INTRODUCTION OF FIRE
Fire is the result of a chemical combustion reaction between oxygen in the atmosphere and fuel in presence of heat causes fire
Definition of Fire: Combustion reaction where heat and flame are evolved.
FUEL + O ⎯ CO + CO
1. The Fire Triangle
The Fire triangle illustrates
the three elements a fire needs to ignite: heat, fuel, and an oxidizing agent (usually oxygen). A fire naturally occurs when the elements are present and combined in the right mixture, meaning that fire is actually an event rather than a thing.
A fire can be prevented or triangle. extinguished by removing any one of the elements in the fire
Figure 1: The Fire Triangle
2. Fire Tetrahedron:
The fire tetrahedron represents the addition of a component in the chemical chain reaction, to the three already present in the fire triangle. Once a fire has started, the resulting exothermic chain reaction sustains the fire and allows it to continue until or unless at least one of the elements of the fire is blocked. According to this concept, 4 elements are necessary to start the fire. They are:
1. Fuel (combustible material and reducing agent)
2. Oxygen or oxidant or oxidizer (from the atmosphere)
3. Heat or source of ignition (necessary to start the fire initially, but maintained by the fire itself once it has started
4. Chain reaction through free radicles to maintain the fire.
Figure 2. Pyramid of fire.
If any one of the above 4 elements is removed, the fire goes out. Therefore, the method of fire extinguishment depends on the:
1. Removing or shutting off the sources of fuel.
2. Excluding oxygen or decreasing it below 14 to 18% by adding inert gases.
3. Removing heat from the fire faster than its liberation or
4. Removing free radicles to discontinue chain reaction and flame propagation. Dry powder chemicals and halogenated hydrocarbons capture free radicals and put out a fire in this way.
Hence, Fire can be defined as:
* Fire is the rapid chemical oxidation-reduction reaction. Oxygen in air acts as an oxidizer and fuel acts as a reducing agent and burning material.
* Fire is the oxidation of substances accompanied by heat, light, and flame.
* Fire is a burning or combustion phenomenon and the combustion may be kinetic or diffusive depending on the homogeneous or inhomogeneous air-fuel mixtures. The combustion may be complete or incomplete. The complete combustion gives products like, CO2, SO2, water vapor, etc., which cannot burn anymore. The incomplete combustion gives CO, alcohol, aldehydes,etc., which can burn furthermore.
Fuels:
2. Gaseous fuels:
* Liquids: gasoline, acetone, ether, pentane
* Solids: plastics, wood dust, fibers, metal particles
* Gases: acetylene, propane, carbon monoxide, hydrogen
Oxidizers
* Gases: oxygen, fluorine, chlorine
* Liquids: hydrogen peroxide, nitric acid, perchloric acid
* Solids: metal peroxides, ammonium nitrite
3. FIRE DEVELOPMENT AND ITS SEVERITY
The lifecycle of fire starts with ignition and grows till it reaches a steady-state of burning. The stages of a fire can be described in terms of heat output. From ignition and through growth there is a steady rise of temperature in the immediate surroundings, the rate of which depends on factors like the type of fuel and the availability of fuel and air. In the case of gas fuel, the heat release rate will rise sharply after ignition and the slope during growth is steep. On the other hand, for a burning piece of wood, the rate of heat release will be much slower. By most standards including the International Fire Service Training Association (IFSTA), there are 4 stages of a fire. The following is a brief overview of each stage.
Incipient Stage:
* This first stage begins when heat, oxygen and a fuel source combine and have a chemical reaction resulting in fire. This is also known as “ignition” and is usually represented by a very small fire that often goes out on its own before the following stages are reached.
* No visible smoke, flame, or more heat developed.
* Invisible particles (<0.3 microns) are generated over a period of minutes, hours or days.
* Ionisation detectors respond to these particles.
Smoldering Stage:
* There is invisible smoke, but no flame and very little heat.
* Visible smoke generation (10% of the flammable
* Vapor from the fuel is visible).
* Little visible flame or noticeable heat.
* Sooth particles produced are of size > 0.3 microns.
* Photoelectric detectors can detect this smoke.
Flame Stage:
* Flame starts after pointing of ignition.
* Smoke decreases and heat increases (flammable vapors are ignited and self-propagating).
* Infrared detectors can detect this stage.
Heat Stage:
* Heat, flame, smoke, and gases are produced in large amounts.
* Thermal detectors respond to this type.
Ex: Burning of wood under atmospheric conditions.
Stages of Fire for Burning Wood.
How Fire Spreads ???
Fire spreads by transferring the heat energy from the flames in three different ways.
* Conduction: The passage of heat energy through or within a material because of direct contact, such as a burning wastebasket heating a nearby couch, which ignites and heats the drapes hanging behind, until they too burst into flames.
* Convection: The flow of fluid or gas from hot areas to cooler areas. The heated air is less dense, and rises, while cooler air descends. A large fire in an open area produces plumes or columns of hot gas and smoke high in the air. But inside a room, those rising gases encounter the ceiling. They travel horizontally along the ceiling forming a thick layer of heated air, which then moves downward.
* Radiation: Heat traveling via electromagnetic waves, without objects or gases carrying it along. Radiated heat goes out in all directions, unnoticed until it strikes an object. Burning buildings can radiate heat to surrounding structures, sometimes even pa sing through glass windows and igniting objects inside.
Modes of Heat transfer.
* The Spread of fire depends on the following factors:
1. The area of the substance-exposed.
2. The amount of heat generated or given off by the burning substance.
3. The ability of the substance to conduct the heat away from the zone of combustion.
4. The atmospheric humidity and the wind velocity.
Thus fire will spread more if the more combustible area is available if more heat is given by the burning material, if more heat conduction is possible and if atmospheric humidity is less and wind speed is high.
Chemistry of Fire:
* Fire or the process of combustion is extraordinarily complex. For a fire to occur, heat, fuel and oxygen and a chemical chain reaction must join in a symbiotic relationship.
* The combustion process is usually associated with the rapid oxidation of fuel by the oxygen in the air. When the combustion process is confined, pressure can increase resulting in an explosion.
* At lower temperatures a similar process taking place over longer periods of time doesn’t result in a fire, called oxidation (e.g. rusting of metals). So, a combustion process intense enough to emit heat and light is fire.
* Fires are of 2 types, which are Flame fires and Surface Fires.
Flame Fires.
Directly burn gaseous or vaporized fuel and include deflagrations. The rate of burning is usually high and a high temperature is produced. There are two types of flame fires as follows:
a) Premixed flame fires arise in gas burners or stoves and are relatively easily controlled.
b)Diffusion flame fires refer to top gases bumming on mixed vapors and air, which are difficult to control.
Surface Fires:
* They occur on the surfaces of solid fuel and are often called a "glow” fire. This occurs at the same temperature as the flame fires. Surface fires can be represented by a triangle composed of heat, fuel, and oxygen, but a flame fire includes an additional component i.e., chemical chain reaction, represented by a tetrahedron. These two fires may occur alone or together.
Surface fires and flame fires.
IMPORTANT DEFINITIONS
Combustion or Fire: Combustion or fire is a chemical reaction in which a substance combines with an oxidant and releases energy. Part of the energy released is used to sustain the reaction.
a) Ignition: Ignition of a flammable mixture may be caused by a flammable mixture coming in contact with a source of ignition with sufficient energy or the gas reaching a temperature high enough to cause the gas to auto-ignite.
b) Auto-Ignition Temperature (AIT): A fixed temperature above which adequate energy is available in the environment to provide an ignition source.
c) Flash Point (FP): The flash point of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air. At the flashpoint, the vapor will burn but only briefly; inadequate vapor is produced to maintain combustion. The flashpoint generally increases with increasing pressure.
d) FirePoint: The fire point is the lowest temperature at which a vapor above a liquid will continue to burn once ignited; the fire point temperature is higher than the flashpoint.
e) Spontaneous Ignition Temperature: This is the lowest temperature at which a substance will burn without the introduction of a flame or other ignition source. This is also called ignition temperature. This means that under certain conditions some materials undergo spontaneous ignition
f) Flammability Limits: Vapor-air mixtures will ignite and burn only over a well-specified range of compositions. The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion. The mixture is also not combustible when the composition is too rich; that is when it is above the upper flammable limit (UFL). A mixture is flammable only when the composition is between the LFL and the UFL. Commonly used units are volume percent fuel (percentage of fuel plus air). Lower explosion limit (LEL) and upper explosion limit (UEL) are used interchangeably with LFL and UFL.
Flammability relations
The above figure is a plot of concentration versus temperature and shows how several of these definitions are related. The exponential curve in Figure 2 represents the saturation vapor pressure curve for the liquid material. Typically, the UFL increases, and the LFL decrease with temperature. The LFL theoretically intersects the saturation vapor pressure curve at the flashpoint; although experimental data do not always agree with this. The auto-ignition temperature is actually the lowest temperature of an auto-ignition region.
g) Ignition Energy:
The minimum ignition energy (MIE) is the minimum energy input required to initiate combustion. All flammable materials (including dust) have MIEs. The MIE depends on the specific chemical or mixture, concentration, pressure, and temperature.
Experimental data indicate that
* The MIE decreases with an increase in pressure,
* The MIE of dust is, in general, at energy levels somewhat higher than combustible gases, and
* An increase in the nitrogen concentration increases the MIE.
Many hydrocarbons have MIEs of about 0.25 mJ. This is low compared with sources of ignition. For example, a static discharge of 22 mJ is initiated by walking across a rug, and an ordinary spark plug has a discharge energy of 25 mJ. Electrostatic discharges, as a result of fluid flow, also have energy levels exceeding the MIEs of flammable materials and can provide an ignition source, contributing to plant explosions.
h) Auto-ignition:
The auto-ignition temperature (AIT) of vapor, sometimes called the spontaneous ignition temperature (SIT), is the temperature at which the vapor ignites spontaneously from the energy of the environment. The auto-ignition temperature is a function of the concentration of vapor, volume of vapor, pressure of the system, presence of catalytic material, and flow conditions. It is essential to experimentally determine AITs at conditions as close as possible to process conditions.
i) Auto-Oxidation:
Auto-oxidation is the process of slow oxidation with accompanying evolution of heat, some- times leading to auto-ignition if the energy is not removed from the system. Liquids with relatively low volatility are particularly susceptible to this problem. Liquids with high volatility are less susceptible to auto-ignition because they self-cool as a result of evaporation.
Many fires are initiated as a result of auto-oxidation, referred to as spontaneous combustion. Some examples of auto-oxidation with a potential for spontaneous combustion include oils on a rag in a warm storage area, insulation on a steam pipe saturated with certain polymers, and filter aid saturated with certain polymers.
j) Adiabatic Compression
An additional means of ignition is an adiabatic compression. For example, gasoline and air in an automobile cylinder will ignite if the vapors are compressed to an adiabatic temperature that exceeds the auto-ignition temperature. This is the cause of pre-ignition knock-in engines that are running too hot and too lean. It is also the reason some overheated engines continue to run after the ignition is turned off.
Several large accidents have been caused by flammable vapors being sucked into the intake of air compressors; subsequent compression resulted in auto-ignition. A compressor is particularly susceptible to auto-ignition if it has a fouled after-cooler. Safeguards must be included in the process design to prevent undesirable fires that can result from adiabatic compression.
Effects of Fire on Personnel:
The fire by which temperature and heat can injure personnel is through burns which can injure the skin and, sometimes, muscles and other tissues below the skin. Skin burns are classified into three degrees of severity.
THE SKIN
The skin protects underlying structures from damage and invasion by harmful organisms; acts as a sense organ; helps in regulating body temperature; stores fat, water, and other substances important to the body; and excretes water and oils. The skin of an adult has an area of about 20 square feet. Its thickness varies from about 1 mm over the eyelid to about 2-3 mm on the back, palms, and soles.
It consists of two main parts, the epidermis (cornea) and dermis (corium). the epidermis is the outer portion, composed of a number of layers of cells. Overall, the epidermis is comparatively thin but of thickness which varies considerably from one part of the body to another, also being thickest on the palms of the hands, the soles of the feet, and other locations subject to rubbing or pressure. It is free of blood vessels. The outer portion of the epidermis is made up of several layers of different kinds of lifeless and scale-like cells. These are sloughed off constantly and replaced by cells from below. The inner layer of the epidermis performs a number of functions. It undergoes frequent cell division, which replaces those cells which are compressed to form the outer layer of the epidermis. Inpocketing of cells from the inner layer of the epidermis extends deep into the dermis to make up the hair follicles, oil and sweat glands, and nails. The inner epidermal cells also contain the pigment, which gives the skin its principal coloration (The outer layer is normally translucent, with a yellowish tinge.)
The dermis (corium) is much thicker than the epidermis. It holds the glands and hair follicles, which are ingrowths from the inner layer of the epidermis, blood and lymph vessels, fat cells, sense organs, and nerves. The fat acts as a cushion against impact injury, assists in preventing excessive loss of body heat, acts as a food reservoir and guards delicate structures by absorbing outside shocks.
Hair is present over most of the body. Each consists of a shaft, which projects beyond the surface, and a root embedded in the skin. The root and two layers of cells from the hair, follicle, and consist of tightly packed dead cells. There are countless of these follicles. Each is associated with an oil (sebaceous) gland, which secrets a film of oil to keep the skin from drying out and cracking. The oil also keeps the hair pliable and moist,
The subcutaneous layer below the dermis contains the sweat glands. There are approximately 2½ million sweat glands all over the body. They are most numerous on the palms of the hands, soles of the fleet, forehead, and in the armpits. The sweat glands pass
perspiration to the surface of the skin, where it evaporates and cools the skin and body. Perspiration is over 99 percent water, with traces of protein and some sodium chloride. Moisture that passes through the sweat glands is brought there through the blood vessels in the dermis. The number of moisture increases as the flow of blood increases due to a warm environment, physical exertion, stress, or illness.
The color of the skin is determined principally by pigments in the second layer of the epidermis. The overlying layer is almost transparent. The blood vessels near the surface also vary the color shade of the skin. When capillaries are contracted (vasoconstrictor), as when affected by contact with a cold surface, the skin appears pale. When the skin is warmed, the capillaries expand (vasodilatation), blood flow increases and the skin appears redder.
First-Degree Burns
First-degree burns involve only redness of the skin, which indicates a mild inflammation. The most common first-degree burn is sunburn. First-degree burns from fires may result from a much greater amount of radiation received during a shorter period of time. However, the two do not vary directly.
Second- Degree Burns.
Second-degree burns are much more serious than first-degree burns. Blisters of the skin will form, and in the server cases, fluid will collect under the skin. The skin beneath the blisters is extremely sensitive.
Third-Degree Burns.
In third-degree burns, the skin, subcutaneous tissue, red blood cells, capillaries, and sometimes muscle are destroyed. Burns may be white, light gray, brown, or even charred black. Third-degree burns from the moist heat, such as scalds or immersion, are usually white, in folds. Cryogenic burns, caused by skin contact with an extremely cold metallic surface, will also cause third-degree burns, destroying skin and underlying tissue, and possibly resulting in gangrene.
EFFECTS ON SKIN IN CONTACT WITH SURFACES AT DIFFERENT TEMPERATURES
Temperature (oF) Sensation or Effect
212 Second - degree burn on 15-seconds contact
180 Second - degree burn on 30-seconds contact
160 Second - degree burn on 60-seconds contact
140 Pain, tissues damage (burns)
120 Pain, burning heat
91+ 4 Warm, “neutral “(physiological zero)
54 Cool
37 Cool heat
32 Pain
Below 32 Pain; tissue damage (freezing)
Classification of Burn Severity:
The classification of burn injuries:
Critical burns
a.Second-degree burns exceeding 30 % of the body surface.
b.Third-degree burns exceeding 10 % of the body surface.
c.Burns complicated by respiratory tract injury, major soft tissue injury, and fractures.
d.Electrical burns.
e.Third-degree burns involving critical areas, for example, the hands, face, or feet.
Moderate burns
a.Superficial second-degree burns of more than 15 % of the body surface.
b.Deep second-degree burns of 15 to 30 % of the body surface.
c.Third-degree burns of less than 10 %, excluding the hands, face, and feet.
Minor burns
a.First-degree burns.
b.Second-degree burns of less than 1 % percent of the body surface.
c.Third-degree burns of less than 2 % of the body surface.
Other Temperature Effects on Personnel
Normal body temperature is approximately 98.6o F. Heat produced by metabolic oxidation of food warms the body and is balanced by heat losses. When there is an imbalance, the net difference between production and loss governs body temperature. In a cold environment, metabolism must be increased to match losses; in warm surroundings, the body must lose any excess or store it. The relationship between the various factors may be expressed by,
S = M – E + R + C + D – K – F
Where
S = heat excess or deficiency
M = heat produced by body metabolism
E = heat loss by evaporation
R = heat loss or gain by radiation
C = heat loss or gain by convection
D = heat loss or gain by conduction
K = heat expended in physical exertion
F = heat loss due to respiration, excretion, urination, etc.
Metabolism always involves the production of heat. An individual has an energy input that depends on the calorie content of the food the individual eats. Any excess over losses is stored in the body as fat; any deficiency must be satisfied by existing stores of fat. If energy from body stores or other sources is not available, body exertion must or will be reduced. As molecules break up in these branched-chain reactions, unstable intermediate products called free radicals are formed, the concentration of which determines the speed of the flames. Extinguishing agents, such as dry chemicals and halogenated hydrocarbons, remove the free radicals in these branched-chain reactions from their normal function as a chain carrier and thus quench the fire.
4. CLASSIFICATION OF FIRES
NFPA 10 classifies fires and fire extinguishers into the following 4 types:
CLASS A: fires in ordinary combustible materials such as wood, paper, textiles, rubber, and many plastics that require the heat-absorbing coolant effect of water or water solutions, the coating effects of certain dry chemicals that retard combustion, or the interruption or the combustion chain reaction by the dry chemical or halogenated agents. The ABC extinguisher can extinguish this type of fire.
CLASS B: Fires inflammable or combustible liquids, greases, petroleum products, solvents and that must be put out by excluding air by inhibiting the release of combustible vapor with AFFF or FFFP agents, or by interrupting the combustion chain reaction.
CLASS C: Fires involve live electrical equipment. The operator safety requires the use of electrically non-conductive extinguishing agents such as dry chemical as halon. When electric equipment is de-energized, extinguishers for Class A or B fires may be used.
CLASS D: Fires in certain combustible metals, such as Sodium, potassium, magnesium, titanium, zirconium that require a heat-absorbing extinguishing medium that does not react with the burning metals.
CLASS K: Fires involving cooking oils. This is the newest of the fire classes.
Fire Classification in India.
Figure 7: Fire Classification UK & USA.
Common Causes for Industrial Fire:
A successful fire prevention program begins with identifying all potential fire hazards. Here's a list of the most common (you may have others to add to the list):
* Scrap and trash. When waste materials are allowed to build up, the danger of fire increases. All it takes is an ignition source to get a fire going, and then the fire has plenty of fuel on which to feed.
* Dust. An excess of dust or powder in the air from wood, plastic, or metal operations can, if ignited, cause an explosion. Combustible dust explosions are among the most destructive and deadly types of workplace accidents.
* Flammable liquids. Improper handling, storage, or disposal of flammable liquids used in production processes, as fuel sources, or for cleaning operations is a leading cause of workplace fires.
* Combustible materials. Ordinary combustibles like paper, cardboard, cloth, and wood, or products made from these materials, can create fire hazards as well. Other combustible materials, such as oily rags or other materials soaked with oil, can spontaneously combust if left carelessly lying around.
* Electrical problems. Overloaded circuits and outlets, damaged wiring, defective switches, and damaged plugs can all lead to dangerous electrical fires. Electriccoffeemakers, fans, space heaters, and other appliances used by employees are also potential fire hazards.
* Sparks may be mechanical, electrical, and static, due to cutting and welding.
* Heat and ignition sources. Any source of heat or ignition (such as a spark) can lead to a fire when combined with combustible or flammable materials.
* Machinery. Inadequately lubricated or dirty machinery can also cause fires, as can mechanical defects.
* Smoking. Although smoking is most likely prohibited except in designated areas, employees may ignore the rules and sneak a smoke in restrooms or some low-traffic hideaway. A smoker might toss a match or cigarette butt into a wastebasket thinking it's extinguished when, in fact, it's still burning.
* Hot surfaces may be due to bearings, and shafting, stoves, heaters, and small appliances, petrol, kerosene, LPG, acetylene or alcohol or alcohol torches, portable furnaces, blowtorches, smoke pipes, chimneys, flues and stacks, stationary heating devices, gas-fired appliances viz. heaters, boilers, salamanders, etc.
* Hazardous chemicals and metals like phosphorous, sodium, potassium, oxidizing materials, nitro-cellulose film, and pyroxylin plastics, fuels, solvents, lubricants, wood, paper, cloth, and rubber products, sprays and mists, LPG, and other flammable or explosive gases are known for fire hazards.
* Spontaneous ignition is due to oxidation of fuel where the air is sufficient but ventilation is insufficient to carry away the heat as fast as it is ignited. Exposure to high temperature and presence of the moisture increases the tendency towards Spontaneous ignition.
* Wet un-slaked lime and sodium chlorate, rags or waste saturated with linseed oil or paint, sawdust, hay grains, etc., and finally divided metals promote spontaneous ignition.Causes of fire
Causes of fire %
Electrical 19
Friction 14
Foreign substance 12
Open flames 9
smoking and matches 8
5.FIRE PREVENTION AND PROTECTION SYSTEMS:
A broad classification of fire system is explained below:
Fire Prevention:
This is an activity directed towards the elimination of possible and potential sources of fires. It mainly indicates the measures to avoid the inception of fires. Where the source cannot be eliminated or avoided, exercise sufficient control to ensure its safe usage. The activity also involves control over the handling, storage, and process of combustibles.
Fire Protection:
This is activity towards limiting the spread of fire to its place of origin by resorting to design, compartmentation, utilization of fire-resistive materials, provisions of safe means of escape, control by portable and fixed automatic extinguishing systems.
Fire Fighting:
This is an activity directed towards provisions of proper fire fighting equipment, proper maintenance, personnel with proper organization, training program, and readiness to fight to fire.
Salvage:
This is an activity to minimize the damage due to fire, smoke, and water to the uninvolved property.
Return to normalcy:
This is a contingency plan where the various steps are laid down to bring back the industry to the productive stage from the crippling damage due to the fire.
6. DETECTION OF FIRE
Basics of detectors and alarms:
* The function of fire detectors is to detect one or more changes in the protected environment indicating the development of a fire condition. Fire detectors are useful devices; they act by sensing either locally or on a panel located elsewhere. Detectors are classified based on either the fire effects they sense or on their working principles.
* The fire detection systems are installed in buildings for:
- The protection of life.
- The protection of property
- A mixture of these purposes, either simultaneously or at different times and places.
* The fire detector is primarily intended to detect changes beyond a threshold value in its immediate environment due to either the effects of or products of fire. These can include the following in the respective sub-stages:
- Invisible products are released at incipient.
- Visible smoke.
- Illumination once flaming starts.
- Gaseous products of combustion like carbon monoxide and carbon dioxide.
- Rise of ambient temperature to asset point or its rapid rate of rising.
Detectors types based on effects:
* Detectors may also be classified as “spot” and “line” types.
* Spot detectors have the sensor located at one point and can sense the actuating condition within a limited radial area around it. Examples of some detectors are bimetallic detectors, fusible alloy detectors, pneumatic rate-of-rise detectors, smoke detectors, and thermoelectric detectors. The area that may be covered by spot detectors will depend on the type of detectors, compartment configurations, ambient conditions, etc.
* Line detectors may be located either at one point or over a continuous distance but can, in both cases, since the actuating conditions over the designed distance. Examples of line-type detectors are rate-of-rise pneumatic tubing detectors, projected beam smoke detectors, and heat-sensitive cables.
* In very spaces like atria, aircraft hangers, and indoor stadia, line detectors are the preferred type and the area covered will also depend on numerous factors.
* The various types of detectors are available operating on principles of thermal expansion, thermoelectric sensitivity, thermos conductivity, or photosensitivity to detect the presence of smoke, increase in temperature, light intensity, or total radiation. Their types are thermal expansion detectors, radiant energy detectors, light interference detectors, and ionization detectors. They should be properly located depending on their range. They give an alarm and cannot extinguish the fire. They make us alert for fire-fighting. Broad classification of sensors based on their detection principle and sensitivity:
Heat Fixed temperature
Rate of rising of temperature
Combination
Rate of compensation
Smoke Ionisation
Photoelectric
Aspirating
Gases Carbon monoxide
Light Visible light – light obscuration
Visible light–scattering
IR radiation
UV radiation
HEAT DETECTOR:
The following effects are utilized as principles of actuation of heat detectors.
- Rise of temperature in the immediate surroundings, and
- Faster rise of ambient temperature than that due to normal atmospheric changes.
Heat detectors are classified into 5 types:
FIXED TEMPERATURE HEAT DETECTOR:
* These are the most common heat detectors in use and are set to operate when the temperature around the detector reaches an asset value that is well above the maximum temperature reached that location under normal conditions. Some sensing elements employed in fixed temperature detectors are:
* Fusible elements made of alloys or eutectic metals that melt at a known temperature and soldered joint type sprinklers.
* Expanding metal or gas contained in a closed place that completes an electric circuit.
* Quartzoid bulb type sprinklers where a liquid expands to shatter the bulb and release water held in the system.
* Bimetallic elements where the differential expansion of the two metals causes the element to bend and make an electric circuit.
* Line detectors are made up of two cables with insulated sheathing that breaks down when heated to complete an alarm circuit.
* The detecting element in a fixed temperature detector must be fully heated to its set temperature for the alarm to be actuated and this makes a fixed temperature detector suited for slow-growing fires. For fast-growing fires, the detector element may not be heated fully as is required for fixed temperature detectors and a dangerous time lag may occur.
* Fixed temperature detectors are suited for spaces that may normally have high ambient temperatures such as in kitchens, garages, or bathrooms.
Bimetallic Elements:
When a bimetallic strip made of two metals with different coefficients of thermal expansion (ex: iron and copper) is heated, one expands more than the other. The differential expansion causes the strip to bend and the effect can be used to close a normally open circuit or open a normally closed circuit. The size of the gap between the contacts determines the temperature to which the strip must be heated to get the desired effect. Detectors using bimetallic strips generally comprise invar (Ni-Fe alloy) for the low expansion component and alloys of Mn- Cu-Ni, Ni-Cr-Fe, or stainless steel for the high expansion component.
Schematic of bimetallic strip fixed type detector.
Expansion of Liquids:
The Quartzoid bulb sprinkler is an established example of a heat detector based on the thermal expansion of liquids. As such, both the liquid and the air bubble contained within the liquid expand when there is a rise in the temperature of the surrounding air. Under sustained free conditions, the glass bulb shatters and water is sprayed over the design area.
Frangible Bulb Sprinkler.
Fusible Elements:
Some alloys and eutectic metals (ex: bismuth, lead, tin, and cadmium) melt at relatively low temperatures (generally in the range of 55-180 0C), a property that has been used in one type of fixed temperature detector. The fusible element can be a solder to hold a spring under tension. When the element fuses, the spring is released to complete a circuit and initiate an alarm. Soldered-joint type sprinklers and automatic fire door closers use fixed temperature detectors based on fusible elements (figure 46). The fusible link type detectors are not reusable.
Typical fusible link sprinkler and door closer.
RATE-OF-RISE TEMPERATURE HEAT DETECTOR
* Fast-growing fires lead to the rapid rise of temperature in the immediate surroundings which a rate of rising heat detector is designed to sense. The internal components of such a detector constantly compare the temperature of the surroundings to a baseline temperature programmed into the detector and the detectors are designed to compensate for any normal variations in ambient temperature.
* A set of predetermined criteria are also programmed into the detector and when the rise in temperature matches these criteria, the alarm is actuated. A typical ROR heat detector responds when the temperature rise is in the range of 7 to 8 0C/min or higher.
* An ROR heat detector is suited for areas that are inaccessible to smoke or where the average temperature remains constant but are likely to give false alarms in case of events like a rush of hot air when oven doors are opened.
Schematic of the rate of rising temperature detector.
COMBINATION HEAT DETECTOR
To take the advantage of the features of both, fixed-temperature and ROR detectors, designers have developed a combination type heat detector. A combination heat detector offers good protection for areas with unstable, high average temperatures, such as rooms housing several ovens that are routinely opened and closed.
RATE COMPENSATION DETECTOR
A rate compensation detector has a big advantage over both fixed temperature and ROR types of detectors because it can accurately sense the surrounding air temperature regardless of the fire growth rate and the system is activated at the set temperature.
Rate Compensation detector.
A typical rate compensation detector consists of a pair of low thermal expansion metal or alloy struts housed in a tubular outer shell of high expansion metal or alloy, which closely follows changes in surrounding air temperature. The struts have contact with each other which stay apart from each other under normal conditions. When exposed to a rapidly growing fire, the outer shell tends to expand, while the inner struts are slow to expand and exert restraint on the outer shell. In due course, the contacts meet to raise an alarm (figure 26).
LINE DETECTOR
It consists of pair of twisted wires of different metals or alloys to give them the right strength, electrical conductivity, and corrosion resistance. Each wire is insulated with a material that loses its insulating property at higher temperatures.
The wire pairs have an outer jacket to protect against damaging environmental conditions (figure 27). When the temperature reaches the limit value along with the line detector, the wires contact each other to create a short circuit and actuate an alarm.
Line detector.
SMOKE DETECTOR
Smoke detectors are intended to respond to a threshold concentration of “smoke”, which includes pyrolysis products, vapors, visible or invisible products of combustion, etc. Types of Smoke detectors.
IONIZATION DETECTOR
a. What is ‘Ionization’?
An atom is made up of protons, electrons, and neutrons, the protons and electrons being in balance as shown below (three of each in this case). If the atom is subjected to radiation from a radioactive source some electrons become detached. As a result, the atom becomes positively charged (i.e., it has more protons than electrons), the ‘free’ electrons quickly link up with other atoms which become negatively charged (i.e., more electrons than protons). These ‘new’ atoms are called ‘Ions’ and the process that creates them is called “ionization”
Ionization chamber
* Ionisation type smoke
detectors contain a small amount of radioactive material
(commonly a foil of americium oxide in a gold matrix) in a silver capsule held inside a palladium chamber. Americium-241 is a good source of alpha particles. The chamber wall is thick enough to retain the radioactive material but thin enough to allow the passage of alpha particles.The ionization chamber contains two metal plates separated by a small distance and connected to a battery. One of the plates carries a positive charge and the other a negative charge. Air molecules between the two plates get ionized as electrons are kicked out of the molecules by alpha particles from the radioactive material.
The mechanism of working of ionization detector.
* Oxygen and nitrogen ions are positively charged being short of electrons and are attached to the cathode. The free electrons are negatively charged and are attracted to the anode. In
a fire condition, smoke particles entering the chamber become attached to the ions
because of electrostatic attraction and slow their movement. This causes a reduction in the current flow. When the current falls below a predetermined level the amplifier senses it and initiates an alarm. They are very sensitive in the early stages of fire when smoke particles are small and must be sited very carefully.
PHOTOELECTRIC DETECTOR
This works on the principle of the amount of light reaching a photoelectric cell. The 2 subclasses of photoelectric type detectors are:
a. LIGHT OBSCURATION TYPE DETECTOR
The light from a source such as an LED is normally incident on a photoelectric receiver (photodiode). When smoke enters the path of light between the source and the receiver, less light reaches the receiver than normally and, when the smoke density/obscuration reaches a set level, an alarm is actuated.
Light obscuration type smoke detector
They are installed with the light source at one end of the area to be protected and the receiver at the other end. They are used to protect large open areas.
b. LIGHT SCATTERING-TYPE DETECTOR
These are built such that under normal non-fire conditions when the detector chamber is free from any smoke particles, light emitted from the light source does not reach the photosensor. When smoke enters the chamber, some of the light is scattered towards the sensor and, when smoke density reaches a threshold value, the light reaching the cell will either create an electrical current in the detector circuit or allow more current to flow through it and an alarm will sound.
Light scattering type Smoke detector.
ASPIRATING DETECTOR
These detectors consists of a detector with a small air pump attached to a piping manifold with perforations used to draw air samples from the surroundings and send it to the sensing element. They can be described as a combination of the point and line type smoke detectors. The air being sampled may be passed through a filter before being analysed for the presence of smoke. Very sensitive sensors are selected for use in aspirating detectors to account for the effects of dilution of smoke with air as it is aspirated.
Aspirating detector.
DUCT DETECTOR
It is used to monitor the quality of air as it travels through building HVAC ductwork. It has the advantage of using a single detector to sample a much greater volume of air than would be possible with an ordinary spot type smoke detector.
Duct detector.
OPTICAL FLAME DETECTOR
* Fires produce both visible and invisible: infrared or ultra-violet radiation and flame detectors are designed to respond to both forms. Flame detectors respond to the production of one or a combination of ultra-violet or infrared spectrums of electromagnetic radiation.
* These detectors are often used in situations where there is a potential for the rapid development of fire such as flammable liquids. These detectors comprise an electronic circuit with an electromagnetic radiation receiver.
* Flame detectors are actuated when they receive electromagnetic radiation from one or more defined wave lengths are received according to their design in the ultra-violet or infrared spectrum.
* Flame detectors can be used to protect large open areas. They act fast but require direct line of sight to all parts of the area covered. Flame detectors respond rapidly to fires in clean burning fuels such as alcohol or methane that would not be detected by smoke detectors. At places where flames may take long time to appear, it may be useful to install flame detectors in conjunction with smoke detectors.
Optical flame detector.
GAS DETECTOR
The electronic circuit is calibrated to a normal range of atmospheric carbon monoxide. When the concentration of carbon monoxide increases the current produced by the cell also increases which in turn creates an alarm signal. Carbon monoxide fire detectors have a relatively short life span 5-10 years and should be maintained strictly in accordance with the manufacturer’s recommendation for optimal performance.
Gas detector.
ALARM SYSTEMS
* A fire alarm system consists of number of devices working together to detect and warn people through visual or audio appliances when smoke, fire, carbon monoxide or other emergencies are present.Alarms may be activated from the heat or smoke detectors.
* The simplest and oldest fire alarm is probably is the fire bell.
* Public address (PA) systems are also widely used in industries in large buildings to alert occupants about an outbreak of fire and to advise them about evacuation or similar actions.
* The break glass fire alarm is a common manually actuated device.
* Alarm systems can be divided into four groups: local, auxiliary, central station, and proprietary.
* All types of alarm systems should be equipped with a signal system that clearly communicates to all persons in the building, plant, or laboratory.
* Whenever an alarm is sounded in any portion of the building or area, all employees must know what the sound means.
Local Alarm Systems
* A local alarm consists simply of bells, horns, lights, sirens, or other warning devices right in the building.
* Local alarms are generally used for life protection – that is, to evacuate everyone and thus limit injury or loss of life from the fire.
* A local alarm can be tied in with another system to call the fire department.
* Local alarm systems are inexpensive, available from a wide range of suppliers, and easy to install.
Auxiliary Alarm Systems
* Auxiliary alarm systems are even less expensive than local alarm systems.
* Such a system simply ties a fire detector to a nearby fire call box. In effect, it becomes a transmit station triggered by fire detectors inside the building.
Central Station Systems
* Central station systems are available in most major cities around the country.
* Operated by trained personnel, a central station continually monitors a number of establishments and, in case of an alarm, calls a nearby fire station and alerts the building’s personnel.
Proprietary Alarm Systems
* Proprietary alarm systems feed alarms to the building’s maintenance force, and,
optionally, to the fire department as well.
FIRE ALARM SYSTEM AND CONTROL PANEL
Typical alarm system
* A fire alarm control panel is a core component of a fire system and can be rightly called
the “CPU” of a fire detection and alarm system.
* The primary purpose of an annunciator panel is to monitor each circuit, zone or detector for condition of an alarm, display the status of that condition and to operate any required output as per system design and are intended to warn occupants of a fire signal and notify emergency response personnel at security or fire station.
* It also may be linked to the public fire brigade station or to fire control systems.
FIRE ALARM SYSTEM
7. CLASSIFICATION OF FIRE EXTINGUISHER
Two types of extinguishers are used to extinguish and control fires:
- Portable fire Extinguisher
- Fixed fire Extinguisher
Portable Fire Extinguishers
As compared to fixed fire extinguishers,portable (first-aid) fire extinguishers are
desirable for quick manual use on small fires and for the period till automatic equipment or outside fire fighters work.Portable fireextinguisher is usually meant for immediate use in the early stage of fire (i.e. Small fires). It is designed to be carried and operated by hand.The total weight of an extinguisher should not be more than 23 kg. The capacity of portable extinguisher varies from 1/2 to 9 Litres per 10 Kg. The choice of a portable extinguisher to be used for a particular risk is to be designed in relation to the nature of fire.
All such extinguishers should be of:
* Reliable make, standard (IS) and properly identified.
* Right type depending upon the class of fire
* Sufficient in number.
* Properly located where they are necessary and readily accessible
* Recharged periodically, inspected and maintained in good working condition.
* Known by operators who are trained to use them.
Type of Extinguishers:
Portable fire extinguishers are classified into 5 different types, according to the extinguishing medium they contain and they are as follows:
- Water type
- Soda acid type
- Carbon dioxide type
- Foam type
- Dry chemical powder type
Common Features of Extinguishers:
The fire extinguisher comprises of the following:
* Pressure gauge
* Safety pin
* Siphon tube
* Hose
* Nozzle
* Gas Cartridge
* Extinguishers are normally operated with the help of gas pressure in the upper part of the container, which forces the extinguishing medium out through a nozzle. Portable extinguishers contain extinguishing media that require a moving force to be expelled. In the case of water, foam or dry chemical powder (DCP) extinguishers, this force is derived from:
a) Chemical Reaction:Two or more chemicals are allowed to react to produce an expellant gas when the operating mechanism is activated.
OR
b) Stored Pressure:The expellant gas is stored with the extinguishing medium in the body of the extinguisher, which is thus permanently pressurized. In the case of carbon dioxide extinguishers, the expellant gas is itself the extinguishing medium.
OR
c) Gas Cartridge:The pressure is produced by means of compressed or liquefied gas released from a gas cartridge fitted into the extinguisher.
Typical components of an extinguisher.
* Metal dry power and halocarbon extinguishers should be charged only with the type of gas mentioned in the label of the extinguishers. CO2 extinguishers are simply CO2 cylinders with an appropriate discharge horn. The size of the cartridge depends on the size of the size of the extinguishers and may accordingly be located inside the extinguisher or attached outside it.
* Stored pressure type extinguishers are filled with dry air or nitrogen (both of specific quality) till the desired pressure is built up in the shell and the container is sealed tight.
* A pressure gauge is mounted at the top the extinguisher shell to indicate availability of adequate pressure for use.
* While actuating, the safety pinis withdrawn and valve leveller is depressed to force out the extinguishing medium through an internal siphon tube into the delivery hose and nozzle.
* Gas cartridge type extinguishers have a CO2cartridge with a sealing disc screwed to the cap of the extinguisher. The plunger has a piercing nail attached to it which punctures the disc of the cartridge when pushed down. CO2gas pressurises the extinguisher up to 6-7 kg/cm2 pressure and expels the contents of the extinguisher as in case of the stored pressure type extinguishers.
* The cartridge must be weighed periodically as per standards and changed if loss is more than 10% of the original. The cartridge must also be replaced after an extinguisher is used. The total weight of a full charged extinguisher should not exceed 23 Kg.
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