9781284172256_SLID_CH05.pptx

CHAPTER 5 Fire Behaviour  Knowledge Objectives Describe the chemistry of fire. List the three states of matter. List the five forms of energy. Explain the concept of the fire triangle. Explain the concept of the fire tetrahedron. Describe the chemistry of combustio...

CHAPTER 5 Fire Behaviour  Knowledge Objectives Describe the chemistry of fire. List the three states of matter. List the five forms of energy. Explain the concept of the fire triangle. Explain the concept of the fire tetrahedron. Describe the chemistry of combustion. Describe the by-products of combustion. Explain how fires are spread by conduction, convection, and radiation.  Knowledge Objectives Define the flow path and describe how it influences the growth of a building fire. Describe the four methods of extinguishing fires. Define Class A, B, C, D, and K fires. Describe the importance of the following characteristics in solid-fuel fires: composition of fuel, amount of fuel, and configuration of fuel. Describe the four stages of fire development: incipient stage, growth stage, fully developed stage, and decay stage.  Knowledge Objectives Define the following terms: thermal layering, neutral plane, roll-over, flashover, backdraft, fuel- limited fires, ventilation-limited fires, and smoke explosion. Describe the conditions that cause thermal layering. Describe the conditions that lead to roll-over. Describe the conditions that lead to flashover. Describe the conditions that lead to a backdraft.  Knowledge Objectives Describe the conditions that lead to rapid fire growth. Describe the conditions that lead to a fuel-limited fire and a ventilation-limited fire. Describe the conditions that lead to a smoke explosion. Describe how fire behaves in modern structures. Describe how the wind effect impacts fire behaviour. Describe the characteristics of liquid-fuel fires. Define the following terms: boiling point, flash point, and fire point.  Knowledge Objectives Describe the characteristics of gas-fuel fires. Explain the concept of vapour density. Explain the concept of flammable range. Define the following terms: lower explosive limit (LE) and upper explosive limit (UEL). Describe the cause and effects of a boiling liquid/expanding vapour explosion (BLEVE). Describe the process of reading smoke. Describe the four key attributes of smoke.  Introduction Fire has been around since the beginning of time. Destruction of lives and property by uncontrolled fires has occurred since ancient times. Total calls in the Province of Ontario increased from 462,542 in 2012 to 494,811 in 2016. Fires with injury, fatality, or dollar loss declined from 11,294 in 2012 to 10,844 in 2016.  The Chemistry of Fire Understanding the conditions needed for a fire to ignite and grow will increase your effectiveness. Being well trained in fire behaviour will allow the fire fighter to control a fire with less water.  What Is Fire? Rapid chemical process that produces heat and usually light – Characterized by the production of a flame Fire requires fuel in the form of combustible vapours. For a fuel to burn, it must first be changed into a flammable vapour.  Matter Atoms and molecules Three states – Solid – Liquid – Gas © Jones & Bartlett Learning  States of Matter Solids have a definite size and shape. – They may change to a liquid or gas when heated. – Cold makes solids brittle, and heat makes them flexible. Liquids assume the shape of their container. – Most will expand and turn into gases. – They have a definite volume. Gases have neither independent shape nor independent volume; expand indefinitely.  Fuels Fuels are materials that store energy. Energy released in the form of heat and light has been stored before it is burned. © Jones & Bartlett Learning  Types of Energy: Chemical Energy created by a chemical reaction Exothermic (fire) versus endothermic (ice) Most chemical reactions occur because bonds are broken as two substances are chemically separated. Heat is produced. If the reaction occurs rapidly or within an enclosed space, the mixture can heat to its ignition temperature and begin to burn. Fire is a result of a chemical reaction.  Types of Energy: Mechanical Converted to heat when two materials rub against each other and create friction – e.g., vehicle tires spinning on pavement Heat is produced when mechanical energy is used to compress air in a compressor.  Types of Energy: Electrical Is converted to heat energy in several different ways Electricity produces heat while flowing through a wire or another conductive material. Examples of electrical energy that can produce enough heat to start a fire include electric heating elements, overloaded wires, electrical arcs, and lightning. Is carried through the electrical wires and can be stored in batteries  Types of Energy: Light Caused by electromagnetic waves packaged in discrete bundles called photons – e.g., radiant energy from the sun Candles, fires, light bulbs, and lasers are common forms of light energy. – Emit both heat and light – Transfer most of their heat via convection and radiation  Types of Energy: Nuclear Created by nuclear fission or fusion Nuclear reactions release large amounts of energy in the form of heat. These reactions can be controlled or uncontrolled. – Both release radioactive material, which can cause injury or death.  Conservation of Energy Energy cannot be created or destroyed by ordinary means. Energy can be converted from one form to another. – Chemical energy in gasoline is converted to mechanical energy when a car moves down the road. – The stored chemical energy in the wood of the house is converted into heat and light energy during the fire.  Conditions Needed for Fire A combustible fuel Oxygen in sufficient quantities A source of heat © Jones & Bartlett Learning  Conditions Needed for Fire A fourth factor must be added to maintain a self-sustaining fire. – Chemical chain reactions If you remove any of these elements, the fire will go out. © Jones & Bartlett Learning  Chemistry of Combustion When one element combines chemically with another, they produce a compound. Oxidation is the process of chemically combining oxygen with another substance to create a new compound. – The process can be slow and does not produce easily measurable heat. Combustion (fire) is a rapid chemical process in which the combination of a substance with oxygen produces heat and light. – Produces numerous toxic by-products  Chemistry of Combustion Pyrolysis is the process that liberates gaseous fuel vapours due to the heating of a solid fuel. Almost all fuel consists of hydrocarbons. Incomplete combustion results in large quantities of deadly © Jones & Bartlett Learning gases and compounds.  Products of Combustion: Smoke Toxic by-product of combustion Composed of: – Particles – Vapours – Gases Particles include: – Unburned – Partially burned – Completely burned  Products of Combustion: Smoke Water and oil-based droplets of water may also become part of the smoke. Smoke contains a wide variety of gases. – Composition of gases in smoke varies greatly Smoke is a form of fuel. Almost all of the gases produced by a fire are toxic. – Includes carbon monoxide, hydrogen cyanide, and phosgene. – Carbon dioxide in smoke displaces oxygen, causing hypoxia.  Products of Combustion: Heat Temperature of the smoke will vary depending on the conditions of the fire and the distance the smoke travels. The inhalation of hot gases in smoke may cause severe injuries in the form of severe burns of the skin and the respiratory tract.  Heat Transfer Measured as energy flow per unit of time Occurs when there is a difference in temperature between two objects Heat will flow from a hotter object to a cooler object until they reach equal temperatures. When two objects have the same temperature, heat transfer does not occur. The rate of heat transfer is dependent on two factors. – The difference in temperature – The ability of the materials to conduct heat  Conduction Process of transferring heat to and through one solid to another Objects that have more tightly packed molecules are more efficient in conducting heat. Dependent on thermal © Jones & Bartlett Learning conductivity, area, and difference in temp  Convection The transfer of heat by the flow of gases or fluid from hotter areas to cooler areas Involves primarily smoke and hot gases generated by the fire Transfer is from a © Jones & Bartlett Learning hotter gas to a cooler surface.  Convection The heat of the fire warms the gases and particles in the smoke. The hotter and less dense column of gases rises and displaces cooler, denser gases downward. In a building, the convection currents generated by the fire rise in the room and travel along the ceiling. This is called the ceiling jet. May carry the fire outside the compartment/room of origin and to other parts of the building – Flow path – Neutral plane  Radiation Transfer of heat in the form of an invisible wave Travels in all directions Not seen or felt until it strikes an object © Jones & Bartlett Learning  Methods of Extinguishment Cool the burning material. Exclude oxygen. Remove fuel. Interrupt the chemical reaction. © Jones & Bartlett Learning  Class A Fire Involve ordinary solid combustibles To extinguish, cool the fuel with water to a temperature below ignition temperature, or use a combination of limiting ventilation and © schankz/Shutterstock applying water.  Class B Fire Involve flammable or combustible liquids and gases To extinguish shut off the fuel supply, or use foam to exclude oxygen from the fuel. © thaloengsak/iStock/Getty Images  Class C Fire Involve energized electrical equipment, power tools, appliances, electronic devices Incorrectly attacking with an extinguishing agent that conducts © mikeledray/Shutterstock, Inc. electricity can result in injury or death.  Class D Fire Involve combustible metals Application of water will result in violent explosions. Must be attacked with special agents © Andrew Lambert Photography/Science Source  Class K Fire Involve combustible cooking oils and fats Special Class K extinguishers are available to handle this type of fire. – Contain wet agents that absorb heat © Kathie Nichols/Shutterstock  Solid Fuels A variety of solid fuels are found in most buildings. – Include wood and wood-based products, fabrics, paper, carpeting, and many petroleum- based fuels, such as plastics/petroleum-based foams Building materials and the building contents will influence how a fire burns. Solid fuels burn when they are heated sufficiently to change them into flammable vapours. Pyrolysis: When fuels are heated, they begin to change chemically and decompose, thereby releasing flammable vapours.  Solid Fuels: Composition Chemical composition has a significant impact on how the fuel burns. Wood is primarily composed of a combustible natural fiber called cellulose. Most modern room contents are manufactured from petroleum products, which generally contain more potential heat energy than natural products. Amount of moisture contained in the fuel  Solid Fuels: Amount In a fuel-limited fire, the fire has sufficient oxygen but limited fuel. All else being equal, when more fuel is available, there is a higher heat release rate (HRR).  Solid Fuels: Configuration Surface-to-mass ratio impacts the ability of the fuel to ignite, the time it takes to be consumed, and the HRR. Orientation of the fuel – Convection carries heat upward; vertical positioning will burn more quickly than horizontal positioning. Continuity of the fuel, or closeness of one piece of fuel to another – Consider horizontal and vertical continuity  Solid-Fuel Fire Development Progresses through four classic stages – Incipient stage – Growth stage – Fully developed stage – Decay stage Courtesy of NIST.  Incipient Stage Occurs when there is an adequate supply of fuel, oxygen, and heat or ignition Fire is small and confined to the initial fuel that was ignited. Low temperature, little threat Courtesy of NIST.  Growth Stage More interaction and more dependent on the environment – Composition of the compartment surfaces and contents – Placement and configuration of the compartment materials – Amount of ventilation Courtesy of NIST.  Types of Fire Conditions Thermal layering: gases forming into layers according to temperature Roll-over: spontaneous ignition of hot gases in the upper levels of a room © Jones & Bartlett Learning. Photographed by Glen E. Ellman. or compartment  Types of Fire Conditions Flashover: transition from a fire that is growing by igniting one Courtesy of Dave Casey. type of fuel to another, to a fire where all of the exposed surfaces have ignited Cannot survive more than a few seconds Courtesy of NIST.  Types of Fire Conditions Backdraft: caused by introduction of oxygen, a change of the ventilation profile, into © Jones & Bartlett Learning. Photographed by Glen E. Ellman. an enclosure where the superheated gases and contents are already hot enough for ignition but do not have sufficient oxygen to combust  Types of Fire Conditions Signs of backdraft – Any confined fire with a large heat build-up – Little or no visible flame from the exterior – A “living fire” – Smoke that seems to be pressurized – Smoke-stained windows – No smoke showing – Turbulent smoke – Thick yellowish smoke  Types of Fire Conditions Rapid fire growth: introducing air into a ventilation- limited fire can result in explosive fire growth. – Fire fighters making entry can introduce enough air to produce rapid fire growth and flashover. Behaviour of ventilation-limited fires: many building fires have a limited supply of oxygen. – Assessing this kind of fire can be a challenge. – Fires in modern residential occupancies are likely to enter a ventilation-limited decay stage prior to the arrival of the first company.  Fully Developed Stage Fire is consuming the maximum amount of fuel possible. Fire may be ventilation-limited or fuel-limited. Not all fires will reach this stage. Courtesy of NIST.  Decay Stage Can occur because of a decreasing fuel supply or because of a limited oxygen supply Active flaming combustion decreases or stops. Courtesy of NIST.  Decay Stage: Oxygen-Limited Rate of combustion slows. Visible flames decrease or disappear. Fuels will continue to pyrolyze and create additional flammable vapours and gases. Rate of pyrolysis will slow as rate of combustion slows, but large quantities of flammable fuel may still be present. If additional oxygen is introduced into the fire compartment, rapid or violent fire growth can develop quickly.  Decay Stage: Smoke Explosion Occurs when a mixture of flammable gases and oxygen are present, usually in a void or other area separate from the fire compartment Conditions needed: – The presence of void spaces – Combustible building materials – A ventilation-controlled fire that produces unburned fuel – A relatively cool temperature There is no change to the ventilation profile; occurs from the travel of smoke within the structure to an ignition source.  Fire Behaviour in Modern Structures Courtesy of UL.  Fire Behaviour in Modern Structures Fire in a modern structure progresses to the fully developed stage quickly. The fire department often arrives on scene while fire is in ventilation-limited decay. Fire fighters open the front door to gain access to house. – This introduces a fresh supply of oxygen to fire. The result is rapid growth of the fire in the form of flashover.  Wind Effect Wind influences fire behaviour. – May affect which side of the structure to enter Size-up of a structure fire must include a wind evaluation © Jones & Bartlett Learning. regardless.  Liquid-Fuel Fires A liquid must be converted to a gaseous state before it will burn. Conditions required for ignition – Fuel-air mixture within flammable limits – An ignition source with sufficient energy – Sustained contact between ignition source and fuel-air mixture  Liquid-Fuel Fires Boiling point is the temperature at which a liquid will give off vapours in a sustained amount and eventually off-gas completely. As the boiling point is reached, the amount of © Jones & Bartlett Learning. flammable vapour generated increases significantly.  Liquid-Fuel Fires Flammability is determined by compound with the lowest ignition temperature. – The amount of liquid that will be vapourized is related to the volatility of the liquid. – Flash point: Lowest temperature at which vapour is produced – Fire point: Lowest temperature at which sufficient vapours are produced – For most materials, fire point is only slightly higher than the flash point.  Vapour Density Vapour density is the weight of a gas fuel. Measures the weight of gas compared with air The weight of air is assigned the value of 1. A gas with a vapour density of 1 will settle. Fire fighters need to understand the vapour density of the escaping fuel so that they can take actions to prevent the ignition of the fuel and allow the gaseous fuel to safely escape into the atmosphere.  Flammable Range Mixtures of flammable gases and air will burn only when they are mixed in certain proportions. The terms flammable range and explosive limits are used interchangeably. The lower explosive limit (LEL) refers to the minimum amount of gaseous fuel that must be present in a mixture for it to be flammable. The upper explosive limit (UEL) of carbon monoxide is 74 percent.  BLEVE Occurs when a vessel © Jones & Bartlett Learning. storing liquid fuel under pressure is heated excessively The vessel can fail, releasing all of the heated fuel in a massive explosion. © Ivan Cholakov/Shutterstock Key to prevention is cooling the top of the tank.  Smoke Reading Potentially enables fire fighter to learn where the fire is, how big it is, and where it is moving Helps mount a more effective attack and may save lives Helpful to think of smoke as fuel – Most of the fuel in a flashover is in the form of smoke. The best place to observe patterns of smoke is outside the fire building.  Step 1: Determine the Key Attributes of Smoke The volume of smoke gives an idea of how much fuel is being heated to the point that it gives off gas. The velocity (speed) at which smoke is leaving the building suggests how much pressure is accumulating inside the building. Smoke density suggests how much fuel is contained in the smoke. Smoke colour gives indication of what stage the fire is in and which substances are burning.  Step 1: Determine the Key Attributes of Smoke Courtesy of Dave Dodson. Courtesy of Keith Muratori.  Step 2: Determine What Is Influencing the Key Attributes The following considerations must be made. – Size of the structure – Wind conditions – Thermal balance – Fire streams – Ventilation openings – Sprinkler systems  Step 3: Determine the Rate of Change Flames indicate what is happening now, whereas smoke gives a more complete picture of the characteristics of the fire and where it is going. The changes to the four key attributes (volume, velocity, density, and colour) indicate changes in the fire.  Step 4: Predict the Event To assess the size and location of the fire: – Consider the key attributes. – Consider the influences of the key attributes. – Determine the rate of change. – This information should help determine location, size, and potential for the fire. – Communicate the key parts of these observations to the company officer. One way to become more proficient in smoke reading is to review videos of fires.  Smoke Reading Through a Door Indications of a hot fire may mean you are dealing with fire in a decay stage. – Sign of great danger – Addition of oxygen results in violent backdraft. – Fires can be dangerous even with little smoke. Watch what the smoke does when you open a door. – Identify the neutral plane.  Summary Fire is a rapid chemical process that produces heat and light. Matter is made up of atoms and molecules. Matter exists in three states: solid, liquid, and gas. – A solid has definite capacity for resisting forces and, under ordinary conditions, retains a definite size and shape. – A liquid assumes the shape of the container in which it is placed. – A gas is a type of liquid that has neither independent shape nor independent volume but rather tends to expand indefinitely.  Summary Fuels are materials that store energy. Energy exists in many forms, including chemical, mechanical, electrical, light, or nuclear. The three basic conditions needed for a fire to occur are fuel, oxygen, and heat. A chemical chain reaction is required to maintain fire.  Summary A by-product of fire is smoke. Smoke includes three major components: small solids (particles), vapours (aerosols), and gases. Smoke consists mainly of unburned forms of hydrocarbon fuels. Fire may be spread by conduction, convection, and radiation. Direct contact is a flame touching a fuel. Conduction is the transfer of heat through matter, like heat traveling up a metal spoon.  Summary Convection is the circulatory movement that occurs in a gas or fluid. Convection currents in a fire involve hot gases generated by the fire that rise because they are lighter, creating a higher pressure. Cooler gases are denser and move to the lower areas of the compartment. Convection pushes hot gases from the fire compartment to other areas of the building and is instrumental in spreading the fire beyond the room of origin. Radiation is the transfer of heat through the emission of energy in the form of electromagnetic waves. Thermal radiation has an important influence on the performance of the fire fighter’s PPE.  Summary The four principal methods of fire extinguishment are cooling the fuel, excluding oxygen, removing the fuel, and interrupting the chemical reaction. Fires are categorized as Class A, Class B, Class C, Class D, and Class K. These classes reflect the type of fuel that is burning and the type of hazard that the fire represents. Most fires encountered by fire fighters involve solid fuels. Solid fuels do not actually burn in a solid state. Instead, they must be heated or pyrolyzed to decompose into a vapour before they will burn.  Summary Three primary factors that influence the combustion of solid fuel fires are the composition of the fuel, the amount of fuel, and the configuration of the fuel. The factors that influence the fuel configuration are the surface- to-mass ratio, the orientation of the fuel, and the continuity of the fuel. There are four stages of fire development: the incipient stage, the growth stage, the fully developed stage, and the decay stage.  Summary A ventilation-limited fire is in a state of decay because there is a limited supply of oxygen available to the fire. The growth of room-and-contents fires depends on the characteristics of the room and the contents of the room. Synthetic products are widely used in today’s homes. The by-products of heated plastics are not only flammable but also toxic. Special conditions within the fire compartment include thermal layering of gases, roll-over, backdraft, flashover, rapid fire growth, and ventilation-limited fires.  Summary Thermal layering is the property of gases in an enclosed space in which they form layers according to their temperature. The hottest gases travel by convection currents to the top level of the room. Roll-over is the ignition of the hot, unburned gases that have accumulated at the top of the fire compartment; this can be a precursor to a flashover. Flashover is the near-simultaneous ignition of most of the exposed combustible materials in an enclosed area.  Summary Backdraft is caused by a change of the ventilation profile, permitting the introduction of oxygen into an enclosure where superheated gases and contents are hot enough for ignition, but the fire does not have sufficient oxygen to cause their combustion. Modern structures tend to be more tightly sealed, be constructed of lighter-weight materials, and contain more plastics. These characteristics can lead to a greater risk of backdrafts when a fire occurs in such a structure.  Summary Liquid-fuel fires require the proper mixture of fuel and air, an ignition source, and contact between the fuel mixture and the ignition. The characteristics of flammable vapours can be described in terms of vapour density and flammability limits. Vapour density reflects the weight of a gas compared to that of air. Flammability limits vary widely for different fuels.  Summary A BLEVE is a catastrophic explosion in a vessel containing both a boiling liquid and a vapour. Assessment of smoke volume, velocity, density, and colour assists fire fighters to potentially predict the location of a fire and its stage of development. Smoke reading requires fire fighters to evaluate the effect of the building, the weather, and ventilation on the smoke.