Dedgning your dust collection system to meet NFPAstqndqrds - Pqrtll

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1 Dedgning yur dust cllectin system t meet NFPAstqndqrds - Pqrtll GaryQ.Jhnsn Wrkplace Expsure Slutins Abut 40 percent f cmbustible dust explsins reprted in the US and Eurpe ver the last 25 years have invlved dust cllectrs. Dust cllectin systems are nw a primary fcus f inspectins required by OSHA's Natinal EmphasisPrgram n safely handling cmbustible dusts.' OSHA als has the authrity t enfrce Natinal Fire htectin Assciatin NFPA) standards fr preventing r prtecting against dust explsins. This tw-part article fcuses n hw yu can design yur dust cllectin system's dust cllectr, ductwrk, and exhaust fan t meet the intent f these NFPA requirements. Part cvered dust explsin basics and hw t pnevent dust accumulatin in system ductwrk; Part II cvers hw t eliminate ignitin surces in the system, hw t use explsin preventin and prtectin methds at the cllectr, and hw t meet additinal NFPA requirements. Eliminating ignitin surces Grunding the system equipment, selecting electrical cmpnents fr yur hazardus area classificatin, and prtecting the exhaust fan are all ways t eliminate ignitin surces in yur dust cllectin system. Grunding equipment. T prevent a static electrical discharge frm prviding an ignitin surce fr a dust explsin, yu must grund the dust cllectr and its cmpnents, the ductwrk, the exhaustfan, and ther system cmpnents t dissipate static electricity. This includes selecting filters with integral grunding straps that prvide a grunding path between the filter and the tubesheet, which als must be grunded. Cmpnent materials must be cnductive, as well. Dn't select duct made f plastic, which is nncnductive. If yur ductwrk includes flexible hse, the hse shuld be mlded with grunding wires, and yu must clamp these wires t the upsfeam and dwnstream metal ducts t grund the hse. Examples f hw t grund typical system cmpnents are shwninfigure 1. The grunding wires n varius cmpnents shuld be visible t peratrs s they can quickly check that the grunding is in place. Operatrs shuld als rutinely check the resistance t grund in the wires t ensure that it's less than 10u hms. Yu can find mre infrmatin n hw t use grunding t minimize these electrstatic hazards in NFPA 77: Recmmended Practice n Static Electricity (2007).2 Selecting electrical cmpnents fr yur hazrdus area classificatin. Yu'll als need t chse elecfrical cmpnents fr yur dust cllecûn system that are certified fr use in yur hazardus lcatin, as detailed in NFPA 499: Recmmended Practice fr the Classfficatin f Cmbustible Dusts andf Hazardus (Classffied) Incatinsfr Electrical Installatins in Chemical Prcess Areas (2008).' In the US, this certificatin is prvidedby Underwriter Labratries and Factry Mutual. The type f hazardus prtectin yurelectrical cmpnents musthave- such as being dust-ignitin-prf r munted in dusttight enclsures - depends n the explsin risk in yur system's lcatin, and these hazardus lcatins are rated by classes, divisins, and grups. Hazardus lcatins with cmbustible dust are Class II. In Class II, Divisin I, lcatins, the dust is present

2 ,..,^".r^"" """?i11 IEE - during nrmal cnditins, and in Class II, Divisin Z,lcatins, the dust is present nly in abnrmal cnditins, such as asystembreakdwn. The additinal grup subclassificatin depends nthe type f dustinthe surrunding envirnment: fr example, Grup E is fr metal dusts, Grup F is fr carbnaceus dusts, and Grup G is fr all ther dusts. T determine the right hazardus area classificatn fr yur system's electrical cmpnents, cnsider yur dust type, dust quantity, whether the system's dust cllectr is inside r utside, and related factrs. Grunding system cmpnents t eliminate ignitin surces Grund clean-air plenum, tubesheet, and difi-air c z d grunding E wires clearly visible t z 0peratrs Grund exhaustfan t and fan mtr à Grund dust cllectr I

3 PBI,I.JANUARY i4ulje Prtecting the exhaustfan.lf yur exhaust fan fails mechanically, the fan impeller can shift and rub r hit the husing. A spark frm such metal-t-metal cntact has enugh energy t ignite a cmbustible dust. T avid this hazard,yu shuld d tw things: First, place the exhaust fan n the dust cllectr's clean side, where it can't cntact dust under nrmal cnditins, as detailed in NFPA 654: Standardfr the Preventin f Fire and Dust Explsins frm the Manufacturing, Prcessing, and Handling f Cmbustible Particulate Slids (2006)'' Secnd, train and equip yur peratrs t practice gd dust cllectr maintenance s they can spt filter leaks early and replace the affected filters befre dust can escape the system. One tl available frm multiple suppliers frhelping the peratrs spt filter leaks befre they cause a prblem is a filter-leak detectin system; the system uses an inductive r tribelectric prbe inserted int the system ductwrk t sense particles escaped frm aleaking filter. Using explsin preventin and prtectin methds at the cllectr T meet NFPA requirements fr prtecting yur dust cllectr frm a dust explsin, yu must use ne (r mre) explsin preventin r prtectin methd, including venting, suppressin, islatin, and thers. Explsin venting is cvered in NFPA 68: Standard n Explsin Prtectin by DeflagratinVenting (2007), and suppressin. islatin. and ther methds are cvered i n N F PA 69 : Standard n Explsin Prtectin Systems (2008).' Yu can see a dust cllectr equipped with varius explsin preventin and prtectin devices infigure 2. Of the several ways t meetnfpa 68 requirements frprtecting yur dust cllectr frm an explsin, explsin venting is the mst cmmn. NFPA 68 venting requirements are described in detulinthe PBE article "Five ways the new explsin venting requirements fr dust cllectrs affect yu."3 As the article states: "The purpse f explsin venting is t save lives, nt prperty. A well-designed explsin vent functins as a weak element in the equipment's pressure envelpe, relieving internal cmbustin pressure t keep the cllectr frm blwing up int pieces....typically, the cllectr is lcated utside and designed t vent away frm buildings and ppulated lcatins." Additinal venting infrmatin in the article highlights NFPA 68 areas that have changed r are f mst imprtance t bulk slids prcessrs, including the perfrmance-based design ptin and sizing vents anddischarge ducts. If yur dust cllectr is indrs and can' t be vented utside thrugh an exterir wall r ceiling, yu must equip it with an explsin preventin methd, such as a suppressin system, that can prevent a dust explsin frm prpagating t cnnected equipment. Yu can use any f several systems described in NFPA 69. One cmmn example is a chemical suppressin system, which senses a develping explsin in the dust cllectr and rapidly injects a chemi-

4 .JANUARY 2tl9 IPBE cal pwder int the develping fireball t stp the flame frnt; this can be a useful retrfit fr a cllectr lcated indrs and t far frm an utside wall t allw explsin venting. Anther suppressin system injects a ht-water mist int the cllectr t stp the flame frnt frm traveling t upstream equipment. Fr a dust cllectin system Dust cllectr equipped with explsin preventin and prtectin devices Explsin vent with deflectr pl Flame-frnt diverter n cllectr's difi-airside I z U d Flatvalvet prtect exhaustfan rventilatin system fr recirculating cleaned air z q È

5 PBEI laruuary ztree 93 that's part f a clsed-circuit prcess, yu may cnsider an explsin preventin methd that uses nitrgen t inert the dust cllectin system; using nitrgen inerting in a clsedcircuit applicatin minimizes nitrgen cnsumptin, which therwise wuld be prhibitively expensive. NFPA 69 als describes methds fr islating upstream and dwnstream equipment frm an explsin in the dust cllectr. One cmmn example is a flame-frnt diverter, which is an explsin vent in the duct (Figure 2). The diverter is typically placed at the dust cllectr's dirty-air inlet when the cllectr is lcated near the dust cllectin system's exhaust fan and ductwrk. When an explsin ccurs in the cllectr, the flame-frnt diverter's vent pens and releases the explsin t the atmsphere, thus islating ther equipment frm the flame frnt. Anther islatin device is a high-speed islatin valve, which is lcated in the dirty-air duct and wired t an explsin detectr in the cllectr. The valve's lcatin n the duct is far enugh frm the explsin detectr t allw the detectr t respnd t an explsin and clse the valve befre the flame frnt can reach upstream equipment. A flat valve (Figure 2) is anther islatin device that prtects the exhaust fan r the ventilatin equipment that recirculates cleaned air t the wrkplace. Placed n the dust cllectr's clean-air side between the fan and cllectr r after the fan, the flat valve wrks like a ball check valve - that is, it's pushed shut by the pressure and airflw changes causedby an explsin in the cllectr. Meeting additinal NFPA requirements Accrding t ur industry's current interpretatin f the NFPA standards cvering dust cllectin system design, yu dn't have t update equipment in yur system each time a particular standard is updated unless yur lcatin's authrity having jurisdictin ("an rgatizatin, ffrce, r individual respnsible fr enfrcing the requirements f a cde r standard" INFPA 68]) decides therwise. Hwever, the standards require yu t fllw certain prcedures related t maintenance, husekeeping, explsin prtectin, and managing system changes, and yu must implement the prcedures retractively. Here are sme f the key retractive requirements in NFPA and654:. Yu must prvide bth initial training and refresher training t emplyees n the established perating and maintenance prcedures fr yur dust cllectin system and explsin preventin and prtectin equipment.. Yu mustprvide husekeeping and cleaning frthe dust cllectin system and surrunding area using prcedures (such as vacuum cleaning) that minimize dust-clud generatin and at afrequency that minimizes dust accumulatin in yur wrkplace. The prtable vacuum cleaners yu use must meet Class II hazardus lcatin requirements when perated in a cmbustible dust hazard area.. Yu must ensure that all dust cllectin system cmpnents are cnductive, bnded (t prtect wrkers frm

6 PBEI.JaNuARY z9 electric shck), and grunded t a resistance fless than 106hms.. Yu must inspect, test, and maintain the dust cllectin system and its explsin preventin and prtectin equipment accrding t the manufacturer's recmmendatins. Yu als must keep recrds f these inspectins and tests and sign ffn them.. Yu need t establish change management prcedures fr the dust cllectin system and its explsin preventin and prtectin equipment and address related technical issues befre making any system changes; after a system change, yu must update the system's design dcumentatin t reflect the chanse. Alsbe aware thatif equipmentin yurdustcllectin system has changed r the dust cllected by yur system has changed since yu initially tested the dust fr cmbustible prperties and cnducted ahazardanalysis f yur system, yu must revisit the hazard analysis t see if yu need t change any f the system's explsinriskmitigatin strategies, such as explsin vent size and lcatin. Dn't frget that NFPA requires that yu keep thehazard analysis up t date fr the life f the prcess it prtects and that yu review and update the analysis at least nce every 5 years. PBE References 1. The OSHA Natinal Emphasis Prgram (NEP) directive n safely handling cmbustible dusts is available at /OshDc/Directive_pdf/CPL_O pdf. 2. Available frm Natinal Fire Prtectin.Assciatin(NFPA), 1 Batterymarch Park, Quincy, M A I : 8OO , f ax ( 3. Lee Mrgan and Terry Supine, "Five ways the new explsin venting requirements fr dust cllectrs affect yôu," Pwder and Bulk Engineering, July 2008, pages 42-49; see "Fr further reading" fr infrmatin n purchasing a cpy f this article. Fr further reading Find mre infrmatin n designing dlst cllectin systems and preventing dust explsins in articles listed under 'Dust cllectin and dust cntrl" and "Safety" in Pwder and Bulk Engineering's crqprehensive article index (later in this issue and at PBE's Web site, and in bks available n the Web site atthe PBE Bkstre. Yu can als purchase cpies f past PBE articles at Gary Q. Ihns n i s princ ipal c nsultant with Wrkplac e Exps ure Slutins, 7 I 72 Willwd Drive, Cincinnati, OH ; phne/fax 5I (gary@wrk expsln.cm, He hlds a master's degree in busine s s administratin frm the University f Scrantn, Scrantn, Pa., and a bachelr's degree in chemical engineering frm Ohi State University in Clumbus.