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Keywords:

  • combustible dust;
  • fire;
  • explosion;
  • standards;
  • regulations;
  • prevention;
  • process safety

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

The National Fire Protection Association (NFPA) continues to play an important role in creating guidance for identifying and controlling hazards associated with combustible dusts. Five separate standards address hazards associated with wood-related materials, metals, agricultural products and foodstuffs, sulfur, and all other combustible dusts (e.g., paper, plastics, chemicals, pharmaceuticals). This article will address a number of developments in NFPA's initiatives addressing combustible dust hazards. Topics to be addressed include:

  • The status of, and recent revisions to, NFPA's combustible dust standards;
  • NFPA's plans for a new combustible dust standard, which will address the common issues associated with controlling combustible dust hazards, leaving the commodity-specific standards to address the issues unique to their scope;
  • A new technical correlating committee, which will help ensure greater consistency between the approaches addressed in the various combustible dust standards;
  • The increasing inclusion of process safety management (PSM) principles in the combustible dust standards; and
  • The increasing allowance provided to users of NFPA standards for the use of risk-based decision-making in the selection of combustible dust hazard control options.

Options exist for AIChE to provide technical support to a number of the above areas. The article will also preview a new Center for Chemical Process Safety project for providing tools for addressing combustible dust hazards. © 2014 American Institute of Chemical Engineers Process Saf Prog 34: 24–30, 2015


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

Explosions and fires involving combustible dusts continue to cause injuries, property loss, and business interruptions all too-frequently. Increased awareness and attention within the public; the press; the insurance industry; and licensing, regulatory, and investigatory agencies has sharpened the focus on common-sense, practical methods for abating combustible dust hazards. This article addresses a number of recent initiatives—with particular, but not sole, focus on NFPA's initiatives addressing combustible dust hazards.

BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

NFPA continues to play an important role in creating guidance for identifying and controlling hazards associated with combustible dusts. There are currently five separate standards addressing the safe handling and processing of combustible dusts, as shown in Table 1.

Table 1. NFPA combustible dust standards.
NFPA Std.Facility/Material CoveredScope of Coverage
61Agricultural and food processing facilities• All facilities that receive, handle, process, dry, blend, use, mill, package, store, or ship dry agricultural bulk materials, their by-products, or dusts that include grains, oilseeds, agricultural seeds, legumes, sugar, flour, spices, feeds, and other related materials
• All facilities designed for manufacturing and handling starch, including drying, grinding, conveying, processing, packaging, and storing dry or modified starch, and dry products and dusts generated from these processes
• Those seed preparation and meal-handling systems of oilseed processing plants not covered by NFPA 36, Standard for Solvent Extraction Plants
484Combustible metals• The production, processing, finishing, handling, recycling, storage, and use of all metals and alloys that are in a form that is capable of combustion or explosion
655Sulfur• Size reduction of sulfur and the handling of sulfur in any form
• Does not apply to the mining of sulfur, recovery of sulfur from process streams, or transportation of sulfur
664Wood processing and woodworking facilities• Industrial, commercial, or institutional facilities that process wood or manufacture wood products, using wood or other cellulosic fiber as a substitute for or additive to wood fiber, and that process wood, creating wood chips, particles, or dust
654Any combustible particulate solid not covered by one of the above standards• All phases of the manufacturing, processing, blending, conveying, repackaging, and handling of combustible particulate solids or hybrid mixtures, regardless of concentration or particle size, where the materials present a fire or explosion hazard

As Table 1 indicates, the first four standards are specific to particular commodities or industries. The fifth standard, NFPA 654, Standard for the Prevention of Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids [1], serves as a “catch all” by covering facilities and materials not explicitly addressed by one of the first four standards. As such, it is currently the most broadly applicable, and most general, of the NFPA combustible dust standards. Materials commonly addressed by NFPA 654 would include, but not be limited to, plastics and rubber products, chemicals, pharmaceuticals, and paper and textile products.

All five standards have been recently revised. The next sections of this article addresses some of the more significant revisions to these documents, and highlights some trends in the evolution of NFPA's combustible dust standards—including the gradual incorporation of selected PSM principles.

THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

The NFPA combustible dust hazard standards emerged in the early 1920s and today retain some of the fundamental principles for hazard identification and control introduced at that time. The first evidence of safety practices for combustible dusts came following the tremendous explosion in 1878 at the Washburn “A” flour mill in Minneapolis, which destroyed the mill and two other nearby mills killing 14 Washburn employees and four other persons in nearby buildings. This incident triggered implementation of safety practices within mills nationwide, such as the addition of ventilation systems designed to limit the hazards from combustible dusts associated with flour and other agricultural processes [2].

Then in 1924, NFPA provided owner/operators a standardized framework, or basis for safety, for protecting industrial operations involving solids production, including all aspects of storage, handling, and use. That framework stressed three primary steps to control the hazards. First, control the formation and release of the particulate materials (manage the formation of the fuel). Second, identify and control potential ignition sources, keeping in mind that ignition source control should not be the only layer of protection implemented. Lastly, recognize that if a combustible dust-fueled fire or explosion occurs, it is essential to limit the spread of any incidents through protection measures such as building construction, equipment isolation, and housekeeping.

The earliest NFPA documents related to combustible dusts were developed for pulverized fuel systems and flour, feed, sugar, and cocoa in 1924. NBFU 60, Regulations for the Installation of Pulverized Fuel Systems, published by the National Board of Fire Underwriters (NBFU) and recommended by NFPA described the types of pulverized fuel systems in use at the time and noted that “…all involve the hazard incident to the creation and possible liberation of combustible dusts unless properly designed, constructed, and operated” [3]. Even this earliest of NFPA documents used construction techniques involving detached, segregated, or separated building design that are common to today's hazardous materials building and fire codes. Pulverizing equipment, dust collection systems, dryers, and other equipment that could contribute to the combustible dust hazard were recommended to be isolated if not located outside the facility. Though no specific thickness for tolerable dust layer accumulations within the facility was provided, the requirement for coal drying and pulverizing plants was that they “…be kept free from even small accumulations of coal dust” [4]. To assist in ensuring that dust accumulations were easily detected, surfaces within the facility “…shall be smooth and of a color, preferably white, that contrasts with the dust” [4]. Housekeeping requirements included intervals frequent enough to prevent accumulations, specifying the use of methods that did not scatter or distribute the dust. Nearly 90 years ago, it was recognized that vacuuming or some similar pneumatic collection system was the preferred method for cleaning accumulated dust, rather than the use of compressed air blowdowns. These requirements remain the same in today's NFPA combustible dust standards.

That same year NBFU published NBFU 61 C, Regulations for the Prevention of Dust Explosions in Flour and Feed Mills, and NBFU 62 A and B, Regulations for the Pulverizing Systems for Sugar and Cocoa [5, 6]. These documents served as the forerunners for today's NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities [7], and included provisions quite similar to those highlighted above in NBFU 60. The Committee on Dust Explosion Hazards recognized in 1925 the importance of housekeeping, maintenance of mechanical equipment (such as screw conveyors and blowers), building construction, and dust collection when establishing control of the potential fire and explosion hazards associated with combustible dusts.

In 1948, NFPA approved Fundamental Principles for the Prevention of Dust Explosions in Industrial Plants, NFPA 63 (which was derived from NBFU 63 first published in 1938 under the same title) [8]. Today, there are currently five combustible dust standards; they address specific dust-type hazards associated with agricultural products and food processing, combustible metals, sulfur, wood working and wood processing, and non-type-specific combustible particulate solids.

NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, applies to facilities that handle, process, store, and ship dry agricultural bulk materials, their by-products, or agricultural dusts [7]. Also, included are those facilities designed for manufacturing and handling starch as well as oilseed processing plants. In 1995, four existing standards dealing with starch processing, grain elevators, feed mills, and food processing were combined into the current single standard covering all applications for the agricultural and food processing facilities.

NFPA 484, Standard for Combustible Metals, applies to the production, processing, finishing, handling, recycling, storage, and use of all metals and alloys that are in a form that is capable of combustion or explosion [9]. This standard also completed a consolidation effort in 2002, whereby six metal-specific standards were combined into this single document. The current edition of the standard incorporates an applicability threshold based on the quantity or amount of metal present within a specific occupancy type. If the quantity of metal is exceeded (much like the Maximum Allowable Quantity concept in fire and building codes) then NFPA 484 applies.

NFPA 655, Standard for Prevention of Sulfur Fires and Explosions, applies to the crushing, grinding, or pulverizing of sulfur and to the handling of sulfur in any form [10]. The 2012 edition incorporated updates intended to bring the combustible dust safety content of the standard into conformance with the recent evolution of many of the other combustible dust standards.

NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, establishes the minimum requirements for fire and explosion prevention and protection of industrial, commercial, or institutional facilities that process wood or manufacture wood products [11]. Similar to NFPA 484, this standard defines an applicability threshold. The standard applies to woodworking operations that occupy areas of more than 465 m2 (5,000 ft2) or where dust producing equipment requires an aggregate dust collection flow rate of more than 2,549 m3/h (1,500 ft3/min).

The one remaining standard, NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, applies to all phases of the manufacturing, processing, and handling of combustible particulate solids or hybrid mixtures where the materials present a fire or explosion hazard [1]. NFPA 654 is the single standard currently published by NFPA that is not dust-type specific. As such, it utilizes a more process safety-oriented philosophy for many of its core requirements.

During the past 2 years, each of the standards just described completed a revision—process safety elements formed the basis for many of the recent changes to the standards. Today, all the standards include requirements that are predicated on the existence of a combustible dust fire or explosion hazard, but previously no clear guidance was given on how to determine whether such hazards existed. With the most recent revisions to NFPA 484, 654, and 664 criteria have been provided for determining whether a dust flash fire or dust explosion hazard exists. Where those conditions exist, additional safeguards are then required to protect the facility, equipment, and personnel.

One difference found in the NFPA standards over the past 20 years is the introduction of safety management system concepts. Safety and loss prevention experts recognize the role of people in the process and address that recognition by considering the influence of the worker on everyday safety. For that reason, NFPA 654 and NFPA 664 began in the 1990s to include process safety concepts within their requirements to highlight that, in addition to the explicit engineered requirements, it was essential also to have a system for managing the various elements of process safety. The elements that currently have been included in the standards are: process hazard analysis (PHA) (or hazard analysis); management of change (MOC); operating procedures; emergency response; incident investigation; inspection, testing and maintenance; and training. More will be provided on this topic later in the article.

THE REGULATORY ENVIRONMENT

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

Following its investigation of three particularly severe combustible dust incidents in 2003, the U.S. Chemical Safety and Hazard Investigation Board (CSB) initiated a broad investigation of dust explosions in general industry. In its report, issued in November 2006, the CSB concluded that combustible dust explosions were a serious hazard in American industry, and that existing efforts inadequately addressed this hazard [12]. The CSB issued a recommendation to the Occupational Safety and Health Administration (OSHA) to develop a comprehensive combustible dust federal standard based, in part, on NFPA dust explosion standards, including NFPA 654 and NFPA 484. At that time, and to this day, the only existing industry specifically covered by a federal standard applicable to combustible dust processes is the grain handling industry. OSHA standard 29 CFR 1910.272 covers grain handling facilities and was issued in the late 1980s [13].

In addition to the recommendation to develop a standard, the CSB also recommended that OSHA establish a Special Emphasis Program (SEP) for combustible dusts. As part of the SEP, OSHA implemented a National Emphasis Program (NEP) focused on increasing its inspections for those industries identified as having the potential for combustible dust hazards. The NEP for combustible dusts was initiated in October 2007 and further expanded in March 2008 following a catastrophic sugar refinery explosion earlier that year.

Then, in April 2009, the Secretary of Labor announced that OSHA would be initiating the rulemaking process for a comprehensive federal standard for combustible dust hazards in general industry. OSHA published the Advanced Notice of Proposed Rulemaking (ANPRM) in October 2009 and called for public comment to be submitted by January 19, 2010 [14]. In addition to soliciting public comments in response to the ANPRM, OSHA conducted three stakeholder meetings in 2009 and 2010 to provide additional opportunities to gather information on the hazards from combustible dusts, the steps various industry stakeholders were already taking to address those hazards, the status of consensus standards, and what was needed from an OSHA standard to protect workers from the fire and explosion hazards posed by combustible dusts. Information was also being gathered through the inspections being conducted as a result of the NEP.

One step in OSHA's regulatory process is the requirement to conduct a review of the potential impact that a new regulation might have on small employers. This review, required by the Small Business Regulatory Enforcement Fairness Act (SBREFA), has been scheduled and rescheduled several times since the end of 2010. While awaiting the SBREFA process to begin, OSHA convened an “expert panel” in May 2011 for the purposes of providing the agency staff with further information regarding combustible dust hazards and possible solutions that should be considered by OSHA in developing the proposed standard. The economic impact on small employers was one of the underlying questions for this special meeting. The SBREFA process remains the next step for OSHA prior to issuance of a Notice of Proposed Rulemaking which would finally provide stakeholders and other interested parties with the first look at specific regulatory text. The SBREFA review is now scheduled to be initiated in December 2014, according to OSHA's current published regulatory agenda [15]. With that proposed schedule, it is not likely that the rulemaking process would be completed before the end of 2015.

TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

During the Public Comment period for OSHA's ANPRM on combustible dusts, some commenters suggested that NFPA's multiple combustible dust standards for the various industries and dust types complicated use of those standards. It was suggested that those standards could be confusing for users and that inconsistent or possibly conflicting requirements existed within the documents. While progress with a comprehensive federal OSHA standard has been slow, and the outcome uncertain, NFPA has taken heed of the public comments and has initiated a restructuring of the current combustible dust committee projects. NFPA acted to establish a new committee structure by creating a Correlating Committee and a new Technical Committee.

The Correlating Committee will enable some of the core topics to be coordinated or correlated between the five current standards so that issues such as the definition of what constitutes a combustible dust and methods for determining that a hazardous condition exists can be reviewed, discussed, and treated in a consistent manner. Underlying NFPA's approach is the belief that fundamental or core requirements exist for all the industrial processes where combustible solids, powders, or dusts are handled or created and, thus, NFPA established the new Technical Committee on Fundamentals of Combustible Dusts. This new committee is charged with developing a standard on the fundamentals of combustible dust that will address those common considerations that apply across the five current standards—agricultural, combustible metals, general, sulfur, and wood processing/woodworking—and then direct the user to the dust-type specific standard for those requirements unique to that industry sector or dust type. The new standard, NFPA 652, Standard on Combustible Dusts, is currently in the first stage of development and is slated for completion in Fall 2015 [16]. Its scope is to provide the basic principles of, and requirements for, identifying and managing the fire and explosion hazards of combustible dusts and particulate solids.

INCLUSION OF PSM PRINCIPLES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

NFPA publications have long been recognized as a source of guidance for the design, construction, and maintenance of buildings; mechanical equipment; safety systems; and personal protective clothing and equipment related to mitigating the risks of fires and explosions. In addition to this “hardware” guidance, NFPA publications have also provided guidance on administrative requirements. Training requirements, for example, have been commonly addressed, where relevant, in NFPA publications. To further illustrate the point, it is noted that the 1982 edition of NFPA 654 incorporated requirements for operating procedures and emergency plans, and for training of personnel on these. The chemical industry tends to aggregate such administrative requirements under the concept of a PSM system.

Several years after the 1992 effective date of the OSHA PSM regulation, 29 CR 1910.119 [17], the NFPA technical committee responsible for NFPA 654 began the development of what would become the 1997 edition of the standard. In doing so, the Committee sought to address the question “Given that PSM is intended to prevent toxic releases and flammable gas explosions, and their unintended consequences, aren't there aspects of PSM that could be equally effective in preventing combustible dust explosions?”

In answer to that question, the Committee strengthened the section on procedures and training in NFPA 654, including explicitly addressing maintenance procedures. The 1997 edition also introduced a requirement for MOC of process materials, technology, equipment, procedures, and facilities. These initial ventures into PSM content were made with the goal that they should not be perceived as imposing a “chemical industry” solution on a variety of industries that were beyond the coverage boundaries of 29 CFR 1910.119. The intent was to add to NFPA 654 a “PSM-lite” treatment of some of the more important aspects of PSM, providing even less specificity than the “performance-based” OSHA PSM regulation.

The 2006 edition of NFPA 654 specified, along with other requirements, that the design of the fire and explosion safety provisions for a facility must be based on a PHA of the facility, the process, and the associated fire or explosion hazards.1 The most recent revision to NFPA 654 (2013 edition) added requirements for incident investigation and the qualification and training of contractors [1]. In addition, the content on emergency procedures and inspection, testing, and preventive maintenance was strengthened.

Fortunately, there is a degree of overlap in the membership of the committees responsible for the five current combustible dust standards. This provides for cross-fertilization of ideas between the committees. As Table 2 illustrates, these process safety principles are being increasingly incorporated in the NFPA combustible dust standards—a trend that is anticipated to continue.

Table 2. Process safety elements incorporated in NFPA combustible dust standards.
NFPA StandardProcess Safety Element
Process Safety InformationHazard AnalysisOperating ProceduresHot WorkManagement of ChangeTrainingInspection, Testing, and Preventive MaintenanceContractorsIncident InvestigationEmergency Procedures
61NNNYNYYNNY
484NYNYYYYNNY
654YYYYYYYYYY
655NYYYYYYYNY
664YYNYYYYYYY

ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

Several additional NFPA sources of information and guidance on addressing combustible dust hazards warrant mention. NFPA and the Fire Protection Research Foundation sponsored symposia addressing combustible dust hazards in 2009, 2010, and 2011. Proceedings from these symposia can be accessed on the NFPA web site [18-20].

In 2012, NFPA published the NFPA® Guide to Combustible Dusts, which was coauthored by the authors of this article and Sam Rodgers (Figure 1) [21]. This 300-page reference provides numerous photographs and case studies of combustible dust incidents. Chapters address the following topics:

  • Explosion fundamentals—extensive background on combustion principles, including discussion of the more commonly applied combustible dust hazard characterization tests
  • PHA—use of PHA to identify, evaluate, and propose controls for combustible dust hazards
  • Fire hazard control—focusing on some of the unique issues involved in combatting fires involving combustible dusts
  • Explosion hazard control—discussion of the application of the various explosion prevention techniques recognized by NFPA
  • Industrial processes—illustrates the application of basic combustible dust safety concepts to some commonly encountered processes and types of equipment
  • Safety management systems—discussion of the application of selected safety management system elements to the control of combustible dust hazards.
image

Figure 1. NFPA guide to combustible dusts. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Download figure to PowerPoint

This book serves to enhance the visibility of combustible dust hazards, as well as to distill highly technical information from multiple standards into a collection of guidance that is accessible and understandable to the nonexpert and end users that are expected to be able to put such guidance into practice.

In addition, the NFPA website provides a number of investigation reports addressing significant combustible dust incidents.

RISK-BASED DECISION MAKING

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

No human endeavor, including the handling and processing of combustible dusts, can be made risk-free [22]. We can, however, seek to mitigate risks to a tolerable level. The NFPA standards described above are certainly sources of risk-mitigation guidance. The requirements in these standards are based on decades-old proven industry practices, more recent technological advances, and the collective experience and judgment of the members of the technical committees responsible for the standards. Implicit in the standards-making process is the concept that conformance with the standard requirements should help the user attain a tolerable level of risk in operations.

NFPA standards have traditionally been compilations of very explicit, specific requirements—“in such-and-such a situation the user shall do this-and-that.”2 NFPA has recognized that the literal requirements of a standard may not always be attainable for all users. For this reason, the front-end “boiler-plate” of an NFPA standard contains an equivalency clause that states, in part:

“Nothing in this standard is intended to prevent the use of systems, methods, or devices of equivalent or superior quality, strength, fire resistance, effectiveness, durability, and safety over those prescribed by this standard” [e.g., 1].

In other words, the user is given the latitude to propose means, alternative to those explicitly established by the standard, to mitigate the risk of operations. Presumably, these alternate means should control the risk to a level no greater than that which would remain had the literal NFPA requirements been implemented. These alternate means are subject to approval by the authority having jurisdiction (AHJ).3

About a decade ago, NFPA began gradually incorporating into its standards a new option for compliance. This new option, the performance-based option, allows the user to bypass certain of the traditional, prescriptive requirements and propose alternative approaches to achieve the objectives established for the standard. Such objectives include protecting life safety, ensuring structural integrity of the facility sufficient to allow for protection or evacuation of occupants, and ensuring continuity of operations to a degree satisfactory to the owner/operator. In application, the performance-based option applies to certain of the engineered controls, and not the administrative controls. For example, the housekeeping and training requirements remain governed by the prescriptive portions of the standard.

In effect, the performance-based option provides more detailed guidance for implementation of the equivalency clause described above. The performance-based option requires that specific fire and explosion scenarios be addressed in the design of the facility and systems, and that quantitative substantiation be provided to show how the performance objectives will be met. This requires documentation of all calculations, references, assumptions, and sources from which material characteristics and other data have been obtained. The AHJ is permitted to retain an independent third-party to review the proposed design.

More recently, some standards such as NFPA 654 have incorporated language such as:

“A documented risk evaluation acceptable to the authority having jurisdiction shall be permitted to be conducted to determine the level of protection to be provided per this chapter.”

The above is quoted from Chapter 7 of NFPA 654 [1], which addresses requirements for fire and explosion protection for specific types of equipment. For example, Chapter 7 might require explosion protection, such as deflagration venting, on a dust collector. The cited paragraph introduces the option for demonstrating, by risk assessment, that explosion protection is not necessary or warranted for a particular dust collector; for example, because the inventory of combustible dust in the collector is so low that the risk of an explosion is low enough to be tolerable.

As will be discussed below, application of the risk-based option places a burden on both the user and the AHJ to understand basic concepts related to risk assessment and risk-based decision making.

AREAS TO ADDRESS GOING FORWARD

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

As NFPA and industry continue to advance practices to control combustible dust hazards, several topics remain in the forefront for research and discussion. Foremost on the list is the development of approaches for assessing whether a dust flash fire or dust explosion hazard condition exists. This is important since many requirements in the NFPA standards only apply to dust fire or explosion hazard areas. The 2013 edition of NFPA 654 includes several options for assessing whether a dust flash fire or dust explosion potential exists in a particular area [1]. These are all premised on scenarios involving the suspension of ambient dust layers to form combustible dust clouds. The Fire Protection Research Foundation sponsored a project, completed in 2011, to study entrainment of combustible dust from layer accumulations [23]. While much has been accomplished, additional work will likely improve the ability to define areas that require protection from dust flash fires and explosions.

Annex A of the 2013 edition of NFPA 654 highlights an issue related to the results from the testing conducted for several combustible dust properties, such as maximum deflagration pressure and deflagration index. Similarly, Chapter 2 of the NFPA® Guide to Combustible Dust describes the potential for obtaining false results—either false positives or false negatives—from screening tests used to determine whether dusts are in fact combustible and, thus, warrant coverage under the NFPA standards. One such test is the Kst test. It is often difficult to infer whether a low Kst value is indicative of a marginally combustible dust that still warrants a degree of explosion protection or, alternatively, whether it is a false positive produced by the nature of the test protocol. Some work on marginally combustible dusts has been reported [24], but additional guidance is needed on this topic.

OSHA has cited employers for failing to provide protective clothing for employees where a dust flash fire exposure potential exists. As a result of recent CSB investigations of the iron dust flash fires from incidents that occurred in early 2011 [25], greater focus has been placed on the use of fire-resistant clothing in combustible metal dust applications. More research on the effectiveness of fire-resistant clothing on various types of dusts may be warranted.

The current revision of NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities [7], is seeking to address the design and installation of deflagration venting for silos. NFPA 68, Standard on Explosion Protection by Deflagration Venting [26], provides the requirements for determining the mandatory vent area for silos and includes calculations that account for the fact that some enclosures containing solids or powders may only be partially filled by combustible dust clouds. However, silos provide particular challenges to deflagration venting and further research on the placement of the vents on silos and limitations of the venting of certain silo designs may be warranted.

Prior definitions of combustible dust included a maximum particle size threshold; for example, 420 microns. However, the most current definition used in NFPA 654 has removed the size specification and placed the discussion of the influence of particle size on the behavior of combustible dusts in Annex A of the standard. During recent discussions within the various NFPA Technical Committees, alternative particle-size thresholds have been suggested. For example, FM Global and operators in Europe are apparently using 500 microns rather than 420, while others suggest the upper limit could be somewhat greater than 500 microns. The concepts of whether there is some particle size, above which combustible dust flash fire and explosion concerns could largely be discounted—and, if so, what that value might be—remain topics for debate.

The concept of risk-based decision making for determining prudent combustible dust hazard mitigation is now embodied in a number of NFPA standards. Additional guidance to owner/operators and AHJs will be required to facilitate the use of this option.

THE ROLE FOR AIChE

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED

Earlier, this article described how “PSM-lite” management system concepts are gradually being incorporated into the NFPA combustible dust standards. The publications of the Center for Chemical Process Safety (CCPS) provide useful sources of guidance for those seeking to implement such “PSM-lite” provisions.

The CCPS publication Guidelines for Safe Handling of Powders and Bulk Solids [27] should be noted as an existing source of valuable information on the mitigation of combustible dust hazards.

As noted above, additional guidance on risk assessment and risk-based decision making will be required to assist those seeking to use the flexibility recently incorporated in the NFPA standards. As a leader in the development of guidance on risk assessment for the process industries, CCPS is an obvious and likely source for additional guidance in this area. CCPS has initiated a project to develop and publish a concept book, tentatively titled A Risk-Based Approach to Assessing, Controlling and Mitigating Dust Fire and Explosion Hazards. It is anticipated that this book will be published in later 2014 or early 2015.

  1. 1

    The 2000 edition of NFPA 654 was an “out-of-cycle” revision that focused on combining NFPA 650 with NFPA 654. No significant consideration of PSM content was added to this revision.

  2. 2

    NFPA standards and codes establish mandatory requirements and are written in the normative form, using “shall.” Other NFPA documents, such as recommended practices, contain nonmandatory guidance.

  3. 3

    NFPA defines the AJH as “An organization, office, or individual responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure” [e.g., 1].

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BACKGROUND ON NFPA COMBUSTIBLE DUST STANDARDS
  5. THE EVOLUTION OF THE NFPA COMBUSTIBLE DUST STANDARDS
  6. THE REGULATORY ENVIRONMENT
  7. TYING IT ALL TOGETHER—THE SEARCH FOR COMMON-SENSE CONSISTENCY
  8. INCLUSION OF PSM PRINCIPLES
  9. ADDITIONAL COMBUSTIBLE DUST SAFETY GUIDANCE FROM NFPA
  10. RISK-BASED DECISION MAKING
  11. AREAS TO ADDRESS GOING FORWARD
  12. THE ROLE FOR AIChE
  13. LITERATURE CITED