By now, most companies and people throughout the manufacturing sector have heard of combustible dust, as it has become a widely discussed topic. As a review, combustible dust fires and explosions are caused when a combustible atmosphere of dust, or a layer of flammable solids, is introduced to an ignition source. These ignitions can be made worse if there are significant levels of dust accumulation present in surrounding areas, and hazard mitigation techniques are not properly utilized. One of the best ways to identify these dust fire and explosion hazards, as well as being the first step in putting a mitigation action plan together is a combustible dust hazard analysis (DHA). Besides being a great first step, it is also a required one according to the NFPA standards 652 and 61.
Dust Hazard Analysis A DHA is a systematic method for conducting a risk analysis of a facility and process to identify any pieces of equipment and areas that pose potential for combustible dust to be present, as well as identifying the potential ignition sources to which the dust can be exposed. According to NFPA standards, the DHA “shall be led by a qualified person, who possessed a recognized degree, certificate, professional standing, or skill and who, by knowledge, training, and experience, has demonstrated the ability to deal with problems related to the subject matter, the work or project.” Besides the notion of a DHA being a good practice and the requirements to have one conducted in accordance with national consensus standards, there is the question of “Can these requirements be enforced?” The answer is yes, but not always in the ways industry typically thinks about safety and enforcement. NFPA writes the standards, but they do not have enforcement powers on their own. The Occupational Safety and Health Administration does not have regulations specifically covering combustible dust, though they do have some industry-specific regulations (e.g.,1910.272 for grain handling facilities), as well as a combustible dust National Emphasis Program. NEP states that OSHA prioritizes combustible dust-related inspections and utilizes the general duty clause to cover combustible dust hazards not otherwise covered by regulatory requirements like lockout/tagout, fire protection, electrical, etc. Also, we have been seeing an increase in insurance carriers requiring industrial facilities conduct a DHA before awarding coverage, and local fire marshals and building code enforcement requiring a DHA be conducted to determine building occupancy codes or before awarding a building permit. Hazards and Associated Mitigation Techniques While there are several potential hazards that translate across the manufacturing sector, especially related to combustible dust fires and explosions, there are some inherent to the pellet industry that correlate to the materials and equipment used, as well as some to the particular processing techniques involved. One such example relates to the widespread use of bucket elevators as a method of mechanical conveyance. Bucket elevators commonly have a hazardous dust atmosphere, as they convey dry material. Additionally, there can be quite a few ignition sources present, from a bearing that may overheat, metal buckets that could generate a spark, tramp metal entering the system, etc. In accordance with NFPA 652, some mitigation techniques include: • Bearings should be external. • Elevators should be outside and equipped with protection and venting. • Magnets at the elevator infeed. • Monitors for bearing temperature, with a cutoff and alarm at 80% speed, monitor at head and tail, pulley alignment. Besides these systems being required by the standards, they are also good industry practices. Using a poly bucket instead of metal will minimize the potential for sparks. Magnets will limit the potential for foreign metal from entering the elevator. Bearing monitors and sensors will stop the system before an overheated bearing causes heat to transfer into the unit. Even the use of explosion relief panels along the outside of the elevator casing will safely relieve the pressure of an explosion in the event of a combustible dust incident, minimizing the cost of repairs and significantly reducing equipment downtime. Along with bucket elevators, dust collection systems that are used to convey sawdust or provide aspiration for equipment are also susceptible to combustible dust hazards. Dust collection systems regularly have a hazardous dust atmosphere during operation and during filter cleaning. All that is needed for a potential catastrophic incident is the introduction of an ignition source. The most effective mitigation method for this hazard is to prevent the ignition source from going through the conveyance line or into the unit. Per NFPA 652, can be done on the upstream equipment with: • The use of magnets and metal detectors to prevent tramp metal from entering the system. • Routine inspections and preventative maintenance to prevent equipment breakdowns and chips from being conveyed. • Ensuring the system is balanced so that tramp metal is not able to be picked up, utilizing spark detection and suppression or extinguishing system, etc. A method to mitigate the severity of an incident is to equip the units with explosion protection and venting (e.g., deflagration relief or rupture panels, flameless vents, chemical suppression, etc.), as well as installing deflagration isolation systems (e.g., backblast dampener, abort gate, etc.) on the inlet line in accordance with NFPA 68 and 69. It cannot be reiterated enough that housekeeping and inspecting the dust collection lines is a vital piece of this puzzle, as it will help prevent a buildup of material that a hot piece of tramp metal or burning ember could make contact with. Another hazard commonly identified in wood pellet and other timber processing facilities is the accumulation of green sawdust and wood material throughout a mill building, especially within storage areas. While green sawdust has on average a moisture content of about 40% to 50%, it does not take long for the material to dry out, making the sawdust easier to ignite. For example, the green end of a mill in Georgia had a significant accumulation of green dust throughout the infeed area of the mill. A DHA was conducted at this mill in the summertime, and dust samples were taken. Samples of fresh green dust and dust that had accumulated over three days were tested. The moisture content of the green dust that had accumulated over three days had reduced from 45% to 3.5%. At 3.5% moisture, the dust is considered dry combustible dust and poses a severe flash fire and secondary explosion hazard. For this reason, housekeeping procedures and schedules are an important aspect to review during a DHA. There are plenty of hazards that could be present in a facility with relation to combustible dust and fires, and there are many different techniques for minimizing or eliminate those hazards. The first step is to have a DHA conducted. After the DHA, review the recommendations for hazard abatement, and determine a feasible priority and corrective action plan to move toward safety and compliance. Author: Adam Haroz Director of Engineering, Conversion Technology Inc. [email protected] 770-263-6330 Ext. 105
Printed in Issue 2, 2022 of Pellet Mill Magazine