January 2, 2006

Fire

Posted in Misc at 19:00 by graham

I went on a basic fire fighting course a long time ago. Here is a write up of the course notes:

Fire is a chemical reaction called combustion (usually oxidation) resulting in the release of heat and light. To initiate and maintain this chemical reaction – i.e. for the outbreak of fire to occur and continue – three elements are required, and the removal of any of these three will extinguish the fire:

  • Fuel: Any combustible substance either solid, liquid or gas. Starving a fire will extinguish it.

  • Heat: The attainment of a certain temperature (the ignition point). Once a fire has started it normally maintains its own heat supply. Cooling a fire will extinguish it.

  • Oxygen: This is usually in plentiful supply as it makes up one fifth of the air we breathe. Blanketing or smothering a fire will extinguish it.

    If the rate of heat generation is less than the rate of heat dissipation, combustion cannot continue. For instance if a match is applied to a block of wood, the heat from the flame is absorbed by the mass of the wood and the heat is insufficient to raise the whole block to its ignition temperature. If the block is reduced to shavings, the surface area of a single shaving is high in relation to its weight, and it will easily catch fire. Gases and flammable vapors are extremely dangerous because of their large surface areas. Water is normally used for cooling a fire as it has a great capacity for absorbing heat, and it is cheap and readily available.

Types of fire fighting equipment

Portable fire extinguishers are ‘first-aid’ fire fighting equipment; they have a limited duration of discharge, and are only for tackling small fires.

Water extinguishers (Red)

The most common type of extinguisher. For use on fires involving combustible materials, such as wood, paper, textiles and fabrics. They remove the heat from the fire by cooling. They should not be used on electrical equipment. To use:

  • Remove the safety pin or cap
  • Operate by squeezing the trigger of the grip mechanism or by striking the plunger
  • Direct the jet of water at the base of the flames
  • Keep moving the jet across the area of the fire in a sweeping motion
  • Only tackle small, minor fires

Carbon dioxide extinguishers (Black)

Best suited to fires in electrical equipment, but will also cope very effectively with flammable liquids. The extinguisher delivers a high concentration of carbon dioxide gas under pressure, producing inert vapor which excludes oxygen and smothers the fire.

The mechanism to operate is similar to that for trigger operated water extinguishers.

Dry powder extinguishers (Blue)

Best suited to larger flammable liquid fires, but can also be used on electrical fires. Powder is expelled from the extinguisher by means of gas pressure. Dry powder is very effective as a knock-down agent for flammable liquid fires (it is used by the fire service in road traffic accidents). However it is also very messy and can damage electrical equipment such as motors.

BCF or Vaporising Liquid extinguishers (Green)

Suitable for us on flammable liquid fires and on electrical fires. The liquid is stored in the extinguisher under nitrogen pressure. When it is expelled, it is vaporised by the heat of the fire producing a smothering effect by reducing the oxygen content. The vaporised liquid also interacts with the process of combustion chemically and this helps extinguish the fire.

Fire fighting

Most fires begin in a very small way and can be put out easily if some form of extinguisher is nearby and there is someone who knows how to operate it. However discretion is essential in deciding the lengths to which ‘first-aid’ fire fighting is carried out. If you do decide to tackle a minor fire follow these rules:

  • Take up a position where access to the fire is unrestricted but where a quick and safe retreat is possible. Outside, this means being up-wind of the fire.
  • Crouching will help the fire fighter to keep clear of smoke and avoid heat. It will also allow a closer approach to the fire.
  • Always ensure that the fire is completely extinguished and not liable to reignite or continue smoldering.
  • Once used an extinguisher should be sent to be recharged.

Spread of fire

Convection is the transport of heat by movement of the heated substance. Over four-fifths of the heat of the fire is carried away by air and other gases in this way. By being heated the air becomes less dense than the surrounding atmosphere and, mixed with gases produced by the fire, moves upwards forming convection currents which carry away with them heat and smoke. The temperature of this rising air is likely to be very high.

Radiation: Objects in the neighborhood of a fire are exposed directly to the radiant heat from its flames and burning fuel. The nearer these are to the fire the greater the intensity of the radiated heat reaching them and the more likely they are of heating to ignition point. This is what happens when clothes drying in front of a fire ignite.

Conduction: Although some metals such as steel will stand up to great heat without igniting, their presence, e.g. in girders or partitions, in a burning structure will not necessarily check a fire. Metal is a very good conductor of heat throughout its length and may cause combustible materials at its other end to smolder until they reach their ignition point. A metal door becoming heated may ignite materials in contact with its other side.

Concluding remarks

At work, at home, on holiday, in any building you occupy for longer than a few hours, ask yourself these questions:

  1. Where is my nearest fire alarm and fire extinguisher ?
  2. Do I know how to operate, and the use of, all the fire fighting equipment ?
  3. Do I know where the nearest fire exit and my assembly point is ?
  4. What action do I take on discovering a fire or hearing the fire alarm bell ?
  5. Do I know the evacuation procedure to follow ?

1 Comment »

  1. denley said,

    May 29, 2006 at 04:50

    I’m love this great website. Many thanks guy

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