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The issue of dust explosions has been a hot topic since the early 20th century. In Dust Explosions, a book published by the National Fire Protection Association (NFPA) in 1922, authors David J. Price and Harold H. Brown acknowledge the need for a vacuum that can withstand the rigors of an industrial environment. They state 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.” In addition, the authors knew that “if there is no accumulation of dust and the plant is perfectly clean, the explosion cannot propagate and the plant will not be destroyed.”
Even without a sufficient vacuum cleaner for industrial environments at the 1922 publication of the book, the authors still warned against using brooms and
compressed air in housekeeping practices because those methods often cause dust to be suspended in the environment during cleaning. This suspended dust could itself ignite or settle back onto floors, equipment and beams, lending itself to potential secondary explosions later.
Primary dust explosions occur when combustible dust is present, forms a dust cloud (in sufficient amounts) in an enclosed environment, and is exposed to an ignition source and oxygen. “The explosion is caused by the rapid pressure rise as a result of the rapid burning of the dust cloud,” says Bill Stevenson, VP of Engineering at Cv Technology and NFPA 654 committee member. “So it has to be in an enclosure. If it were outside, you’d just have a big flash.”
Stevenson further explains that if a layer of combustible dust was 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 throw the same dust 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-type effect that can cause further explosions. To use the same example of the smoldering dust on the desk, if you waved a piece of paper to make the particles airborne, a dust cloud could form and explode.
According to the NFPA Fact Sheet “U.S. Industrial and Manufacturing Property Structure Fires,” “U.S. fire departments responded to an estimated average of 10,500 structure fires in industrial and manufacturing properties per year in 2003-2006,” averaging 29 fires per day in the industrial sector. Of those fires, 29% involved shop tools or industrial equipment, and the manufacturing area was the leading origin of the fires.
As shown in Figure 1, the explosion pentagon includes the three elements of the fire triangle: fuel (combustible dust), ignition source (heat) and an oxidizer (air). Two additional elements are required: the dispersion of dust particles (in sufficient quantity and concentration) and the confinement of the dust cloud (vessel, area or building). If one of these elements is missing, a fire or explosion cannot occur.
While it is difficult to remove air and fuel from the triangle, the first rule of fire prevention—and therefore explosion prevention—is to eliminate the ignition source. While most machinery manufacturers design equipment with safety in mind, mechanical equipment is capable of malfunctioning, overheating and causing ignitions. 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 equally important to prevent dust explosions.
Industrial vacuum cleaners that are used 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,” says Stevenson.
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 buildup 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 OSHA has issued numerous citations for using standard vacuum cleaners where Class II Div 2 equipment is required.
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 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. But if you eliminate the fugitive dust, it cannot create a secondary dust explosion.”
Bobbitt sees a lot of standard shop-type vacuums in plants. “There are so many problems with them,” he says. “They themselves are hazards in an industrial environment.” First and foremost, they are not grounded or classified for Class II Div 2 areas. In addition, they often shock workers and clog easily, so workers don’t want to use them. If workers don’t use them, fugitive dust is accumulating in the plant.
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 electric vacuums can be designed for Class II Div 2 environments, the most economical solution for cleaning combustible fugitive dust is an air-operated vacuum.
Air-operated vacuums use no electricity and have no moving parts. Grounding 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, 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-gal drum eliminates the potential for arcing.
Not only are air-operated vacuums for combustible dust safer in terms of grounding, they also work more efficiently in the industrial environment. Bobbitt says on a recent visit to a coal-fired electric power plant, he was shown five different electric vacuums sitting in a warehouse not being used. After 20 minutes, the filters in these vacuums would bind and workers didn’t want to use them because they would have to lift the head from the vacuum and tap the cake off before they could get any more suction.
According to Bobbitt, the power plant and two sister facilities “now use an air-powered model* with a pulse cleaning system on the filters. With the push of a button, the dust is released from the filter and workers can resume cleaning.”
Fugitive dust “is a moving target that changes depending on the nature of the process and how well plants manage keeping the dust contained,” says Stevenson. Most NFPA guidelines for combustible dust state that a layer of dust the thickness of a paperclip is enough to cause a significant secondary explosion. The problem, he says, “is that it doesn’t account for the different Kst values between different dusts. Some are more reactive than others. Some are more easily suspended into a cloud. Some tests found that, depending on the type of dust, even half of the thickness of a paper clip would be too much.”
Kst values classify dusts according to their explosivity—the rate of pressure rise of a dust in the test vessel upon being ignited. In situations where many different dusts are handled, testing all of them can be prohibitively expensive. For instance, in a high-performance rubber plant where several different products are manufactured within the same plant, the dust in each area of the plant may have different Kst values. For these situations, it is recommended that manufacturers work with an expert in the field to select samples for testing that represent the worst-case scenario.
According to Bobbitt, this is why companies may need a Class II Div 2 vacuum in a non-Class II Div 2 area. “You might have explosive dust in small quantities,
“The one thing you can do very simply and easily is to keep everything clean—it is as simple as that."
and it might take a very hot and prolonged source of ignition, but with the new combustible dust initiative facilities need to be very careful that they comply because there is a lot of question as to what compliance means. Although the regulations for combustible dust aren’t real clear, I find that a lot of companies are simply just trying to get better at general housecleaning.”
Cv Technology’s Stevenson agrees. “The one thing you can do very simply and easily is to keep everything clean—it is as simple as that,” he says. “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.”
For more information, contact Vac-U-Max at 69 William St., Belleville, NJ 07109; call (800) VAC-U-MAX or (973) 759-4600; email firstname.lastname@example.org; or visit www.vac-u-max.com.