Dust Gets Its Due

Industry is placing greater emphasis on addressing combustion hazards.

By Seán Ottewell, Editor at Large

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"Those numbers suggest the various industries have 'coped' with the hazard and operated around the hazard. At this stage, it would appear that industry is no longer looking to cope with the problem and is resolute in finding common understanding about the phenomenon that can lead to more effective hazard assessment and control solutions."

In Colonna's experience, two issues commonly crop up in question-and-answer sessions these days — and both are core to coming to grips with combustible dust hazards.

The most common relates to the hazard identification process for examining a facility and any combustible particulates that might be involved as process input, intermediate or final product. This generally requires testing of a sample and then interpreting the results.

One property commonly determined is the dust's deflagration index, KSt , which indicates how rapidly a pressure front forms and moves through the combustion zone.

"Often, those interpreting the data are seeking guidance on the interpretation for the value reported. Their question asks whether a 'low value' for the KSt means there is no problem from the dust in terms of explosibility. The answer to that question is NO. There is no target or threshold value below which no hazard is assumed. Lower values of the index just mean the rate of rise of the pressure with respect to time will be 'slower' (in a relative sense) than other materials that have higher values for the index. The damage pattern from a low KSt value dust is different than the damage experienced when the index is greater, but the overpressure is still present and capable of destroying structures — and the fireball also poses a hazard to both personnel and the facility."

The second issue involves the characterization of the dust hazard condition or area. In many cases this currently is based on the thickness of the layer of accumulated dust. Colonna explains that NFPA 654 presents an equation that enables adjusting the permitted or target layer thickness based on the specific material's bulk density — how packed or non-packed a settled accumulation could be — while still yielding an equivalent mass per area. The layer thickness as used today serves as a trigger for various protective measures as well as housekeeping. The prorating equation in the standard permits users to adjust their layer thickness (ultimately that accumulated mass) based on the specific bulk density.

"So, materials like tissue paper, which have a very low bulk density compared to wood and other materials, when evaluated for layer thickness on the basis of the bulk density prorating equation are recognized as accumulating in a less packed or thicker layer while still yielding the same amount of accumulated dust mass across a given area. The equation method for establishing the layer thickness doesn't penalize those materials whose bulk density is low relative to other materials by requiring them to comply with the same layer thickness as more densely layered dusts," he says.

Lately, the chemical industry also has become concerned about various regulatory standards and consolidations currently afoot. Much of this revolves around OSHA's notice of its intention to develop a federal standard to address workplace safety requirements to protect workers from combustible dust fire and explosion hazard conditions.

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