By Jeffrey Wanko
OSHA is targeting scrap recycling facilities under its national emphasis program on combustible dust. Know your risks and what you can do to abate them.
On Jan. 29, 2003, an explosion and fire killed six people, injured 38 more, and destroyed a building belonging to West Pharmaceutical Services in North Carolina. The facility produced medical supplies such as rubber seals for syringes and vials. When the company manufactured its rubber sheets, it would apply a finely powdered polyethylene to them to prevent them from sticking together. Over time, the plastic powder settled in large amounts above a suspended acoustical tile ceiling in the production area. An initiating event disturbed the accumulated dust and caused the explosion.
Another major explosion happened Feb. 7, 2008, at the Imperial Sugar refining facility in Georgia. Sugar crystals and dust had accumulated in large quantities throughout the buildings. When an initiating event disturbed accumulated sugar dust that had settled on the surface of an elevated conveyor, it exploded and created multiple secondary explosions and fires that killed 14 people, injured 36, and destroyed a major portion of the complex.
The Occupational Safety and Health Administration created its Combustible Dust National Emphasis Program in 2007 and increased its enforcement activities after the Imperial Sugar explosion in 2008. The Combustible Dust NEP targets the scrap recycling industry as one likely to have combustible dust hazards. Although scrap recycling might not be the first industry that comes to mind when you think about combustible dust hazards, those hazards can exist in any industrial operation that involves manipulation or size reduction of a combustible solid that creates dust.
What Is Combustible Dust?
The National Fire Protection Association defines combustible dust as a finely divided combustible particulate solid that presents a flash-fire hazard or explosion hazard when suspended in air or in the process-specific oxidizing medium over a range of concentrations. This definition identifies the conditions that must exist for there to be a combustible dust hazard:
• The dust must be solid and must be something that can burn.
• It must be finely divided. The general rule of thumb is that the particle size does not exceed 420 microns, which is a little less than ½ of a millimeter.
• It must be suspended in air or in some other oxidizer. A pile of finely divided solid material on the floor at ground level does not pose a hazard.
• It must exist in that suspension at a specific range of concentrations. Much like combustible vapors and gases, combustible dusts exhibit lower and upper explosive limits. The lower limit is called the minimum explosible concentration.
The scrap recycling industry handles and processes combustible solids every day. Plastics, wood (including paper), and aluminum are some of the most common combustible solids. Scrap processing can create the finely divided and small particle sizes meeting the definition of combustible dust. Sorting and size reduction, such as through shredding, compaction, and milling, while not intended to create particles 420 microns or smaller, invariably do create such particles.
For dust explosions to occur, all of the above conditions must exist in addition to the presence of a competent ignition source and confinement of the suspended dust. Generally, it’s assumed that ignition sources are always present. These might include electrical equipment and wiring or metal striking metal. Confinement can occur within scrap-processing and dust-handling equipment such as hammermills, dust collectors, and air-
handling duct work. Confinement also occurs in buildings or rooms—indoor facilities are typically where the most hazardous conditions exist. The most catastrophic combustible dust incidents over the past 20 years have occurred because dust escaped from industrial processes and settled in large quantities within rooms. Escaped dust (also known as fugitive dust) settling on elevated surfaces readily becomes suspended in air and contributes to the severity of a combustible dust incident.
Hazard identification and evaluation are key to protecting workers from combustible dust hazards. You can’t control a hazard without first identifying and evaluating it. If your workplace has potential combustible dust hazards, take steps such as these to identify and evaluate the risk.
• Identify areas where combustible solids are processed. Is it likely the process produces a dust?
• Identify where dust accumulates. Is dust contained to process equipment? Does dust leak and accumulate in work areas? Does dust accumulate on elevated surfaces?
• If accumulations exist, where are they and how deep are they? Do they exceed a depth similar to the thickness of a dime? How widespread are they?
• Where possible, safely collect dust samples. Dusts of similar material may have significantly different properties depending on where the dust accumulates and how long it is permitted to accumulate.
• Test dusts at a lab that can perform test methodologies in accordance with ASTM E1226—the Standard Test Method for Explosibility of Dust Clouds.
Your answers to these observations and test results will help you evaluate the hazard and apply appropriate hazard abatement methods, which could be engineering controls, procedural changes, or a mixture of both. Engineering controls could include deflagration venting designed in accordance with NFPA 68—Standard on Explosion Protection by Deflagration Venting, or explosion protection systems designed in accordance with NFPA 69—Standard on Explosion Prevention Systems.
Housekeeping to Reduce Risk
Regular cleaning and housekeeping can reduce the risks associated with combustible dusts. Don’t permit dusts to accumulate in any significant quantities. An effective housekeeping program may include, but is not limited to, the following:
• Remove dust accumulations regularly.
• Maintain the staffing needed to sustain the cleaning schedule.
• Use vacuum cleaning where possible to minimize the creation of dust clouds, provided that the vacuum cleaning equipment is approved for the hazardous location.
• Where vacuuming is impractical, clean by gently sweeping.
Keep in mind that dusts often accumulate in elevated and difficult-to-access locations. Where this occurs, engineering solutions might be the best approach, first, to prevent dust from escaping from processing equipment, and second, to prevent dust from accumulating by eliminating horizontal surfaces or other spots where it can rest. Resources from the National Fire Protection Association (www.nfpa.org) and FM Global (www.fmglobal.com) can give you a better understanding of the hazards and control methods.
OSHA Enforcement Activity
The Combustible Dust NEP provides a framework for how OSHA conducts an inspection of an establishment where combustible dust hazards exist or are thought to exist. The intent of the emphasis program is to inspect establishments in many different industries where combustible dust hazards may exist and work collaboratively to reach an abatement strategy that protects workers from the risk of fire and explosion.
The program provides details on the type of facilities OSHA targets and how it selects a particular establishment for inspection. When it selects a facility, an OSHA compliance officer will enter the facility and conduct an opening conference to discuss the scope of the inspection and request written program materials. After the opening conference, the compliance officer will conduct a walk-around inspection to view the areas where you process or handle combustible dusts. The officer will likely collect samples, interview workers, and review engineering drawings of dust-control and dust-handling measures. After all of these activities, the compliance officer will conduct a closing conference to discuss his or her observations and possible violations. You will not receive any citations during the closing conference; all citations must be approved and delivered by the OSHA area office director.
An effective safety and health management system includes identification and control of all workplace hazards, including those that may seem unlikely, like a dust explosion. The consequences of a combustible dust incident are often catastrophic and devastating. Fortunately, combustible dust hazards are detectable and preventable with effective management. Taking the time to evaluate your facility’s operations for combustible dust hazards will pay dividends for your continued operations and success.
Jeffrey Wanko is director of chemical process safety and enforcement initiatives for the U.S. Occupational Safety and Health Administration. He has adapted this article from his presentation at the spring 2020 ISRI Safety and Environmental Council meeting.
Taking Action on Combustible Dust
To meet the National Fire Protection Association’s NFPA 652—Standard on the Fundamentals of Combustible Dust, facilities must complete an initial dust hazard analysis by Sept. 7, 2020, and review and update their analysis once every five years. The Occupational Safety and Health Administration does not have its own combustible dust standard, but it has said it will use its general duty clause along with NFPA 654—Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids for enforcement, according to an Aug. 16 bulletin from ISRI’s environment, health, and safety staff. For links to a 30-minute video from the U.S. Chemical Safety and Hazard Investigation Board on combustible dust hazards and a list of questions from the NEP standard that OSHA enforcement officers are likely to ask when evaluating your facility’s combustible dust hazards, ISRI members can visit isri.org/news-publications/news-details/2020/08/16/dust-hazard-analysis-deadline-is-september-7.