About40percentofcombustibledustexplosionsre-ported in the US andEurope over the last 25 yearshave involved dust collectors. Dust collection sys-tems are now a primary focus of inspections re-quiredbyOSHA’sNationalEmphasis Programonsafely handling combustible dusts.1 OSHAalso hastheauthoritytoenforceNationalFireProtectionAs-sociation (NFPA) standards for preventing or pro-tectingagainstdustexplosions.Thistwo-partarticlefocuses on how you can design your dust collectionsystem’s dust collector, ductwork, and exhaust fantomeet the intentof theseNFPArequirements.PartIIwillappear inJanuary.

Theexplosionhazardsposedbydustscommonlyhan-dled in bulk solids plants can be surprising. In fact,most natural or synthetic organic dusts and some

metal dusts canexplodeunder the right conditions.Youcanfind a limited list of combustible dusts and their explosiondata in the appendix to theNFPAstandard focusingondustexplosion hazards,NFPA 68: Standard on Explosion Pro-tectionbyDeflagrationVenting (2007),2 and inRolfK.Eck-hoff’sbookDustExplosions in theProcess Industries.3

While such published data can give you some idea of yourdust’s explosion hazards, using this data for designing ex-plosion prevention or protection equipment for your dustisn’t recommended. Your processing conditions and yourdust’s characteristics — such as its particle size distribu-

tion — differ from those for the published data for thesame material, producing different combustible dust re-sults. The onlyway to determine your dust’s combustibil-ity is to have aqualified laboratory run explosion tests on arepresentative sample of the dust. Then, tomeetNFPA re-quirements, you’ll need to commission a hazard analysisof your dust collection system to document that its designmitigates the explosion risk posedbyyour dust. (Formoreinformation, see reference 4.)

Somedust explosionbasics

The five elements required for a dust explosion canbepic-tured as a pentagon, as shown in Figure 1. The three ele-ments labeled in black are those in the familiar firetriangle: fuel (combustible dust), an ignition source, and

Designing your dust collection system to meetNFPA standards — Part I

GaryQ.Johnson WorkplaceExposureSolutions

www.powderbulk.comAs appeared in PBE December 2008

Figure 1

Dust explosion pentagon

Ignitionsource

Confinementofdustcloudinequipmentorbuilding

Fuel(combustibledust)

Oxygen

Dustdispersionatorgreaterthandust’sMEC

oxygen. For a dust explosion, twomore elements (labeledin red) are required: dust dispersion at or greater than thedust’s minimum explosible concentration (the lowest dustconcentration thatwill propagate a combustible dust defla-gration or explosion; MEC) and confinement of the dustcloudwithin equipment or abuilding.

Put simply, a dust explosion occurs when an ignitionsource touches a dust cloud with a concentration at orgreater than the dust’s MEC. A dust cloud with this con-centration can result when a layer of dust thicker than 1⁄32inch on equipment, piping, overhead conduit, or similarcomponents is pushed into the air by some event, such asthe pressure wave from a relief device’s operation. Whenan ignition source — such as a spark or the flame frontfrom an equipment explosion — touches the cloud, thedust can explodewith devastating impact, as evidencedbythe fatal results of the sugar refinery explosion in Georgialast February. To mitigate your dust collection system’sexplosion risk, you need to focus on preventing dust accu-mulation in the system, preventing ignition, andprovidingexplosion prevention or protection at the collector — allcoveredbyNFPAstandards.

Even when a dust collector is equipped with an explosionvent that works properly, the ductwork in the dust collec-tion system can propagate a collector dust explosionthroughout a process area. An investigation into one suchcase revealed that a contributing factor was the ignitionand explosion of dust that had accumulated in the duct-workbecauseof the system’s inadequate conveyingveloc-ity.5 Another contributing factor was the lack offlame-front-isolation devices in the collector’s dirty-airinlet and the clean-air outlet for recirculating air to thebuilding. Such devices could have prevented the flamefront in the collector fromentering the inlet duct and re-en-tering the building through the outlet duct.

In this case as inmany others, following the requirementsinNFPA standards formitigating explosion risks in a dustcollection systemcouldhaveprevented the dust explosionfrompropagating beyond the dust collector. In the follow-ing sections, we’ll look at how you can design your dustcollection system to meet the NFPA standards. Informa-tion covers preventing dust accumulation in ductwork,eliminating ignition sources, and using explosion preven-tion and protection methods at the collector. [Editor’snote:Capture hooddesign, another important factor in de-signing a safe dust collection system, is beyond this arti-cle’s scope; for more information, see the later section“For further reading”or contact the author.]

Preventingdust accumulation inductwork

Toprevent dust fromaccumulating in your dust collectionsystem’s ductwork and becoming fuel for an explosion,youmust design all ducts in the systemwith twoprinciplesin mind, as described inNFPA 654: Standard for the Pre-

ventionofFire andDustExplosions from theManufactur-ing,Processing, andHandlingofCombustibleParticulateSolids (2006).2 First, the conveying air velocity must beadequate throughout theduct. Second, at pointswhere twoairstreamsmerge, the duct sectionsmust join in away thatmaintains this velocity.

If your plant handles a combustible dust, a visitingOSHAinspector will ask whether the conveying air velocitythrough your dust collection system is adequate — andwill ask you to prove it. Why is this velocity important?Keeping the conveyingair velocity in everypart of theductwithin a reasonable rangewill prevent two problems: Toolowan air velocitywill cause the dust to drop out of the airand build up inside the duct, and, depending on the dust’scharacteristics, too high an air velocity will waste energy,erode the duct, or, if the dust is moist or sticky, cause thedust to smear on the ductwall.

If your plant handles a combustible dust, a visitingOSHA inspector will ask whether the conveying airvelocity through your dust collection system isadequate — and will ask you to prove it.

A conveying air velocity between 3,500 and 4,000 fpm(17.5 and 20m/s) is a reasonable starting point for design-ing your system. Then, based on supporting data aboutyour application, you can speed or slow the conveying airto the system’s optimal velocity. For instance, if your sys-tem handles an extremely fine, lightweight material thatwon’t clump together, like cotton dust, you can slow thevelocity to 3,000 fpm; if youhandle a veryheavymaterial,like lead dust, you may need to increase the velocity to4,500 to 5,000 fpm. For guidance in determining the opti-mal velocity for your application, see Table 5-1 in theAmerican Conference of Governmental Industrial Hy-gienists’ Industrial Ventilation: A Manual of Recom-mended Practice for Design (26th edition, 2007),6 whichlistsminimumduct design air velocities formanydusts.

How duct sections are joined in your system also affectsthe conveying air velocity. If incorrectly designed, thepoint where ducts join to merge two airstreams can slowthe air velocity, in turn causing the dust to drop out and ac-cumulate in theduct.Youcanprevent this problembycon-necting eachbranchduct to a15-degree tapered expansionon the main duct, which enlarges the main duct diameterto a size appropriate for the merged airstreams. A relatedproblem is that dust particles can drop out of the airstreamwhen a branch duct joins the main duct at too great anangle. The momentum of the conveyed dust particlescauses them towant tomove in a straight line, sowhenoneduct joins another at a sharp angle, the particles have to

change direction and slow down. Avoid this problem bydesigning the branch duct entry with no more than a 30-degree angle to themainduct.

Failing to practice these duct design principles can lead toany of several problems that produce a slower-than-re-quired conveying air velocity in your ducts. Following aresome visual clues that indicate the air velocity in the ductsisn’t high enough to prevent dust fromdropping out of theair. Under each clue, ways to remedy the problem and getadequate conveying air velocity though the ducts are de-scribed.

Clue1:Mainduct diameter doesn’t enlargeafter branchjunctions. In Figure 2a, two8-inch-diameter branchductsjoin an 8-inch-diameter main duct, and the main duct’sdownstream diameter is the same after each junction. AtA, before the first branch junction, the 4,200-fpm air ve-locity required to convey the dust is reasonable and can beachieved by the system’s design airflowof 1,500 cfm.Butbecause the duct diameter doesn’t enlarge after the branchjunctions, the required air velocity increases exponen-tially: It’s 8,400 fpm at B, after the first branch junction,which would require a 3,000-cfm airflow, and it’s 16,800fpmatC, after the second junction,whichwould require a4,500-cfm airflow. However, 5,500 fpm is the practicalupper limit for air velocity in system ductwork. To meetthe velocity requirements in this duct arrangement, thesystem would require a major upgrade of the exhaust fanand electric power,which is impractical.

Solution: The more economical solution is to enlarge thedownstream duct. This will solve the problem that resultsfrom not upgrading the exhaust fan— that is, that C getsmost of the airflow, B gets some, andA gets very little. Toensure that the duct’s diameter is large enough after abranch duct joins it, follow this rule of thumb: The sum ofthe areas of the upstream branch ducts should roughlyequal the area of the downstreamduct. Based on the equa-tion duct area = � � (diameter/2)2, this rule can be re-stated as: the sum of the squares of the upstream branchduct diameters should roughly approximate the square ofthe downstreammainduct diameter. Thus, atB:

82 +82 =128�112 or 121

so the main duct diameter at B should be changed to 11inches. Then, atC, two solutions are possible:

112 +82 =185�132 or 169

112 +82 =185�142 or 196

so themain duct diameter at C should be changed to 13 or14 inches, depending on your application’s conveying ve-locity requirements.

Clue 2:Main duct is blanked off. In Figure 2b, an 8-inch-diameter main duct, A, is blanked off. A 4-inch-diameterbranch duct, B, joins themain duct at aY junction that en-

larges from 8 to 9 inches, and the downstreammain duct,C, is 9 inches in diameter. Before the blank flangewas in-stalled, the system’s design airflowmet the air velocity re-quirements at A (4,200 fpm [1,500-cfm airflow]), B(3,900 fpm [350-cfm airflow]), and C (4,100 fpm [1,850-cfm airflow]). But withA blanked off, the required air ve-locity through the 4-inch-diameter duct (B) is now 3,900fpm (350-cfm airflow). The exhaust fan might be able topull an airflow of nomore than 600 cfm through B and C,which would drop the air velocity at C from the required4,100 fpm to270 fpm.

Solutions:Two solutions are possible: You can replace allthe duct between B and the system’s dust collector withsmaller duct to achieve an adequate conveying velocity.Or, as a much cheaper alternative, you can remove theblank flange and replace it with an orifice plate that deliv-ers 1,500-cfm airflow at the system’s available static pres-sure; the orifice plate has a hole at its center that’s sized tomeet the system’s airflowandpressure drop requirements.

Clue 3: Poor duct junctions don’tmaintain conveying ve-locity. Let’s look at two examples of this problem. In thefirst, shown in Figure 2c, an 8-inch-diameter duct sectionabruptly joins a 20-inch-diameter section. At the system’s1,400-cfm design airflow, the conveying air velocity is4,000 fpm in the 8-inch section, but it drops abruptly to 650fpmin the20-inchsection,whichwill cause thedust todropoutof theair.Solution: In this case, the solution is to replacethe20-inchduct sectionwith8-inchduct.Theductdiametershould stay at 8 inches until the next branch junction; afterthat junction, theduct shouldbeenlarged tomaintain theairvelocity, following the ruleof thumbunderClue1.

Another poorduct junction is shown inFigure2d.Here, an8-inch-diameter branch duct, A, joins an 8-inch-diametermain duct at a 90-degree angle, forming a T junction. Thesystem’s 1,400-cfm design airflow can produce the re-quired 4,000-fpm conveying air velocity through A andand section B upstream from the junction without a prob-lem. But section C downstream from the junction wouldrequire 8,000 fpm (at an airflow of 2,800 cfm) to conveythe dust through the duct and past the T junction.Meetingthis impossibly high air velocity requirement would de-mand an unreasonably high fan energy, and the duct atboth B and Cwould probably plugwith dust. Solution: Inthis case, replacing theT junctionwith a30-degreeY junc-tion that enlarges to a downstream diameter of 11 inches(again following the rule of thumb inClue1)willmaintainthe system’s 4,000-fpmconveying air velocity.

Clue 4: Ductwork includes too much flexible hose. InFigure 2e, flexible hose has been used in place of metalduct as a quick way to connect two duct sections. How-ever, dust builds up more easily on the hose’s corrugatedinside surface thanon smoothmetal duct.Thehose’s inter-nal resistance also ismore than twice that of smoothmetal

duct, so with the hose bends acting as elbows, the hose’sequivalent length is much greater than its actual length.The system’s exhaust fanmay not be large enough to pro-

vide the speed necessary to overcome this additional air-flow resistance, and the result is low air velocity thatcauses dust to dropout of the air andplug the ducts.

Figure 2

Visualclues toductproblems thatcausedustaccumulation

a.Mainductdiameterdoesn’t enlargeafterbranch junctions

b.Mainduct isblankedoff

c.Smaller-diameterductabruptly joins larger-diameterduct

d.Branchduct joinsmainductatT junction

e.Ductwork includes toomuch flexiblehose

f.Ductblastgate isn’t locked inposition

Branchjunctions

Blankflange

4 inches

8 inches

9 inches

8 inches

20 inches

AB

C

A

BC

A

B

A

B

C

T junction 30-degreeY junction

Blastgateinunlockedposition

Solution:Replace the flexible hose with sections ofmetalduct that are clamped together. You should use flexiblehose in the system only with equipment that must move,such as connecting metal duct to the capture hood for aloss-in-weight feeder that rests on load cells; see NFPA654 formore information.

Clue 5: Duct blast gate isn’t locked in position. Blastgates in ducts add artificial airflow resistance to balancethe airflow in individual duct branches. For eachblast gate,only one position is correct to balance the airflow in allbranches. In Figure 2f, the blast gate has been adjusted tosend more airflow into this branch, which steals airflowfromother branches.

Solution:Set this andother duct blast gates tomeet the sys-tem’s design airflow and then lock the gates in place. Youcan avoid this problem altogether by designing the dustcollection system for the correct airflow balance withoutusingblast gates,which is calledbalancebydesign.

Clue 6: The pressure drop across the filter media ishigher than the design pressure drop.Figure 3 shows theairflow resistance increasing in abaghousedust collectionsystem in which the pressure drop across the bag filtermedia exceeds thedesignpressuredrop. (Pressuredrop, ordifferential pressure, is the difference in the static pres-suresmeasuredon the clean anddirty sides of the dust col-lector; themore dust collected on the filters, the higher thepressure drop will be.) In Figure 3, the design pressuredrop across the bag filters is the system’s design staticpressure (11 inches water column) minus the static pres-sure required tomove the air from the longest branch ductto the baghouse inlet (7 inches), which equals 4 incheswater column. The lineQ1 represents the design airflowthe fan shoulddeliver,with the line’s topwhite portion rep-resenting the design pressure drop though the media. ButQ2 represents the airflow the fan actually delivers,which islower than the design level because the actual pressuredrop through themedia (shownby the line’swhite portion)exceeds the design level, increasing airflow resistance.

The system exhaust fan’s operating curve shows howmuchair the fan canmove (airflow, representedby thehor-izontal axis) at different static pressures (on the verticalaxis).Asyou can see, this curve shifts to the left of the sys-tem operating point — showing that the fan delivers lessairflow than the system requires — because the higherpressure drop has increased the airflow resistance acrossthe system. Because the fan can’t deliver the required air-flow, the conveying air velocity in the ductwork slows andleads tomore dust droppingout in the ducts.

Solutions: The solution to high pressure drop across themedia depends on your application. Assuming you’ve se-lected the right air-to-cloth ratio (the airflow in cubic feet

per minute divided by the square feet of filter media sur-face area) for your dust collector, properly starting up thecollectorwhen thenew filters are installedwill provide thebest long-term performance. Once the new filters are in-

Figure 3

Effectonairflowofhighpressuredropacross the filtermedia

Staticpressure(in

inches

watercolumn)

Airflow (inactualcubic feetperminute)

7

11

Designstaticpressure

Designstaticpressuretomoveairfrom longestductbranch tobaghouse inlet

Exhaust fanoperation

Highpressuredropshiftsfanoperationcurve to left

Systemoperatingpoint

Q2 Q1

stalled, you should also condition them (also called pre-coating or seeding) before your dust collection systemgoes back online; this will build up an initial dust cake onthe media that resists blinding and prevents high pressuredrop. (For more information, see reference 7.)With olderfilters, increasing the cleaning frequency or replacing thefilters more often can control the pressure drop across themedia.Another solution is to replaceyour filterswithonesthat have a larger surface area to better handle your dust-laden airflow.

Clue7:Dust cloud is first signof trouble.Figure4showsadust cloud surrounding a vibrating conveyor that deliverspowder toa sifter; thecloudhasdevelopedbecause thecap-ture hood over the equipment isn’t drawing the dust-ladenair into the dust collection system. The dust cloud—a po-tential explosionhazard—couldbe the result ofductplug-ging, filter blinding, or other problems, any ofwhich couldreduceairflow through the system.Unfortunately, thisdustcloud is the first sign of trouble because the dust collectionsystempressure, airflow, andotherdata aren’tmonitored.

Solution:You need tomake routine measurements of sta-tic pressure and airflow at appropriate points in the dustcollection system, aswell asmeasure each capture hood’sface velocity (the air velocity at the inlet opening). Suchsystemmonitoringwill reveal anychanges inpressure, air-

flow, or face velocity from the system’s baseline perfor-mancedata.Byhelpingyou spot such changes early,mon-itoring allows you to catch a small problem before it cancreate a hazardous dust cloud in your process area.

What is baseline performance data? It’s documentedproof of your dust collection system’s performance atstartup (or after any significant system modification),which demonstrates that the system can deliver the designairflow at every capture hood or other dust-controlledopening. This is one of the requirements of NFPA 91:Standard for Exhaust Systems for Air Conveying of Va-pors, Gases, Mists, and Noncombustible ParticulateSolids (2004)2, which is incorporated by reference intoNFPA 654. In addition to verifying the system design, thebaseline data provides a reference point for systemmoni-toring. Baseline data documentation is powerful evidenceto showanOSHAinspector that your systemhas adequateconveying velocities. The design documentation youshould keep on file includes the system schematic, a tablelisting locations and dimensions of air-balancing devices(such as blast gates), the as-built system’s static pressurebalance calculations for sizing the exhaust fan, and the de-sign bases and specifications for system equipment. (Formore information, see reference 8.) Turning the baselineperformance data over to the operators once the system isonline allows them to use the data to monitor system per-formance andkeep the systemworkingover the long term.

In some cases, comparing this baseline data to current op-erating datamay help you determine that the original sys-tem design can no longer handle your application’schanged field conditions and requirements. In this case,you’ll have to redesign the system tomeet thenewrequire-ments. Several situations requiringyou to test the dust col-lection system todemonstrate that itworks as designedarelisted inNFPA91. PBE

Nextmonth: In Part II, sectionswill cover how to eliminateignition sources in the systemandhow touse explosionpre-vention and protectionmethods at the collector. A final sec-tionwillexplainhowtomeetadditionalNFPArequirements.

References1. The OSHA National Emphasis Program (NEP) directive on safelyhandling combustible dusts is available at http://www.osha.gov/OshDoc/Directive_pdf/CPL_03-00-008.pdf.

2. Available from National Fire Protection Association (NFPA), 1Batterymarch Park, Quincy, MA 02169-7471; 800-244-3555, fax617-770-0700 (www.nfpa.org).

3. Rolf K. Eckhoff, Dust Explosions in the Process Industries, ElsevierPublishing, third edition, 2003; available from the PBE Bookstore atwww.powderbulk.com.

4. Lee Morgan and Terry Supine, “Five ways the new explosion ventingrequirements for dust collectors affect you,” Powder and BulkEngineering, July 2008, pages 42-49; see “For further reading” forinformation on purchasing a copy of this article.

5. Investigation Report: Combustible Dust Hazard Study, Report 2006-H-1, US Chemical Safety and Hazard Investigation Board,November 2006 (www.chemsafety.gov).

6. Available from American Conference of Governmental IndustrialHygienists, 1330 Kemper Meadow Drive, Cincinnati, OH 45240;513-742-6163, fax 513-742-3355 (www.acgih.org, mail@acgih.org).

7. Ed Ravert, “Precoating new filters for better airflow, longer filterlife,” Powder and Bulk Engineering, October 2006, pages 27-33; see“For further reading” for information on purchasing a copy of thisarticle.

8. Gary Q. Johnson, “Dust control system design: Allowing for yourrange of process conditions and establishing baseline performancevalues,” Powder and Bulk Engineering, October 2005, pages 39-48;see “For further reading” for information on purchasing a copy of thisarticle.

For further reading

Find more information on designing dust collection sys-tems and preventing dust explosions in articles listedunder “Dust collection and dust control” and “Safety” inPowder and Bulk Engineering’s comprehensive articleindex (later in this issue and at PBE’s Web site, www.powderbulk.com) and in books available on theWeb siteat the PBE Bookstore. You can also purchase copies ofpastPBE articles atwww.powderbulk.com.

Gary Q. Johnson is principal consultant with WorkplaceExposure Solutions, 7172 Willowood Drive, Cincinnati,OH 45241; phone/fax 513-777-4626 (gary@workexposoln.com, www.workexposoln.com). He holds a master’sdegree in business administration from the University ofScranton, Scranton, Pa., and a bachelor’s degree in chemi-cal engineering fromOhioStateUniversity inColumbus.

Figure 4

Lackof systemmonitoringmakesdustcloud first signof trouble

About40percentofcombustibledustexplosionsre-ported in the US andEurope over the last 25 yearshave involved dust collectors. Dust collection sys-tems are now a primary focus of inspections re-quiredbyOSHA’sNationalEmphasis Programonsafely handling combustible dusts.1 OSHAalso hastheauthoritytoenforceNationalFireProtectionAs-sociation (NFPA) standards for preventing or pro-tectingagainstdustexplosions.Thistwo-partarticlefocuses on how you can design your dust collectionsystem’s dust collector, ductwork, and exhaust fantomeet the intentof theseNFPArequirements.PartI(December)covereddustexplosionbasicsandhowto prevent dust accumulation in system ductwork;Part II covers how to eliminate ignition sources inthe system, how to use explosion prevention andprotectionmethodsatthecollector,andhowtomeetadditionalNFPArequirements.

Eliminating ignition sources

Grounding the system equipment, selecting electricalcomponents for your hazardous area classification, andprotecting the exhaust fan are all ways to eliminate igni-tion sources in your dust collection system.

Grounding equipment. To prevent a static electrical dis-charge fromproviding an ignition source for a dust explo-sion, you must ground the dust collector and itscomponents, the ductwork, the exhaust fan, and other sys-tem components to dissipate static electricity. This in-

cludes selecting filters with integral grounding straps thatprovide a grounding path between the filter and thetubesheet, which alsomust be grounded. Componentma-terialsmust be conductive, aswell.Don’t select ductmadeof plastic, which is nonconductive. If your ductwork in-cludes flexible hose, the hose should be molded withgrounding wires, and you must clamp these wires to theupstreamanddownstreammetal ducts to ground the hose.Examples of how to ground typical system componentsare shown inFigure 1.

The grounding wires on various components should bevisible to operators so they can quickly check that thegrounding is in place. Operators should also routinelycheck the resistance to ground in the wires to ensure thatit’s less than 106 ohms. You can find more information onhow to use grounding tominimize these electrostatic haz-ards inNFPA 77: Recommended Practice on Static Elec-tricity (2007).2

Selecting electrical components for your hazardous areaclassification.You’ll alsoneed tochooseelectrical compo-nents for your dust collection system that are certified foruse in your hazardous location, as detailed inNFPA 499:Recommended Practice for the Classification of Com-bustibleDusts and ofHazardous (Classified) Locations forElectrical Installations inChemicalProcessAreas (2008).2

In theUS, thiscertification isprovidedbyUnderwriterLab-oratoriesandFactoryMutual.The typeofhazardousprotec-tionyour electrical componentsmust have—suchasbeingdust-ignition-proof or mounted in dust-tight enclosures—dependson theexplosion risk inyoursystem’s location,andthesehazardous locationsare ratedbyclasses,divisions,andgroups. Hazardous locations with combustible dust areClass II. InClass II,Division I, locations, the dust is present

Designing your dust collection system to meetNFPA standards — Part II

GaryQ.Johnson WorkplaceExposureSolutions

As appeared in www.powderbulk.comPBE January 2009

during normal conditions, and in Class II, Division 2, loca-tions, the dust is present only in abnormal conditions, suchasasystembreakdown.Theadditionalgroupsubclassifica-tiondependson the typeofdust in the surroundingenviron-ment: for example, Group E is for metal dusts, Group F is

for carbonaceous dusts, and Group G is for all other dusts.Todetermine the righthazardousareaclassificationforyoursystem’s electrical components, consider your dust type,dust quantity, whether the system’s dust collector is insideoroutside, and related factors.

Protecting the exhaust fan. If your exhaust fan fails me-chanically, the fan impeller can shift and rub or hit thehousing. A spark from such metal-to-metal contact hasenough energy to ignite a combustible dust. To avoid thishazard, you should do two things: First, place the exhaustfanon thedust collector’s clean side,where it can’t contactdust under normal conditions, as detailed in NFPA 654:Standard for the Prevention of Fire and Dust Explosionsfrom the Manufacturing, Processing, and Handling ofCombustible Particulate Solids (2006).2 Second, train andequip your operators to practice good dust collectormain-tenance so they can spot filter leaks early and replace theaffected filters before dust can escape the system.One toolavailable frommultiple suppliers for helping theoperatorsspot filter leaks before they cause a problem is a filter-leakdetection system; the system uses an inductive or tribo-electric probe inserted into the system ductwork to senseparticles escaped froma leaking filter.

Using explosionprevention andprotectionmethodsat the collector

Tomeet NFPA requirements for protecting your dust col-lector from a dust explosion, you must use one (or more)

Figure 1

Grounding system components to eliminate ignition sources

PhotocourtesyofJ.O.A.NorthAmerica,Charlotte,N.C.

Duct groundingwires clearlyvisible tooperators

Use filters groundedto tubesheet

Ground clean-air plenum,tubesheet, and dirty-air plenum

Ground each sectionof conductive duct

Ground exhaust fanand fan motor

Grounddust collector

explosion prevention or protection method, includingventing, suppression, isolation, and others. Explosionventing is covered in NFPA 68: Standard on ExplosionProtection by Deflagration Venting (2007), and suppres-sion, isolation, andothermethods are covered inNFPA69:

Standard on Explosion Protection Systems (2008).2 Youcan see a dust collector equipped with various explosionprevention andprotectiondevices inFigure 2.

Of the severalways tomeetNFPA68requirements forpro-tecting your dust collector from an explosion, explosionventing is the most common. NFPA 68 venting require-ments are described in detail in thePBE article “Fivewaysthe newexplosion venting requirements for dust collectorsaffect you.”3 As the article states: “The purpose of explo-sion venting is to save lives, not property. Awell-designedexplosion vent functions as a weak element in the equip-ment’s pressure envelope, relieving internal combustionpressure to keep the collector fromblowing up into pieces....Typically, the collector is locatedoutside anddesigned tovent away from buildings and populated locations.” Addi-tional venting information in the article highlights NFPA68 areas that have changed or are of most importance tobulk solids processors, including the performance-baseddesignoptionand sizingvents anddischargeducts.

If yourdust collector is indoors andcan’t beventedoutsidethrough an exterior wall or ceiling, youmust equip it withan explosion prevention method, such as a suppressionsystem, that can prevent a dust explosion from propagat-ing to connected equipment. You can use any of severalsystemsdescribed inNFPA69.Onecommonexample is achemical suppression system, which senses a developingexplosion in the dust collector and rapidly injects a chemi-

lates cleaned air to the workplace. Placed on the dust col-lector’s clean-air side between the fan and collector orafter the fan, the float valve works like a ball check valve

— that is, it’s pushed shut by the pressure and airflowchanges causedby an explosion in the collector.

Meeting additionalNFPArequirements

According to our industry’s current interpretation of theNFPA standards covering dust collection system design,you don’t have to update equipment in your system eachtimeaparticular standard is updatedunless your location’sauthority having jurisdiction (“an organization, office, orindividual responsible for enforcing the requirements of acode or standard” [NFPA 68]) decides otherwise. How-ever, the standards require you to follow certain proce-dures related to maintenance, housekeeping, explosionprotection, and managing system changes, and you mustimplement the procedures retroactively.

Here are some of the key retroactive requirements inNFPA68, 69, and654:

•Youmust provideboth initial training and refresher train-ing to employees on the established operating andmain-tenance procedures for your dust collection system andexplosionprevention andprotection equipment.

•Youmust providehousekeepingandcleaning for thedustcollection systemand surrounding areausingprocedures(suchasvacuumcleaning) thatminimizedust-cloudgen-eration andat a frequency thatminimizes dust accumula-

Figure 2

Dust collector equipped with explosion prevention and protection devices

Flame-frontdiverter oncollector’sdirty-air side

Explosion ventwith deflector plate

Float valve toprotect exhaust fanor ventilation systemfor recirculatingcleaned air

PhotocourtesyofJ.O.A.NorthAmerica,Charlotte,N.C.

cal powder into the developing fireball to stop the flamefront; this can be a useful retrofit for a collector located in-doors and too far from an outside wall to allow explosion

venting. Another suppression system injects a hot-watermist into the collector to stop the flame front from travel-ing to upstream equipment. For a dust collection systemthat’s part of a closed-circuit process, youmayconsider anexplosionpreventionmethod that usesnitrogen to inert thedust collection system;usingnitrogen inerting in a closed-circuit application minimizes nitrogen consumption,whichotherwisewouldbeprohibitively expensive.

NFPA 69 also describes methods for isolating upstreamand downstream equipment from an explosion in the dustcollector. One common example is a flame-front diverter,which is an explosion vent in the duct (Figure 2). The di-verter is typically placed at the dust collector’s dirty-airinlet when the collector is located near the dust collectionsystem’s exhaust fan and ductwork. When an explosionoccurs in the collector, the flame-front diverter’s ventopens and releases the explosion to the atmosphere, thusisolating other equipment from the flame front. Anotherisolation device is a high-speed isolation valve, which islocated in the dirty-air duct and wired to an explosion de-tector in the collector. The valve’s location on the duct isfar enough from the explosion detector to allow the detec-tor to respond to an explosion and close the valve beforethe flame front can reach upstream equipment. A floatvalve (Figure 2) is another isolation device that protectsthe exhaust fan or the ventilation equipment that recircu-

tion in your workplace. The portable vacuum cleanersyou use must meet Class II hazardous location require-mentswhenoperated in a combustible dust hazard area.

•You must ensure that all dust collection system compo-nents are conductive, bonded (to protect workers fromelectric shock), and grounded to a resistance of less than106 ohms.

•You must inspect, test, and maintain the dust collectionsystem and its explosion prevention and protectionequipment according to the manufacturer’s recommen-dations. You alsomust keep records of these inspectionsand tests and signoff on them.

•You need to establish change management proceduresfor the dust collection system and its explosion preven-tion and protection equipment and address related tech-nical issues before making any system changes; after asystem change, you must update the system’s designdocumentation to reflect the change.

Alsobeaware that if equipment inyourdust collection sys-tem has changed or the dust collected by your system haschanged since you initially tested the dust for combustibleproperties andconductedahazardanalysis of your system,you must revisit the hazard analysis to see if you need tochangeanyof the system’sexplosion riskmitigation strate-gies, such as explosion vent size and location.Don’t forgetthatNFPA requires that you keep the hazard analysis up todate for the lifeof theprocess it protects and thatyou reviewandupdate the analysis at least onceevery5years. PBE

References1. The OSHA National Emphasis Program (NEP) directive on safelyhandling combustible dusts is available at http://www.osha.gov/OshDoc/Directive_pdf/CPL_03-00-008.pdf.

2. Available from National Fire Protection Association (NFPA), 1Batterymarch Park, Quincy, MA 02169-7471; 800-244-3555, fax617-770-0700 (www.nfpa.org).

3. Lee Morgan and Terry Supine, “Five ways the new explosion ventingrequirements for dust collectors affect you,” Powder and BulkEngineering, July 2008, pages 42-49; see “For further reading” forinformation on purchasing a copy of this article.

For further reading

Find more information on designing dust collection sys-tems and preventing dust explosions in articles listedunder “Dust collection and dust control” and “Safety” inPowder and Bulk Engineering’s comprehensive articleindex (later in this issue and at PBE’s Web site,www.powderbulk.com)and inbooks available on theWebsite at thePBEBookstore.Youcanalsopurchase copies ofpastPBE articles atwww.powderbulk.com.

Gary Q. Johnson is principal consultant with WorkplaceExposure Solutions, 7172 Willowood Drive, Cincinnati,OH 45241; phone/fax 513-777-4626 (gary@workexposoln.com,www.workexposoln.com).He holds amas-ter’s degree inbusiness administration from theUniversityof Scranton, Scranton, Pa., and a bachelor’s degree inchemical engineering from Ohio State University inColumbus.