Dust Explosions
When a nuisance becomes deadly

NFPA Journal®, November/December 2008

By Amy Beasley Spencer

Many people never knew sugar could explode until February 7, 2008, when an explosion rocked the Imperial Sugar Company in Port Wentworth, Georgia, killing 13 people and injuring 40. Located just outside Savannah, this 91-year-old facility processed granulated and powdered sugar.

While this incident is still under investigation by the U.S. Chemical Safety and Hazard Investigation Board (CSB), the Occupational Safety and Health Administration (OSHA) reported that the initial explosion occurred in a bucket elevator. They believe that activities in the silo created an accumulation of suspended, combustible sugar dust. The explosion started the fire and caused the partial collapse of the four-story building. The initial explosion put more combustible dust into the atmosphere, causing a series of secondary explosions that traveled in succession through the silo gallery and the tunnel.  Fire from the secondary explosions spread through the packing houses and into adjacent buildings.

But the explosion at Imperial Sugar was far from the only appalling dust explosion in the United States in recent years. According to the CSB, there were more than 280 dust explosions from 1980 to 2005, resulting in 119 deaths and 718 injuries. Yet many workers, even those well trained in the safety routines of their facility, are unaware of the hazards of dust.

In 2006, the CSB recognized this knowledge gap and issued a report recommending that OSHA regulate dusts in general industry, based on NFPA standards.1 OSHA has yet to indicate they will initiate a regulatory development project for dusts, but the Imperial Sugar explosion spurred Congress to become involved in the ongoing attempt to compel OSHA to create a dust regulation. As evidence of the need for government regulations, OSHA inspected Imperial Sugar in 2000 and did not issue a single citation, according to Edwin Foulke, assistant secretary of Labor for Occupational Safety and Health, in his testimony before the Senate Subcommittee on Health Education, Labor, and Pensions on July 29, 2008.

Causes of dust explosions
Part of the problem with regulating dust explosions is the confusion about which dusts can explode and under what conditions. Even how much dust is a hazard is still unknown.

A large number of solid substances become explosive when in the form of a fine dust. These substances include organic materials, such as grain, sugar, wood, and coal; synthetic organics, such as plastics, dyes, foams, pharmaceuticals, and chemicals; and combustible metals, such as aluminum, magnesium, zinc, and iron. Not explosive are mineral dusts, such as silicates, sulphates, nitrates, carbonates, and phosphates; salt; gypsum; sand; limestone; and cement.

Generally speaking, the smaller the dust particle, the bigger the hazard. An often-used rule of thumb is that dust of 420 microns particle size (40 mesh sieve) may serve as the fuel in a dust explosion. As a frame of reference, normal table sugar ranges from about 850 microns to 150 microns, with the bulk of sugar between 350 and 450 microns, according to Paul Caulkins, manager of Quality Assurance for Imperial Sugar. Quite frequently, dusts are a mixture, or distribution, of sizes. For mixtures, another rule of thumb is that only 2 percent of the dust needs to be 420 microns or less to be potentially explosive. Fibers, due to their nonspherical shape, may not pass through the 40 mesh sieve, but they can still be a problem, as evidenced by the 1995 Malden Mills explosion in Lawrence, Massachusetts, which involved fabric fibers. (For more on this incident, see sidebar on the next page.)

Five conditions raise the risk of a dust explosion. If a combustible dust is suspended in air or another oxidizing medium, is at the minimum explosible concentration (MEC), is in the presence of an ignition source, and is confined, the conditions are ripe for a dust explosion. Removing at least one element of the dust pentagon is the control strategy in NFPA dust standards.

NFPA’s principal dust document, NFPA 654, Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, covers dust hazard processes and is referenced in the other NFPA dust documents. The commodity-specific dust documents covering coal, sulfur, combustible metals, wood dust, and agricultural dust incorporate hazard identification and control measures similar to those found in NFPA 654, but also address requirements unique to the particular dust. NFPA provides comprehensive coverage of dust hazards in seven dust-related documents and has done so since 1923 with NFPA 61, Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities (see box on the next page).

You may obtain additional dust hazard information from OSHA, which developed a Safety and Health Information Bulletin in 2005 and instituted a National Emphasis Program in 2007.

Is housekeeping the “easy” answer?
Just because the end product at your facility doesn’t involve dusts, it is not necessarily safe from this hazard. When evaluating your facility’s dust hazard, you should consider your entire process, including the raw materials, ingredients, and intermediate products, as well as process by-products. If you work with any solid combustible components of any size or shape, then handling, conveying, or otherwise manipulating these materials anywhere in your process can create combustible dusts. Whenever larger particles are abraded, ground, cut, or sanded, dusts can be created. Even if the solids are typically stored or handled when wet, the same hazard potential arises when those materials become dry. For these reasons, it is essential that your hazard assessment identify all combustible particulate solids in your processes.

Dust layers as thin as 1/32 of an inch (0.8 millimeters) or 1/16 of an inch (1.6 millimeters) can be a problem, so if you see dust, don’t ignore it. Clean it up and examine where it’s coming from. Seal all openings in processing and conveying equipment to prevent the release of dusts into the work environment. When you inspect your workplace, the boundaries will typically include the obvious, visible areas, but you must also consider overhead beams, electrical cable trays, lighting fixtures, equipment surfaces, and “invisible” areas such as concealed areas above false ceilings.

The dust you can’t see is even more dangerous than the dust you can see, because it is more frequently overlooked. Consider the 2003 incident at the West Pharmaceuticals, Inc., in Kinston, North Carolina. In photos taken before the explosion, the facility’s general working area appeared relatively dust-free. But the manufacturing process released dusts that accumulated unseen above a drop ceiling. When the accumulated dust was suspended, concentrated, and ignited, it resulted in an explosion that killed 6 and injured 38.

Good housekeeping might seem like an easy answer to keeping your facility safe, but creating and implementing an effective routine can be complex. Proper housekeeping represents an essential element of a dust hazard safety management program because it limits the fuel that potentially can become airborne and contribute to an explosion.

However, housekeeping is only part of a well-rounded dust management program. Even round-the-clock clean up in a facility where dusts are generated and released at an excessive rate might not be adequate to eliminate the risk. You may still have a layer capable of fueling an explosion.

If you can ensure the dust is never released outside the processing, conveying, or collection equipment, the system will be inherently safer. As much as practicable, containment is important, along with dust collection systems. Conveying and collection equipment should be installed and maintained in a manner that keeps it from becoming an ignition source. Obviously, fire detection and suppression systems in these areas are considered an integral part of fire protection and prevention methods.

Another important concept you must address when reviewing housekeeping is the danger presented by secondary explosions. Secondary explosions occur when the blast wave from the initial explosion, which might not have even involved dust, causes accumulated dust in other areas—perhaps remote, unrelated process areas—to become airborne and explode. This can create a domino effect, possibly leading to more explosions moving through the plant.

The secondary explosion phenomenon highlights the importance of proper housekeeping, not only in the area where dust hazard processes occur, but throughout the facility.

If proper and consistent housekeeping isn’t difficult enough, this preventive measure itself can be the cause of the explosion if it is not performed properly, as was evidenced by the explosion at CTA Acoustics, Inc. in Corbin, Kentucky, in 2003. An oven door was left open and heat ignited a small dispersion of dust that was generated by regular cleaning. (For more on this explosion, see sidebar, right.)

Anecdotally, many dust-industry employees tell investigators of “puffs,” or small explosions, that “happen all the time.” These puffs are warnings that should not be ignored, as they may be a precursor to a larger and potentially deadly explosion. Employees must be trained to recognize the conditions that signal the development of a dangerous situation.

Ensuring your facility is dust-safe
Management’s enthusiasm, or lack thereof, for safety programs is crucial. In the case of Imperial Sugar, that lack may have the difference between life and death.

At the July 29, 2008 Senate hearing, Graham H. Graham, the recently hired vice-president of Operations for Imperial Sugar, testified that he had warned upper management about the dangers of sugar dust at their two facilities. In mid-January, Graham met with the management team at Port Wentworth and told them that
“…some of them would probably not be coming home soon because they would be in the city morgue due to the potential explosive properties…due to the excessive volume of dusts….”

He recounted details of the horrendous conditions at the plant and how Imperial Sugar’s CEO told him just a month before the explosion that, regarding improving the plant’s dust-related conditions, “…his passion was extreme and he had to temper it….” Because of this incident and others, the U.S. House of Representatives has directed OSHA to create a dust regulation through the passage of the Worker Protection Against Combustible Dust Explosion and Fires Act (H.R. 5522).

The NFPA standards that could have prevented those explosions exist, but they are not mandatory nationwide. Whether OSHA will rely on them isn’t known, so facilities shouldn’t wait for them to regulate dusts. The tragic incidents reported by the CSB in their dust study and magnified by this most recent incident provide us with what educators call a “teachable moment.” Let’s not miss this opportunity to increase awareness and enhance the understanding of the fundamentals that characterize this problem. Like Mr. Graham, let’s all be passionate. Ensure your employees are safe and don’t temper your passion for safety.

References
1. U.S Chemical Safety and Hazard Investigation Board Investigation Report: Combustible Dust Hazard Study, 2008.