FIRE BASICS
WHAT EXACTLY IS FIRE?
Whether you’re an expert like the professionals at Red Hawk Fire Protection or just someone learning about fire safety, it’s important to know that fire is an observable outcome of a specific chemical reaction called combustion. It arises when oxygen from the surrounding air interacts with a particular fuel. The resulting substances produced through this reaction are entirely distinct from the initial materials.
For combustion to take place, the fuel needs to be raised to its ignition temperature. As long as there is an adequate supply of heat, fuel, and oxygen, the reaction will continue. This combination of factors is commonly referred to as the fire tetrahedron.
THE STAGES OF FIRE
To effectively manage a fire, a team of firefighters will perform a brief evaluation of potential hazards. When confronted with a blaze, it is crucial to comprehend the various phases it undergoes. Identifying the specific stage of fire development aids firefighters in selecting an appropriate strategy to extinguish the flames.
To gain control over a fire, a firefighting team will perform a brief evaluation of potential risks. When facing a flame, it is crucial to understand the distinct phases of fire. Recognizing the developmental stage of the fire enables firefighters to select an appropriate strategy for extinguishing the flame. This knowledge empowers them to identify the most effective extinguishing method, minimize risks to personnel, and predict the fire's progression.
STAGE 1: INCIPIENT Stage:
An incipient fire refers to a flame that is in its initial stage of development. Fires at this early stage can be suppressed or managed using portable firefighting equipment. However, once a fire has advanced to a stage where visibility is hindered by smoke or structural firefighting measures are necessary, it can no longer be classified as an incipient fire.
STAGE 2: GROWTH Stage:
During the growth stage, the fire increases fuel consumption and creates more heat and smoke.
Flames that are small and have not spread extensively
Manageable smoke levels
Low heat emission
FLASHOVER occurs between GROWTH and FULLY DEVELOPED stages. NO ONE survives FLASHOVER.
STAGE 3: FULLY DEVELOPED Stage:
The fully developed stage occurs when the maximum amount of fuel and oxidizers are consumed, and the highest heat release rate is produced.
The flame continues to grow due to an ample supply of oxygen and fuel.
A distinct layer of smoke forms above the flame.
The room temperature rises.
Condensation vanishes from windows.
Possible brown stains may emerge on window glaze.
Cracks may develop in windows.
darkened or black smoke
dense smoke
substantial heat
blackened window glazing
visible exterior flames
flames obscured by smoke
STAGE 4: DECAY Stage:
Usually the longest stage of a fire, the decay stage is characterized by a significant decrease in oxygen or fuel, putting an end to the fire. Two common dangers during this stage are first – the existence of non-flaming combustibles, which can potentially start a new fire if not fully extinguished. Second, there is the danger of a backdraft when oxygen is reintroduced to a volatile, confined space.
fire is slowed down by a reduction of fuel or insufficient oxygen
usually the longest stage of the fire
non-flaming combustibles, which can potentially start a new fire if not fully extinguished
danger of a backdraft when oxygen is reintroduced to a volatile, confined space
HOW FIRE SPREADS
Fire propagates by transferring the heat energy emitted by the flames through three distinct methods:
CONDUCTION: the transmission of heat energy within a material or through direct contact. For instance, a burning wastebasket can transfer heat to a nearby couch through contact, causing the couch to ignite. As the fire spreads, the heated couch then transmits heat to the drapes hanging behind it, leading to their ignition and subsequent combustion.
CONVECTION: the movement of fluid or gas from hotter regions to cooler regions. As heated air becomes less dense, it rises, while cooler air descends. In an open area, a substantial fire generates a plume or column of hot gas and smoke that ascends into the air. However, in an enclosed room, these rising gases come into contact with the ceiling. They then move horizontally along the ceiling, creating a dense layer of heated air, which eventually descends.
RADIATION: the transmission of heat through electromagnetic waves, without the need for objects or gases to carry it. Radiated heat disperses in all directions until it encounters an object. In the case of burning buildings, heat can radiate and extend to surrounding structures, potentially passing through glass windows and igniting objects within them.
SPECIAL CIRCUMSTANCES
ROLLOVER happens when ignited fire gases or partially burned fuels ascend towards the ceiling and disperse horizontally. Subsequently, the smoke seems to spontaneously ignite. If no action is taken to ventilate the room or reduce the air temperature, this situation progresses into flashover.
FLASHOVER is the abrupt and simultaneous ignition of everything within a room. This phenomenon unfolds in the following manner:
Hot gases rise and disperse across the ceiling and walls.
Radiating heat descends and intensifies, causing all flammable objects to reach their ignition temperatures and ignite.
Within a matter of seconds, temperatures escalate to as high as 1,000 degrees Fahrenheit. Even firefighters wearing full protective gear are unlikely to survive a flashover.
Firefighters undergo training to identify the indicators that precede a flashover:
Dense black smoke characterized by tightly curled patterns ("black fire").
Dense black smoke forcefully emanating from a doorway or window opening.
Accumulated smoke at a low level, even reaching as low as a doorknob, with visible fire below.
BACKDRAFT refers to an explosion that transpires when oxygen is introduced into a space filled with hot gases. The progression of a backdraft occurs as follows:
A fire within a confined area consumes available oxygen.
Visible flames diminish, while solid fuels smolder and emit hot, flammable gases that accumulate and fill the room.
As the temperature rises, the gases expand and create pressure, causing pulsations against doors and windows. The building's exterior may exhibit a breathing or throbbing appearance.
If an opening is created to allow oxygen entry, the highly vaporized fuel rapidly ignites, triggering the pressurized gases to explode out through the opening. This results in the formation of a rolling fireball.
FIRE CLASSIFICATIONS
Class A: Includes ordinary combustible materials like wood, cloth, paper, rubber, and many plastics. These materials burn with embers and leave behind ash. They can be extinguished by cooling the fuel below its ignition temperature. Water and other effective extinguishing agents can be used.
Class B: Involves flammable liquids that burn at room temperature and combustible liquids that require heat to ignite. Examples include petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols, and flammable gases. These pose a high fire hazard, and water may not effectively extinguish them. Creating a barrier between the fuel and oxygen, such as using a layer of foam, can be effective.
Class C: Encompasses fuels that would fall under Class A or B, but they involve energized electrical equipment. Special techniques and agents are necessary to extinguish these fires, with carbon dioxide or dry chemical agents being commonly used. Water is highly dangerous in these situations due to its conductivity of electricity.
Class D: Includes combustible metals like magnesium, titanium, zirconium, sodium, lithium, and potassium, which are commonly found in various applications, including cars. Due to the extremely high flame temperatures, water can break down into hydrogen and oxygen, potentially intensifying the burning or causing explosions. Extinguishing these fires requires special powders based on sodium chloride or other salts, as well as clean dry sand.
Class K: Designates fires in cooking appliances involving combustible cooking media such as vegetable or animal oils and fats.