Basics of Fire Behavior and Protection Systems Development

Learning Objectives

• Describe the difference between fire and combustion.
• Identify and describe the elements of the fire triangle.
• Identify and describe the elements of the fire tetrahedron.
• List and describe the different types of fire.
• List and describe the different stages of fire.
• List and describe the mechanisms of heat transfer.
• List and describe the methods used to extinguish fires.
• List and describe the classes of fire and their relationship to extinguishing agents.

Case Study: Shasta County Wildfire (July 2018)

• Event Details: On July 23, 2018, in Shasta County, California, environmental conditions (hot, dry, abundant fuel, erratic wind) led to a massive wildfire.
• Ignition Source: Sparks emanating from a tire rim of a trailer with a flat tire.
• Spread and Intensity:
  • The fire spread over $20,000\,\text{acres}$ in the first $3$ days.
  • On July 26, it doubled to nearly $49,000\,\text{acres}$.
  • A deadly fire tornado raged for approximately $30\,\text{minutes}$.
  • The fire jumped the Sacramento River, a natural fire break.
• Total Impact (after 5 weeks):
  • $229,651\,\text{acres}$ burned.
  • $1881\,\text{structures}$ damaged or destroyed.
  • $11\,\text{people}$ injured.
  • $3\,\text{firefighters}$ and $5\,\text{civilians}$ killed.
  • Damage estimates: Over $\$1.6\,\text{billion}$.
  • Suppression costs: Just under $\$159\,\text{million}$.
• Relative Significance: At the time, it was the sixth most destructive wildfire in California. By November 2018, fires in Butte County (the most destructive/deadly, killing $85\,\text{people}$ and destroying $18,804\,\text{structures}$) and Ventura County (the seventh most destructive, destroying $1,643\,\text{structures}$ and killing $3\,\text{people}$) surpassed or joined it in rankings.

Introduction to Fire Protection Systems and Hazard Evaluation

• Engineering fire protection systems requires understanding fire behavior relative to the type and size of the hazard.
• Hazard Evaluation Factors:
  • Occupancy and use conditions.
  • Construction materials, products, and finishes.
  • Quantity, arrangement, and type of items (fuel load).
  • Methods of handling or storing items.
• Fuel Load: Defined as the amount of combustible and flammable materials within an area; a primary factor in determining a building's hazard classification.
• System Reevaluation: Systems must be reevaluated if a building's occupancy, hazards, or fuel loads change (e.g., a warehouse changing contents or storage methods).

Defining Combustion and Fire

• Combustion: A chemical reaction where a combustible material and an oxidizing agent (typically oxygen in the air) produce heat/energy and other products. It is self-sustaining if heat is generated faster than it dissipates.
• Fire: A rapid, self-sustaining chemical reaction that produces energy in the forms of heat, light, and flame. It requires fuel, a heat source, and an oxidizer.
• The Key Difference: In combustion, released energy stays in the reaction to continue it. Fire is a specific form of combustion that emits and dissipates energy as heat and light.

The Models of Combustion: Fire Triangle and Fire Tetrahedron

• The Fire Triangle: A geometric depiction of three essential elements:
  1. Fuel
  2. Oxygen (Air)
  3. Heat (Energy)
  • Eliminating any one side extinguishes the fire. Proportions determine if the fire smolders, burns slowly, or burns rapidly.
• The Fire Tetrahedron: A more sophisticated four-sided model developed in the late 20th century. It includes the original three legs plus a fourth:
  • Chemical Chain Reaction: This element explains why some agents extinguish fires without cooling or removing fuel/oxygen—they interrupt the self-sustaining chain reaction.
  • While the triangle depicts what starts a fire, the tetrahedron depicts what sustains it once ignition occurs.

Discernible Types of Fire and Flame

• Diffusion Flame: The most recognizable fire (candles, matches, campfires, structure fires). It is a gaseous reaction where gas fuel and oxygen move into the reaction zone from opposite sides.
• Smoldering: Combustion without flame (charcoal, cigarettes). Air flows over the surface slowly. It can transition to flaming fire if oxygen or heat increases. It may be the beginning or end phase of a fire event.
• Spontaneous Combustion (Self-Heating): Fire starting without an external heat source. Occurs when oxidation in a heat-limiting environment increases local temperatures. Examples include cotton rags with linseed oil (hours to ignite) or wet hay (weeks to ignite).
• Premixed Flame: Fuel and air mix before ignition (gas stoves, Bunsen burners, internal combustion engines). Requires the fuel to be within its flammable limits.
  • Acetylene Example: Flammable limits are between $2.5\%$ and $100\%$.

The Stages of Fire Development

• Incipient Stage: Initial stage where fuel, oxygen, and heat join. Heat generation exceeds dissipation, promoting the chain reaction. Transition to growth begins here.
• Growth Stage (Free Burning Stage): Fire survives by consuming local air and materials. Factors affecting speed:
  • Oxygen Supply: Oxygen-enriched atmospheres or chemical oxidizers cause rapid development.
  • Fuel Form: $100\,\text{lb}$ of hardwood releases approximately $860,000\,Btu$ ($8600\,Btu/lb \times 100\,lb$). Tightly stacked wood burns slowly; cribbed wood (aerated) burns rapidly; sawdust suspended in air can explode.
  • Compartment Geometry: Low ceilings reflect radiant heat; insulation prevents dissipation.
• Flashover: The transition between growth and fully developed stages. All surfaces in a space reach their ignition temperature simultaneously. Survival chances in a flashover area are essentially zero.
• Fully Developed Stage: The fire consumes all available fuel/oxygen and releases maximum heat. If ventilation is limited, unburned gases are produced and may migrate to adjacent areas to start new fires.
• Smoldering/Decay Stage: Occurs when fuel is exhausted or oxygen drops below $16\%$. Fire subsides into glowing combustion. High levels of carbon monoxide and toxic gases remain.

Mechanisms of Heat Transfer

• Conduction: Transfer of heat via direct contact. Dense materials (copper, aluminum) are better conductors than porous ones (wood, fiberglass, air).
• Convection: Transfer of heat through a medium like liquid or gas. Creates convection currents in buildings and can generate hurricane-force winds in large conflagrations.
• Radiation: Transfer of heat via electromagnetic energy (light). It travels in a straight line and is a major factor in flashover and fire spread between buildings.

Methods of Fire Extinguishment

• Cooling: Primarily using water. $1\,\text{gallon}$ of water absorbs about $8000\,Btu$. Note: Ordinary combustibles release $7000$-$8000\,Btu/lb$, but plastics can release double that, requiring twice the water.
• Oxygen Reduction: Starving the fire by controlling air intake, using foam blankets, or displacing oxygen with carbon dioxide ($CO_2$).
• Removing/Interrupting Fuel Supply: Shutting off gas lines, using "backfires" in wildlands, or pre-wetting adjacent fuel with sprinklers.
• Interrupting the Chain Reaction: Using dry chemicals, halon, or clean agents to chemically decompose or isolate combustion products.

Classification of Fires

• Class A: Ordinary combustibles (wood, paper, cloth, rubber, many plastics). Water is most effective. Rubber tires release $12,000$-$16,000\,Btu/lb$.
• Class B: Flammable/combustible liquids and gases (gasoline, oils, alcohols). Extinguished by smothering (carbon dioxide, foam) or interrupting the chain reaction.
• Class C: Energized electrical equipment. Requires nonconductive agents. If de-energized, the fire reclassifies (usually to Class A).
• Class D: Combustible metals (magnesium, titanium, sodium, lithium, etc.). Requires dry powder agents. Water application can be extremely hazardous.
• Class K: Cooking oils and fats (vegetable/animal fats). Established via UL 300 testing due to high-efficiency fryers and vegetable oils. Extinguished via saponification (converting fats to soapy foam).

International Fire Classifications

• Europe/Australia System:
  • Class A: Ordinary Combustibles.
  • Class B: Flammable Liquids.
  • Class C: Flammable Gases.
  • Class D: Combustible Metals.
  • Class E: Live Electrical Equipment.
  • Class F: Oil and Fats.

Key Terminology

• Autoignition Temperature: Lowest temperature at which a material ignites in air without a spark or flame.
• British Thermal Unit (Btu): Heat required to raise $1\,\text{lb}$ of water $1\,^{\circ}F$ at $1\,\text{atm}$ and $60\,^{\circ}F$ ($= 1055\,J = 1.055\,kJ = 252.15\,cal$).
• Fire Protection System: Device or system designed to detect, control, or extinguish fire.
• Saponification: Process where fatty acids in cooking media react with extinguishing agents to form foam.
• UL: Independent world leader in fire and safety testing (formerly Underwriters Laboratories).

Questions & Discussion

Case Study Discussion Questions

1. What impact does the public’s desire to build at the urban/wildland interface have on the fire service’s ability to deal with wildfires?
2. What effects do you think climate change and the environment have on wildland fires?
3. What type of planning, technology, and resources would help to reduce losses associated with wildland fires?
4. Should state or federal agencies attempt to enact legislation to restrict a developer’s or landowner’s right to build on property or require stricter development and building codes when building in the urban/wildland interface?
5. What role do you think the insurance industry plays in this discussion?

Challenging Questions

1. Define the terms fire and combustion and explain the difference between them.
2. Explain the term oxidation.
3. Compare and contrast the fire triangle and the fire tetrahedron.
4. Using the fire tetrahedron, describe four methods of fire extinguishment.
5. Differentiate the classifications of fire and give a practical example of each.
6. List the types of fire and explain each.
7. List the stages of fire development and explain each.
8. What are the factors that contribute to flashover?
9. Fuel dispensers at service stations are equipped with signs that include instructions and safety warnings. Use the fire tetrahedron to explain the reasons for these warnings.
10. Each year, wildland fires destroy thousands of homes. Using the modes of heat transfer for guidance, develop a list of strategies to reduce the amount of damage.