Use Of Fire Extinguishers
Fire is the whole of occurrences that accompany uncontrolled hazardous oxidation. It results from a chemical reaction between a fuel and oxygen and it is usually accompanied by heat and smoke generation and by emission of light or flames.
A fire can start only when the following elements are combined:
Together they form the so-called Triangle of Fire, where each leg represents a parameter.
Removal or elimination of one of the parameters prevents or ceases fire. Insight in these parameters allows for fire prevention and for efficient extinguishing of fire.
To understand fire prevention; it is important to recognize that every fire consists of a chemical reaction between a fuel and oxygen, activated by some form of energy.
With reference to the fire triangle, the principles of fire extinguishing and of different means for fire extinguishing can be deducted. Elimination of one of the three parameters 'fuel, oxygen, heat source) will halt the reaction. Every means of fire extinguishing destroys the triangle of fire.
Extinguishing a fire can be accomplished by:
The type of the fire and the properties of the burning materials determine the method of choice to extinguish the fire.
Fires are classified in different categories, that feature each appropriate methods of extinguishing.
Class A comprises fires of solid fuels, such as wood, paper, cardboard, coal, fabrics, etc. (burning dry matter).
Appropriate means of extinguishing:: water, foam or some special powders.
Class B comprises fires of liquid fuels - more precisely of the vapors in equilibrium with the liquids -, such as oil, gasoline, alcohol, paints, varnishes, tar, grease,Ö Contrary to Class A fires, Class B fires proceed at the surface of the fuel, where to a more or lesser degree the vapor phase of the liquid acts as the actual fuel.
Appropriate means of extinguishing:: powders, CO2, foam.
Class C comprises fires of flammable gases, such as natural gas, methane, propane, butane,Ö
Appropriate means of extinguishing:: FIRST OF ALL, THE GAS SUPPLY SHOULD BE CUT. The fire will extinguish automatically. If this is impossible, the fires can be kept under control by powders, CO2, halon. It is dangerous to extinguish the fire as long as the gas supply is not cut, for accumulation of gas may pose an explosion hazard.
Class D comprises fires of chemicals, such as flammable metals (magnesium, sodium, titanium,Ö).
Appropriate means of extinguishing: commonly used methods are inappropriate and may worsen things. Powders and dry sand are excellent materials.
Class E comprises fires in electrical apparatuses, such as motors, transformers, switch boards, cables,Ö
Appropriate means of extinguishing: NEVER USE CONDUCTIVE MATERIALS? SUCH AS WATER OR FOAMS. Preferred methods use powders, CO2 or halon. Occasionally non conductive high pressure methods can be used.
Knowledge of the procedures to operate fire extinguishers is important to enable swift reaction to a developing fire, before it becomes to big to handle.
Apparatuses containing powder or CO2 are idealto deal with beginning fires. These apparatuses are abundant in the laboratories. Please note they are effective only when used to extinguish small scale fires or developing fires.
Powder is forced out under pressure and forms a dense cloud around the burning mass. The powder contains approximately 80% of NaHCO3. When in contact with fire, it dissociates in CO2 and H2O, thus depleting the surrounding atmosphere from the oxygen necessary to sustain the fire and at the same time lowering the temperature by absorbing energy to dissociate. In addition, a layer of powder will be deposited on the mass. Direct contact with the surrounding air becomes impossible, thus preventing re-ignition of the fuel.
Powder fire extinguishers have a range of 6 to 8 meters, but the nozzle should not be more than 3 to 4 meters away from the fire to be effective.
CO2-apparatuses don't leave traces as do
powder types. Hence they are more suitable to deal with
fires in rooms containing sensitive measuring and control
equipment, communication devices and the like. Powder spread
over the room requires thorough and expensive cleaning.
CO2 is an inert gas and disappears in the
CO2 extinguishers contain CO2
gas under a pressure of approximately 60
kg/cm2. When released to atmospheric pressure,
the temperature of the gas drops to ñ79°C.
On the one hand, the temperature of the burning mass can be lowered beneath the ignition temperature of the fuel, thus extinguishing the fire. More importantly however,
CO2 gas is inert and heavier than air, so it displaces the source of oxygen to the fire and builds a protective blanket over the fuel
Water acts as a coolant. It has the most pronounced cooling effect of all extinguishing means through its high specific heat and low boiling point of 100°C. It succeeds in dropping the fuel temperature to below the ignition temperature, thus preventing further propagation of fire. Evaporation of water requires lots of energy to be delivered by the burning mass. Spraying water is more efficient than jetting, because a larger amount of water can come into contact with the fire and extract heat from it. Steam formation has a secondary effect of displacing oxygen, thus suffocating the fire. One liter of water can produce 1,650 liters of steam.
Advantages of water
Disadvantages of water
When a fire develops, time is of the essence.
Automated systems are most effective the sooner they are
activated upon onset of a fire..
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