Page 20 - Occupational Health & Safety, January/February 2020

COMBUSTIBLE DUST
The 1/32” idea is addressed in NFPA 654, Standard for the Pre- vention of Fire and Dust Explosions from the Manufacturing, Pro- cessing, and Handling of Combustible Particulate Solids, which is referenced in NFPA 652. Called the “layer depth criterion method,” it is one of the four acceptable ways to determine dust flash-fire or dust explosion hazard areas.4
Using this method, a hazard area exists when the average dust layer thickness external to process equipment exceeds the layer depth criterion which, for materials with a bulk density of 75 lb/ft or greater, is 1/32”. For less dense materials, the standard provides an equation that can be used to increase the layer depth criterion.
Once you have the layer depth criterion for your material, you can determine your risk based on how much of your building or room meets that criterion. Here are the four conditions that indi- cate a hazard:
■ The total area of nonseparated dust accumulations exceeding the layer depth criterion is greater than five of the footprint area.
■ The area of any single nonseparated dust accumulation ex- ceeding the layer depth criterion is greater than 1,000 square feet.
■ The total volume of nonseparated dust accumulations is greater than the layer depth criterion multiplied by five percent of the footprint area.
■ The total volume of any single nonseparated dust accumula- tion is greater than the layer depth criterion multiplied by 1,000 square feet.
Note that these requirements are based on footprint area, not just the floor. Dust can also settle on beam flanges, piping, duct- work, equipment, suspended ceilings, light fixtures, and walls. In its Combustible Dust National Emphasis Program, OSHA notes: “Rough calculations show that the available surface area of bar joists is approximately five percent of the floor area and the equiva- lent surface area for steel beams can be as high as 10 percent.”5 All of these surfaces need to be included in the calculations along with the floor.
Cleaning Equipment Selection and Certification
NFPA 652 specifies the cleaning methods and equipment types per- mitted in combustible dust environments. The preferred cleaning methods are vacuuming, sweeping, and water washdown. Com- pressed air blowdown is only permitted after the preferred methods have been used and other safety precautions have been taken.
Plant managers often underestimate the direct and indirect costs of purchasing inefficient cleaning equipment. Brooms don’t eliminate dust; they just move it around. Shop-style vacuums aren’t built for industrial environments, so their motors burn out quickly, making replacement a frequently-occurring expense. That’s why vacuuming with an industrial vacuum cleaner is the best and most cost-effective cleaning method for combatting combustible dust.
The NFPA does not certify vacuum equipment, but NFPA 652 does provide requirements for vacuum design. For the sake of space, we won’t list them here, but you can find them in Sec- tion 8.4.2.2.1.7. The standard also outlines when equipment must be certified for Class II electrically classified (hazardous) locations.
Here are some things to keep in mind as you evaluate industrial vacuum equipment:
■ Certified equipment must be third-party certified by a Na- tionally Recognized Testing Laboratory (NRTL).6 Unlike in Eu-
rope, in the United States there is no such thing as self-certification for electrical equipment used in hazardous environments.7
■ Certification needs to apply to the entire machine, not just the electrical components. For example, one of the NFPA 652 vac- uum design requirements is that the wands and attachments must be bonded and grounded.8
■ There are times when certified equipment is required even in non-classified locations. In the past, companies relied on location class to determine equipment selection. Today, it’s recognized that a combustible dust hazard could be present in a non-classified loca- tion (e.g., a combustible dust spill). As a result, equipment selection needs to be based on the dust conditions, rather than the location.
■ There is no NRTL certification for pneumatic equipment. NFPA 652 doesn’t mention pneumatic equipment, either to ap- prove it or to restrict it. Pneumatic vacuums can meet the NFPA 652 design requirements for use in combustible dust areas, but they can’t be certified for classified locations.
What Other AHJs Assess Combustible Dust Compliance?
In the intro, we mentioned OSHA and other authorities having jurisdiction (AHJs). While OSHA compliance is often the main focus (perhaps because large violations make the news), there are other state and local bodies—including labor departments, build- ing commissions, and fire marshals—that establish and enforce combustible dust regulations.9 In addition to government agencies, insurance inspectors also want to make sure companies are pro- tecting their assets.
Fortunately, like OSHA, most other AHJs have adopted codes based on NFPA standards, so your compliance efforts will cover all of the bases. However, you should still contact all AHJs in your area to guarantee your i’s are dotted and your t’s are crossed.
Stephen Watkins is an Application Engineer for Nilfisk, Inc. Stephen works directly with Nilfisk’s salesforce to better understand manu- facturers’ housekeeping needs and solve their unique challenges. His technical expertise has played a key role in developing Nilfisk’s indus- trial vacuum product line, as well as technical documents and articles that educate and raise awareness on combustible dust.
REFERENCES
1. https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list- of-codes-and-standards/detail?code=652
2. https://dustsafetyscience.com/2019-report-summary/
3. https://ohsonline.com/webcasts/2019/07/3-steps-to-combustible-dust-
compliance-in-manufacturing.aspx?admgarea=webinars&tc=page0
4. https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list- of-codes-and-standards/detail?code=654
5. https://www.osha.gov/OshDoc/Directive_pdf/CPL_03-00-008.pdf 6. https://www.osha.gov/dts/otpca/nrtl/nrtllist.html
7. https://news.nilfiskcfm.com/2017/03/atex-certification-applicable- north-america/
8. https://news.nilfiskcfm.com/2016/11/vacuum-cleaner-nfpa-652- compliant/
9. https://news.nilfiskcfm.com/2019/05/combustible-dust-regulations- codes-and-resources-by-state/
16 Occupational Health & Safety | JANUARY/FEBRUARY 2020
www.ohsonline.com