Transcript Chapter 5

5
Fire Behavior
5
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 roomand-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
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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
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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.