CRITERIA 1.1 – The causes of fires are identified in accordance with industry practice

Conduction

Put a metal spoon into a cup of hot tea or coffee. Notice that after the spoon has been in the cup for a short time that the teaspoon gets hot. The spoon gets hot because the heat is conducted from the hot water to the spoon. A pan in the oven is also heated by conduction.

What would happen if one end of a big metal beam was placed in a fire in the workplace? Heat would be conducted from the fire along the beam to the other end of the beam. You would not be able to touch the other end of the beam because it would be too hot. The heat has been transferred from molecule to molecule, from the flame, through the bar, to the hand, the transfer being through direct contact.

Conduction then is the transmission of heat from a hot material to a cold material by molecular vibration.

Cavemen boiled water by heating stones in the fire and dropping them into a container of water.  Today, we apply heat to the steel tubes of a boiler to heat up the water contained in the tubes.

Conduction may occur in solids, liquids or gasses, although it is most clearly present in solids.  The ability to conduct heat varies between materials.

Aluminium has about half the thermal conductivity of silver, while iron has about one eight.

Radiation

The sun shines and warms us every day. Heat from the sun reaches the people on earth through radiation. A fire will also warm you by radiation. Now if this heat gets too much, you can burn yourself. In the same way, a fire can give off enough heat to start another fire.

On a cold winters evening a fire is made in an open fireplace.  The flame is not touched, so there is no conductor.  The air has not had time to become warm, also bear in mind that the air moves into the fireplace and out of the chimney, so we can safely say that there was no convection.

How did the warming take place?

The theory is that everybody emits and receives heat waves at all times. These waves are similar to light waves, traveling through space as energy impulses whose wavelength depends on the temperature and other physical conditions of the emitting body. When these energy waves meet a cooler body, they are absorbed, thereby warming the body they strike.

This method of heat transfer is radiation.

The radiant heat from the sun travel unobstructed through the great outer reaches of the universe and reaches our world with their energy unimpaired. Immediately upon contact with our atmosphere, absorption begins and the air above the earth, as well as the earth itself is warmed.

Radiation has been described as being dependent on the temperature and other physical conditions of the radiating body. This is equally true of the receiving body.  Bright smooth surfaces radiate and receive poorly, dark rough surfaces are efficient radiators and receivers of heat waves.

We paint tanks white or silver to reflect the heat waves.  A barefooted boy will say there is nothing hotter than the surface of a black, tar topped road on a summer’s day.  Black oily spots on the ground also become very hot in the sun.

Convection

The movement of hot air can cause enough heat to start a fire. This is known as convection.

This occurs only in liquids and gasses, never in solids.  When liquid or gas is heated it expands and becomes less dense.  The lighter fluid rises being displaced by cooler fluid.  This becomes heated and so circulation is set up.

In a fire situation in a building, convection currents can convey gasses produced by combustion upward through stairwells and lift shafts, thereby spreading the fire to upper parts of the building.

How do fires start?

A fire needs oxygen, fuel and a source of heat to burn, air which contains oxygen, is all around us in the workplace, there are many items that could burn – this is the fuel needed to start a fire.

Heat can come from four main sources, namely:

• Naked flames
• Hot surfaces
• Mechanically generated sparks
• Electrically generated sparks

The only way to make sure a fire is not started in the workplace is to control the source of heat.

The following are examples of sources of heat that could accidently ignite a fire:

• Smokers’ cigarettes, cigarette lighters and matches
• Open flames
• Hot processes (such as welding or grinding work)
• Engines, boilers or machinery
• Faulty or misused electrical equipment
• Lighting equipment and lamps
• Obstruction of equipment ventilation
• Friction, for example from loose bearings or drive belts
• Static electricity
• Metal tools striking each other and creating a spark
• A magnifying glass in the sun

A fire needs three elements to be present, if it is to continue burning, hence the name fire triangle.

The three elements are:

• Oxygen
• Fuel
• Heat or energy

Consider the three points of this triangle:

Oxygen

Oxygen is present in the air that we breathe. Fire needs oxygen to burn. Oxygen in the air combines chemically with the fuel, and the rapid oxidation gives off heat. If you can remove the oxygen, the fire will go out. This could be done by throwing sand on the fire or covering it with foam from a fire extinguisher. There are however certain materials such as celluloid which contains sufficient oxygen to support combustion even when the atmospheric oxygen is removed.

Fuel could be:

Flammable liquids like petrol, oils, paraffin, paints, solvents and cooking oils/fats

Flammable gases like natural gas, LPG, acetylene

Solid stuff that can burn like paper, wood, furniture, clothing, plastics and coal.

Any of the substances listed above can combine with oxygen and burn. If you can cut off or remove the fuel supply, the fire will go out. For example, turning off the valve in the pipeline that is feeding the fire.

Heat

The third point of the triangle is heat. Fuel and oxygen without heat, will not ignite. The fuel needs heat at a specific temperature – its ignition temperature – before it will burn. For the fire to continue burning, it needs to maintain that heat. If the fuel can be cooled down enough, the fire will be extinguished. For example, pouring water onto a wood fire cools the wood.

When fuel is heated, it gives off vapours and it is these vapours that burn. This rapid oxidation produces heat and light (flames). The fuel could be a solid (e.g. wood) or a liquid (e.g. petrol).  Remove one of these elements in the fire triangle, and the fire will go out.

The temperature at which material will burn fall into three categories:

Flash point

The lowest temperature, at which a substance gives of sufficient flammable vapour to produce a momentary flash on the application of a small flame.

Fire point

The lowest temperature at which heat generated by combustion of the flammable vapour is capable of producing sufficient vapour to enable combustion to continue.

The difference between Flash point and Fire point is that Flash point temperature is only required to produce vapour to enable a momentary flash to take place, whereas Fire point temperature has to be high enough to produce sufficient vapour to maintain combustion.

Spontaneous Ignition

The lowest temperature at which the spontaneous ignition of a substance can take place.  Carbon is the principal ingredient of all organic matter and has the characteristic of wanting to chemically combine with the oxygen in air. The surrounding air absorbs the heat generated by this reaction. (Atmosphere).

Tight compaction and confined spaces however cause the temperature to rise, which in turn accelerates the chemical reaction, until a temperature is reached where the substance bursts into flames. The temperature is called the IGNITION TEMPERATURE.  This is not a fixed temperature and varies a lot due to factors such as surrounding temperatures etc.

Oily rags in a lubrication pit may readily absorb oxygen and generate enough heat to reach ignition.

The uncontrolled mixing (or accidental contact) of acids and oils could generate rapid temperature increases readily reaching the ignition temperature.

A compost heap may undergo oxidation under influence of microorganisms.  If the generated heat is not dissipated rapidly enough, the ignition temperature may be reached.

Ignition Sources

We now see that a fire is not necessarily started by an open flame or spark but by heat.

The main sources of ignition are the following:

Electrical

21 % caused by poorly maintained electrical appliances (especially motors), overloaded circuits, and abuse of appliances, using the wrong or poorly maintained appliances in hazardous areas and to a lesser extent, static sparks.

Friction

14 % caused by hot bearings, miss-aligned or broken machine parts, choking or jamming of materials and poor adjustments of power drives and conveyors.

Foreign substances

12 % caused by particles of metal or stone mixed with materials being processed causing machine sparks.

Open flames

9 % caused by mainly the abuse and misuse of cutting and welding torches, gas and oil burners and petrol / paraffin torches.

Smoking and matches

8 % caused by smoking where combustible materials are present.

Spontaneous ignition

8 % caused by oily waste and rubbish deposits in dryers, ducks, fuels and low-grade material wastes in storage and materials exposed to some form of heating.

Hot surfaces

7 % caused by materials exposed to heat of boilers and furnaces, hot ducts, pipes and fuels, electrical irons and lamps and hot metal being processed.

Combustion sparks

6 % caused by sparks released from rubbish burning, furnaces, fireboxes, process equipment and industrial trucks and tractors.

Overheated material

3 % caused by normal process temperature especially those involving heated flammable liquids and substances in dryers.

Other

12 % caused by other less common or unknown causes.

Classification of Ignition Sources

For the sake of simplicity therefore mentioned ignition sources could be classified as follows:

• Spontaneous ignition
• Open flames
• Electrical sparks or Arcs
• Hot surfaces
• Mechanical sparks