Dust Explosions

1
Dust Explosions

• Safe handling of solids

2
Dust Explosion Control

• Introduction
• Basic concepts of dust explosions
• Ignition sources
• Electrostatic ignitions
• Deadly Dust II (Video)

3
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

4
Flammable gases vapors

• 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.

5
Explosive Dusts Hexagon

• For 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 higher moisture
  content, then the dust burning process is
  extinguished.

6
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.

7
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.

8
Prevention of Dust Explosions

• Eliminate fuel
• Prevent dust suspensions
• Add moisture
• Keep fuel below LFL
• Reduce oxygen below MOC
• Eliminate ignition sources

9
Flammable Dusts

• Acetamide
• Adipic Acid
• Aluminum
• Barley
• Carbon
• Cellulose
• Coffee
• Corn
• Epoxy Resin
• Iron

• Milk
• Nylon
• Paper
• Polystyrene
• Starch
• Steel
• Sucrose
• Wheat
• Wood
• Zinc

10
Minimum Ignition Energies

• Dusts
• Aluminum 10 mJ
• Corn (2moisture) 110 mJ
• Epoxy Resin lt10 mJ
• Milk Powder 50 mJ
• Sugar 30 mJ
• Sulphur lt10 mJ

• Versus Vapors
• Acetone 1 mJ
• Acrolein 0.1 mJ
• Benzene 0.2 mJ
• Carbon Disulfide lt0.1 mJ
• Heptane 0.2 mJ
• Toluene 0.2 mJ

11
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.

12
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.

13
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

14
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

15
Dust Explosion Control

• Introduction
• Basic concepts of dust explosions
• Ignition sources
• Electrostatic ignitions
• Accumulation of charges
• Electrostatic discharges
• Deadly Dust II (Video)

16
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
• Contact and Frictional
• Double layer
• Induction
• Transport

17
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

18
Double layer charging

• Caused by friction at interfaces on a microscopic
  scale.
• Liquid-liquid
• Solid-liquid
• Solid-solid
• Gas-liquid
• Gas-solid

19
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.

20
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.

21
Dust Explosion Control

• Introduction
• Basic concepts of dust explosions
• Ignition sources
• Electrostatic ignitions
• Accumulation of charges
• Electrostatic discharges
• Deadly Dust II (Video)

22
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
• Sparks
• Propagating brush
• Brush
• Corona
• Conical pile (Maurer)

23
Spark discharges

• Discharges between two conductors.
• Very energetic with energies ranging up to 10
  Joules.
• Can ignite flammable gases and dusts

24
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.

25
Brush discharge

• Occurs between non-conductor and a conductor.
• Energetic lt 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

26
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 non-incendive to dusts.

27
Conical pile discharge (Maurer discharge)

• Occur between sliding solids and charged air.
• Vessels larger than 1 m3.
• Nonconductive particles with resistance greater
  than 1010Ohmm
• Particles larger than 1mm diameter
• Relatively fast filling rate, greater than 0.5
  kg/s
• Energetic 1 Joule
• Can ignite flammable dusts and vapors

28
Preventing Electrostatic Ignitions

• Handout gives a thought process procedure to
  prevent electrostatic ignitions from dust or
  dust/flammable vapor systems.

29
Deadly Dusts II

• 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.