Fire Safety Awareness : Behaviour of Fuels In Combution

In last blog “Fire Safety Awareness: Basic Chemistry of fire”, I have discussed basic fire chemistry, Basic fuel properties and Classification of fire. In this blog, I am going to discuss important thing  Before going to the topic of ‘fire prevention’ and ‘fire protection’. These two important things are

  • The behaviour of fuel in combustion and
  •  Basic Flammability Principles

The Behaviour of Fuel In Combustion:

To understand the fire and fire hazard, it is of import that one has to know the behaviour of particular fuel during its combustion process. Let’s start with the liquid fuel

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.Naptha, Petrol 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 specified 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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Basic Flammability Principles:

The science of fire protection rests upon the following principles:

  • An oxidizing agent, a combustible material and an ignition source is essential for combustion.
  • The combustible material must be heated to its piloted ignition temperature before it can be ignited or support flame spread.
  • The subsequent burning of a combustible is governed by the heat feedback from the flames to pyrolyzing or vaporizing combustible.
  • The burning will continue until:
  1. Combustible material is consumed; or
  2. Oxidizing agent concentration is lowered to below the concentration necessary to support combustion; or
  3. Sufficient heat is removed or prevented from reaching the combustible material to prevent further fuel paralysis; or
  4. The flames are chemically inhibited or sufficiently cooled to prevent further reaction.

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