Safety in the Use of ChemicalsChapter 7Fire Fighting |
As you have learned, many chemicals can catch fire and some can cause explosions. A person using chemicals must always be prepared for the day when something goes terribly wrong and the chemical catches fire. If action is taken in time, most fires can be controlled at once; otherwise, if they spread, they can result in heavy losses to life and property. The aims of any fire protection programme should be these:
- Prevention of fire
- Evacuation of people in case of fire
- Fire fighting
- Calling outside help if required
Fire alarms
The first action which should be taken when a fire is detected is to raise an alarm. An alarm system should be installed where there is a danger of fire. This need not be very sophisticated, if the danger of fire is not very great. At the sound of the alarm, everyone (except those concerned with fire fighting) should leave the building and assemble on a previously decided point, in order to be counted. Effort should be made to install an automatic alarm system which detects smoke or heat and sounds the alarm.
Fire fighting techniques
It is important to use correct techniques for fighting fires caused by chemicals. Water, traditionally used for fire fighting, may in fact increase the intensity and damage from fires, in certain cases. Suppose, some gasoline spills on the floor and catches fire. If you try to throw water on it, it is likely that the fire will spread, as gasoline, being lighter then water, will float on the surface of water.
Still, water is the most useful medium for extinguishing fires, as it not only puts out the fire, but also cools down the materials, thus preventing fire from spreading. The common fire fighting devices can be divided into two categories a) fixed systems and b) portable systems.
Sprinklers
A sprinkler system sprays water at high pressure at the fire and douses and extinguishes it by flooding the area. The system consists of a variety of pipes, nozzles, valves and water storage facilities. The water supply system should be such that it is not interrupted even when repairs are being carried out. These are considered the most effective fire fighting equipment. They are very effective against most kinds of fire, especially those involving combustible materials like paper, wood and cotton.
Water spray
The spray system is similar to the sprinkling system, but the nozzles at the ends of pipes are different. They create a spray of water. Such sprayers can be either at fixed points or can be directed at specific areas manually. These are used to extinguish existing fires or to prevent fires from spreading by cooling down containers and areas where flammable and combustibles materials may have been stored.
Water based systems should not be used to extinguish fires of electrical equipment, as water is a conductor of electricity; it may aggravate the fire, besides damaging the equipment. Such systems should also not be used when there is a danger of reaction between the chemicals and water, which may lead to spread of fire.
Carbon dioxide
Carbon dioxide gas can be used to combat most kinds of fires, especially those involving flammable liquids, solids and electrical materials. Gas under pressure is released in the area where fire is burning; it is heavier than air and forms a barrier between the fire and the air and thus cuts off the supply of oxygen, essential for the burning process. It leaves no residues, so damage to the equipment is averted. It is not recommended in certain types of fires like those of active metals, metal hydrides and chemicals supplying heir own oxygen.
Dry chemicals
Many types of chemicals are used to fight fires involving flammable liquids and electrical equipment. The Chemicals used for this purpose include bicarbonates and ammonium phosphates, mixed with other fire extinguishing chemicals. Sand is also used to douse many fires. In fact, it is a very cost-effective way of fighting small fires.
Foams
Foam created either by chemical or mechanical means is sprayed on the burning fire. Foam spraying systems are generally set up like sprinklers. These systems create a layer of foam on top of the burning material which helps in extinguishing the fire by removing the supply of oxygen.
Portable extinguishers
These can be in the form of cylinders containing halogenated hydrocarbons or certain other chemical, or they may simply be buckets of sand. These extinguishers can
be carried either manually or on a carrier to the scene of fire and are very useful in extinguishing small fires.
If you happen to see a portable fire extinguisher in your school or workplace, try to read the label. It will usually indicate that the extinguisher is effective in controlling fires of certain categories or classes, such as A, B or C, or all categories. These classes are in fact assigned to fires according to the nature of the materials which are burning. A description of the different classes of fires and the extinguishing agents required to combat them is given in Table 7.1.
Table 7.1 Classes of fires and extinguishing materials required for them
| Class of fire | Types of materials which may cause it | Extinguishing media required to fight this class of fire |
| A | Ordinary combustible materials like wood, paper, rags, rubbish, rubber and plastics | Water spray and sprinkling system. Special dry chemicals douse the flames quickly and prevent the spreading of fire |
| B | Flammable or combustible gases and liquids like gasoline, kerosene, thinners, paints, grease and similar materials | Materials which limit the supply of air, like carbon dioxide, dry chemicals, foam and halogenated hydrocarbons. Use of water may spread the fire; however fine water spray can be used to cool the containers which are likely to catch fire |
| C | Fires involving or near electrical equipment | Non-conducting materials like carbon dioxide or dry powders should be used. Water and foam should not be used as they may cause short circuiting, electrical shock and damage to the equipment |
| D | Fires involving active metals like magnesium, sodium, titanium potassium, zirconium | Special extinguishing agents are available for such fires; normal extinguishing agents should not be used as they may increase the intensity of the fire |
Exercise
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