The Finer Points of Combustible Dust Compliance Requirements

The Finer Points of Combustible Dust Compliance Requirements

Companies that don’t recognize the risks of combustible dust and fail to mitigate them are quite literally playing with fire.

If you’re involved in any kind of industrial processing, then by now you should be familiar with NFPA 652, the National Fire Protection Association’s Standard on the Fundamentals of Combustible Dust.1 First issued in 2016 and updated for 2019, NFPA 652 sets the requirements used by OSHA and other authorities having jurisdiction (AHJs) to determine if companies are doing what they need to do to keep their workers safe from combustible dust hazards.

Judging by the 2019 Mid-Year Combustible Dust Incident Report, many companies still haven’t taken the required compliance steps.2 Between January and July of this year in the United States and Canada alone, there were 92 fires and 20 explosions, resulting in 36 injuries and one fatality. At least four of the incidents resulted in losses of more than $1 million. Keep in mind that these are only the incidents that were reported in publicly-available sources, like news outlets. The real number is undoubtedly much higher.

Almost any dust except regular dirt can be combustible under the right conditions, and every small fire has the potential to become a catastrophe. Companies that don’t recognize these risks and fail to mitigate them are quite literally playing with fire.

Many articles in this publication have addressed the broad strokes of NFPA requirements. In this article, we’ll dig into some specific issues that give rise to the most common questions when performing cleaning compliance assessments at industrial processing facilities.

When to Use Historical or Published Data to Determine Dust Combustibility or Explosibility
The first thing facilities need to do is to determine whether their dust is combustible or explosible. This is required even if you’ve never had a combustible dust incident.

NFPA 652 provides two methods for evaluating your dust:

  • Using historical facility data or published data that are deemed to be representative of current materials and process conditions
  • Using analysis of representative samples, i.e., dust testing

Obviously, the first option here is the easiest. There is published data on just about every type of material used in processing facilities (see NFPA 652, Appendix A). The trick is that the available data may not be “representative of current materials and process conditions.”

In a recent webinar on combustible dust compliance, Nilfisk’s director of engineering, Norman Nowosinski, gave the example of spice mixes.3 A company might have historical or published data on each spice (e.g., salt, pepper, garlic powder) that goes into a mix. But the properties of the mixture dust will be different from the properties of the individual ingredient dust, so each mixture still needs to be tested.

The bottom line is that you can use historical or published data as long as you can prove that the properties of your dust are the same as the previously tested dust. If they aren’t the same, you’ll need to take the testing route.

The 1/32” Criterion for Dust Accumulation
You may have heard about the 1/32” requirement, aka the paper clip test. This is the idea that dust should not be allowed to accumulate more than 1/32”, or roughly the thickness of a paper clip. Another way of thinking about this is that if there’s so much dust that you can’t see the color of the underlying surface, you should clean immediately. This is a useful rule of thumb, and cleaning whenever you see more than 1/32” of dust certainly isn’t a bad idea. But it’s also not the complete story.

The 1/32” idea is addressed in NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, 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 indicate 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 exceeding 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 accumulation 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, ductwork, 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 equivalent 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 permitted in combustible dust environments. The preferred cleaning methods are vacuuming, sweeping, and water washdown. Compressed 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 Section 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 Nationally Recognized Testing Laboratory (NRTL).6 Unlike in Europe, 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 vacuum 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 location (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 approve 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, building 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 protecting 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.

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This article originally appeared in the January/February 2020 issue of Occupational Health & Safety.

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