This is a touchy topic and greatly misunderstood. Today, plant operators recognize that there is more danger from lawyers, on this issue, than from actual explosions in the dust collector. In fact, the accumulation of dust in the plant itself is a greater danger for conflagration than a properly engineered dust collection system. Go to our website and view the videos on “Combustible dust in the workplace” by 60 Minutes on CBS and the U.S. Chemical Safety and Hazard Investigation Board.
Go to view .... Combustible dust videos
Explosions in Dust Collectors
Explosions in pulse jet collectors invariably are when cleaning off-line and can be prevented by sound practice. We run into a risky process when we write the safe procedure because it is application dependent and relies on common sense of the operators. If your design is good engineering, there will be no explosion. All explosions in pulse jet dust collectors, we have investigated (about 100 or so), have been clear cut stupidity. Among the typical ones; horseplay, ignoring warnings posted on equipment and disgruntled employee sabotage.
The most risky application is a mechanical cleaning (shaker style) dust collector when cleaned off-line, and, you can only clean them off-line. A dangerous spark is generated by static charges produced during the shaker action. If I were there, I would tell someone else to turn off the collector or put a long delay on the shaker actuator while I go to the restroom. There is no particular reason to warrant us to observe an explosion first hand. I am cowardly since I heal from injuries slowly.
Advanced Technology vs Poorly Designed Dust Collectors
An overwhelming number of explosions have occurred on badly designed collectors with bottom inlets in which the fine dust has difficulty in making its way to the hopper until the fan is shut down. Advanced technology dust collectors (such as ULTRA-FLOW), with their high side inlet and high ratio cleaning system, have some marked advantages that further reduce the risks involved in explosions. One major advantage is the extremely high efficiency of these designs which prevents dust being returned to the plant, thereby reducing the hazard of accumulated dust referred to the introductory paragraph of this bulletin. The cleaning system more thoroughly cleans the bags and the inventory of dust on the bags is very low, and usually not sufficient to cause the dust concentration to go above the lower explosive limit in the event of an explosion front traveling into the collector from the inlet ducts. With these collectors, the fire and explosion generally occurs outside the collector, in the ductwork, and is drawn into the collector. In normal operation there is only a very small part of the collector that passes through the lower explosive limit. This consists of a narrow band about 1/8 to 1/4 inches thick that surrounds the bag when it is cleaned. Eggshell or singed finishes on the filter bag is recommended to further reduce dust inventory on the bags.
Placing the explosion vent below the filters (i.e. in the hopper) is a bad idea. It allows the pressure to build up in the housing before it can be released by an undersized vent in the hopper. This is especially true with conventional dust collector designs that have venturis restricting the neck of the filter cage at the tube-sheet. With advanced technology designs (having no venturi), we have 12-15 times more open area to the outlet which in itself is a natural explosion vent. We place the vent in the housing side where it offers the most protection by venting the explosion immediately where it occurs.
Woodworking
Because of the NFPA rules do not directly apply to dust collectors, there is much latitude in their interpretation. The solution is to apply sound engineering to assess the risk and to provide equipment suitable for a particular service. Venting woodworking applications is probably the largest number of installations in the dust collector industry. Explosions have occurred and the venting has been quite effective in controlling them.
The norm in the industry, for the last 30 or so years, has been to provide a 60:1 vent ratio. This has been sufficient for this service. ULTRA-FLOW uses a standard 20:1 vent ratio for its explosion vents, which further protects against the harmful effects of an explosion.
Vent Ratio
This was developed by UL labs. It is the ratio of the volume of the dirty air compartment of a dust collector to the area of the explosion vent. For example; a cylindrical bag dust collector with (24) 6 inch by 6 foot bags, dirty air housing size of 6ft x 4ft x 6ft, hopper which is 1/3 x ( 6 x 4 x6 ). The gross volume of the collector = 192 cu.ft. The volume of the bags is 24 x 1.2 cu.ft./ bag = 28.2. The volume of the collector = 192 – 28.2 = 164 cu.ft. Therefore, if we want a vent ratio of 20:1; 164/20 = 8 sq.ft. of explosion vent.
Ultra-Flow dust collectors use a vent ratio of 20:1. In general the insurance companies determine the specification that they want and we supply it accordingly. In the end, good engineering is the key.
Kst Ratings
This issue is very complex and not as easy as just meeting a “Kst” deflagration rating. It is an NFPA 68 test requirement for ideal lab conditions. “Kst” refers to the rate of pressure rise in an explosion. Unfortunately defining of the number is difficult since NFPA never really measure it except when they use a sealed globe enclosure, stir the dust in it and then try to ignite it with a sparkplug. This is not the real world of dust collectors.
A more accurate test was performed by AAF specifically on dust collectors. See the “Combustible Dusts” chart at the end of this bulletin. That chart shows the “Explosion Pressure” or burst pressure where theoretically a dust collector will blow apart in an explosion. If a dust collector is built of 12 gage steel to withstand +/- 20 SP (inWG). The burst pressure is usually a factor of 4 times that or 80 psi. As an example, for wood dust it was determined that a vent ratio of only 180:1 was safe in a dust collector. The chart says the burst pressure would be 35psi which is less than the 80psi allowed. Ultra-Flow uses a 20:1 vent ratio, therefore it is 9 times that value, so, you are as safe as you can get. No matter what you do, there will always be some risk. All we can do is make it inconsequential. As mentioned above, there is a far greater risk from dust in the plant than you will find in the dust collector itself. Look on the home page of our website for the news reports on “Combustible Dusts in the workplace”.
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