Flames: Drx/Dreamstime.com; Steel texture: Sharpshot/Dreamstime.com; Chapter opener photo: Lakeview Images/ShutterStock, Inc.Combustion, Fire, and Flammability OBJECTIVES After studying this chapter, you should be able to: Describe how the U.S. fre incidence database enables development of a national profle of fres and fre losses. Defne the process of combustion. Explain fammability, in terms of both fre properties and practical application. Explain the nonfaming and faming stages of fre. Discuss the fre tetrahedron and explain how it is a focus for a unifed view of fre initiation, growth, and termination. Discuss the terms fre consequences, hazard, and risk. 6 9780763757175_CH06_Printer.indd 77 13/11/13 3:14 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONIntroduction Understandingfretypesandthecharacteristicsofeachtypeisfundamentalin improvingfresafety.TheUnitedStateshasauniquecapabilitytotrackthefre-quency,severity,andnatureoffres.TheNationalFireIncident ReportingSystem (NFIRS) is the worlds largest national database of freincident information [ 1 ]. Each year, approximately 23,000 fre departments in 50 states and the District of Columbia report information on fres in their jurisdictions. The input data to NFIRS refect 75 percent of all reported fres that occur annually. The U.S. Fire Administra-tion, the U.S. Consumer Product Safety Commission, and theNational Fire Protection Association publish complementary analysesoftheNFIRSdata.Theseanalysesenablethefresafetycommunitytoquantifythelargestcontributorstothe United States fre problem, identify emerging threats to fre safety, and estimate the effectiveness of new standards and code provisions. To illustrate the power of this database, consider the analysis of fres involving residential upholsteredfurniture and beds as the frst item ignited (Table 6-1). The two shaded rows inTable6- 1 show that, while these fres represented only 5 percent of all residential fres, they led to one-third of all fre deaths. A subsequent analysis by John Hall of NFPA [ 2 ] esti-mated the fre losses for fres in which residential upholstered furniture was not the frst item ignited, but was the principal ampliferofthehazardresultingfromtheinitialignitionofanotheritem.Theseadditionalcontributionstofrelossare refected in the upper Upholstered Furniture row. (A similar analysis has not been performed for mattresses and bedding.) The combined data show that furniture and bed fres are seven times more deadly and three times more likely to result in an injury than the average household fre. Figure 6-1 shows that NFIRS compilesinformation that allows identifcation of the prevalent ignition sources. Cigarette ignition of upholstered furniture is the largest single cause of fre deaths [ 3 ]. Figure 6-2 demonstrates yet another dimension of the datanamely, that nearly two-thirds of the U.S. fre deaths that began with upholsteredfurniture resulted from fres that had extended beyond the room of fre origin. These fres are interpreted as having passed the point of room fashover. The knowledge gained from these analyses enables fre scientists to examine the most relevant fre phenomena, the fre service to align its response capability and operating procedures with the most important fres, and fre codes and stan-dards developers and regulators to guide themarketplace toward effective solutions to freproblems.Supporting all these members of the fre safety community is a common understanding of fre and its consequences. This chapter begins the presentation of thatunderstanding. Annual U.S. Residential Fire Losses, 20052009, by First Item Ignited (Plus Direct Involvement of Upholstered Furniture)Table 6-1Fires Deaths InjuriesDirect Property Loss ($ M)Total 373,900 2650 12,890 7100Upholstered furniture [3] 9240 (2%) 630 (24%) 966 (7%) 442 (8%)7040 (2%) 500 (19%) 690 (5%) 442 (6%)Mattresses and bedding [4] 10,260 (3%) 371 (14%) 1340 (10%) 385 (5%)9780763757175_CH06_Printer.indd 78 13/11/13 3:28 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION10% 20% 30% 40% 50% 60% 70%0Playing with heat sourceElectrical distribution andlighting equipmentHeating equipmentHot ember or ashCandleIntentionalSmoking materialsFiresCivilian deaths Civilian injuriesProperty damage28%58%38%24%13%6%8%9%10%6%12%10%10%7%10%6%9%7%6%6%9%11%10%6%8%5%9%5%Figure 6-1Ignition sources in home structure fres that began with upholstered furniture, 20052009.Reprinted with permission from SFPE Handbook of Fire Protection Engineering, 4th edition; Society of Fire Protection Engineers, Boston, p. vii, Copyright 2008, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and offcial position of the NFPA or the referenced subject, which is represented only by the standard in its entirety.CombustionCombustionisanexothermicchemicalreaction between a fuel and an oxidizer resulting in the gener-ation of substantive heat and often light. Interestingly, dictionariesoftenusethesamewordstodefnefre. Fire generally conveys a connotation of being poten-tially uncontained, unwanted, and destructivewhich isthecontextforthistext.(Therearesomecasesof positioned,intentional,andbenefcialfres,suchas those set to clear the rubble from last years crops.)Chapter 6 Combustion, Fire, and Flammability 799780763757175_CH06_Printer.indd 79 13/11/13 3:18 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONTounderstandthecontentofthisandthefol-lowingchapters,itisimportanttounderstandthe defnition of combustion. Each of the following words or phrases is central to differentiating combustion in general, and fre in particular, from other processes.Chemicalreaction.Combustioninvolves chemicalchange:thearrangementsofatoms inthe combustionproductsaredifferent from the arrangements in the reactants. Thus, whilealiquidmaterialmaybevaporized during combustion, the phase change in itself is not combustion.Exothermic.Theoverallcombustionreac-tiongivesoffheat.Thisheatcanraisethe temperatureofadditionalreactants,thereby continuingthecombustionprocess;expand gases that drive turbines; or burn skin.Fuel.Combustiblematerialscanbegases (e.g.,methane),liquids(e.g.,gasoline),or solids(e.g.,wood).Noncombustiblemateri-als, such as helium, water, and stone, do not satisfythisdefnition.Thenature,intensity, anddurationofthecombustionareinpart determined by the fuel chemistry and supply.Oxidizer.Themostcommonoxidizerin combustion is the oxygen present in air, and this is especially true for fres. As is true for the fuel, the nature, intensity, and duration ofthecombustionareinpartdetermined by the oxygen supply. A restricted supply of air or the depletion of oxygen in the air by the combustion, referred to as vitiation, can reduce the burning rate and alter the nature ofthecombustionproducts.Combustion FiresCivilian deaths 0% 10% 20% 30% 40% 50% 60%Beyond building of originConfined to building of originConfined to floor of originConfined to room of originConfined to object of origin24%6%34%25%11%15%28%48%3%5%Figure 6-2Home structure fres that began with upholstered furniture by extent of fame damage, 20052009.Reprinted with permission from SFPE Handbook of Fire Protection Engineering, 4th edition; Society of Fire Protection Engineers, Boston, p. 5, Copyright 2008, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and offcial position of the NFPA or the referenced subject, which is represented only by the standard in its entirety.80 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 80 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONdoesnotnecessarilyrequireoxygenmole-cules or even oxygen atoms. An example of the former is the use of potassium perchlo-rate (KClO 4 ) in freworks and munitions; an example of the latter is the reaction of CF 3 Br (halon1301,aneffective extinguishantfor hydrocarbonfres)with magnesium.Other chemicalsself- combustthesearepre-sented in the chapter on solid combustibles. Substantiveheat .Combustionreactionsare asubsetofexothermicoxidationreactions. Someexothermicoxidationreactionsare extremelyslow.Forexample,rustingnever increasesthetemperatureofthemetalmore thanadegreeorsoabovethatofthesur-roundings and may go unnoticed for months. By contrast, a combustion reaction generates heatfasterthanitisdissipated,causinga substantialtemperaturerise(atleasttensof degrees Celsius, and often 1000 C or more). Light .Whenthecombustion-generatedtem-perature is high enough, visible light is emitted from the combustion reaction zone. This can be seen as a glowing solid, such as a freplace ember, or in the gas phase, such as a fame. Ascanbegatheredfromthesedefnitions, theinitiationandcontinuanceofcombustionor afrerequirefourcomponents,asdepictedinthe fre tetrahedron ( Figure 6-3 ):1. Fuel 2. Oxidant 3. Elevated temperature (heat) 4. Chemical chain reaction The balance among these components is critical. Too smallaconcentrationoffuelmaynotbesuffcient topropagatecombustion,evenifplentyofairis available. Similarly, fuel and air may not react if the temperature is too low. HeatOxidizing agentUninhibitedchemical chainreactionsFuel(reducing agent) Figure6- 3 The fre tetrahedron. Chapter 6 Combustion, Fire, and Flammability 819780763757175_CH06_Printer.indd 81 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION The following is a brief presentation of combus-tionasitpertainstofres.Formoredetailedinfor-mation, refer to references such as [ 5 ], [ 6 ], and [ 7 ]. Flaming and Nonfaming Combustion ISO 19706 defnes a set of phenomenological stages of a fre [ 8 ]:1. Nonfaming a. Self-sustained(smolderingcombustion; also called glowing combustion) b. Oxidativepyrolysisfromexternally applied thermal radiation or conduction c. Anaerobic(oxidizer-free)pyrolysisfrom externallyappliedthermalradiationor conduction 2. Well ventilated faming free burning 3. Underventilated faming a. Small, localized fre, generally in a poorly ventilated compartment b. Post fashover fre The existence, sequencing, and duration of these phe-nomenologicalstagescanvaryfromfretofre.The roles of these stages are best explained using examples.Figure 6-4 depictsthetimelinefortwohypo-thetical fres. The curve in blue is for a fre occurring in a typical residential bedroom with the door open 012TimeHeat Release Rate/mwA BCDEF E FDCHGGFigure6- 4Rate of heat release from two bedroom fres under different ventilation conditions. Blue curve: Door open and windows initially closed. Red curve: Door and windows initially closed. Dashed red curve: Door and windows remain closed.A. Smoldering ignition of the bed by a cigarette; some oxidative pyrolysisB. Smoldering transition to well ventilated faming; some oxidative pyrolysisC. Room fashover; ventilation-limited faming; extensive pyrolysisD. Window breaks from the heat; burning no longer ventilation-limitedE. Maximum mass burning rate; extensive pyrolysisF.Fuel begins to run out; extensive pyrolysisG. Flames out; rubble smolderingH. Door is opened82 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 82 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONand the windows closed. The curves in red are for a fre in the same room with the door initially closed. Thefollowingsectionsofthischapterprovidea general description of the stages of these fres. More detailedinformationonprinciplesoffrephenom-ena is presented in the subsequent chapters.Fire Initiation We are surrounded by combustible items (e.g., trees, beds,clothes,cars,andevenourownbodies)that are in contact with the oxygen in air. Why do these fuelsnotigniteandburn?Howcanreactantsfor which we can write a balanced, exothermic chemical equationcoexist without consuming each other? Theanswerstothesequestionslieinchemical kinetics, as introduced in thePhysical and Chemical Change chapter. At room temperature, very few colli-sions between fuel molecules and oxygen molecules have enough energy to break chemical bonds. At the macroscopic level, this means that the reaction rates of common combustibles with oxygen are extremely slow at room temperature. However, these reaction ratesincreasesharplywithincreasingtemperature. If a suffciently high temperature is created in even a small region where both the combustible material and oxygen are present,ignition will occur. The oxi-dationprocesswillaccelerateinthisregion,gener-ating its own heat and supply of free atoms and free radicals. The heat will then spread to adjacent areas, and the combustion will be sustainedthat is, there will be a fre. Dependingontheignitionsource,afrecan beginassmolderingcombustion,perhapspro-gressinglatertofaming,orthefrecanbegin directly as faming.Smolderingisthemostcommoninitialstage of combustion in fres that lead to injury or death (Figure6-1).Smokingmaterials(cigarettes),hot embersandashes,spaceheaters,andoverheated electrical equipment are potential ignition sources forsmoldering.Onceignited,upholsteredfurni-ture containing cotton fabrics and cotton or poly-urethane foam padding can smolder for an hour or more. A large pile of wood chips, sawdust, or coal can smolder for weeks or even months. Peat felds have smoldered for decades. Smoldering generally is limited to porous materi-als that can form a carbonaceous char when heated. The oxygen in the air slowly diffuses into the pores of the material, where it reacts directly with the solid carbon and generates heat. These porous materials are poor conductors of heat (i.e.,good thermal insu-lators); therefore, even though the combustion reac-tionoccursslowly,enoughheatisretainedinthe reaction zone to maintain the elevated temperature neededtosustainthereaction.Thereactionzone within the material can be glowing red, but typically is not readily visible from the outside. Thereactionzoneinasmolderingmattress might spread a foot or more from the ignition point Figure 6-5or might even consume the entire mat-tress. At some point, the mattress might suddenly burst into fames. This change in the nature of the combustion typically results from an increase in the temperature of the smoldering zone, often a result of an increase in air fow across the furniture. The mass rate of burning (and the rate of heat release) duringfamingcombustionismanytimeshigher than during smoldering combustion.During the smoldering process, adjacent mate-rial is also being pyrolyzed by the heat being gen-eratedorbytheheatfromtheignitionsource. Pyrolysisisdifferentfromsmolderinginthat pyrolysisstopswhentheheatsourceisremoved, while smoldering generates suffcient heat to con-tinue without external heat input. The combustion productsfromsmolderingandpyrolysisalsoare different.Pyrolysisoccurringdueto,e.g.,aspace heater cooking a nearby chair, is oxidative pyrol-ysis, since there is ample surrounding air. Anaero-bic pyrolysis is likely when overheated household powercablemaintainscontactwithwoodinthe stud space behind gypsum wallboard. Afameistheresultofrapidgaseousoxidation reactions.FromEquation 3- 2inthePhysicaland ChemicalChangechapter,thetemperaturerisefor a given heat generation is T =H/mc p . For typical combustion reactions, H is quite large and both m(themassofairbeingheated)andcp(the specifc Chapter 6 Combustion, Fire, and Flammability 839780763757175_CH06_Printer.indd 83 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONheatofair,showninthePhysicalandChemical Change chapter) are small. Thus, the temperature rise islarge.Theyellow-orangeglowofthesootinthe fame is characteristic of temperatures near 1500 K.Flames from a condensed (solid or liquid) fuel needgaseousfueltocontinueburning.Whena condensed fuel, such as a candle or a pool of gas-oline, burns, a portion of the heat of the gaseous flame is transferred to the fuel, causing it to gasify. Whenthisgasificationoccurswithoutchemical decomposition of the molecules, it is simple evap-oration. If chemical decomposition occurs, which is typical of solid fuels, the process is pyrolysis.Fire SpreadThe spread mechanism of a fre depends heavily on whetherthecombustiblesaregaseous,liquid,or solid.Inthecaseofafammablegas,thespreadis determined by the extent of mixing of the gas with airpriortocombustionaswellasbythedegreeof motion and turbulence of the gas. For a liquid, the spread of combustion depends on whether the com-bustion occurs in a still pool of liquid, a fowing liq-uid, a spray, a thin flm, or a foam.A solid might be in the form of a powder, a thin sheet (e.g., paper), or a thick solid object. The speed offrespreadoverasolidcanbeasfastasseveral Figure 6-5Firefghters attending to a smoldering mattress during a training session.Courtesy of the Upper Allen Fire Department.84 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 84 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONmeters per second or as slow as a fraction of a mil-limeterpersecond,dependingontheconditions. Combustionspreadsmuchmorerapidlyoverthin solids and solids with high surface areas. The spread rate on a vertical solid surface is much faster in the upwarddirectionthaninthehorizontalordown-warddirections.Thefrespreadrateoverahori-zontalsurfacedependsonwhetheraircurrentsare moving toward or away from the combustion zone. Inmostcasesoffrespread,thebasicmecha-nism is the same. A portion of the heat produced by aburning item is transferred, by radiation, conduc-tion, orconvection, to an unburned part of the item ortoanearbycombustiblethatisnotyetburning. This cool fuel is heated to the temperature at which its pyrolysis is fast enough that the concentration of combustible vapor above the fuel surface exceeds its leanfammabilitylimit.Thefamesthenextendto this region and begin to pyrolyze the next area of cool fuel. Fire also can spread by the melting and dripping of burning material, or by airborne frebrands. Fire VentilationFlamingcombustiongeneratesthelargereleaseof heat that can lead to room fashover, with the degree ofventilationoftendeterminingwhetherfashover is reached or not. When a faming fre is small and issurroundedbyfreshair,thefrecanentrainall the oxygen it needs. The fre is said to be well venti-lated. As the fre grows, it can reach a point at which itisconsumingalltheairthatcanentertheroom through,e.g.,anopendoor.Atthispoint,thefre cannotburnanyfaster;itisventilation-limited.If the fre is burning in a closed room, the ambient air becomes oxygen-depleted, and this fre also becomes ventilation limited. During the course of a fre, a ventilation-limited conditioncanchangesuddenlytoamorenearly ventilatedcondition.Thiscanoccurwhenthefre breaks a window, an occupant opens a door, or fre-fghters vent a roof. At this time, the heat release rate of the fre rises quickly to a level determined by the size and location of the new opening. Fire Termination The fre tetrahedron (Figure 6-3) shows that combus-tion requires supplies of fuel and oxidant, a high tem-perature, and reactions that proceed fast enough at this high temperature to generate heat as fast as it is dissi-pated, so that the reaction zone will not cool down. Any actionthatsuffcientlyupsetsthis balancewillextin-guish the fre. In practice, most frefghting approaches affect more than one component of the fre tetrahedron. Amoredetailedpresentationoffreextinguishment appears in the Fire FightingChemicals chapter. Adding a coolant absorbs heat from and reduces the temperature of the combusting system; that is, the cool-ant serves as a heat sink. The coolant can be anymaterial with a relatively high heat capacity. It is not necessary for the coolant to absorb heat as fast as the heat is being generated, because the reaction zone in a fre is already losing some heat to the cooler surroundings. In some cases, only modest additional heat dissipation is needed to tip the balance toward extinguishment. Extinguishmentcanbeaccomplishedbycooling eitherthegaseouscombustionzoneorthesolidor liquidcombustible.Intheformercase,thecoolingis accomplished by introducing a gas or liquid with a high heatcapacityintothefamezone.Thismechanismis how carbon dioxide extinguishers work, in addition to displacing or diluting the supply ofoxygen. In the latter case, the coolant is applied to the fuel surface, prevent-ing the production ofcombustible vapors. This is what you accomplish when you douse a campfre with water. As a simple demonstration of extinguishment by cooling, support a thick, 20-cm-long vertical strip of cardboard from the top and ignite the bottom with a matchFigure 6-6 . The fame will attach to the bottom of the paper and spread rapidly to the top of the strip. Then repeat the experiment, but frst soak the top half of the cardboard with water. When the fame reaches thewetsection,itisextinguished.Thewateracts asaheatsinkandupsetsthebalancebetweenheat production and heat loss. The fame cannot heat the cardboard to its pyrolysis temperature in the available time because of heat absorption by the water. Extinguishmentalsocanbeaccomplishedby introducingabarrierbetweenthecombustible Chapter 6 Combustion, Fire, and Flammability 859780763757175_CH06_Printer.indd 85 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION surface and the fame, the source of heat. For exam-ple, applying a layer of aqueous foam to a fuel spill cools the surface, shields the surface from the fame radiation, and blocks any gasifed fuel from replen-ishing the fameFigure 6-7 . In addition, the poten-tial fuel may be separated from the fre by placing a freblanketoverthecombustibleitem.Somefre- retardant additives, when heated, expand to form an insulating layer over the material they are protecting. Anothermeansofextinguishingafreisto reducetheavailabilityofoxygenbyclosingthe openingstoaburningcompartment.Thispractice constricts the infow of air, while diluting the exist-ingairwith noncombustiblecombustionproducts, suchas carbondioxide.Figure 6-8 showsasimple experimentthatdemonstratestheprincipleofthis approach.Intheleftdrawing,theglassbeakeris heldabovethefuelsurface.Freshairisentrained into the base of the fame, and the candle continues toburn.Intherightdrawing, thebeakerhasbeen lowered.Lessairisentrainedintothe baseofthe fame, and the atmosphere surrounding the fame is depleted in oxygen and rich in carbon dioxide from thecombustion,causingthefametoextinguish. WetMatch Match Figure6- 6 Demonstration of fre extinguishment by absorption of heat. 86 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 86 13/11/13 3:19 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION Note The same approaches to interference with the fre tetrahedron can control a fre, keeping it at a manageable level until frst responders arrive on the scene. One of the functions of an automatic sprinkler system is to contain fre spread when extinguishment is not possible. Texture: Eky Studio/ShutterStock, Inc.; Steel: Sharpshot/Dreamstime.com Figure6- 7 Fire extinguishment with foam. Bortel Pavel/ShutterStock, Inc. A BFigure 6-8Extinguishment of a wax candle fame by surrounding it with its own combustion products. A: Freely burning candle; B: Extinguished candle.Whenextinguishingafrebyeliminatinganyven-tilation,theairtightintegrityofthecompartment needstobe maintaineduntilthefreiscompletely out and all hot surfaces have cooled below ignition temperatures. Otherwise, opening a door will intro-ducefreshairintoahot,fuel-richenvironment. Chapter 6 Combustion, Fire, and Flammability 879780763757175_CH06_Printer.indd 87 13/11/13 3:20 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONTheignitedfamesmaythenexpandviolentlyout thedoorway,harminganywhoarenearbythe dangerousconditionknownas backdraft .Thisis graphically shown inFigure 6-9 , where two photo-graphs,takenonlysecondsafterthecompartment windowwasopened,showtherapidandextreme hazard of a backdraft. Thechemicalreactionsinfamesaresustained by free radicals. Some bromine-, chlorine- and phos-phorus-containing chemicals reduce the concentra-tion of free radicals when added to fames and slow downthecombustionreactions.Examplesinclude halonfreextinguishantsandammoniumphos-phate, which is dropped onto forest fres. Theseapproachescanalsopreventignition. Afuse or circuit breaker, recognizing that too much currentisbeingdrawn,canpreventanelectrical ignitionbytrippingbeforeanyfuelishotenough toignite.Somegaseousfresuppressantsinerta spacebyincreasingtheheatcapacityoftheairor bychemicallyinterferingwiththeignitionprocess beforecombustioncanbesustained(addressedin theFire Fighting Chemicals chapter). Inmanyreal-worldfres,thehazardcontin-uesevenafterthefameshavebeenextinguished. Although the combustion of liquid and gaseous fuels ends very soon after the fames have been quenched, some solid fuel fres leave a hot, porous rubble that can continue to smolder for days. This smoldering is fundamentally similar to the smoldering that occurs intheearlieststageofafre.However,themassof smoldering material is considerably larger. Two Examples of Room Fires For decades, fre scientists have conducted real-scale frestolearnhowfresigniteandprogresstothe point where lives and the integrity of the building are threatened.Figure 6-10showsasequenceofframes fromsuchafretest.Thefurnishingsinthefamily room are a sofa and a loveseat fanking an end table with a lamp. There are draperies behind the sofa. The fre begins in the corner of the sofa,perhaps astheresultofacigarettethatfellintothecrack betweentheseat,arm,andbackcushioning.The initialsmolderingtransitionstofaming,slowat frstbutgrowing.Atabout60seconds,thefame isashighasthebackofthesofa,andwispyblack smoke is rising toward the ceiling. By 1:30, a black smoke layer has spread across the entire room. The bottomofthelayerisabout1.5mfromthefoor. At2:50,intheupperlayer,famescanbarelybe seen, but are spreading across the ceiling. This is the Figure 6-9Demonstration of a backdraft. Jones & Bartlett Learning. Photographed by Glen E. Ellman.88 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 88 13/11/13 3:20 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION phenomenon called fameover or rollover. The air inthelowerpartoftheroom(awayfromthefre itself) is still relatively cool and clear. However, just after3 minutes,theupperlayerisnowsohotthat theradiantenergyisintenseenoughtoignitethe loveseat in a location that is away from the burning sofa. This is the phenomenon called fashover, the point at which all the combustibles in the room are afame. The pyrolysis rate of the sofas is very high, and there is not enough oxygen in the room to burn all the gaseous fuel that is being generated.During the early faming, the bottom of the upper layerdescendsbelowthetopofthedoorframe. Someofthesmokefowsoutoftheroomthrough theupperpartofthedoorway,whilefreshairis drawninthroughthelowerpart.Astheburning Figure 6-10Scenes from a video of a furniture fre in a family room.Courtesy of National Institute of Standards and Technology.Chapter 6 Combustion, Fire, and Flammability 899780763757175_CH06_Printer.indd 89 13/11/13 3:20 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONaccelerates,thesetwofowsincreasedramatically, andthesmokefromtheupperlayerisdriveninto other parts of the building. This threatens the escape capabilityofpeoplenotintheinitialfreroom.At 3:14, the unburned gasesjetting through the door-way are hot enough that they react with the oxygen outside.Theturbulentfamessurgingthroughthe doorwaycanspreadthefretoadjacentcompart-ments.Atthistime,approximately3minutesafter the frst fames became evident, the building and all its occupants are at risk. Thegrowthrateofthisfreandthetimeto fashovermaybesurprising,butarenotatall unusual.Infact,somefresreachseriouspropor-tions much faster. Figure 6-11 shows images of a dry Christmas tree fre in a small living room. The tree is ignited near the foor, perhaps by an overheated stringofdecorativelights.Theblacksmokelayer isrollingacrosstheceilinganddownthewalls only 8 seconds after ignition. One of the chairs has ignited by 17 seconds, and the room reaches fash-over in 26 seconds. By 40 seconds, even the foor-ing is burning. If the door had been closed in either of these two fres,theintensethermalradiationfromtheupper layerwouldhavepyrolyzedmaterialatthesame timethattheburningdepletedtheoxygeninthe room. Shortly after opening the door, the emerging fames due to the backdraft would look similar to the fnal frame inFigure6- 10 . Flammability In the broadest sense, we as a community use the term fammabilitytoconnotehowvigorouslysomething burns.Theterm combustibilityisalsousedinsome casestodescribethefammabilityofcertainbuild-ingconstructionmaterials.Asweseekamorepre-cisedefnitionoffammability,however,the degree ofagreementdiminishes.Becausethenextthree chapters treat the fre characteristics of gaseous, liq-uid, and solid combustibles, here are some thoughts forconsiderationasyoureadaboutfreproperties andtheirmeaninginthecontextof fammability. Restassured that there is no simplistic resolution of thedisagreementsaboutdefnitions;however,peo-plewhowillbepractitionersinfresafetyneedto understand the phenomena they will face. A frst approach to defning fammability derives fromwhatoureyestellus.Doessomethingignite? Doesitkeepburning?Waterdoesnotignite,soit isnotfammable.Gasolineisdesignedtoburnin engines, so it is fammable. Hold a match to a 2 4 stud and it will not ignite, yet we know wood burns because we have seen houses destroyed by fre. Thissuggestsasecondapproachtodefning fammability, one that lies in themeasurement of fre behavior.Classically,anapparatusisconstructed thatappearstoreplicateafresituationthathas caused considerableharm.Anignitionsource ofachosen size is placed in contact with a specimen ofachosensizeforachosenduration.Aquantity, such as the time to ignition or self-extinguishment, the rate of fame spread, or the opaqueness of smoke, is measured. Based on this value, the product from which the test specimen was cut is assigned a fam-mability rating. More recently, apparatus have been designed to quantify material fre properties, such as the rate of heat release or the minimum radiant fux necessarytospreadfames.Reference [9 ]compiles nearlyallofthestandardfretestsusedinNorth America. When one of these tests is accepted, and the use ofthetestresultisincorporatedintopractice,the testoutputbecomesthedefnitionoffammability. Commercialmaterialsandfnishedproductsare then designed to the test, and product specifcations are set according to the test. AscanbeseeninReference [9] ,multipletest methods may exist for the same property (e.g., igni-tion delay time, rate of fame spread). These do not always give the same result for a given product, nor do they always rank products in the same order. This textisintendedtoprovideasuffcientknowledge ofthescientifcprinciplesunderlyingfrethatthe readers can understand the concepts andmetrics of fammabilityandrecognizethoseactionsthatcan improve a nations fre profle and those that do not. 90 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 90 13/11/13 3:20 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONFigure 6-11Frames from a video of a Christmas tree fre in a family room.Courtesy of National Institute of Standards and Technology.Chapter 6 Combustion, Fire, and Flammability 919780763757175_CH06_Printer.indd 91 13/11/13 3:21 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION Fire Consequences, Hazard, Risk, and Flashover Three defnitions regarding the outcomes of fres are used throughout this text (and in the fre literature as a whole): Afre hazard is a condition with the potential to create harm under a specifed set of condi-tions called afre scenario . An example of a fre scenarioisanintoxicatedperson,aloneinan apartment, smoking in bed. The hazard is that hecoulddropthelitcigaretteontothebed, where it ignites the bedclothes and mattress. A freconsequenceisafrehazardquan-tifedastoitsseverity.Inthisexample,the consequence of the fre could be the smokers death from inhalation of smoke from a smol-dering pillow. A freriskisthecombinationofconse-quences,eachmultipliedbythelikelihood (probability)ofeachscenariooccurringand the probability that the scenario will result in eachconsequence.AsevidencedintheU.S. frelossstatistics,theriskoffredeathfrom smokinginbedisgreaterthantheriskof death from smoking outdoors. The consequences of a fre are sharply increased if the fre in a room proceeds to fashover. Inphysical terms,fashoveristhepointatwhichthe thermal radiation from a fre and the hot smoke layer becomes suffcientlyintense(inexcessofapproximately 20 kW/m)thatnearlyallcombustible surfacesin thecompartmentignitesimultaneously.Thispoint occurs when the temperature of the hot smoke layer under the ceiling approaches 1100 F (600 C). Before a room reaches fashover, the fre hazard is mostly limited to the people within the room and to the room contents, because the fre is localized and littlesmokeorheatleavestheroom.(Theremight be suffcient smoke fow through a doorway to acti-vateasmokealarmthatisproperlyplacedoutside the room.) After fashover, the fow of heat and toxic gasesintootherspacesinthebuildingisgreatly accelerated.IntheUnitedStates,mostfredeaths occur outside the room of fre origin from fres that have proceeded past fashover. A principal goal of fre safety science, technology, and engineering practice is to reduce the likelihood of fashover. This is equivalent to managing the heat release rate that might occur in a room. Theminimum heat release rate for fashover increases with the size oftheroomanddependsonthe ventilationinthe room. If too little ventilation is available, the fre is starved for air and goes out or the burning generates too little heat for fashover. If the ventilation is exces-sive, the excess air fow dilutes and cools the smoke, soalargerrateofheatreleaseisneededtoreach thecriticaltemperature conditionfor fashover. Thematerialsof constructionandthicknessofthe ceilingandupperwallsareotherimportantfactors indetermining whether fashover will occur and, if so, how soon. Reference [2]hascompiledquantitativerela-tionshipsthatcanbeusedtocalculatethecritical fresizeforfashover.Apointofreferenceisthat aheatreleaserateof1MWwillbringanordinary residentialbedroomtofashover,ifthisintensityis sustainedlongenoughfortheextensiveradiative ignitiontooccur,whichcanbeaslittleas1min-ute.As seen inFigure 6-10 , an upholstered sofa can bringasmalllivingroomtofashoverinapproxi-mately 3minutes from the time of ignition. Note The following data put the concepts of fre risk and fre consequences into perspective. With approximately 400,000 residential fres reported each year and about 100 million households in the United States, it is likely that each of us will see and hear fre trucks in our neighborhood in our lifetime. These fres are consequential to both civilians and fre fghters: someone in the United States dies or is injured in a fre every 5 minutes [ 10 ]. Texture: Eky Studio/ShutterStock, Inc.; Steel: Sharpshot/Dreamstime.com92 Principles of Fire Behavior and Combustion9780763757175_CH06_Printer.indd 92 13/11/13 3:21 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONFlames: Drx/Dreamstime.com; Steel texture: Sharpshot/Dreamstime.com Chapter Summary Analysis of the data compiled in the National Fire Incident Reporting System database can identifytherelativefrequencyofvarious typesoffres.Theknowledgegainedfrom suchanalysesenablesfrescientists,thefre service,frecodesandstandardsdevelopers, andregulatorstoprovideguidancegeared toward reducing losses from fres. Combustionisanexothermicchemicalreac-tionbetweenafuelandanoxidizerresulting in the generation of substantive heat and often light.Interestingly,dictionariesoftenusethe same defnition for fre . Fire is a form of uncon-tained, unwanted, and destructive combustion. The fammability of a material or product can bedefnedbywhetheritignitesandburns, althoughfammabilityismorecommonly defnedintermsofameasuredpropertyin a fre test. A fre can proceed through both nonfaming andfamingstages.Thenonfamingstages consistofsmolderingandpyrolysis(with orwithouttheparticipationofoxygen);the faming stages include well-ventilated, under-ventilated, and post-fashover fre. Theinitiationandcontinuanceofafre requirethefourcomponentsthatmakeup thefretetrahedron:fuel,oxidant,heat,and chemical chain reaction. The reactions that initiate a fre proceed very slowlyunlessthefuelisheatedtohundreds of degrees Celsius. Common ignition sources include cigarettes, matches, and sparks; fric-tion;overheatedelectricalcircuits;welding operations; space heaters; and lightning. Forafretospread,someoftheenthalpy from the ignition region must heat unburned materialtoatemperatureatwhichitcan burn. Fire also can spread by the melting and drippingofburningmaterialorbyairborne frebrands. A fre can be extinguished by cooling, reduc-ing the oxygen supply, separating the fuel and theoxidizer,anddecreasingtheconcentra-tion of the fame-propagating free radicals. Firehazardisthepotentialforharmfroma particular fre scenario. A fre consequence is a specifc quantifed hazard. Fire risk combines the severity of consequences and the probabil-ity that the consequences will occur. There is a sharp increase in the consequences and risk from a fre in a room if it proceeds to fashover. Key Terms backdraftIntense fames emanating from a just opened doorway that introduced fresh air to a fre that had been oxygen starved. freconsequenceAspecifc,quantifedfre hazard. frehazardAconditionwiththepotentialto createharmfulconsequencesunderaspeci-fed fre scenario. freriskTheundesiredconsequencesofa fremultipliedbythelikelihoodoftheir happening. frescenarioAspecifedsetoffreconditions, includingdetailsofthefresiteandits condition,thecombustibleitems,thenum-ber and characteristics of the occupants, and anythingelsethatmightaffecttheoutcome of the fre. fashoverTheoften-suddentransitionfrom local burning to almost simultaneousignition of (nearly) all of the exposed combustibles in a confned area. glowingA descriptor of smoldering combustion whenitisaccompaniedbyvisiblethermal radiation. ignitionThe onset of combustion. 9780763757175_CH06_Printer.indd 93 13/11/13 3:21 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTIONFlames: Drx/Dreamstime.com; Steel texture: Sharpshot/Dreamstime.com pyrolysisTheanaerobicoroxidativedecom-positionofagas,liquid,orsolidintoother molecules when heated. rollover (fameover)The stage of astructure fre when fre gases in a room or otherenclosed area ignite. smoldering (nonfaming combustion)The slow, low-temperature,famelesscombustionofa solid. vitiationThe depletion of oxygen and incorpo-rationoftheresultingcombustionproducts in air by a fre. Challenging Questions1. When is oxidationnot combustion? When is combustionnot oxidation? 2. Givethreeexamplesofcombustionother than fre. 3. Which kinds of materials can undergo smol-dering combustion? 4. Name six ways in which a fre can originate. 5. What is the basic mechanism of fre spread? 6. What are the four fundamental ways of extin-guishing a fre? 7. Perform the experiment shown inFigure 6- 6 . A strip of the cardboard from the back of an 8in.11in.notepadisagoodtestsub-ject, and ordinary kitchen tongs can be used toholdthecardboardsafely.Whatisthe generalconceptdemonstratedbythistest? ( Note:Conductanyexperimentswithfre outsideandawayfromadjacentstructures and materialsthatcouldignite.Wearleather gloves and have a pail of water handy.) References 1. National Fire Incident Reporting System website. http://www.usfa.fema.gov/freservice/nfrs 2. Pitts ,W. M. ( 2013 ).Summary and Conclusions of a Work-shoponQuantifyingtheContributionofFlamingResidential UpholsteredFurnituretoFireLossesintheUnitedStates (NIST Technical Note 1757 Revised).Gaithersburg, MD : National Institute of Standards and Technology . 3. Ahrens ,M. ( 2011 ).Home Fires ThatBegan with Upholstered Furniture .Quincy, MA :National Fire Protection Association . 4. Evarts , B.( 2011 ). HomeStructureFiresThatBeganwith MattressesandBedding . Quincy,MA : NationalFire Protection Association . 5. Drysdale , D.( 2011 ). AnIntroductiontoFireDynamics , 3rd ed.New York, NY :John Wiley . 6. DiNenno , P.J. , ed.( 2008 ). SFPEHandbookofFire Protection Engineering ,4th ed.Quincy, MA :National Fire Protection Association . 7. Glassman ,I. ( 1996 ).Combustion ,3rd ed.New York, NY : Academic Press . 8. ISO19706:GuidelinesforAssessingtheFireThreatto People . ( 2011 ).Geneva,Switzerland :International Stan-dards Organization . 9. ASTMFireStandardsandRelatedMaterials , 7thed. ( 2007 ). WestConshohocken,PA : ASTMInternational . www.ASTM.org 10. Karter , M.J. ,Jr.( 2011 ). FireLossintheUnitedStates during2010 . Quincy,MA : NationalFireProtection Association . 9780763757175_CH06_Printer.indd 94 13/11/13 3:22 AM Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION