Protecting The Plant From Combustible Dust
The issue of dust explosions has been a hot topic since the early 20th century. In a book published by the NFPA in 1922 titled Dust Explosions, authors David J. Price and Harold H. Brown acknowledge the need for a vacuum that can withstand the rigors of an industrial environment. Despite every precaution to capture dust at the source, says the book, small amounts of it “will get out into the atmosphere of the mill and gather on floors, walls and ledges.”
Even without a sufficient vacuum cleaner for industrial environments at the 1922 publishing 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, which could itself ignite, or would 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, with an ignition source and oxygen.
Bill Stevenson, VP of Engineering at Cv Technology and NFPA 654 committee member, says, “the explosion is caused by the rapid pressure rise as a result of the rapid burning of the dust cloud. So it has to be in an enclosure. If it were outside you’d just have a big flash.”
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. 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.
According to an NFPA Fact Sheet, U.S. fire departments responded to an estimated average of 10,500 structure fires in industrial and manufacturing properties per year in 2003-2006. Of those fires, 29 percent involved shop tools or industrial equipment, and the manufacturing area was the leading origin of the fires.
Controlling The Pentagon
The explosion pentagon includes the three elements of the fire triangle: fuel (combustible dust), ignition source (heat), and an oxidizer (air), but needs two additional elements — dispersion of dust particles (in sufficient quantity and concentration), and the confinement of the dust cloud (vessel, area, or building).
If one of the 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, heating up, and causing ignitions.
Although every precaution is taken to eliminate ignition sources to prevent fires, and dust collection equipment 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.
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 OSHA has issued numerous citations for using standard vacuum cleaners where Class II Div 2 equipment is required.
Tools For Combustible Dust
Bill Bobbitt, of Bobbitt Associates Environmental Systems, has been working in the clean air industry for over 25 years. “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,” he says.
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. 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 VAC-U-MAX does make electric vacuums designed for Class II Div 2 environments, the most economical solution for cleaning combustible fugitive dust is its 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 conducts 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.
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,” says Stevenson. Most NFPA guidelines for combustible dust state that a layer of dust the thickness of a paperclip is enough dust 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 in each area. For this circumstance it is recommended to work with an expert in the field to select samples for test that represent the worst case.
This is why, 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 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.”
Doan Pendleton is an expert in vacuum technology with over 20 years of experience. For more information about combustible dust industrial vacuum cleaners, visit www.vac-u-max.com.