Read 11_mar_Safety_First.pdf text version

Hazards are ever-present in the steel plant environment, and a heightened awareness and emphasis on safety is a necessary priority for our industry. This monthly column, written by members of the AIST Safety & Health Technology Committee, focuses on procedures and practices to promote a safe working environment for everyone.

Combustible Dust in the Steel Industry -- OSHA NEP Recognition, Evaluation and Control

Recognition

Although the steel industry has experienced few events resulting in the devastating effects of combustible dust explosions, the Occupational Safety and Health Administration (OSHA) has included the industry in the National Emphasis Program (NEP), OSHA Directive CPL 03-00-008. Therefore, the steel industry must be aware of the aspects of the NEP, what OSHA is looking for and using to enforce this program, how to recognize and minimize unknown potential dust explosion conditions, how to increase employee safety, and how to prepare for an OSHA visit. Since OSHA has no specific standard in place to regulate combustible dust, National Fire Protection Association (NFPA) consensus codes and standards for the processing and handling of combustible particulate solids are utilized for compliance. Specific NFPA codes and standards considered applicable to the steel industry are listed below: · NFPA 654 (2006 edition), "Standard for the Prevention of Fire and Dust Explosions From the Manufacturing, Processing, and Handling of Combustible Particulate Solids." · NFPA 68 (2007 edition), "Standard on Explosion Protection by Deflagration Venting." · NFPA 69 (2008 edition), "Standard on Explosion Prevention Systems." · NFPA 70 (2008 edition), "National Electrical Code." · NFPA 499 (2008 edition), "Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas." In addition to the NFPA codes, OSHA inspection procedures and criteria based on the NEP may utilize the following OSHA regulations: · · · · · 29 CFR 1910.307 and 399, Subpart S -- Electrical. 29 CFR 1910.94, Ventilation. 29 CFR 1910.22, Housekeeping. 29 CFR 1910.176(c), Storage. 29 CFR 1910.132(a), PPE. · 29 CFR 1910.178, Powered Industrial Trucks. · 29 CFR 1910.253, Welding, Cutting, Brazing. · 29 CFR 1910.5(a)(1), General Duty. The Chemical Safety Board (CSB) November 2006 for U.S. Dust Incidents 1980­2005 study found: · Overall awareness and understanding of dust explosion hazards is lacking. · Training is needed. · Secondary explosions are most often responsible for damage and injuries. · Existing codes and standards are good but not understood or applied consistently. With the overall lack of awareness and understanding of dust explosion hazards, the recognition of areas or processes that may pose a dust explosion hazard starts with identifying product stream materials, including alloys, additives (bulk dust products), process waste streams (grinding, sawing, etc.) and other dust-potential materials. These materials may be classified as combustible dust. Where or how these materials are used or produced needs to be assessed as to their accumulation on surfaces; otherwise, the dust collection system, if applicable, needs to be assessed. Accumulation of combustible dust of greater than 1/32 inch (thickness of a paper clip) on surfaces is the rule of thumb found in NFPA 654 and used by OSHA in their inspections. These accumulations add significantly to combustible dust deflagration risks; the greater the accumulation, the greater the risk. Areas or processes may include meltshops (see Figures 1­3), including alloy or additive storage or transport areas, BOFs, EAFs; ladle refining; caster runout torch cut or other metal powder cutting processes; slab/billet conditioning (cutting/ grinding) or other processes that produce dust from the product. Also included for integrated mills are coke plant coal and coke handling processes.

Evaluation

Once risk areas and processes are identified, evaluate the area against the applicable OSHA regulations and NFPA codes

Figure 1 -- Meltshop area adjacent to the EAF.

Figure 2 -- An elevated surface in the meltshop.

Figure 3 -- The alloy addition area of the meltshop.

March 2011 31

Safety First

Table 1

Dust Explosion Class

St 0 St 1 St 2

Kst (bar·m/s)

0 >0 and 200 >200 and 300 >300

Characteristics

No explosion Weak explosion Strong explosion Very strong explosion

Figure 4 -- Graphite mixing area/dust collector in a pipe mill. mentioned, and characterize the dust for combustibility. The areas should also be evaluated for housekeeping issues (accumulations on upper surfaces), ignition sources (open flames, welding/torch cutting, resistance heating, friction sparks, high heat), electrical classification (NEMA 12 or NFPA 499 Class II) of enclosures/equipment, wiring and outlets/lighting. Other issues for evaluation are the use and class of powered industrial trucks, selection of employee personal protective equipment (fire-retardant clothing), storage considerations, and the use of ventilation (dust collectors, baghouses). Several characteristics affect the flammability of dust: temperature, particle size, moisture content, turbulence, pressure (confinement) and the presence of oxidants. Similar to conditions for a fire, a dust explosion requires five basic conditions: a fuel (combustible dust), in a concentration in air at a concentration within explosible range, a particle size distribution capable of propagating flame, an ignition source must be present, and the atmosphere must support combustion (O2). The dust needs to be characterized as to whether it is explosive. Two analytical procedures may be used: (a) the screening combustibility test (A/B Test), where the dust is determined to be combustible or not, but does not provide a value as to the severity of an explosion; and (b) the Kst Test (20-liter test sphere per ASTM E1226), which determines the combustibility and deflagration index (explosion severity). It should be noted that the dust for both procedures is dried and screened for optimal conditions to support ignition. There has been some controversy over whether the 20-liter test sphere has been "over-driving," or causing, a false positive result for weakly explosible dust. However, this is an established ASTM method. The alternative test, the A/B method, utilizes a significantly larger test vessel, a 1 m3 sphere. However, this test method is not ASTM, and has false negative results because of the non-uniform distribution of the dust inside the vessel due to the increased size. The ASTM method may over-rate weakly explosible dust and subsequently increase the likelihood of increased mitigation and controls. Even so, a false negative result with the alternative method may mischaracterize a potentially explosible dust and result in a catastrophic event due to the lack of mitigation and controls. The relationship between Kst value and the physical severity of the dust explosion is given by Table 1, which is taken from OSHA Directive Number CPL-03-00-008. OSHA inspections will have the Kst Test, not the A/B Test, performed on suspected dust. If the Kst value is greater than 1, a citation may be issued. The A/B Test is a relatively less expensive procedure and requires less dust to perform, whereas the Kst Test is expensive and requires a sample approximately the size of a 1-pound coffee can.

32 Iron & Steel Technology

St 3

In addition to these two testing procedures, the combustibility of the dust may also have other characteristics that will aid in determining the likelihood or mitigation of explosions: · Minimum Ignition Temperature (MIT), which is not a fundamental property but depends on particle size and moisture content. · Minimum Ignition Energy (MIE), which is the minimum spark energy needed to ignite an optimum concentration of the material using a capacitive spark. · Minimum Explosible Concentration (MEC), which is the minimum concentration in air to support ignition.

Control

The mitigation of explosive dust combines basic safety practices and technology by eliminating ignition sources (heat sources, mechanical friction, friction sparks, electrical sparks, static electricity). Heat sources can be controlled by preventing the overloading of the processing plant in which the internal buildup will both reduce heat loss from material and increase operating temperature above "normal." Consider installing overload protection devices on drive motors, isolating or shielding hot surfaces, preventing/removing dust accumulation on hot surfaces, and using approved electrical equipment (with the correct temperature rating). If the material is subjected to heat as part of the normal process (e.g., during drying), the temperature should be maintained below the self-heating temperature of the material. Friction sources can be controlled by preventing foreign material from entering the system when such foreign material presents an ignition hazard. Consider the use of screens, electromagnets, pneumatic separators, etc. Prevent overheating due to misalignment, loose objects, belt-slip/rubbing, etc., by regular inspection and maintenance of the plant. Hot work operations should be controlled by a hot work permit system in accordance with NFPA 51B, "Standard for Fire Prevention During Cutting, Welding, and Other Hot Work." Impact sparks can occur, for example, when operators use, drop or otherwise strike metal equipment with metal tools or objects. Minimize the likelihood of impact sparks through proper tool selection (non-sparking), techniques to prevent dropping tools (straps), and employee training. Electrical equipment can be a major source of ignition, particularly when incorrectly specified electrical equipment presents hot surfaces and sparks for flammable gases, vapors and dusts. The electrical equipment installed must be suitable for the environment -- defined hazardous areas -- in which it is used. Electrical area classifications are defined under Article 500 of the National Electrical Code (NFPA 70) to prevent electrical equipment from providing a means of ignition for

an ignitable atmosphere. There are two classes of hazardous locations: · Class 1 -- Flammable vapors and gases (NFPA 497). · Class 2 -- Combustible dust (NFPA 499), in which there are two divisions: Division 1 -- Normally or frequently present. Division 2 -- Not normally present but possible. These class locations are further divided into groups: · Group E -- Atmospheres containing combustible metal dusts, including aluminum, magnesium and their commercial alloys, and other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in the use of electrical equipment. · Group F -- Atmospheres containing combustible carbonaceous dusts that have more than 8% total entrapped volatiles (see ASTM D 3175, "Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke," for coal and coke dusts) or that have been sensitized by other materials so that they present an explosion hazard. Coal, carbon black, charcoal and coke dusts are examples of carbonaceous dusts. Static electricity, usually generated when any two materials make and then break contact, although not as prevalent, is a source of ignition. The buildup of the charge on electrically isolated conductors and/or on insulating materials can give rise to electrostatic discharges. Depending on the energy of the discharge, a flammable atmosphere can be ignited. Consider isolating conductive metals by grounding, and check ground connections regularly. Resistance to ground should be checked. If R > 10 ohm, direct ground connection is required. During normal activity, the potential of the human body can reach 10­15 kV, and the energy of a possible spark can reach 20­30 mJ. On those occasions, personnel should be grounded so that their resistance to ground is < 1 x 108 ohm. Static-dissipative footwear may be used. Resistance of the floor/surface on which the operator is standing should also be < 1 x 108 ohm, and where risk of exposure to electricity mains exists, resistance to ground should be 5 x 104 ohm < R < 1 x 108 ohm. Non-conductive or insulating material may be utilized; however, these materials can build up static charge, insulation of conductive materials, and charge retention on liquids and powders in non-conductive containers. Grounding of nonconductive materials would not facilitate the relaxation of electrostatic charges to ground. Consider conductive or staticdissipative materials.

When it comes to explosion prevention and protection techniques, preventive measures alone may not ensure an adequate level of safety. Protective measures should be taken as well. These measures include: · Containment by explosion-resistant construction. · Design based on ASME Boiler and Pressure Vessel Code, Section VIII, Division I. · Explosion suppression by injecting a suppressant, NFPA 69. · Explosion venting to a safe place, NFPA 68. So what may be done? Perform a site-wide dust hazard assessment and evaluate the hazard assessment and cost feasibility. If needed, based on the assessment evaluation, draft and implement a written combustible dust control program, which may include a combustible dust committee. Conduct on-site dust hazard explosion inspection training, and revise the contractor safety handbook to address combustible dust issues. In addition, develop an awareness training program for employees so that, at a minimum, persons who access combustible dust areas will receive combustible dust hazard awarenesslevel training. The training should occur: · · · · · Upon initial hire. Upon assignment or reassignment. Annually to refresh employees' knowledge. When combustible dust hazards or processes change. When contractors are working in and around combustible dust areas of the known combustible dust hazards. Precautions related to the contractor's work and work area should be well known.

-- Barry J. Momyer, CIH, is executive vice president and chief operating officer of AM Health and Safety Inc., Pittsburgh, Pa. If you have questions about this topic or other safety issues, please contact [email protected] Please include your full name, company name, mailing address and e-mail in all correspondence.

March 2011 33

Information

3 pages

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

2235