Fire, or combustion, is when fuel reacts with oxygen to release heat energy. Combustion can be slow or fast depending on the amount of oxygen available. Combustion that results in a flame is very fast and is called burning. Combustion can only occur between gases.
The fuel must be heated to its ignition temperature for combustion to occur. The reaction will keep going as long as there is enough heat, fuel and oxygen. This is known as the fire triangle.
Fire Triangle:
The fire triangle, or combustion triangle, is the three components needed to ignite and sustain a fire. The three ingredients of a fire triangle are heat, fuel, and oxygen.
If just one of these components is removed, the fire triangle will collapse, and the fire will be extinguished.
Let’s explore these components in more detail:
1. Heat
A source of heat is required in order for ignition to occur, and different materials have different ‘flash points’ e.g. the lowest temperature at which they ignite.
Unfortunately, combustion reactions also produce heat as they burn, further increasing the temperature of the fuel. For some types of fire, the heat can be cooled with the application of water.
2. Fuel
A fire cannot begin if there is no material to burn. Homes and businesses are full of flammable materials, such as paper, oil, wood and fabrics. Any of these can serve as a fuel for a fire. Some materials burn more easily than others. Fuels are probably the most difficult ‘side’ of the fire triangle to remove, so it’s wise to store them appropriately to prevent them from becoming a fire hazard.
3. Oxygen
To sustain the combustion reaction, oxygen (or an oxidizing agent) is needed, as it reacts with the burning fuel to release heat and CO2. Earth’s atmosphere consists of 21% oxygen, so there is plenty available to trigger a fire if the other two components are present.
Fire blankets and certain fire extinguishers remove the oxygen ‘side’ of the triangle by removing it or displacing it, causing suffocation and thereby ceasing the combustion reaction.
Fire Facts
Fire normally generates three different types of hazards: heat, oxygen depletion and smoke.
Heat: The most obvious hazard is heat. Although the majority of fire deaths are caused by smoke, many deaths and severe injuries are caused by burns. When the temperature of skin reaches 133 degrees, it’s associated with pain.
A room fire can range from 212 degrees at floor level to 2192 degrees at the ceiling.
As temperatures rise above 260 degrees, skin will burn with permanent injuries, and if extremely hot air is inhaled it can scorch internal organs.
Oxygen Depletion: A decrease in the partial pressure of oxygen (PO2) can cause serious harm to the brain. As fire roars and grows, it consumes enormous amounts of Oxygen. I.e. a small fire in a normal house will normally consume all the oxygen inside the home in under 30 seconds.
The brain needs a constant supply of oxygen. If the oxygen intake is interrupted for more than 3 minutes the brain can suffer irreversible damage.
Smoke: Smoke is all the airborne products of the pyrolysis and combustion of materials, and it can be very toxic. It’s particles, gases such as carbon monoxide, volatilized organic molecules, aerosols and free radicals are deadly when inhaled.
A fire often gives off a dark, thick smoke. In a serious fire, it can be hard to see what’s ahead and where you’re going. Breathing in even the smallest amount of the toxic smoke can disorient a person quickly, causing them to pass out.
How to prevent and stop the spread of fires
Heat: Heat usually originates from people smoking, equipment malfunctions, hot work, neglected electrical, or arson. Heat is occasionally inevitable; in which case it is important that this heat is kept away from fuel or that it’s being carefully managed under a controlled environment.
Fuel: As previously mentioned, flammable materials (fuel) are materials that burn readily in a normal atmosphere. It’s important that all these flammable materials are identified and that appropriate measures are taken to control them. You need to store these materials appropriately, and in appropriate quantities, furthermore you need to maintain good housekeeping. For extra volatile flammable materials, make sure that these are stored with extra precautions.
Oxygen: The air we breathe contains ~21% of oxygen. With just a small increase to say 23% oxygen – a fire will burn hotter and more fiercely. With just a little more oxygen in the air, a fire can become almost impossible to put out.
If you’ve got any equipment that could be leaking oxygen, you need to ensure that these are safeguarded properly.
Fire Extinguishers: Choosing a fire extinguisher is often easier said than done, especially if you’re working in an environment where the source of the fire could be a range of different things.
Nonetheless, it is extremely important to be able to choose the right fire extinguisher, or you can end up getting seriously hurt.
Classifications of Fires and Extinguishers
Class A
Class A fires involve ordinary combustible materials, such as cloth, wood, paper, rubber, and many plastics. Extinguishers with an A rating are designed to extinguish fires involving these ordinary combustible materials.
Class B
Class B fires involve flammable and combustible liquids such as gasoline, alcohol, oil-based paints, lacquers. Therefore, extinguishers with a B rating are designed to extinguish fires involving flammable and combustible liquids.
Note: Do not attempt to extinguish a fire involving flammable gas unless there is reasonable assurance the source of fuel can be promptly shut off. In fact, if the only fuel burning is the leaking gas, the best method for extinguishing the fire is to shut off the fuel supply. Extinguishing a flammable gas fire, without shutting off the fuel, will allow unburned gas to escape into the atmosphere, which may permit a dangerous accumulation of gas to develop, and an explosion may occur if the gas is exposed to an ignition source.
Class C
Class C fires involve energized electrical equipment. Extinguishers with a C rating are designed for use with fires involving energized electrical equipment.
Class D
Class D fires involve combustible metals, such as magnesium, titanium, and sodium. Extinguishers with a D rating are designed to extinguish fires involving combustible metals.
Note: Common extinguishing agents may react with a combustible metal fire causing the severity of the fire to increase. The most common method for extinguishing a combustible metal fire is to cover the burning material with a dry powder, such as sand, which will not react with the material.
Class K
Extinguishers with a K rating are designed to extinguish fires involving vegetable oils, animal oils, or fats utilized in commercial cooking appliances.
Note: Extinguishers with a K rating are normally required where deep-fryers and/or griddles are utilized to prepare large quantities of food. An example would be a commercial kitchen similar to those found in restaurants and cafeterias.
Multipurpose Extinguishers
Most portable extinguishers are rated for use with more than one classification of fire. For example, an extinguisher with a BC rating is suitable for use with fires involving flammable liquids and energized electrical equipment. An extinguisher with an ABC rating is suitable for use with fires involving ordinary combustibles, flammable liquids, and energized electrical equipment. An extinguisher that is rated for use with multiple hazards should include a symbol for each hazard type.
Use of Extinguishers
In the event of a fire, one employee should get the nearest fire extinguisher and use it to attempt to put the fire out. All other employees in the immediate area should prepare to evacuate if needed. All other employees in the building need to be advised that a fire is in progress.
The employee attempting to extinguish the fire will break the safety seal on the handle and pull the pin. He will then aim his extinguisher at the base of the fire and discharge it with a sweeping motion from side to side; continuing until the fire is out or the extinguisher is emptied. It is a good idea to turn the fire extinguisher upside down a couple times before actuating it. This will loosen any settled agent make the extinguisher last longer.
Remember that a standard fire extinguisher will be emptied in about 10 to 15 seconds. Multiple extinguishers can be used to put out a small fire. If extinguishers are not able to put out the fire or they serve to spread the fire, all employees must evacuate the area and let fire fighters manage the fire.