DUST CONTROL OSHA has cmmenced rulemaking t develp a cmbustible dust standard fr general industry. Cmbustible dust can cause catastrphic explsins like the 2008 disaster at a sugar refinery that killed 14 wrkers and seriusly injured dzens mre. Deadly cmbustible dust fires and explsins can be caused by a wide array f materials and prcesses in many industries. Materials that may frm cmbustible dust include wd, cal, plastics, spice, starch, flur, feed, grain, fertilizer, tbacc, paper, sap, rubber, drugs, dyes, certain textiles, and metals. Cmbustible dust is lsely defined as material capable f passing thrugh a U.S. N. 40 standard sieve. While several OSHA standards address aspects f this hazard, the Agency des nt have a cmprehensive standard that addresses cmbustible dust. In situatins where OSHA has nt develped specific standards, emplyers are respnsible fr cmplying with the OSH Act s General Duty Clause (Sectin 5.a.1). The General Duty Clause states that each emplyer shall furnish a place f emplyment which is free frm recgnized hazards that are causing r are likely t cause death r serius physical harm t his emplyees. The fllwing elements are necessary fr OSHA t cite a General Duty Clause vilatin: Emplyees are expsed t a hazard; The hazard is recgnized; The hazard is likely t cause death r serius physical harm; and There is a feasible and useful methd t crrect the hazard. OSHA has subsequently included cmbustible dust as meeting the definitin f a hazardus chemical in the Hazard Cmmunicatin standard; cmbustible dust hazards must be addressed n labels and SDSs by 2015. Label elements include the signal wrd warning and the hazard statement May frm cmbustible dust cncentratins in the air. The ease f ignitin and severity f cmbustible dust explsin are typically influenced by particle size. Finer particles are mre explsive because they have large surface areas relative t their weight, allwing them t rapidly react with xygen when dispersed in air and ignite. Cmbustible dusts with an average particle size smaller than 420 micrns are cnsidered by mst surces t be explsive unless testing prves therwise. T put the size f a micrn in rugh perspective, the particle size f table salt is arund 100 micrns. NFPA defines dust categries as: Green material: Wd particulate that has an average misture cntent equal t r greater than 25 percent by weight
Deflagrable wd dust: Wd particulate that will prpagate a flame frnt, thus presenting a fire r explsin hazard, when suspended in air, r the prcess-specific xidizing medium ver a range f cncentratins, regardless r particle size r shape; wd particulate with a mass median particle size f 500 micrns r smaller (ie: material that will pass thrugh a US N. 35 standard sieve), having a misture cntent f less than 25 percent. Dry Nndeflagrable wd dust: Wd particulate with a mass median particle size greater than 500 micrns (ie: material that will nt pass thrugh a US N. 35 standard sieve), having a misture cntent f less than 25 percent. Even seemingly small amunts f accumulated dust can cause catastrphic damage. Accrding t the NFPA, a catastrphic explsin can ccur frm as little as 1/32 f an inch f accumulated dust (arund the thickness f a dime) cvering just 5 percent f a rm s surface. Per NFPA a deflagratin hazard is deemed t exist where the layer f accumulated fugitive deflagrable wd dust n upward facing surfaces exceeds 1/8 ver 5% f the area r 1,000 sq. ft. whichever is smaller. Per NFPA, fire and deflagratin hazards shall be deemed nnexistent where nly green material is cllected r cnveyed and cnstructin f the equipment handling and string the material is all nncmbustible. Like all fires, a dust fire ccurs when fuel (the cmbustible dust) is expsed t heat (an ignitin surce) in the presence f xygen (air). Remving any ne f these elements f the classic fire triangle eliminates the pssibility f a fire. A dust explsin requires the simultaneus presence f tw additinal elements dust suspensin and cnfinement. Suspended dust burns mre rapidly and cnfinement allws fr pressure buildup. Remval f either the suspensin r the cnfinement elements prevents an explsin, althugh a fire may still ccur. There are tw types f dust explsins, primary and secndary. Secndary explsins are ften the mst damaging. A primary dust explsin ccurs when a dust suspensin within a rm is ignited and expldes. The pressure wave frm the first (r primary) explsin ften shakes lse dust frm flat building surfaces frming a clud that can be ignited by the flame fllwing it (the secndary explsin). The Chemical Safety Bard says the best way t prevent secndary dust explsins is t minimize dust accumulatins. Ensuring gd husekeeping, designing and maintaining equipment t prevent dust leaks, using dust cllectrs, eliminating flat surfaces and ther areas where dust can accumulate, and sealing hard t clean areas can effectively prevent secndary dust explsins. Using prper equipment and techniques t clean cmbustible dust are essential, fr example minimizing dust cluds and making sure that vacuum cleaners are apprved fr cmbustible dust lcatins. The CSB ntes that sme incidents invlved dust explsins that spread thrugh pipes r vent ducts, frm ne piece f equipment t ther equipment r ther areas f the facility. In many cases, the pressure can increase as the explsin mves frm ne lcatin t the next, increasing the damage. The NFPA standards fr dust cllectrs cnsider the risk f www.wdriskmanagement.cm
prpagatin, with recmmendatins t prvide islatin valves r distance t minimize chances f a dust explsin spreading t areas where wrkers may be present. The fllwing tw pages represent a checklist fr a cmbustible dust prgram and the minimum requirements fr design. Any fluctuatins frm these requirements require a written recmmendatin be made. www.wdriskmanagement.cm
CHECKLIST FOR A COMBUSTIBLE DUST PROGRAM Dust hazard assessments OSHA recmmends that facilities carefully identify the fllwing t assess their ptential fr dust explsins. A qualified team f managers shuld identify the fllwing t assess their facilities ptential fr dust explsin: Materials that can be cmbustible when finely divided; Prcesses which use, cnsume, r prduce cmbustible dusts; Open areas where cmbustible dusts may build up; Areas requiring special electrical equipment classificatin (Class II lcatins) due t the presence (r ptential presence) r cmbustible dust; Hidden areas where cmbustible dusts may accumulate; Means by which dust may be dispersed in the air; and Ptential ignitin surces (fr example, welding, cigarettes, ht surfaces, and electrical/switch/utlet activatin). Cntrlling Dust Implement a hazardus dust inspectin, testing, husekeeping and cntrl prgram; Use prper dust cllectin systems and filters; Minimize the escape f dust frm prcess equipment r ventilating systems; Use surfaces that minimize dust accumulatin and facilitate cleaning; Prvide access t all hidden areas t permit inspectin; Inspect fr dust residues in pen and hidden areas at regular intervals; If ignitin surces are present, use cleaning methds that d nt generate dust cluds; Use nly vacuum cleaners apprved fr dust cllectin; and Lcate relief valves away frm dust depsits. www.wdriskmanagement.
Ignitin cntrl Use apprpriate electrical equipment and wiring methds; Cntrl static electricity, including bnding f equipment t grund; Cntrl smking, pen flames, and sparks; Cntrl mechanical sparks and frictin; Use separatr devices t remve freign materials capable f igniting cmbustibles frm prcess materials; Separate heated surfaces and heating systems frm dusts; Select and use industrial trucks prperly; Use cartridge activated tls prperly; and Use an equipment preventative maintenance prgram. Injury and damage cntrl Separatin f the hazard (islate with distance); Segregatin f the hazard (islate with a barrier); Deflagratin islatin/venting; Pressure relief venting fr equipment; Directing vents away frm wrk areas; Specialized fire suppressin systems; Explsin prtectin systems; Spark/ember detectin fr suppressin activatin; Emergency actin plan develpment; and Maintenance f emergency exit rutes Wrkers are the first line f defense in preventing and mitigating fires and explsins. The peple clsest t the surce f the hazard can be instrumental in recgnizing unsafe cnditins. Emplyees with hazardus chemicals (including cmbustible dusts) in their wrkplaces are required t cmply with 29 CFR 1910.1200, the Hazard Cmmunicatins Standard. This includes having labels n cntainers f hazardus chemicals, using safety data sheets, and prviding emplyee training. Initial and refresher training shall be prvided t emplyees wh are invlved in perating, maintaining and supervising facilities that handle www.wdriskmanagement.cm
cmbustible particulate slids. Initial and refresher training shall ensure that all emplyees are knwledgeable abut the fllwing: (1) Hazards f their wrkplace (2) General rientatin, including plant safety r u les (3) Prcess descriptin (4) Equipment peratin, safe startup and shutdwn, and respnse t upset cnditins (5) The necessity fr prper functining f related fire and explsin prtectin systems (6) Equipment maintenance requirements and practices (7) Husekeeping requirements (8) Emergency respnse plans www.wdriskmanagement.cm
Dust cllectin system design requirements All ductwrk shall be sized t prvide the air vlume and velcity necessary t keep the duct interir clean and free f residual material. The air material separatr and ductwrk must be f nncmbustible materials with the exceptin f the filter media which shall be permitted t be cnstructed f cmbustible materials. The last pickup t the duct shuld be at least 20 frm the duct building exit. The sil/hpper bttm shall be slped. The cllectr shall be equipped with blast panels. Access drs r penings shall be prvided t permit inspectin, cleaning and maintenance. Ducts shall be prtected per the fllwing: Ducts with a fire hazard: Ducts cnveying dry material released by equipment (five majr machine r mre perating at the same time) shall be designed and cnstructed in accrdance with ne f the fllwing: Equipped with a listed spark detectin and extinguishing system installed dwnstream frm the last material entry pint and upstream f any cllectin equipment. Equipped with a listed spark detectin system actuating a high-speed abrt gate, prvided the abrt gate can perate fast enugh t intercept and divert burning embers t atmsphere befre they can enter any cllectin r strage equipment. Recycling f air material separatr exhaust fr prcessing f dry wd t building shall be permitted nly if the fllwing cnditins are met: Fr dust cllectin systems f capacity less than r equal t 5000 cfm the system shuld have a spark detectin and extinguishing system installed. Fr dust cllectin systems f capacity greater than 5000 cfm, the system shall be equipped with a listed spark detectin and extinguishing system lcated n the duct upstream frm the dust cllectr and dwnstream frm the last material entry pint and the exhaust air duct cnveying the recycled air back t the building shall be equipped with a high-speed abrt gate activated by the spark detectr and prvided with a manual reset. A dcumented maintenance and husekeeping prgram must be in place. The inspectin, testing, and maintenance prgram shall include the fllwing: www.wdriskmanagement.cm
(1) Fire and explsin prtectin and preventin equipment in accrdance with applicable NFPA standard (2) Dust cntrl equipment (n fugitive dust release) (3) Husekeeping (at least semi-annually) (4) Ptential ignitin surces (must fllw ht wrk permit prgram) (5) Electrical, prcess, and mechanical equipment (must clean electrical system at least annually) Lubricatin f bearings and check f fans, blwers, and mtrs fr excessive heat n a peridic basis. www.wdriskmanagement.cm