Regardless of how much prevention is employed to mitigate combustible dust in coal-fired power plants, fugitive dust coal dust is pervasive and employing industrial vacuum cleaners, built from the bottom up to be used in Class II Div 2 areas, should be the first defense in housekeeping routines to prevent catastrophic explosions and comply with regulatory agencies.
In coal fired power plants mechanical transfer points are leading sources for airborne fugitive dust; but, since coal dust travels quickly over large areas with minimal airflow, fugitive combustible dust settles in many areas.
Primary dust explosions occur when combustible dust is present, forms a dust cloud (in sufficient amounts), in an enclosed environment, with an ignition source and oxygen.
If one were to put a flame to a layer of combustible dust on a desk, the dust would burn, but not explode. However, fanning the dust with a piece of paper to make the dust particles airborne, would create a dust cloud and blow up.
Catastrophic secondary explosions occur when the force from the primary explosion dislodges fugitive dust that is allowed to accumulate on walls, floors, and horizontal surfaces such as equipment ledges, above suspended ceilings and other concealed surfaces, producing more dust clouds, and creating a domino type effect causing further explosions.
Bill Bobbitt of Bobit Associates Environmental Systems, who’s been working in the clean air industry for over 25 years, says, “I always tell my clients, it not a matter of if, but when. Conditions have to be perfect and that ‘when’ can be 30 years from now, or it could be next week. But if you eliminate the fugitive dust, it cannot create a secondary dust explosion.”
Perils of Standard Shop Type Vacuums
Any time there is powder flowing in one direction through a plastic vacuum-cleaning hose it can create a significant static electric charge. In addition, there is the possibility that there may be static electricity build-up on individual dust particles. If a charged, ungrounded hose used to vacuum combustible dust were to contact an object that was grounded, the static electricity could then arc and trigger a violent explosion. This is why OSHA has issued numerous citations for using standard vacuum cleaners where Class II Div 2 equipment is required.
Bobbitt sees a lot of standard shop type vacuums in plants. "There are so many problems with them. They themselves are hazards in an industrial environment," he says. First and foremost, they are not grounded or classified for Class II Div 2 areas, they shock workers, they clog easily and the workers don’t want to use them, and if workers don’t use them, fugitive dust is accumulating in the plant.
Bobbitt says during a recent presentation to Kansas City Light and Power’s Coal Handling Group, where safety professionals from each of the power stations get together to discuss proactive solutions, he was shown five different expensive Class II Div 2 electric vacuums sitting in a warehouse not being used because after 20 minutes, the filters would bind and workers just didn’t want to use them because they would have to lift the head from the vacuum and tap the cake off before they would get any more suction.
The power plant, and five sister facilities, “now use a Class II Div 2 air powered VAC-U-MAX model with a pulse cleaning system on the filters, that with the push of a button releases the dust from the filter and they can resume cleaning,” says Bobbitt.
VAC-U-MAX, located in Belleville, NJ, developed the first air operated industrial vacuum in 1954 and has been the pioneer in solving vacuum-related challenges in a wide range of manufacturing and industrial settings.
Redundantly Grounded Class II Div 2 Industrial Vacuum Cleaners
Employing an industrial vacuum cleaner that is redundantly grounded in five different ways, “eliminates the possibility of any kind of explosion from the vacuum,” says Bobbitt. Although Vac-u-Max does produce electric vacuums designed for Class II Div 2 environments, the most economical solution for cleaning combustible fugitive coal dust is the company’s air-operated vacuums.
Beyond the fact that air operated vacuums use no electricity and have no moving parts, the first of the five ways that VAC-U-MAX vacuums are grounded begins with the air line that supplies the compressed air to the units.
Because most plants have compressed air lines made from iron that conduct electricity, the company’s air operated vacuums use static conductive high-pressure compressed air lines. In addition to the static conductive air lines, static conductive hoses, filters and casters are employed to further reduce risk. A grounding lug and strap that travels from the vacuum head down to the 55-gallon drum eliminates the potential for arcing.
Air operated vacuums for combustible dust are safer in terms of grounding and they also work more efficiently in the industrial environment.
Bobbitt says that when you are dealing with explosive dust, you may need a Class II Div 2 vacuum in a non Class II Div 2 area. “You might have explosive dust small quantities, and it might take a very hot and prolonged source of ignition, but with OSHA’s Combustible Dust NEP, facilities need to be very careful that they comply because there is a lot of question as to what compliance means.”
According to OSHA, housekeeping ranked second in citations under the NEP “with respect to combustible dust related hazards.” In addition, OSHA issued General Duty Clause citations for the practice of blowing dust with an air compressor, and not using electrical equipment that was designed for hazardous (classified) locations. In the Electric Services Industry Group, during October 2010 through September 2011, the General Duty Clause was one of the top ten most frequently cited by Federal OSHA.
“Although the regulations for combustible dust aren’t real clear,” Bobbitt says, “I find that a lot of companies are simply just trying to get better at general housecleaning.”
For more information about combustible dust industrial vacuum cleaners or to learn about VAC-U-MAX pneumatic conveying systems, write to them at 69 William Street, Belleville, NJ 07109; call 1-800-VAC-U-MAX (800) 822-8629 or (973) 759-4600; e-mail firstname.lastname@example.org; or visit their website www.vac-u-max.com.
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