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Fire Safety Management > Explosions > Flashcards

Explosions Flashcards

0
Q

When and where was the first recorded incidence of dust explosion

A

1785 italian flour mill

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1
Q

When can dust cause an explosion

A

if its combustible
if its capable of being airborne
if it has a size distribution capable of flame propagation
if its within exposable range
if an ignition source is present
if the atmosphere contains sufficient oxygen to support and sustain combustion

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2
Q

Where are dust explosions a risk

A

in the grain industry. 50 explosions per year

they can also occur whenever a processes uses particlate matters either as feedstock, intermediates or products

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3
Q

What is an explosion

A

A sudden release of energy resulting from a chemical reaction that leads to sudden and significant pressure rise

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4
Q

What are the 2 types of explosions? what is the difference between them and which is more destructive?

A

Detonations and deflagrations

Detonations: flame front speed is greater than explosion medium speed of sound –> more destructive

Deflagration: flame front speed is less than speed of sound

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5
Q

What are dust explosions unlikely to cause and why

A

detonations because of the relatively slow process of combusting solid particles

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6
Q

What is a dust explosion

A

Similar to VCE

flammable mixture is ignited resulting in a rapid pressure increase and fire moving through the cloud

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7
Q

When does a dust explosion occur

A

when a combustible material is dispersed in air forming a flammable cloud and a flame propagates through it

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8
Q

What is the relationship between surface area and burn rate

A

rate increases as surface area increases

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9
Q

Under what conditions will the burning reaction continue

A

when amount of heat released is sufficient to cause further combustion.

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10
Q

what happens when heat release produces more combustion than it took to generate

A

fire will grow very rapidly and if its fast enough, the fire will become an explosion

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11
Q

Relationship between surface area and DCE

A

the higher the surface area, the more likely the dust will be involved in DCE but it is possible for very small particles to become lumps and reduce risk

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12
Q

what is the main factor in the estimation of the likelihood of dust explosion

A

particle size and specific surface area

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13
Q

What are other factors affecting ignition sensitivity and explosion

A

dust chemical compostion and moisture content
chem composition, P and T of gas
particle shape and size distribution
radiative heat transfer from flame

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14
Q

What are dust cloud factors that influence likelihood of dust explosion

A

degree of dispersion of dust cloud
concentration distribution in dust cloud
turbulence in dust cloud

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15
Q

What do dust explosions generally arise from

A

combustion of fuel, oxygen, oxides and heat

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16
Q

what other materials can be involved in dust explosions

A

not stable oxides

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17
Q

what compounds are unable to produce dust explosions

A 
silicates
sulphates
nitrates
carbonates
phosphates
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18
Q

what 2 compounds do metal dusts react exothermically with in special cases

A

N2

CO2

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19
Q

Which industies have no risk of dust explosions

A

cement manufacture
sand quarrying
limestone exacavation

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20
Q

Which materials can cause dust explosions

A

natural organic: grain, linen, sugar
synthetic organc: plastics, pesticides
coal and peat
metals: aluminum, zinc, iron

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21
Q

What is the severity of the resulting explostion related to

A

the heat released in the combustion of these materials. Examine the amount of heat released per mole of O2

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22
Q

What is the relationship between heat of combustion and dust explosion

A

the higher the heat of combustion, the greater the chance for a dust explosion

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23
Q

From an energetic viewpoint, metals are the least dangerous. T/F

A

False. Most dangerous

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24
Q

What is the equation used for ideal gases and what does it allow us to do/estimate

A

P=(TnR)/V

Relates heat release to pressure increase. As T increases, so does P

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25
Q

When can DCE occur

A

When dust concentration is within certain limits. LC: 50-100 mg/m3 and UC: 2-3 kg/m3

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26
Q

How can we tell if a cloud is explosive

A

The visibility through the dust cloud even at low concentrations will be impaired

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27
Q

What is a primary explosion

A

Dust explosions that start within a piece of equipment. Concentrations needed for a dust explosion are rarely seen outside of process vessels

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28
Q

What is the main difference between dust explosion and gas/vapor explosions

A

gas/vapor explosions rarely happen inside vessels due to lack of air to support the explosion; whereas, with dusts, it is generally suspended in air in process equipment which allows DE to occur

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29
Q

What is a secondary explosion

A

When lying dust is disturbed by the primary explosion and forms a second dust cloud which is then ignited by the heat released form the pimrary explosion

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30
Q

What is the issue with small amounts of lying dust

A

Small amounts of lying dust occupy very little space but if disturbed can easily form dangerous clouds

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31
Q

List possible ignition sources

A 
Open flames 
Hot surfaces
Heat from mechanical impacts
Electrical and Electrostatic discharges
Smoldering or burning dust
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32
Q

Why are open flames an ignition source that must be controlled/avoided

A

flames from welding/cutting operations are easily sufficient to trigger a dust explosion

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33
Q

What is the relationship between oxygen and heat required for explosion

A

The more o2 present, the less heat is required to trigger an explosion. this is why Oxy-acetylene torches are very dangerous - oxygen suppliers

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34
Q

How do we control open flames

A

we prohibit the use of cutting or welding torches on working dust processing units.
Smoking is also prohibited anywhere in plant

35
Q

Why is smoking a hazard

A

A match flame generates 100W of heat energy which is enough to ignite a dust cloud

36
Q

How can hot surfaces ignite dust explosions

A
  1. causing lying dust to burn

2. direct contact with dust cloud

37
Q

What are the chances of igniting a dust cloud using a hot surface

A

less likely as the surface temperature must be over 400 C to be an ignition source

38
Q

What is heat from mechanical impacts also known as

A

friction sparks

39
Q

When are friction sparks dangerous

A

when they are between 2 metals as these provide enough heat to start a dust explosion

40
Q

What causes friction sparks

A

falling objects (nuts/bolts) or moving equipment contacting plant items

41
Q

how are electrical sparks a dust explosion hazard

A

A spark of anything over 1mJ can ignite a dust cloud and this is easily produced if a plant equipment fails. and especially high voltage devices or severed cable

42
Q

What are electrical sparks given off from

A

faulty electrical equipment

43
Q

When can electostatic discharges cause dust explosions

A

where equipment has not been grounded. The static electricity build up can be large

44
Q

When is smoldering/burning dust a hazard

A

when dust is deposited in heaps under certain conditions and slowly and internally combusts.

45
Q

What does the low heat transfer through dusts in heap cause

A

temperature in heap will rise causing further burning as long as oxygen is present

46
Q

When are the heaps a DE hazard

A

When disturbed but these combustions require ignition themselves

47
Q

What is the only way to get data concering a specific dust

A

Analytical testing

48
Q

Dust can sill can an explosion if its not combustible. T/F

A

False

49
Q

What is dust chemistry affected by

A

the elements that make up the molecule and their specific configuration in the molecule

50
Q

What is the most fundamental consideration in investigating DE and why

A

Dust chemistry as it directly influences the thermodynamis, how much heat is released, kinetics, heat release speed, which all in turn affect explosion severity

51
Q

What does moisture content of dust affect

A

The ability of a dust cloud to be ignired and its ability to sustain an explosion

52
Q

What will increasing moisture content of dust to

A

It pushes the ignition energy up exponentially. Once evaporated, water vapor mixes with gases making them less reactive and increasing intermolecular cohersion, thus larger particle size

53
Q

How does size affect possibility of DE

A

If its in big lumps it will not cause DE even if its combustible

54
Q

Do dusts become more and more explosive down to a certan size limit then plateau?

A

Yes

55
Q

It is not a linear relationship between explosibility and size/surface area of dusts. T/F

A

True

56
Q

Why does surface area affact violence of explosion

A

the particle size/ surface area influences speed at which volatiles are extracted from particle before they burn

57
Q

What are UCL and LCL dictated by

A

UCL - min amount of O2 required for explosion

LCL - min quantity of particles needed to sustain combustion

58
Q

Will a more turbulent cloud result in a less or more severe explosion. Why?

A

More severe because the more homogenous concentrations and lower degree of dispersion, the flame front will more faster through dust cloud

59
Q

Is a less turbulent cloud less and more easily ignited. Why?

A

More as heat dissipation is at a lower rate so the initial heat release is more locally concentrated leading to a higher probability of ignition from an input of energy

60
Q

How does O2 in air affect explosion severity

A

Less O2 causes explosion to be less severe as it limits combustion rate

61
Q

How can you minimize the possibility of dust explosions due to process vessels

A

Limit O2 in process vessels

62
Q

Will a more evenly dispersed dust burn less or more easily

A

More

63
Q

How do dispersion and degree of agglomeration affect combustion?

A

they change the effective local dust concentrations and particle size respectively

64
Q

How does temperature affect dust cloud ignitability

A

at higher initial temp, more easily ignited

65
Q

What does increasing the pressure in dust cloud do to the explosion severity

A

it becomes more violent because essentially all the combustibles are closer together

66
Q

What does high temperature reduce

A

minimum dust concentration for explosion

67
Q

What does increasing pressure lower

A

Required ignition energy

68
Q

What does addition of fuel gas or vapor lower? what does it raise?

A

ingition energy for a pure dust cloud massively. Max explosion pressure

69
Q

What is a satisfactory protection against dust explosions

A

elimination of ignition sources - process and general work

70
Q

Why should gas cutting torches that use excess O2 be avoided

A

they can cause ignition to be easier than it would be in air

71
Q

How can you minimize the change of ignition from hot surfaces

A

removal all dust before performing hot work
shield or isolate hot surfaces
use equipment with low chance of overheating
regular and thorough inspectin and maintenance
use electrical equipment approved for use in presence of combustible dust

72
Q

How can you minimize the change of ignition from mechanical impacts

A

Remove foreign objects from process streams as soon a possible
Avoid any construction materials that could give sparks or flashes
Inspect odd noises to see if they are caused by impacts, and if so remedy the situation in a safe manner

73
Q

How can you minimize the change of ignition from electrical/electrostatic sparks

A

 Use conducting materials for plant items to avoid charge buildup  Earth any equipment that may become charged  Earth coarse non-conducting powders via an earth rod through their storage vessel  If in doubt, ground it.

74
Q

What are 3 major ways to avoid creating explosive dust cloud without modifying dust itself

A

 Add inert gas to the atmosphere
Ensure that the dust is outside of the combustible concentration limits
Add inert dust

75
Q

What does adding inert gas do

A

lowers O2 content in a process area to min the chance of dust explosion. easiest way.
N, CO2, water vapor

76
Q

How can we ensure that the dust is outside of explosible concentrations

A

extremely difficult to do but achieved through plant and equipment design; very difficult if very fine powders are involved

77
Q

What is explosion venting

A

The cheapest, most effecitve way to relieve pressure. It is difficult to vent correctly to allow sufficient presure relief

78
Q

How is venting designed

A

it must allow sufficient outflow of burnt dust and air out to relieve the pressure being generated by the heat of the explosion. designed so pressure can be kept below some upper value

79
Q

What are some hazards of venting

A

Ejection of flames from vent opening
Emission of blast waves from the vent opening
Emission of solid objects (parts of the vessel, vent covers, etc.)

80
Q

What are the 3 major considerations of fast fire suppression systems

A

Extinguishing agent is permanently pressurized

Large diameter discharge orifice High speed valve opening (usually via a detonated charge)

81
Q

What does housekeeping refer to

A

removal of dust accumulation within plant

82
Q

Howdoes addition of liquid control dust

A

Increases likelihood of agglomerate production and can also lessen amount of fine dust created in particle collisions

83
Q

What are factors to consider in plant layour

A

Away from other buildings and actual parts of plant as far apart as possible
If hazardous part is indoors, there must be protection (blast wall) for rest of areas and sufficient venting. Safe escape routes, fire reistant materials for construction, doors and high quality electrical insulation

84
Q

What are human factors to consider

A

Communcation is very important. Workers must know and be motivated and have the confidence to make decisions when needed. There must be training and instruction regarding DE

Fire Safety Management (6 decks)

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