NOTE: In talking with Jim Gallup, P.E., CSP, this past week during the winter ASSE Board of Directors meeting, he mentioned this article and offered to have it re-published here in my blog. Jim is the ASSE Region II Vice President, and this article is from the most recent Region II Newsletter. Thanks for sharing this important information concerning Combustible dust.
Many of the deadliest explosions in the past fifteen years have involved combustible dusts. To address this dangerous issue, in 2008, the Occupational Safety and Health Administration (OSHA) established a National Emphasis Program that included a field directive for the inspection of sites handling combustible dusts. OSHA recently published an Advance Notice of Proposed Rulemaking to consider the option of writing a comprehensive standard on combustible dusts.
Many common solids can generate combustible dusts during cutting, grinding or buffing operations. We know that flour will explode if suspended. Metals such as aluminum form ignitable clouds of the dust if suspended. Plastics, textile, and candy manufacturing can all produce combustible dusts.
Destructive combustible dust explosions require two additional elements beyond the three typical flammable elements; heat, oxygen and fuel. For a fast fire in dust to become a destructive dust explosion, the dust must also be suspended and confined or enclosed.
The deadliest dust explosions usually involve two back-to-back explosions. The first is localized at the point where the dust is generated. The second, and often more destructive explosion, is a result of the first explosion which serves as the ignition source for the second explosion. The initial explosion creates a blast wave that lifts and suspends accumulated fugitive dust on beams and other horizontal high surfaces above the floor. The newly suspended dust ignites and a larger secondary explosion occurs.
How can we determine if industrially created dust is a problem? First, any process that generates a visible dust cloud will be suspect. Visible clouds may not be in the flammable range, but visible clouds indicate a suspended dust. Usually these processes have a means to collect the dusts generated. A hood, duct, separator, and fan are most often present. Secondly, any process that generates enough fugitive dust to deposit 1/16-inch or more of dust outside of the collection system will be suspect. Additionally, dust deposited on surfaces above the floor should be suspect.
The next step will be to take samples and have the samples tested for combustibility. If the dust is combustible, the degree of explosibility and several other factors, including minimum ignition temperature and minimum dust density, require testing in order to establish the potential need for corrective measures. The full range of testing is expensive, on the order of $10,000 per sample. Once the testing of a sample is completed, the safety professional will know not only the minimum cloud concentrations needed for an explosion, but also the heat source required, the maximum pressure generated by an enclosed explosion, and the rate that the maximum pressure is generated.
Controls will range from relocating collection equipment outside to providing explosion relief devices and explosion suppression systems. Sources of ignition may need to be controlled or eliminated. Fugitive emissions (uncontrolled dust) may need to be evaluated and controlled. Limiting total quantities of dust by frequent cleaning may prove necessary. Some facilities permit the use of compressed air for cleaning, which may not be acceptable if blowing the dust would suspend the dust in high concentrations. And, importantly, training of affected employees may be needed. As you might guess the analysis of all factors is not simple.
For more information on dust explosion prevention, visit OSHA at www.orc-dc.com/?q=node/3207. There are several NFPA standards such as NFPA 484 for metal dusts, and 654 for non-metal dusts covering proper handling of combustible dusts. Factory Mutual Data Sheet 7-76 also has information.