Fire Safety Awareness : Chemistry of Fire

Dear Friends,

Due to Lack of Fire Safety Awareness in General Public, Whenever accidental fire takes place, we suffer immense economical as well emotional losses. Economic losses can be recovering as time passes but overcoming the emotional losses is a tough job and some time they are of permanent nature. One can reestablish their business but cannot bring back the life of their dearest one who loses his life in an accidental fire incident. Our aim is to train, educate and raise awareness about health and safety among the most vulnerable people within the community in order to reduce the suffering caused by fires and other emergencies in order to minimize the risks associated with fire. This website aims to mitigate the risk of fire leading to loss of life, property and damage. One important objective of this website is to teach the public to become self-sufficient in saving their own lives and assisting others in emergency cases. Let’s Start our mission fire safety Awareness with discussing the basic chemistry of fire, Important fuel properties, Classification of fire and Behaviour of different fuel in combustion.

What is Fire?

Chemistry of Fire :

Fire is a chemical process/reaction. In this reaction, Reactants are fuel, heat and oxygen. And Products are heat, light, smoke and other toxic gases. In other words, Fire/Combustion is a chemical process caused by the combination of one or more substances with oxygen. One should know following things while understanding fire

  • In order to start this chemical process, normally an initial source of heat is required
  • During the process, fuel and oxygen are consumed and as a result heat, light, smoke and toxic gases or combination of these are released.
  • A fire can be started and sustained only when fuel and oxygen are available in the correct ratio.
  • Some solids can burn directly by glowing combustion or smouldering but in the flaming combustion of solids and liquid fuels, vaporization takes place before burning. This is well illustrated with below wood slab example.

For Example, The wood slab is initially heated by the external source of heat. As its surface temperature approaches the boiling point of water, gases (steam) slowly evolve from wood. As slab temperature increases above the boiling point of water, the drying process penetrates deeper into the wood interior and with continuing heating, the surface colour begins to discolour at 3000C  temperature. Then the wood slab releases combustible gases while leaving a residue (black carbonaceous – char). After active decomposition of matter, combustible gases evolve rapidly enough to support gas phase combustion

FUEL PROPERTIES : 

While understanding the fire, it is important to know the basic properties of fuels. By knowing the properties of fuel one can recognize the risk associated and help to prevent accidental fires.

  • Fire Point: The lowest temperature at which liquid fuel produces sufficient flammable vapour to produce sustained combustion after the removal of the pilot source
  • Flash Point: The lowest temperature at which liquid fuel produces enough vapours to produce a flash through the application of a small ignition source near the surface of the liquids.
  • Ignition Temperature: The temperature to which solid fuel has to be heated to initiate sustained combustion, once ignited
  • Auto Ignition Temperature: The temperature at which the heat evolved by a material decomposing under the influence of heat is sufficient to bring about combustion without application of an external source of ignition.
  • Flammable Limits: The flammability limits are the minimum and maximum concentration of a gas or vapour in air which can be ignited and sustain a self-propagating flame.
  • Flammable Range: The range of the flammable vapour or gas/air mixture between the lower and upper flammable limits. When the mixture temperature is increased, the flammability ranges widens; when the mixture temperature is decreased, the range narrows
  • Combustible Material: A product is called combustible when its flash point is 380C or higher
  • Flammable Material: A product is called Flammable when its flash point is less than 380C. 

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CLASSIFICATION OF FIRE :

There is two type of classifications available. One is European Fire Classification and Other is US Fire Classification. In India, we follow European Fire Classification

  • CLASSIFICATION OF FIRE (European Classification)

A-Fires involving ordinary combustible materials

B-Fires in flammable/combustible liquids,    varnishes, paints etc.

C-Fires involving gases

D-Fire involving metals

E-Fire involving live electrical appliances

  • US Classification

A-Fires involving carbonaceous substances

B-Fires in flammable/combustible liquids and gases

C-Fire involving metals

D-Fire involving the live electrical appliances

BEHAVIOUR OF FUEL IN COMBUSTION :

Liquids: A combustible liquid does not directly combine with oxygen when burning because a vapour has to be produced at the surface of the product before it will burn. Some flammable liquid i.e.Naphtha, Petrol etc. vapourise well below ambient temperature and concentration may be such that vapour air mixture can be readily ignited by a heat source without being heated initially. The temperature of burning liquid will increase gradually because of the heat radiation from the fire and volume of vapour produced is directly related to the surface area of the liquid. After reaching the boiling point temperature of the fuel, if more heat is passed to fuel, the temperature will not rise but more liquid will be evaporated. Flammable vapours will burn only when the concentration of the vapours is within the specific minimum and maximum limits with air (Lower Flammable Limit / Upper Flammable Limit).

Gases: Gases are already in the vapour phase and ignited in one single stage without preliminary warming up. Some gases are heavier than air, others are lighter. The lighter ones will rise into the atmosphere and after having reached the lower flammability limits can longer be ignited. During the time that the gas concentration is between the upper and lower flammable limits, a spark or tiny flame is sufficient to ignite the vapour.

Solids: Solids need an external source of heat to raise the temperature at which they will produce the flammable vapours and few solid materials (Celluloid) can ever burn in absence of air. Solids produce vapours due to its decomposition during the initial phase of warming-up, which create smoke. The fire condition will be reached at a much higher temperature and in this case, a fire condition can be avoided by timely removing the heat source.

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