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Learn the essence of fire science, including prevention methods, the components of combustion, and the combustion process in gas and liquid forms to safeguard life and property.
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Fire Science • Fire Science is the science of preventing and mitigating the adverse effects of fire. • Prevention decreases the likelihood that an emergency will occur. • Mitigation actions are steps that eliminate or reduce the loss of life or property damage. • Fire-science programs cover all aspects of fire prevention, control, response and post-incident activities.
Definition of Fire • Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products • Usually fire occurs when a source of heat comes in contact with combustible material in presence of air.
Fire Elements Three (3) elements are required for combustion to occur : • Fuel – to vapourise and burn • Oxygen – to combine with fuel vapour • Heat – to raise the temperature of the fuel vapour to its ignition temperature
Fire triangle • Fire triangle or combustion triangle is a simple model for understanding the ingredients necessary for occurrence of a fire.
Fire triangle….. • The triangle illustrates a fire requires three elements : heat,fuel and oxygen. • The fire is prevented or extinguished by removing any one of them. A fire naturally occurs when elements are combined in right mixture.
Fire triangle……. • Fire cannot begin if either heat or oxygen is insufficient. • When oxygen-fuel mixture is in right proportion and the heat source is of adequate intensity to initiate ignition of oxygen fuel mixture, fire will occur.
Fire tetrahedron • Fire tetrahedron is an addition to the fire triangle. It adds the requirement for the presence of the chemical reaction which is the process of fire.
Combustion Process…. • The occurrence of combustion is associated with a spontaneous acceleration of the rate of oxidation reaction in the system. The process of spontaneous acceleration of the rate of oxidation turning into combustion can be attributed to three basic processes:-- -- heat -- chain -- heat-chain reaction
Combustion Process…… • The heat theory explains the autoignition by the activation of oxidation and increased chemical reaction due to heat. • The chain theory states self- ignition to the branching of the chains of chemical reaction. • In practice, combustion process occurs by heat-chain mechanism.
Combustion of gases • Fire and explosion hazards from gases are determined by : • the concentration limits of ignition (more commonly known as the explosive range), ii) ignition source intensity, iii) combustion temperature and iv) rate of flame spreading.
Combustion of gases…. • A gaseous combustible mixture will burn only if it contains the right proportion of fuel. The lowest amount of fuel at which combustion may take place is known as the lower concentration limit of the explosive range. The highest concentration of fuel in a combustible mixture at which combustion may still take place is called the upper concentration limit of the explosive range. The range of concentration is called the explosive range (or range of ignition).
Combustion of gases……. • The minimum intensity of ignition is the lowest value of the electric discharge spark intensity sufficient to ignite the most rapidly ignitable mixture of the given gas, vapor or dust with air. • A gas is termed combustible if its mixture with air ignites at temperatures below 55°C. • Gases such as acetylene, hydrogen and hydrogen sulfide which have a wide explosive range, low lower limit of the explosive range, low ignition intensity and high flame speed are the most dangerous.
Combustion of liquids • All combustible liquids ,are known to evaporate and combustion takes place only in the vapor phase over the liquid surface. • The amount of vapor formed dependson the temperature of the liquid. • Combustion will take place only when the concentration of vapor reaches the explosive range limit.
Combustion of liquids……. • The lowest liquid temperature at which the proportion of vapors in air is sufficient for the mixture to ignite in the presence of an open flame heat source without consequent steady state combustion is known as the flash temperature (or flash point). • A liquid is termed combustible if it keeps burning when ignited and after the ignition source has been withdrawn. • The flash point of such liquids is not above 61°C (in closed crucible) or not above 66°C (in an open crucible).
Combustion of liquids……. • The flash point is known as the temperature at which a liquid become extremely dangerous as regards fires. It has, therefore, been adopted for classifying combustible liquids by their hazard of fire. • Fire and explosion risks involved in liquids can be characterized also by the temperature limits of the explosive range of their vapors.
Combustion of liquids……. • The temperature of a liquid at which the concentration of its saturated vapour in a limited air volume is capable of igniting in the presence of a source of ignition (heat) is known as the lower temperature limit of the explosive range. • The temperature of the liquid at which the concentration of its saturated vapors in a limited air volume is still capable of igniting in the presence of an ignition source is called the upper temperature limit of the explosive range.
Combustion of dusts • Dust, by fire hazard, is many times more dangerous than the material (product) from which it is formed. This is explained by a greater specific surface of dust particles as compared with that of a solid body. • The properties of pulverized material are quite different from those of the same material in a solid state. The oxidation surface is so large that the heat losses into the environment are minimum and the dust becomes liable to self-ignition (dust of aluminum, iron, brass).
Combustion of dusts……. • Like the combustible air, gas and air vapor mixtures, dusts may explode when their concentrations in the air reaches the explosive range. • The lower explosive limit of dusts is most important characteristic because dust concentrations corresponding to this limit may be formed both inside the machine treating solid materials, and almost everywhere in production premises
Combustion of dusts…….. • Dust is said to be more dangerous, the lower its lower explosive limit and the lower its auto - ignition temperature. • The risk of explosion are high with dusts whose lower concentration explosive limit is below 65g/m3 (Pulverized sulfur, sugar, flour).
Fire Behaviour • Fires behave differently. Some burn slowly and evenly; others are extremely hot, burning violently and quickly. • Different fires have different coloured flames. Some fires start easily; others don’t. Some fires produce deadly gases and smoke which may be the cause of death if not ventilated
Fire Behaviour…… • The behaviour of the fire often depends on the fuel. Other factors or variables may include where the fuel is situated and how near it is to other fuels, the weather (especially wind and relative humidity), oxygen concentration. • Different fuelscatch fire at different temperatures. It takes a certain amount of heat energy to change any particular material into a gas (if it is not already). Then it takes more heat energy to trigger the reaction with oxygen
Fire Behaviour…… • The amount of heat produced depends on the molecules that make up the fuel. The most flammable fuels are hydrocarbons (contain carbon and hydrogen) that recombine with oxygen quite easily to form carbon dioxide, water and other gases. • Size or surface area of fueldetermine how quickly a fuel catches fire and burns For example, large pieces of wood take a lot longer to absorb heat energy to ignition temperature. Thin wood catches fire easily because it heats up easily
Fire Behaviour…… • The bigger the area of the surface of the fuel, the more oxygen molecules can collide with the surface. The more oxygen molecules that collide per second with the fuel, the faster the combustion reaction is. • A powder has the largest surface area and will have the fastest reaction rate
Fire Behaviour…… • Amount of fuel available to burn is known as the fuel load. The bigger the fuel load, the more intense the fire will be in terms of heat energy output. • Oxygen availability will affect the rate of burning. A low concentration of oxygen will slow the burning right down.
Fire Behaviour…… • An example of dangerous fire behaviour that can occur in a situation where there is a low concentration of oxygen is called backdraught (an explosive surge in a fire produced by the sudden mixing of air with other combustible gases). • This is when an enclosed fire has used up most of the oxygen and is just smouldering. If there is a sudden influx of oxygen (like someone opening a door or window), the fire will immediately explode into flame
Fire Behaviour…… • Relative Humidity reflects the amount of moisture in the air. If relative humidity is low, it will contribute to the drying of fuels. If it is high, fuels will absorb moisture from the air, making ignition more difficult. • Wind is a major factor in determining fire spread. Wind affects the rate of oxygen supply to the burning fuel (controlling combustion) and it tilts the flame forward so that unburned fuel receives energy by radiation and convection at an increased speed. Wind can also dry out the fuel
Common gases and smoke • The particular gases produced by a fire depend mainly on the fuel. • The most common hazardous gases are carbon dioxide (CO2), the product of complete combustion, and carbon monoxide (CO), the product of incomplete combustion.
Common gases and smoke…….. • Carbon monoxide is the more dangerous of the two. When air mixed with carbon monoxide is inhaled, the blood absorbs the CO. • The result is an oxygen deficiency in the brain and body. Exposure to a 1.3% concentration of CO will cause unconsciousness in two or three breaths and death in a few minutes.
Common gases and smoke…….. • Carbon dioxide works on the respiratory system. CO2 concentrations in the air reduce the amount of oxygen that is absorbed in the lungs. The body responds with rapid and deep breathing, which is a signal that the respiratory system is not receiving sufficient oxygen. • When the oxygen content of air drops from its normal level of 21% to about 15%, human muscular control is reduced. At 10% to 14% oxygen in air, fatigue sets in. Unconsciousness usually results from oxygen concentrations below 10%.
Common gases and smoke…….. • Depending upon the fuel source, there may be several other gases generated by a fire that are of equal concern. e.g. i) Hydrogen cyanide (HCN) is produced when nylon, wool etc. are involved in fire. Hydrogen cyanide arrest the activity of all forms of living cells of body muscles and nerves. ii) Hydrogen chloride (HCl) is produced when polyvinyl chloride (PVC) is decomposed at fires. If inhaled, HCl will damage the upper respiratory tract and lead to asphyxiation or death.
Common gases and smoke…….. • Smoke is a visible product of fire that adds to the problem of breathing. It is made up of carbon and other unburned substances in the form of suspended particles. • It also carries the vapors of water, acids and other chemicals, which can be poisonous or irritating when inhaled
Smoke detector • A smoke detector is a device that detects smoke, typically as an indicator of fire. Smoke detectors are typically housed in a disk-shaped plastic enclosure about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick. • Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while others use both detection methods to increase sensitivity to smoke.
Smoke detector…….. • All smoke detectors consist of two basic part : a sensor to sense the smoke and a very loud electronic horn to wake people up. Smoke detectors can run off of a 9-volt battery or 120-volt house current. • Test requirements for smoke detectors fall broadly into two categories - Functional (or Operational) Testing and Calibration (or Sensitivity) Testing. Functional checking is accomplished by introducing a smoke, from the protected area through the vents of a detector to the sensor (s). Sensitivity testing confirms whether the detector’s performance is within the acceptable parameters.
Smoke detector…….. • While regular testing is essential in making sure that the smoke detector is working properly, regular maintenance is a very good idea as well to ensue that no dust, hair or fiber particles are hiding the grates in the smoke detector, preventing the detector to detect a problem, or even cause damage to the interior workings.
Smoke Exhaust System • Smoke Exhaust is used to remove smoke from buildings to enable emergency evacuation as well as improved firefighting. • Smoke Exhaust System is a mechanical or gravity system intended to convey smoke from one portion of a building to the outdoors and usually includes a venting system, as well as exhaust fans.
Fire door • Fire door is a door with a fire-resistance rating (the duration for which a fire protectionsystem can withstand a standard fire resistance test) and used as part of a fire protection system to reduce the spread of fire or smoke between compartments and to enable safe egress from a building.
Fire door……. • Fire doors may be made of a combination of materials, such as: i) timber ii) steel iii) gypsum (calcium sulphate ) iv) vermiculite (natural mineral that expands with the application of heat) V) glasssections • Fire doors maintain its structural integrity for a period of time in the event of a fire.
Exit sign • An exit sign is a device in a building denoting the location of the emergency exit, guiding people to the closest exit in case emergency including fire.. Most relevant codes (fire, building, health or safety) require exit signs to be permanently lit. • Exit signs shall be preferably in pictogram form, with text supplement.
Exit sign……. • Since visibility may be reduced in a fire, due to smoke or failure of electric lighting, the sign is often permanently illuminated, usually by : i) electric light, with the building's emergency lighting circuits providing back-up power from a UPS and/or a generator in case normal power fails , or ii) electric light, with a local rechargeable power source
Emergency lighting • An emergency light is a battery-backed lighting device that comes on automatically when a building experiences a power failure as a result of a fire or a power cut to avoid darkness and a possible danger to occupants either throuh physical danger or panic. • The system normally used is lead acid batteries to store a full 120-volt charge. • Electrical devices are used to switch on the lights and battery supply in the event of a power failure automatically
Treatment following inhalation of gases and smoke • Treatment varies with the severity of the damage caused. • The primary focus of treatment is to maintain an open airway and provide an adequate level of oxygen. • If the airway is open and stable, the individual may be given high-flow humidified 100 percent oxygen by mask. • If swelling of the airway tissues is closing off the airway, the person may require the insertion of a breathing tube to artificially maintain an open airway.
Treatment following inhalation of gases and smoke.. • Oxygen is often the only medication necessary. However, people who have a cough with wheezing (bronchospasm), indicating that the bronchial airways are narrowed or blocked, may be given a bronchodilator to relax the muscles and increase ventilation • Hyperbaric oxygen therapy ( treatment of the entire body with 100-percent oxygen at greater than normal atmospheric pressures ) may be used to treat smoke inhalation, resulting in severe carbon monoxide or cyanidepoisoning. This treatment requires a special chamber in which the person receives pure oxygen at three times the normal atmospheric pressure, thus receiving more oxygen faster to overcome loss of consciousness and other associated illnesses.
Treatment following inhalation of gases and smoke.. • There are also antidotes for specific poisonous gases in the blood e.g. hydroxocobalamin against hydrogen cyanide, dosage is dependent upon the level indicated by blood tests. Oxygen works as an antidote against carbon monoxide as it increases the removal of carbon monoxide from hemoglobin, in turn providing the body with normal levels of oxygen. • Carbon dioxide is an asphyxiant gas and not classified as toxic or harmful. There is no antidote to be administered to counteract the effects of inhalation of hydrogen chloride. In case of severe inhalation exposure, humidified supplemental oxygen should be administered.
Extinguishment Theory • The extinguishment of fire is based on an interruption of one or more of the essential elements in the combustion process. • With flaming combustion the fire may be extinguished by reducing temperature, eliminating fuel or oxygen, or by stopping the chemical chain reaction. • If a fire is in the smoldering mode of combustion, only three extinguishment options exist reduction of temperature, elimination of fuel or oxygen
Extinguishment Theory……. • Extinguishment by Temperature Reduction One of the most common methods of extinguishment is by cooling with water. The process of extinguishment by cooling is dependent on cooling the fuel to a point where it does not produce sufficient vapour to burn. If we look at fuel types and vapour production, we find that solid fuels and liquid fuels with high flash points can be extinguished by cooling. Low flashpoint liquids and flammable gases cannot be extinguished by cooling with water as vapour production cannot be sufficiently reduced.
Extinguishment Theory……. • Extinguishment by Fuel Removal In some cases, a fire is effectively extinguished by removing the fuel source. This may be accomplished by stopping the flow of liquid or gaseous fuel or by removing solid fuel in the path of the fire.
Extinguishment Theory……. • Extinguishment by Oxygen Dilution The method of extinguishment by oxygen dilution is the reduction of the oxygen concentration to the fire area. This can be accomplished by introducing an inert gas into the fire or by separating the oxygen from the fuel. Carbon dioxide or nitrogen, are the two most common extinguishing agents.
Extinguishment Theory……. • Extinguishment by Chemical flame Inhibition Some extinguishing agents, such as dry chemicals interrupt the flame producing chemical reaction, resulting in rapid extinguishment. This method of extinguishment is effective only on gas and liquid fuels as they cannot burn in the smoldering mode of combustion.
