Chapter 07


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Chapter 07

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Junk science is that which is unproven, founded on speculation, conjecture, and outdated concepts and principles 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