multiplying ignition sensi- tivity and explosion severity according to Table 2. An example of this approach is given below for testing results from a metalcast- ing facility with gray iron bond dust:
Ignition Sensitivity = 610 x 55 x 60(1) = 0.01 400 x 10,000 x 40(2)
Explosion Severity = 1.4 x 19(2) = 0.007 7.9 x 491(1)
Subscript 1 refers to the
appropriate values for Pitts- burgh seam coal (ignition values from NMAB report #353-3, June 1, 1989; sever- ity values from Chilworth CTI 20 liter testing). Subscript 2 refers to the
T(c) (Minimum Ignition Temperature) = 601oC
E (Minimum Ignition Energy) = 55 mJ
M(c) (Minimum Explosive Concentration) = 60 g/m3
P(max) (Maximum Explosive Pressure) = 1.4 barg
P (Maximum KST) = 19 bar-m/second
The results in this example indicate an explosivity index of 0.0007.
Controlling the Hazards Metalcasters can take a number
of steps to control combustible dust hazards. First, perform the following actions to determine the presence of combustible dust in your facility: • Review the materials for combus- tibility. Some Material Safety Data Sheets now contain information about combustibility, but absence of this information does not necessarily mean no hazard exists.
• Review metalcasting processes to identify those that create dust. The dust may be generated by the opera- tion (grinding, sawing or melting), shipping or handling (dust in boxes, bags or hoppers), or abrasion by process equipment (powered trucks
34
Additional Information on Combustible Dust
In addition to NFPA 484, 499 and 654 mentioned in this
article, several other references may provide useful informa- tion to properly deal with the hazards of combustible dust: • NFPA 664—Standard for the Prevention of Fires and Ex- plosions in Wood Processing and Woodworking Facilities
• NFPA 68—Guide for Venting of Defl agrations (2002 Edition)
• NFPA 69—Standard on Explosion Prevention Systems • Aluminum Association, Guide F-1—Guidelines for Handling Fines Generated During Various Aluminum Fabricating Operations (
www.aluminum.org/Content/ NavigationMenu/TheIndustry/PowderandPaste/Fines- Guidelines.pdf)
• FM Global, Data Sheet No. 7-76—Prevention and Mitigation of Combustible Dust Explosions and Fire (2006 Edition)
• OSHA National Emphasis Program (
www.osha.gov/ OshDoc/Directive_pdf/CPL_03-00-008.pdf)
values for the specifi c dust in question.
running over material on the fl oor, sieves or attrition mills).
• Look for fugitive dust sources. Fine dust can accumulate on rafters, build- ing trusses or ventilation ductwork. Piles of dust on the fl oor can indicate a fugitive dust emission source.
• Determine if the dust is combustible. The most common concern is with dusts that contain organic materials or fi ne metal particles. If you determine the existence of combustible dust in your plant, take the following additional steps:
• Conduct further testing to determine the degree of the hazard. The manu- facturer of your dust col- lection equipment may be able to assist you in this process.
• Review the design of the dust collection system to determine if explosion relief vent- ing is needed. Again, the equipment manu- facturer will be a good source of information.
• Control fugitive emissions: • Seal leaks in exhaust system ducts.
• Ensure adequate dust capture at the point of operation.
• Be alert to process- es that can separate fi ne dust from larger particles.
• Implement a rigorous house- keeping program to control dust accumulation.
• Do not use compressed air for clean- ing combustible dust.
MC About the Authors
George Hutcheson, CIH, is an industrial hygienist for John Deere Waterloo Works, Waterloo, Iowa. Tom Slavin, CIH, CSP is global safety and health director, based in Chicago, for Navistar Inc., Warrenville, Ill.
For More Information
“Avoiding Fatalities,” S. Gibbs, MODERN CASTING, October 2008.
To control dust hazards, look for fugitive dust sources and determine if the material is com- bustible. If so, review the design of your dust collection system and make changes as needed.
MODERN CASTING / February 2011
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