NFPA 654 provides a framework from which to address combustible dust hazard identification and control. Currently developed by the Committee on Handling and Conveying of Dusts, Vapors, and Gases, NFPA 654 prescribes a step-by-step approach, beginning with characterizing the properties of the combustible particulate solid. These steps form the core requirements of the standard and are the subject of many of the changes implemented by the committee during the current revision process. As part of the regularly scheduled revision, the committee finalized:
1) proposed changes to the definition of combustible dust
2) requirements applicable to the hazard assessment
3) explosion isolation
4) fugitive dust control and housekeeping
5) control of ignition hazards
6) Incident Investigation
7) contractors
Definition of Combustible Dust: Beginning in the 1970s, the definition of combustible dust included criteria based on particle size that was established as 420 microns or smaller in diameter. The size criteria provided a qualitative basis to evaluate solids that are more likely to form dust clouds. A U.S. Standard Sieve is used to screen for the proper particle size threshold but can exclude non-spherical solids, such as flakes, chips, and fibers. The Committee proposes to revise the definition for 2011 as follows: “Combustible dust is a combustible particulate solid that presents a fire or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations, regardless of particle size or shape.”
Hazard Assessment: The most significant revision introduced in the new edition deals with defining those areas where either hazard exists. The standard relies on a trigger that prompts the application of specific protective measures once it has been determined that an explosion or flash fire hazard exists. Since the 1980s, the qualitative trigger has been the nominal layer thickness of combustible dust accumulations. NFPA 654 has used a 132-inch (2.5-centimeter) dust accumulation to define a hazardous area. Though simple in principle, the dust accumulation layer thickness approach raises a number of questions during implementation. One of the most common questions relates to the differences in accumulation layers that can apply based on the bulk density of the specific dust, which is best described as how compressed a dust layer becomes. The hazard areas in the proposed new edition are now based on two sets of equations: a simplified set for both dust explosion and dust flash-fire hazards, and a theoretical set of equations that are derived from the partial volume methodology defined in NFPA 68, Explosion Protection by Deflagration Venting. The simple equations reduce both relationships to the product of a numerical constant and the floor area of the facility. The simple equations establish where a dust hazard exists without extensive dust testing or building analysis. From earlier editions of the standard, such a criterion involved a dust accumulation of 1 kilogram/square meter distributed over a surface area equivalent to 5 percent of a building. The committee’s goal was to retain some of the traditional qualitative criteria related to the 132-inch (2.5-centimeter) definition. The committee derived the two simplified equations by applying conservative properties to the complete (theoretical) equations with the results being single constants. The complete equations for dust explosion hazard area and dust flash-fire hazard area originate from the partial volume equation in NFPA 68, which adjusts the amount of venting needed when the design scenario presumes the combustible dust mixture fills only a part of the enclosure.