Explosions Flashcards
When and where was the first recorded incidence of dust explosion
1785 italian flour mill
When can dust cause an explosion
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
Where are dust explosions a risk
in the grain industry. 50 explosions per year
they can also occur whenever a processes uses particlate matters either as feedstock, intermediates or products
What is an explosion
A sudden release of energy resulting from a chemical reaction that leads to sudden and significant pressure rise
What are the 2 types of explosions? what is the difference between them and which is more destructive?
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
What are dust explosions unlikely to cause and why
detonations because of the relatively slow process of combusting solid particles
What is a dust explosion
Similar to VCE
flammable mixture is ignited resulting in a rapid pressure increase and fire moving through the cloud
When does a dust explosion occur
when a combustible material is dispersed in air forming a flammable cloud and a flame propagates through it
What is the relationship between surface area and burn rate
rate increases as surface area increases
Under what conditions will the burning reaction continue
when amount of heat released is sufficient to cause further combustion.
what happens when heat release produces more combustion than it took to generate
fire will grow very rapidly and if its fast enough, the fire will become an explosion
Relationship between surface area and DCE
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
what is the main factor in the estimation of the likelihood of dust explosion
particle size and specific surface area
What are other factors affecting ignition sensitivity and explosion
dust chemical compostion and moisture content
chem composition, P and T of gas
particle shape and size distribution
radiative heat transfer from flame
What are dust cloud factors that influence likelihood of dust explosion
degree of dispersion of dust cloud
concentration distribution in dust cloud
turbulence in dust cloud
What do dust explosions generally arise from
combustion of fuel, oxygen, oxides and heat
what other materials can be involved in dust explosions
not stable oxides
what compounds are unable to produce dust explosions
silicates sulphates nitrates carbonates phosphates
what 2 compounds do metal dusts react exothermically with in special cases
N2
CO2
Which industies have no risk of dust explosions
cement manufacture
sand quarrying
limestone exacavation
Which materials can cause dust explosions
natural organic: grain, linen, sugar
synthetic organc: plastics, pesticides
coal and peat
metals: aluminum, zinc, iron
What is the severity of the resulting explostion related to
the heat released in the combustion of these materials. Examine the amount of heat released per mole of O2
What is the relationship between heat of combustion and dust explosion
the higher the heat of combustion, the greater the chance for a dust explosion
From an energetic viewpoint, metals are the least dangerous. T/F
False. Most dangerous
What is the equation used for ideal gases and what does it allow us to do/estimate
P=(TnR)/V
Relates heat release to pressure increase. As T increases, so does P
When can DCE occur
When dust concentration is within certain limits. LC: 50-100 mg/m3 and UC: 2-3 kg/m3
How can we tell if a cloud is explosive
The visibility through the dust cloud even at low concentrations will be impaired
What is a primary explosion
Dust explosions that start within a piece of equipment. Concentrations needed for a dust explosion are rarely seen outside of process vessels
What is the main difference between dust explosion and gas/vapor explosions
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
What is a secondary explosion
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
What is the issue with small amounts of lying dust
Small amounts of lying dust occupy very little space but if disturbed can easily form dangerous clouds
List possible ignition sources
Open flames Hot surfaces Heat from mechanical impacts Electrical and Electrostatic discharges Smoldering or burning dust
Why are open flames an ignition source that must be controlled/avoided
flames from welding/cutting operations are easily sufficient to trigger a dust explosion
What is the relationship between oxygen and heat required for explosion
The more o2 present, the less heat is required to trigger an explosion. this is why Oxy-acetylene torches are very dangerous - oxygen suppliers
How do we control open flames
we prohibit the use of cutting or welding torches on working dust processing units.
Smoking is also prohibited anywhere in plant
Why is smoking a hazard
A match flame generates 100W of heat energy which is enough to ignite a dust cloud
How can hot surfaces ignite dust explosions
- causing lying dust to burn
2. direct contact with dust cloud
What are the chances of igniting a dust cloud using a hot surface
less likely as the surface temperature must be over 400 C to be an ignition source
What is heat from mechanical impacts also known as
friction sparks
When are friction sparks dangerous
when they are between 2 metals as these provide enough heat to start a dust explosion
What causes friction sparks
falling objects (nuts/bolts) or moving equipment contacting plant items
how are electrical sparks a dust explosion hazard
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
What are electrical sparks given off from
faulty electrical equipment
When can electostatic discharges cause dust explosions
where equipment has not been grounded. The static electricity build up can be large
When is smoldering/burning dust a hazard
when dust is deposited in heaps under certain conditions and slowly and internally combusts.
What does the low heat transfer through dusts in heap cause
temperature in heap will rise causing further burning as long as oxygen is present
When are the heaps a DE hazard
When disturbed but these combustions require ignition themselves
What is the only way to get data concering a specific dust
Analytical testing
Dust can sill can an explosion if its not combustible. T/F
False
What is dust chemistry affected by
the elements that make up the molecule and their specific configuration in the molecule
What is the most fundamental consideration in investigating DE and why
Dust chemistry as it directly influences the thermodynamis, how much heat is released, kinetics, heat release speed, which all in turn affect explosion severity
What does moisture content of dust affect
The ability of a dust cloud to be ignired and its ability to sustain an explosion
What will increasing moisture content of dust to
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
How does size affect possibility of DE
If its in big lumps it will not cause DE even if its combustible
Do dusts become more and more explosive down to a certan size limit then plateau?
Yes
It is not a linear relationship between explosibility and size/surface area of dusts. T/F
True
Why does surface area affact violence of explosion
the particle size/ surface area influences speed at which volatiles are extracted from particle before they burn
What are UCL and LCL dictated by
UCL - min amount of O2 required for explosion
LCL - min quantity of particles needed to sustain combustion
Will a more turbulent cloud result in a less or more severe explosion. Why?
More severe because the more homogenous concentrations and lower degree of dispersion, the flame front will more faster through dust cloud
Is a less turbulent cloud less and more easily ignited. Why?
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
How does O2 in air affect explosion severity
Less O2 causes explosion to be less severe as it limits combustion rate
How can you minimize the possibility of dust explosions due to process vessels
Limit O2 in process vessels
Will a more evenly dispersed dust burn less or more easily
More
How do dispersion and degree of agglomeration affect combustion?
they change the effective local dust concentrations and particle size respectively
How does temperature affect dust cloud ignitability
at higher initial temp, more easily ignited
What does increasing the pressure in dust cloud do to the explosion severity
it becomes more violent because essentially all the combustibles are closer together
What does high temperature reduce
minimum dust concentration for explosion
What does increasing pressure lower
Required ignition energy
What does addition of fuel gas or vapor lower? what does it raise?
ingition energy for a pure dust cloud massively. Max explosion pressure
What is a satisfactory protection against dust explosions
elimination of ignition sources - process and general work
Why should gas cutting torches that use excess O2 be avoided
they can cause ignition to be easier than it would be in air
How can you minimize the change of ignition from hot surfaces
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
How can you minimize the change of ignition from mechanical impacts
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
How can you minimize the change of ignition from electrical/electrostatic sparks
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.
What are 3 major ways to avoid creating explosive dust cloud without modifying dust itself
Add inert gas to the atmosphere
Ensure that the dust is outside of the combustible concentration limits
Add inert dust
What does adding inert gas do
lowers O2 content in a process area to min the chance of dust explosion. easiest way.
N, CO2, water vapor
How can we ensure that the dust is outside of explosible concentrations
extremely difficult to do but achieved through plant and equipment design; very difficult if very fine powders are involved
What is explosion venting
The cheapest, most effecitve way to relieve pressure. It is difficult to vent correctly to allow sufficient presure relief
How is venting designed
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
What are some hazards of venting
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.)
What are the 3 major considerations of fast fire suppression systems
Extinguishing agent is permanently pressurized
Large diameter discharge orifice High speed valve opening (usually via a detonated charge)
What does housekeeping refer to
removal of dust accumulation within plant
Howdoes addition of liquid control dust
Increases likelihood of agglomerate production and can also lessen amount of fine dust created in particle collisions
What are factors to consider in plant layour
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
What are human factors to consider
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
