OSHA Hazard Information Bulletins
October 6, 1998
| MEMORANDUM FOR: | REGIONAL ADMINISTRATORS
STATE DESIGNEES
CONSULTATION PROGRAM MANAGERS |
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| FROM: | STEVEN F. WITT
Director
Directorate of Technical Support
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| SUBJECT: | Hazard Information
Bulletin(1) - Dust
Explosion Hazard in Certain Textile Processes |
Following a catastrophic explosion2
and fire, the Methuen Area Office has brought to our attention a potentially serious
safety hazard associated with a process known as "flocking". It was discovered that
under certain conditions(3), dusts
produced from processed and treated nylon fiber may be combustible, ignitable and
explosive as defined in 29 CFR 1910.399 and the National Fire Protection Association
70, National Electrical Code (NEC). There is a direct correlation between particle size
and its explosive hazard. The smaller the particle, the more reactive the dust. As the
materials become smaller, they disperse and remain suspended more easily, increasing the
potential for ignition and propagation of the reaction.
Flocking is the precision application of finely-cut fibers to various substrates for
decorative and functional uses. These substrates can be such common items as upholstery
and drapery fabrics, carpeting, toys, belts, ribbon, vinyl, picture frames, rubber seals,
metals, foam, cardboard, paper, and automotive components. The fibers are usually cotton,
acrylic, polyester, rayon, or nylon. Some functional uses are noise reduction, insulation,
friction modification, and surface protection.
In general, the industry process entails dyeing the raw fiber, cutting the fibers to product
specifications, chemically treating to prevent the individual fibers from clumping/sticking
together, creating a fiber/air mixture, and dropping the fibers through an electrical field.
Fine dust may be produced during processing and handling of fibers. Coatings applied to fibers
may be dislodged, and if allowed to accumulate, may contribute to a dust explosion. The ignition
source can be arcs from the corona associated with the electrical grid or arcs from the high
voltage equipment used in the production process. There are indications that small fires can
develop in the process area from fiber accumulation on the electrical grid or equipment which
can then become the ignition source. Static discharge may also be an ignition source.
To minimize the potential for personal injury or loss of life and property damage, we recommend
the following:
- A hazard assessment should be conducted by each flock employer, and appropriate measures
should be taken to minimize the risk of ignition from ignition sources. The assessment, in
part, should evaluate methods of detection and suppression (such as CO2 systems or
chemical suppression systems), extinguishing systems, and venting systems, among others. The
employer should verify facility compliance with all aspects of 29 CFR 1910.307 "Hazardous
(classified) locations";
- As part of the process of determining compliance with 29 CFR 1910.307, the employer should
conduct the testing outlined in National Materials Advisory Board (NMAB) 353-3-80 on representative
samples of the flock materials to determine if the dust produced from the handling and processing
of the materials meets the Class II hazardous materials definition(4)
[Note: Some explosive dusts, not considered to be Class II dusts, present an explosion hazard in the
workplace in the presence of stronger ignition sources such as small fires. Additional testing may
be required to determine the explosive hazard of dusts that are not in the Class II category. Appropriate
American Society for Testing and Materials (ASTM) or Pittsburgh Research Laboratory (formerly part of
Bureau of Mines) testing procedures should be conducted.];
- Suitable electrical equipment designed for Class II locations should be used where needed
based on the material's explosibility properties as determined by the NMAB 353-3-80 testing;
- Appropriate electrical equipment should be used in locations that are hazardous because of
the presence of easily ignitible fibers or flyings but in which such fibers or flyings are not
likely to be in suspension in the air in quantities sufficient to produce ignitible mixtures
(Class III locations);
- Aggressive housekeeping procedures, especially in the "flocking" area should be implemented;
- Strict control of fugitive/stray fibers by enclosure of process equipment should be considered
if determined not to increase the hazard potential; and
- All appropriate equipment and components in the application/process area should be installed
in accordance with OSHA and NEC requirements.
Please distribute this bulletin to all area offices and appropriate labor and industry groups.
Bibliography
American Society for Testing and Materials, Committee E-27, Hazard Potential of Chemicals
Annual Book of ASTM Standards, Volume 14.02, 1997.
Cashdollar, K. L., and M. Hertzburg, U.S. Department of Interior, Bureau of Mines, 20-l
Explosibility Test Chamber for Dusts and Gases, Rev. Sci. Instrum., Vol. 56, 592-602, 1985
U.S. Department of Interior, Bureau of Mines, RI 5624, Laboratory Equipment and Test Procedures
for Evaluating Explosibility of Dusts.
National Academy of Sciences, National Materials Advisory Board, NMAB 353-3-80 Classification of
Combustible Dusts in Accordance with the NEC.
National Fire Protection Association, NFPA 70, National Electrical Code.
National Fire Protection Association, NFPA 497M, Manual for Classification of Gases, Vapors, and
Dusts for Electrical Equipment in Hazardous (Classified) Locations.
National Fire Protection Association, Fire Protection Handbook, Seventeenth Edition,
Section 3, Chapter 12.
Footnote (1) The Directorate of Technical Support issues Hazard Information
Bulletins (HIB) in accordance with OSHA Instruction CPL 2.65 to provide relevant information regarding
unrecognized or misunderstood health hazards, inadequacies of materials, devices, techniques, and
safety engineering controls. HIBs are initiated based on information provided by the field staff,
studies, reports and concerns expressed by safety and health professionals, employers, and the public.
Bulletins are developed based on a thorough evaluation of available facts and in coordination with
appropriate parties. (Back to Text)
Footnote (2) In the instance where there was an explosion, the dust involved
was from treated and processed nylon, 1.3mm(0.051 in.) long and 1.3 denier. The fiber was dropped
through an electrical field grid, using direct current at 15,000 - 30,000 volts at less than 3 milliamps,
onto a latex-coated woven fabric. (Back to
Text)
Footnote (3) The OSHA Salt Lake Technical Center (SLTC) conducted tests on dusts
produced from treated and untreated nylon flock using SLTC's 20-liter explosibility test chamber and
also NMAB 353-3-80. The test results indicated that the dust associated with some treated nylon flocking
materials can be explosive. As for the dust from the untreated nylon flock, it could not be positively
concluded that it is not explosive. (Back
to Text)
Footnote (4) Class II dust is defined by NMAB 353-3-80, Classification of Combustible
Dusts in Accordance with the National Electrical Code, as having an ignition sensitivity equal to or greater
than 0.2 or explosion severity equal to or greater than 0.5.
Ignition sensitivity and explosion severity are defined by the NMAB 353-3-80 as follows:
| Ignition sensitivity = |
(TcxExMc)1 |
| |
(TcxExMc)2 |
| Explosion severity = |
(PmaxxP)2 |
| |
(PmaxxP)1 |
| where |
Pmax= |
Maximum Explosive Pressure; |
P = Maximum Rate Pressure Rise; |
| |
Tc = |
Minimum Ignition Temperature; |
E = Minimum Ignition Energy; |
| |
Mc = |
Minimum Explosive Concentration. |
|
Subscript 1 in the equation above refers to the appropriate values for Pittsburgh seam coal, the standard
dust used by the Pittsburgh Research Laboratory (formerly part of the Bureau of Mines) which is part of
the National Institute for Occupational Safety and Health, Department of Health and Human Service.
Subscript 2 refers to the values for the specific dust in question.
Materials which have an ignition sensitivity greater than, or equal to 0.2 or an explosion severity
of greater than or equal to 0.5 are class II dusts, and as such, trigger the requirements found in 29 CFR
1910.307 "Hazardous (classified) locations)." (Back to Text)
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