BASIC FIRE SCIENCE

BASIC FIRE SCIENCE

A.   FIRE TRIANGLE

Four things must be present at the same time in order to produce fire:

 
1.    Enough oxygen (O2) to sustain combustion,
·         Oxygen (O2) is an element from the atmospheric air that supports the combustion process.  It makes up 21% of the total air mixture.
2.    Enough heat to raise the material to its ignition temperature,
·         Heat is a form of energy which may be produced by mechanical or chemical reaction and transmitted via radiation, conduction and/or convection.
3.    Some sort of fuel or combustible material,  
·         Fuels are combustibles that may exist in solid, liquid, molten and/or gaseous states. Examples of fuels are paper products, wood products, molten chemicals, flammable fluids and gases.
4.    The chemical, exothermic reaction known as fire.

Fire occurs when fuel and oxygen come into contact with substantial amount of heat energy. This will cause a chain reaction to take place which will in turn produce sufficient heat energy to propagate the burning in the presence of subsequent fuel and oxygen.

Oxygen(O2), heat, and fuel are frequently referred to as the "fire triangle." Add in the fourth element, the chemical reaction, and we actually have a fire "tetrahedron."

Combustion will continue as long as all these factors are present. Removal of one of the factors will lead to the collapse of the "triangle/ tetrahedron" and combustion stops, and we will not have a fire or the fire will be extinguished.

B.   METHODS OF EXTINGUISHING FIRE

Fire Safety, at its most basic, is based upon the principle of keeping fuel sources and ignition sources separate.

Fire can be extinguished with one or a combination of the following methods:

 
1.    Starvation: Removal of the combustible substances.
 
2.    Smothering: Limiting the amount of oxygen (O2) that supports combustion.
 
3.    Cooling: Reduce the temperature to the point of fire extinction.
 
4.    Breaking (blocking) or interfering with the chemical reaction occurring in the flame.
  

C.   BASIC STAGES OF FIRE DEVELOPMENT

Fir
Fires can be commonly seen to develop in the following stages:
1.    Initial
The fire develops from its point of origin. If the combustion reaction is capable of emitting further heat in the presence of subsequent fuel and oxygen, the reaction will propagate and temperature will continue to rise.
 
2.    Flashover
This is the transition stage between the Initial and the full development stage. The fire spreads rapidly and large merged flames are produced. Flashover will especially occur in a confined space fire.
 
3.    Full
Fire will be fully developed burning at/near its development maximum intensity.
 
4.    Decay
Fire is slowing down due to depletion of Oxygen and/or fuel. Introduction of subsequent Fuel and/or Oxygen will however propagate a second initial development.

D.   CLASSIFICATION OF FIRE

The different types of fire are classified into the following "Classes":
 
1.    Class A : For fires involving the burning of ordinary combustible materials like wood, paper, cloth, furnishing, plastics and rubber.
 
2.    Class B : For fires involving flammable liquids, solvents, oils, paints, thinner and flammable gases.
 
3.    Class C : For fires involving live electrical equipment like electrical mains, transformers and electrical appliances, etc and where extinguishing medium used must be electrically non-conductive. (If the electrical equipment is de-energised, extinguishers for Classes A and B can be used.)
 
4.    Class D : For fires involving combustible metals, eg. Potassium (K), magnesium(Mg), titanium(Ti), sodium(Na), lithium(Li) and zirconium.

 E.   IGNITION AND COMBUSTION

1.    IGNITION
 
i.      Ignition is the process of starting a combustion process through the input of energy.  
 
ii.     If the substance is a liquid, ignition can only occur when
·         There exists a gas/air mixture whose composition lies within the flammable range for that substance. A small ignition source, e.g. a spark or small flame, will then start ignition.
·         Enough vapour is given off the vapour surface so that a vapour/air mixture is formed whose properties lie within the flammable range for that substance. The flash point is the temperature at which the liquid gives off just enough vapour to form this flammable vapour/air mixture. For temperatures above the flash point a small igniting source will start ignition.
 
iii.    If the substance is a solid, ignition will only start when either:
·         The solid is heated so that destructive decomposition starts and flammable vapours are given off. When these vapours form a flammable vapour/air mixture combustion can be started if a small flame is applied. Most organic materials will come under third category. The size of ignition source needed will depend on the physical form and shape of the material. If it is in the form which has a high surface area to mass ratio only a small igniting source is required.
·         The solid is heated in air so that the surface oxidation is initiated of sufficient vigour to be self-sustaining reaction. Carbon and most metals are examples f this type of combustion. A high surface area to mass ratio is needed for ignition and this means that the metal has to be in he form of finely divided powder. An intense source of heat is required in most cases to ignite solids which burn this way.
 
 
2.    COMBUSTION
 
i.      Combustion is a reaction between a substance and oxygen in which heat is given out, i.e. it is an exothermic reaction, and enough heat must be given out either to maintain the temperature of the reactants or to continuously increase their temperatures so setting up a chain reaction.
 
ii.     In some cases the reaction between a substance and oxygen requires heat to be applied to maintain the reaction and the reaction stops when the applied heat is removed, this is not combustion.
 
iii.    All combustible materials burn as vapour phase reactions, i.e. they are reactions between two gases, one of which is usually oxygen. The combustible materials which are usually solid will, because of high temperatures involved, either melt and then vapourise and burn, or decompose by pyrolysis and give off flammable vapours which will then burn. There are exceptions and there can be reactions between oxygen and solid surfaces which are vigorous and exothermic and so can be termed combustion, e.g. carbon and certain metals. Liquids also vapourise before combustion takes place.
 
iv.   The following are the products of a combustion process:
 
Thermal           -           Flame & Heat
 
Non-Thermal   -           Smoke & Gases
 

·         Damage to property is due mainly to thermal effects although smoke and gases can also cause considerable damage. Some of the gases produced during combustion are corrosive, or become corrosive in solution with water.
 
·         Most fire deaths or injuries, however, may be attributed to smoke or gases arising from smoke inhalation and suffocation. Smoke can also cause panic among occupants which is likely to cause further injuries and death. Heat and flamers are primary causes of fewer deaths or injuries than smoke or gases.
 
·         The heat generated in the process of combustion can completely destroy a building. Steel will have lost two-thirds of its strength by the time it has been heated to 600 ° C. This is by no means uncommon temperature in a domestic fire. Concrete is more resistant material; but as reinforced concrete depends on steel for its tensile strength, there needs to be sufficient insulation of the steel to prevent it reaching its critical temperature. Timber, of course, burns but is a very good structural material as burning occurs at a fairly constant rate and so structural timbers can be oversized to provide a known measure of fire resistance. Bricks provide one of the best fire-resistant materials as they have already been kiln-fired at high temperature during manufacture.