By Zach Hachmeister, Director of Operations, Fauske & Associates, LLC
Dust in industrial settings can present a variety of hazards, but the two that are most prevalent relate to the toxicity and explosive characteristics of the material. Both hazards are addressed in different fashions; however, there is some correlation between the two.
For toxicity purposes, government agencies such as the American Conference of Governmental Industrial Hygienists (ACGIH) and Occupational Safety Health Associates (OSHA) have developed threshold limit values (TLV) or permissible exposure levels (PEL) for a wide range of materials. These values set a safe level at which employees can be exposed to a specific material over a given amount of time and are usually reported in parts per million (ppm) or mg/m3. When addressing explosion hazards, the potential for an explosion does not begin until dust concentrations suspended in air reach a value that is typically above 20 g/m3. To help put this in perspective, a 25 watt light bulb positioned one meter away would be barely visible in a dust cloud that has a concentration of 60 g/m3. Keeping that in mind, it is also possible to see dust particles suspended in air and still be below the threshold value set by OSHA.
In accident investigations, dust explosions are usually referred to as either a primary or secondary explosion. A primary dust explosion is one that occurs inside a process vessel such as a dust collector and the fuel is usually the actual process dust. Secondary explosions often follow a primary explosion. Essentially, the pressure wave from the primary explosion lofts fugitive material that was previously deposited on overhead surfaces. The lofted material then forms a cloud and eventually finds an ignition source resulting in a secondary explosion. One of the more recent, well known dust explosions was the incident that occurred at Imperial Sugar where a series of secondary explosions demolished the facility causing over $270 million dollars in damage.
The relationship between the toxicity and explosion hazards primarily relates to secondary dust explosions and fugitive dust deposits. An extremely low dust concentration from a hygiene perspective is usually an indication of good dust control, especially if these measurements are taken near process equipment. Good dust control leads to lower fugitive dust deposits which can alleviate housekeeping duties and ultimately reduce secondary dust explosion hazards. However, using hygiene measurements to solely address dust explosion hazards is not a safe approach. The reason this is not acceptable is that it will not always tell the story of what is on elevated horizontal surfaces such as overhead I-beams or duct work. It also does not address primary dust explosion hazards, which are those that exist inside process equipment.
In order to mitigate the hazards associated with dusts in the work place, it is not only important to look at hygiene levels but also to evaluate the explosive characteristics of the materials being processed. A good starting point is to have dust explosibility screening tests conducted on samples that are taken from process equipment such as dust collectors or mills. It is also a good practice to evaluate the fugitive dust deposits on overhead supports. If the materials are identified as non-explosible then the main safety concern may be reduced to toxicity effects. However, if the dust is found to be explosible then explosion protection equipment should be installed on process equipment and housekeeping efforts should be capable of keeping fugitive dust deposits to a minimum.
If you have any further questions regarding the how to assess dust explosion hazards in the work place, please don't hesitate to contact me at firstname.lastname@example.org or 630-887-5223. www.fauske.com