Fires 6 - Flammability Limits

Question 1

What state reaction are flames?

Answer 1

gaseous reactions

Question 2

What state can be flammable and produce flaming combustions and what can’t be?

how does the ones that can’t be work?

Answer 2

gases can be flammable

gases can produce flaming combustions - solids and liquids cannot

liquids cannot be flammable but can release vapours which then burn with flames
- once flame begins, heat of flame releases more vapours
- called a pool fire (need to know and quantify how material makes gas and then the flames)

Question 3

define vapour pressure by IUPAC definition

define vapour pressure by normal definition

what is the boiling point?

what effect does amount of vapour have on amount to burn?

what other way can vapour pressure be explained in simple terms?

Answer 3

the pressure exerted by a pure substance (at a given temperature) in a system containing only the vapour and condensed phase (liquid or solid) of the substance

the partial atmospheric pressure exerted by the vapours of a liquid (how much pressure comes from evaporating gases)
- when something changes from liquid to gas there is a pressure change - the gas will exert pressure on the surroundings

boiling point is when vapour pressure = atmospheric pressure

more vapour (more volatile) = more to burn (more energy present for combustion)

vapour pressure can be explained in simple terms as the extent to which the liquid becomes gas

Question 4

Why is combustion only possible within certain ranges?

Answer 4

because our combustion reactions need to be in the right stoichiometry (correct ratio of fuel to oxygen) - combustion is only possible within certain ranges
- too low - not enough fuel
- too high - not enough oxygen and too much fuel

Question 5

What can be said about the flammability limits in open and closed systems?

Answer 5

closed system: flammability limit = explosive limit

open system: other factors (such as turbulence) play a role and affect the UFL and LFL

Question 6

What alters the vapour pressure and therefore likelihood of a fire?

Answer 6

temperature alters vapour pressure and therefore likelihood of fire

higher temperature = volatilise easier

Question 7

what is lower and upper flammability limits? explain these.

Answer 7

below LEL/LFL: fuel lean where the concentration of flammable gas is too low so it cannot burn
- the enthalpy of combustion does not generate enough heat to sustain the flame

at LEL/LFL: gas will ignite and burn
- it is the lowest concentration of flammable gas in air that can sustain combustion

within LEL and UEL: there is an optimal idealised ratio which causes the most powerful explosion/idealised combustion

UEL/UFL: the highest concentration of flammable gas in air that can sustain combustion

above UEL/UFL: fuel rich where concentration of flammable gas is too high so it cannot burn
- there is not enough oxygen to sustain the reaction
- e.g. a flooded petrol engine

These limits vary for different fuels - some have massive/small flammability ranges

Question 8

What can be inferred from the plot of concentration of fuel vapour and of oxygen as a function of the distance from the fuel surface?

Answer 8

narrow distance where we get idealised ratio and will see idealised combustion - this will determine the shape of the flame

Question 9

Where are the LEL and UEL on a candle?

Answer 9

top of flame (lean) = lower explosive limit

bottom of flame (rich) = upper explosive limit

Question 10

What is the method to calculate flammability limits?

For what type of fuels is this not accurate?

Answer 10

CH4 + 2O2 –> CO2 + 2H2O
- stoichiometric molecular ratio CH4: O2 is 1:2
- ideal gas laws : pV = nRT so the volume relates to the number of molecules, not their size
- therefore the stoichiometric volume ratio CH4:O2 is also 1:2
- O2 comprises 21% of air
- so stoichiometric volume ratio methane:air is therefore
- 1:2(100/21) = 1:200/21 = 1:9.52
- so 1 part in 10.52 of the mixture is methane, which is 9.5% methane, 90.5% air
- whites rule of thumb is that the lower flammability limit for a gaseous hydrocarbon is about half the stoichiometric concentration

it is only empirical not exact
- petrol, kerosene and other distillate fuels are complex mixtures (made up of multiple components that have own flammability limits) so not pure chemical substances so for these materials it is more complicated

Question 11

What can be said about using calculations to predict LFL and UFL?

Answer 11

calculations are indicators but do not always translate to real world situations

Question 12

What type of fuels do measured LFL and UFL apply to?

Answer 12

fuels in air (21% oxygen) at 1 atmosphere pressure
- changing the pressure will change the combustion properties

Question 13

What happens if a flammable vapour is pre-mixed with air within its limits of flammability and then ignited?

Answer 13

an explosion will result

Question 14

What happens when vapour is mixed with air in stoichiometric ratio and low/high ratio of vapour to air?

Answer 14

vapour is mixed with air in its stoichiometric ratio: most powerful explosion

mixture with low ratio of vapour to air: cause a lean explosion
- powerful but brief and may not result in subsequent fire

mixture with high ratio of vapour to air: cause a flaming rolling explosion often followed by a fierce fire

Question 15

what can be said about the comparison between vapour phase vs condensed phase explosions?

Answer 15

vapour phase explosions not as powerful as condensed phase explosions

Question 16

What can be said about flammability limits for other concentrations of oxygen (other than 21%)?

Answer 16

if the oxygen conc falls below about 15% (e.g. when a fire is in a closed room) the rate of combustion will decrease

at low enough oxygen concentrations the flames will die out and be replaced by smouldering or glowing combustion on surface of solids

this oxygen conc this occurs at depends on the flammability limits of the fuel and the temp of combustion gases

Question 17

define flash point

Answer 17

the minimum temp at which the vapour (produced by the liquid) concentration reaches the LFL then it can be ignited momentarily in air

Question 18

What does the vapour pressure of a liquid increase with?

Answer 18

temperature

Question 19

What is the ignition source?

What happens when the ignition source is removed? What influences this?

Answer 19

it is external - small flame, glowing wire, electrical spark etc. which supplies the initial activation energy

when removed, the resultant flame does this self-sustain at this temperature
- 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 of production of more vapour

Question 20

Define fire point

Describe what must happen when the ignition source is removed

Answer 20

the minimum temperature at which sufficient vapour is produced by a liquid to sustain combustion after ignition in air

after ignition source removed
- heat produced by combustion must balance heat loss from the flame so the temperature does not drop
- vapour must also be produced at a rate sufficient to maintain concentration above the LFL otherwise end up with fuel lean

Question 21

What is the value of the fire point usually compared to the flash point?

Answer 21

fire point is usually a few degrees higher than the flash point but not always

Question 22

What is the correlation between flash point and vapour pressure?

Answer 22

low flash point correlates with high vapour pressure (i.e. high volatility)

Question 23

What happens when some volatile liquids have flash points below or significantly above ambient temperature?

Answer 23

below e.g. petrol:
- they can be ignited at even cold temperatures
- they represent significant fire hazards if ignited

above e.g. diesel, ethylene glycol (antifreeze):
- cannot be ignited in bulk unless pre-heated to a temp above their fire point

Question 24

What is the best flash/fire points according to arsonists?

Answer 24

best accelerants have low flash/fire points

but low flash/fire points = low volatility

arsonists look for easy of accessibility, cost of fuel, and ease to stockpile fuel

e.g. more likely to buy petrol than diesel as easier to ignite

Question 25

how to measure flash point and fire point?

Answer 25

• test fuel is placed in container and liquid fuel is slowly and uniformly heated with its temperature monitored
• periodically as the temperature is increased, an ignition source is inserted into the container
• the lowest temp where a flash occurs is defined as flash point temp (this is technically difficult to do accurately, the vapour must be at the measured temperature)
• it isn’t representative of a real-world scenario

Question 26

What can an absorbent material act as?

Answer 26

a wick
- it immobilises the liquid phase in the vicinity of the flame so it can be heated locally to a high temperature

Question 27

Why are real world systems more complex?

Answer 27

sometimes higher or lower observed flash point

• mixtures
• waste fuels
• used lubricating oils
• mixtures of petroleum liquids with solids
• organic-solvent based paints, which form surface films

Question 28

Define ignition temperature

What does it reflect?

Answer 28

also called auto-ignition or spontaneous ignition temperature

it is the temperature at which the fuel will ignite without any additional source (flame, spark)

it reflects the activation energy more than volatility

Question 29

What are heats of combustions often quoted in?

Answer 29

kJ kg -1

Question 30

What do many fuels consist of?

Answer 30

a mixture of different chemicals (e.g. petrol)

investigators can identify chemicals

many fuels and building materials have complex compositions that cannot be defined on a molecular or molar basis so it is natural to work in mass terms
- can get associated energy outputted in terms of mass

Question 31

define distillate fuel

Answer 31

a fuel derived from distillation (evaporation of fuel, when hit right evaporation temperature, it condenses and can collect it as fractions, chamber has temperature difference - cool at top and hot at bottom)

in reality we distil across a wide range

Question 32

define aliphatic

Answer 32

means non aromatic (if something isn’t aromatic, it is aliphatic)

aliphatic can mean cyclic
aliphatic species can have conjugation

Question 33

define aromatic

Answer 33

means it has delocalised conjugated rings

anything that has 4n + 2 pi electrons where n = 0, 1, 2, 3, etc (n = number of carbons)

Question 34

list some hydrocarbon mixtures

Answer 34

petrol
paraffin (kerosene)
diesel
petroleum ether
benzine
white spirit
turpentine or turps
methylated spirits
thinners or paint thinners

Question 35

define petrol

Answer 35

a distillate fuel, fractionated over a temperature range

• mainly aliphatic hydrocarbons (some branched/cyclic) with some aromatics and alkylated aromatics
• straight chain alkanes absent or present in very low amounts in these fuels as their autoignition temperatures are too low

Question 36

what does 95 octane mean?

Answer 36

it is petrol octane rating - it compares technical combustion properties with those of iso-octane

• combustion properties of 95 octane are 95 % similar to those of iso-octane

Question 37

define paraffin (kerosene)

Answer 37

• mainly aliphatic HCs (straight chain and branched) with some alkenes (olefins)
• C10 - C16 usually centred around C12
• used as heating oils, jet fuels

Question 38

define diesel

Answer 38

• aliphatic HCs (straight chain and branched)
• C12 - C25 and above much wider range - centred around C18
• used in diesel engines and as heating oils

Question 39

define petroleum ether

Answer 39

• mixture of aliphatic hydrocarbons
• not ethers - name is misleading
• mainly C5 and C6
• chemical and spectroscopic solvent

Question 40

define benzine

Answer 40

• an aliphatic fraction - not benzene
• aka ligroin
• used as a solvent

Question 41

define white spirit

Answer 41

• a mixture of saturated aliphatic and alicyclic C7 - C12 HC’s with maximum content of 25 % of C7 to C12 alkylated aromatics
• used in paints and varnishes, in cleaning products and as a degreasing solvent

Question 42

define turpentine or turps

Answer 42

• natural olefinic (alkene) oil obtained by distillation of pine resin
• used as a solvent and in thinning paints

Question 43

define methylated spirits

Answer 43

• ethanol containing enough methanol to stop (most) people drinking it
• used as cleaning solvent and to thin paints

Question 44

define thinners/paint thinners

Answer 44

• any organic solvent used to dissolve paint
• should be volatile enough to evaporate away as the paint dries
• modern paints are increasingly aqueous based for environmental reasons

Question 45

what is the use of hydrocarbon mixtures by arsonists?

Answer 45

• used as accelerants by arsonists
• they can be bought in large quantities (some in ordinary retail stores) without arousing suspicion - it is not illegal to possess them
• less volatile accelerants are best from the arsonists viewpoint, especially if a delayed timing ignition device is to be used to start the fire

Question 46

What do different fuels have that are different?

Answer 46

different fuels will have different profiles they will leave on ground/whatever they are poured on

Question 47

How do viscous liquids spread compared to non-viscous liquids?

Answer 47

viscous liquids (e.g. diesel) spread less far (deeper pool) than those which flow more easily (e.g. methanol)

Question 48

how do porous surfaces absorb liquids?

Answer 48

porous surfaces (raw wood, concrete, carpet) absorb liquids and make smaller pools but evaporation is faster due to wick effect

porous surfaces can provide samples for lab analysis
- some of fuel is protected on surface
- latent heat of evaporation protects area under pool from damage to some extent

Question 49

What can gas lines be?

what about closed lines? what is the hazard with these?

what else can occur with these and how?

Answer 49

gas lines can be a source of readily ignitable fuel

however closed lines are safe because there is no oxygen (too rich)
- if there are any breaches then oxygen can enter

escape can occur due to poor joints, corroded pipes, mechanical or thermal failure

Question 50

what are some sources of readily available gas fuels?

Answer 50

• natural gas (methane)
• liquid petroleum gas (LPG)

aerosol products (lubricants, paints, insecticides, air fresheners, hair spray etc.) are pressurised with dimethyl ether or mixtures of butane, propane, isobutane

Question 51

what state of fuel is viscosity more of a consideration for?

Answer 51

liquid fuels

Question 52

what does the vapour/gas density relative to air determine?

give examples

Answer 52

vapour/gas density relative to air will determine how it fills a room in the absence of significant air movement
- petrol sits low in room
- natural gas rises
- LPG goes below ground

Question 53

what does vapour density determine?

what does <1 and >1 mean?

Answer 53

determine which levels will be inclined to explode

it is the ratio of the (average) molecular mass of the gas/vapour to the average molecular mass of air (29)

lower than 1 - lighter than air
higher than 1 - heavier than air

Question 54

What does the flammability of a liwuid depend on?

Answer 54

the flammability of a liquid depends on its volatility (measured by its vapour pressure and flash point) and its activation energy (measured by ignition temperature)