The United States Occupational Safety and Health Administration has issued a memo to provide guidance around new combustible dust classification.
The memo is designed to help health and safety officers determine whether manufacturers or importers have properly classified their products for combustible dust hazards under the revised Hazard Communication Standard (HCS).
Combustible dust is a solid combustible material, composed of distinct pieces or particles, which "presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape." A number of voluntary standards prepared by the National Fire Protection Association (NFPA), FM-Global, and ASTM International suggest various tests, data, and criteria that may be used to determine whether a material presents a combustible dust hazard.
On March 26, 2012, OSHA amended the HCS to align with the Globally Harmonized System for the Classification and Labelling of Chemicals (GHS). However, the GHS does not contain a classification for combustible dust hazards, and to maintain coverage of this hazard under the HCS, OSHA amended the standard's definition of "hazardous chemical" to include "combustible dust."
Under the HCS, classifiers are required to "evaluate chemicals produced in their workplaces or imported by them to classify the chemicals in accordance with this section." Any such classification must "identify and consider the full range of available scientific literature and other evidence concerning the potential hazards."
However, there "is no requirement to test the chemical to determine how to classify its hazards.” The classifier must consider not only the hazards of the chemical in the form it is shipped, but also consider the hazards that arise under normal conditions of use and foreseeable emergencies.
For combustible dusts, often the best information is actual experience with the product. If the classifier knows that its product has been involved in a deflagration or dust explosion event, the classifier should classify the product as a combustible dust unless the classifier can show that the conditions surrounding the event are not expected in normal conditions of use or foreseeable emergencies.
In the absence of information on a deflagration or dust explosion event, classifiers may use one or more of the following approaches in determining whether such hazards exist, depending on the information that is available:
All of the voluntary standards recognize that reliable test data for a material, based on scientifically validated tests, is strong evidence for determining whether a material presents a combustible dust hazard and should be used for classification if available. Reliable screening tests, such as that described in ASTM E12263 , showing a positive normalized rate of pressure rise (Kst), and tests for Class II dusts may be used to determine whether a material presents a combustible dust hazard, and classification should be based on such data if it is available.
Many voluntary standards recognize the ASTM E1226 and E15154 methods as reliable means to establish a combustible dust hazard. When performing inspections of classifiers CSHOs must obtain and evaluate any appropriate and available test results for the product to ensure the classification accurately reflects the hazards of the chemical.
Published test results
NFPA 615 , 686 , 4847 , and 4998 publish lists of test results for various materials. Though the NFPA documents caution care in the use of these results because the extent of explosibility can vary even for different dusts of the same solid material, they nonetheless can "aid in the determination of the potential for a dust hazard to be present in [an] enclosure." NFPA 61, A.6.2.1 (2013).
As a part of a poster about combustible dust hazards, OSHA has published a list of combustible materials based on the information provided in the NFPA standards (
Dust particle size
Dusts of combustible material with a particle size of less than 420 microns can be presumed to be combustible dusts. However, certain particles, such as fibers, flakes, and agglomerations of smaller particles, may not pass a No. 40 sieve but still have a surface-area-to-volume ratio sufficient to pose a deflagration hazard. In the most recent revisions, the explanatory notes in many of the NFPA standards have moved from a 420 to 500 micron size threshold. See NFPA 484 (2013), NFPA 654 (2013), NFPA 66411 (2012) and FM Global Data Sheet 7-76 (2013)12 .
View the entire memo here: