COMBUSTIBLE DUST

Combustible Dust: An Old Hazard Reemerges | FEATURE

Control of dust to prevent explosions is a complex science, well developed and documented in numerous publications.

On Feb. 7, 2008, a massive explosion at the Imperial Sugar refinery

in Port Wentworth, Ga., killed 14 employees and injured many others.

The cause of the explosion is less important than the fact that old

lessons had been forgotten. The refinery did not follow practices

long known to be necessary to safe industrial processing of organic

and other materials that produce combustible dust. Tried and true

methods of preventing such tragedies include simple things such

as good housekeeping, adequate design of dust control ventilation

systems, and adequate bonding and grounding for control of static

electricity—as well as more sophisticated engineering design

practices and controls.

Neglect of Established SafetyPractices Can Lead to Disaster

March 2009 • The Synergist 27

An Old Hazard Reemerges

BY DAVID G. SARVADI AND CHRISTOPHER D. WALKER

28 The Synergist • March 2009

FEATURE | Combustible Dust: An Old Hazard Reemerges

Combustible dust as a potential explosionhazard is not a new subject. A NationalFire Protection Association (NFPA) reportfrom 1957 states that, up to December1956, 1,120 dust explosions had killed676 people and injured over 1,700.1 Theseevents occurred not only in grain eleva-tors and sugar refineries—establishmentswhere combustible dust was a commonlyrecognized hazard—but also in fertilizerplants, malt houses, woodworking plants,and cotton mills. Irving Hartman’s chap-ter on combustible dust in the 1958 edi-tion of Patty’s Industrial Hygiene andToxicology revealed the presence of corkdust, coffee and spice dust, bark dust,rubber dust, phonograph record dust,and other dust in places where explo-sions occurred. Hartman also identifiednew industries and processes as poten-tial sources of combustible dust and de-scribed numerous sources of ignition inexisting plants.

Reports from the Chemical SafetyBoard on recent explosions clarify thatcombustible dust exists in many kinds ofindustrial operations, including mining,food processing, and manufacturing. Thecharacteristics of the substance producingthe dust, operating conditions, and thesurrounding environment are all factorsthat can lead to dust explosions. Moisturehas an effect, as does static electricity. Butthe real impact of a dust explosion comesafter the shock wave passes, when rafters,building structures, and other flat, dust-covered surfaces become airborne. Theseevents cause secondary and sometimestertiary explosions involving a greaterarea and a much larger amount of dust.

Knowing the DangerControl of dust to prevent explosions is acomplex science, well developed and doc-umented in numerous publications fromNFPA and other sources. The OSHASafety and Health Information Bulletin(SHIB) “Combustible Dust in Industry:Preventing and Mitigating the Effects ofFire and Explosions” contains an excel-lent summary of the technical issues ofcombustible dust and a long list ofsources for additional information. TheSHIB is available at www.osha.gov/dts/shib/shib073105.html.

Engineers are key to recognizing andcontrolling combustible dust hazards.NFPA standard 654, Standard for the

Prevention of Fire and Dust Explosionsfrom the Manufacturing, Processing, andHandling of Combustible ParticulateSolids, contains many technical specifica-tions and criteria for designing and oper-ating facilities where combustible dusthazards may exist. But without objectivecriteria for determining whether a facilityhas a potential combustible dust hazard,applying the standard is problematic.NFPA 654 defines the scope of its cover-age as “all phases of the manufacturing,processing, blending, pneumatic convey-ing, repackaging, and handling of com-bustible particulate solids or hybridmixtures, regardless of concentration orparticle size, where the materials presenta fire or explosion hazard.” Combustibledust is present in many processes, butwhether it presents a fire or explosionhazard is difficult to determine. This un-certainty undermines attempts to create alegally enforceable standard.

According to NFPA, “any industrialprocess that reduces a combustible materialand some normally noncombustible mate-rials to a finely divided state presents apotential for a serious fire or explosion.”Because of their experience in classifyinghazardous locations for insurance pur-poses, insurance carriers and organizations

like Factory Mutual can help determinewhether a facility has a potential com-bustible dust hazard.

Compliance IssuesIn March 2008, OSHA reissued its compli-ance instruction titled “Combustible DustNational Emphasis Program” (CPL 03-00-008). The program trains OSHA inspectorsin combustible dust hazards and instructsthem to consider whether a plant has ahistory of fires, material safety data sheets(MSDS), the presence of dust accumula-tions and other sources of data. In loca-tions where dust is visible, OSHAinspectors apply the NFPA criterion (5 per-cent of the surface area of a facility up to1,000 square feet with dust at least 1⁄32 inchdeep), which triggers more in-depth as-sessments. Samples must be collected andtested at the Salt Lake City Laboratoryusing standard ASTM methods for deter-mining characteristics related to dust com-bustibility and explosion hazards.

As explained in CPL 03-00-008, ClassII locations—those made hazardous by thepresence of combustible dust—are dividedinto three groups:

• Group E. Atmospheres containingcombustible metal dusts, including

The real impact of a dust explosion comes after the shockwave passes, when rafters, building structures, and otherflat, dust-covered surfaces become airborne. These eventscause secondary and sometimestertiary explosions involving agreater area and a much largeramount of dust.

March 2009 • The Synergist 29

Combustible Dust: An Old Hazard Reemerges | FEATURE

aluminum, magnesium, and their com-mercial alloys, and other combustibledusts whose particle size, abrasiveness,and conductivity present similar haz-ards in the use of electrical equipment.

• Group F. Atmospheres containingcombustible carbonaceous dusts thathave more than 8 percent total en-trapped volatiles (see ASTM D 3175,Standard Test Method for VolatileMatter in the Analysis Sample of Coaland Coke, for coal and coke dusts) orthat have been sensitized by other ma-terials so that they present an explo-sion hazard. Coal, carbon black,charcoal, and coke dusts are examplesof carbonaceous dusts.

• Group G. Atmospheres containingother combustible dusts, includingflour, grain, wood flour, plastic andchemicals.

In the enforcement context, OSHA canissue citations for existing standards,such as failure to maintain adequatehousekeeping by keeping the workplacegenerally clean. For storage areas, theworkplace must be kept “free from accu-mulation of materials that constitute haz-ards from. . . fire, explosion. . . .” OtherOSHA standards that can be cited forcombustible dust-related hazards includethose on emergency action plans, pow-ered industrial trucks (operation of vehi-cles not rated for the location), theelectrical standard, and ventilation stan-dards. In addition, the general dutyclause can be used where the employerfails to follow widely accepted practicesfor prevention of combustible dust explo-sions and exposures.

OSHA inspectors also review designcriteria and systems for explosion vent-ing and prevention in dust handling andprocessing systems. Inspections may beproblematic for employers who havedust collectors inside their buildings.Building codes generally prevent suchlocations, but the NFPA standard is notretroactive—unless the local authoritydecides to impose the standard on thefacilities within its jurisdiction. Nor-mally, the NFPA standard requires em-ployers to update to the new criteriawhen major renovations or replace-ments occur.

The full text of CPL 03-00-008 isavailable from www.osha.gov.

Standard PracticesThose responsible for preparing MSDSshould be aware of the hazards of com-bustible dust. The MSDS should charac-terize the propensity of a product toproduce combustible dust, refer to appro-priate additional sources, and describe theprocessing conditions that produce finelydivided dust, which is the most signifi-cant hazard. But an MSDS cannot be-come an encyclopedia of combustibledust hazards, just as it cannot become atreatise on the toxicology of a substanceor product. Relying on MSDS for suffi-cient information on combustible dusthazards would be foolhardy.

Simple steps can reduce the potentialrisks of combustible dust. Many preven-tive measures require sophisticated engi-neering assessment and design, but theseshould be supplemented by standard op-erating practices, such as the following:

• Keep all sources of ignition out of haz-ardous areas.

• Assure that controls for static electric-ity are functional and in use.

• Keep areas and equipment clean andminimize the accumulation of dust.

• Seal areas that are inaccessible forcleaning to prevent dust from entering.Check regularly for dust above dropceilings or in unsealed areas.

• Check dust collection systems to makesure all leaks are sealed and dust is notaccumulating in the ductwork. Checkbonding and grounding of all dust sys-tem components.

• Implement an effective housekeepingprogram with regular cleaning fre-quencies for floors and work areas. Es-tablish a regular program to cleanhorizontal surfaces, such as ducts,pipes, hoods, ledges and beams, thatare not part of the daily or weeklycleaning activity.

• Use cleaning methods that limit disper-sal of dust in the air—either wet meth-ods or vacuum methods.

• Include mission critical equipment anddust controls in a regular inspectionand maintenance program.

• Train and retrain employees in thehazards of combustible dust, and em-phasize the importance of good house-keeping and other proper operationalprocedures.

Controlling combustible dust hazardsdoes not require developing new tech-nologies or scientific information, but itdoes require diligence and attention todetail. Periodically revisit the principlesof combustible dust control, and makesure the entire organization is aware ofthe hazards of combustible dust and theconsequences of failing to follow thesesimple rules.

David G. Sarvadi, JD, CIH, is an attorney with Kellerand Heckman LLP. He can be reached atsarvadi@khlaw.com or (202) 434-4249.

Christopher D. Walker, JD, CIH, CSP, is a staff scien-tist and OSHA Regulatory Specialist with Keller andHeckman LLP.

References

1. Hartman, Irving: “CombustibleDust.” In Patty’s Industrial Hygieneand Toxicology. Ed.: Frank A. Patty.2nd revised ed. John Wiley & Sons,New York (1958).