INTRODUCTION OF FIRE
There are three types of hazards viz., fire, explosion, and toxic release occur in the industry in particular chemical process industries. In many instances, more than one of these hazards occurs in tandem with each other.
Fires occur in industry, more frequently than explosions and toxic releases, although the consequences in terms of loss of life are generally less. Hoverer, if the ignition of escaping flammable material is delayed, an unconfined vapour cloud of flammable material may be formed, which if ignited, can cause violent explosion, resulting in extensive damage. The fire is the first major hazard in the chemical / process industries and causes more serious accidents than explosions or toxic release, although the accidents in which the greatest loss of life and the damage occur are generally caused by explosion.
Fire is normally regarded as having a disaster potential less than explosion or toxic release. It may be remembered here that the world’s worst industrial disaster – Bhopal Gas Tragedy – occurred on the night of 2 – 3 December 1984, due to toxic release of the chemical “methyl isocyanate” and over 5,00,000 people were exposed to this chemical. However, ‘toxic releases’ are not dealt in this article. The scope of this article is confined to Fire and Explosion hazards only.
DEFINITION OF FIRE
Fire is a rapid chemical reaction of oxidant with fuel accompanied by the release of energy, indicated by incandescence or flame.
Combustion Charactersitics
For a fire to happen, the following elements are essential
Oxidiser to sustain combustion.
Heat to reach ignition temperature.
Fuel or combustible material.
This results in a chemical chain reaction that starts a fire.
Removing any of these elements will extinguish the fire.
HEAT TRANSFER
During a fire, heat transfer occurs by
Conduction– transfer of heat within the material itself.
Convection – transfer of heat by the physical movement of hot masses of air.
Radiation – refers to the emission of heat in the form of electromagnetic waves.
IGNITION
Ignition is the process of initiating self-sustained combustion.
The ignition temperature of a substance is the minimum temperature to which it must be heated for it to ignite.
SOURCE OF IGNITION
Ignition can occur by –
- Electrically powered equipment-Arcing, damaged wiring, over heating of cables due to excess loads, loose electrical connections, heat from electric bulbs, etc.
- Open flame
- Hot surfaces
- Sparks from welding operations
- A chemical reaction between incompatible chemicals
- Smoking
- Batteries
FLASH POINT
In a flammable liquid fire, it is the vapors released from the surface of the liquid that burns.
Flashpoint is the lowest temperature at which a liquid produces enough vapor to form an ignitable mixture.
E.g., Diethyl ether (-45 O C)
Methyl alcohol (11.1OC)
Lower the flashpoint of a flammable liquid, the greater the hazard.
CLASSIFICATION OF FIRE
On the basis of the type of fuel, fires are classified into the following
Class A Fires — Solid combustible materials of organic nature such as wood, paper, rubber, plastics, etc.
Class B Fires — Flammable Liquids & Natural Gas.
Class C Fires —
Class D Fires — Combustible Metals, such as magnesium, sodium, potassium, etc.
EXTINGUISHING FIRE
Fire can be extinguished by
- Cooling – The fuel by removing heat (e.g., by applying water).
2. Smothering – By cutting off oxygen supply (e.g., by applying the foam, carbon dioxide).
3. Starving – The fire by removing the fuel.(e.g., stopping gas flow during a pipeline fire).
Inhibition – By stopping the chain reaction.(e.g., by applying dry chemical powder).
FIRE EXTINGUISHERS
Type of extinguishers and the classes of fire for which they can be used
WATER EXTINGUISHER
They are used for Class A fires.
•Water removes heat and extinguishes the fire.
•Water must not be used on fires involving live electrical equipment as it can cause electrocution.
•Water must not be used on metal fires.
CARBON DI OXIDE EXTINGUISHER
- CO2 extinguishers are mainly used for Class B and C fires.
- CO2 is not suitable for fires involving metals.
- It’s a principal advantage is that it does not leave any residue.
- Can be used on electrical/electronic equipment.
- CO2 extinguishes the fire by displacing oxygen in the surrounding air.
DRY CHEMICAL POWDER
- The main base chemicals used in DCP extinguishers are sodium bicarbonate and potassium bicarbonate.
DCP extinguishers put out the fire by coating the fuel surface with chemical powder.
This separates the fuel from the oxygen in the air and prevents vapor formation. - The powder also interrupts the chemical chain reaction of fire.
The disadvantage is that it leaves residue particularly making it difficult to clean up in case of sensitive equipment.
PASS TECHNIQUES
PULL THE SAFETY PIN
AIM AT THE BASE OF FIRE
SQUEEZE
SWEEP ONE SIDE TO ANOTHER SIDE
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