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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