxygen in the surrounding air.

The second stage of fire growth is free or open burning. When the temperature of a fire gets high enough, visible flames can be seen. The visible burning at this stage is still limited to the immediate area of origin. The combustible process continues to release more heat, which heats nearby objects to their ignition temperature, and they begin burning. In a wildland fire the surrounding growth will ignite and the flames will spread, quickly if wind and dry growth are present. A structure fire is different, because the gaseous products of combustion, most of which are flammable and lighter than air, rise and are contained in the upper levels of the structure. When this occurs, the structure fire is at a critical point, either the fire has insufficient oxygen available to burn and it progresses back to the smoldering stage, or it has sufficient oxygen available to move on to the next stage.

The third stage of fire growth is called flashover. It is the most significant moment of any structure fire. As combustible gases are produced by the two previous stages they are not wholly consumed. They rise and form a superheated gas layer at the ceiling. As the volume of this gas layer increases, it begins to bank down to the floor, heating all combustible objects regardless of their proximity to the burning object. In a typical structure fire, the gas layer at the ceiling can quickly reach temperatures of 1500 degrees Fahrenheit. If there is enough existing oxygen, usually near floor level, flashover occurs and everything in the room breaks out into open flame at once. The instantaneous eruption into flame generates a tremendous amount of heat, smoke, and pressure with enough force to push beyond the room of origin through doors and windows. The combustion process then speeds up because it has an even greater amount of heat to move to unburned objects.
Flashover is a critical stage of fire growth for two reasons. First, no unprotected living thing in a room where flashover occurs will survive and the chance of saving lives drops dramatically. Second, flashover creates a huge jump in the rate of combustion, and a significantly greater amount of water is needed to reduce the burning material below its ignition temperature. A post-flashover fire burns hotter and moves faster, requires more resources for fire attack, and compounds the problems of search and rescue, exposure protection, and containment.

The Significance of Flashover
A typical objective of fire service agencies is to maintain enough strategically located personnel and equipment so that the minimum acceptable response force can reach a reasonable number of fire scenes before flashover is likely. But when is flashover likely? Fire growth occurs exponentially, which means it doubles itself every second during free burning if enough fuel is available. It is generally accepted among fire service professionals that flashover will occur between four and ten minutes from ignition of the fire. This time frame is illustrated on what is known as the Time and Temperature Curve.
It is clear that the stage of a fire affects staffing and equipment needs. These needs can be reasonably predicted for different risk levels and fire stages. The ability to correlate staffing and equipment requirements with fires according to their stage of growth and risk hazard is the basis for this response coverage study.

The Tactical Objective of Fire Suppression
There are six tactical objectives to be accomplished at every fire regardless of size or intensity. They are:

Rescue: The saving of lives endangered by fire or accident.
Exposure: Property that may be endangered by a fire in another area of the structure.
Confinement: Preventing fire from extending to uninvolved areas of other structures.
Extinguishment: Putting out flames, but also completely controlling a fire so that no excess heat or smoke remains.
Ventilation: Removing super heated smoke or gases in a burning building to prevent explosive concentrations and flashover, and to permit advancement of hose lines into effective positions to affect extinguishment.
Property Conservation: Reducing additional losses from smoke, water, and weather during and following fires. Attempting to reduce emotional distress caused by the devastating effects of fire and personal loss.
Overhaul: Carefully scrutinizing the area and contents involved in a fire for any hidden or remaining trace of fire or embers in order to eliminate further damage or rekindle. Rendering property to a safe condition before turning property back over to owner and occupant.

 

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