What is an acceptable thickness of accumulated combustible dust in an industrial facility?
Combustible dusts present both flash fire and explosion hazards. Companies which generate, handle, process, store, or distribute combustible dusts need to cost-effectively manage these hazards. In the United States, the Occupational Health and Safety Administration (OSHA) has stepped up its enforcement activity and is conducting inspections at these locations to verify that the facility is being operated in accordance with recognized and generally accepted good engineering practices (RAGAGEP). The combustible dust safety standard from the National Fire Protection Association, NFPA 654 (1) is often cited as the RAGAGEP for combustible dust risk management. One aspect of combustible dust risk management is the monitoring and control of fugitive dust accumulation on horizontal surfaces. NFPA 654 gives specific guidance on how to determine an acceptable level of combustible dust accumulation using different risk scenarios. These acceptable levels or thresholds were only recently added as requirements in the 2013 edition of NFPA 654 and there is debate as to whether they are accurate. An examination of this guidance reveals that it is very conservative because it omits consideration of several distinct events necessary for a dust deflagration or flash fire to occur.
NFPA 654, 2013 edition presents four techniques to determine if a flash fire or explosion hazard exists in a building or enclosure. These are: the layer depth criterion, Mass Method A, Mass method B, and Risk Evaluation. The standard gives explicit directions on how to calculate critical layer thickness using the first three methods. The standard does not give guidance on how to conduct a risk evaluation. In this paper we present a risk evaluation based on the NFPA 654 layer depth criteria. We formulate the dust accumulation scenario as a sequence of distinct events, estimate probabilities for each event, and illustrate how the NFPA 654 guidelines generally skew the layer depth criteria towards lower values. It is argued that the NFPA 654 guidance may result in layer depth criteria that are too conservatively low for facilities that manage marginally combustible dusts. In those facilities a more quantitative risk analysis will likely yield better, i.e., more practical criteria.