One of the objectives of the standard is to prevent injury from explosions through the design, construction, and maintenance of facilities, processes, and equipment and the implementation of management systems. The standard directly addresses confinement, one of the necessary elements for a combustible dust explosion according to the dust explosion pentagon. NFPA 652 defines an enclosure as a “confined or partially confined volume.” When a combustible dust ignites in a confined area, pressure may build until the structure of the space fails, resulting in the sudden release of pressure, which may result in the resuspension of dust in the air and secondary explosions. THE DUST HAZARD ANALYSIS DEFINED Central to NFPA 652 is the requirement for owners or operators of facilities with potentially combustible dust to conduct a Dust Hazard Analysis. A DHA is intended to determine combustibility hazards; identify and assess fire, flash fire, and explosion hazards; manage those hazards; and communicate the hazards to affected personnel. In the written discussion of the origin and development of NFPA 652, facility owners’ and operators’ lack of awareness of combustible dust hazards was the driving force in establishing the DHA as a fundamental step in creating a plan for safeguarding facilities that handle, store, or process combustible dusts. The standard sets a deadline of Sept. 7, 2020, for completion of the DHA. NFPA 652 provides some clarity on the issue of defining a combustible dust atmosphere, in the sense that it requires the owner or operator of a facility with potentially combustible dust to, among other things, determine the combustibility and explosibility hazards of the materials it handles. Chapter 5 of the standard, “Hazard Identification,” requires the owner or operator to determine whether the materials are combustible or explosible and, if so, to “characterize their properties” in support of the development of a DHA. NFPA 652 permits two options for determining the combustibility or explosibility of dust: the use of historical or published data, or the analysis of representative samples of the dust in question.  There is already much published data with respect to the combustibility of various types of dusts. OSHA published a combustible dust poster (PDF) that identifies 67 agricultural products or dusts, 12 carbonaceous dusts, 14 chemical dusts, 5 metal dusts, and 18 plastic dusts that, if processed in powdered form, pose a hazard of combustible dust explosion. More specific information about more than 7,000 combustible dusts is available from the Institute for Occupational Safety and Health of the German Social Accident Insurance, which developed a database of the combustion and explosion characteristics of dusts, also known as the GESTIS-DUST-EX database. This database provides information about a particular dust’s particle size, explosibility class (based on the rate of pressure rise), and minimum ignition energy. The database does not, however, provide the Minimum Explosible Concentration (MEC) of the dusts, a piece of data that could be invaluable in determining whether an explosion hazard actually exists in a given situation. According to NFPA 652, if the owner or operator chooses the testing option, explosibility of the dust may be determined either by the “go/no-go” screening test methods in ASTM E1226, Standard Test Method for Explosibility of Dust Clouds, or ASTM E1515, Standard Test Method for Minimum Explosible Concentration of Combustible Dusts. The ASTM E1226 test simply determines whether a sample of dust has the potential to be explosive, and the ASTM E1515 test determines the minimum concentration for explosivity of a dust cloud. So, an owner or operator of a facility in which potentially combustible dusts are handled or stored is now directed to define the minimum concentration of the dust suspended in air that could lead to a deflagration. 

As discussed, the current OSHA confined space standard defines a hazardous atmosphere with respect to combustible dust at a concentration that meets or exceeds its lower explosive limit. With NFPA 652, the owner or operator of a confined space containing combustible dust, such as a grain mill, for example, must now define the actual concentration of that dust that may cause an explosion. This information could then be used in confined space situations to more clearly define the level of hazard from airborne combustible dusts, which is a huge improvement from the “obscures vision at a distance of 5 feet” definition. In the case of potentially explosive gas or vapor scenarios in confined spaces, OSHA defines a hazardous atmosphere at a gas or vapor concentration in excess of 10 percent of its lower flammable limit, implying a safety factor of 10 from the actual explosive concentration. So, even if the concentration of dust that may give rise to an explosion in a confined space is known, the definition of combustible dust in OSHA’s Confined Spaces standard fails to adequately protect confined space entrants from explosion hazards, as there is no safety factor built into the definition. Once a dust is determined to be explosible or combustible, the owner or operator is required to perform a DHA. The DHA is the real meat of NFPA 652: it requires an owner or operator, now armed with information that the dusts they process are potentially explosive, to dive further into their operations to identify parts of the process that are vulnerable to dust fires or explosions. Chapter 7 of NFPA 652 describes the DHA process, which is required for all new processes and facility “compartments.” A compartment is defined in the standard as a subdivision of an enclosure, which itself is defined as a confined or partially confined volume. The DHA must assess the fire, deflagration, and explosion hazards in each facility compartment, and provide specific recommendations to manage those hazards. In general, the DHA must identify and evaluate the process or facility areas where fire, flash fire, and explosion hazards exist; where those hazards are present, the DHA must identify specific scenarios that may give rise to a fire and deflagration. The evaluation of fire and deflagration scenarios must include the identification of safe operating ranges, the safeguards that are in place to manage those fire and explosion scenarios, and recommendations for additional controls, including an implementation plan. Where combustible dust is present or potentially present in a process system, each part of the system must be evaluated for the potential to transport dust between parts of the system, the potential for fugitive dust to enter a building or building compartments, and the potential for deflagration propagation between parts of the process system. To be considered a fire hazard, an oxidizing atmosphere and a credible ignition source must be present. Where a sufficient quantity of combustible dust is present to propagate a deflagration, the addition of a credible suspension mechanism, such as vibration or air movement, constitutes a dust deflagration hazard. THE BEST HOPE In October 2018, a CSB study of combustible dust incidents found that an average of 9.5 incidents per year occurred during the period 2006 through 2017, a slight improvement on the annual average of 11 incidents the agency identified during 1980–2005. But the hazard clearly remains. Unless and until OSHA promulgates a combustible dust standard, the NFPA 652 requirement for a Dust Hazard Analysis is perhaps the best hope for prevention of future combustible dust incidents.   RUSH BOWERS, CIH, CSP, is senior industrial hygiene consultant at Terracon Consultants Inc. in Midvale, Utah. Send feedback to The Synergist.