MODULE 2: TYPE’s OF FIRE

Outcome

Identify the type of fire, its context and select the appropriate fire fighting procedure

Assessment criteria

An understanding of procedures for dealing with fires in the workplace is demonstrated.

Notes

ü  The nature of the fire is correctly identified and its context correctly assessed

ü  Hazards are isolated

ü  Respond to ‘what if’ and `why’ questions covering:

ü  Applicable chemistry of fires

ü  Methods of fire transmission

ü  Types of fires

Outcome range

ü  Types of fires include organic materials/fibre materials, flammable liquids and gases, live electrical equipment, combustible metal fires.

ü  Fire contexts include the nature of the area where the fire is, adjacent hazards, structural materials.

ü  Methods of fire transmission include conduction, induction, radiation.

ü  Combustible hazards include combustible materials and temperatures, compressed containers, electrical equipment, structure and layout of site.

Spread of fire and heat radiation

How fires behave

A candle burning in a room without draughts produces a steady flame.  The flame’s heat vaporises just enough candle wax to keep the flame burning at the same height.

Uncontrolled fires, on the other hand, fuel themselves by vaporising the solid or liquid materials they find in their path. A house fire or forest fire may begin with easily ignitable materials.  As the fire grows, it radiates more heat.  This contributes to further growth, and the process accelerates as long as fuel and oxygen remain available.  In a house fire, a phenomenon known as flash-over occurs when all the surfaces in a room reach their ignition temperature.

At this point, a relatively small fire suddenly ignites the remaining materials, filling the room with flames.  In a forest fire, leaves, twigs and other materials along the ground usually make up the fuel.  But wind and certain types of terrain may cause a forest fire to spread along the tops of trees.  Because fires can grow quickly and suddenly, professional fire fighters should immediately be called to control them.

Flames spreading over a surface

A veldt fire is a good example of flames that spread from the ignition source to other flammable material by means of flammable substances on the surface such as vegetation of liquids lighter than water

Falling Objects

Two examples of the way a fire can spread from falling objects are the following:

Items stacked near other flammable items that are on fire fall over and ignite the stacks in their immediate environment, and parts of a burning object that break off and fall to a lower level.

General Sources Of Ignition

Studies done in recent years have indicated that there are 16 sources of ignition, 14 of which relate directly to human actions.

Five of the ignition sources are more commonly found (not necessarily in higher percentage) and are discussed in more detail later.

The ignition sources are reflected below as percentages:

Electricity

Electricity is the single biggest cause of fires.

Electrical fires are caused by the following:

ü  Overloading of circuits

ü  Over-fusing and bridging over circuits

ü  Defective thermal appliances

ü  Overheating of appliances (ventilation systems blocked, obstructed)

ü  Loose connections

ü  Cables running under carpets

ü  Radiant effect from heaters

ü  Heated Surfaces

Surfaces may become hot because of the following:

ü  Friction

ü  Welding and cutting

ü  Heaters and stoves

ü  Overheated appliances (obstructed ventilation systems)

ü  Hot processes (steam pipes with items stacked on top of or against them)

ü  The bottom of a pot which has been removed from a stove and placed on material with an ignition point lower than the heat radiated by the pot

ü  Electric irons that have not been switched off

ü  Soldering irons left in unsafe positions during use

ü  Containers with hazardous contents that are placed on hot surfaces, for example acetylene, ignite at about 305 °C or petrol (low octane) at about 400 °C.

Open flames

The main cause of the direct spread of flames is human negligence and   people’s poor knowledge of the material that they use, disregard for the wind direction and the environment in which work is done.

Rubbish bins

Rubbish bins with tightly fitting lids should be a standard feature in factories and plants.  The convection effect that heat has on material lifts up material from an open rubbish bin and drops it onto other flammable material.  The deposited material has the same effect as that of hot surfaces.

Dust as a fire and explosive hazard

Although dust is usually not regarded as a danger in fire prevention, in the right suspension and under the right conditions dust can explode and be extremely destructive.

Dust control is a statutory requirement in safety legislation.  In addition, dust control is also a requirement laid down by insurance companies in cases where explosives are formed or released as a product or by-product.

Materials that can react explosively

Organic materials with high carbon content are extremely explosive, for example:

ü  Sugar

ü  Coal dust

ü  Dextrin

ü  Starch

ü  Cocoa

Factors affecting the risk attached to dust

Fineness: The fineness of the dust particles determines the explosive hazard since the finer the particles, the more easily they can catch fire.

Suspension in the atmosphere: As is the case with gases and vapours, dust has to mix in the right proportions with air if it is to fall inside the explosive limit.  The dust in the air will explode if conditions are favorable and an ignition source with sufficient energy is present.

Characteristics of dust explosions

An enormous amount of energy is released during dust explosions, often with more serious consequences than those resulting from gas and vapours explosions.  Dust explosions cause severe structural damage and the explosion is not easily brought under control.

Precautionary measures against dust explosions

Special electrical equipment as prescribed in SABS codes must be installed.

Dust has to be extracted where it forms and a local exhaust system should preferably be used.  The places where dust can accumulate should be regularly cleaned with a suction machine.

Under no circumstances should dust be blown. The blowing method increases static electricity and at the same time the dust is blown into suspension in the atmosphere.  This could result in the lowest explosive limit of the dust being reached and dust exploding through a static discharge.  Control hot work in environments where dust is formed.  Earthling and interconnecting are essential in areas where dangerous dust is present.

Classification of fires

Fires are classified into four types: those in solids, e.g., wood, paper, and cloth; those in flammable liquids, e.g., gasoline, alcohol, oils, lacquers, and paints; those in electrical apparatus; and those in flammable metals such as magnesium. These are called, respectively; class A, B, C, and D fires.

Note:   Only class A fires can be extinguished with water.  Water is denser than most liquids and therefore cannot be used on class B fires.  Because water is a conductor of electricity it can not be used on a class C fire.  Most chemical substances react with water and therefore water cannot be used on a class D fire.

Fire-Extinguishing Mediums

The effectiveness of a fire extinguisher is largely determined by the design and manufacture of the equipment.  The choice of a fire extinguisher is determined by its contents and operation.

Water

Dry Powder (smothering)

Foam (smothering and cooling)

Carbonic acid gas (smothering, quenching and cooling)