Combustible Dust. Overview

Transcription

1 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 1

2 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 2

3 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 ftdeep Imperial Sugar Plant 3

4 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 4

5 Dust Explosion 5 required ingredients What is a Combustible Dust NFPA 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 ed. Stated A finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the processspecific 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 ExplosibleConcentration (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/m 3 These concentrations are sufficiently high that a 2m thick cloud can prevent seeing a 25watt bulb on the other side of the cloud. 5

6 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 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 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. 6

7 NFPA 68 Chemical Dusts Reprinted with permission from NFPA 68, Explosion Protection by Deflagration Venting, Copyright 2007, National Fire Protection Association, Quincy, MA 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 7

8 Cyclone Enclosureless Dust Collector Enclosureless Dust Collector 8

9 Enclosed Dust Collector Central Vacuum 9

10 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 % in dryers 28% in silos 11% in dust collectors 10

11 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 11

12 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 12

13 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 BulkingBrush 1-10 Piles of powders with resistivities>10 9 resistance-m in hopper or silo Propagating Brush Boots, plastic pipe or duct Spark >10,000 Ungrounded conductor, e.g. bag house, cage, or person (packager) Powder Painting 13

14 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 2 nd vessel produces a much more rapid rate of burning and associated pressure rise. This can render explosion venting or suppression systems ineffective in the 2 nd vessel. Why explosion isolation systems are needed to protect individual vessels. Pressure Development in Dust Deflagration Vessel Suppression Canister 14

15 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 ft 2 for areas greater than 20,000 ft 2 Quantity 1/32 starting point 1/8 for wood dust 15

16 Housekeeping Wood Dust The Typical Explosion Event Initial Internal Deflagration Dust clouds caused by Elastic Rebound Process Equipment Time, msec. 16

17 The Typical Explosion Event Containment Failure from Initial Deflagration Dust Clouds Caused by Elastic Rebound Process Equipment Time, msec. The Typical Explosion Event Dust Clouds Caused by Elastic Rebound Process Equipment Secondary Deflagration Initiated Time, msec. The Typical Explosion Event Process Equipment Secondary Deflagration Propagates through Interior Time, msec. 17

18 The Typical Explosion Event Process Equipment Secondary Deflagration Vents from Structure Time, msec. The Typical Explosion Event Secondary Deflagration Causes Collapse and Residual Fires Time, msec. Diagrams Courtesy of John M. Cholin, P.E., FSFPE, J.M. Cholin Consultants, Inc. 18

19 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 5a Housekeeping Personal Protective Equipment LO/TO, esp. with suppression system canisters Class II locations, electrical Over-exposures to total dust Hazard Communication, training on the physical hazards of chemicals/dust in workplace. Questions 19