Introduction Basic concepts of dust explosions Ignition sources Electrostatic ignitions Deadly Dust II (Video) Flammable gases & vapors Explosive Dusts Hexagon Effects of suspension Prevention of Dust Explosions Flammable Dusts Minimum Ignition Energies Dust Explosion Control

1. Dust ExplosionsDust Explosions
Safe handling of solids

2. Dust Explosion ControlDust Explosion Control
 Introduction
 Basic concepts of dust explosions Basic concepts of dust explosions
 Ignition sources
El t t ti i iti Electrostatic ignitions
 Deadly Dust II (Video)

3. Solids HandlingSolids Handling
 The safe handling of solids in becoming more important
because the production and the processing of solids is
increasingincreasing.
 More chemicals are being produced and handled as
solids to eliminate reactions with volatile and hazardoussolids 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 More chemicals are transported in reusable super
sacks”

4. Flammable gases & vaporsFlammable gases & vapors
 When dealing with flammable
gases and vapors, the
generally accepted majorgenerally accepted major
requirements for a fire or
explosion are fuel, oxygen
and ignition.
 In chemical industry they try
to eliminate or reduce one orto eliminate or reduce one or
more of the sides of the
triangle.

5. Explosive Dusts HexagonExplosive Dusts Hexagon
For Dust explosions there isFor Dust explosions there is
a more complex situation
needed before an
explosion occurs.
 Fuel – any dust such as,
chemicals, grain, wood dust,
flour, polymers, lint etc.
 Moisture – when fuel contains a Moisture – when fuel contains a
higher moisture content, then the
dust burning process is
ti i h dextinguished.

6. Explosive Dust Hexagon (cont)Explosive Dust Hexagon (cont)
 Dust and Air Suspension:
Particles must be below a certain minimum size toParticles must be below a certain minimum size to
be able to be suspended.
Particle loading (concentration) must be betweeng ( )
certain limits:
– Lower 20 to 60 g/m3
– Upper 2 to 6 kg/m3
Dust loading must be fairly uniform to be explosive.

7. Effects of suspensionEffects of suspension
 In the upper picture a bin
(with a vent) that
contains dust is ignitedcontains dust is ignited.
 In the lower picture, an
additional pile of dustadditional pile of dust
was located in the path
of the venting flame.
 The dust become
suspended and caused a
secondary explosionsecondary explosion.

8. Prevention of Dust ExplosionsPrevention of Dust Explosions
 Eliminate fuel
 Prevent dust suspensions Prevent dust suspensions
 Add moisture
K f l b l LFL Keep fuel below LFL
 Reduce oxygen below MOC
 Eliminate ignition sources

9. Flammable DustsFlammable Dusts
 Acetamide
 Adipic Acid
 Aluminum
 Milk
 Nylon
 Paper Aluminum
 Barley
 Carbon
 Paper
 Polystyrene
 Starch
 Cellulose
 Coffee
C
 Steel
 Sucrose
Wh t Corn
 Epoxy Resin
 Iron
 Wheat
 Wood
 Zinc Iron  Zinc

10. Minimum Ignition EnergiesMinimum Ignition Energies
 Dusts
Aluminum 10 mJ
 Versus Vapors
Acetone 1 mJ
Corn (2%moisture) 110 mJ
Epoxy Resin <10 mJ
Acrolein 0.1 mJ
Benzene 0.2 mJ
Milk Powder 50 mJ
Sugar 30 mJ
Sulphur <10 mJ
Carbon Disulfide <0.1 mJ
Heptane 0.2 mJ
Toluene 0 2 mJSulphur <10 mJ Toluene 0.2 mJ

11. Elimination of suspensionsElimination 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
l i i f h f ili ill h dexplosion in one part of the facility will cause the dust to
be suspended and exacerbate the hazard.
Oft i t i dd d t lid t i i i Often moisture is added to solids to minimize
suspensions and also to reduce explosivity.

12. Keeping fuel below LFLKeeping 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 toas a bag house, cyclone or electrostatic precipitator to
remove and collect the dust.
 Pelletize solids to minimize amount of material in air 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 timeanother time.

13. Common Ignition SourcesCommon Ignition Sources
 A United Kingdom study of dust explosions and
fires cited the following causes of ignitiong g
sources
Mechanical 18% Welding 7%Mechanical 18%
Overheating 17%
Open flames 15%
g
Electrical 3%
Other 29%Open flames 15%
Static Electricity 11%
Other 29%

14. Static Electricity DischargesStatic Electricity Discharges
 Static electricity is thee fourth largest cause of
ignition sources in dust explosions.g p
 Because of the nature of solids, the handling and
transportation of solids can actually be the causetransportation of solids can actually be the cause
of the static electricity

15. Dust Explosion ControlDust Explosion Control
 Introduction
 Basic concepts of dust explosions Basic concepts of dust explosions
 Ignition sources
El t t ti i iti Electrostatic ignitions
Accumulation of charges
Electrostatic discharges
 Deadly Dust II (Video)

16. Fundamentals of Static ElectricityFundamentals 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 sourcethen serves as an ignition source.
 One method to prevent static electricity is to prevent the
accumulation of chargeaccumulation of charge.
 Charge Accumulation:
Contact and Frictional
Double layer
Induction
T tTransport

17. Contact and Frictional ChargingContact and Frictional Charging
 Dust transport
e.g. pneumatic transport of powders/solidse.g. pneumatic transport of powders/solids
 Pouring powders
e g pouring solids down chutes or troughse.g. pouring solids down chutes or troughs
 Gears and belts
t ti h f f te.g. transporting charges from one surface to
another

18. Double layer chargingDouble layer charging
 Caused by friction at interfaces on a microscopic
scale.
 Liquid-liquid
 Solid liquid Solid-liquid
 Solid-solid
 Gas-liquid
 Gas-solid

19. Induction chargingInduction charging
 Occurs when an isolated Occurs when an isolated
conductor is subject to a electric
field. Charges of different
polarity are induced on oppositepolarity are induced on opposite
sides. If an earthed electrode
touches or approaches the body
then the charges closest tothen the charges closest to
electrode flow away leaving the
body with a net charge of
opposite signopposite sign.
 Occurs by walking across
carpet.
N d ti h Nonconductive shoes are a
problem.

20. Charging by TransportCharging by Transport
 Results from a charged dust, liquid or solid
particles settling onto a surface and transportingp g p g
their charges to this new surface.
 The rate of charge accumulation is a function of The rate of charge accumulation is a function of
the rate of transportation.
 Lightening is an example of this type of charging Lightening is an example of this type of charging
phenomenon.

21. Dust Explosion ControlDust Explosion Control
 Introduction
 Basic concepts of dust explosions Basic concepts of dust explosions
 Ignition sources
El t t ti i iti Electrostatic ignitions
Accumulation of charges
Electrostatic discharges
 Deadly Dust II (Video)

22. Electrostatic IgnitionsElectrostatic Ignitions
 Static electric ignitions are the result of transferring the
accumulated charges to another surface via a discharge.
Th l t d h b f l l k d t The accumulated charge may be safely leaked away to
earth by grounding.
 If energy of discharge exceeds MIE then fire or explosion If energy of discharge exceeds MIE then fire or explosion.
 Static electric Discharges
SparksSparks
Propagating brush
Brush
Corona
Conical pile (Maurer)

23. Spark dischargesSpark discharges
 Discharges between two
conductors.
 Very energetic with
energies ranging up to
10 J l10 Joules.
 Can ignite flammable
d d tgases and dusts

24. Propagating brush dischargePropagating brush discharge
 Occur between a conductor
and a non-conductive lining.
 Very energetic can be Very energetic, can be
greater than 100 Joules.
 Major contributor to static
l t i it i itielectricity ignitions.
 If breakdown voltage of lining
is less than 4 kV, then
propagating brush
discharges are not possible
because charge will pass
through lining.

25. Brush dischargeBrush discharge
 Occurs between non- Occurs between non
conductor and a conductor.
 Energetic < 5mJ
N d ti li i Nonconductive lining or
surface must have a
breakdown voltage greater
th 4kV d thi kthan 4kV and a thickness
greater than 2mm.
 Can ignite flammable vaporsg p
but rarely ignites flammable
dusts.
 Nonconductive coating can Nonconductive coating can
be a layer of the powdered
solid

26. Corona dischargeCorona discharge
 Corona discharge similar to
brush discharge but occurs
when electrode morewhen electrode more
pointed.
 Occurs over a longer period
of time than a spark and mayof time than a spark and may
give faint glow and hissing
sound.
 C i iti f Can cause ignition of
flammable gas mixtures with
low MIE.
 Usually considered non-
incendive to dusts.

27. Conical pile discharge (Maurer discharge)Conical pile discharge (Maurer discharge)
 Occur between sliding solidsg
and charged air.
 Vessels larger than 1 m3.
 Nonconductive particles with Nonconductive particles with
resistance greater than
1010Ohm•m
P ti l l th 1 Particles larger than 1mm
diameter
 Relatively fast filling rate,y g
greater than 0.5 kg/s
 Energetic ~ 1 Joule
 Can ignite flammable dusts and Can ignite flammable dusts and
vapors

28. Preventing Electrostatic IgnitionsPreventing Electrostatic IgnitionsPreventing Electrostatic IgnitionsPreventing Electrostatic Ignitions
 Handout gives a
thought process
procedure to
preventprevent
electrostatic
ignitions from dustignitions from dust
or dust/flammable
vapor systems.p y

29. Deadly Dusts IIDeadly Dusts II
 Opening scene is an actual
explosion captured by a TV
photographer filming an Ad.photographer filming an Ad.
 Made for grain handlers and deals
primarily with grain silos.
 T k thi id l t t To make this video relevant to
other industries, every time the
word grain is used, substitute it
ith h i l d t fl t hwith chemical dusts, flour, starch,
pharmaceutical dusts, fibers,
polymers, plastics, etc.