5 Fire Behavior

Objectives (1 of 4) Describe the chemistry of fire. Define the three states of matter. Describe how energy and work are interrelated. Describe the conditions needed for a fire. Explain the chemistry of combustion. Describe the products of combustion.

Objectives (2 of 4) Explain how fires can spread by conduction, convection, and radiation. Describe the four methods of extinguishing fires. Define Class A, B, C, D, and K fires. Describe the characteristics of solid-fuel fires.

Objectives (3 of 4) Describe the ignition phase, growth phase, fully developed phase, and decay phase of a fire. Describe the characteristics of a room-and-contents fire. Explain the causes and characteristics of flameover, flashover, thermal layering, and backdraft.

Objectives (4 of 4) Describe the characteristics of liquid-fuel fires. Define the characteristics of gas-fuel fires. Describe the causes and effects of a boiling liquid expanding vapor explosion (BLEVE). Describe the process of reading smoke.

Introduction Fire has been around since the beginning of time. Destruction of lives and property by uncontrolled fires has occurred since just as long.

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 utilizing less water.

What Is Fire? Rapid chemical process that produces heat and usually light Fire is neither solid nor liquid. Wood is a solid, gasoline is a liquid, and propane is a gas—but they all burn.

Matter Atoms and molecules Three states Solid Liquid Gas

Solids Definite shape Stokes most uncontrolled fires Expands when heated and contracts when cooled

Liquids Assume the shape of their containers Most will turn into gases when sufficiently heated Has a definite volume

Gases Have neither independent shape nor volume Expand indefinitely Mixture of gases in air maintain a constant composition 21% Oxygen 78% Nitrogen 1% Other gases

Fuel Form of energy Energy released in the form of heat and light has been stored before it is burned

Types of Energy Chemical Mechanical Electrical Light Nuclear

Chemical Energy Energy created by a chemical reaction. Some of these reactions produce heat and are referred to as exothermic reactions. Some of these reactions absorb heat and are referred to as endothermic reactions.

Mechanical Energy Converted to heat when two materials rub against each other and create friction Heat is also produced when mechanical energy is used to compress air in a compressor.

Electrical Energy Produces heat while flowing through a wire or another conductive material Other examples of electrical energy Heating elements Overloaded wires Electrical arcs Lightning

Light Energy Caused by electromagnetic waves packaged in discrete bundles called photons Examples of light energy Candles Light bulbs Lasers

Nuclear Energy Created by nuclear fission or fusion Controlled (nuclear power plant) Uncontrolled (atomic bomb explosion) Release radioactive material

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 along a road.

Conditions Needed for Fire Three basic factors required for combustion: Fuel Oxygen Heat Chemical chain reactions keep the fire burning.

Chemistry of Combustion (1 of 2) Compounds of atoms and molecules Almost all fuels are hydrocarbons Consist of both hydrogen and carbon atoms Wide variety of other molecules that release toxic by-products Incomplete combustion produces large quantities of deadly gases

Chemistry of Combustion (2 of 2) Oxidation Combustion Pyrolysis

Products of Combustion Combustion produces smoke and heat. Specific products depend on: Fuel Temperature Amount of oxygen available Few fires consume all available fuel.

Smoke Airborne products of combustion Consists of: Ashes Gases Aerosols Inhalation of smoke can cause severe injuries.

Smoke Contents (1 of 2) Particles Vapors Solid matter consisting of unburned, partially, or completely burned substances Vapors Small droplets of liquids suspended in air Oils from the fuel or water from suppression efforts

Smoke Contents (2 of 2) Gases Most gases produced by fire are toxic. Common gases include: Carbon monoxide Hydrogen cyanide Phosgene

Fire Spread Three methods of fire spread: Conduction Convection Radiation

Conduction Heat transferred from one molecule to another (direct contact) Good conductors absorb heat and transfer it throughout the object.

Convection Circulatory movement in areas of differing temperatures Creates convection currents

Convection Within a Room Hot gases rise, then travel along the ceiling. Convection may carry the fire outside the room of origin

Radiation Transfer of heat in the form of an invisible wave Travels in all directions Is not seen or felt until it strikes an object and heats its surface

Methods of Extinguishment Cool the burning material. Exclude oxygen. Remove fuel. Break the chemical reaction.

Classes of Fire Five classes of fires: Class A Class B Class C Class D Class K

Class A Involve ordinary solid combustibles Cool the fuel with water Wood Paper Cloth Cool the fuel with water

Class B Involve flammable or combustible liquids Gasoline Kerosene Oils Shut off the fuel supply or use foam to exclude oxygen from the fuel

Class C Involve energized electrical equipment Attacking a Class C fire with an extinguishing agent that conducts electricity can result in injury or death.

Class D Involve combustible metals Sodium Magnesium Titanium The application of water will result in violent explosions Must be attacked with special agents

Class K Involve combustible cooking oils and fats Special extinguishers are available to handle this type of fire.

Solid Fuels Most fires encountered involve solid fuels. Do not actually burn in the solid state Must be heated or pyrolyzed to decompose into vapor May change directly from a solid to a gas Wood does not have a fixed ignition temperature

Solid-Fuel Fire Development Four distinct phases: Ignition Growth Fully developed Decay

Ignition Phase Fuel, heat, and oxygen are present. Flame produces a small amount of radiated energy. Convection and radiation heat the fuel.

Growth Phase Kindling starts to burn, increasing convection of hot gases upward. Energy radiates in all directions. Major growth in an upward direction

Fully Developed Phase Produces the maximum rate of burning Fire will burn as long as fuel and oxygen remain.

Decay Phase Fuel is nearly exhausted Rate of burning slows Flames become smoldering embers

Key Principles of Solid-Fuel Fire Development (1 of 2) Hot gases and flame tend to rise. Convection is the primary factor in spreading the fire upward. Downward spread occurs primarily from radiation and falling chunks of flaming material. If there is no remaining fuel, the fire will go out.

Key Principles of Solid-Fuel Fire Development (2 of 2) Variations in the direction of fire spread occur if air currents deflect the flame. The total material burned reflects the intensity of the heat and the duration of the exposure to the heat. An adequate supply of oxygen must be available to fuel a free-burning fire.

Room Contents Synthetic products prevalent today made from petroleum products. These produce dense smoke that can be highly toxic. Newer paints Carpets Furniture

Ignition Phase Flame begins small and localized Convection of hot gases is the primary means of fire growth Fire could probably be extinguished with a portable fire extinguisher

Growth Phase Additional fuel is drawn into the fire. Convection current carries hot gases to the ceiling Flames spread upward and outward Radiation starts to play a greater role Growth is limited by the fuel and oxygen available

Fully Developed Phase Flammable materials are pyrolyzed. Volatile gases are being released. Flashover All combustible materials in a room ignite at once. Temperatures can reach 1000 °F. Fire fighters cannot survive for more than a few seconds in a flashover

Decay Phase Burning decreases to the point of smoldering fuel May continue to produce a large volume of toxic gases

Special Considerations Three conditions Flameover (or rollover) Thermal layering Backdraft

Flameover (Rollover) Flaming ignition of hot gases layered in a developing room or compartment fire Flames can extend throughout the room at the ceiling level

Thermal Layering Gases rise and form layers Thermal balance Water applied to a fire creating steam Steam displaces hot gases at the top of the room Ventilate while attacking the fire Avoid directing water at the ceiling

Backdraft (1 of 3) Requires a “closed box” Explosion that occurs when oxygen is suddenly admitted to a confined area that is very hot and filled with combustible vapors © Dennis Wetherhold, Jr.

Backdraft (2 of 3) Signs of an impending backdraft: Confined fire with a large heat build-up Little visible flame from the exterior “Living fire” Pressurized smoke Smoke-stained windows Turbulent smoke Ugly yellowish smoke

Backdraft (3 of 3) Prevention of backdrafts: Ventilate at a high level to allow superheated gases to escape before or just as additional oxygen is introduced. Well-coordinated fire attack

Liquid-Fuel Fires (1 of 2) 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 (2 of 2) Flammability is determined by the compound with the lowest ignition temperature Flash point is the lowest temperature at which vapor is produced Flame point (or fire point) is the lowest temperature at which sufficient vapors are produced

Gas-Fuel Fires (1 of 2) Vapor Density Weight of a gas fuel Gas with vapor density less than 1 will rise. Gas with vapor density greater than 1 will settle. Knowing vapor density helps predict where the danger of ignition will be.

Gas-Fuel Fires (2 of 2) Flammability limits Below the lower flammability limit Too little fuel = too lean Above the upper flammability limit Too much fuel = too rich

BLEVE (1 of 2) Boiling liquid, expanding vapor explosion Occurs when a vessel storing liquid fuel under pressure is heated excessively

BLEVE (2 of 2) Vessel is heated. Internal pressure rises past ability to vent. Temperature exceeds the boiling point of the liquid causing the vessel to fail. Liquid immediately turns into a rapidly expanding cloud of vapor. Vapor ignites into a huge fireball.

Smoke Reading (1 of 4) Enables the fire fighter to learn where the fire is, how big it is, and where it is moving Fires are dynamic events. Smoke is the fuel all around you at a fire. The best place to observe patterns of smoke is outside of the fire building.

Smoke Reading (2 of 4) Determining the key attributes of smoke Smoke volume Smoke velocity Smoke density Smoke color Black fire

Smoke Reading (3 of 4) Determine the influences on the key attributes Size of the structure Wind conditions Thermal balance Fire streams Ventilation openings Sprinkler systems

Smoke Reading (4 of 4) Determine the rate of change Predict the event Changes in the four key attributes indicate changes in the fire Predict the event Consider the key attributes, what influences them, and their rate of change Communicate key parts to the company officer

Smoke Reading Through a Door If smoke exits through the top half and clean air enters through the bottom half If smoke rises and the opening clears If smoke thins, but still fills the door

Summary (1 of 3) Characteristics of solids, liquids, and gases are different. Fire triangle and fire tetrahedron represent conditions necessary for combustion. Five classes of fire require specific extinguishing methods.

Summary (2 of 3) Knowledge of fire spread Typical fires pass through four distinct phases. Liquid-fuel fires, gas- fuel fires, and interior fires have unique characteristics.

Summary (3 of 3) Flameover, themal layering, and backdraft are conditions that threaten fire fighters and victims. Smoke reading enables the fire fighter to learn where the fire is, how big it is, and where it is moving.