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  • Joe Mlynek, CSP, OHST

GRAIN DUST EXPLOSIONS

Secondary Explosions


Grain dust explosions often result in fatalities, injuries, and significant property damage. OSHA’s Grain Handling Standard 29 CFR Part 1910.272 addresses grain dust explosion prevention and includes a requirement for a written housekeeping program to address fugitive grain dust.


One of the primary goals of OSHA’s housekeeping program requirement is to reduce the potential for secondary explosions. Secondary explosions occur when the blast or pressure wave from the primary explosion propagates into areas such as gallery floors, tunnels, and intermediate floors of the headhouse causing layers of dust to become suspended. The pressure wave travels away from the primary explosion at approximately 1,000 feet per second followed by the flame wave at 10-100 feet per second. The pressure wave places dust into suspension and the slower flame wave ignites these concentrations resulting in powerful secondary explosions. Primary explosions generate pressures around two psi while secondary explosions can generate pressures in excess of 100 psi. For reference, the rupture strength of equipment such as bucket elevator legs and conveyance is around two psi while the rupture strength for concrete is roughly 25 psi.


OSHA requires that the housekeeping program be documented and specifies the frequency and cleaning methods used, such as vacuuming or sweeping, to effectively reduce dust accumulation. Keep in mind that portable vacuums must be rated for Class II, Group G hazardous locations. The standard also requires that the housekeeping program immediately address dust accumulations at priority areas using an action level of 1/8 inch (.32 cm) of accumulated dust. Priority areas are defined as floors within 35 feet of an inside bucket elevator, floors of enclosed areas containing grinding equipment, and floors of enclosed areas containing grain dryers inside the facility. An inside bucket elevator means a bucket elevator that has the boot and more than 20 percent of the total leg height inside the grain elevator structure. Bucket elevators with leg casings that are inside of rail or truck dump sheds with the remainder of the legs outside of the grain elevator structure are not considered inside bucket elevators.


In addition, the housekeeping program should define schedules for cleaning dust accumulations from motors, critical bearings, and other ignition sources in the work area. Areas around bucket elevators, milling machinery, and similar equipment should be prioritized in the cleaning schedule. Housekeeping efforts must also address “hard-to-reach” areas. This may include upper surfaces of equipment, spouting, ledges, and walls. These efforts often require the availability of work platforms, portable ladders, mobile scaffolding, and long handled tools.


Using compressed air for cleaning is acceptable but does pose significant risk since these efforts place dust into suspension. For this reason, OSHA requires that the use of compressed air to blow dust from ledges, walls, and other areas only be permitted when all machinery that presents an ignition source is shut down, and all other known potential ignition sources in the area are removed or controlled. While not required, many companies implement the use of “blow down” or compressed air cleaning permits. The permit requires an evaluation of the housekeeping area, identification and elimination of ignition sources, equipment shutdown, and management approval prior to use of compressed air for cleaning inside of enclosed areas (tunnels, boot pits, headhouse, gallery floors, etc.).


Written housekeeping programs should also specify the schedules and control measures for controlling dust emitted from grain handling systems. This includes efforts for identifying and addressing point sources of dust such as holes in spouting, bucket elevator casings, pneumatic dust collection system piping, and conveyance.


Companies should evaluate their program on a routine basis to ensure that it:

  • Defines the frequency and methods used to remove grain dust accumulation.

  • Determines the locations of priority areas as defined by the OSHA standard including the requirements for cleaning “hard-to-reach” areas and critical equipment that can pose an ignition source.

  • Requires documentation of housekeeping inspections and cleaning efforts.

  • Establishes routine inspections to identify and correct point sources of fugitive dust.

  • Requires employee training on the requirements within the program.


Grain dust explosions are preventable. Review your housekeeping program to ensure that it provides an effective strategy for addressing grain dust accumulations at your location(s). After the strategy is developed, it all comes down to execution.



 

Joe Mlynek is a partner and subject matter expert at Safety Made Simple, LLC. He has over 20 years of experience in safety at the corporate level and as a consultant. He is a Certified Safety Professional (CSP) and Occupational Safety and Health Technician (OHST). Joe can be reached at joe.mlynek@safetymadesimple.com

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