Transcript
© 2009 Delmar, Cengage Learning
Science, Methodology, and Fire Behavior
Chapter 7
© 2009 Delmar, Cengage Learning
Objectives
• Describe the concept of scientific methodology• Describe the aspects of fire behavior • Describe and understand the concept of heating
at the molecular level• Describe and understand the concept of heat
transfer as it relates to the fire investigator• Describe and understand the concept of flame
spread and complications associated with flashover
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Case Study
• A garage door is open and flames can be seen from floor to ceiling inside the garage
• The homeowner explained that he was using diesel fuel to clean some engine parts
• He bent over and the cigarette in his mouth fell into the bucket causing an explosion
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Case Study (cont’d.)
• Local police investigator, who had no fire experience, accepted that explanation and closed the case– However, based on the private investigators report,
the insurance company denied the claim
• Diesel fuel is combustible, but it is difficult at best to get it to ignite from a cigarette– Under conditions at this fire scene, it would be even
less likely to ignite
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Introduction
• To conduct detailed fire investigations effectively, you need to understand topics in chemistry and physics
• Good science is that which is based on proven and reproducible scientific principles
• Junk science is that which is unproven, founded on speculation, conjecture, and outdated concepts and principles
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Scientific Methodology
• One aid is to use a systematic approach using the scientific method – Was not invented by the fire service but takes a
fundamental principle from the scientific community
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Scientific Methodology (cont’d.)
• Steps in the scientific method– Recognize the Need– Define the Problem – Collect Data – Analyze the Data – Develop a Hypothesis – Test the Hypothesis– Select a Final Hypothesis
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Presumption of the Fire Cause
• Just as dangerous as labeling a fire based on limited knowledge after an extensive investigation is presuming the fire cause before starting the investigation
• Fundamental problem is that an individual may look only for evidence that supports the presumption – This is why many fire investigators want to look at the
fire scene before conducting their interviews
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Systematic Search
• Investigator should do a systematic search of the fire scene – Working from the least damaged area to the most
damaged area– Starting from the outside and working to the interior– Working in a circle, clockwise or counterclockwise;
either, as long as that investigator does it the same each and every time
• No one procedure or process can fit each and every situation
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Chemistry of Fire for the Fire Investigator
Figure 7-2 The firetriangle and thefire tetrahedron arethe basic modelsthat explain how toextinguish a fire aswell as how a firestarted.
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Fire Tetrahedron
• Fire service uses a model called the fire tetrahedron, which is a four-sided solid object with four triangular faces – The firefighter uses this concept to show how a
fire is extinguished
– The fire investigator must use this model to determine how the fire started
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Self-Sustained Chain Reaction
• Using a piece of paper as an example the following happens: – A burning match in close proximity heats the paper,
exciting the molecules– This begins to breakdown the paper, resulting in
combustible gases being given off– The gas ignites from the match or from reaching its
ignition temperature
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Self-Sustained Chain Reaction (cont’d.)
– When the gas ignites, that heat impacts the surface of the paper, causing the release of more combustible gases
– More surface of the paper is now burning, impacting an even larger area on the paper, igniting and giving off more vapor
– This chain reaction continues until the fire is extinguished or the fuel (paper) is consumed
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Oxygen
• Fires cannot occur without an oxidizer– Atmosphere is usually primary oxidizer – Many compounds can give off oxygen in sufficient
amounts to allow combustion• Ammonium nitrate fertilizer used in OKC bombing
– The rate of combustion is based on amount of oxygen available
– If there are reports of white, intense heat, far from ordinary combustion, the investigator should consider the proximity of an oxidizer
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Fuels
• A fuel is anything that can burn • Physical state of the fuel affects the combustion
process– Most fuels have to be gases
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Fuels (cont’d.)
• Solids– Solids have a physical size and shape – To ignite, solids must be heated to the point of
decomposing and producing vapors
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Fuels (cont’d.)
• Liquids– Flash point: sufficient vapors are given off to support
a flaming fire across the surface of the liquid – Fire point: liquid generates sufficient vapors to allow
the flame to continue to burn– Auto ignition temperature: minimum temperature at
which properly proportioned mixture of air and vapor will ignite with no external source
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Fuels (cont’d.)
• Gases– Relatively few gases are ignitable at room
temperature– Vapor density is the weight of a gas when compared
to air – Both gases and vapors can ignite only when mixed
with an appropriate amount of oxygen– Lean: not enough gas– Rich: too much gas
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Heat and Temperature
• Molecules are always in motion– This is always producing energy in the form of heat
• Thermal runaway: more and more energy is created that eventually results in the release of heat and light (fire)
• Temperature is a measurement of the amount of heat
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Ignition Temperature and Ignition Energy
• Ignition temperature is the minimum temperature a substance must attain before ignition can occur– Most solids and liquids need to be heated
• Amount of energy also important
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Sources of Heat
• Mechanical– Mechanical heat is the heat of friction– Another form of mechanical heat is the heat of
compression
• Chemical– A mixture of two or more chemicals can create heat
and sometimes cause ignition – Spontaneous heating can occur from biological action
as well as chemical• Can lead to spontaneous ignition
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Sources of Heat (cont’d.)
• Electrical– Electricity and electrical devices are heat producers – Electrical sources can be as small as a static
electrical arc or as massive as a lightning bolt
• Nuclear– By its very nature, radioactive materials are unstable
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Heat Transfer
• Conduction– The transfer of heat through a solid object is
considered to be conduction• Metals are good conductors
• Convection– Transfer of heat through the movement of liquid or
gases is convection • Movement of heat up an elevator shaft
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Heat Transfer
• Radiation– Radiation is the transmission of energy through
electromagnetic waves
Figure 7-10 Examplesof heat transfer in fire.
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Thermal Layering
• Plume: the column of smoke, hot gases, and flames that rises above a fire
• Rising gases form thermal layers of varying temperatures
• Create burn patterns that investigators can study
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Heat Release Rate
• Amount of energy produced over a period of time is considered to be the energy release rate or the heat release rate
• Knowing the rate for various objects can help the investigator in analyzing the fire scene
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Compartment Fires
• Configuration, construction, and contents of a compartment affect the growth and development of a fire
• In the beginning, heat and smoke become buoyant and rise to the ceiling
• Fire progresses, with smoke and heat escaping from openings in the room of origin
• Air entrainment: new air being introduced– Keeps combustion process going
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Compartment Fires (cont’d.)
• Flame over: where the gases in the upper layer ignite, sending flames rolling across the buoyant layer – Could be precursor to flashover (all the combustible
materials in the room ignite)
• Many fires start to decay before flashover due to consumption of the fuel
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Compartment Fires (cont’d.)
• If there is adequate ventilation, flashover will not occur since escaping heat will prevent the room from reaching flashover temperature fire will eventually decay
• Flashover: a transition where all materials in the compartment reach ignition temperature
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Compartment Fires (cont’d.)
• On occasion, a compartment may have sufficient heat for combustion, and fuel is present, especially in the form of un-ignited gases, but there is no oxygen– If air rushes in, it could create a condition called a
backdraft
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Summary
• Fire investigators need not be scientists to investigate fires– However, scientific knowledge is necessary
• Investigators must use good science, which is science based on accepted principles and facts• Knowledge of fire science principles are
essential• Investigators must constantly challenge what
they have learned in the past
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