Transcript Dust Explosions

Dust Explosions
Safe handling of solids
Dust Explosion Control
 Introduction
 Basic concepts of dust explosions
 Ignition sources
 Electrostatic ignitions
 Deadly Dust II (Video)
Solids Handling
The safe handling of solids in becoming more important
because the production and the processing of solids is
increasing.
 More chemicals are being produced and handled as
solids to eliminate reactions with volatile and hazardous
solvents.
 Emphasis to produce products as powders (versus
liquids) to eliminate the need to handle empty
containers.
 More chemicals are transported in reusable “super
sacks”
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Flammable gases & vapors
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When dealing with flammable
gases and vapors, the
generally accepted major
requirements for a fire or
explosion are fuel, oxygen
and ignition.
In chemical industry they try
to eliminate or reduce one or
more of the sides of the
triangle.
Explosive Dusts Hexagon
For Dust explosions there is
a more complex situation
needed before an
explosion occurs.
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Fuel – any dust such as,
chemicals, grain, wood dust,
flour, polymers, lint etc.
Moisture – when fuel contains a
higher moisture content, then the
dust burning process is
extinguished.
Explosive Dust Hexagon (cont)
 Dust and Air Suspension:
Particles must be below a certain minimum size to
be able to be suspended.
Particle loading (concentration) must be between
certain limits:
– Lower 20 to 60 g/m3
– Upper 2 to 6 kg/m3
Dust loading must be fairly uniform to be explosive.
Effects of suspension
In the upper picture a bin
(with a vent) that
contains dust is ignited.
 In the lower picture, an
additional pile of dust
was located in the path
of the venting flame.
 The dust become
suspended and caused a
secondary explosion.
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Prevention of Dust Explosions
 Eliminate fuel
 Prevent dust suspensions
 Add moisture
 Keep fuel below LFL
 Reduce oxygen below MOC
 Eliminate ignition sources
Flammable Dusts
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Acetamide
Adipic Acid
Aluminum
Barley
Carbon
Cellulose
Coffee
Corn
Epoxy Resin
Iron
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Milk
Nylon
Paper
Polystyrene
Starch
Steel
Sucrose
Wheat
Wood
Zinc
Minimum Ignition Energies
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Dusts
Aluminum
Corn (2%moisture)
Epoxy Resin
Milk Powder
Sugar
Sulphur
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10 mJ
110 mJ
<10 mJ
50 mJ
30 mJ
<10 mJ
Versus Vapors
Acetone
1 mJ
Acrolein
0.1 mJ
Benzene
0.2 mJ
Carbon Disulfide <0.1 mJ
Heptane
0.2 mJ
Toluene
0.2 mJ
Elimination of suspensions
Good housekeeping – If dust is not lying around it
cannot get suspended which results in an explosive
situation.
 Dust on beams is especially a problem since an
explosion in one part of the facility will cause the dust to
be suspended and exacerbate the hazard.
 Often moisture is added to solids to minimize
suspensions and also to reduce explosivity.
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Keeping fuel below LFL
Venting of area and/or hooding used to collect dust.
The collected dust is sent to a collections system such
as a bag house, cyclone or electrostatic precipitator to
remove and collect the dust.
 Pelletize solids to minimize amount of material in air
suspendable size range.
 Inerting the area where dust will be present to reduce
oxygen to below MOC. Often not practical in large
systems. Inerting processes will be discussed at
another time.
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Common Ignition Sources
 A United Kingdom study of dust explosions and
fires cited the following causes of ignition
sources
Mechanical 18%
Overheating 17%
Open flames 15%
Static Electricity 11%
Welding 7%
Electrical 3%
Other 29%
Static Electricity Discharges
 Static electricity is thee fourth largest cause of
ignition sources in dust explosions.
 Because of the nature of solids, the handling and
transportation of solids can actually be the cause
of the static electricity
Dust Explosion Control
 Introduction
 Basic concepts of dust explosions
 Ignition sources
 Electrostatic ignitions
Accumulation of charges
Electrostatic discharges
 Deadly Dust II (Video)
Fundamentals of Static Electricity
Handling solids often leads to the accumulation of static
electricity. This accumulation can lead to a spark that
then serves as an ignition source.
 One method to prevent static electricity is to prevent the
accumulation of charge.
 Charge Accumulation:
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Contact and Frictional
Double layer
Induction
Transport
Contact and Frictional Charging
 Dust transport
e.g. pneumatic transport of powders/solids
 Pouring powders
e.g. pouring solids down chutes or troughs
 Gears and belts
e.g. transporting charges from one surface to
another
Double layer charging
 Caused by friction at interfaces on a microscopic
scale.
 Liquid-liquid
 Solid-liquid
 Solid-solid
 Gas-liquid
 Gas-solid
Induction charging
Occurs when an isolated
conductor is subject to a electric
field. Charges of different
polarity are induced on opposite
sides. If an earthed electrode
touches or approaches the body
then the charges closest to
electrode flow away leaving the
body with a net charge of
opposite sign.
 Occurs by walking across
carpet.
 Nonconductive shoes are a
problem.
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Charging by Transport
 Results from a charged dust, liquid or solid
particles settling onto a surface and transporting
their charges to this new surface.
 The rate of charge accumulation is a function of
the rate of transportation.
 Lightening is an example of this type of charging
phenomenon.
Dust Explosion Control
 Introduction
 Basic concepts of dust explosions
 Ignition sources
 Electrostatic ignitions
Accumulation of charges
Electrostatic discharges
 Deadly Dust II (Video)
Electrostatic Ignitions
Static electric ignitions are the result of transferring the
accumulated charges to another surface via a discharge.
 The accumulated charge may be safely leaked away to
earth by grounding.
 If energy of discharge exceeds MIE then fire or explosion.
 Static electric Discharges
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Sparks
Propagating brush
Brush
Corona
Conical pile (Maurer)
Spark discharges
Discharges between two
conductors.
 Very energetic with
energies ranging up to
10 Joules.
 Can ignite flammable
gases and dusts
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Propagating brush discharge
Occur between a conductor
and a non-conductive lining.
 Very energetic, can be
greater than 100 Joules.
 Major contributor to static
electricity ignitions.
 If breakdown voltage of lining
is less than 4 kV, then
propagating brush
discharges are not possible
because charge will pass
through lining.
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Brush discharge
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Occurs between nonconductor and a conductor.
Energetic < 5mJ
Nonconductive lining or
surface must have a
breakdown voltage greater
than 4kV and a thickness
greater than 2mm.
Can ignite flammable vapors
but rarely ignites flammable
dusts.
Nonconductive coating can
be a layer of the powdered
solid
Corona discharge
Corona discharge similar to
brush discharge but occurs
when electrode more
pointed.
 Occurs over a longer period
of time than a spark and may
give faint glow and hissing
sound.
 Can cause ignition of
flammable gas mixtures with
low MIE.
 Usually considered nonincendive to dusts.
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Conical pile discharge (Maurer discharge)
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Occur between sliding solids
and charged air.
Vessels larger than 1 m3.
Nonconductive particles with
resistance greater than
1010Ohm•m
Particles larger than 1mm
diameter
Relatively fast filling rate,
greater than 0.5 kg/s
Energetic ~ 1 Joule
Can ignite flammable dusts and
vapors
Preventing Electrostatic Ignitions
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Handout gives a
thought process
procedure to
prevent
electrostatic
ignitions from dust
or dust/flammable
vapor systems.
Deadly Dusts II
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Opening scene is an actual
explosion captured by a TV
photographer filming an Ad.
Made for grain handlers and deals
primarily with grain silos.
To make this video relevant to
other industries, every time the
work grain is used, substitute it
with chemical dusts, flour, starch,
pharmaceutical dusts, fibers,
polymers, plastics, etc.