Combustion & Fire Behaviour

Question 1

What is Combustion

Answer 1

Combustion is a irreversible chemical reaction between oxygen and fuel, which produces heat and light

Question 2

What are the components of the triangle of combustion

Answer 2

Oxygen
Heat
Fuel
Also Passive agents

Question 3

For flaming combustion to take place what is required

Answer 3

a solid or liquid must be converted into a gas (fuel).

Oxygen, heat and fuel must be present in the correct proportions.

Question 4

What can also be Passives in a fire compartment

Answer 4

compartment boundaries, absorb heat initially, and then radiate heat into compartment accelerating fire development.

Question 5

What is the most influential passive

Answer 5

The compartments’ structure and contents

Question 6

Water is a very effective passive agent?

Answer 6

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

Question 7

What is Pyrolysis?

Answer 7

Pyrolysis is the chemical decomposition of a substance by heat

Question 8

As Pyrolysis occurs what happens

Answer 8

Different gases are given off as the chemical composition of a substance is altered

Question 9

what is chemical formula for wood

Answer 9

C6 H10 O5

Question 10

At what temperature does wood break down

Answer 10

180 -215 degrees

Question 11

How does Wood break down

Answer 11

Atoms become unstable they fire off and join other different atoms forming molecules

Question 12

1 Carbon atom ( C ) + 1 Oxygen atom ( O ) =

Answer 12

CO = Carbon Monoxide, very easily produced, the most prevalent fire gas

Question 13

What is the Vapour density of

Carbon Dioxide

Answer 13

1.5 times that of air

Question 14

Carbon Monoxide – Vapour Density

Answer 14

0.97

Question 15

Soot gives off

Answer 15

Carbon Particles cooling converting back to solid

Question 16

All gases will obey laws of chemistry and will have an auto ignition temp CO of What

Answer 16

609 degrees

Question 17

Pyrolysis ?

Answer 17

Fuel breaks down produces flammable gas

Question 18

What are the three mix levels

Flammable range between 0 – 100%

Answer 18

Lower Explosive Limit
Ideal Mixture
Upper Explosive Limit

Question 19

Explain: Lower Explosive Limit

Answer 19

The lowest concentration of a fuel in air that will just support a self propagating flame (Oxygen rich, blue flame)

Question 20

Explain: Ideal Mixture (Stoiciometric mix)

Answer 20

The point along the flammable range of a gas, that will produce the most energy / force

Question 21

Explain: Upper Explosive Limit

Answer 21

The highest concentration of a fuel in air that will just support a self propagating flame (fuel rich, yellow flame)

Question 22

Limits of flammability

for Propane

Answer 22

Propane: 2% to 10% and 4% is the ideal mix

Question 23

Limits of flammability

for Acetylene

Answer 23

Acetylene: 2% to 88% and 7% is the ideal mix

Question 24

Limits of flammability

for Carbon Monoxide

Answer 24

Carbon Monoxide: 12% to 76% and 29% is the ideal mix

Question 25

What is a Flashover?

Answer 25

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

Question 26

Common signs of an impending flashover

Answer 26

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

Question 27

Flashover Actions:

Answer 27

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).

Question 28

What is a Backdraught?

Answer 28

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.

Question 29

Backdraught occurs as:

Answer 29

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

Question 30

Common external signs of a possible backdraught:

Answer 30

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)

Question 31

Common internal signs of a possible backdraught:

Answer 31

Neutral plane almost at floor level

No visible flame

Question 32

Backdraught

Actions:

Answer 32

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).

Question 33

Remember, backdraughts occur:

Answer 33

Can only occur when fresh air is permitted to enter the compartment
Tend not to occur spontaneously – can take minutes.

Question 34

What is a Fire Gas Explosion?

Answer 34

A Fire Gas Explosion is ignition of fire gases that have accumulated outside original fire compartment.

Question 35

Common signs of an impending fire gas explosion:

Answer 35

Heavily smoke logged compartment.

No signs of fire

Question 36

Fire Gas Explosion

Actions:

Answer 36

Pulse spray into the ceiling – cool gasses.

Consider Ventilation to remove gases (only on the incident commander’s orders.)