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What is fire and when does it occur? |
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Fire is a self sustaining chemical reaction & normally occurs when following three elements are present. |
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A) Heat B) Fuel C) Oxygen |
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In the presence of all three of these elements, a fire can start. Fire is traditionally is a self-sustaining reaction, hence the rate of combustion increases 8 to 10 times per minute. After 10-15minutes of starting, the fire is too large for the use of portable extinguishers. That is why portable appliances are first aid fire extinguishing devices to be used in the incipient stages of a fire. Fire is traditionally represented as a triangle with heat, fuel & oxygen as it's arms. Hence removal of one of the elements of the triangle causes the combustion process to collapse and thus extinguish the fire. |
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How can fire be extinguished ? |
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The process of extinguishing can be as follows: |
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A) REMOVE HEAT : Cooling by applying heat absorbing agent e.g. water. |
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B) REMOVE FUEL : Closing the valve in leaking Hydrocarbon or gas pipe line. |
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C) REMOVE OXYGEN : Smothering by using suitable agent e.g. foam or carbon dioxide. |
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In addition to above inhibition of the combustion reaction chemical inhibition of the combustion chain reaction will also cause the fire to cease. |
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D) STOP THE REACTION : By using Dry Chemical powder or Halon gas. |
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CLASSIFICATION OF HAZARD |
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THE CLASSES OF FIRE: |
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a) Fires involving Carbonaceous solid combustibles |
"A" Eg. Wood, cloth, paper, rubber, plastic |
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b) Fires involving Hydrocarbon liquids |
"B" Eg. Petrol, diesel, kerosene |
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c) Fires involving Gaseous fuel |
"C" Eg.
Acetylene, LPG |
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d) Fires involving Electrical short circuits |
"E" Eg. Naked wiring, over-loading leading
to short circuits & combustibles in the vicinity, catching fire. |
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CLASSIFICATION OF FIRE |
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a) Surface Fire: Fire generally involving solid materials in which flaming take place on the surface of the fuel e.g. loose paper. |
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b) Deep Seated: fire involving class A materials in which shouldering combustion has taken place long before flaming e.g. computer panel Cards, logs of wood. |
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c) Spill or pool fire: Fire involving class B liquid spills, usually on the ground to form a pool. |
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PRINCIPAL CAUSES OF FIRE: |
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Fire
can be principally caused by: |
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a) |
Arson |
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b) |
Electric sparks/Short circuits |
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c) |
Over heating of electrical equipment |
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d) |
Gas
Cutting & Welding |
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e) |
Smoking |
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f) |
Naked wiring |
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g) |
Rubbish bumming |
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h) |
Bad house keeping in industries |
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i) |
Spontaneous
Combustion |
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(A) Water: |
Water is the best and cheapest method to extinguish "A" class fires. Plenty of water is readily available. A good stream of water which is generally recommended for Class "A" fires penetrates and extinguishes even deep-seated fires. Water, if used in the form of High Velocity Water Spray, can be used to extinguish oil fires also e.g. transformer fires. |
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(B) Foam: |
Foam is an aggregate of gas filled bubbles formed from aqueous solution and is lighter than the lightest flammable liquid. It forms a coherent blanket on the surface of burning liquid and inhibits generation of flam-mable vapours from the surface. It also cuts off the access of combustible vapour to air (oxygen), thus smothering flames. The water contents of the foam cools the burning fuel surface by absorbing the heat. |
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(C) Dry Chemical Powder: |
There are different types of Dry Chemical Powders used to extinguish different classes of fires. The extinguishments of fire is based on the chain breaking mechanism of the fire. It is based on the theory that free radicals are present in the combustion zone and that the reaction of the particles with each other are necessary for continued burning. The discharge of dry chemical into the flames prevents reactive particles form coming together and continuing the combustion chain reaction. Discharge of dry chemical products also produces a cloud powder between the flame and the fuel which shields the fuel from some of the heat radiated by the flames. |
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(D) Carbon-Dioxide: |
Carbon-dioxide has a number of properties that make it disable fire extinguishing agent. It is non-combustible and it does not react with most substances. Also since carbon-dioxide is a gas it can penetrate and spread to all parts of the fire area. It leaves no residue thus eliminating clean up due to agent itself. Since it is heavier than air it displaces air from the burning surface and settles below the air thus preventing oxygen supply to the fire. |
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