If you work in a facility that creates dust, there are several questions you may be asking yourself regarding combustible dust. First, what is a combustible dust? The Canadian Centre for Occupational Health and Safety (CCOHS) provides the following:
“Essentially, a combustible dust is any fine material that has the ability to catch fire and explode when mixed with air. Some of these materials are not ‘normally’ combustible, but they can burn or explode if the particles are the right size and in the right concentration. Therefore, any activity that creates dust should be investigated to see if there is a risk of that dust being combustible. The build-up of even a very small amount of dust can cause serious damage.”
The Combustible Dust National Emphasis Program references 1/32” dust accumulation levels. This is based on some assumptions: uniform distribution of the dust and a bulk density of 75 lb/ft3. However, not all dust meets this density (some are lighter, some are heavier). NFPA 654 allows the dust accumulations to exceed the layer depth criteria of 1/32” for materials with bulk densities less than 75 lb/ft3, under certain conditions. Additionally, NFPA 654 states that a dust explosion hazard and dust flash fire hazard exist in any building or room where any of the following conditions exists:
• The total area of nonseparated dust accumulations exceeding the layer depth (LD) criterion is greater than five percent of the footprint area.
• The area of any single nonseparated dust accumulation exceeding the LD criterion is greater than 1,000 ft2.
The total volume of nonseparated dust accumulations is greater than the LD criterion multiplied by five percent of the footprint area.
•The total volume of any single nonseparated dust accumulation is greater than the LD criterion multiplied by 1,000 ft2.
CALCULATE ALLOWABLE COMBUSTIBLE DUST LEVELS
To calculate the level of dust accumulations that may be allowed, you must know the bulk density. Bulk densities depend on many factors, including the type of material (e.g., wood, paper, plastic, other organics, metal, etc.), the dust particle size, and the dust particle shape. OSHA compliance officers must take into consideration the bulk density of the dust prior to determining if there is a violation and subsequent citation.
Very low bulk density materials (e.g., paper dust and corn dust—25 lb/ft3 and 16 lb/ft3, respectively) may not create a deflagration (heating until it burns away rapidly) hazard at levels above 1/32”. Therefore, facilities should determine their dust bulk densities in order to determine the level of accumulation that does not present a combustible dust hazard. One of the easiest ways to determine bulk densities is to use an online resource, such as:
The referenced bulk densities can then be plugged into the equation in Fig. 1 to determine the allowed LD.
EXAMPLE OF ALLOWED
For example, for paper dust with a bulk density of 25 lb/ft3, the LD criterion is approximately 1/10”. Therefore, in this example, a dust explosion hazard and dust flash fire hazard are deemed to exist if:
• The total area of nonseparated dust accumulation exceeding 1/10” over greater than 5 percent of the footprint area, or
• The area of any single nonseparated dust accumulation exceeding 1/10” is greater than 1,000 ft2, or
• The total volume of nonseparated dust accumulations is greater than 1/10” multiplied by five percent of the footprint area, or
• The total volume of any single nonseparated dust accumulation is greater than 1/10” multiplied by 1,000 ft2.
HOW DOES OSHA EVALUATE
Note that OSHA compliance officers will likely only send samples out for bulk density determination if the material is light (e.g., paper dust, fabric fibers) and the levels of accumulation exceed ¼” over 5 percent of the floor area of a room or building, or 1,000 ft2, whichever is less. However, if dust accumulations are documented to exceed 1” in depth (extending over 5 percent of the floor area or a room/building, or 1,000 ft2, whichever is less), samples of dust don’t need to be submitted for bulk density determination. The OSHA compliance officer can use the referenced sources as guidance to determine the approximate values for LD and can issue citations without the bulk density determination.
JOIN NEW DUST
TAPPI’s Tissue Division has launched a new committee focused on reducing dust in tissue mills to increase safety and product quality. The Dust Mitigation Committee was formed to create a common industry platform for learning and the potential to create unified guidelines to support these efforts. Everyone from the full production process is invited. To join, email [email protected]
George Bower, CIH, CSP, senior safety and industrial hygiene consultant with S&ME, is a former safety and health director at a large southeastern paper mill; he currently serves multiple pulp and paper companies. Contact him at [email protected] or 843.452.4604.