Protecting the Plant from Catastrophic Combustible Dust Explosions
Industrial vacuums are the right tool for preventing secondary explosions.
- By Doan Pendleton
- Sep 01, 2020
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. They stated 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.”
Even without a sufficient vacuum cleaner for industrial environments in 1922, 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 and could 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), is in an enclosed environment and there is an ignition source and oxygen present.
“The explosion is caused by the rapid pressure rise as a result of the rapid burning of the dust cloud,” Bill Stevenson, VP of Engineering at Cv Technology and NFPA 654 committee member, said. “So, it has to be in an enclosure. If it were outside you’d just have a big flash.”
Cv Technology is a Florida-based corporation dedicated to the prevention, protection and mitigation of industrial dust explosions and related fires.
Stevenson further 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 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 titled, 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 percent involved shop tools or industrial equipment, and the manufacturing area was the leading origin of the fires.
Controlling the Explosion Pentagon
The explosion pentagon includes the three elements of the fire triangle: fuel (combustible dust), ignition source (heat) and an oxidizer (air). However, and explosion also 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 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. This is equally important in preventing dust explosions.
Industrial vacuum cleaners to control fugitive combustible dust should be suitable for use in Class II, Division 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,” said 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 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 Division 2 equipment is required.
The Right Tool for Combustible Dust
Bill Bobbitt of Bobit Associates Environmental Systems has been working in the clean air industry for over 25 years. He always tells his clients, it isn’t a matter of it, but when.
“Conditions have to be perfect and that ‘when’ can be 30 years from now, or it could be next week,” Bobbitt said. “But if you eliminate the fugitive dust, it cannot create a secondary dust explosion.”
Bobbitt sees many standard shop type vacuums in plants. “There are so many problems with them. They themselves are hazards in an industrial environment,” Bobbit said.
First and foremost, they are not grounded or classified for Class II Division 2 areas. They also shock workers, clog easily and the workers do not want to use them. If workers do not 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.
Although some manufacturers like VAC-U-MAX do make electric vacuums designed for Class II Division 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 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, 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, but they also work more efficiently in the industrial environment. Bobbitt said 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 because after 20 minutes, the filters would bind and workers did not want to use them because they would have to lift the head from the vacuum and tap the cake off to get any more suction.
Compliance When Regulations Are Not 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,” said 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 “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,” he said.
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. In this instance, 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 said, that when you are dealing with explosive dust, you may need a Class II Division 2 vacuum in a non Class II Division 2 area.
“You might have explosive dust small quantities, and it might take a very hot and prolonged source of ignition,” Bobbit said. “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 regulations for combustible dust change, Bobbitt said, “I find that a lot of companies are simply just trying to get better at general housecleaning.”
Cv Technology’s Stevenson agrees with the practice of keeping things clean.
“The one thing you can do very simply and easily is to keep everything clean—it is as simple as that,” Stevenson said. “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 and those are pretty straight forward easy things that everyone can do.”
This article originally appeared in the September 2020 issue of Occupational Health & Safety.