Minimize Dust Explosion Hazards

On January 29, 2003, a massive explosion rocked the West Pharmaceutical facility in Kinston, N.C. The blast killed six plant workers, injured dozens more, cost the economically depressed town more than two hundred jobs, and destroyed millions of dollars worth of property. Such was the devastation that few clues, and sadly fewer close eyewitnesses, survived.

What is known for sure is that the Kinston facility used moistened polyethylene powder to manufacture plastic, medical-grade stoppers and syringes. Armed with that information and knowledge of the plant's construction, investigators pieced together a plausible theory about what went wrong. Stephen Selk, an engineer with the U.S. Chemical Safety and Hazard Investigation Board (CSB), Washington, D.C. and CSB's lead investigator at Kinston, theorized that when the polyethylene powder dried, plastic dust as fine as talcum powder probably migrated upward and settled, over many weeks or months, in the area above ceiling panels. A spark of some sort " from electrical equipment, a static discharge, or some heat source " probably ignited a small quantity of the dust. The shock wave from this initial blast may have dispersed the remaining reservoir of powder, which was ignited by the flame front that followed, creating a much larger, more deadly secondary blast. Selk and others familiar with the West disaster believe that a secondary blast was probably responsible for obliterating the Kinston site.

Dust explosions are more insidious, and potentially more dangerous, than vapor blow-ups. As fluids, flammable gases mix and diffuse through the atmosphere more or less homogeneously, through predictable concentration gradients. As a result, vapor explosions tend to involve a single, relatively localized blast. Because they are solids, dusts and powders settle heterogeneously onto equipment, in passageways, and in nooks and crannies of the plant superstructure. When airborne particles ignite, the blast front propels settled dust into the air. The flame front from the initial fire then ignites the newly airborne material to cause successive explosions. The primary explosion typically damages a limited area; secondary blasts tend to be much more destructive.

The Kinston facility was technically a rubber compounding and forming plant, not a pharmaceutical manufacturing operation. However, the sheer devastation of January 29 should cause every process industry that works with powders to take notice: Any finely divided organic material is a potential dust explosion hazard.

The National Fire Protection Association (NFPA) defines any organic powder with an average particle size of 420 microns or less as a potential hazard. according to Amy Spencer, a spokeswoman for the organization. "The smaller the particle, and the higher its concentration, the greater the hazard," she says.

With dust, what you can't see is sometimes more dangerous than what you can. Obvious accumulations of dust or powder are routinely vacuumed away at most manufacturing facilities. But according to NFPA, a nearly invisible one-thirty-second of an inch layer of dust can be hazardous. Even if dust present is not sufficient to blow the roof off a building, it can serve as a trigger for other explosions. Dust or powder need not even precipitate the "big bang."

"Dust explosions are usually secondary events," Selk explains. "A small explosion that is not dust-related can send dust airborne, where it gets ignited by the fire front from the first explosion or from an independent ignition source."

Fuel, oxidant, and ignition source are the three ingredients required by all explosions. Almost any organic material and many metals serve as fuel which is why so many different industries need to become aware of the dangers associated with errant powder and dust. Fire safety experts recommend understanding your powders as much as possible: How flammable are they, in what concentrations, and under which conditions? Pharmaceutical processors should take special care in any area where dust and powder are stored, processed, or handled, including process filters, conveying systems, transfer lines and tubes, vacuum systems, mixing and blending operations, filling equipment, and even bulk storage containers.

No single fire/explosion prevention strategy fits all process situations. Often, engineers adopt a combination of approaches that minimize risk factors (fuel, oxidant, and/or ignition) at appropriate times and locations within a process.

As one safety engineer who preferred anonymity (due to his involvement with the West explosion) notes, engineers first must identify danger zones and the risks each of the three critical explosion factors pose in those areas. "By calculating the risks and costs associated with various mitigation strategies, it's possible to arrive at an explosion prevention strategy that mitigates potential risks intelligently and economically."

Risk assessment can go a long way towards implementing a smart, economic explosion strategy. Bill Stevenson, general manager at safety consultant CV Technology, mentions a Midwestern manufacturer who spent $25,000 per year maintaining an active explosion suppression system on a spray drier, unaware that dust concentrations barely surpassed 20% of the material's minimum explosive concentration, or MEC. "They hadn't even considered that their risk didn't pose any hazard whatsoever," Stevenson comments.