Dust, Revised
Sharpening the definition of "combustible dust," and other proposed changes to the 2011 edition of NFPA 654.
NFPA Journal , March/April 2010
By Guy Colonna
On January 29, 2003, a giant explosion ripped through the West Pharmaceutical Plant in Kinston, North Carolina, killing six and injuring dozens. Less than a month later, on February 20, an explosion and fire killed seven at CTA Acoustics in Corbin, Kentucky. And on October 29 of that year, an explosion at the Hayes Lemmerz International plant in Huntington, Indiana, damaged the facility and severely burned two workers, one of whom later died.
All three incidents were blamed on combustible dust. The U.S. Chemical Safety and Hazard Investigation Board (CSB) found that the explosion at West Pharmaceutical, which made pharmaceutical equipment, was the result of the ignition of polyethylene dust. At CTA Acoustics, which produced fiberglass insulation for the automotive industry, the CSB determined that resin dust in a production area was probably ignited by flames from a malfunctioning oven. And at Hayes Lemmerz, which made cast-aluminum automotive wheels, the CSB attributed the explosion to accumulated aluminum dust.
The trio of explosions heralded an awakening in the industrial and enforcement communities to the dangers of combustible dusts. In its investigation reports from these incidents, the CSB highlighted NFPA’s five combustible dust standards, including NFPA 654, Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. In each report, the CSB stated that compliance with the provisions of the voluntary consensus standard could have limited the consequences of these incidents. The level of awareness about combustible dust hazards continued to grow when 14 workers perished in the February 2008 fire and explosion at the Imperial Sugar refinery in Port Wentworth, Georgia.
NFPA 654, which provides requirements for hazard identification and hazard control to protect against combustible dust fires and explosions, has often been characterized as the fundamental dust explosion prevention standard, since it applies to non-specific combustible dusts, including plastics, pharmaceuticals, textile fibers, and paper products. Today, 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 proposed changes to the definition of combustible dust and to requirements applicable to the hazard assessment, explosion isolation, fugitive dust control and housekeeping, control of ignition hazards, and contractors.
Sharpening the definition
Hazard identification begins with recognizing what materials are combustible dusts. NFPA 654 currently includes a broader concept—combustible particulate solid—to stress that even larger solid particles can, through handling or processing, be broken down into the finely divided solid form that creates the most critical condition associated with combustible dusts. The dusts become a problem when they are suspended in air at the optimum concentration in the presence of a viable ignition source, and where confinement occurs, either in process equipment or a building or structure. Without all these conditions, the dust or solid is still capable of combustion, but the nature of the combustion does not yield an explosion. The importance of the combustible dust characterization is evident through the history of the revisions to NFPA 654 and the changes to the specific 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 addressed this exclusion with a revision of the definition in the 2006 edition of the standard. The Committee proposes to further 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.” Annex material supports the definition by explaining the traditional use of the particle size threshold and how the new definition addresses the importance of surface-area-to-volume ratio in determining the behavior of various solids.
NFPA 654 protects lives and property from dust explosion hazards and flash fire hazards. 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.
The committee acknowledged industry best practice by including a new section on incident investigation. An investigation will be required with this new provision any time a fire or explosion causes property damage, production shutdown time, or injury. A similar requirement already exists in NFPA 664, Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities. This provision ensures that owner/operators will respond to incidents and learn from them.
The importance of housekeeping
The investigations conducted by the CSB and the Occupational Health and Safety Administration (OSHA) into the recent combustible dust incidents reveal a lack of housekeeping as a contributing factor in many of the incidents. With combustible dust explosions, the secondary explosions are often the cause of the most significant damage. Secondary explosions result from the introduction of the accumulated dust into the dust cloud that can then be ignited by the initial source of the fire or explosion. Thus, housekeeping represents a major factor in controlling the amount of accumulated dust that could be entrained into a potential secondary explosion.
As a result, the revised provisions in NFPA 654 focus on facilities that are intended to operate with less than the dust accumulation levels defined in the new dust hazard evaluation equations. A planned inspection process must be implemented in order to evaluate the dust accumulation rates in the facility and to determine the housekeeping frequency needed to maintain accumulations below established thresholds. The standard now also provides an option for those facilities that are intended to operate above the dust accumulation levels, such as facilities that require a documented risk evaluation to determine the housekeeping frequency that ensures the accumulated levels can be controlled by installed protective measures.
In a related matter, the committee has also clarified the hierarchy to be followed during cleaning. Since the 1970s, the standard has provided guidance regarding the use of the most common cleaning methods, including sweeping, vacuum cleaning, blowing down, or water wash-downs, but without any preference. As the investigation reports demonstrate, improper cleaning methods can be as potentially dangerous as no cleaning. The committee established in this revision that vacuum cleaning is the preferred method, to be followed by sweeping or water wash-down. Blow down using compressed air or steam is permitted, but only after the other methods have been used or when the other methods result in a greater personal safety risk. Use of the blow down method requires that other factors be considered, including a restriction on the air or steam pressure and the use of protected electrical equipment in accordance with NFPA 70®, National Electrical Code®.
The committee added a new section on contractors and subcontractors to address another factor common to industrial incidents, including those involving combustible dust fires and explosions. These new provisions require contractors to possess appropriate credentials attesting to their qualifications and to be specifically trained in the installation, testing, and maintenance of systems and equipment on which they are working. In addition, the contractors must be trained on facility safe work practices, such as lockout/tagout, hot work, housekeeping, and personal protective equipment use, and the emergency response and evacuation plan.
There are a number of other changes proposed to the revision of NFPA 654 that have not been included in this discussion. For more details on the complete changes and proposed new text, the Report on Proposals (ROP) and Report on Comments (ROC) for the Annual 2010 revision cycle for NFPA 654 can be found at www.nfpa.org/654.
The next step in the revision process will be the submission of any Notice of Intent to Make a Motion (NITMAM), which must be provided to NFPA Standards Administration by the April 9, 2010, deadline. Two other NFPA combustible dust projects, NFPA 664 and NFPA 484, Combustible Metals, are beginning the revision cycle. Both these documents will be reporting in the Annual 2011 revision cycle. In addition to the activity on these NFPA standards, OSHA will continue its efforts to develop and publish a comprehensive federal standard for combustible dust hazard process safety in the workplace. The recent tragic losses suffered by the various industries reported by the CSB and OSHA over the past decade highlight the importance of the work of the NFPA committees as they develop the best practices that protect life safety and property so that these practices can be implemented and enforced.
Guy Colonna is division manager of Industrial and Chemical Engineering at NFPA.