Rachelle Andreasen, Contributor
The above question is a regularly searched internet question. In an effort to help customers understand the importance of evaluating “dust” hazards within their facility, the people at Fauske have taken a current ASTM method and modified the purpose to answer: “Is my material a dust?”
ASTM E2316 Standard Test Method for Determination of Particles Resulting from the Attrition of Granular Pesticides was originally authored to provide information on health hazards, such as inhalation risks based on the amount of pesticide dust present within a working area. The method looks at the original size of the dust and simulates breakage from normal manufacturing and handling processes. The fines generated from this testing procedure are labeled as the “fines from attrition.”
As the amount of dust increases, the risk increases for not only inhalation concerns, but for dust explosion hazards as well. As previously mentioned, this test procedure was modified to simulate the amount of dust that could be generated by transport via pneumatic and mechanical means within a facility or in containers on the road (i.e., sea or air).
Most recent revisions of the NFPA standards related to dust define a “dust” as a particulate with a particle size of 500 μm or smaller. For this reason, this analysis was performed by taking a sample of material and sieving the material to less than 500 μm to remove the inherent fines (i.e., the powder/dust at the bottom of the bag of cereal). Once the inherent fines were removed, the material was placed within a glass jar with an equal weight of glass beads. The material was tumbled with the glass beads for approximately 4,500 rotations, thus creating an attrition scenario. Once again, the material was sieved to less than 500 μm to remove the fines from attrition. The total quantity of fines then becomes an estimate of the amount of powder/dust that can be present in your material after transport.
As the histogram below indicates, the commercial particle size of granulated sugar was 54% less than 500 μm (see Figure 1). After the tumbling process, the material was determined to have a particle size of 62% less than 500 μm (see figure 2). The table below also details the data generated from the analysis. The fines percent was increased by approximately 8%, which is nearly a 15% increase in fines.
The data generated in this analysis clearly shows that even though your material may be in granular form (or larger), the potential for particle attrition based on your or the end-users’ process should be evaluated.
There is not a definitive particle size that governs whether or not a material is explosible in dust cloud form. The explosion characteristics can be altered based on a materials particle size distribution, moisture content and even particle morphology. Care should be taken when operating within a facility with explosible dust.
Understanding the combustible nature of your materials helps us identify which materials are combustible and where credible explosion or fire hazards exist in your facility. Safety professionals will work with you to identify key data points to ensure that the information is used to make decisions aimed at improving the safety of your facility.
Per NFPA 652, an explosion hazard exists in a process or facility when a sufficient amount of combustible dust, a credible ignition source, an oxidizing atmosphere and a credible means for dispersion are all present for a given scenario. A few key words jump out in this definition; sufficient and credible. Safety professionals identify hazardous areas by focusing on credible scenarios where a sufficient amount of dust is or could be present.
The ultimate goal of a DHA is to protect your employees and assets from the effects of combustible dust fires and explosions. Properly characterizing your materials and identifying credible hazards in your process environment enables you to begin the process of risk reduction or mitigation. WMHS
By Rachelle Andreasen, Manager, Dust Testing Operations, Earl Johnson, Lab Technician and Ashok Ghose Dastidar, Ph.D., MBA, Vice President, Dust & Flammability Testing and Consulting Services, Fauske & Associates, LLC