NFPA 484 partial excerpt

 

6.3.2 Dust-Producing Operations.

 

6.3.2.1 (footnote 1) Machines that produce fine particles of aluminum shall be provided with hoods, capture devices, or enclosures that are connected to a dust collection system having suction and capture velocity to collect and transport all the dust produced.

 

6.3.2.2 Hoods and enclosures shall be designed and maintained so that the fine particles will either fall or be projected into the hoods and enclosures in the direction of airflow.

 

6.3.2.3 (footnote 2) Special attention shall be given to the location of all the dust-producing machines with the respect to the location of the dust collection system to ensure that the connecting ducts will be as straight and as short as possible.

 

6.3.2.4 Grinding operations shall not be served by the same dust collection system as buffing and polishing operations.

 

6.3.2.5 (footnote 3) Dry-type dust collectors shall be located outside of buildings.

 

6.3.2.5.1 (footnote 4) Individual machines with portable dust collection capability shall be permitted to be used indoors when the object being processed or finishing is incapable of being moved to a properly arranged fixed hood or enclosure and shall incorporate the safeguards in 6.3.2.5.1 (A) through 6.3.2.5.1 (D).

 

(A) The operation of portable dust collection devices shall be subject to a hazards analysis to ensure that the risk to personnel and operations from flash fire and shrapnel is minimized.

 

(B) Personnel protective clothing shall comply with 6.6.2 (see next page for 6.6.2)

 

(C) The collector shall be designed to dissipate static electricity.

 

(D) Collector retention capacity shall be limited to 0.45kg (1 lb).

 

6.3.2.5.2 (footnote 6) Dry-type collectors shall be provided with barriers or other means for protection of personnel.

 

6.3.2.5.3 (footnote 7) The area around the collector shall be posted with a sign that reads as follows:

 

Caution: This dust collector can contain explosible dust. Keep outside the marked area while equipment is operating.

 

6.3.2.6 (footnote 8) Dust collections systems shall be dedicated to collection of aluminum or aluminum alloy dust only.

 

6.3.2.6.1 Grinders, buffers and associated equipment with dust collectors utilized for processing aluminum shall be provided with a placard that reads as follows:

 

Warning: Aluminum Metal Only - Fire or Explosion Can Result with Other Metals.

 

6.3.2.6.2 If the combustible aluminum dust collection system is to be used for other materials, the system shall be dissembled and thoroughly cleaned of all incompatible materials prior to and after its use.

 

6.6.2 Personal Protective Equipment. [As referred to by section 6.3.2.5.1 (B)]

 

6.6.2.1 Outer clothing shall be clean, flame retardant, and non–static generating where combustible aluminum dust is present and shall be designed to be easily removable.

 

(A) Tightly woven, smooth fabrics treated with a flame-retardant chemical and from which dust can readily be brushed shall be used if necessary.

 

(B) Wool, silk, or synthetic fabrics that can accumulate high static electric charges shall not be used.

 

6.6.2.2 Work clothing shall be designed to minimize the accumulations of combustible aluminum dust (e.g., trousers shall not have cuffs).

 

6.6.2.3 (footnote 8) Safety shoes shall be static-dissipating, where necessary, shall have no exposed metal, and shall be appropriate for the type of operation taking place.

 

6.6.2.4 (footnote 9) Clothing Fires.

 

6.6.2.4.1 Emergency procedures for handling clothing fires shall be established.

 

6.6.2.4.2 If deluge showers are installed, they shall be located away from dry aluminum powder–processing and aluminum powder–handling areas.

 

NFPA Footnotes

 

1. A.6.3.2.1 MECs for combustible metal dusts in air are published in U.S. Bureau of Mines, RI 6516, and “Explosibility of Metal Powders.” Although the metal dust–air suspension normally can be held below the MEC in the conveying system, the suspension can pass through the flammable range in the collector at the end of the system.

 

2. A.6.3.2.3 Often, individual wet-type dust collectors can be provided for each dust-producing machine so that ductwork connecting the hood or enclosure of the machine to the collector is as short as possible.

 

3. A.6.3.2.5 Figure A.6.3.2.5 (below) is an example of a media-type dust collector. The figure shows major system components.

 

 

4. A.6.3.2.5.1 See figure A.6.3.2.5.1 (below)

 

 

5. A.6.3.2.5.2 A high-efficiency cyclone-type collector presents less hazard than a bag- or media-type collector and, except for extremely fine powders, will usually operate with fairly high collection efficiency. Where cyclones are used, the exhaust fan discharges to the atmosphere away from other operations. It should be recognized that there will be some instances in which a centrifugal-type collector can be followed by a fabric-type, bag-type, or media-type collector or by a scrubber-type collector where particulate emissions are kept at a low level. The hazards of each collector should be recognized, and protection against the hazards should be provided. In each instance, the fan will be the last element downstream in the system. Because of the extreme hazard involved with a bag- or media-type collector, consideration should be given to a multiple-series cyclone with a liquid final stage.

 

Industry experience has clearly demonstrated that an eventual explosion can be expected where a bag- or media-type collector is used to collect aluminum fines. Seldom, if ever, can the source of ignition be positively identified. In those unusual instances when it is necessary to collect very small fines for a specific commercial product, it is customary for the producer to employ a bag- or media-type collector. With the knowledge that strong explosive potential is present, the producer will locate the bag- or media-type collector a safe distance from buildings and personnel. If a bag- or media-type collector is used, the shaking system or dust removal system can be such as to minimize sparking due to frictional contact or impact. Pneumatic- or pulse-type cleaning is more desirable, because no mechanical moving parts are involved in the dusty atmosphere. If the bags are provided with grounding wires, they can be positively grounded through a low-resistance path to ground. Where bags are used, it is customary that the baghouse be protected by an alarm to indicate excessive pressure drop across the bags. An excess air–temperature alarm is also frequently employed. A bag- or media-type collector is customarily located at least 15 m (50 ft) from any other building or operation. It is not customary to permit personnel to be within 15m (50 ft) of the collector during operation or when shaking bags. Explosion vents are usually built into the system, as described in NFPA 68, Guide for Venting of Deflagrations. Care should be exercised in locating the vents because of the possibility of blast damage to personnel or adjacent structures.

 

6. A.6.3.2.5.3 For the method to calculate the length of a fireball issuing from a vented collector, see NFPA 68, Guide for Venting of Deflagrations.

 

7. A.6.3.2.6 Under certain circumstances, such as impact with rusted iron or steel, aluminum cannot safely be considered to be non-sparking, since a minor thermite reaction can be initiated. For details, refer to Eisner, “Aluminum and the Gas Ignition Risk.”

 

8. A.6.6.2.3 Safety shoes that meet the following guidelines should be worn by all operating personnel, except those persons who are required to work on electrical circuits or equipment. (For information, see ANSI Z41, Personal Protection—Protective Footwear.) The following are important elements for safe shoes:

 

(1) Soles should be resistant to embedding particles and to petroleum solvents, if used.

 

(2) Soles and heels should be attached by sewing or pegging.

 

(3) Nails, metal cleats, or metal plates should not be used.

 

(4) Safety toe caps should be completely covered with a scuff-resistant material.

 

(5) Soles and heels should be static-dissipating.

 

9. A.6.6.2.4 Fire blankets have been found to be effective for extinguishing clothing fires. They should be distributed in areas where water is excluded from the plant area.

 

Source: NFPA 484-Standard for Combustible Metal 2006 Edition, Copyright © 2005, National Fire Protection Association, All Rights Reserved. Page 484-15 and page 484-70; ANSI Publication. American National Standards Institute, Inc., 25 West 43rd Street, 4th Floor, New York, NY 10036.

 

For more information on the new OSHA standards visit www.osha.gov/index.html.