Fire Behavior

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Fire Behavior

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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.

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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.

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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.

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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.

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Introduction

• Fire has been around since the beginning of time.

• Destruction of lives and property by uncontrolled fires has occurred since just as long.

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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.

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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.

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Matter

• Atoms and molecules• Three states

– Solid– Liquid– Gas

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Solids

• Definite shape

• Stokes most uncontrolled fires

• Expands when heated and contracts when cooled

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Liquids

• Assume the shape of their containers

• Most will turn into gases when sufficiently heated

• Has a definite volume

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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

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Fuel

• Form of energy

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

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Types of Energy

• Chemical• Mechanical• Electrical• Light • Nuclear

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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.

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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.

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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

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Light Energy

• Caused by electromagnetic waves packaged in discrete bundles called photons

• Examples of light energy– Candles– Light bulbs– Lasers

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Nuclear Energy

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

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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.

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Conditions Needed for Fire

• Three basic factors required for combustion:– Fuel– Oxygen– Heat

• Chemical chain reactions keep the fire burning.

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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

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Chemistry of Combustion (2 of 2)

• Oxidation

• Combustion

• Pyrolysis

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Products of Combustion

• Combustion produces smoke and heat.

• Specific products depend on:– Fuel– Temperature– Amount of oxygen available

• Few fires consume all available fuel.

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Smoke

• Airborne products of combustion

• Consists of:– Ashes– Gases– Aerosols

• Inhalation of smoke can cause severe injuries.

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Smoke Contents (1 of 2)

• Particles– 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

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Smoke Contents (2 of 2)

• Gases– Most gases produced by fire are toxic.– Common gases include:

• Carbon monoxide• Hydrogen cyanide• Phosgene

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Fire Spread

• Three methods of fire spread:– Conduction– Convection– Radiation

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Conduction

• Heat transferred from one molecule to another (direct contact)

• Good conductors absorb heat and transfer it throughout the object.

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Convection

• Circulatory movement in areas of differing temperatures

• Creates convection currents

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Convection Within a Room

• Hot gases rise, then travel along the ceiling.

• Convection may carry the fire outside the room of origin

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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

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Methods of Extinguishment

• Cool the burning material.

• Exclude oxygen.

• Remove fuel.

• Break the chemical reaction.

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Classes of Fire

• Five classes of fires:– Class A– Class B– Class C– Class D– Class K

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Class A

• Involve ordinary solid combustibles– Wood– Paper– Cloth

• Cool the fuel with water

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Class B

• Involve flammable or combustible liquids– Gasoline– Kerosene– Oils

• Shut off the fuel supply or use foam to exclude oxygen from the fuel

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Class C

• Involve energized electrical equipment

• Attacking a Class C fire with an extinguishing agent that conducts electricity can result in injury or death.

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Class D

• Involve combustible metals– Sodium– Magnesium– Titanium

• The application of water will result in violent explosions

• Must be attacked with special agents

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Class K

• Involve combustible cooking oils and fats

• Special extinguishers are available to handle this type of fire.

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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

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Solid-Fuel Fire Development

• Four distinct phases:– Ignition– Growth– Fully developed– Decay

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Ignition Phase

• Fuel, heat, and oxygen are present.

• Flame produces a small amount of radiated energy.

• Convection and radiation heat the fuel.

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Growth Phase

• Kindling starts to burn, increasing convection of hot gases upward.

• Energy radiates in all directions.

• Major growth in an upward direction

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Fully Developed Phase

• Produces the maximum rate of burning

• Fire will burn as long as fuel and oxygen remain.

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Decay Phase

• Fuel is nearly exhausted

• Rate of burning slows• Flames become

smoldering embers

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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.

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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.

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Room Contents

• Synthetic products prevalent today made from petroleum products.– These produce dense smoke that can be

highly toxic.

• Newer paints

• Carpets

• Furniture

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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

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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

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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

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Decay Phase

• Burning decreases to the point of smoldering fuel

• May continue to produce a large volume of toxic gases

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Special Considerations

• Three conditions – Flameover (or rollover)– Thermal layering– Backdraft

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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BLEVE (1 of 2)

• Boiling liquid, expanding vapor explosion

• Occurs when a vessel storing liquid fuel under pressure is heated excessively

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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.

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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.

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Smoke Reading (2 of 4)

• Determining the key attributes of smoke– Smoke volume– Smoke velocity– Smoke density– Smoke color

• Black fire

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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

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Smoke Reading (4 of 4)

• Determine the rate of change– 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

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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

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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.

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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.

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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.