Lecture 17

Question 1

what is vapour pressure

when vapour pressure = atmospheric pressure what does this mean

Answer 1

the partial atmospheric pressure exerted by vapours of a liquid (how much pressure comes from the evaporating gases)

it is the definition of boiling point

Question 2

the more the vapour the more ….

Answer 2

there is to burn

Question 3

if something has a higher vapour pressure what does this mean

Answer 3

volatises more and more quickly

Question 4

are these normally open or closed systems
a) explosions
b) combustions

Answer 4

a) closed
b) open

Question 5

in a closed system what is the explosive limit the same as

why is this not the case for an open system

Answer 5

the flammability limit

in an open system there are other things that can have an influence these limits

Question 6

what is one factor that can alter the vapour pressure

Answer 6

temperature

Question 7

what is meant by the right stoichiometry

Answer 7

the correct ratio of oxygen to fuel
2:1 - fuel:oxygen ratio

Question 8

in between what limits is the idealised combustion

Answer 8

between the lower and upper explosion limits

Question 9

what type of combustion do we get above the upper explosion limit

Answer 9

an incomplete combustion

Question 10

what do the upper and lower flammability limits vary with (2)

Answer 10

fuel type and the conditions of the combustion

Question 11

what is meant by the lower flammability or the lower explosion limit

what is below this called

Answer 11

the lowest concentration of flammable gas in air that can sustain combustion

below this limit is called fuel lean or oxygen rich (not enough fuel or too much oxygen)

if the conc of flammable gas is too low it can’t burn

Question 11

how is the stoichiometric concentration of a hydrocarbon fuel be predicted

why is this a problem for fuels such as petrol

Answer 11

• by measuring the upper and lower flammability in air
• the LEL (lower limit) tends to be about 1/2 of the stoichiometric concertation

the calculations are indicators and don’t always translate to real world situations particularly for complex mixtures such as petrol

Question 11

what is meant by the upper flammability or explosion limit

what is above this called

Answer 11

the highest concentration of flammable gas in air that can sustain combustion

above this is fuel-rich or oxygen lean (too much fuel or not enough oxygen)

if the conc of flammable gas is too high it can’t burn

Question 12

what is whites rule of thumb

Answer 12

the LEL (lower limit) of a gaseous hydrocarbons tends to be about 1/2 of the stoichiometric concertation

Question 13

when does an explosion result from a flammable vapour

Answer 13

when the vapour is pre-mixed with air within its flammability limits and then ignited

Question 14

when do the most powerful explosions occur

Answer 14

when the vapour is mixed with air in it’s stoichiometric ratio

Question 15

what is meant by a lean explosion

Answer 15

when there is a mixture with a low ratio of vapour to air

these tend to be powerful but brief and may not result in a subsequent fire

Question 16

are vapour phase explosions or condensed phase explosions more powerful

Answer 16

condensed phase are more powerful

Question 16

what does a mixture with a high ratio of vapour to air cause

Answer 16

a flaming, rolling explosion often followed by a fierce fire

Question 17

what do the flammability limits represent

Answer 17

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

Question 18

what percentage of oxygen concentration in air decreases the rate of combustion

Answer 18

regular % = 21%

if oxygen conc falls below about 15% the rate of combustion decreases

Question 19

when does the concentration of oxygen in the atmosphere normally change

Answer 19

with altitude or when an explosions occurs

Question 20

at low enough oxygen concentrations what happens to a flame

Answer 20

it will die out and be replaced by a smouldering/glowing combustion at the surface of the solid fuel

Question 21

what does the oxygen concentration depend on

Answer 21

the flammability limit of the fuel and the temperature of the combustion gases

Question 22

what is the flash point

Answer 22

the minimum temperature at which the vapour produced by a liquid can be ignited momentarily in air the temp at which the vapour concentration reaches the lower flammability limit then it can be ignited

Question 23

what does the vapour pressure of a liquid increase with

Answer 23

temperature

Question 24

is the ignition source external or internal to a hydrocarbon fuel how can this be applied

Answer 24

external this can be applied with a small flame, a glowing wire or electrical spark for example anything that supplies the initial activation energy and is then quickly removed

Question 24

give 4 factors that influence the sustainability of the fire

Answer 24

1. the heat capacity of combustion products 2. the heat generated from the enthalpy of combustion 3. the rate of heat loss from the flame by radiation 4. the kinetic rate of production of more vapour

Question 25

when the ignition source is removed what must the removed heat by the combustion balance what must also be produced at a sufficient rate

Answer 25

the heat loss from the flame so the temp doesn't drop vapour to maintain the vapour conc above the lower flammability limit - this depends on the liquid volatility

Question 26

what is the fire point

Answer 26

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

Question 27

what types of volatile liquids represent significant fire hazards

Answer 27

those with flash/fire points below ambient temperature

Question 27

what does a low flash point correlate to in terms of vapour pressure

Answer 27

high vapour pressure or high volatility

Question 28

when can a volatile liquid not be ignited in bulk how can this be managed

Answer 28

when the flash/fire points are significantly above ambient temperature the fuel can be pre-heated to a temperature above their fire point

Question 29

from an arsonists point of view what characteristics do the best accelerants have (4)

Answer 29

low flash/fire points ease of access cost ability to stock pile

Question 30

low flash point = ..... fire risk

Answer 30

high

Question 31

how can the presence of a wick help with creating a flame

Answer 31

this is an absorbent material that promotes the volatility of certain fuels from solid to gas phase to they are ignited more easily

Question 32

what is the ignition temperature what are two other names for ignition temperature

Answer 32

the temp at which a fuel will ignite without any additional source e.g flame or spark auto-ignition or spontaneous ignition temperature these reflect the activation energy more than the volatility

Question 33

what si the heat of combustion normally given in (untis) why is this

Answer 33

kJ per Kg because many fuels are complex mixtures so can not be defined on a molecular or molar basis so we work in mass

Question 34

how are distillate fuels derived

Answer 34

they have undergone distillation = a separation into slightly less complex mixture (evaporation then cooled and condensed when the bp is reached)

Question 35

give some examples of fuels that are complex hydrocarbon mixtures (9) where are these commonly used by arsonists

Answer 35

petrol paraffin diesel petroleum ether benzine white spirit turpentine or turps methylated spirits thinners or paint thinner these are used as accelerants by arsonists as they can be bought in large quantities without suspicion, it is not illegal to posses them

Question 36

for an arsonist why is a less volatile accelerant preferred

Answer 36

a delayed timing is good for arsonists so they can escape before ignition preventing injury or being caught

Question 36

what is the term given as to why the area under a pool of liquid is protected from damage of a fire

Answer 36

latent heat of evaporation

Question 37

how can lower viscosity liquids be a problem for arsonists

Answer 37

they spread more easily and can splash when used they tend to absorb into porous substrates more so can be collected for forensics - therefore best area to sample is where source of fire was

Question 38

what can gas lines be a source of why are closed lines more safe

Answer 38

readily ignitable fuel because there is no oxygen available but breaches in closed lines can cause fires e.g eroded pipes, poor joints, mechanical or thermal failure

Question 39

give three examples of available gas fuels

Answer 39

natural gas = methane LPG = liquid petroleum gas = mix of propane and butane aerosol products = are pressurised with dimethyl ehter or mixtures of butane, propane or isobutane

Question 40

what determines how a gas/vapour fills a room what does this influence in terms of a fire

Answer 40

the density of it compared to air in the room influencing which levels are likely to explode

Question 41

what is meant by vapour density

Answer 41

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