Combustion & Fire Behaviour Flashcards
What is Combustion
Combustion is a irreversible chemical reaction between oxygen and fuel, which produces heat and light
What are the components of the triangle of combustion
Oxygen
Heat
Fuel
Also Passive agents
For flaming combustion to take place what is required
a solid or liquid must be converted into a gas (fuel).
Oxygen, heat and fuel must be present in the correct proportions.
What can also be Passives in a fire compartment
compartment boundaries, absorb heat initially, and then radiate heat into compartment accelerating fire development.
What is the most influential passive
The compartments’ structure and contents
Water is a very effective passive agent?
Water is very effective passive agent but will have a latent heat of vaporisation i.e. the change of state from a liquid to a gas
What is Pyrolysis?
Pyrolysis is the chemical decomposition of a substance by heat
As Pyrolysis occurs what happens
Different gases are given off as the chemical composition of a substance is altered
what is chemical formula for wood
C6 H10 O5
At what temperature does wood break down
180 -215 degrees
How does Wood break down
Atoms become unstable they fire off and join other different atoms forming molecules
1 Carbon atom ( C ) + 1 Oxygen atom ( O ) =
CO = Carbon Monoxide, very easily produced, the most prevalent fire gas
What is the Vapour density of
Carbon Dioxide
1.5 times that of air
Carbon Monoxide – Vapour Density
0.97
Soot gives off
Carbon Particles cooling converting back to solid
All gases will obey laws of chemistry and will have an auto ignition temp CO of What
609 degrees
Pyrolysis ?
Fuel breaks down produces flammable gas
What are the three mix levels
Flammable range between 0 – 100%
Lower Explosive Limit
Ideal Mixture
Upper Explosive Limit
Explain: Lower Explosive Limit
The lowest concentration of a fuel in air that will just support a self propagating flame (Oxygen rich, blue flame)
Explain: Ideal Mixture (Stoiciometric mix)
The point along the flammable range of a gas, that will produce the most energy / force
Explain: Upper Explosive Limit
The highest concentration of a fuel in air that will just support a self propagating flame (fuel rich, yellow flame)
Limits of flammability
for Propane
Propane: 2% to 10% and 4% is the ideal mix
Limits of flammability
for Acetylene
Acetylene: 2% to 88% and 7% is the ideal mix
Limits of flammability
for Carbon Monoxide
Carbon Monoxide: 12% to 76% and 29% is the ideal mix
What is a Flashover?
A Flashover is the sudden and sustained transition of a growing fire to a fully developed fire
Or fire in a room = a room on fire
Common signs of an impending flashover
Intense heat, forcing crews low
750-1200 degrees
Flames in gas layer (‘dancing angels’)
Lowering of neutral plane (due to over pressure)
All exposed surfaces pyrolysing
Flashover Actions:
Use door entry procedure to gain entry to room
Pulse spray in to ceiling – cool gases/reduce pyrolysis
Stay low
Consider withdrawing and fire-fighting outside the compartment
Consider ventilation to remove gases (only on Incident Commander’s orders).
What is a Backdraught?
A Backdraught is a compartment fire that due to limited ventilation has allowed a build up of fire gases which, with the addition of air, causes a deflagration to occur.
Backdraught occurs as:
Gases reach their flammable range. The closer the dilution of a gas to its IM will dictate the velocity of the backdraught. Deflagration will be a sub sonic ignition up to 340 metres per second
Common external signs of a possible backdraught:
Thick ‘cauliflower - like’ fire gases forced out under pressure
Air being drawn in rapidly through any opening
Pulsing or ‘breathing’ gases from any opening
Blackened windows
No visible flame
Door extremely hot (Painted, PVC or Metal)
Common internal signs of a possible backdraught:
Neutral plane almost at floor level
No visible flame
Backdraught
Actions:
Stay low
Cover door with charged hose
Spray gases exiting openings and around openings
Only open door when teams are in a safe position and retain hold of door – be ready to force it closed
Pulse spray interior gases to cool and dilute
Consider making high level external opening, cover this opening with a jet. (on IC’s instructions only).
Remember, backdraughts occur:
Can only occur when fresh air is permitted to enter the compartment
Tend not to occur spontaneously – can take minutes.
What is a Fire Gas Explosion?
A Fire Gas Explosion is ignition of fire gases that have accumulated outside original fire compartment.
Common signs of an impending fire gas explosion:
Heavily smoke logged compartment.
No signs of fire
Fire Gas Explosion
Actions:
Pulse spray into the ceiling – cool gasses.
Consider Ventilation to remove gases (only on the incident commander’s orders.)
