Report of Committee on Dust Explosion Hazards

A. R. Albrecht, Dm~ Chemical Co. R. W. Andrews, Jr., Pittsburgh, PA John D. Coons, Factory Mutual Research Corp.

Correlating Committee

Murray Jacobson, Chairman Mine Safety and Health Administration

Robert P. Benedetti, Secretary National Fire Protection Association

(Nonvoting)

John Nagy, Library, PA Robert W. Nelson, Industrial Risk Insurers Max Spencer, Continental Grain Co.

Technical Committee on

Metal Dusts

John D. Coons, Factory Mutual Research Corp Vincent E. Furnas, Jr., Louisville, KY

Rep. Reynolds Metal Co. Robert W. Nelson, Industrial Risk Insurers

Parker Peterson, Fenwa] Rep. Fire Equipment Manfacturers Assn.

Harry W. Cooley, Aluminum Co. of America (Alternate to R. W. Andrews, Jr.)

J.P. Gi l l is , Fenwa7 Inc. (Alternate to Parker Peterson)

R.W. Andrews, Jr., Chairman Pittsburgh, PA

Rep. American Society of Mechanical Engineers

J.M. Robinson, Aetna Life and Casualty Co. Joseph B. Scheller, Silberline Manufacturing Co.

Rep. Aluminum Assn. Frank E. Tibbetts, Wollaston Alloys

Rep. American Foundrymen's Society

Alternates

Michael E. Weiskopf, Industrial Risk Insurers (Alternate to R. W. Nelson)

Thls l i s t represents the membership at the time the Committee was balloted on the two-part Technical Committee Report. Since that time, changes in membership may have occurred.

Part I , prepared by the Technical Committee on Metal Dusts, proposes for adoption a complete revision of NFPA 65, Standard for the Processing and Finishing of Aluminum. NFPA 65 is puslished in Volume 5 of the 1979 National Fire Codes and in separate pamphlet Form.

Part ! has been submitted to letter ballot of the Technical Committee on Metal Dusts which consists of 8 voting members. Of these, 6 voted affirmatively, 1 abstained (Mr. Furnas), and 1 did not return a ballot (Mr. Tibbetts).

Part I has also been submitted to letter ballot of the Correlating Committee on Dust Explosion Hazards which consists of 7 voting members, all of whom voted affirmatively.

Part I I , also prepared by the Technical Committee on Metal Dusts, proposes for adoption a complete revision of NFPA 651, Standard for the Manufacture of Aluminum and Magnesium Powder. NFPA 651 is published in Volume 5 of the 1979 National Fire Codes and in separate pamphlet form.

Part I I has been submitted to letter ballot of the Technical Committee on Metal Dusts which consists of 8 voting members. Of these 6 voted affirmatively, 1 abstained {Mr. Furnas), and 1 did not return a ballot (Mr. Tibbetts).

Part I I has also been submitted to letter ballot of the Correlating Committee on Dust Explosion Hazards which consists of 7 voting members, all of whom voted affirmativly.

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65-1 - (Entire Standard): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Revise the entire 1975 edition of NFPA 65 as shown in the reprinted draft of the standard. SUBSTANTIATION: This total revision complies with the NFPA Style Manual. Also, the standard has been edi tor ial ly updated and reorganized for clar i ty and simplicity. COMMITTEE ACTION: Accept.

Standard for the Processing and Finishing of Aluminum

NFPA 65-1980

i NOTICE

An asterisk I*) following the number or letter designating a paragraph indicates explanatory material on that paragraph in Appendix A.

i Chapter I General

] - i Scope.

I-1.1 This standard shall apply to industrial operations where aluminum or aluminum alloys are subjected to processing or finishing operations which produce fine metallic powder or dust. Such operations include, but are not limited to, grinding, buffing, and polishing.1

I-1.2 This standard shall also apply to the handling and storage of aluminum powder by users of such material. This standard shall not, however, apply to the manufacture of such powder. (See Standard for the Manufacture of Aluminum and Magnesium Powder, NFPA 651.)

]-2 Purpose. The purpose of this standard is to minimize the occurrence of and resulting damage from f i re and explosion in areas where aluminum dust is produced or handled.

i-3 Retroactivity,

I-3.1 Unless otherwise stated, the requirements of this standard shall not be applied retroactively.

I-3.2 Existing plants, equipment, structures, and installations which do not comply s t r ic t ly with the requirements of this standard shall be considered to be in compliance i f i t can be shown that equivalent protection has been provided or that no specific hazard will be created or continued through noncompliance.

Chapter 2 Dust Collection

2-i General.

2-1.1 Machines which produce fine particles of aluminum shall be provided with hoods or enclosures connected to a dust collection system having sufficient suction and capture velocity to collect and transport all the dust produced. Hoods and enclosures shall be designed and maintained so that the fine particles will either fal l or be projected into them in the direction of airflow.

2-1.2" Special attention shall be given to the location of all dust-producing machines with respect to the location of the dust collection system to insure that the connecting ducts will be as straight and as short as possible.

2-1.3 Grinding operations shall not be served by the same dust collection system as buffing and polishing operations.

2-1.4 Grinding, polishing, or finishing operations shall not be located within 20 f t (6.1 m) of washing or cleaning fac i l i t ies which employ combustible or flammable solvents.

2-1.5 Dry-type dust collectors shall be located outside of buildings.

2-1.6 All dust collection systems shall be installed in accordance with Standard for the Installation of Blower and Exhaust Systems for Dust, Stock, and Vapor Removal or Conveying, NFPA 91.

2-2 Ducts and Ductwork.

2-2.1 Ducts shall be designed to maintain a velocity of not less than 4,500 ft/min (1372.5 m/min) to insure the transport of

I Where the term "aluminum" is used in this standard, i t also includes aluminum alloys.

40

both coarse and fine particles and to insure re-entrainment i f , for any reason, the particles should fal l out before delivery to the collector (for example, in the event of power fai lure).

2-2.2* Ducts shall be designed to handle a volumetric flow rate sufficient to maintain a dust loading safely below the lower explosive l imit.

2-2.3* Ducts shall be as short and as straight as possible.

2-2.4 Ducts shall have as few bends and irregularities as possible so that free airflow is not interfered with.

2-2.5 Ducts shall be of substantial metal construction and shall be carefully fabricated and assembled with smooth interior surfaces and with internal lap joints facing the direction of airflow. There shall be no unused capped outlets, pockets, or other dead-end spaces which might allow accumulations of dust. Duct seams shall be oriented in a direction away from normal working personnel.

2-2.6 Duct systems, dust collectors, and dust-producing machinery shall be bonded and grounded to minimize accumulation of static electric charge (see Recommended Practice on Static Electricity, NFPA 77).

2-3 Wet-Type Dust Collectors.

2-3.1 Where tests prove that the collector's efficiency is great enough to provide both personnel and property safety, the cleaned air may be allowed to return to the work area. (See A-2-3.2.1.)

2-3.2 I f an exhaust vent is provided to the outside of the building, i t shall be securely fastened, be as short and straight as possible, and shall be designed to withstand an

linternal explosion of 100 psig (689 kPa). pressure

Exception: Where explosion venting to outdoors, designed according to information in the Guide for Explosion Venting, NFPA 68, is provided, a lower design pressure may be used.

2-3.2.1" The exhaust vent shall be inspected and cleaned frequently to prevent buildup of highly combustible deposits of metal dusts on the interior surfaces of the duct.

2-3.3 The dust collector shall be arranged so that contact between dust particles and parts moving at high speeds is prevented. The blower for drawing the dust laden air into the collector shall be located on the clean air side of the collector.

2-3.4 The dust collector shall be arranged so that the dust laden air stream will be thoroughly scrubbed by the liquid to achieve the desired efficiency.

2-3.5* Sludge shall be removed from the collector on a regular schedule to insure proper and safe operation of the equipment. Sludge shall be disposed of in accordance with the requirements of Section 2-5.

2-3.6* The sump of water wet-type dust collectors shall be ventilated at all times. When the dust collector is not in operation, this ventilation shall be provided by an independent blower or by an unimpeded gravity vent. Gravity vents shall be provided with means to automatically open when the dust collector is shut down.

2-4 Dry-Type Dust Collectors.

2-4.1 Fabric or f i l t e r media-type collectors or electrostatic collectors shall not be used.

2-4.2 Cyclone separators or improved cyclone separators or centrifugal separators shall preferably have exit velocities no greater than 500 ft/min (152.5 m/min).

2-4.3 Special care shall be taken to insure the internal cleanliness of dry dust collection systems at all times and to avoid the accumulation of material except in the discharge hoppers of the dust collectors. Accumulation or condensation of water shall be prevented.

2-4.4 Dust shall be removed from dry collectors at least once each day and at more frequent intervals i f conditions warrant. Extreme care shall be taken in removing dust from the collectors to avoid creating dust clouds. The material shall be discharged into metal containers which shall be promptly and t ight ly covered to avoid the creation of airborne "fugitive dust." Waste material shall be mixed with an inert material, such as

I dry sand, in a ratio of 5 parts inert material to I part metal dust, and then deposited in a licensed landfi l l where i t shall be immediately covered.

2-4.5* Dry dust collectors and other equipment where an explosion hazard exists shall be provided with explosion rel ief wherever practical. Extreme care shall be taken in the selection of the type and location of vents or weak sections of

the collector to minimize injury to personnel and blast damage to nearby equipment or structures. Explosion vents shall be positioned so that a potential blast will not be directed towards any combustible or frangible structure. (For information on design of explosion vents, see the Guide for Explosion Venting, NFPA 68.)

2-4.6 When repairs on dry dust collectors are necessary, the collectors shall be emptied and residual accumulations of dust thoroughly removed (s~e Section 3-i). Duct work leading into the collector shall be disconnected and blanked off before repair work is starte~.

2-4.7 The interior of hoods and ducts shall be regularly cleaned wherever there is a possibil i ty of buildup of wax or l in t .

2-5 Disposal of Sludge from Water Wet Collect.~rs.

2-5.1 Sludge from water wet-type dust collectars shall be removed at least once each day or more frequently i f conditions warrant.

2-5.2 Covered, vented metal containers, preferably holding not more than 50 Ibs (22.7 kg) each, shall be used to transport the collected sludge for disposal. (See A-2-3.5.)

2-5.3 Sludge shall be disposed of in one of the following methods:

2-5.3.1 Sludge may te mixed with sand in a ratio of 5 parts sand to 1 part sludge, and then shall be discarded in a protected dump.

2-5.3.2* Sludge may be dumped in an open pit . The pit shall be fenced or guarded from publ ic access.

2-5.4 Smoking or open Flames shall be prohibi ted in the disposal area and throughout the disposal process.

Chapter 3 Fire Safe Practices

3-1 Housekeeping.

3-1.1 Good housekeeping shall be practiced i r the ent i re work area. Dust shall not be allowed to accumulate on f loors , exposed bui ld ing st ructura l members, piping, conduit, or ductwork.

3-1.2 Cleaning shall be done with a soft brush or squeegee and pickup shall be done with nonsparking conductive scoops. Synthetic fiber brisl;le brushes and plastic or other nonconductive scoops shall not be used because of their tendency to accumulate strong static electric charges.

3-1.3 Only vacuum cleaners specifically approved for use with reactive commbustible metal dusts shall be used to pick up small quantities of loose dust. Standard commercfa" industrial vacuum cleaners shall not be used, as they are not safe for use with combustible metals.

3-1.3.1 All spills ~hall f i r s t be cleaned by scooping and mild brushing.

3-1.4 The vacuum cleaner, its piping, suction hose, and tools shall be electr ical ly conductive and shall be grounded to prevent static electric sparks. The vacuum cleaning tools shall be made of conductiv~ nonsparking materials. (See Recommended Practice on Static Electr icity, NFPA 77.)

3-2 Electrical Equipment.

3-2.1 All electrical equipment and wiring exposed to dust or dust-laden atmospheres shall be approved for use in Class I I , Group E, Division i locations. Installation ~f such electrical equipment and wiring shall comply with the pravisions of Article 502 of the National Electrical Code~ NFPA 70.

I Exception: Control equipment meeting the requirements of the Standard for Purged and Pressurized Electrical Equipment, NFPA 496, or of the Standard for Intr insical ly Safe Apparatus for Use in Class I, I I and IlI~ Division I Hazardous Locations, NFPA 493, need not comply with 3-2.1.

3-2.2 All electrical equipment and wiring not exposed to dust or dust-laden atmospheres shall be installed in accordance with applicable requirements of the National Electrical Code, NFPA 70.

3-2.3 All electrical equipment shall be inspected and cleaned at regular intervals.

3-2.4 Extension cords, lamps, and pendant lighting fixtures shall be of a type approved for the intended use.

3~2.5 All electrical equipment shall be adeeuately grounded by permanent ground wires to minimize accumulation of static electric charges. !See Recommended Practice on Static Electr icity, NFPA 77.)

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3-3* Solvent Cleaning.

3-3.1 Cleaning solvents shall not be recovered on the site unless approval is obtained from the authority having jurisdiction.

3-3.2 Operations which may produce sparks shall be separated from any cleaning equipment using flammable solvents.

3-4 Elimination of Ignition Sources.

3-4.1 No open flames shall be allowed in buildings where aluminum dust is produced. No smoking shall be allowed in such buildings.

3-4.2 Grinding wheels shall not be dressed while on a spindle protected by an aluminum dust collection system.

3-4.3 Powder actuated tools shall not be used in buildings where aluminum dust is created or handled.

Exception: I f the use of powder actuated tools becomes absolutely necessary, the same precautions as outlined in 3-4.4 shall be taken.

3-4.4 Electric or gas welding or cutting or maintenance operations which could produce hot particles, such as dr i l l ing, sawing, or chipping, shall not be permitted in areas where an aluminum dust hazard exists unless the following precautions are taken:

3-4.4.1 All machinery in the area where such operations are to be performed shall be shut down.

3-4.4.2 All ductwork of dust collection systems shall be cleaned and the dust collectors shall be emptied and cleaned.

3-4.4.3 All accumulations of aluminum dust shall be thoroughly cleaned and removed from the area. The area shall also be free of all solvents and solvent vapors.

Chapter 4 Process Handling and Storage of Aluminum Powder and Paste

4-1" Scope. The requirements of this chapter shall apply to both regular and "nondusting" grades of aluminum powder.

4-2 Handling of Powder and Paste.

4-2.1 Where aluminum powder or paste is used or handled, good housekeeping practices shall be maintained.

4-2.2 Aluminum powder and paste shall be handled so as to avoid spillage and the creation of airborne dust.

4-2.3 Scoops, shovels, and scrapers used in the handling of aluminum powder and paste shall be electr ical ly conductive and shall be grounded when necessary. Tools shall be made of minimum-sparking materials.

4-2.4 Vehicles used in areas where aluminum powder or paste is handled shall have nonsparking wheels.

4-3 Storage of Powder and Paste.

4-3.1 When aluminum powder or paste is stored in sealed containers of a type approved by the U.S. Department of Transportation (DOT), the practices listed below shall be followed.

(a) Containers from which a portion of powder or paste has been removed shall be carefully covered and resealed.

(by Containers shall be kept free of contact with water or moisture.

(c) Aluminum powder or paste packed in sealed containers of a type approved by DOT shall be shipped in accordance with a l l applicable regulat ions of the U.S. Department of Transportation.

(d) Aluminum powder or paste packed in sealed containers of a type approved by DOT may be stored in commercial or public warehouses i f they are of f i r e - r e s i s t i v e , noncombustible, or l imited-combustible construct ion.

(e)* Aluminum powder or paste shall not be stored in the same area as oxid iz ing agents or combustible mater ial .

(f) When storing aluminum powder or paste in sealed containers, care shall be exercised to assure secure piling. Ample clear aisle space shall be provided around stored aluminum powder or paste.

{g) Leakage or condensation from roof, steam or water lines, or radiators shall be avoided.

(h) Smoking and open flames shall be prohibi ted in areas where aluminum powder or paste is stored.

Chapter 5" Fire Protection

fi-1 Portable Fire Extinguishers.

5-1.1" Only f i r e extinguishers approved for Class D f i res shall be permitted in areas where metal dusts are present.

5-1.2" Only extinguishers approved for Class B f i res shall be provided in areas where solvent cleaning and washing is performed. Conspicuous signs shall be posted adjacent to such extinguishers stat ing that these extinguishers shall not be used for metal dust f i res .

Exception: Haloqenated ext inguishing agents shall not be used.

5-1.3 Al l applicable requirements of the Standard for Portable Fire Extinguishers, NFPA 10, shall be complied with.

5-2 Bulk Extinauishing Mater ia ls.

5-2.1" Metal dust f i res on reasonably f l a t surfaces may be contro l led by placing a r ing dam of dry sand or approved dry ext inguishing material around the burning area. This shall be done with areat care to insure that not even a small dust cloud is created.

5-2.2* Long-handled shovels or scoops of nonsparking material shall be provided for the appl icat ion of dry ext inguishing agents.

5-3 Equipment Shutdown. In the event of f i re , all fans and machinery, including dust collection systems, shall be shut down. Drafts shall be avoided wherever possible.

5-4* Work Clothing. Work clothing shall be made of smooth, hard-finished, closely woven fabric and shall not be made of synthetic fabrics which tend to accumulate static electric charges. Trousers shall have no cuffs where dust might accumulate.

Appendix A

This Appendix is not a part of the requirements {recommendations) of this NFPA document . . . but is included for information purposes only.

A-2-1.2 Often, individual wet-type dust collectors can be provided for each dust-producing machine so that duct work connecting the hood or enclosure of the machine to the collector is as short as possible.

A-2-2.2 The U.S. Bureau of Mines Report of Investigation RI-6516, Explosibil ity of Metal Powders, reports the results of tests conducted on 89 different samples of aluminum powders of various grades and sizes. In general, the explosibi l i ty index was found to be greater than 10, the highest reference value used. Minimum ignition energies for dust clouds ranged upwards from 15 mill i joules, while minimum ignition energies for dust layers ranged upwards from 2 milli joules. Ignition temperatures ranged upwards from 320oc. Minimum explosive concentrations ranged upwards from 0.040 oz per cu f t (40 g/m3). Rate of pressure rise can be as high as 20,000 psi/sec (137 800 kPa/secl and maximum explosion pressures can exceed 90 psig (620 kPa).

A-2-2.3 Short, straiqht ducts reduce the explosion hazard and minimize the likelihood of accumulations of dry dust. Also, accumulations of tallow, wax, or oil with metallic fines and l int can be readily seen and more easily removed.

A-2-3.2.1 The humid air of the wet-type dust collector wets the fine particles which pass through the collector so that the particles agglomerate and tend to build up a cake or a sponge-like deposit ("sludge"), which is highly combustible, on the inner wall of the exhaust duct. For this reason, a wet-type collector complying with 2-3.4 is preferred.

A-2-3.5 I t should be remembered that wetted dust not submerged under a cover of water is highly f la~able and very dangerous.

J A-2-3.6 The reaction of aluminum with water produces hydrogen, which is highly flammable.

A-2-4.5 Explosion venting is especially important for aluminum dust, due to the high maximum explosion pressures reached and the extremely high rate of pressure rise.

A-2-5.3.2 The f i re and explosion hazards of the sludge must be kept in mind when this method of sludge disposal is used.

A-3-3 Tests have shown that aluminum powder, in contact with methyl chloride, carbon tetrachloride, or mixtures of carbon tetrachloride and chloroform, is capable of explosion. Also, aluminum in contact with methyl chloride may form spontaneously combustible aluminum methyl. These and other test results are described in Reactions of Aluminum and Magnesium with Certain

Chlorinated Hydrocarbons, U. L. Bu l le t in of Research No. 34, avai lable from Underwrlters Laboratories Inc. , 333 Pfinosten Road, Northbrook, IL, 60062. For these reasons halogenated ext inguishing agents must not be used on a f i r e involv inq combustible metals.

A-4-I Certain "nondusting" grades of aluminum f lake powder are being produced. These tend to reduce the hazards of inadvertently-caused dust clouds. They are as combustible as regular grades of f lake powder and, once levigated into a cloud, exh ib i t the same e x p l o s i b l l i t y characterlst~cs. For these reasons, the same precautions must be observed as for normal irades of powder.

~-4-3.1(e) This requirement is to protect the alumlnum powder or paste from ign i t ion by an exposure f i r e .

A-5 The use of Fine dry sand, preferably less than 20 mesh, or other approved powder is an e f fec t i ve method of iso la t ing incipient f i res in aluminum dust. An ample supply of such material should be kept in covered bins or receptacles located in the operating areas where they can be reached at a l l times. A long-handled shovel of nOnsparking metal should be provided at each such receptacle to af ford a ready means of laying the material around the perimeter of the f i r e .

Nearly a l l vaporizing l i qu id f i r e ext inguishing agents react v i o l en t l y with burning aluminum, usual ly serving to great ly in tens i f y the f i r e and sometimes resul t ing in explosion. (See A-3-3. )

Water streams should not be used. Not only w~ll water react with aluminum to give o f f h ighly flammable hydrogen, but the impact of the water stream or the reaction or both can l i f t enough dust into the a i r to produce a strong dust explosion.

I A-5-1.1 Since these extinguishers project a dry powder under the impulse of a gaseous propel lant , extreme care must be taken in the i r use to avoid creating even a small dust cloud. Where possible, the stream of powder should be directed above the f i r e , al lowing the powder to se t t le down on the burning material via g rav i ty .

A-5-1.2 Class B ext inguishing agents w i l l usual ly great ly accelerate metal dust f i res and may cause the burning metal to explode.

A-5-2.1 A Quiescent f i r e in aluminum dust tends to build a hard crust of aluminum oxide which excludes oxygen and causes the f i r e to se l f -ex t ingu ish.

A-5-2.2 Under certa in circumstances, p r i nc ipa l l y impact with rusted iron or steel , where a minor thermite reaction can be in i ta ted, aluminum cannot safely be considered to be nonsparking. For deta i ls , re fer to:

Eisner, H.S. "Aluminum and the Gas Ign i t ion Risk," The ~ i n e e r , London, Feb. 17, 1967.

Gibson et al , "Fire Hazards in Chemical Plant from Fr ic t ion Sparks Involv ing the Thermite Reaction," Industr ia l Chemists En~ineerin~ S vmposium Series, No. 25, London 1968.

A-5-d Work c loth ing without external pockets is preferred.

65-2 - (Ent i re standard): Reject SUBMITTER: Edward S. Naidus, APC Corporation and Fire Equipment l~la'n'u-f~ac't-6rers Assoc. RECOMMENDATION: I . Al l required explosion r e l i e f vents and associated components shall be of an approved type. Approval by the o f f i c i a l having j u r i sd i c t i on shall be based on acceptable performance tests and standards ( fo r the ent i re assembled device and not only unassembled components) by a qua l i f ied test ing organizat ion such as Fenwal Laboratories, Ashland, Massachusetts, Factory Mutual Laboratories, Norwood, Massachusetts, or others.

2. Tests shall include both pressure and time measurements expressed as average maximum pressure (at the instant of release) in Ibs/sq f t (kg/sqm.) and as average time (mil l iseconds) to release (from ign i t ion time to release time). The integrated product of "maximum pressure" mul t ip l ied by "release time" is cal led "impulse" and should range below 10% of the estimated "impulse" load resistance of the bui ld ing. (Example: most bui ldings of conventional steel and masonry block can res is t an "impulse" of i00 Ibs/sq f t for 0.5 seconds).

The vents should open f u l l y at not over 30 I b . / s q . f t . in not over 50 mil l iseconds.

3. Unit vents shall not be used for access and when in closed posit ion shall not have f ixed openings {to the outside) in the unit greater than I% of the vent area. 4. Unit vents mounted on a roof shall have provisions for

rest ra in ing a 200 pound person from f a l l i n g through the vent in the nonactivated condit ion.

5. Mater ials of construct ion shall be durable and funct ional (without frequent inspection and maintenance) in the expected

42

environment of weather, corrosion, temperature, igni t ion sources and mechanical loading as well as any known special hazards. The releasable portions of the vent shall be tethered to minimize " f l y i ng obje(t" hazards and shall not give r ise to pro jec t i les or shards that may cause in jury. SUBSTANTIATION: Standards for explosion venting have been expressed in rat ios of vent area to building volume in NFPA standards and various model code provisions. However, deta i ls as to explosion vent performance have not usually been stated and the approving o f f i c i a l has been forced to re ly on general pr incip les. In recent: years, standards and test performance have been dewloped to the point where basic engineering informatlon can now be furnished to architects, builders, englneers and code o f= ic ia ls . (See Referencesl

Pef: !. Accidental Explo~ipns - H. A. Streblow Nasa Cr 134779 -

June, 1975 2. Factory Mutual System - Loss Preventlon Data 7-76 (August,

1976) p. 16 3. Dust Explosions ~nd Fires - K. N. Palmer - Chapman & Hall -

London (1973) I t is now known thal damage to buildings or large structures

by internal pressure waves depends on a characLerist ic time response of the buildlnq, When the pressure w~ve moves more rap id ly (most explosions) than the building caq red is t r ibute the energy of the pressure wave (the building is t~o r ig id or not f l e x i b l e enough) then a portion or al l of the building collapses, The purpose of an explosion vent is to reduce the total energy on the building before the building reaches the force x time impulse th , t w i l l cause damage. The character is t ic time response decreases with increasing r i g i d i t y but for conventional masonry, s!eel or timbered structures, the time constant w i l l range close to 1/2 second. Therefore, venting must occur well belo~ 1/2 second af ter igni t ion to be of value. Use of vents for uses other than primary emergency explosion r e l i e f may cause excessive wear, damage or other performance impairment.

Current safety objectives for construction, maintenance or emergency personnel are met by providinq a safety structure around or over the vent.

For specialized structures, such as ducts over 6' in diameter or 4' x 4' in area, Elevator legs, conveyor throughways, etc. , unit vents should be placed on a l l ex ter ior faces and the vents should extend the fu~l width of the structure. The vents should have a smallest dimension of 3' (I meter) and a dimension ra t io of not over 2.

Structures with L/D greater than 3 require higher vent rat ios than near-cubical buildings. (Example: Elevator legs should have vents no less than 20' on center). COMMITTEE ACTION: Reject. COMMITTEE COMMENT: The proposed material is more properly wi th in the scope of Guide for Explosion Venting, NFPA 68. Addl t ional ly , the pr(~posal would rul~ out other proven e f fec t ive methods of explosion venting.

65-3 - (2-2.5): Accept SUBMITTER: Technica~ Committee on Metal Dusts, RECOMMENDATION: Add the fol lowing reguirement to 2-2.5: "Duct Teams shall be orienl:ed in a direct ion away from normal working personnel." SUBSTANTIATION: Thi; requirement w i l l ensure that the force of an explosion which ruptures the duct work w i l be directed in a d i rect ion away from ~reas occupied by normal uperating personnel, COMMITTEE ACTION: Accept.

65-4 - (2-3.2): Acc,~pt SUBMITEER: Technical Committee on Metal Dusts RECOMMENDATION: Increase the desian pressure from 75 psiq to IO0 psig. SUBSTANTIATION: Thi~ requirement establishes an exhaust vent more able to withstand internal explosion. COMMITTEE ACTION: Accept.

65-7 - (2-5.4): Accept SUBMITTER: Technical Committee on Metal Dusts ~ A T I O N : Add new subsection 2-5.4 to read: "Smoking or open flames shall be prohibited in the disposal area and throughout the disposal process," SUBSTANTIATION: This requirement increases the safety of the sludge disposal procedure. COMMITTEE ACTION: Accept.

65-8 - (3-2.1): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Add an exception to allow use of i n t r i n s i ca l l y safe e lec t r ic~ l equipment and purged/pressurized e lec t r ica l equipment. SUBSTANTIATION: The exception allows the use of other modes of providing safe e lec t r i ca l equipment that are recognlzed by Ar t ic le 500 of the Natlonal Electr ical Code, NFPA 70. COMMITTEE ACTION: Accept.

65-9 - (5-1.2): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Add an exception to prohib i t the use of halogenated extinguishing agents on aluminum f i r es . SUBSTANTIATION: This recognizes the violent reaction between burning aluminum and halogenated agents. COMMITTEE ACTION: Accept.

65-]0 - (A-2-3.6): Accept SUBMITTER: Technical Committee on Metal Dusts ~ A T I O N : Add an appendix item to explain that aluminum reacts with w-ater to produce hydrogen. SUBSTANTIATION: This explains the requirement for venting of the sump of water-wet dust col lectors. COMMITTEE ACTION: Accept.

65-I i - (A-4-I) : Accept SUBMITTER: Technical Committee on Metal Dusts ~ A T I O N : Add an appendix item to 4-1 to read as shown in the reprinted draf t of NFPA 65. SUBSTANTIATION: This appendlx item explains that, while "non-dusting" grades of aluminum f lake powder are safer, they s t i l l require the same precautions as for normal grades. COMMITTEE ACTION: Accept.

66-12 - (A-4-3.1(e)): Accept SUBMITTER: Technical Committee on Metal Dusts ~ A T I O N : Add the fol lowing appendix item: "This requirement is to protect the aluminum oowder or paste from igni t ion by an exposure f i r e . " SUBSTANTIATION: This appendix item explains the reason for storing aluminum powder and paste separately from combustible or oxidizers. COMMITTEE ACTION: Accept.

65-13 - (A-5-I .1) : Accept SUBMITTER: Technical Committee on Metal Dusts ~ A T I O N : Add an appendix item A-5- I . I to read as shown in the reprinted draf t of NFPA 65. SUBSTANTIATION: This appendix item provides guidance on the use of dry powder extinguishments. COMMITTEE ACTION: Accept.

65-5 - (2-4.4): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Include a recommended ra t io of 5 parts inert ~a ter ia l to I part metal dust for disposal of waste metal dust. SUBSTANTIATION: This provides guidance as to the amount of ~ a ~ needed for safe disposal, COMMITTEE ACTION: #ccept.

65-6 - (2-5.3): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Delete old 252(3) which read: "(Sludge) can be spread over the ground in an isolated area, where i t w i l l oxidize. An area used For th is purpose shall be fenced or guarded from public access. SUBSTANTIATION: This procedure is considered too hazardous for sludge disposal, COMMITTEE ACTION: #,ccept.

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651-1 - (Entire Standard): Accept SUBMITTER: Technical Committee on Metal Dusts ~ A T I O N : Revise the entire 1974 edition of NFPA 651 as shown in the reprinted draft. SUBSTANTIATION: This total revision complies with the NFPA Style Manual. Also, NFPA 551 has been edltorial ly updated and reorganized for clar i fy and simplicity. COMMITTEE ACTION: Accept.

Standard for the Manufacture of Aluminum and Magnesium Powder

NFPA 651-1980

NOTICE

An asterisk (*) following the number or letter designating a Iparagraph indicates explanatory material on that paragraph in ~ppend ix A.

Chapter 1 General

I-1 Scope.

1-1.1" This standard shall apply to manufacturing fac i l i t ies whlch produce light metal flake powder, or paste, atomized light metal granules, or the dust of any light metal alloy that is explosive in an environmental atmosphere.

1-1.2 This standard shall not apply to the production of waste metal dust by operations such as grinding, buffing, ahd polishing of semi-finished l ight metal products. (See NFPA 65, Standard for the Processing and Finishing of Aluminum, and NFPA 48, Standard for the Storage, Handling, and Processing of Magnesium.)

1-2 Purpose. The objective of this standard is to minimize the occurrence of and resulting damage from f i re and explosion in areas where light ~ t a l powder products are manufactured.

I-3 Retroactfvity.

I-3.1 Unless otherwlse stated, the requirements of this standard shall net be applied retroactively.

1-3.2 Existing plants, equipment, structures, and installations which do not comply strictly with the requirements of this standard shall be considered to be in cdmpllance if it can be shown that equivalent protection has been provided or that no specific hazard will be created or continued through noncompliance.

I-4 Definitions.

1-4.1 The term "light metal powder," as used in this standardj shall mean aluminum and magnesium powders only.

Chapter 2 Location and Construction of Light Metal Powder Production Plants

2-I Location.

2.1.1 Light metal powder production plants shall be located on a site large enough so that the buildings in which powder is manufactured will be at least 300 ft (91.4 m) from any occupied structure, such as public buildings, dwellings, business or manufacturing establishments, other than those buildings which are a part of the light metal powder production plant.

2-1.2 Different production operations shall be located in separate buildings located at least 50 ft (15.24 m) from each other.

Exception: Where two buildings are less than 50 ft (15.24 m) apart, one of the facing walls shall be capable of resisting a blast pressure of 2.0 psig (13.8 kPa) and shall be nonload-bearing, noncombustible, dust-tight, and without openings. (See also 2-5.3.)

2-1.3 Electric or steam power generators shell be housed in a separate building (or buildlngs)Iocated at least 100 f t (30.5 m) from any building containing a dust explosion hazard.

2-2 Security.

2-2.1 The site on whTch the powder production plant is located shall be surrounded by strong fencing at least 6 f t (2 m) high wlth suitable entrance gates.

2-2.2 All gates that are not kept locked shall be under the supervision-of a guard.

2-3 Building Construction.

2-3.1 All buildings used for the manufacture, packing, or loading for shipment of l ight metal powders shall, where

pART I [

practical, be single story, without basements, constructed of noncombustible materials throughout, and have nonload-bearing walls. The buildings shall be designed so that a11 internal surfaces are readily accessible to fac i l i t a te cleaning.

2-3.2 All buildings used for the manufacture of l ight metal powders shall be subdivided into as many small units as practical by blast resistant, nonload-beartng, noncombustible, dust-tight walls having no openings.

2-3°3 All walls of areas where dust may be produced which are net of ~no l i t h i c construction shall have al l masonry Joints thoroughly slushed with mortar and trowelled smooth so as to leave no inter ior or exterior voids where l ight metal powder may in f i l t r a te and accumulate.

2-3.4 Floors shall be hard-surfaced and nonsltp, Installed with a minimum number of joints in which l ight metal powder may col lect. The requirements of this section shall also apply to elevated platforms, balconies, f loors, or gratings. (See Appendix B. )

2-3.5 Roofs of buildings which house dust-produclug operations shall be supported on girders or structural members designed to minimize surfaces on which dust may col lect. Where such surfaces are unavoidably present, they shall be covered by a

, smooth concrete, plaster, or noocoa~)ustible mastic f i l l e t having a mfnimml slope of 550 to the horizontal.

2-3.6 Roof decks shall be as hght as practical, noncombustible, and be so arranged that they wi l l be blown o f f by an internal explosion.

2-3.6.1 Any sheet metal used as roof decking shall be well sealed to prevent leakage. Alternatively, sheet metal may be

y alvantzed and mall maintained to prevent corrosion which could ead to leakage.

2-4 Ooors and Windows.

Z-4.1 All door and window frames shall be metal.

2-4.2 Each room shall have at least two widely separated exits to exit corridors or to the outside. All doors in interior openings shall be approved self-closing f i re doors, installed in accordance with NFPASO, Standard for Fire Doors and Windows. Hardware for ~rgency exit doors shall conform to requirements of NFPABO and of NFPA 101, Life Safety Cod~).

2-4.2.1 Emergency exit doors shall be provided froeI all areas, including balconies and elevated platforms.

2-4.3 Windows shall be installed so that they automatically open in case of internal explosion. They shall be hinged at the top, shall open outward, and shall be held in place by f r i c t ion latches. (See Guide for Explosion Venting, NFPA 68.)

2-5 Cmmunication Between Buildings.

2-5.1 Buildings separated by not less than 50 f t (15.24 m) or small units of one major process section may communicate through enclosed passageways of noncombustible construction.

2-5.2 Enclosed passageways, leading from production or storage areas shall be specifically designed to relieve internal pressure from an explosion and shall be protected by automatic self-closing Class A f i re doors.

2-5.3 When two buildings are less than 50 f t (15.24 m) apart, only one of the facing walls shall have windows and doors. (See also Exception to 2-1.2)

2-5.4 All enclosed passageways shall be provided with an exi t door leading to the outside.

2-6 Grounding and Lightnin 9 Protection.

2-6.1 All steel process equipment and all building steel shall be bonded and grounded to a suitable 9round connection outside the building in accordance with NFPA 78, Lightning Protection Code.

2-6.2 Lightning rods shall be provided for al l boiler stacks and chimneys and for the high points of all buildings.

2-6.3 Power lines shall be adequately protected afainst lightning (See NFPA 78, Lightning Protection Code.)

2-6.4 A lightnlng arrestor system shall be provided around or within the building area of such capacity as to fu l ly protect al l buildings from lightning.

2-7 Electrical Power.

2-7.1 All electrical equipment and wiring shall be installed in accordance with NFPA 70, National Electrical Code~.

2-7.2 All parts of manufacturing buildings shall be considered

44

Class I I , Group E, Division i locations, as described in Articles 500 and 502 ol ~ NFPA 70, National Electrical Code.

Exception No. I : Offices and similar areas so occupied and segregated as to be reasonably free from dust and so classed by the authority having jur isd ic t ion.

I Exception No. 2: Control equipment meeting the requirements of NPFA 496, Standard for Purged and Pressurized Electrical Equipment, or NFPA 493, Standard for In t r ins ica l l y Safe Apparatus for Use in Class I , I I and I l l , Division I Hazardous Locations.

2-7.3 Electrical equipment that is not suitable for Class [ I , Group E atmospheres, sqall be located in a dust-free area.

2-7.4* Each building shall be provided with remote manual cutoff of all electr ical power. The remote manual cutoff shall be located at least 10 f t (3.05 m) from the nearest opening in the affected building.

2-7.4.1 Provisions shal! also be made for remote manual cutoff of all electr ical power to manufacturing areas from one or more central locations, such as off ices, watchman's booth, or other appropriate locations.

2-7.5 All manufacturing buildings shall be provided with low voltage (24 to 32 volts) emergency l ighting systems capable of providing I it-candle 110,7 lux) of illumination for all aisles leading to emergency exits. The emergency lighting systems shall be energized automatically on loss of electrical power to the building. (See 14-2.9 of Life Safety Code, NFPA 101.)

2-7.6 Electrical equipment shall be inspected and cleaned at least once each year or more frequently i f conditions warrant.

2-7.7 Flashlights and storage battery lamps may be used i f approved for the locations in which they are used.

Chapter 3 Machinery and Operations

3-i General Precauti()ns.

3-1.1 In powder hand ing or manufacturing buildings and in the operation of dust conveying systems, every precaution shall be taken to avoid the production of sparks from static e lec t r i c i t y , electr ical faul ts, or impact (e.g., iron or steel art ic les on stones, on each other, or on concrete).

3-1.2 Water leakage in or into any building where i t can contact l ight metal powder shall be prevented to avoid possible spontaneous heating and ignit ion.

3-1.3 Electrical hea:ing to a high temperature of any wire or resistance element or load in an area containing a dust hazard shall be prevented.

3-1.4" Serious local Friction heating in any machine located in an area containing a Just hazard shall be prevented.

3-2 Requirements for Machinery.

3-2.1 All dust-producing machines and conveyors shall be constructed so that escape of dust is minimized.

3-2.2 All machinery shall be bonded and grounded to minimize accumulation of stat ic electr ic charge. (See Recommended Practice on Static E lect r ic i ty , NFPA 77.) This requirement shall be appl icable to stamp mortars, mi l ls , fans and conveyors in a l l areas where dust is produced or handled, f in ish ing and pol ishing equipment, f i l t e r s , dr iers, dust screens, f ixed storage bins, and dust col lect ion and transport systems of a l l types. (See also 2-6.1.)

3-2.3* Ball or ro l l e r bearings, properly sealed against dust, shall be used for shafts and high speed equipment. Where exposed bearings must be used, they shall be protected as well as possible to prevert ingress of l i gh t metal dust.

3-2.4 Internal mach!ne clearances shall be maintained to prevent internal rubbing or jamming.

3-2.5 Approved magnetic separators or approwd pneumatic separators or screens shall be installed ahead of mil ls, stamps, or pulverizers wherever there is any poss ib i l i ty that tramp metal or other foreign objects may be introduced into the manufacturing operation.

3-2.5.1 Electromagnets shall be approved for use in Class I I , Group E, Divis ion 1 atmospheres.

3-3 Heating of Light Metal Powder Production Buildings.

3-3.1 Heating of bui ldings shall be done by hot a i r heating systems or by bare pipe heating systems using steam or hot water as the heat t ransfer medium. The ai r may be heated by steam or lot water coils located in a dust-free area adjacent to the room or building where heated air is required.

45

3-3.2 Fans or blowers used to convey the heated air shall also be located in a dust-free location. The air supply shall be taken from outside or from a dust-free location.

3-3.3 Make-up air for building heating shall have a dew point low enough to insure that no free moisture can condense at any point where the air is in contact with l ight metal dust or powder.

3-3.4 The requirements of 3-3.1, 3-3.2, and 3-3.3 shall not apply to areas where metal is melted for purposes of atomization.

3-4 Start-Up Operations. All machines shall be thoroughly cleaned and absolutely dry before they are charged with metal and placed in operation.

3-5 Charging and Discharging Light Metal Powders.

3-5.1 All containers shall be sealed with metall ic, waterproof covers while in storage or t ransi t .

3-5.2 When charging l ight metal powders to machines (or discharging from), the containers shall be posi t ively grounded by a conducting cable from the container to a suitable ground connection.

3-6* Packing and Storage. Light metal powder shall be packed into steel drums or other closed containers acceptable to the U.S. Department of Transportation (DOT). The containers shall be tightly sealed and stored in a dry location until ready for shipment or repacking.

3-6.2 Light metal powder product shall be shipped as soon as possible.

3-7* Wet Milling of Light Metal Powder.

3-7.1 Where light metal is milled or comminuted in the presence of an inert l iquid, the oxygen content shall be maintained at no less than 8 percent.

3-7.2 Where l ight metal is slurried in tanks or processed in blenders or other equipment in the presence of an inert l iquid, the oxygen content shall be maintained at no less than 4 percent.

3-7.3 The dew point of the controlled oxygen atmosphere in 3-7.1 and 3-7.2 shall be maintained substantial ly below the point where condensation could occur.

3-7.4 Bearings of wet mills shall be grounded across the lubricating f i lm by use of current collector brushes.

3-7.5 Adequate vent i lat ion, forced or natural, shall be maintained in areas where solvents are handled.

3-7.6 Solvent or slurry pumps shall be installed with proper controls to insure that they are shut down should they run dry.

3-7.7 All alarms and electr ical equipment shall be installed in accordance with appropriate provisions of the National Electrical Code, NFPA 70.

Chapter 4 In-Plant Conveying of Light Metal Powder

4-I Wheeled Containers.

4-1.1 Movable containers for in-plant transportation of l ight metal powders shall be cgnstructed ent i re ly of nonferrous minimum-sparking metal or of nonmagnetic minimum-sparking stainless steel.

4-1.2 Drums approved by the U.S. Department of Transportation (DOT) for shipment of l ight metal powders may be used when moved on 2- or 4-wheeled trucks or when moved on pallets by l i f t trucks.

4-1.3 All wheeled containers, hand trucks, and l i f t trucks shall have nonsparking, stat ic conductive t i res and wheels which have been bonded through or around the lubricating f i lm in the bearings.

4-2 Pneumatic Conveying.

4-2.1 Conveyor ducts shall be fabricated of nonferrous minimum-sparking metal or of nonmagnetic minimum-sparking stainless steel.

4-2.2 Ducts shall be e lec t r ica l ly bonded and grounded to minimize accumulation of static electr ic charge. (See Recommended Practice on Static E lect r ic i ty , NFPA 77.)

442.3 Plastics or other nonconductive ducts or duct liners shall not be used.

4-2.4* Inert gas explosion prevention systems shall be used in any pneumatic conveying system where the concentration of l ight metal powder is or may be within the explosive range. (See Standard on Explosion Prevention Systems, NFPA 69.)

4-2.4.1" The inert gas used shall be based on such gases as nitrogen, argon, or helium, and shall have an oxygen concentration appropriate to the inert ing gas and the par t ic le size of the metal dust, but in no case less than 1 percent.

4-2.4.2 The inert gas shall contain no carbon monoxide.

4-2.4.3 The inert gas shall have a dew point such that no free moisture can condense or accumulate at any point in the system.

4-2,4.4 The inert gas stream shall be continuously monitored for oxygen content and shall be arranged to sound an alarm i f the oxygen content is not within the prescribed range.

4-2.4.5 The inert gas for magnesium dust shall not contain carbon dioxide or nitrogen.

4-2.5* Where the conveying duct is exposed to weather or moisture, i t shall be moisture t ight .

4-2.6 A minimum conveying velocity of 4500 ft/min (1371 m/min) shall be maintained throughout the conveying system to prevent the accumulation of dust at any point and to pick up any dust or powder that may drop out during an unscheduled system stoppage.

4-2.7 I f the conveying gas is inducted into the system in a r e l a t i v e l y warm environment and the ducts and col lectors are r e l a t i v e l y cold, the ducts and the col lectors shall be ei ther insulated or provided with heating so that the gas temperature does not f a l l below the dew point, causing condensation.

4-2.8* I f the conveying gas is a i r , the metal dust - to-a i r ra t io throughout the conveying system shall be held safely below the minimum explosive concentration of the metal dust.

4-2.8.1" I f the dust is collected in a l iqu id, such as in a spray tower, any l iqu id used shall not have a f lashpoint below lOO°F (37.8oc) and shall be nonreactive with metal dust or reactive at a control led rate under favorable operating conditions. The l iqu id remaining in or on the product shall be compatible with subsequent processing requirements.

4-3 Ductwork for Conveying Systems.

4-3.1 Wherever practical, explosion vents, openings protected by antiflashback swing valves, or rupture diaphragms shall be provided on ductwork. Relief shall be to the outside of the building. (See Guide for Explosion Venting, NFPA 68.)

4-3.1.1 The inert ia of swing valves shall be limited to the minimum required.

4-3.2 Wherever damage to other property or injury to personnel may result from the rupture of the ductwork or where explosion re l ie f vents cannot provide suf f ic ient pressure re l ie f , the ductwork shall be designed to withstand a suddenly applied internal pressure of at least 100 psig (689 kPa).

4-3.2.1 I f a portion of the ductwork is so located that no damage to property or injury to personnel wi l l result from its bursting, that portion may be of l ight construction so as to intent ional ly f a i l , thereby acting as an auxi l iary explosion vent for the system.

4-4 Fan Construction and Arrangement.

4-4.1 Blades and housings of fans used to move air or inert gas in conveying ducts shall be constructed of conductive, nonsparking metal such as bronze, nomagnetic stainless steel or aluminum.

4-4.2 In no case shall the design allow the transported dust or powder to pass through the fan before entering the f inal collector.

4-4.3 Personnel shall not be permitted within 50 f t (15.5 m) of the fan while i t is operating. No maintenance shall be performed on the fan unt i l i t is shut down.

4-4.3.1 I f personnel must approach the fan while i t is operating, such as for a pressure test, i t shall be done under the direct supervision of competent technical personnel and with the knowledge and approval of operating management.

4-4.4* Fans shall be located outside of al l manufacturing buildings and so located that entrance of dust iS minimized.

4-4.5* Fans shall be equipped with ball or ro l ler bearings. Bearings shall be equipped with suitable temperature- indicating devices and shall be arranged to sound an alarm in case of over-temperature.

4-4.6 Fans shall be e l e c t r i c a l l y interlocked with dust- producing machinery so that the machines are shut down i f the fan stops.

4B

Chapter 5 Dust Collection

5 - i * Dust Collectors.

5-!.1 Dry-type dust col lectors shall be located outside, in a sage location, and shall be provided with suitable barricades or other means for protection of personnel.

5-1.2 Dust col lectors shall be constructed of nonferrous, mfnimum-sparking metal or of nonmagnetic, minimum-sparking stainless steel.

5- i .3 Duct work shall comply with the provisions of Section 4-3.

5-1.4 The ent i re dust col lecion system, including the dust co l lec tor , shall be completely bonded and grounded to minimize accumulation of s tat ic e lec t r i c charge. (See Recommended Practice on Stat ic E lec t r i c i t y , NFPA 77.)

5-2 Fans and Other Air Moving Equipment.

5-2.1 Fans and other equipment for moving a i r small be located so that entrance of dust into the fan housing is minimized.

5-2.1.1 Where the requirement of 5-2.1 cannot be met, the a i r moving equipment may be enclosed by construction designed to

I res is t a suddenly applied internal pressure of 100 psig (689 kPa). The housing of the a i r moving equipment must be provided with explosion venting to the outside.

5-2.2 Fans shall be provided with ball or r o l l e r bearings.

5-3 High Temperature Warning.

5-3.1 Cyclone or other dry-type col lectors shall be equipped with suitable instruments for recording the inside temperature. An over-temperature alarm or warning device shall be included and the l im i t sett ing shall be safely below the maximum service temperature of the f i l t e r medium or safely below the igni t ion temperature of the dust layer or dust cloud, whichever is lowest.

5-3.2 Alarms and actuating equipment shall be suitable for use in Ciass I I , Group E, Division 1 locations or shall be located in a dust-free area.

5-3.3 Al l such instruments shall give indication and alarm at easi ly observed central locations.

5-4* Dust Collecting F i l t e r Medium. Dust col lect ing f i l t e r medium made from synthetic fabrics which accumulate high stat ic e lec t r i c charges shall not be used.

Chapter 6 Prevention of Dust Accumulations

6-1 General.

6-1.1 Dust shall not be permitted to accumulate. Spi l ls shall be removed at once, using conductive, nonsparking scoops and soft brooms or brushes having natural f iber br is t les . Final cleanup may be accomplished using a vacuum cleaning system.

6-1.2 The use of water for cleaning shall not be allowed in manufacturing areas unless the fol lowing requirements are met:

(a) I t has been ascertained by competent technical personnel that the use of water wil l be the safest method of cleaning in the shortest exposure time.

(b) Operating management has fu l l knowledge and granted approval of i ts use. (c) Adequate vent i lat ion, either natural or forced, is

available to maintain the hydrogen concentration safely below the lower explosive l imi t .

(d) Complete drainage of al l water and powder to a safe, remote area is available.

6-2* Vacuum Cleaning Systems.

6-2.1 Vacuum cleaning systems shall only be used for removal of dust accumulations too small or too dispersed to be thoroughly removed by hand-brushing.

6-2.2 Vacuum cleaning systems shall be ef fect ive ly grounded and bonded to minimize accumulation of stat ic electr ic charge. (See Recommended Practice on Static E lect r ic i ty , NFPA 77.)

6-2.3 I f located in a dust-producing building or in an area where dust can accumulate, the vacuum cleaning system's electr ical equipment shall be suitable for Class I I , Group E, Division 1 locations.

6-2.4 Vacuum cleaner hoses shall be conductive and nozzles or f i t t i ngs shall be made of conductive, nonsparking material.

6-2.5 Dust picked up by the vacuum cleaning system shall be discharged into a suitable receptacle or collector located outside the building.

6-2.6 Portable vacuum cleaners may be used only i f approved for use in Class I I , Group E, Division I locations or shall be nonelec t r ica l ly powereJ.

6-3 Cleaning Frequency.

6-3.1 Supervisors shall be a le r t to p~event the accumulation of excessive dust on any portions of buildings or machinery not regular ly cleaned in dai ly operations.

6-3.2 Regular periodic cleaning, with al l machinery idle and power of f , shall be carried out as frequently as conditions warrant, but in no case less than once weekly.

6-3.3 Machinery shall be inspected once weekly and shall be cleaned i f the unit has been in operation durio~ that time.

Chapter 7* Fire Fighting Procedures

7-i Dry Powders.

7-1.1 An incipient f i re shall be ringed with a dam of dry sand, dry inert granular material or powder, or approved dry powder extinguishing agent. Extreme care shall be exercised during application to avoid any disturbance of the l ight metal powder which could cause a dust cloud.

7-1.2 The dry material shall be careful ly applied with a nonsparking metal scoop or shovel.

7-1.3 Care shall be (xercised to eliminate drafts by shutting of f fans and machinery and by closing doors and windows.

7-1.4 Areas where dry l ight metal powders are produced or handled shall not haw. f i re extinguishers rated for Class A, B, or C f i res. An ample and readily available supply of dry extinguishing materiat suitable for use with ccmbustible metals, (such as Class D extinguishing agent) shall be provided and suitable tools for application shall be kept if the same locations. The dry extinguishing material shall be stored in such a manner that i t remains clean and dry.

7-1.5 In cases of f i res involving magnesium powder or dust, i t is permissable to use dry magnesium foundry fILx as the extinguishing agent. The procedures stated in 7-1.! through 7-1.3 shall apply to such use.

7-2* Solvent-Wetted Powders.

7-2.1 A f i re occurring while the l ight metal powder is in slurry form may be fought using Class B extinguishing agents.

Exception: Halogenated extinguishing agents shall not be used.

7-2.2 A f i re occurring in semi-wet material or f i l ter-cake shall be fought using suitable dry extinguishing material.

7-2.3 Carbon dioxide or nitrogen shall not be used on f ires involving magnesium in any form.

7-2.4* Where carbon dioxide is used to extinguish f i res involving aluminum, tlle residual material shall be immediately covered with dry sand or with other suitable dry extinguishing material and the enti~-e mass shall be allowed :o cool unt i l i t reaches ambient tempe'^ature. When the material has cooled and i t has been determined that there are no hot spots, the covered material shall be careful ly removed for disposal. I t shall be handled in small quantities in covered containers, preferably not more Lhan 2 gal (11.4 dm3)each in 5-gal (!g.O-dm3) containers.

7-2.5 Manual water aaplication shall only be used on a solvent-metal powder Fire as a last resort , when other methods of control have fa i leJ and the f i r e shows evidence of going out of control . Only low ve loc i ty spray or fog nozzels shall be used, Extreme care shall be exercised to avoi l creating a dust cloud. Once water is used, i ts use shall be continued unt i l the f i r e is extinguished or unt i l the area becomes untenable.

7-2.5.1 After extinguishment, the area shall be immediately cleaned of a l l wetted powder, paste, or s lurry.

7-2.5.2 Adequate vent i la t ion shall be provided during cleanup to avoid concentrations of hydrogen from the exothermic reaction of the l ight metal with water.

7-2.5.3 Suitable drainage provisions to a safe area away from manufacturing buildinqs shall be providedi

7-3 Automatic Sprinkler Protection.

7-3.1 Automatic spr inkler protection may be used in areas where solvents are stored or used or where l ight metal powders are stored in sealed metal containers.

7-3.2 Automatic spr inkler systems shall be designed and insta l led in accordance with NFPA 13, the Standard for the Ins ta l la t ion of Spr i rk ler Systems.

47

7-3.2.1 The selection, design, and instal ]at ion of automatic sprinkler systems shall be made only with the guidance of experts who have knowledge of the special hazards of the particular l ight metal powder involved.

7-3.3 Attention shall be given to employee training and organizational planning to insure safe and proper evacuation of the protected area within the time l imi ts of the system operation.

7-4 Fire Fighting Organization.

7-4.1 Work assignments shall be so planned that an organized crew, trained in f i r e f ight ing, is in or close to the hazardous area at a l l times during operation.

7-4.2 Only trained personnel shall be permitted to engage {n f i r e control ac t i v i t y . All others shall be evacuated from the aree,

7-4.3 Fire f ight ing personnel shall be given regular and consistent t ra ining in the extinguishment of test f i r es set in a safe location away from manufacturing buildings. Training shall include al l possible contingencies.

7-4.4* I f professional or volunteer f i r e f ighters are admitted onto the property in the event of a f i r e emergency, their' a c t i v i t y shall be directed by the on-site ranking off icer" of the trained plant f i r e f ighters.

Chapter 8 Safety Reauirements

8-1 Protectlve Clothing for Workers.

8-! .1 Outer clothing shall be clean, flame resistant , nonstatic-generating and shall be designed to be easi ly removable. T ight ly woven, smooth fabr ics treated with a flame retardant chemical, i f necessary, shall be used.

8-1.2 Work clothing shall have no external pockets. Trousers shall not have cuffs.

8-1.3 Metal l ic fasteners shall be avoided where possible.

8-1.4 Safety shoes meeting the fol lowing requirements shall be worn by a l l operating personnel except those persons who are required to work on e lec t r i ca l c i rcu i t s or equipment.

(a) Soles shall be resistant to embedd:ng part ic les and to petroleum solvents, i f used.

(b) Soles and heels shall be attached by sewing or pegging. I Nails, metal cleats, or metal plates shall not be used. ~ Safety toe caps shall be completely covered with a

scuf f - res is tant material, (el Soles and heels shall be s ta t ic conductive.

8-2 Smoking and Matches.

8-2.1" Smoking materials~ matches, and l ighters shall only be allowed Rn the "change house" at the building entrance,

8-2.2 Smoking materials, matches, and l lghters shall not be carried or used by employees or v i s i to rs about the premises adjacent to or within any building in which l ight metal powder is produced, handled, or loaded for shipment.

8-2.3 The "change house" building shall be of f i r e - r e s i s t w e construction, shall be normally accessible only from a single entrance, and shall be located at or near the entrance to the premises.

8-3 Open Flames, Cutting and Welding Equipment, Powder-Operated Tools.

8-3.1" Maintenance workers and furnace or boi ler operators shall be furnished such safe igni t ion tools as thei r duties require.

8-3.2* Cutting, welding, soldering, or" brazing shall not be permitted in buildings housing powder-producing or handling machinery unless operations are completely shut down.

8-3.2.1 All machinery in the area where the hot work is to be done shall be completely shut down.

8-3.2.2 The ent i re area and machinery located in the area shall be thoroughly cleaned.

8-3.3 Powder-operated tools shall not be used in areas where a dust explosion hazard may exist unless the procedures of 8-3.2.1 and 8-3.2.2 are followed. After use of such tools, a careful check shall be made to insure that no cartr idges or charges are l e f t in the area, where they could enter equipment or be accidental ly discharged af ter operations have resumed.

8-3.4 A!umlnum, copper, or bronze metal tools, including shovels and scoops, shall be used in a l l buildings where a dust explosion hazard exists. Iron, steel, o n other spark-producing tools shall not be used.

Exception: In areas where the procedures of 8-3.2.1 and 8-3.2.2 are fol lowed.

~-3.5 Dismantling of any no-longer-used powder-producing, handling, co l lec t ing , or conveying equipment, e i ther indoors or outdoors, shall only be done with tools meeting the requirements of 8-3.4.

8-3 .6* Nonsparking tools and other hard objects shall not be allowed to s t r i ke any other hard surface in an area containing a dust pxplosion hazard.

8-3.7 Powder or dust sweepings and otheK material swept from f loors , machines, etc. shall be care fu l l y screened to remove foreign material i f the sweepinqs are to be returned to any machine for processing.

R-4 Employee Inst ruct ion.

8-4.1 All employees shall be care fu l l y and thoroughly instructed by the i r supervisors regarding the hazards of the i r working environment and the i r behavior and procedures in case of f i r e or explosion.

8-4.2 All employees shall be shown the location of e lec t r i ca l switches and alarms, f i r s t aid equipment, safety equipment, and f i r e ext inguishing equipment.

~-4.3 Al l employees shall be taught the permissible methods for f i gh t ing inc ip ient f i r es in pastes and for iso la t ing l i gh t metal powder' f i r es ,

8-4.4 The hazards involved in causing dust clouds and the danger of applying l iqu ids onto an inc ip ient f i r e shall be explained.

8-4.5 S t r i c t d isc ip l ine and scrupulous housekeepinq shall be maintained at al l times.

8-~ Periodic Inspection.

8-5.] A thorough systematic inspection shall be made at regular intervals not to exceed 1 month.

8-5.2 Two or more competent persons shall conduct each inspection and the record of the i r f indings and recommendations shall be permanently recorded in the pr incipal plant o f f i ce .

8-5.3 The inspection shall include the fo l lowing: (a) general safety precautions;

I b) f i r e f igh t ing equipment; c) f i r s t aid equipment;

Id) housekeeping; (e) e lec t r i ca l and mechanical equipment; ( f ) procedures.

3-5.4 Indicat ing and recording instruments and alarm devices shall be checked da i ly and the results recorded. Instruments shall be cal ibrated every 6 months.

8-6 Deluge Showers. Deluge showers actuated by hinged f loor valves shall be insta l led at st rategic locations immediately outside c r i t i c a l working areas to immediately douse clothing f i res .

8-7 Safety Blankets. Safety blankets shall be provided throughout the plant area.

Appendix A

This Appendix is not a part of the requirements (recommendations) of this NFPA document...but is included for information purposes only.

A-1-I .1 Certain "nondusting" grades of aluminum f lake powder are being produced. These tend to reduce the hazards from inadvertently-caused dust clouds. However, they are as combustible as regular grades of flake powder. Although they exhibit less tendency to be dispersed into a dust cloud, the same precautions described in this standard should be observed.

A-2-7.4 Provisions may also be made to automatically cut o f f e lectr ical power and lighting c ircui ts in manufacturing buildings when one or more safety sensing devices are activated by high pressure, low airf low, abnormal oxygen content, excessive vibration, or other pertinent factors being monitored. Alternatively, these sensing devices may he arranged to sound an alarm in those locations where prompt corrective action can be taken.

A-3-I.4 Temperature-sensing elements connected to alarms or machine stop switches may be employed for locations where overheating of bearings or other elements may be anticipated.

A-3-2.3 Plain bearinqs must not be used because of the d i f f i c u l t y of maintaining proper lubrication to prevent overheating. Outboard bearings are used where practical because i t is easier to check for overheating. In those instances where

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dust tends to penetrate bearings a continuous flow of inert gas ( i i /2 to 5 percent oxygen) can be employed to pressurize the hearings and seals, or a compatible l iqu id may be used in the same manner.

A-3-6 Open bin storage is not desirable. Storage bins can be sealed and they can be purged with inert gas pr ior to f i l l i n g . Once f i l l e d , the bins can be maintained inert by a sui table gas as detai led in 4-2.4.

A-3-7 When l i gh t metal is mil led in the presence of a l iqu id which is chemically iner t with respect to the metal, the ai r -dust explosion hazard is eliminated. When the resul t ing product is subsequently exposed to a i r , any unoxidized surfaces produced during mi l l ing w i l l react and may generate enough heat to cause ign i t ion . To prevent th is , i t is imperative that a contro l led amount of oxygen be present in the mi l l ing operation and in s lur r ies ahead of f i l t e r s and blenders, so that new surfaces are oxidized as they are formed. The addit ion of a mi l l ing agent, such as stearic acid, does not el iminate the need for th is added oxygen.

A-4-2.4 Light metal and l i gh t metal a l loy powders are produced by various mechanical means of par t i c le size degradation. These processes, as well as certain f in ish ing and transport ing operations, tend to expose a continuously increasing area of new metal surface. Most metals immediately undergo a surface reaction with avai lable atmospheric oxygen which forms a protect ive coating of metal oxide that serves as an impervious layer to inhibi t further oxidation. This reaction is exothermic. I f a f ine or thin lightweight part icle having a large surface area of "new" metal is suddenly exposed to the atmosphere, suff ic ient heat w i l l be generated to raise its temperature to the ignition point. Completely inert gas cannot be used as an inerting medium since

the metal powder would eventually, at some point in the process, be exposed to the atmosphere, at which time the unreacted surfaces would be oxidized; enough heat would be produced to i n i t i a t e e i ther a f i r e or an explosion. To provide maximum safety, a means for the control led oxidat ion of newly exposed surfaces is provided by regulat ing the oxygen concentration in the inert gas. The mixture serves to control the rate of oxidat ion, while mater ia l l y reducing the f i r e and explosion hazard.

A-4-2.4.1 Oxygen l imi ts of 3 to 5 percent have been maintained in aluminum powder systems using a control led f lue gas. Other l imi ts are applicable where other inert gases are used. Refer to Inf lammabi l i ty and E x p l o s i b i l i t y of Metal Powders, RI 3722, U.S. Bureau of Mines, 4800 Forbes Ave., Pi t tsburgh, PA.

A-4-2.5 Any moisture entering the system can react with the l i gh t metal dust, generating heat and serving as a potent ia l source of ign i t ion .

A-4-2.8 These minimum explosive concentrations are published in E x p l o s i b i l i t y of Metal Powders, RI 6516, U.S. Bureau of Mines, 4800 Forbes Ave., Pit tsburgh, PA. Although the metal dust-a i r suspension may be held below the explosive concentration in the conveying system, the suspension w i l l necessari ly pass through the explosive range in the co l lector at the end of the system unless the dust is col lected in l iqu id , such as in a spray tower.

A-4-2.8.1 Such wet co l lec t ion is not always possible or desirable.

A-4-4.4 Ul t imate ly , a l l fans in dust co l lector systems accumulate su f f i c ien t dust to become a potent ia l explosion hazard.

A-4-4.5 Fans may also be provided with v ib ra t ion- ind ica t ing devices, arranged to sound an alarm or to provide shutdown or both in event of blade or rotor imbalance or bearing or dr ive problems.

A-5-1 A high e f f i c iency cyclone-type co l lector presents less hazard than a bag-type co l lector and, except for extremely f ine powders, w i l l usual ly operate with f a i r l y high co l lect ion e f f i c iency . When cyclones are used, the exhaust fan discharges to atmosphere away from other operations. I t should be recognized that there w i l l be some instances in which a cent r i fuga l - type co l lector may be followed by a fabr ic or bag co l lector or by a scrubber-type co l lec tor where par t icu la te emissions must be kept at a low level . The hazards of each co l lec tor must be recognized and protected against. In each instance, the fan w i l l be the last element downstream in the system. Because of the i n i t i a l capital cost of an e f f ic ient bag col lector, the operating maintenance expense, the downtime cost, and the extreme hazard involved, a rea l is t i c evaluation of the losses from a multiple series cyclone with a l iquid f inal stage should be seriously considered. Industry experience has clearly demonstrated that an eventual

explosion can be expected when a bag collector is used to col lect aluminum or magnesium fines. Seldom, i t ever, can the source of ignition be posi t ively identif ied. In those unusual instances when i t becomes necessary to col lect extreme fines for a specifiG commercial product, i t is customary for the producer to employ a bag col lector. With the knowledge that strong

explosiv~ potential is present, he w i l l locate the bag collector a SaTe dls~ance from bLildings and personnel. Usually he wi l l surround the collector with a strong steel plate barricade to direct tbe force of thE, explosion upward and w i l l cover i t with a loose, lightweight, ~latertight cover which can blow of f to provide venting. Under these very special conditions, when an explosion occurs, only a physical loss is incurred. Conventional sheet metdl baghouse enclosures are not suitable.

I f a bag co l lector is used, the shaking system or dust ~emoval system can be such as to ~inimize sparking due to f r i c t i o n a l contact or impact. Pneumatic or pulse-type shaking is more desirable because no mechanical moving parts are involved in the dusty atmosphere. I f R he bags are provlded with grounding wires, they can be posctively grounded through a low resistance oath to ground. When bags are used, i t is customary that the baghouse be protected by an alarm to indicate excessive pressure drop across the bags. An eEcess air temperaturE, alarm is also frequently employed. A bag collector is customarily located at least 50 f t (15.24 m) :rom any other building or operation. I t is not customary to pe-mit personnel to be withm 50 i t (15.24 m) of the collector during operation or when shaking bags. Explosion vents are usually bui l t into the system, as described in NFPA 68, ~uide for Explosion Venting. Care is customarily exercised in locating the vents because of the poss ib i l i ty of blast damage to personnel or adjacent structures.

A-5-4 Some coiiector aags nr screens have fine, noninsulated • wire enmeshed into or ,ioven with the cloth or otherwise fastened to i t . These are alwavs securely grounded. ~t should be pointed out that this .s not a posit ive guarantee of s ta t ic charge removal because there is no dependable force to cause the charges to move across the nonconducting area oF the fabr ic to the grounded wires. Often, a substantial potential di f ference can be measured.

A-6-? Permanently instal led vacuum cleaning systems provide the maximum safety because the dust col lect ing device and the exhaust blower can be lorated in a safe location outside the dust-producing area. The dust co l lector should be located outside the bui ld ing, preferably more than 50 f t I15.24 m) away. I f the co l lector is located closer than 50 f t (15.24 m), i t is usually surrounded by a strong steel shield, cy l indr ica l in shape and open at the top, or closed with a l igh t , unfastened covey'. The shield is closed at the bottom and designed to withstand a blast pressure of 200 psig (1378 kPa). Such a protect ive barricade w i l l d i rect an explosion harmlessly upward and w i l l protect both property and personnel. All suction lines should be provided with explosion vents and anti-f lashback valves.

A-7 S~nce i t is almost impossible to extinguish a massive f i r e in dry l ight metal po~der, the f i r e problem resolves i t s e l f into the control of f i res ~n the incipient stage. The requirements of Section 7- i must bE followed i f the f i r e is to be control led qulckly. This is esp~cial ly true with regard to the application of the extinguishing material, as even a minor dust cloud can explode v io len t l y . A properly rlnged f i r e w i l l develop a hard crust of metal oxide

which w i l l u l t imate ly exlude enough oxygen to cause self-extinguishment. I t is customary practice, a f ter dispensing the extinguishing matEri~l, to leave the area, closing al l doors leading to the area ard sealing them with sand. The area should not be re-entered unt ' l combustion has stopped and the material has cooled.

A-7-2 Mi l l ing of a'uminum with combustible colvents is practiced in the manufacture of aluminum f lake used in pigments and powders. The materlal is handled as a s lurry during processing. Some of l;he product is marketed as a paste; other oortions are f i l t e r e d dried, sometimes polished, and sold as dry ~lake powder. The solvents employed are generally moderately high f lash point naphthas. A f i r e n an aluminum powder s lu r ry is pr im,v' i iy a solvent f i r e and (:an be fought using Class B extinguishing agents, except for halogenated extinguishing agents. Major producers usually employ f ixed extinguishing systems of

carbon dioxide or foam in this area, Some Class B portable extinguishers are provided also. Obviously, judgement must be used in determining whether Class B extinguishinq agents can be safely used, I f the extinguishing agent is careful ly app]ied, i t w i l l be very evident i f i t accelerates the ":ire, I f i t does, i ts use should be discontinued and a Class D e:<tinguishing agent or dry inert qranular material used. A f i r e in f i l t e r cake, a solvent-wetted but semi-dry material containing aluminum, may be a solvent f i r e or i t may at some point exhibit the characteristic of a p)wder f i r e at which time it must be treated as such. I f the aluminum metal has ignited, i': may continue to burn under a crust without flames.

A-7-2.4 Carbon dioxide may be used to extinguish f i res involving solvent-wetted aluminum. However, reJgnitJon may occur due to high localized heat or spontaneous heating. To avoid re ign i t ion, the residual material must be immediately smothered.

A-7-~.4 Professional or volunteer f i r e f ighters from outside the plant cannot be expected to be trained for the speci f ic f i r e and l i f e hazards associated with l ight metal powder

manufacturing. The requirement that the# be directed by the plant's f i r e fighting of f icer is in the interest of their own safety.

A-8-2.1 Employees may bring smoking materials in their street clothes provided they remain in the "change house."

A-8-3.1 Ignition tools w i l l generally be the f l i n t -and - f i l e type used for l ighting torches or furnace burners.

A-8-3.2 Attention is called to the hazardous conditions that may exist both inside and outside the plant i f cutt ing torches are used to dismantle dust col lectors or powder-producing machinery before a l l dust accumulations have been removed.

I t is a commonly recbgnized practice that operators of cutt ing or welding torches be required to obtain a wr i t ten permit from the safety or f i r e protection o f f i ce r of the plant before using the i r equipment under any condition aY'ound l ight metal powder plants.

A-8-3.6 Attention is d;rected to the fact that, under certain circumstances, p r inc ipa l l y impact with rusty iron or steel , where a minor thermite reaction can be in i t l a ted , aluminum cannot safely be considered to be nonsparking. For deta i ls , refer to "Aluminum and the Gas Igni t ion Risk," by H.S. Eisner, published in the Engineer (London, Feb. 17, 1967), and "Fire Hazards in Chemical Plant from Fr ic t ion Sparks Involving the Thermite Reaction," by N. Gibson, F.C, Lloyd, and G.R. Perry, published i n ~ o s i u m Series No. 25, I n s t i t u t i on of Chemical Engineers ( L o n d ~

Appendix B E lec t r i ca l l y Conductive Floors

This Appendix is not a part of the requirements (reco~nendations) of this NFPA document...but is included for information purposes only.

B-I General.

B- I . I E lec t r i ca l l y conductive f loor ing is often employed in l ight metal powder plants, although i t is recognized that i t is d i f f i c u l t to maintain the conduct iv i ty of the f l oo r over a period of time using current ly avai lable materials. Careful examination of the detai ls of this standard w i l l disclose the logic of the use of conductive f loor ing materials.

B-1.2 The surface of a conductive f l oo r w i l l provide a path of moderate e lec t r i ca l conduct iv i ty between a l l persons and portable equipment making contact with the f loor , thus preventing the accumulation of dangerous e lec t ros ta t i c charges.

B-1.3 The maximum resistance of a conductive f l oo r is usually less than 1,000,000 ohms, as measured between two electrodes placed three feet apart at any two points on the ~loor. The minimum resistance is usually greater than 25,000 ohms, as measured between a ground connection and an electrode placed at any location on the f loor . This minimum resistance value provides protection for personnel against e lec t r i ca l shocks. Resistance values are checked at regular intervals, usually once each month.

B-2 Testing for Minimum and Maximum Resistance. The fol lowing equipment and procedures are accepted practice.

B-2.1 Each electrode w i l l welgh 5 Ibs (2.26 kg) and w i l l have a dry, f l a t , c i rcu la r contact area 2.5 in. (63.5 mm) in diameter. The electrode w i l l consist of a surface of aluminum f o i l 0,00005 to 0.0001 in, (0.00127 to 0,00254 mm) thick, backed by a layer of rubber 0.25 in. (6.35 mm) thick and measuring 40 to 60 durometer hardness, as determined by a Shore Type A Durometer or equivalent.1

B-2.2 Resistance may be measured with a suitably calibrated ohmmeter which can operate on a nominal open c i rcu i t output voltage of 500 volts dc and a short c i rcu i t current of 2.5 to 10.0 m amp.

8-2.3 Measurements may be made at f ive or more locations in each room and the results averaged.2

B-2.4 For compliance with the maximum resistance l imi t , the average of al l measurements should be less than 1,000,000 ohms.

I ASTM 02240-68, Method of Test for Indentation of RuPber by Means of a Durometer, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

? I f resistance changes appreciably with time during a measurement, the value observed af ter the voltage has been applied for about f i ve min may be considered the measured value.

49

B-2.5 For compllance with the minimum resistance l im i t , no individual measurement should be less than 10,000 ohms and the average of not less than f~ve measurements should be qreater than ?~,NO0 ohms.

B-2.6 Where resistance to ground is measured, two measurements are customarily made at each location, with the test leads interchanged at the instruments between the two measurements. The awrage of the twn measurements is taken as the resistance to ground at that Incation. Measurements are customarily taken with the electrodp or electrodes more than 3 f t (0.92 m) from any qround connection or qrounded object resting on the f loor .

651-? - (Entire Standard): Reject SUBMITTER: Edward S. Naidus, APC Corp. & Fire Fauipment Manufacturers Assoc. RECOMMENDATION: i . All required explosion r e l i e f vents and ~ ~ p o n e n t s shall be of an approved type. Approval by the o f f i c i a l hawnq ju r i sd ic t i on shall be based on acceptable performance tests and standards ( for the ent i re assembled device and not only unassembled components) hy a qual i f ied testing organization such as Fenwal Laboratories, Ashland, Massachusetts, Factory Mutual Laboratories, Norwood, Massachusetts, o~ others.

2. Tests shall include both pressure and time measurements expressed as average maximum pressure (at the instant of release) in oounds/sq, f t . {kg/sqm.) and as average time (mill iseconds) to release (from igni t ion time to release time). The integrated product of "maximum pressure" mult ip l ied by "release time" is called "impulse" and should range below 10% of the estimated "impulse" load reslstance of the buildlng. (Example: most buildings of conventional steel and masonry block can resist an "impulse" of 100 Ib./sq. f t . for 0.5 seconds).

The .ents should open f u l l y at not over 30 Ib./sq. f t . in not over 50 mill iseconds. 3. Unit vents shall not bp used for access and when in closed

position shall not have f ixed openinqs (to the outside) in the unit greater than I% of the vent area. a. Unit vents mounted on a roof shall have provisions for

restrainlng a 200 pound person from fa l l i ng through the vent in the non-activated condition. 5. Materidls of construction shall be durable and functional

(without frequent inspection and maintenance) in the expected environment of weather, corrosion, temperature, igni t ion sources and mechanical load~nq as well as any known special hazards. The releasable portions of the vent shall be tethered to minimize " f l y ing object" hazards and shall not give r ise to pro ject i les or shards that may cause injury. SUBSTANTIATION: Standards for explosion venting have been expressed in rat ios of vent area to building volume in NFPA standards and various model code provisions. However, detai ls as to explosion vent performance have not usually been stated and the approving o f f i c i a l has been forced to re ly on general pr inciples. In recent years, standards and test performance have been developed to the point where basic engineering information can now be furnished to architects, builders, engineers and code o f f i c i a l s . (See references)

Ref: I. Accidental Explosions - H. A. Streblow Nasa Cr I34779 -

June, 1975 2. Factory Mutual System - Loss Prevention Data 7-76 (August,

1976) p. 16 3. Dust Explosions and Fires - K. N. Palmer - Chapman & Hall -

London I t is now known that damage to buildings or large structures

by internal pressure waves depends on a character is t ic time response of the building. When the pressure wave moves more rapidly (most explosions) %han the building can red is t r ibute the energy of the pressure wave (the building is too r ig id or not f l ex ib le enough) then a portion or al l of the building collapses. The purpose of an explosion vent is to reduce the total energy on the building before the building reaches the force x time impulse that w i l l cause damage. The character is t ic time response decreases with increasing r i g i d i t y but fo r conventional masonry, steel or timbered structures, the time constant w i l l range close to i /2 second. Therefore, venting must occur well below I/2 second af ter igni t ion to be of value. Use of vents for uses other than primary emergency explosion r e l i e f may cause excessive wear, damage or other performance impairment.

Current safety objectives for construction, maintenance or emergency personnel are met by providing a safety structure around or over the vent.

For specialized structures, such as ducts over 6' in diameter or 4' x 4' in area, elevator legs, conveyor throughways, etc. , unit vents should be placed on al l ex ter ior faces and the vents should extend the f u l l width of the structure. The vents should have a smallest dimension of 3' (I meter) and a dimension ra t i o of not over 2.

Structures with L/D greater than 3 require higher vent rat ios than near-cubical buildings. (Example: Elevator legs should have vents no less than 20' on center). COMMITTEE ACTION: Reject.

COMMITTEE COMMENT: The proposed materlal is more properly within the scope of the Guide for Explosion Ventinq, NFPA 65. Addi t ional ly , the proposal would rule out other proven e~fective methods of explosion venting.

651-3 - (2-7.21: Accept SUBMITTER: Technical Committee on Metal Ousts ~ A T I O N : Add an exception to allow use of i n t r l ns ,ca l l v ~ -eTec t - r~c -a l equipment and purged/pressurized e lec t r ica l equipment. SUBSTANTIATION: The exception allows the use of other mod#~ of D ~ s ~ e lec t r i ca l equipment that are recognlzed by Ar t ic le 500 of the National Electr~cal Code, NFPA 70. COMMITTEE ACTION: Accept.

651-4 - (3-3.1): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Add new 3-3.1 to read as shown in t~e reDrlnted draf t of NFPA 651. SUBSTANTIATION: This new section more c lear ly e~plains how production buildings may be safely heated and the heat tranqfer medium to be used. COMMITTEE ACTION: Accept.

651-5 - (5-2.1.1): Accept SUBMITTER: Technical Committee on Metal Dusts RECOMMENDATION: Decrease the required desiqn pressure for' internal explosion resistance from 200 psi to 100 psi. SUBSTANTIATIO_N: This change is in agreement with 4-3.2 and is considered adequate in re la t ion to the requirements in Section 5-I. COMMITTEE ACTION: Accept.

651-6 - (8-7): Accept SUBMITTER: Technical Committee on Metal Dusts I~TE'~R)I~E'I~ATION: Add the fo l lowing: 8-7 Safety Blankets. Safety blankets shall be provided throughout the plant area, SUBSTANTIATION: This new requirement augments Section 8-6 which requires Deluge Showers. COMMITTEE ACTION: Accept,

50