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fires and explosions > Lecture 17 > Flashcards

Lecture 17 Flashcards

1
Q

what is vapour pressure

when vapour pressure = atmospheric pressure what does this mean

A

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

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2
Q

the more the vapour the more ….

A

there is to burn

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3
Q

if something has a higher vapour pressure what does this mean

A

volatises more and more quickly

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4
Q

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

A

a) closed
b) open

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5
Q

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

why is this not the case for an open system

A

the flammability limit

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

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6
Q

what is one factor that can alter the vapour pressure

A

temperature

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7
Q

what is meant by the right stoichiometry

A

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

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8
Q

in between what limits is the idealised combustion

A

between the lower and upper explosion limits

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9
Q

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

A

an incomplete combustion

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10
Q

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

A

fuel type and the conditions of the combustion

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11
Q

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

what is below this called

A

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

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11
Q

how is the stoichiometric concentration of a hydrocarbon fuel be predicted

why is this a problem for fuels such as petrol

A
  • 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

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11
Q

what is meant by the upper flammability or explosion limit

what is above this called

A

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

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12
Q

what is whites rule of thumb

A

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

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13
Q

when does an explosion result from a flammable vapour

A

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

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14
Q

when do the most powerful explosions occur

A

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

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15
Q

what is meant by a lean explosion

A

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

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16
Q

are vapour phase explosions or condensed phase explosions more powerful

A

condensed phase are more powerful

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16
Q

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

A

a flaming, rolling explosion often followed by a fierce fire

17
Q

what do the flammability limits represent

A

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

18
Q

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

A

regular % = 21%

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

19
Q

when does the concentration of oxygen in the atmosphere normally change

A

with altitude or when an explosions occurs

20
Q

at low enough oxygen concentrations what happens to a flame

A

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

21
Q

what does the oxygen concentration depend on

A

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

22
Q

what is the flash point

A

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

23
Q

what does the vapour pressure of a liquid increase with

A

temperature

24
Q

is the ignition source external or internal to a hydrocarbon fuel

how can this be applied

A

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

24
Q

give 4 factors that influence the sustainability of the fire

A
  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
25
Q

when the ignition source is removed what must the removed heat by the combustion balance

what must also be produced at a sufficient rate

A

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

26
Q

what is the fire point

A

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

27
Q

what types of volatile liquids represent significant fire hazards

A

those with flash/fire points below ambient temperature

27
Q

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

A

high vapour pressure or high volatility

28
Q

when can a volatile liquid not be ignited in bulk

how can this be managed

A

when the flash/fire points are significantly above ambient temperature

the fuel can be pre-heated to a temperature above their fire point

29
Q

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

A

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

30
Q

low flash point = ….. fire risk

A

high

31
Q

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

A

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

32
Q

what is the ignition temperature

what are two other names for ignition temperature

A

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

33
Q

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

why is this

A

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

34
Q

how are distillate fuels derived

A

they have undergone distillation = a separation into slightly less complex mixture

(evaporation then cooled and condensed when the bp is reached)

35
Q

give some examples of fuels that are complex hydrocarbon mixtures (9)

where are these commonly used by arsonists

A

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

36
Q

for an arsonist why is a less volatile accelerant preferred

A

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

36
Q

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

A

latent heat of evaporation

37
Q

how can lower viscosity liquids be a problem for arsonists

A

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

38
Q

what can gas lines be a source of

why are closed lines more safe

A

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

39
Q

give three examples of available gas fuels

A

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

40
Q

what determines how a gas/vapour fills a room

what does this influence in terms of a fire

A

the density of it compared to air in the room

influencing which levels are likely to explode

41
Q

what is meant by vapour density

A

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

fires and explosions (20 decks)

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