Module 2 – Fire Chemistry

What is Fire?

Fire is a by-product of combustion, and combustion is a result of rapid oxidation.

Triangle of Fire

  1. Fuel Removed “Starving”

  2. Oxygen Removed “Smothering”

  3. Heat Removed “Cooling”

Fire Triangle

  • Fuel can be solid, liquid or gas. Only gases burn. The heat turns solids and liquids into gas.
  • Oxygen in the air, and oxygen in other materials, drives the process of oxidation.
  • The normal percentage of oxygen in the atmosphere is 21%. If the oxygen level drops to 15% or below, then it will not support combustion.
  • A spark or flame is all it takes. It can be a self-heating or chemical heat. The heat causes the vaporisation.

Ignition Sources

  • Electric current
  • Naked flames
  • Sunlight (solar)
  • Sparks
  • Chemicals friction (mechanical)
  • Nuclear static electricity
  • Hot surface

Conduction – The Spread of Fire

  • Direct Contact
  • Fire can spread by pipes connecting different areas

Convection - The Spread of Fire

  • Heat transferred by movement of air or liquid
  • Fire spreads up buildings by heated air

Radiation - The Spread of Fire

  • Infrared
  • Can travel across open spaces
  • Creates self-sustaining fire

Sources of Ignition

  • Chemical – Reaction giving off heat i.e. hay, oily rags
  • Electrical – Short circuit, overload circuit, static, heaters
  • Mechanical – Mechanical breaks, drive belt rubbing off a guard

Fuel

Substances which can provide the “FUEL” for Fire / Explosion

  • Flammable gases
  • Flammable liquids
  • Flammable solids
  • Spontaneously combustible substances
  • “Dangerous when wet” substances
  • Organic peroxide

Flammable Liquids

Broadly divided into two categories:

  • Liquids fuels – Most derived from petroleum e.g. petrol, kerosene
  • Solvents and raw materials – Used in the manufacture of other chemicals. e.g. methanol, ethanol, acetone and toluene

Definitions

  • Flash point – Temperature at which a liquid gives off sufficient vapour to flash momentarily when ignited by a flame or spark
  • Fire point – When liquid is heated past it’s flash point, it will reach a temperature where sufficient vapour is given off to maintain combustion
  • Ignition temperature – The temperature at which the molecules of a substance are able to combine with the oxygen molecules in the air

Sources of Heat

  • Cookers
  • Heaters
  • Fireplace
  • Damaged wiring

Sources of Fuel

  • Flammable liquids
  • Clothing
  • Furniture
  • Curtains
  • People

Development of a Fire

Backdraft - Introduction of Oxygen

Fire Safety

Fire safety is based upon the principle of keeping fuel and ignition sources apart.

HUMAN CARELESSNESS IS A MAJOR CAUSE OF FIRE!

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