How to Manage Combustible Dust

The October 2011 explosion of a Kansas grain elevator that killed six people tragically highlights one of the most serious hazards feed and grain processing companies manage every day — combustible dust. The loss of an employee and community member is certainly the ultimate price paid for ineffective dust control, but loss of equipment and product, as well as nonfatal injuries, can also occur.

Nearly 280 dust fires and explosions occurred in U.S. industrial facilities from 1980 to 2005, resulting in approximately 119 fatalities and over 700 injuries, according to the Chemical Safety and Hazard Investigation Board (CSB).

Protecting your facility from a potentially lethal situation starts with understanding the cause of an explosion and designing an effective prevention plan.

Explosion pentagon

What does it take for an explosion to occur? David Grandaw, regional sales manager for Fenwal, Ashland, MA, says, “The rule of thumb is if it can burn as a solid, it can explode in dust form. All it takes is having the right factors in place.”

Often referred to as the “explosion pentagon,” the five conditions that must come together to create a dust explosion are:

• Fuel — dust or fine particles
• Dispersion — dust suspension in air
• Oxygen
• Confinement — inside a vessel or room
• Ignition Source

Dust collection

Managing dust — the fuel source — is one of the most important steps toward explosion prevention. An effectively designed dust mitigation approach combines exhaust fans, dust collectors and housekeeping.

The National Fire Protection Association (NFPA) is a national safety standard setting organization that has developed a standard for combustible dust hazards in industrial facilities. Two standards — NFPA 61 (Agricultural and Food Products) and NFPA 654 (Combustible Particulate Solids) — are what OSHA relies on to inspect grain processing facilities.

NFPA 654 says companies should not employ open loading processes to minimize the release of dust.

However, many applications in a grain processing facility cannot be done in a closed environment. Guy Colonna, NFPA’s division manager, industrial and chemical engineering, says the standard’s alternate option is to use a hood or exhaust system to send dust to a central dust collector or a smaller, process-dedicated collector.

Kirt Boston, Industrial Air Filtration program manager for Donaldson Torit of Minneapolis, MN, says now more than ever, dust collectors are designed with safety in mind.

“As recently as 10 years ago, it was commonplace for all processes to be connected through ductwork leading to one central dust collector,” explains Boston. “The danger is if a collector explosion occurs, fire goes back into the ductwork, involving other systems that otherwise might not have had contact with the explosion.”

To reduce the chance of an explosion traveling throughout the facility, Boston suggests using smaller, dedicated collectors designed specifically for installation near a piece of equipment that run concurrently with the process.

Investing in higher performing dust collectors also helps reduce the housekeeping load a facility has to manage, a main provision in NFPA 654.

Housekeeping measures

Dispersion — dust suspension in the atmosphere — the second factor in the explosion pentagon, can be controlled with various housekeeping measures. If an explosion occurs, dust accumulated on the floor and in the beams and rafters can be knocked loose and become suspended in the air to provide fuel for a secondary, and often more powerful blast.

NFPA 654’s stance on housekeeping is that a hazard is present when accumulation is higher than 1/32 of an inch, but permits users to adjust their layer thickness based on the bulk density of the dust. For the grain handling industry, OSHA’s regulation in 29 CFR 1910.272 establishes a layer depth of 1/8 inch because the typical grain product has a bulk density less than that in NFPA 654 and, therefore, the pro-rated layer depth is increased.