Fires 6

Question 1

What is vapour pressure?

Answer 1

Partial atmospheric pressure exerted by the vapours of a liquid
* When vapour pressure is equal to the atmospheric pressure, this is the definition of boiling point
* The more vapour (i.e. the more volatile) the more there is to burn (energy for combustion)

Question 2

What are flammability limits?

Answer 2

• Because our combustion reactions need to be in the right stoichiometry, combustion is only possible within certain regions
• In a closed system, the explosive limit will be the same as the flammability limit
• In an open system, other factors (such as turbulence) can interfere
• Temperature alters the vapour pressure and so the likelihood of fire

Question 3

Lower flammability limit

Answer 3

• The lowest concentration of flammable gas in air that can sustain combustion
• Below this is fuel/lean/oxygen rich
• No ignition will occur below the LEL no matter how much energy is given
• Not enough fuel
• If volatilisation is promoted, this will bring it above the LEL which will allow for combustion

Question 4

Upper flammability limit

Answer 4

• The highest concentration of flammable gas in air that can sustain combustion
• Gas will burn but will ususally be incomplete combustion above the UEL
• Not enough oxygen above this to sustain a reaction (fuel rich)
• Slow but not ideal combustion
• As the fuel gets used up we will reach between the flammability range

Question 5

How to work out the stoichiometric concentration of a fuel?

Answer 5

1. Balance the combustion equation and work out the molecular ratio between the fuel and oxygen (O2) - x : y
2. Oxygen comprises 21% of air, x : y(100/21)

Question 6

How to work out the lower flammability limit?

Answer 6

Divide the stoichiometric concentration by 2
* LEL tends to be half of the calculated stoichiometric conc

Question 7

What will a mixture with a low ratio of vapour to air cause?

Answer 7

Lean explosion
* very powerful, but brief and may not result in a subsequent fire

Question 8

What does a mixture with a high ratio of vapour to air cause?

Answer 8

Flaming, rolling explosion often followed by a fierce fire

Question 9

What are the flammability limits?

Definition

Answer 9

The range of concentrations of fuel vapour capable of burning in air (21% oxygen)

Question 10

What will happen at other % of oxygen in air?

Answer 10

• If the concentration falls below 15% the rate of combustion will decrease
• At low enough oxygen concentrations, the flames will die out and be replaced by smoulder (or glowing) combustion at the surface of solids
• The oxygen conc depends on the flammability limits of the fuel and the temperature of the combustion gases

Question 11

What is the flash point?

Answer 11

• The minimum temperature at which the vapour produced by a liquid can be ignited momentarly in air
• The temp at which the vapour conc reaches the LFL

Question 12

What factors influence the flash point?

Answer 12

• The heat generated from enthalpy of combustion
• The heat capacity of the combustion products
• The rate of heat loss from the flame by radiation
• The kinetic rate production of more vapour

Question 13

What is the fire point?

Answer 13

• The minimum temperature at which sufficient vapour is produced by liquid to sustain combustion after ignition in air
• Usually a few degrees higher than the flash point

Question 14

What factors influence the fire point?

Answer 14

The volatility of the liquid
* vapour must be produced at a rate sufficient to maintain concentration above the lower flammability limit

Question 15

What is best for an arsonist in regards to flash and fire point and why?

Answer 15

Low fire and flash point
* Low flash point temps correlates with high vapour pressure (high volatility)

Question 16

How do you measure the flash point and fire point?

Answer 16

• Place test fuel into a container
• Liquid fuel is slowly and uniformly heated with its temp measured
• Periodically as the temp is increased, an ignition source is inserted into the container
• The lowest temp where a flash occurs is defined as the flash point temp

Question 17

Why is it difficult to measure the flash and fire point?

Answer 17

• The vapour must be at the measured temperature
• Also requires the presence of air
• Difficult to get defined experimental condition - even harder to represent real life scenarios

Question 18

Define auto-ignition or spontaneous ignition temperature

Answer 18

The temp at which the fuel will ignite without any additional source
* Reflects activation energy more than volatility

Question 19

What is the unit for heats of combustion?

Answer 19

kJ kg^-1

Question 20

What is aliphatic?

Answer 20

• Non-aromatic
• Cyclic
• Have conjugation

Question 21

What is aromatic?

Answer 21

• Delocalised conjugated rings
• 4n + 2 pi electrons where n is carbons

Question 22

Examples of hydrocarbon mixtures

Answer 22

• Petrol
• Paraffin
• Diesel
• Petroleum ether
• Benzine
• White spirit
• Turpentine
• Methylated spirits
• Paint thinners

Question 23

How liquids behave in fires

Answer 23

• Viscous liquids spread less far than those which flow more easily
• Porous surfaces absorb liquids and make smaller pools, but evaporation is faster due to wick effect
• Porous surfaces can provide samples for lab analysis
• Latent heat of evaporation protects the area under the pool from damage to some extent

Question 24

Examples of gas fuels

Answer 24

• Methane
• Liquid petroleum gas
• Aerosol products

Question 25

How do gas fuels behave in a fire?

Answer 25

• The density of the vapours relative to air will determine how it fills a room in the absence of significant air movement
• This determines which levels will be inclined to explode

Question 26

What is vapour density?

Answer 26

The ratio of the average molecular mass of the gas/vapour to the average molecular mass to air