By David Kennedy, General Manager, Vacuum Cleaning Division, VAC-U-MAX The issue of dust explosions has been a hot topic since the early Twentieth Century. In a book published by the National Fire Protection Association (NFPA) in 1922, titled “Dust Explosions,” the authors, David J. Price and Harold H. Brown, acknowledged the need for a vacuum that can withstand the rigors of an industrial environment stating that despite every precaution to capture dust at the source, small amounts of it “will get out into the atmosphere of the mill and gather on floors, walls and ledges.” The authors of the book knew then, as it still stands today, “if there is no accumulation of dust and the plant is perfectly clean, the explosion cannot propagate and the plant will not be destroyed.” 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. Bill Stevenson, vice president, engineering, Cv Technology, and NFPA 654 Committee member, explained that if there was a layer of combustible dust sitting on a desk, “you could get it to burn by putting a flame to it, but it wouldn’t explode. If you took the torch away, it would smolder and most would self extinguish. But, if you take the same dust, throw it in the air and then light it on fire, it would literally blow up in your face.” Catastrophic secondary explosions occur when the force from the primary explosion dislodges fugitive dust, producing more dust clouds, and creating a domino effect that can cause further explosions. So if you took that same dust smoldering on the desk and waved a piece of paper to make the particles airborne, a dust cloud could form and explode. Although every precaution is taken to eliminate ignition sources to prevent fires and dust-collection equipment is designed to safely contain most of the dust in the plant, manufacturers must make housekeeping for fugitive dust that can be formed into a dust cloud equally important to prevent dust explosions. Industrial vacuum cleaners to control fugitive combustible dust should be suitable for use in Class II Div 2 areas. “Vacuum cleaners in particular are vulnerable to ignition and that is why there are only a few companies that know how to do that properly and VAC-U-MAX is one of them that does it right,” said Stevenson. “They take extraordinary care to make sure there is no chance for the product to come into contact with anything ignitable.” 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 powder were to contact an object that was grounded, the static electricity could then arc and trigger a violent explosion. This is why Occupational Safety and Health Administration (OSHA) has issued numerous citations for using standard vacuum cleaners where Class II Div 2 equipment is required. Right Tool For Combustible Dust Bill Bobbitt, Bobit Associates Environmental Systems, who’s been working in the clean-air industry for over 25 years, said he always tells his clients “it is 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.” If you eliminate the fugitive dust, there can be no secondary dust explosion. Employing an industrial vacuum cleaner that is redundantly grounded in five different ways, “eliminates the possibility of any kind of explosion from the vacuum,” he explained. Beyond the fact that air-operated vacuums use no electricity and have no moving parts, the first of the five ways that our 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 conducts electricity, our air-operated vacuums use static-conductive, high-pressure, compressed-air lines. Static conductive hoses, filters and casters are employed to further reduce risk in addition to the static-conductive air lines, 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 these also work more efficiently in the industrial environment. On a recent visit to a coal-fired electric power plant, Bobbitt was shown five different electric vacuums sitting in a warehouse and not being used. The workers said that after 20 minutes, the filters would bind and they 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 now two sister facilities, Bobbitt said, “now use an 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 so they can resume cleaning.” Compliance When Regulations Aren’t Clear Fugitive dust “is a moving target that changes depending on the nature of the process and how well plants manage keeping the dust contained,” explained Stevenson. “The one thing you can do very simply and easily is to keep everything clean -- it is as simple as that. If you clean the place up and protect your dust collectors, you’ve gone a long way toward minimizing the chance for an explosion even if you do nothing else. Those are pretty straightforward, easy things that everyone can do.” Editor’s Note: David Kennedy is the general manager of the vacuum cleaning division at VAC-U-MAX. He has been assisting the manufacturer's design and engineering in heavy-duty, industrial-vacuum cleaning systems for over 20 years. VAC-U-MAX, the pioneer in industrial vacuum cleaning and pneumatic conveying since 1954, makes a line of combustible dust vacuums that are redundantly grounded and ideal for combustible dust. For more information, 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 email@example.com; or visit the website.
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