Assessing the Potential for a Dust Explosion and Navigating the Current Regulatory Environment

Assessing the Potential for a Dust

Explosion and Navigating the

Current Regulatory Environment

Ali Reza, P.E., CFI

David Clayton, Ph.D., P.E.

Exponent, Inc.

5401 McConnell Avenue

Los Angeles, CA 90066

(310) 754-2700

[email protected]

Ali Reza, P.E., CFI is a Principal Engineer at Exponent and a member of several

NFPA committees, including the committee responsible for the NFPA 654,

NFPA 655 and NFPA 91 standards. He is experienced in evaluating site-specific

procedures, including engineering controls and regulatory requirements, designed

to mitigate or prevent dust explosions. Mr. Reza has investigated and testified

regarding several large dust explosions over the last 30 years.

David Clayton, Ph.D., P.E., is a Senior Engineer at Exponent and works with Mr.

Reza in the engineering investigation and prevention of dust explosions. Dr. Clayton

is an alternate member on two technical committees responsible for NFPA 61, 654,

655 and 91.

Assessing the Potential for a Dust Explosion and Navigating the...

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Assessing the Potential for a Dust Explosion and

Navigating the Current Regulatory Environment

I. Introduction ...169

II. Combustible Dusts and Hazardous (Classified) Locations ...169

III. Elements of a Dust Explosion ...170

IV. Assessing the Hazard of a Combustible Dust ...171

V. Enforcement of Dust Explosion Hazard Regulations ...172

VI. Conclusion ...173

VII. Bibliography ...174

Assessing the Potential for a Dust Explosion and Navigating the...

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Assessing the Potential for a Dust Explosion and

Navigating the Current Regulatory Environment

I. Introduction

Many industries produce and/or handle combustible dusts. These include but are not limited to agri-culture, chemicals, food products, forest and furniture products, metal processing, paper products, pharma-ceuticals, plastics, recycling, wood products, and coal processing facilities. Ignition of accumulated dust can cause an explosion that can result in catastrophic injuries to personnel and significant damage or destruc-tion to property. Accordingly, understanding how to effectively mitigate dust hazards is of significant interest to industry and safety personnel. Although there are no comprehensive federal regulations or requirements regarding combustible dust, the National Fire Protection Association (NFPA) has created a series of consensus standards that have been adopted by many states. State and Federal Occupational Safety and Health Admin-istrations (OSHA) typically attempt to apply the provisions of these standards under Housekeeping require-ments or the General Duty Clause when evaluating dust explosion hazards.

II. Combustible Dusts and Hazardous (Classified) Locations

To provide a clear picture of what constitutes a combustible dust and how dust hazards are classified, some basic information is helpful in first understanding how the NFPA classifies combustible dust.

A combustible dust is defined as a “finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations” (NFPA 654). For example, dusts comprised of metal, wood, coal and other carbon, plas-tics, biosolids, sugar, flour, paper, soap, and many textile materials can cause dust explosions (OSHA, 2008). Combustible dusts are divided into three main groups: Group E includes metal dusts such as aluminum, mag-nesium, or others that present similar hazards; Group F includes various forms of combustible carbonaceous dusts; and Group G includes all other combustible dusts not included in Groups E and F (NFPA 499).

Similarly, the National Electric Code (NEC) defines three types of hazardous locations: Class I loca-tions are hazardous due to the presence of a flammable gas or vapor; Class II localoca-tions are hazardous due to the presence of a combustible dust; and Class III locations are hazardous due to the presence of easily-ignit-able fibers. These locations are further subdivided on whether the hazardous substance is present under nor-mal operating conditions (Division 1) or can be expected due to abnornor-mal operations (Division 2).

Table 1 below summarizes these hazardous locations (NFPA 70).

Table 1. Summary of hazardous locations

Summary of Class I, II, and III Hazardous Locations

Classes Groups Divisions

1 2

I. Gases,

vapors, and

Liquids

and hazardous Not normally present in an explosive concentration (but may accidentally exist)

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II. Dusts

(conductive, and explosive)

F: Carbon dusts (some are conductive, and all are explosive)

G: Flour, starch, grain, combustible plastic or chemical dust (explosive)

Ignitable quantities of dust normally are or may be in suspension, or conductive dust may be present

Dust not normally suspended in an ignitable concentration (but may accidentally exist). Dust layers are present.

III. Fibers and

flyings

Handled or used in

manufacturing Stored or handled in storage (exclusive of manufacturing)

III. Elements of a Dust Explosion

A dust explosion requires five concurrent elements: — Combustible dust

— Oxidizer (e.g., air) — Ignition source — Dispersion of the dust — Confinement

Class II, Division 1 and Class II, Division 2 locations can exist in a wide variety of industrial settings and, in the absence of a rigorous housekeeping program or a properly designed, maintained and operating dust collection system, combustible dust can accumulate on horizontal surfaces such as floors, ducts, pipes, conduit, structural members, equipment, and even above ceilings.

Dust explosions typically begin with a primary event wherein accumulated dust is dislodged, form-ing a small dust cloud. This can occur when a sudden flush of air passes over a surface with standform-ing dust or due to mechanical vibration. This primary cloud can ignite if exposed to a suitable local ignition source. The propagating pressure wave and expanding gases from this initial event can dislodge dust accumulated on sur-faces both near and far from where the initial ignition occurred. The fireball from the primary deflagration can then ignite nearby dust dislodged by the propagating pressure wave. The second fireball so generated can then create a third and larger dust cloud, which can also ignite (Bartknecht, 1989; Eckhoff, 1984).

If the amount of accumulated dust is sufficient, this cascading sequence can lead to catastrophic propagating explosions throughout an entire facility, leading to complete structural collapse and residual fires. Mitigating the potential for a dust explosion when finely divided solids are used or generated during a par-ticular process therefore requires deliberate and systematic engineering controls that remove at least one, and preferably several of the concurrent elements listed above. Since it is expensive to conduct plant operations within an inerting environment, removing the oxidizer is often not a practical option. Similarly, confinement is often provided by the structure of the plant building and accumulated dust can become dispersed when it is first ignited. Accordingly, preventing accumulations of combustible dust via rigorous housekeeping, install-ing dust removal systems, controllinstall-ing ignition sources by requirinstall-ing NEC-compliant electrical equipment, and conducting regular preventative maintenance on rotating machinery and limiting hot-work in areas where dust has accumulated are the preferred mitigation options. Specialized equipment that prevents the

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gation or quenches the initial ignition, thereby preventing a cascading sequence of explosions, has also been successfully employed.

IV. Assessing the Hazard of a Combustible Dust

To assess whether the presence of a specific dust creates an explosion hazard, three specific proper-ties must be evaluated. 1) Flammability: does the material support combustion and under what conditions? 2) Ignition sensitivity: what temperature or energy is required for ignition? 3) Consequences: what is the rate of flame spread and how quickly does the pressure rise during an explosion? Unless data for a specific mate-rial are already available, these properties can only be determined via a series of laboratory tests. If a matemate-rial can burn, its dust will also be combustible. While finer particles generally create more violent explosions, a facility has to be very careful in concluding that they do not have a dust explosion hazard just because they do not generate sufficiently fine particles. In the absence of adequate housekeeping, accumulated powder can be mechanically broken into progressively finer dust that selectively accumulates in higher locations.

The ignition sensitivity is determined from experimentally measured values for Minimum Ignition Energy (MIE), Minimum Explosible Concentration (MEC), or Minimum Ignition Temperature (MIT). Dusts that have a low MIE value (typically less than 25 mJ) should also be characterized to determine their electro-static properties because they might accumulate sufficient charge to produce a spark that has sufficient energy to cause ignition. Electrostatic tests might include a chargeability test to determine how quickly the material can acquire a charge and charge relaxation time or powder volume resistivity tests to determine how easily the material can dissipate the charge.

The explosion severity is also experimentally measured by recording the rate of pressure rise and the maximum pressure obtained when the dust is ignited. The dust deflagration index, K_St (in units of bar*m/s), measures the relative explosion severity as compared to other dusts. Table 2 below shows the four dust explo-sion classes derived from the value of K_St.

According to OSHA, General Duty Clause citations may be issued for deflagration and explosion haz-ards if K_St values for collected dust samples are greater than zero (OSHA, 2008). However, such citations can be reversed if a facility can demonstrate that the MIE of the dust is much higher than likely ignition sources in the vicinity of the dust. Similar arguments can be made if the MEC or MIT exceed likely values in the process areas. Such arguments typically require very strict housekeeping procedures to minimize combustible dust accumulations. However, these efforts can be cost effective when compared to moving or making significant changes to dust collection equipment.

Table 2. Dust explosion class based on K

Dust Explosion Class K_St (bar*m/s) Characteristic

St 0 0 No explosion

St 1 0 < K_St ≤ 200 Weak explosion St 2 200 ≤ K_St ≤ 300 Strong explosion St 3 300 ≤ K_St Very strong explosion

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V. Enforcement of Dust Explosion Hazard Regulations

NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing,

Process-ing, and Handling of Combustible Particulate Solids, officially adopted for the first time in 1945, is recognized

as the primary dust fire and explosion prevention standard in the United States. The standard provides general guidance regarding construction requirements, separation distances between dust handling and processing areas, venting and isolation of equipment, control of ignition sources, housekeeping, employee training, man-agement of change and inspection and maintenance requirements. Additional NFPA industry specific stand-ards serve the agriculture, metals, coal, wood, and sulfur industries.

The NFPA standards are continuously updated based on research and experience from industry, reg-ulators and law enforcement personnel and experts. For example, some of the key changes in the 2013 edition of NFPA 654 include

— Revised separation distance requirements — New enclosure-less dust collector requirements — Expanded housekeeping requirements

— Revised vacuum cleaner requirements

— New chapter on fire protection system impairments — New chapter on requirements for contractors

— Personal protective equipment (PPE) requirements per NFPA 2113 — New incident investigation requirements

Although NFPA does not require that all changes should be retroactive, the evolving nature of these standards makes it necessary for management to continuously assess their facilities and make appropriate changes in their processes and procedures.

Many building and fire codes contain some guidance on dust explosion protection, and the majority of states adopt certain editions of the NFPA standards related to minimizing dust hazards. However, there is no explicit federal requirement for evaluating dust explosion hazards or standard that requires dust collection or explosion mitigation equipment.

Following a series of very large dust explosions from 1999 to 2003, the United States Chemical Safety Board (CSB) examined all recorded dust explosions in the U.S. between 1980 and 2005. They concluded that during this period there were at least 281 combustible dust explosions and fires that caused at least 119 fatali-ties, 718 injuries, and damage in excess of $100 million (CSB, 2006). The CSB study demonstrated that United States industries and facilities did not comply with generally recognized standards of care and that the good engineering practices recommendations in NFPA 654 were implemented sporadically and infrequently. Insur-ance and government investigators did not appropriately recognize dust explosion hazards because they had limited resources, insufficient training, and their enforcement efforts did not focus on these hazards. In the absence of a uniform National Standard, OSHA was also limited in the violations that they could cite for, even if they observed a dust explosion hazard. This prompted the CSB to recommend that OSHA develop a univer-sally applicable, combustible dust standard.

The overall goal of OSHA’s rulemaking is to develop a standard that will comprehensively address the fire and explosion hazards of combustible dust because existing standards address some, but not all, of the elements needed to protect workers from these hazards [Federal Register, 2009]. In 2008 and again in 2009 and 2011, the United States Congress introduced house resolutions (H.R. 5522 in 2008, H.R. 849 in 2009, and H.R. 552 in 2011) to protect workers against combustible dust fires and explosions by requiring the

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tary of Labor to issue a regulatory standard. Although none of these bills passed, OSHA has since released an advanced notice of rule-making, drafted the preliminary regulatory text and conducted a preliminary eco-nomic analysis. However, a comprehensive general industry regulation for the prevention of dust explosions has not yet been agreed upon and, in the interim, OSHA has initiated a Combustible Dust National Emphasis Program (NEP) that became effective in October 2007. They announced that they would inspect facilities that generate or handle combustible dusts that pose a fire or explosion hazard. The NEP identified sixteen tries More Frequent and/or High Consequence Combustible Dust Explosions/Fires” and forty-eight “Indus-tries that may have Potential for Combustible Dust Explosions/Fires” (OSHA, 2008).

Through early 2011, OSHA had conducted approximately 2,300 inspections under the NEP directive, resulting in over 10,000 violations and $23 million in penalties (Kanth, 2011). Grain handling facilities are explicitly covered by 29 CFR 1910.272. For the other industries, OSHA typically issues citations under 29 CFR 1910.22 (housekeeping), 29 CFR 1910.307 (classified electrical equipment) or 29 CFR 1910.176(c) (house-keeping in storage areas). If laboratory testing indicates the dust is combustible and, during their inspection, they determine that accumulations of dust exist on various surfaces within the facility, OSHA can also issue citations under Section 5(a)(1) of the Occupational Safety Health Act, the General Duty Clause (OSHA, 2008).

§5(a)(1) Each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or likely to cause death or serious physical harm to his employees.

Inspections conducted under the Combustible Dust NEP allow OSHA to identify structural defi-ciencies within the dust hazard mitigation and assessment plan within a facility. Accordingly, common NEP inspection observations and violations include (Kanth, 2011):

— Dust collectors located inside buildings without proper explosion protection; — Lack of explosion isolation between equipment;

— Hazardous levels of dust accumulations due to poor housekeeping;

— Explosion vents directed towards work areas or other potentially occupied areas; — Lack of explosion relief venting;

— Horizontal surfaces at elevated locations not minimized to reduce dust accumulation; — Electrical equipment not approved for locations handling combustible dusts;

— Equipment not designed and maintained to minimize dust emissions; — Portable vacuum cleaners not approved for Class II locations;

— Hot work performed in combustible dust handling areas without proper hot work procedures; — Enclosure-less systems not properly located and maintained;

— Use of compressed air to clean combustible dusts with potential ignition sources nearby; — Recycling of dust collector air back into work area without proper safety devices; — Improper ductwork not capable of minimizing static electricity accumulation.

VI. Conclusion

Even without a comprehensive general industry regulation for the prevention of dust explosions, state and federal regulators have started to aggressively inspect facilities that create combustible dust during their operations. A systematic hazards assessment that is based on NFPA 654 can assist plant administrators in determining whether the dust generated at their plant is explosible and in proactively identifying potential

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hazards and non-compliance with the NFPA 654 standard. In the event an entity is cited by OSHA or the local authorities, potential corrective actions can range from a greater (and documented) emphasis on housekeep-ing to expensive equipment retrofit with dust collection or explosion suppression equipment.

VII. Bibliography

Bartknecht, W., 1989, Dust Explosions: Course, Prevention, Protection. Springer-Verlag.

Code of Federal Regulations, Title 29: Labor, Part 1910 – Occupational Safety and Health Standards, Section 22 – General requirements.

Code of Federal Regulations, Title 29: Labor, Part 1910 – Occupational Safety and Health Standards, Section 176 – Handling materials – general.

Code of Federal Regulations – Title 29: Labor, Part 1910 – Occupational Safety and Health Standards, Section 272 – Grain handling facilities.

Code of Federal Regulations – Title 29: Labor, Part 1910 – Occupational Safety and Health Standards, Section 307 – Hazardous (classified) locations.

Eckhoff, R.K., 1984, Dust Explosions in the Process Industries. Oxford: Butterworth-Heineman. 74 FR 54335, October 21, 2009.

Kanth, S., 2011, OSHA Combustible Dust National Emphasis Program and Rulemaking Efforts. National Fire Protection Association, 2013, NFPA 61 – Standard for the Prevention of Fires and Dust

Explosions in Agricultural and Food Processing Facilities.

National Fire Protection Association, 2011, NFPA 70 – National Electric Code

National Fire Protection Association, 2010, NFPA 120 – Standard for Fire Prevention and Control in

Coal Mines.

National Fire Protection Association, 2012, NFPA 484 – Standard for Combustible Metals.

National Fire Protection Association, 2013, NFPA 499 – Recommended Practice for the Classification

of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.

National Fire Protection Association, 2013, NFPA 654 – Standard for the Prevention of Dust

Explo-sions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids.

National Fire Protection Association, 2012, NFPA 655 – Standard for Prevention of Sulfur Fires and

Explosions.

National Fire Protection Association, 2012, NFPA 664 – Standard for the Prevention of Fires and

Explosions in Wood Processing and Woodworking Facilities.

Occupational Health and Safety Administration, 2008, OSHA, CPL-03-00-008 – Combustible Dust

National Emphasis Program.

United States Chemical Safety Board, 2006, CSB Investigation Report: Combustible Dust Hazard