CHAPTER 5 Fire Behavior - PowerPoint Presentation

CHAPTER 5 Fire Behavior

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, rollover, flashover, backdraft, fuel-limited fires, ventilation-limited fires, and smoke explosion . Describe the conditions that cause thermal layering. Describe the conditions that lead to rollover. 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 behavior. 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 vapor 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 vapor 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. The United States has a higher mortality rate from fires than many other developed countries that keep fire statistics. In 2016, there were 3390 civilian fire deaths and 14,650 civilian fire injuries. Public fire departments responded to 1,342,000 fires.

The Chemistry of Fire Understanding the conditions needed for a fire to ignite and grow will increase your effectiveness. Being well trained in fire behavior 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 vapors. For a fuel to burn, it must first be changed into a flammable vapor.

Matter Atoms and molecules Three states Solid Liquid Gas © Jones & Bartlett Learning

States of MatterSolids 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 conve c tion 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 CombustionPyrolysis is the process that liberates gaseous fuel vapors due to the heating of a solid fuel. Almost all fuel consists of hydrocarbons. Incomplete combustion results in large quantities of deadly gases and compounds. © Jones & Bartlett Learning

Products of Combustion: SmokeToxic by-product of combustion Composed of: Particles Vapors Gases Particles include: Unburned Partially burned Completely burned

Products of Combustion: SmokeWater 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: HeatTemperature 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 TransferMeasured 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

ConductionProcess 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 conductivity, area, and difference in temp © Jones & Bartlett Learning

ConvectionThe 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 hotter gas to a cooler surface. © Jones & Bartlett Learning

ConvectionThe 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. May carry the fire outside the compartment/room of origin and to other parts of the building Flow path Neutral plane

RadiationTransfer of heat in the form of an invisible waveTravels in all directions Not seen or felt until it strikes an object © Jones & Bartlett Learning

Methods of ExtinguishmentCool the burning material. Exclude oxygen. Remove fuel. Interrupt the chemical reaction. © Jones & Bartlett Learning

Class A FireInvolve ordinary solid combustiblesTo extinguish, cool the fuel with water to a temperature below ignition temperature, or use a combination of limiting ventilation and applying water. © schankz / Shutterstock

Class B FireInvolve flammable or combustible liquids and gasesTo extinguish shut off the fuel supply, or use foam to exclude oxygen from the fuel. © iStock /Getty Images Plus/ thaloengsak

Class C FireInvolve energized electrical equipment, power tools, appliances, electronic devices Incorrectly attacking with an extinguishing agent that conducts electricity can result in injury or death. © mikeledray /Shutterstock, Inc.

Class D FireInvolve combustible metalsApplication of water will result in violent explosions. Must be attacked with special agents © Andrew Lambert Photography/Photo Researchers, Inc.

Class K FireInvolve combustible cooking oils and fatsSpecial Class K extinguishers are available to handle this type of fire. Contain wet agents that absorb heat © Kathie Nichols/ Shutterstock

Solid FuelsA 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 vapors. Pyrolysis: When fuels are heated, they begin to change chemically and decompose, thereby releasing flammable vapors.

Solid Fuels: CompositionChemical 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: AmountIn 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: ConfigurationSurface-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 DevelopmentProgresses through four classic stages Incipient stage Growth stage Fully developed stage Decay stage Courtesy of NIST.

Incipient StageOccurs 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 StageMore 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 ConditionsThermal layering: gases forming into layers according to temperatureRollover: spontaneous ignition of hot gases in the upper levels of a room or compartment © Jones & Bartlett Learning. Photographed by Glen E. Ellman .

Types of Fire ConditionsFlashover: transition from a fire that is growing by igniting one 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 Dave Casey. Courtesy of NIST.

Types of Fire ConditionsBackdraft: caused by introduction of oxygen, a change of the ventilation profile, into an enclosure where the superheated gases and contents are already hot enough for ignition but do not have sufficient oxygen to combust © Jones & Bartlett Learning. Photographed by Glen E. Ellman .

Types of Fire ConditionsSigns of backdraftAny 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 ConditionsRapid 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. Behavior of ventilation-limited fires: m any 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 engine company.

Fully Developed StageFire 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 StageCan 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-LimitedRate of combustion slows.Visible flames decrease or disappear. Fuels will continue to pyrol y ze and create additional flammable vapors 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 ExplosionOccurs 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 Behavior in Modern Structures Courtesy of UL.

Fire Behavior 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 behavior. May affect which side of the structure to enter Size-up of a structure fire must include a wind evaluation regardless. © Jones & Bartlett Learning.

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 vapors in a sustained amount and eventually off-gas completely. As the boiling point is reached, the amount of flammable vapor generated increases significantly. © Jones & Bartlett Learning.

Liquid-Fuel Fires Flammability is determined by compound with the lowest ignition temperature. The amount of liquid that will be vaporized is related to the volatility of the liquid. Flash point: Lowest temperature at which vapor is produced Fire point: Lowest temperature at which sufficient vapors are produced For most materials, fire point is only slightly higher than the flash point.

Vapor Density Vapor 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 vapor density of <1 will rise. A gas with a vapor density >1 will settle. Fire fighters need to understand the vapor 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 RangeMixtures 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 storing liquid fuel under pressure is heated excessively The vessel can fail, releasing all of the heated fuel in a massive explosion . Key to prevention is cooling the top of the tank. © Ivan Cholakov / Shutterstock , Inc. © Jones & Bartlett Learning.

Smoke ReadingPotentially 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 color g ives 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 AttributesThe 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 ChangeFlames 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 color) indicate changes in the fire.

Step 4: Predict the EventTo 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 DoorIndications 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), vapors (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 vapor 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, rollover, 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. Rollover 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 vapors can be described in terms of vapor density and flammability limits. Vapor 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 vapor. Assessment of smoke volume, velocity, density, and color 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.