Designing Your Dust Collection System to Meet NFPA Standards — Part I
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As appeared in PBE December 2008 www.powderbulk.com Designing your dust collection system to meet NFPA standards — Part I Gary Q. Johnson Workplace Exposure Solutions tion — differ from those for the published data for the same material, producing different combustible dust re - About 40 percent of combustible dust explosions re - sults. The only way to determine your dust’s combustibil - ported in the US and Europe over the last 25 years ity is to have a qualified laboratory run explosion tests on a have involved dust collectors. Dust collection sys - representative sample of the dust. Then, to meet NFPA re - tems are now a primary focus of inspections re - quirements, you’ll need to commission a hazard analysis quired by OSHA’s National Emphasis Program on of your dust collection system to document that its design safely handling combustible dusts. 1 OSHA also has mitigates the explosion risk posed by your dust. (For more information, see reference 4.) the authority to enforce National Fire Protection As - sociation (NFPA) standards for preventing or pro - tecting against dust explosions. This two-part article Some dust explosion basics focuses on how you can design your dust collection The five elements required for a dust explosion can be pic - system’s dust collector, ductwork, and exhaust fan tured as a pentagon, as shown in Figure 1. The three ele - to meet the intent of these NFPA requirements. Part ments labeled in black are those in the familiar fire II will appear in January. triangle: fuel (combustible dust), an ignition source, and Figure 1 he explosion hazards posed by dusts commonly han - Dust explosion pentagon dled in bulk solids plants can be surprising. In fact, Ignition Tmost natural or synthetic organic dusts and some source metal dusts can explode under the right conditions. You can find a limited list of combustible dusts and their explosion data in the appendix to the NFPA standard focusing on dust Confinement explosion hazards, NFPA 68: Standard on Explosion Pro - Dust dispersion of dust cloud 2 at or greater in equipment tection by Deflagration Venting (2007), and in Rolf K. Eck - than dust’s MEC hoff’s book Dust Explosions in the Process Industries. 3 or building While such published data can give you some idea of your dust’s explosion hazards, using this data for designing ex - plosion prevention or protection equipment for your dust Fuel Oxygen isn’t recommended. Your processing conditions and your (combustible dust) dust’s characteristics — such as its particle size distribu - oxygen. For a dust explosion, two more elements (labeled vention of Fire and Dust Explosions from the Manufactur - in red) are required: dust dispersion at or greater than the ing, Processing, and Handling of Combustible Particulate dust’s minimum explosible concentration (the lowest dust Solids (2006). 2 First, the conveying air velocity must be concentration that will propagate a combustible dust defla - adequate throughout the duct. Second, at points where two gration or explosion; MEC) and confinement of the dust airstreams merge, the duct sections must join in a way that cloud within equipment or a building. maintains this velocity. Put simply, a dust explosion occurs when an ignition If your plant handles a combustible dust, a visiting OSHA source touches a dust cloud with a concentration at or inspector will ask whether the conveying air velocity greater than the dust’s MEC. A dust cloud with this con - through your dust collection system is adequate — and 1 centration can result when a layer of dust thicker than ⁄32 will ask you to prove it. Why is this velocity important? inch on equipment, piping, overhead conduit, or similar Keeping the conveying air velocity in every part of the duct components is pushed into the air by some event, such as within a reasonable range will prevent two problems: Too the pressure wave from a relief device’s operation. When low an air velocity will cause the dust to drop out of the air an ignition source — such as a spark or the flame front and build up inside the duct, and, depending on the dust’s from an equipment explosion — touches the cloud, the characteristics, too high an air velocity will waste energy, dust can explode with devastating impact, as evidenced by erode the duct, or, if the dust is moist or sticky, cause the the fatal results of the sugar refinery explosion in Georgia dust to smear on the duct wall. last February. To mitigate your dust collection system’s explosion risk, you need to focus on preventing dust accu - mulation in the system, preventing ignition, and providing explosion prevention or protection at the collector — all covered by NFPA standards. If your plant handles a combustible dust, a visiting OSHA inspector will ask whether the conveying air Even when a dust collector is equipped with an explosion velocity through your dust collection system is vent that works properly, the ductwork in the dust collec - adequate — and will ask you to prove it. tion system can propagate a collector dust explosion throughout a process area. An investigation into one such case revealed that a contributing factor was the ignition and explosion of dust that had accumulated in the duct - A conveying air velocity between 3,500 and 4,000 fpm work because of the system’s inadequate conveying veloc - (17.5 and 20 m/s) is a reasonable starting point for design - 5 ity. Another contributing factor was the lack of ing your system. Then, based on supporting data about flame-front-isolation devices in the collector’s dirty-air your application, you can speed or slow the conveying air inlet and the clean-air outlet for recirculating air to the to the system’s optimal velocity. For instance, if your sys - building. Such devices could have prevented the flame tem handles an extremely fine, lightweight material that front in the collector from entering the inlet duct and re-en - won’t clump together, like cotton dust, you can slow the tering the building through the outlet duct. velocity to 3,000 fpm; if you handle a very heavy material, like lead dust, you may need to increase the velocity to In this case as in many others, following the requirements 4,500 to 5,000 fpm. For guidance in determining the opti - in NFPA standards for mitigating explosion risks in a dust mal velocity for your application, see Table 5-1 in the collection system could have prevented the dust explosion American Conference of Governmental Industrial Hy - from propagating beyond the dust collector. In the follow - gienists’ Industrial Ventilation: A Manual of Recom - 6 ing sections, we’ll look at how you can design your dust mended Practice for Design (26th edition, 2007), which collection system to meet the NFPA standards. Informa - lists minimum duct design air velocities for many dusts. tion covers preventing dust accumulation in ductwork, eliminating ignition sources, and using explosion preven - How duct sections are joined in your system also affects tion and protection methods at the collector. [ Editor’s the conveying air velocity. If incorrectly designed, the note: Capture hood design, another important factor in de - point where ducts join to merge two airstreams can slow signing a safe dust collection system, is beyond this arti - the air velocity, in turn causing the dust to drop out and ac - cle’s scope; for more information, see the later section cumulate in the duct. You can prevent this problem by con - “For further reading” or contact the author.] necting each branch duct to a 15-degree tapered expansion on the main duct, which enlarges the main duct diameter to a size appropriate for the merged airstreams. A related Preventing dust accumulation in ductwork problem is that dust particles can drop out of the airstream To prevent dust from accumulating in your dust collection when a branch duct joins the main duct at too great an system’s ductwork and becoming fuel for an explosion, angle. The momentum of the conveyed dust particles you must design all ducts in the system with two principles causes them to want to move in a straight line, so when one in mind, as described in NFPA 654: Standard for the Pre - duct joins another at a sharp angle, the particles have to change direction and slow down. Avoid this problem by larges from 8 to 9 inches, and the downstream main duct, designing the branch duct entry with no more than a 30- C, is 9 inches in diameter. Before the blank flange was in - degree angle to the main duct. stalled, the system’s design airflow met the air velocity re - quirements at A (4,200 fpm [1,500-cfm airflow]), B (3,900 fpm [350-cfm airflow]), and C (4,100 fpm [1,850- Failing to practice these duct design principles can lead to cfm airflow]). But with A blanked off, the required air ve - any of several problems that produce a slower-than-re - locity through the 4-inch-diameter duct (B) is now 3,900 quired conveying air velocity in your ducts. Following are fpm (350-cfm airflow). The exhaust fan might be able to some visual clues that indicate the air velocity in the ducts pull an airflow of no more than 600 cfm through B and C, isn’t high enough to prevent dust from dropping out of the which would drop the air velocity at C from the required air. Under each clue, ways to remedy the problem and get 4,100 fpm to 270 fpm. adequate conveying air velocity though the ducts are de - scribed.