SOURCES OF FIRE HAZARDS

SOURCES OF FIRE HAZARDS

Sources of fire hazards can be classified based on the triangle of fire, namely from materials, oxidants and heat energy (Tharmarajan 2007). Each of these elements is described below;

1.  Hazards of materials

Hazards of materials can be further classified into wood and wood-based products, plastics, textiles, liquids and gases. These are further described below.

2.Woods and wood based products

Wood and numerous wood-based products such as paper, cellulose-based fibrous materials and many others, are indeed ubiquitous. They are invariably involved in almost all kinds of fires. Therefore, understanding of their fire characteristics is important for fire protection.

The chemical content of dry wood and wood-based products is relatively simple. Carbon (50%), oxygen (40%) and hydrogen (6%) are the most abundant elements with nitrogen and mineral ash making up the remainder. However, these few elements are combined to form a large number of substances, of which cellulose (50%), lignin (26%) and extractable (1%) are the major components. Wood also contains water, either as moisture or absorbed water in wood cells. Whereas moisture is readily removed on heating wood and wood products above 380 K, absorbed water remains even after prolonged heating. Apparently, dry wood may still contain considerable amounts of water (5 – 6%) (Stroup David W, 2007).

Wood and wood based products are combustible. They can burn in different modes such as smouldering, charring, ignition followed by flames or burning with a lot of smoke (Leonard Y, 2007). Smoke produced from burning wood has a characteristic recognizable odour. In comparison with other solids, the toxicity of smoke produced from wood burning is not pronounced. Except for carbon monoxide, other toxic gases are either absent or only present in traces.

3. Plastics

Plastics are used practically everywhere, namely in building construction, homes, offices, shops, schools, hospitals etc. A lot of equipment’s and applications, furniture, wall coverings, curtains, textiles and many other products either are made completely of plastic or contain some plastic parts.

All plastics, regardless of chemical characteristics, being organic compounds are combustible. Various flame retardants can considerably reduce flammability but cannot completely stop combustion. Ignition temperatures of plastics are somewhat higher in comparison with wood and wood – based products. However, the rates of flame spread are generally much higher than wood.

The burning of plastic s rapidly produces smoke which is usually dense, contains a lot of soot and has a dark colour. It has been found in many cases that the inhibition of flammability of plastics by flame retardants increases smoke production (Stollard p, 1994). Thermoplastics soften on heating before reaching ignition temperature and harden on cooling. At higher temperatures, thermoplastics melt and flow. This characteristic of plastics is potentially hazardous since the flaming liquid may drip and thus spread the fire.

Another hazard is that during the burning process, some plastics release corrosive and toxic gases such as HCL, HF, HBr, HCN and NH₃(Johnson L.E, 1994). The conditions which enhance the emission of such gases during a fire are low ventilation and lengthy fire growth which releases fire temperatures to the point of an easy break of the polymer matrix and the release of simple gaseous constituents.

4. Textiles

The widespread use of textiles in daily life, coupled with the fact that nearly alltextiles are combustible, explain the leading role of textile fires in fire deaths. More than 50% of fatal incidents involve a fabric. As to the type of fabric first ignited, artificial fibres, cotton and rayon comprise the largest percentage (41%), whereas wool and wool mixtures are very rarely first ignited (1%). This marked difference is due to the differing ignition temperatures. While cotton and most artificial fibres ignite at relatively low temperatures (520 – 670 K), the ignition temperature for natural protein-based fabrics such as wool, silk and cashmere is between 840 and 880K (FN Spon, 1994).

The fire characteristics of textiles depend on the nature and proportion of individual fibres, on their weight and the method of blending. Textiles based on cellulosic fibres such as cotton and jute behaves differently in fires compared to protein fibres. The latter ones do not burn readily; shrink at temperatures approaching their decomposition temperature and burn more slowly. Common artificial fibres such as nylon and rayon burn similarly to protein-based natural fibres. Thus, they are relatively safer compared to fabrics containing cellulose. However, when exposed to heat, artificial fibres often melt and stick to the skin. Therefore, they should not be employed for protective clothing.

Synthetic fabrics can be hazardous in some special atmospheres, such asoxygen-enriched atmospheres or atmospheres containing flammable gases and vapours because of the accumulation of static electricity (Drysdale D, 2007). The discharge of this electricity to the ground or other objects may produce a spark of sufficient energy to ignite a flammable gas. In such situations, electrically conductive fabrics should be employed.

5.  Liquids

Flammable and combustible liquids are among the most fire hazardous materials. Fire statistics record that fires involving a liquid are the most frequent ones. Flammable and combustible liquids are the most hazardous in all instances when the liquid is exposed to air, such as in spillages (Drucker, 2004). The fire and explosioncharacteristics of a liquid can be described using a number of parameters. Some of them are applicable to solid and gaseous materials as well. Thus, most of the hazardsare similar to the ones already described earlier.

6. Gases

For fire protection purposes a gas may be defined as any substance which exists in a gaseous state at normal temperature and pressure. Since at these conditions many substances may exist as either liquids or gases, depending on the partial pressures of their vapours, it is generally accepted that all those liquids which exert a relatively high vapour pressure may be regarded as gases (Frantzich, 1997).

In most situations, gases are used in large volumes. Since gases are much lighter than liquids and solids, the only practical means of having a reasonable quantity of gas on hand is either by gas compression in containers or by filling the containers with liquefied gas. Both these forms of gas-packing present hazards. Fire hazards of gases are very similar to those of liquids. This is not surprising since in fire hazards of liquids it is the vapour of a particular liquid which is hazardous, rather than the liquid phase itself. Thus, most of the hazards are also similar to the ones already described previously.

Thus, most of the hazards are also similar to the ones already described other properties. The hazard stems from properties such as toxicity, reactivity and chemical inertness (Hakan, 2007). However, the basic hazard of all gases and vapours, regardless of their chemical composition, is related to the change of gas pressure with changing temperature. From classical gas laws, the doubling of temperature leads to a doubling of gas pressure. Hence, gas containers would normally be ruptured in the event of fire.