1、STD-AA F-1-ENGL 2000 Ob04500 00L8b70 728 Guidelines for Handling Aluminum Fines Generated During Various Aluminum Fabricating Operations The Aluminum Association Incorporaied The opinions and recommendations in this publication are based on information believed to be reliable and are offered in good
2、 faith but without guarantee. The precise causes of dust explosions, and the conditions under which the problems may arise, are not fully known. The operational conditions which exist in individual plants and facilities vary widely, and thus no definitive standards exist for dust collection and disp
3、osal. Accordingly, The Aluminum Association and its member companies assume no responsibility or liability for the completeness of the data or the general applicability of the opinions and recommendations herein, which are based on state- of-art knowledge but may not be appropriate in all situations
4、. Users of this publication should adapt the recommendations herein, as appropriate, to the precise conditions of the individual facility and should always exercise independent discretion in establishing plant or facility operating procedures. No warranty, express or implied, is made of this informa
5、tiori by The Aluminum Association or by any of its member companies. 1 This brochure discusses the potential problems involved in various aspects of aluminum fabricating which generate fine aluminum particles. It also discusses methods of particle collection and handling designed to reduce the risk
6、of fire or explosion. The information is similar, in general, to that presented in NFPA 65, ?Standard for Processing and Finishing of Aluminum.? However, it is not written to displace any aspect of the Standard, but rather to support and perhaps supplement it. In addition, it is not meant to cover a
7、luminum powder and paste products. These are discussed in The Aluminum Association brochure No, TR-2, ?Recommendations for Storage and Handling of Aluminum Powders and Paste.? The fines and how they are generated During fabricating operations aluminum fines may be generated by such activities as gri
8、nding, sawing, cutting, sanding or scratch brushing and at least some of them will be fine enough to be potentially explosible. The term ?dust? or ?powder? is frequently used to describe such particles. It is difficult to be specific about dimensions, but if all the particles are larger than 500 mic
9、rons, an explosion will probably not be sustained. It is more likely, however, that a mixture of coarser and finer material will be generated, and if at least 15% to 20% of the mixture is finer than 44 microns (325 mesh), then it has the potential for explosion. Finer particles burn readily when the
10、ir ignition point is reached, and tend to ignite the coarser particles as well. In the case of aluminum, an explosion can result if ignition occurs while the particles are suspended in air as a dust cloud, since the burning extends from one particle to another with extreme speed. Examples of potenti
11、al sources of ignition are open flames, welding equipment and cutting torches, matches and cigarettes, faulty electrical equipment and static electrical discharges. Such conditions must be avoided in areas where dust producing operations are carried out. Measuring the hazard and how it may change Th
12、e degree of hazard involved in any given operation can be determined by laboratory explosibility testing conducted by qualified personnel. The U.S. Bureau of Mines in Pittsburgh can advise the location of proper testing facilities. Later, if any changes are made in the process which generates the fi
13、nes, fresh samples should always be submitted for re-determination of the minimum explosive concentration and ignition energy. Changes in speed, alloy, lubricant or abrasive used may cause finer particles to develop and thus increase the potential hazard. General criteria for an aluminum dust explos
14、ion Several conditions must be met before a dust explosion can take place: i. The dust has to be combustible. 2. It must be suspended in air. 3. It must be fine enough to propagate flame. STD-AA F-L-EMGL 2000 Ob04500 OOltb73 437 = 4. The concentration of the suspended dust must be within the explosi
15、ble range. 5. An ignition source contacting the dust suspension must have enough energy to initiate flame propagation, that is, combustion of the particles. 6. Enough oxygen or other oxidizer must be available to support and sustain combustion of the dust suspension. What must be done Dust collectio
16、n systems must be installed which will safely capture potentially explosive aluminum fines. They may be of the wet or dry type, and must be so arranged that enclosures or exhaust hoods will provide efficient pick up of the fines from the machine or other equipment from which they are generated. The
17、fines must either fall or be projected into the hoods or enclosures in the direction of the air flow. Fines will then be transported through ducting to the collector itself by means of positive air flow created by an exhaust fan. DUST COLLECTION AND DISPOSAL Either dry-type or wet-type dust collecto
18、rs may be used and the following recommendations apply to both. The hoods, enclosures, duct work and collectors should be constructed of rustproof and non-sparking metal. To ensure the ductwork is as short as possible, each dust producing machine area should have a separate pick up hood or enclosure
19、 connected to its own dust collector located outside the building. Metallic dusts from grinding, sawing or cutting should be picked up by one collection system, while dusts from buffing and polishing should be picked up by a different system. This will prevent the mixing of explosive and flammable d
20、usts. To prevent a build up of static electrical charges, the entire system should be thoroughly grounded, including the machine generating the dust, the conveying ductwork and the collector. Detailed guidelines are given in National Fire Protection Association Standard No. 77, “Static Electricity.”
21、 It is important that both the coarser and finer particles be moved efficiently to the dust collector and therefore a minimum velocity of 4500 f.p.m. should be maintained in the conveying ductwork. The concentration of aluminum fines in the air in the ductwork should be safely below the lower explos
22、ive limit. This is reported by the U.S. Bureau of Mines to be 0.040 oz. per Cu. ft. (Report of Investigation RI-65 16, Explosibility of Metal Powders). The environmental aspects of plant disposal are beyond the scope of this brochure. However, aluminum fines may be recycled through companies that pr
23、ocess aluminum scrap and/or aluminum dross. Contact The Aluminum Association for names and addresses of these firms. Dry type dust collectors Dry dust collectors should be located outside the building so that potential damage to equipment or injury to personnel is minimized. Electrostatic and fabric
24、 or filter media collectors (bag houses) should not be used due to the high risk of static electrical charge build up. High collection efficiency cyclone collectors should be used. Ducts should have a minimum number of bends so that free airflow can be maintained at all times. They should be of rust
25、proof and non-sparking metal with smooth interior surfaces and lap joints facing the direction of air flow. There should be no configurations of any kind which might allow accumulation of dust. As outlined in National Fire Protection Association Standard No. 68, “Deflagration Venting,” collectors sh
26、ould have explosion vents installed to release an explosion pressure wave. They should be positioned so that a blast will not be directed toward personnel or other buildings which may bum or be structurally damaged. There should be at least regular daily removal of the dust from the collectors, and
27、more often if required by the amount accumulated. It should be handled very carefully to prevent dust clouds from forming, and be discharged into metal containers which can be tightly covered immediately. Before making repairs, the entire system should be emptied and cleaned by trained, authorized e
28、mployees, and the ducting capped off. Wet type dust collectors A roto-clone type collector should be considered. It should be a high efficiency type (dust collection efficiency of at least 99.5%). It is important that the air stream containing the dust be thoroughly scrubbed by the wet collector liq
29、uid so that the particles are trapped efficiently. Otherwise damp, fine particles may agglomerate and form a sludge on the interior of the exhaust vent. If an exhaust vent is used, it should be straight and of minimum length. It should be able to withstand an internal explosion pressure of at least
30、100 psi, as recommended in NFPA Standard No. 651, “Manufacture of Aluminum and Magnesium Powder.” The exhaust vent should have regular inspections and be cleaned frequently enough to prevent the buildup of a combustible cake of metal dust on its inner wall. Tests should be conducted to determine whe
31、ther the collectors efficiency is high enough to prevent a buildup of metal dust sludge on the inner wall of the exhaust duct. If it is, then the cleaned exhaust air may be able to be returned to the work area. Check local and state regulations about permission to do so. Parts moving at high speed s
32、hould not be allowed to contact dust particles. The exhaust fan pulling the air stream and dust into the collector should be located on the collectors clean air side. STDmAA F-1-ENGL SOO0 Ob04500 OOltlb75 20T The scrubber exhaust fan should run three to four minutes before the process begins, so tha
33、t there will be positive ventilation in the collector area and any hydrogen present will be purged. Interlocks should prevent the process from beginning to operate unless the exhaust is functioning and there is enough water in the scrubber. If already operating, the process should shut down if the w
34、ater pressure should fail. The scrubber exhaust fan should operate for at least two hours after the process has stopped. The sump should be automatically ventilated when the collector is shut down so that any hydrogen produced by an aluminum-water reaction will be exhausted. Another blower may be us
35、ed, or an unimpeded gravity vent, which should be designed to open automatically when the dust collector is not operating. Sludge should be removed from the wet collector at least once each day. FIRE PREVENTION AND FIRE PROTECTION Sources of Ignition These may include, but are not limited to, cuttin
36、g torches, welding activity, gas fired heating equipment, malfunctioning equipment, cigarettes and matches, glowing electrical elements and static electrical discharges. NFPA 65, Standard for “Processing and Finishing of Aluminum” recommends that where aluminum dust is generated, all electrical equi
37、pment and wiring should be in accordance with the “National Electrical Code,” NFPA 70. All equipment used in the dust generating process must be thoroughly grounded to remove static electricity. “Recommended Practice on Static Electricity,” NFPA 77, should be followed. Inspection and cleaning of all
38、 electrical equipment must be done regularly and frequently (at least weekly). Ground connections should be checked visually on a daily basis by the operators. Should sparks develop in any part of the operation, all equipment must be shut down immediately to determine the source and take corrective
39、action. All flammable solvents and solvent vapors should be kept at least 20 feet away from operating areas, as recommended in NFPA Standard No. 65, “Processing and Finishing of Aluminum.” If maintenance is planned and welding or cutting is required, all machinery, ductwork and collectors must be sh
40、ut down, emptied and cleaned, and all aluminum dust removed from the area and properly disposed of. The interior surfaces of all equipment must be carefully inspected to check for possible accumulations of fine powder, particularly at elbows, welded joints, etc. These surfaces must be wiped clean. E
41、mployees must be trained to understand the possible hazards involved and to react to an incident intelligently. All activities involving sparks or open flames must be constantly supervised and welding or burning permits should be issued. All of these recommendations also apply when gun-type power op
42、erated tools are to be used. In addition, when the work has been completed, no cartridges or charges should be left in the area, since they might discharge when the equipment begins operations again. Housekeeping Good housekeeping must be carefully carried out in the entire operating area at all tim
43、es. Dust accumulations must not be allowed to build up on floors, piping, ductwork, conduit, exposed building structural members or walls. Ductwork interiors must be checked often and cleaned as required. Cleaning tools must be a soft natural fiber brush or squeegee with nonsparking, conductive scoo
44、ps used to pick up the collected material. Synthetic fiber bristle brushes and plastic or other nonconductive scoops must not be used, since they tend to accumulate strong static charges. Standard commercial industrial vacuum cleaners must not be used during cleaning. Only special machines specifica
45、lly approved for use with combustible metal dusts are acceptable. Controlling and extinguishing an aluminum powder fire When aluminum fines are involved in a fire, only Class D dry powder type portable extinguishers should be used. The powder discharges from the extinguisher under some pressure and
46、could create an aluminum dust cloud. Therefore, it is important to try to direct the stream of powder above the fire and allow the powder to settle by gravity onto the burning material. In addition, fine dry sand, preferably finer than 20 mesh, and other approved dry powders, are effective materials
47、 for isolating an aluminum fines fire. A plentiful supply should be easily available in the operating area and stored in covered bins or drums. Long handled shovels should be available at each bin or drum, and should be constructed of non-sparking metal. The shovels can be used to safely lay the san
48、d or powder around the edge or perimeter of the fire. Aluminum fines can form explosible or combustible mixtures with various chlorinated or brominated hydrocarbons such as carbon tetrachloride, methyl chloride, and mixtures of carbon tetrachloride and chloroform. Therefore, halogenated extinguishin
49、g agents such as Halon should not be used in aluminum powder fires. A water stream should not be used, since its impact on the fire may cause the formation of an explosible aluminum dust cloud, In addition, water will react with the aluminum fines to form flammable hydrogen gas. Qualified supervisory personnel should decide in advance whether it is better to attempt to fight the fire or to withdraw to a safe location and allow it to bum itself out. STD*AA F-1-ENGL 2000 Ob04500 O038677 082 The Incorporated Aluminum Association 900 19th Street, N.W., Washington, D.C. 20006
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