session #47 combustible dust (wacker) - wmc€¦ · electrostatic discharge type of discharge max....
TRANSCRIPT
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Combustible Dust
Michael Wacker
Industrial Hygienist
Madison, WI Area OSHA Office
Overview
• History of Combustible dust
• What is combustible dust
• Some recognized hazards
• OSHAs Combustible Dust NEP
• What does OSHA look for during a combustible
dust inspection.
• Possible Citations
History of Combustible Dust• Beginning with the processing of grain and wood
• 1948 “Dust Hazards in the Confectionery Industry”
• 1977 Farmer’s Export Company
• 1980 Amoco Chemical Company
• 2003 CTA Acoustic
• 2008 Imperial Sugar Plant Explosion
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Farmer’s Export Company
• December 28, 1977-Galveston, TX
• Spark near grain elevator caused explosion
• 18 died, 35 injured
Amoco Chemical Company
• October 21, 1980-New Castle, DE
• Polypropylene unit exploded
• 6 died, 100 or more injured
• Damage estimated at $45million
CTA Acoustics
• February 20, 2003-Corbin, KY
• Phenolic resin powder dust accumulated
• Dust ignited due to malfunctioning oven
• 7 died
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CTA Acoutics
Imperial Sugar Company
• Feb 7, 2008 Dust Explosion killing 14 people and
injuring 36
• March 7, 2008
– OSHA sends letter to Imperial Sugar CEO
• Urged to eliminate combustible dust at Gramercy, LA plant
• March 14,2008
– OSHA inspects Gramercy, LA plant
• Inspectors found combustible dust
• Dust ranging from an inch to 4 ft deep
Imperial Sugar Plant
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Some Materials that may be
Combustible Dust
• Any dust that explodes
• Common dusts:
• Wood
• Food (starch, flour, whey, sugar, freeze dried microbes)
• Metals (aluminum, steel)
• Dried blood
• Petroleum based products, rubber, plastics (even fire retardant plastic)
• Powder Paint
What is not combustible
• Green Wood (more than 25% moisture content)
• Silica
• Cement
• Sand/Gravel
OSHA Combustible Dust NEP
• The dust has to be combustible
• The dust has to be dispersed in air or another
oxidant, and the concentration of the this
dispersed dust is at or above the MEC.
• The combustible mixture is dispersed within a
confined enclosure.
• There is an ignition source
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Dust Explosion
5 required ingredients
What is a Combustible Dust
• NFPA 654 2004 ed. Stated “any finely divided solid material that is 420 microns or smaller in diameter and present a fire or explosion hazard when dispersed in air.”
• NFPA 654 2013 ed. “Stated A finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.”
• Why the Change: many combustible fiber segments, flat platelets, and agglomerates do not readily pass through a No. 40 sieve, but they can be dispersed to form a combustible dust cloud
Minimum Explosible Concentration
(MEC)
• Concentration depends on the combustible dust.
• MEC values are not very sensitive to particulate diameter for diameters less than about 60um, but increase significantly with increasing diameter about this approximately threshold.
• Majority of materials have an MEC between 30 to 125 g/m3
• These concentrations are sufficiently high that a 2m thick cloud can prevent seeing a 25watt bulb on the other side of the cloud.
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Explosible Range
Source: Dust Explosions in the Process Industries, Second Edition, Rolf K Eckhoff
NFPA 68 Agricultural Products
Reprinted with permission from NFPA 68, Explosion Protection by Deflagration Venting, Copyright 2007, National Fire Protection Association, Quincy, MA 02169. This reprinted
material is not the complete and official position of the National Fire Protection Association on the referenced subject which is represented only by the standard in its entirety.
NFPA 68 Plastic Dusts
Reprinted with permission from NFPA 68, Explosion Protection by Deflagration Venting, Copyright 2007, National Fire Protection Association, Quincy, MA 02169. This reprinted
material is not the complete and official position of the National Fire Protection Association on the referenced subject which is represented only by the standard in its entirety.
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NFPA 68 Chemical Dusts
Reprinted with permission from NFPA 68, Explosion Protection by Deflagration Venting, Copyright 2007, National Fire Protection Association, Quincy, MA 02169. This reprinted
material is not the complete and official position of the National Fire Protection Association on the referenced subject which is represented only by the standard in its entirety.
The combustible mixture is dispersed
within a confined enclosure.
• Silos
• Cyclone
• Dust Collector
• Central Vacuuming system
• Hopper/bin
• Spray Dryers
• Bucket Elevator Legs
• Conveyors: drag, screw, pneumatic
Silo
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Cyclone
Enclosureless Dust Collector
Enclosureless Dust Collector
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Enclosed Dust Collector
Central Vacuum
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Ignition Sources
• Burning embers/Flames/Sparks
• Hot Temperature
• Self Heating
• Impact/Friction
• Electrical Equipment
• Electrostatic Discharge
Burning Embers and Hot Temperatures
• Smoldering nests were the most prevalent causes
of dust explosion in 426 German dust explosions
from 1965 to 1985
• 29% in dryers
• 28% in silos
• 11% in dust collectors
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Self Heating
• ABS resin powder
• Activated charcoal (esp. Powder River Basin Coal)
• Various chemical intermediates
• Freshly mft. Dried wood chips, anhydrous calcium
hypochlorite
• Organic Peroxides
• Agricultural Materials: Bagasse, soybeans
Impact/Friction
• When the tip speed is greater than 10m/s and
the fill rate is less than 70%, there is a high
probability of a dust cloud ignition.
Electrical Equipment
• Electrical equipment and wiring can potentially
ignite dust clouds by sparks, arcs, or heated
surfaces
• Class II Div. 1: location which combustible dust is in
the air under normal operating conditions.
• Class II Div. 2: location which combustible dust is
due to abnormal operations may be present in the
air in quantities sufficient to produce explosive or
ignitable mixture
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Class II Dust
• Dust particles smaller than 200 mesh may be
classified as a class II dust and electrical
components need to be rated for Class II dust.
• Wood dust: cutting with a blade with more than
60 teeth may create Class II dust.
Wood dust, Class II dust
Class II Dust
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Class II Dust
Electrostatic Discharge
Type of Discharge Max. Energy (mJ) Examples
Corona 0.1 Wires, type D bulk bags
Brush 1-3 Flexible boots and socks
Bulking Brush 1-10 Piles of powders with
resistivities>109 resistance-m
in hopper or silo
Propagating Brush 1000-3000 Boots, plastic pipe or duct
Spark >10,000 Ungrounded conductor, e.g.
bag house, cage, or person
(packager)
Powder Painting
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Pressure Development in Dust
Deflagration
• Propagation pressure can more than double
• Propagation and piling does not always occur.
• When it does: jet flame ignition of dust cloud in the
2nd vessel produces a much more rapid rate of
burning and associated pressure rise.
• This can render explosion venting or suppression
systems ineffective in the 2nd vessel.
• Why explosion isolation systems are needed to
protect individual vessels.
Pressure Development in Dust
Deflagration
Vessel Suppression Canister
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Propagation Isolation Canister
Secondary Dust Explosion
(Housekeeping)
• Cause of most casualties
• Occurs when dust deposits on exposed surfaces in
the building are lifted by the blast wave emanating
form the breached equipment/enclosure
Housekeeping
• Accumulation on surfaces, esp. high horizontal
surfaces
• 5% or more of the facility foot print
• Or 1000 ft2 for areas greater than 20,000 ft2
• Quantity
• 1/32” starting point
• 1/8” for wood dust
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Housekeeping
Wood Dust
The “Typical” Explosion Event
Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325
Process
Equipment
Initial
Internal
Deflagration
Dust clouds caused
by Elastic Rebound
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The “Typical” Explosion Event
Process
Equipment
Containment
Failure from Initial
Deflagration
Dust Clouds Caused
by Elastic Rebound
Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325
The “Typical” Explosion Event
Process
Equipment
Secondary Deflagration
Initiated
Dust Clouds Caused
by Elastic Rebound
Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325
The “Typical” Explosion Event
Process
Equipment
Secondary Deflagration
Propagates through Interior
Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325
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The “Typical” Explosion Event
Process
Equipment
Secondary Deflagration
Vents from Structure
Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325
The “Typical” Explosion Event
Secondary Deflagration
Causes Collapse and Residual Fires
Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325
Diagrams Courtesy of John M. Cholin, P.E., FSFPE, J.M. Cholin Consultants, Inc.
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Summary
• Know your Dust…is it combustible, class II
• Dust Collection system: size and location of dust collector(s) and other assoicated vessels
• Ducting: metal, PVC pipes, bonded
• Conveying system
• Explosion protection-blast panels, suppression system
• Propagation isolation system
• Electrical and other ignition sources
• Housekeeping...
Possible OSHA Citations
• 5a1
• 1910.22 Housekeeping
• 1910.132 Personal Protective Equipment
• 1910.147 LO/TO, esp. with suppression system
canisters.
• 1910.307 Class II locations, electrical
• 1910.1000 Over-exposures to total dust
• 1910.1200 Hazard Communication, training on the
physical hazards of chemicals/dust in workplace.
Questions