FIRE - Anotomy and its classification

Fire is an exothermic chemical reaction in which combustible material combines with oxygen in the atmosphere to give out heat and flame. But once fire has begun, the heat produced may often be sufficient to support or even accelerate the process. The materials involved in the combustion process may be in solid, liquid or gaseous state. They are also identified as two or three-dimensional

Two-dimensional fires are those where fire and fuel are on a single plane or flat surface. Ground fire or trench fire fall in this group of fires.Three - dimensional fires are complicated fires of falling liquid streams or fuel under pressure escaping from a container. Fire resulting from leak of transformer oil falls under this category.
Fire is not happen by their own self. It require some elements,Four factors/ Elements must be required in order before combustion occurs.

Heat-Sufficient to raise the temperature to ignition point Oxygen at sufficient quantity FuelChemical Chain reaction of above three mixtures.
These Four elements may be represented in the form of a Tetrahedron.

  Just as removal of one or more sides of a tetrahedron causes its breakdown, the removal of one or more factors causes the fire to cease.

Stages of Fire
The Fire Triangle/Fire Tetrahedron

Except explosion, most fires have humble starting and grow through the following four stages of combustions:

Incipient stage: At this stage decomposition is occurring at the surface of the fuel due to the influence of some form of heat. Products of combustion given off at this stage are invisible to the eye.
Smouldering Stage: At this Stage, up to 10% of the decomposing products released at the surface of the fuel are visible.
Flaming Stage: Vapours from the decomposing fuel have ignited and are at the stage where flames are self-propagating.
Heat stage: At this stage the burning has progressed to the point where the fire is still small but generating sufficient heat to warm the air immediately around the fire sending warm products of combustion upwards by convection. The above principle is used for the selection of fire detectors.

Classification of Fire as per Bureau of Indian Standards (BIS)
Combustible substance can be present in three states - solid, liquid and gaseous state. The burning of most materials produces a flame, this occurs when vapour or gas given off by the liquid or solid materials is ignited. Depending upon the nature and state of the burning material, fires are generally classified into A, B, C or D type.

Class - A: Fire
These are fires involving solid materials, normally of carbon compounds in which combustion.
Generally occurs with the formation of glowing embers. These types of fires are most common.Examples of Class A - fires are fires involving wood, rags, paper, plastic coal etc.

Class - B: Fire
These fires involve flammable liquids or liquefiable solids. That is, fires occurring in vapour-air mixture over the surface of flammable liquids such as gasoline, oils, paints & thinners. These types of fires occur in storage tanks and chemical storages.

Class - C: Fire
These are fires involving liquefiable gases like propane, butane, LPG, H2, etc.

Class - D: Fire
These are fires that occur in combustible metals such as magnesium, Titanium, Sodium etc. For Controlling such fires, special extinguishing agents/equipment and techniques are to be used.