Details

CLASSIFICATION OF FIRES

TYPES OF FIRES & METHOD TO QUENCH IT

Classification of Fires
Indian Standards Specifications classifies fires as follows
(1) Class 'A ‘fires: These fires involve combustible materials e.g. timber, coal, rubber, conveyor belt, other carbonaceous material.
(2)  Class 'B' fires: These fires involve inflammable liquids e.g. lubricating oils, diesel, petrol and other fuel oils, greases, etc.
(3)  Class 'C' fires: These fires involve gaseous fuels like LPG gas; butane, etc.
(4)   Class 'D' fires: These are metal fires such as melting iron, etc.
(5)    Class   'E' Fires:  These fires involve live electrical equipments such as electric motors, generators, cables, oil-filled transformers, circuit breakers, electronic equipment, etc.
Class 'A' fires are generally quenched by water. Stone dust and sand may be used if the fire is on a small scale and in its early stage. Overhead fires cannot however be tackled by stone dust and sand.
Class 'B' fires are quenched by an extinguishing agent which has a blanketing or smothering action and foam extinguishers are best suited for this purpose. Water, if used for extinguishing, spreads the oil and the fire along with it.
Class ‘E’ fires require use of extinguishing agent which is not a conductor of electricity. Water, with its normal impurities is a conductor of electricity and should not be used. A foam extinguisher is also not recommended but sand is suitable for smothering such fires in the initial stages on a small scale and the best extinguisher is the carbon dioxide gas. One of the advantages of CO3 extinguisher is that it has no adverse effect on the insulation or, other working parts of the equipment.

Portable fire extinguishers are of the following types:
 
1.    Soda acid
2.    Water-CO2.
3.    Foam
4.    CO2 gas.
5.    Dry powder.
6.    C. T. C.
7.    B. C. F.
 

Soda acid extinguisher
It consists of a lead coated steel cylinder full of solution of soda bicarbonate and a hermetically sealed bottle of sulphuric acid placed inside a cage in the extinguisher. A spring loaded plunger protruding outside the cylinder is so placed that when the plunger is struck against a hard surface it breaks the glass bottle of acid and the sulphuric acid reacts with sodium bicarbo¬nate solution generating CO., gas. The gas under pressure forces the mixture of water and sodium bicarbonate up a tube and a nozzle to a distance of 6-8 m for initial 60 seconds in a 9-litre extinguisher and the total duration of discharge is 60-120 sec. The CO2 expels the soda bicarbonate solution and it also gets partially dissolved in the water and helps in the quenching of fire when it is liberated as the water evaporates. Cooling action of water is the main factor in quenching the fire; CO2 helps it only partially. The jet of the solution should be aimed at the base of the flames.
Such fire extinguisher is suitable for only class ' A ' fires and is not recommended for class ' B ' fires. It is prohibited by the mining regulations for quenching class * E ' fires.
The extinguisher can be operated cither in an inverted or upright position depending upon the model and instructions for operation are stated -on the extinguisher itself. In general soda acid extinguishers of conical shape should be held in upright position and of cylindrical shape in inverted position during use. The conical shaped extinguishers are gradually going out of use. The extinguisher can be refilled by the user at the site if refills are kept in stock.
An extinguisher of 9-lit. capacity weighs 15 kg when full.
The soda acid extinguisher is also available as wheel moun¬ted mobile model. One model of 45 lit. capacity has 10-12 m range of discharge for initial 100 -sec. and total duration of dis¬charge is 100-150 sec. During operation the mobile model has to be kept upright. The hose provided on it directs the jet in any direction.

Water-CO2 extinguisher
It consists of a steel cylinder coated inside with copper and containing water and a charge of high pressure CO3 in a separate small sealed container of copper placed within the main cylinder. A spring loaded plunger protruding outside the main cylinder is provided at the top of this small CO2 container. To operate, strike the plunger against a hard surface. This punctures the CO2 container and the gas pressure forces the water out of the cylinder to nearly 12 m.
It is effective for class' A ' fires only but not recommended for class * B ' fires and totally prohibited for class ' C ' fires. The small copper cylinder containing CO, under pressure forms the refill and during use, specially in underground mines, the extin¬guisher can be recharged with water and the refill.

Foam extinguisher
In this extinguisher the foam produced during its operation forms a blanket on the flame and smothers the fire by stopping access of air. A foam type extinguisher is not permitted for dealing with class ' C ' fires as the foam is a conductor of electri¬city but it is well suited for class ' A ' and class * B ' fires.
There are two types of foam type fire extinguishers:
1     Chemical foam extinguishers and
2    Air foam extinguishers.
The former type is more common and it consists essentially of a vessel containing a solution of soda bicarbonate enclosing a sealed container of aluminium sulphate solution or similar foam forming solution along with a stabilising agent. During use the seal of the inside container (containing foam forming chemicals) can be released by turning a spring loaded plunger rod attached lo it. When the seal is thus released and the extinguisher turned upside down chemicals of the two containers react to give CO3 in the form of foam bubbles which are forced out of the nozzle under pressure. The foam has a smothering and wetting action with the flame. All foam type extinguishers have to be held up¬side down during use. The commonly used extinguisher of 9-litre capacity produces about 75 litres of dense foam. In "Firex" foam type extinguishers the foam refill basically consists of two packets of powder i. e. aluminium sulphate and sodium bicarbo¬nate. Refilling can be done by the user at the site. The total duration of discharge of foam in a 9-litre extinguisher is 50-90 seconds with an effective range of discharge 6-8 m for initial 40 seconds. The wheel mounted mobile extinguishers are of 45 litre or 135/150 litre capacity. The 45-litre capacity extinguisher pro¬duces 360 litres of dense foam and has duration of discharge as 75-120 seconds and effective range of discharge U-14 m for initial 75 seconds. The 135/150-litre extinguisher has 90-135 seconds duration of discharge and the effective range is H-17 m for initial 100 seconds.

CO2 extinguisher
It consists of a cylinder having one to five kg of CO2 in liquified form under high pressure f70 kg/cm8) which can be released during use by operation of a wheel valve mounted on its top as in the case of the familiar Indane gas cylinder used for domestic cooking. This is by far the simplest portable fire extin-guisher and has the advantage that it can be used for fire of any class including electrical fires. No residue is left on the extingui¬shed material. The gas penetrates to otherwise inaccessible places. CO2 has the property of quenching the fire and the gas leaving the extinguisher is at a very low temperature so that it exerts a cooling effect on the burning material.
All CO2 gas type extinguishers are provided with funnel shaped discharge horns and the CO2 leaves the extinguisher in the form of a dense but very cold white cloud. Such extinguisher can however be used where air current is not brisk. In a well ventilated underground roadway the CO2 may be carried away by air current rendering the extinguisher ineffective. The refilling of Co2 can be done only at the manufacturer's factory. During operation the extinguisher has to be held upright. One should not enter the premises affected by fire immediately after use of the CO2 gas extinguisher.
The CO2 gas extinguishers are available with capacities of 7 kg, 10 kg, 20 kg, or 45 kg in trolley mounted models. Smaller size extinguisher of 2 kg capacity can be carried on a coal cutting machine for use in the event of fire in the undercut of a coal cutting machine.

Dry powder type extinguisher
An extinguisher of this type can be used for dealing with lire of any type including electrical fire. It contains a fire quen¬ching powder and a container of high pressure CO2, both placed inside a cylinder. The dry powder is a mixture of chemicals consi¬sting of mono ammonium phosphate as the main ingredient and other chemicals which have anti-setting and water repellent prope¬rties. The powder has very little cooling effect on the fire but it forms an adhering glassy crust of metamorphic acid over the combustible material and retards further combustion. The fire quenching powder goes by the trade name ‘Firex-Nugas’ in the Firex make fire extinguishers. The powder is water repellent, non corrosive, non toxic, non conductor of electricity and will not deteriorate or cake when stored. It is free flowing at all tempera¬tures. The extinguisher can be recharged on the spot by the user with suitable refills. In some models, e.g. Minimax make, the CO2 gas cartridge is specially designed for withdrawal and inspection.
Dry chemical fire extinguishers are available in capacities of 1 to 12 kg of dry powder. The trolley mounted units are avai¬lable in 25 and 70 kg capacity models.
To put the extinguisher in operation, strike the knob or plunger rod against a hard surface. This releases the high pressure CO8 into the chamber containing dry chemical powder which is shot out of the cylinder under the gas pressure. The powder is discharged in a fan shaped cloud and it settles on the surface of the burning material or liquid and stops access of oxygen to it. The “squeeze grip” valve at the end of a hose pipe enables the operator to control the discharge of the powder and in some smaller models pressure may be released without further dischar¬ge of powder by inverting the extinguisher. For quenching the fire the extinguisher has to be held erect and not upside down.

C. T. C, Fire extinguishers
Carbon tetrachloride is a liquid at normal temperature but it evaporates easily. Its cooling effect is high and when the liquid comes in contact with burning material the cooling effect quenches the fire. C. T. C. is a non-conductor of electricity and therefore the C. T. C. type extinguisher can be used for electrical fires in generators, sub-stations, transformer rooms, etc. When evaporated CTC forms fumes which are toxic; hence the extinguisher should not be used in confined space and in underground mines. The CTC fire extinguishers are used on motor cars or automobiles as handy units in 2-litre capacity models. The liquid is discharged by gas kept under pressure inside the cylinder.
These extinguishers are being gradually withdrawn from use.

B. C. F. Fire extinguishers
In recent years a liquid called B.C.F. (bromochlorodifluoromethane) is gaining popularity as a fire extinguishing medium in place of C, T. C.; B . C. F. is a clear, colourless liquid at normal atmospheric temperature with the following properties.
Formula        : CBr C1F2,
Molecular weight    : 165.4
Boiling point        : 80° C
Freezing point        : minus 4°C
Odour            : similar to the halogen group of compounds
Liquid sp. gr. at 21 °C    : 1.83.
Effect on human flesh    : BCF does not present any hazard by absorption through the skin. Having a low boiling point the liquid evaporates quickly and gives a chilling sensation if spilled onto the skin but it cannot cause low temperature ' burns ' as CO2 does.
Effect on metals and non-metals; BCF is chemically very stable and it shows fewer tendencies to cause corrosion like CTC does. In the absence of water BCF can be stored satisfactorily in containers made from most common metals. BCF causes-natural rubber to swell but its effect on nitrated rubber is very small and these can be used satisfactorily for gaskets and joint rings.
Electrical conductivity: BCF is a non-conductor of electri¬city and can therefore be used to deal with class 'E' fires.
B.C.F. is also known as Halon. Halon 1211 is used for extinguishing fires.
The suitability of different types of fire extinguishers is summarised as follows:
Class A fire - Soda acid type;
Class B - Foam type;
Class C - Dry chemical powder;
Class D - Special dry chemical powder;
Class E - CO type and BCF. Halon 1211 BCF type is recommended also for fires involving electronic equipment. It is imported and may not be available easily these days.

Grahm’s Ratio
The production of CO in relation to the oxygen consumed by the oxidized material is a
function of temperature and therefore indicates the average intensity character of a fire.
Value of Grahm’s Ratio quoted in a number of technical reference are :
0.4 % or less - indicates normal value
0.5 % - indicates necessity for a through check-up
01 % - indicates existence of heating
02 % - indicates serious heating approaches active fire
03 % & above - indicate active fire with certainty
More than 07 % - may occur for blazing fire