The Right Tool for Combustible Dust
- By Doan Pendleton
- Jun 01, 2012
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, 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."
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 and 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 and forms a dust cloud (in sufficient amounts) in an enclosed environment, with 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. So it has to be in an enclosure," said Bill Stevenson, vice president of engineering at Cv Technology Inc. and an NFPA 654 committee member. "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 explained that a layer of combustible dust on a desk could be ignited 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 of effect that can cause further explosions. Taking that same dust smoldering on the desk and waving a piece of paper to make the particles airborne could form an explosive dust cloud.
An NFPA Fact Sheet titled "U.S. Industrial and Manufacturing Property Structure Fires" says U.S. fire departments responded to an estimated average of 10,500 structure fires in industrial and manufacturing properties per year in 2003-2006, an average of 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) -- with two additional necessary 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.
Most machinery manufacturers design equipment with safety in mind, but 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 equally important, in order 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," Stevenson said. "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 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.
The Right Tool for Combustible Dust
"I always tell my clients, it's not a matter of if, but when," said Bill Bobbitt of Bobit Associates Environmental Systems, who has worked in the clean air industry for more than 25 years. "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 said he 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 said. First and foremost, they are not grounded or classified for Class II Div 2 areas. They shock workers, they clog easily, and 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," said Bobbitt.
Air operated vacuums for combustible dust are safer in terms of grounding, and they also work more efficiently in the industrial environment. During a recent visit to a coal-fired electric power plant, Bobbitt said he was shown five different electric vacuums sitting in a warehouse and not being used because, after 20 minutes, the filters would bind and workers just didn't want to use them. They would have to lift the head from the vacuum and tap the cake off before they would get any more suction. That power plant and two sister facilities now use an air-powered model, he said.
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," Stevenson said.
Most NFPA guidelines for combustible dust state that a layer of dust the thickness of a paper clip is enough to cause a significant secondary explosion. The problem "is that it doesn't account for the different Kst values between different dusts," he said. "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 this circumstance, it is recommended to work with an expert in the field to select samples for testing that represent the worst case.
This is why, when dealing with explosive dust, you may need a Class II Div 2 vacuum in a non-Class II Div 2 area, Bobbitt said. "You might have explosive dust small quantities, and it might take a very hot and prolonged source of ignition," he 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 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 agreed. "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. And those are pretty straightforward, easy things that everyone can do."
This article originally appeared in the June 2012 issue of Occupational Health & Safety.