Fire Behaviour

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INTRODUCTION TO FIRE BEHAVIOUR

• Module 10 Bradley Quinn

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Introduction
As a firefighter, you must have a good
understanding of what makes fire, what a fire
needs to get started, what it feeds on, what
makes it most dangerous, and how to put it out.
By knowing these key principles, you can act in
safer ways and help your colleagues to be safer.
You will also then have the key to fire ground
decision-making and risk management.
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What is fire?

• Three things interact to create fire. When you
  know what these three things are and how they
  work together, you are better able to choose the
  right plan for putting the fire out.
•  
• Three things are necessary for fire
•  1. heat (enough heat energy to make the fuel
  burn)
•  2. fuel (this can be a solid, such as wood, a
  liquid, such as petrol, or a gas, such as LPG)
•  3. a supporter of combustion (usually oxygen).
•  

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What is fire?

• Fire is an exothermic reaction which produces
  light, flames and other products such as
• smoke and fire gases.
•  HEAT
• TYPE OF HEAT ENERGY WHAT IT DEVELOPS
  FROM
• CHEMICAL
  Burning or heating, including the
• Heat given off from fire
  heat produced when material
• 
  breaks down or
  dissolves. 
•  ELECTRICAL
  Resistance, arcing, static
• Heat from a fault
  electricity, leaking electrical current,
• 
  dielectric
  heating (microwave) 
• MECHANICAL
  Friction or pressure
• Heat from moving parts in
• a car engine.

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Fuel 

• Fuel is anything that will burn, such as wood,
  fabric, or cooking oil.
• However, it is only the vapour from the fuel that
  actually burns.
• Heat energy can change solid or liquid fuel into
  flammable vapour.
•  A fuel must be changed from a solid or liquid
  into a gas
• (or vapour) before combustion can occur.
• The changing of solid fuels into flammable
  vapours is called pyrolysis.
•  
• Supporter of combustion
•  Oxygen is the most common supporter of
  combustion.

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THE FIRE TRIANGLE

• Three things heat, fuel, and a combustion
  supporter form the fire triangle.
• Hypoxia is the medical condition of
  your body not receiving enough oxygen.
• This can be the result of poisonous
  gases from the fire preventing the body from
• carrying or using oxygen in the air.
• The hot, poisonous gases produced by a
  fire are what make smoke so dangerous.
• In this example, oxygen, chemical heat, and wood
  are interacting to create the conditions for a
  fire.
•  
• Both people and fires need oxygen from the air to
  survive. If all the
• oxygen in a compartment was used up, both the
  people and the fire in the compartment would
  eventually die.

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The Fire Triangle
Cooling
Smothering
Oxygen
Oxidiser
(Water)
(Foam)
Supporter of Combustion
Heat
Fuel
Starving
(Fire Break)
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COMBUSTION

• What is combustion?
•   Combustion is what we call the process of a
  fuel catching fire. It is a chemical chain
  reaction in which a fuel is oxidized to produce
  heat and light. This heat helps to keep the
  process going.
•  
•   What happens during combustion?
•   When you add heat to an object, for example
  wood, it will continue to absorb heat until it
  starts to decompose and produce flammable vapors.
  This process
• is called pyrolysis.
•  
• The flammable vapours then mix with the
  oxygen
• in the air.

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COMBUSTION (CONT)

• When there is enough fuel vapor mixing with
  the air and there is an ignition source present,
  the vapor will ignite briefly. This is the
  flashpoint.
•   As the heat builds and an ignition source
  continues to be applied, the log will continue to
  burn.
•   When the log continues to burn without the
  application of the ignition source, the
  combustion chemical chain reaction has started
  and the fire is self sustaining. This is called
  the fire point.
•  
• The flame provides a heat and ignition
  source, which creates more fuel vapors from
  available fuel, for example, the remaining parts
  of the log or another log on top of it.
•  
• This chemical chain reaction will keep going
  until one of the elements of the fire triangle
  runs out or is taken away.

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COMBUSTION OF DIFFERENT TYPES OF FUELS

• Solids
•   Examples of common solid fuels are coal and
  wood. Solid fuels usually follow the combustion
  process above.
•   Liquids
•   Some liquids are more volatile than others.
•   More volatile liquids, such as petrol, give
  off flammable vapors at room temperature.
•   Less volatile liquids, such as oil, need
  heat energy to give off flammable vapors.
•   Miscibility is the ability of a liquid to mix
  with water. This is important to think
• about when dealing with liquid fuels.
•   Many flammable liquids do not mix with water.
  This means you cannot
• use water to put these out if they are on
  fire. If water is applied to the fire it will
  sink below the flammable liquid. Foam is better
  for extinguishing these types of fires.
•  
•  

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COMBUSTION OF DIFFERENT TYPES OF FUELS

• Gases
•    There are many flammable gases, such as LPG.
  Flammable gases are usually stored in pressurized
  vessels. Gases are already in a vapor state,
  which makes them very dangerous if there is an
  ignition source present.

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Methods of Heat Transfer
Fire Science
No. 14
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Conduction
Fire Science
No. 15
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Convection
Fire Science
No. 16
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Radiation
Fire Science
No. 17
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Direct Flame Contact (Direct Burning)
Fire Science
No. 18
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Classes of Fire
Fire Science
No. 10
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How a compartment fire grows and develops

• PHASES OF A FIRE
•   The fuel type and quantity of fuel in an
  area, room, or space can be important in
  determining what type of fire results and how
  intense it is. The size, shape, and location of
  fuels can affect how quickly fire grows.
•  
• A compartment is any enclosed space with a
  floor, walls and ceiling. Compartments can also
  be a room within a structure that has already
  been vented. For example, a bedroom within a
  house.
• Fires goes through four phases
•   Phase 1 Incipient
• In the incipient phase, fuel, oxygen and heat
  come together and ignition occurs.
•  

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• Phase 2 Growth, sometimes including flashover
  stage
• The fire begins
• Air movement increases
•  

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Overpressure zone

• 3. Two zones form
• 4. The fire gets hotter

Over pressure zone
Under pressure zone
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• 5. Flashover period starts
• 6. Flashover develops

Pyrolisis occurs
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• 7. Flashover occurs

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Phase 3 Fully developed (steady state)
Phase 4 Decay
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SIGNS OF FLASHOVER

• If you can get to a fire in the growth phase,
  there are ways to control the fire in the
  compartment so that you can conduct search and
  rescue or find the location of the fire and
  extinguish it. When you are fighting a structural
  fire, you need to be able to recognize the
  warning signs of flashover or back-draught.
• Reducing the risk of flashover
•  If you cool the overpressure zone where the
  hottest gases are, you can reduce the risk of
  flashover.
•  Action
• Set your nozzle to a spray pattern. Aim the spray
  into overpressure zone (overhead).
• You want to create the smallest drops of water
  you can that will get through the fire gas layer.
• This cools the flammable gases below their
  ignition temperature. Use only a small amount at
  a time.
•  Short applications let you see the effect of
  each spray so you dont use too much.
• Water will turn to steam that expands in the
  overpressure zone. Too much steam can force hot
  steam, smoke, and flame downwards on to
  firefighters.
•  
• 6. Be sure to set your nozzle to a spray
  pattern. The thousands of smaller water droplets
  in the medium spray pattern are better for
  cooling the overpressure zone than a single hose
  stream.

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BACK DRAUGHT AND FIRE GAS EXPLOSIONS
How a backdraught occurs
A closed compartment will become under-ventilated
as the fuels burn. The fire will die down to
smoldering and some of the combustion gases will
not be burnt. The only thing that has been
removed is the air.
When an opening is made, air moves into the
compartment.
If the compartment is hot enough when the air
mixes with the heat and un-burnt gases in the
compartment, they ignite explosively. If the
un-burnt gases are not above ignition point, they
can be ignited by an ignition source. These
explosive ignitions are called back draught. The
explosion expands rapidly within the
compartment. It usually travels back to the
source of the air (the opening).
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• THE WARNING SIGNS OF BACKDRAUGHT
•  
• Smoke is puffing out around windows, doors, and
  eaves (the fire is searching for more oxygen).
•  
• The spaces around the doors and windows are
  making a rattling or whistling noise.
•  
• The doors and windows look like they are moving
  in and out slightly.
•  
• There is little or no visible fire within the
  compartment.
•  
• Sounds from within the compartment seem muffled
  (because heavy smoke dampens noise).
•  
• Dense grey-yellow smoke becomes darker as it
  exits the building.
•  
• There is a sudden inrush of air when an opening
  is made.
•  
• Blackened windows and blistering on the door
  (this indicates extreme heat)

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PREVENTING AND REDUCING THE RISK OF BACK DRAUGHT

•   
• ACTIONS TO TAKE BEFORE ENTERING OR MAKING AN
  OPENING IN A COMPARTMENT
• Check for warning signs (see above).
•  
• Before opening a door, cover it with a charged
  delivery capable of a minimum flow of 440 litres
  a
• minute.
• 2. If there is a build-up of smoke outside
  the compartment, spray these gases before opening
  the door.
• 3. Be ready to close the door quickly if
  back draught conditions exist.
• 4. Maintain a safe and reasonable distance
  if you are not directly involved with fighting
  the fire.
• You need a charged delivery stream
  in position for cover before attempting
  ventilation.
•   Options to reduce the risk of
  back draught
• 5. If you can, keep the doors closed and
  stay out of the room.
• 6. Use correct door entry procedure.
• Cool the environment. This may be a good option
  in situations where you can make a small opening
  for a hose
• spray. (Flammable gases will still
  remain in the compartment.)
• 8. Remove the flammable gases (ventilation).
• 9. This is the best option after youve
  cooled the environment because it removes a lot
  of heat and reduces smoke damage.

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FIRE GAS EXPLOSIONS

• A fire gas explosion is where flammable fire
  gases burst into flame, either by finding an
  ignition source or reaching auto-ignition
  temperature.

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• Thank You
• Any Question