Fire is a chemical reaction initiated by presence of heat energy in which a substance combines with oxygen (from air). The process involves giving out heat energy (exothermic reaction), light and sometimes sound.
A combustible substance – fuel The following are essential for fire:
Oxygen, Nitrogen (e.g. Magnesium burns in Nitrogen)
Heat source, spark or flame
Process of chain reaction.
Types of Combustion:
Rapid: Gas is ignited, producing heat and light
Spontaneous: Without the application of external heat.
Explosion: Combustion in confined space under pressure. Heat, light and sound is produced.
Stages of Fire:
Incipient stage: Preheating and gasification (slow pyrolysis) is in progress. Invisible pyrolysis produces gas.
Submicron size: Aerosols (tiny particles) are found in the vicinity of fire.
Smouldering stage: Lasting for 4 hours – gas and smoke.
Radiation stage: Convective heat
Heat stage: Heat, flame, smoke, toxic gas – for few seconds.
Development of a Fire
A fire develops typically in four stages, and fire detectors are designed to detect some characteristic effect of one or more of these stages:* Incipient stage. No visible smoke, no flame and very little heat. A significant amount of invisible (but sometimes smellable) combustion particles may be created. This stage usually develops slowly.* Smouldering stage. Smoke, but no flame and little heat.* Flame stage. Visible flame, more heat, often less or no smoke, particularly with flammable liquids and gas fires.* Heat stage. Large amounts of heat, flame, smoke and toxic gases are produced. The transition from the previous stage can be very fast.
Classes of fire
To make it easier to select the appropriate extinguishing media according to the nature of the material undergoing combustion, fires are arranged in ‘Classes’.
Class A | Wood, paper, textile, rubbish, grass etc. | Water is the best extinguishing medium for Class A. Some Dry Chemical Powder(DCP) are also used. |
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Class B | Flammable liquids. (Oils, petrol, varnishes, paints, solvents, grease.) | Foam is the best extinguishing medium for Class B fires. Its reactivity should be ensured in case of Chemical fires. |
Class C | Flammable gases | Dry Chemical Powder is widely used for extinguishing gaseous fires. Its ability to cut the chain reaction in the combustion process makes it suitable for the purpose. |
Class D | Burning metal viz. Magnesium, Aluminium, Zinc, | Steam, Dry Chemical Powder be used against metal fires. |
Class E | Fires of electrical origin involving transformers, circuit breakers, switchgears | Dry sand may be used. CO2-type extinguisher to be used. DO NOT USE WATER. |
Class F | Cooking oil, fats (animal and vegetable) | Wet Chemicals to cool and emulsify. |
EXTINGUISHING A FIRE
a) Starving: By removal of the fuel material
b) Smothering: Removing the air (oxygen) supply
c) Cooling: Turning of the source of heat.
Fire Extinguishers: The following is a quick guide to help choose the right type of extinguisher.
Class A extinguishers are for ordinary combustible materials such as paper, wood, cardboard, and most plastics. The numerical rating on these types of extinguishers indicates the amount of water it holds and the amount of fire it can extinguish. Geometric symbol (green triangle)
Class B fires involve flammable or combustible liquids such as gasoline, kerosene, grease. The numerical rating for class B extinguishers indicates the approximate number of square feet of fire it can extinguish. Geometric symbol (red square)
Class C for fires of gaseous origin, using dry chemical powder.
Class D fire extinguishers are commonly found in a chemical laboratory. They are for fires that involve combustible metals, such as magnesium, titanium, potassium and sodium. These types of extinguishers also have no numerical rating, nor are they given a multi-purpose rating – they are designed for class D fires only. Geometric symbol (Yellow Decagon)
Class E fires involve electrical equipment, such as appliances, wiring, circuit breakers and outlets. Never use water to extinguish class E fires – the risk of electrical shock is far too great! Class E extinguishers do not have a numerical rating. The E classification means the extinguishing agent is non-conductive. Geometric symbol (blue circle)
Class F fire extinguishers are for fires that involve cooking oils, trans-fats, or fats in cooking appliances and are typically found in restaurant and cafeteria kitchens. Geometric symbol (black hexagon)
Types of Portable Fire extinguishers:
Spray Foam (Cream) type extinguishers: Here a chemical foam is made to form over the fire so that oxygen supply is cut off and fire dies down.
Dry Chemical Powder type extinguisher: Here the fire is controlled by the inhibiting action of the dry chemical powder which breaks the chain reaction.
Fire Detection Systems
Detection systems are based on smoke, heat, flame, gas leakage, combustion, water flow.
Conduction/Convection Heat detection: Housing of detector has a fusible element that melts at a specified temperature causing electrical contact to activate a fire alarm.
Smoke detector: These are used to detect fires in the incipient stages.
Heat detectors: They respond to high temperatures caused by fire.
Flame detector: Detects light from flames where rapid development of flame occurs.
Electromagnetic detectors: They employ photocells sensitive to infrared and ultraviolet light emitted by the fire.
Water flow detector: It indicates because the other detector types (smoke, flame or heat) is activated and starts the water sprinkler system.
Tips on Kitchen Fire Safety
Extra care should be taken when working in the kitchen in order to avoid the onset of a fire. Here are some safety tips:
Wear appropriate clothing, not long, flowing sleeves or open, loose-fitting shirts that can come in contact with hot burners.
Wear your long hair up and do not leave any strands hanging.
Never take your eyes off hot oil because it can ignite in an instant. If it catches fire, immediately place a cover on the pan.
Always unplug electrical cords from all ancillary appliances. A plug can ignite even though the equipment is turned off. Any power surge or unforeseen problem with electricity can cause a fire.
Do not use electrical appliances near water and keep cords away from all heat sources.
Turn pot handles inward on the hot stove so as to avoid bumping into them accidentally and spilling their contents.
Do not place potholders, boxes, towels, cutting boards, or plastic utensils and containers near cooking areas.
Keep the kitchen work area clean, free from grease and burnt food.
Keep a box of baking soda on the kitchen counter while cooking as it can quickly put out a small fire at its onset.
Make sure all appliances are in good working condition. At the first sign of a problem, have it fixed or get rid of it.
Fire Extinguishers:
Fire extinguishers contain different chemicals, depending on the application. Handheld extinguishers, which are commonly sold at hardware stores for use in the kitchen or garage, are pressurized with nitrogen or carbon dioxide (CO2) to propel a stream of fire-squelching agent to the fire. The active material may be a powder such as potassium bicarbonate (KHCO3), liquid water, an evaporating fluorocarbon or the propelling agent itself. The most effective and common fluorocarbon used until recently for this application had been bromochlorodifluoromethane (CF2ClBr), referred to as halon 1211. By international agreement, however, production of all types of halons ceased in 1994 because the bromine and chlorine atoms in the chemical were found to migrate over time to the stratosphere, where they react to deplete ozone in a very efficient catalytic cycle.
Many fire extinguishing systems are built into the building or other structure being protected. Water sprinklers are by far the most common type of fixed system because they are inexpensive, highly reliable and safe for people. But water damage cannot always be tolerated (say, in a computer room or electrical fire); it is sometimes ineffective (a fuel storage system); and it is impractical where weight and space are limited (in an airplane). In these situations, fire extinguishers use different materials–ones that flood a protected compartment with a fire-fighting gas. CO2 works well, but is fatal at the concentrations necessary to extinguish a fire, and so cannot be used where people will be present. Bromotrifluoromethane (CF3Br, or halon 1301) is a close cousin to halon 1211, but has a much lower boiling point and toxic level–properties that have made halon 1301 the firefighting chemical of choice for applications where sprinklers cannot be used. Manufacturers have introduced new families of chemicals containing no chlorine or bromine, called hydrofluorocarbons (HFCs),that have physical properties similar to the halons and no ozone depletion potential. But lacking Br or Cl atoms, the HFCs cannot disrupt the combustion reaction to the same degree. HFCs extinguish fires in a manner similar to CO2 or N2–by absorbing heat and reducing the concentration of oxygen. Even so, several different companies are marketing such HFCs as CHF3, C2HF5, and C3HF7 for a variety of applications.
Most often, simple fire extinguishers use sodium carbonate and dilute sulfuric acid to produce carbon dioxide, that extinguishes fire. small pouch carries the acid within a matrix of sodium carbonate. The pouch prevents the acid to come in contact with sodium carbonate outside. When the bottle is shaken, the two chemicals come in contact with each other, liberating carbon dioxide and water droplets that extinguishers.Another type of fire extinguisher contains compressed Carbon Tetrachloride in it. When the nozzle is pushed, an aerosol of carbon tetrachloride is liberated that suppresses and extinguishes fire.
This type has a major advantage that it can extinguish fires of oils, electric circuits, etc.Other forms of fire extinguishers use forced water to extinguish fire.
Types of extinguishers
Multi-Purpose Dry Chemical (A, B, C)
A dry chemical agent called mono ammonium phosphate. The chemical is non-conductive and can be mildly corrosive if moisture is present. In order to avoid corrosion, it is necessary to scrub and thoroughly cleanup the contacted area once the fire is out. A dry chemical fire extinguisher is usually used in schools, general offices, hospitals, homes.
Regular Dry Chemical (B, C)
A dry chemical agent called sodium bicarbonate. It is non-toxic, non-conductive and non-corrosive. It is easy to cleanup, requiring only vacuuming, sweeping or flushing with water. Extinguishers with sodium bicarbonate are usually used in residential kitchens, laboratories, garages.
Carbon Dioxide (B, C)
Carbon dioxide removes oxygen to stop a fire but has limited range. It is environmentally friendly and leaves no residue, so cleanup is unnecessary. Extinguishers with carbon dioxide are usually used in contamination-sensitive places such as computer rooms, labs, food storage areas, processing plants, etc.
Halotron (A, B, C)
A vaporizing liquid that is ozone friendly and leaves no residue. Because it requires no cleanup, fire extinguishers with halotron are ideal for computer rooms, telecommunication areas, theaters, etc.
Foam (A, B)
Foam floats on flammable liquids to tame the fire and helps prevent reflashes. To cleanup the affected area, it must be washed away and left to evaporate. Fire extinguishers with foam are usually used in garages, homes, vehicles, workshops.
Purple K Dry Chemical (B, C)
A dry chemical called potassium bicarbonate. It is non-conductive and non-corrosive. Clean up requires vacuuming, sweeping or flushing with water. Extinguishers with potassium bicarbonate are usually used in military facilities, oil companies, vehicles, etc.
Water (A)
The most common agent is water; however, it cannot be used for class B or C fires because it is conductive. Water-based fire extinguishers are usually used in stockrooms, schools, offices, etc.
Wet Chemical fire extinguishers (F)
The potassium acetate based agent discharges as a fine mist which forms a soapy foam that suppresses any vapors and steam or the risk of fire reflash as it extinguishes the fire. Class K fire extinguishers can usually be found in commercial cooking areas such as restaurants and cafeterias.
Area to be Covered under Fire Detection and Protection System
Fire Detection System | Area to be Covered |
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Smoke Detection System |
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Linear heat sensing cable detection system |
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Quartzoid bulb heat detection system |
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Infra-red Detectors |
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Design Philosophy of Fire Detection and Alarm System
The PLC based control panel at pump houses shall indicate the status of each pump, system pressure, operation of hydrant or spray system, failure of starting of pumps, healthiness and failure of batteries/chargers, main supply, low level/very low level of water in the water supply system, status of compressors, selection station of batteries and chargers of panels and diesel engines etc. Alarms from these panels shall also be available at fire station in a repeater panel and at main fire panel alarm in control room.
The PLC based addressable type panels at central Control rooms shall receive signal from sensor from various areas/equipments of the respective units.
The Microprocessor based addressable panel shall evaluate the signals received from detectors, transmit the fire or trouble alarms (audio-visual) to prearranged points, supervise and monitor the complete fire detection and extinguishing circuits, initiate control functions like closure of fire doors, shutdown of draft fans, air conditioning and ventilation plant/equipment, closure of fire dampers in A/C and ventilation system etc. Opening smoke extraction vents, switching on smoke extraction equipment emergency lighting, tripping of transformer lockout relays etc.
All the circuits from the detectors to the panels and the circuits from the panels to the actuating devices (such as solenoid valves, deluge valves, push buttons etc) shall be in closed loop type and in close watch for open-and-short-circuiting.
Facilities shall be provided on the main fire alarm panel for sensitivity adjustment, isolation of detectors etc from the panel.
Panel of fire detection, alarm monitoring and annunciation system shall operate on DC-supply, along with associated rectifier bank. Further each panel shall have the provision for batteries for charging.
Each set of battery shall have the capacity for a period of 12 hours from the instant of charger/AC failure.
Exit signs shall be painted o 18 gauge cold rolled sheet steel. This sheet should be suitable for fixing on walls or hanging from ceiling. Fire Exit/No Fire Exit shall be displayed on both sides of the Exit signs.
Fire alarm system shall be designed to provide round the clock security against fire in the complete power plant area.
Microprocessor based addressable analogue type fire alarm and detection system to be used for various building/areas to detect and provide alarm signal in the main fire alarm panel to be located in the central control room. Fire alarm will be repeated in a repeater alarm panel in the fire station. Main fire alarm panel annunciation shall be located in the control building. A repeater panel shall be provided in the fire station. Total no of annunciation shall be based on the specific plant requirement
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