Triangle of Fire

Before we can understand the basics of practical fire fighting, first we must understand what combustion is and what factors are required to be present in order to achieve combustion.

Combustion can be defined simply as being “A Chemical Re-action Evolving both Heat and Light Energy”

The three factors required to be present in order to achieve combustion are;

  • FUEL
  • HEAT
  • OXYGEN
fire triangle

FUEL

All matter exists in one of three states – solids, liquids or gaseous form and under the correct conditions almost anything will burn as fuel.

The burning rate of a fuel depends on configuration or state of division. Finely divided fuels such as dust, powders or shavings will absorb heat more than bulky materials because of the greater area exposed to heat and therefore will liberate flammable vapours more quickly and so burning more readily.

Flammable liquids release vapour in much the same way as fuel. The rate of release is greater for liquids than solids since liquids has less closely packed molecules and will vaporise more readily. The ease or degree of vaporisation will depend on the product, for example petrol will vaporise more rapidly than fuel oils which in turn will release vapours more readily than lubricating oil.

Flammable gasses are already in the required vapour state. Only the proper inter-mix with oxygen and heat are required to achieve ignition.
A flammable gas must mix with oxygen within its flammable range otherwise ignition will not take place.

Heat

Heat is required to act on a fuel in order to commence the chemical re-action which will produce the flammable vapours required for combustion. The amount of heat required to raise a substance to its ignition temperature will vary depending on the substance involved.

Oxygen

Oxygen is a supporter of combustion an must be present before combustion can be achieved. Normally a 16 – 20% concentrate of oxygen is required to support combustion. But there are products which when subjected to heat and the subsequent chemical decomposition will liberate its own oxygen supply.

When all three factors are present, Fuel, Heat and Oxygen in the correct proportions , we will have combustion, this is known as the TRIANGLE OF FIRE. Take away one of these factors and you will extinguish the fire.

Temperatures

There are three critical temperatures we must be aware of if we are to understand and control the fire fighting operations.

  • FLASH POINT
  • FIRE POINT
  • SPONTANEOUS IGNITION TEMPERATURE

FLASH POINT

Can be determined as being the lowest temperature at which there is sufficient vaporisation of the substance to produce a vapour which will flash momentarily on application of a test flame.

FIRE POINT

Can be defined as being the lowest temperature at which heat from the combustion of a burning vapour is capable of producing sufficient? vapour to enable combustion to continue. Once this temperature is reached then we have reached a point of accelerating combustion and the fire will rapidly grow in intensity.

SPONTANEOUS IGNITION TEMPERATURE {S.I.G}

Can simply defined as being the lowest temperature at which the substance will ignite spontaneously.

Spontaneous Combustion can take place in certain organic materials based on carbon which will react with oxygen

FIRE SPREAD

Having achieved combustion by all three legs of the triangle we now have to deal with FIRE SPREAD.

This can be achieved by one or more of the following

  • RADIATION
  • CONVECTION
  • CONDUCTION

RADIATION

Heat radiation is the transfer of heat from one source to another across an intervening space, no material substance being involved. When heat comes into contact with a body it is absorbed , reflected or transmitted. Absorbed heat raises the temperature of the absorbing body and if this? continues may result with fire spread some distance away from the fire source.

CONVECTION

Convection of heat through the motion of heated matter, i.e. through the motion of smoke, air gasses etc, produced by fire. The fire produces lighter than air gasses that will rise towards the higher part of the building. As these hot gasses rise then the cool gasses and air will fall, so feeding the fire with a convection draught cycle and also risk spreading the fire at higher levels.

CONDUCTION

Conduction is the transfer of heat through a solid body. Metal is an excellent conductor, and heat transfer by conduction is a real hazard.

With timely and careful application of a water spray to affected areas then the heat transfer can be greatly retarded.

EXTINCTION OF FIRE

Having created a situation where we now have a fire situation on hand, the question raised is how it best can be extinguished.

This can be achieved by attacking the Triangle of combustion or breaking the chain reaction? known as the FIRE TETRAHEDRON.

Remove one or more of the legs of the Fire Triangle or break the chain reaction sequence and the fire will be extinguished.

COOLING

This method is the cheapest and most commonly used method of Fire extinguishment. The base of the fire is attacked with water to destroy the ability of the fire to sustain itself. Water is a very effective heat absorber and when applied properly it absorbs the heat from the fuel and will cool the burning substance below its critical fire temperature so to reduce the amount of flammable vapours given off to sustain combustion.

SMOTHERING

The exclusion of oxygen from a fire will bring about its extinction.
This can be achieved by;

  • Use of inert gasses systems
  • Use of fixed Installation systems
  • Carbon Dioxide
  • Foam
  • Fire Blankets

Care must be taken as smothering a fire produces little or no cooling effect and if the fire is opened up too early flash over conditions may occur with the inrush of oxygen.

By smothering we effectively break the fire triangle of fire by excluding Oxygen from the fire.

STARVATION

Removal of a fuel from a fire in any situation will lead to the extinction of a fire.

By starvation we effectively beak the fire triangle by starving the fire of fuel.

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