Page 8 - OHS, May 2021

COMBUSTIBLE DUST
What is the Big Deal about Combustible Dust Vacuum Cleaners?
Industrial vacuum cleaners require expert design and the options to remove fugitive combustible dust that cause catastrophic secondary dust explosions.
SBY DOAN PENDLETON
tandard industrial vacuum cleaners are vulnerable to ignitionmakingthemunsuitable for combustible dusts. Any time
there is powder flowing in one direction through a plastic vacuum-cleaning hose it can create significant static electric charge. If a charged, ungrounded hose used to vacuum combustible dust powder were to come into contact with a grounded object, the static electricity could arc and trigger a violent explosion.
There is also the possibility that there may be static electricity build-up on individual dust particles sucked into the vacuum which is another reason why combustible dust vacuum cleaners need redundant grounding in five different ways, eliminating the possibility of any kind of explosion from the vacuum cleaner.
Combustible dust vacuum cleaners are designed to remove fugitive combustible dust safely and efficiently. They include portable drum style vacuums, stationary continuous duty central vacuum systems and portable breakaway vacuum cleaners that are an economical alternative to central vacuum cleaning systems.
Fugitive dust, or particulate matter (PM), is basically any solid or liquid suspended in the air through wind or human interaction. While half of all fugitive dusts are greater than 10 microns (human hair is 70 microns) and settle on surfaces rather quickly, the other half are smaller than 10 microns (not visible to the naked eye) and can remain suspended in the air for days or weeks settling on the top of equipment, rafters and ducting.
NFPA 652 defines combustible dust as, “a finely divided combustible particulate that presents a flash-fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.”
Combustible dust flash fires occur when a fourth element, the suspension of fuel (dust) into an environment, is introduced to the three elements of the fire triangle, fuel (combustible dust), ignition source (heat) and an oxidizer (usually air).
Dust explosions occur when a fifth element, an enclosed space, is present, forming the dust explosion pentagon comprised of fuel, ignition source, an oxidizer, dispersion of dust particles (in sufficient quantity and concentration) and a confined area (vessel, area or building).
Catastrophic secondary explosions occur when the force from a primary blast wave from an explosion dislodges fugitive dust from surfaces and disperses them into the air, producing dust clouds that ignite, creating a domino-type effect throughout a facility.
If you mitigate the accumulation of fugitive combustible dust to limits below a sufficient quantity and concentration, secondary dust explosions are unlikely.
While each combustible dust has different thresholds that constitute sufficient quantities and concentrations that lead to explosions, OSHA guidelines state, “in general, a thickness greater than 1/32 of an inch is cause for concern when the surface area covered by settled dust exceeds five percent of the floor area in a given room.”
Many facilities appear to be free of concentrations of dust at eye level, but the culprit behind many secondary dust explosions is the accumulation of dust in out of sight places such as overhead beams, joists, duct work and the tops of equipment. According to OSHA’s National Emphasis Directive on Combustible Dusts CPL 03-00-008 section IX.E.3.c, all surfaces need to be included in the hazard evaluation noting that the available surface area of bar joists, steel beams, tops of machinery and rough wall surfaces serve as significant dust accumulation points.1
As of September 2020, NFPA 652 requires organizations that deal with combustible dusts to have a Dust Hazards Analysis (DHA) on hand, that identifies and evaluates fire, flash fire and explosion hazards throughout a facility as well as safeguards in place that adhere to the general requirements for mitigation, including “housekeeping thresholds for dust accumulation levels.”
Broadly speaking, vacuum cleaning is the preferred method to remove combustible dust accumulations. Sweeping creates airborne dust, moving that dust somewhere else, but industrial vacuums suck up and contain all the dust.
Prior to OSHA’s National Emphasis Program (NEP) on Combustible Dust, some facilities would attempt to use shop-type vacuums similar to what people have in their garages to remove combustible dust from surfaces. Those types of vacuums not only create sparking hazards but are ineffective at sucking up fine dust particles or heavy materials and often create their own dust clouds when operating.
With combustible dust present in a facility, the use of an intrinsically safe vacuum that adheres to Class II Division 2 design may be necessary even in non- Class II Division 2 areas. Early in the NEP OSHA issued numerous citations for using standard vacuum cleaners where Class II Division 2 equipment is required.
Every plant has unique processes and thresholds when it comes to combustible dust. There is no one size fits all vacuum cleaning applications and no single standard, or one industrial vacuum cleaner that can meet the requirements
8 Occupational Health & Safety | MAY 2021
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Image Courtesy of Pendleton