Over the years, combustible dusts have been involved in a number of fires and explosions. In October 2007, the United States Occupational, Safety and Health Administration (OSHA) initiated its National Emphasis Program (NEP) for combustible dust to begin inspecting facilities that generate or handle combustible dusts.

According to OSHA, a “study conducted by the U.S. Chemical Safety and Hazard Investigation Board (CSB) found that nearly 280 dust fires and explosions have occurred in U.S. industrial facilities over the past 25 years, resulting in 119 fatalities and over 700 injuries.” One of the most recent incidents occurred at the Imperial Sugar refinery in Port Wentworth, Georgia, U.S.A. in February 2008, where approximately thirteen fatalities occurred.

Secondary Dust Explosions

Although dust explosions can be quite severe, secondary dust explosions can be catastrophic. Secondary dust explosions, as explained by OSHA, occur when an initial explosion takes place in processing equipment or in an area where fugitive dust has accumulated. This initial event may dislodge more accumulated dust into the air or damage a primary containment system (such as a duct, vessel, or dust collector). As a result, the additional dust dispersed into the air, if ignited, may cause one or more secondary explosions. These can be far more destructive than a primary explosion due to the increased quantity and concentration of dispersed combustible dust.

Source: U.S. OSHA

Recommendations

It is clear that controlling dust “outside the pipe” is mandatory in preventing catastrophic events. Additional recommendations for preventing secondary dust explosions provided by OSHA and the U.S. National Fire Prevention Association (NFPA) include

• Implement a housekeeping program, with processes for inspection and controls.  OSHA, citing NFPA 654 guidelines, has referenced 1/32”, or about the thickness of a paperclip, as the threshold. Cleaning should be conducted as frequently as necessary to maintain accumulations below this level.
• Inspect for dust in open and hidden areas at regular intervals. Some areas to inspect include
  • Structural steel members.
  • Conduit and pipe racks.
  • Cable trays.
  • Floors.
  • Areas above ceilings.
  • Areas on and around equipment.
• Use proper dust collection systems and filters. The design should prevent dust from settling in the duct work and providing another fuel source. Any additional ventilation pick-up points should not be added to a dust collection system without an engineering analysis.
• Clean or change dust collector filter elements as needed, and monitor the pressure drop to verify that dust collector elements are not plugged.
• Minimize the escape of dust from process equipment or ventilation systems. 
• Use surfaces that minimize dust accumulation and facilitate cleaning. Flat, horizontal surfaces provide easy locations for dust to accumulate. Consider providing “boxing” around structural steel components.  
• Do not use cleaning methods or work practices such as air hoses or high-velocity fans that generate dust clouds.
• Use only vacuum systems approved for dust collection.
• Locate relief valves away from dust deposits. Should a relief device open, the shock effect could cause accumulated dust to become suspended.

Summary

In summary, fugitive dust clouds and dust accumulations create an environment that could lead to secondary dust explosions, which are typically more damaging than the initial explosive event, and could result in catastrophic consequences. In order to prevent dust explosions, a program should be put in place to identify problem areas and implement corrective actions.

Additional Resources

• U.S. OSHA National Emphasis Program for Combustible Dust.
• NFPA 654, NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids.

About the Author

Dave Scott is employed by Hexion Specialty Chemicals and is their Global Risk Engineering Manager.

Image: United States OSHA, Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions  (http://www.osha.gov/dts/shib/shib073105.html).