Although EXAIR Static Eliminators are not recommended for use in flammable areas, I receive a good amount of calls for static removal in flammable areas. Thus the purpose of my blog this week is not to promote EXAIR products but to educate on the perils of materials that under the right conditions are the leading contributors to explosions.
In the U.S. alone, during the period 1980 to 2005, the Chemical Safety Board (CSB) reports 281 explosions caused by ignitable combustible dust atmospheres. They resulted in 199 fatalities and 718 injuries. (ref 1) Similar data was compiled in the UK, where the Health and Safety Executive reported 303 explosions over a nine-year period. And German records show 426 similar incidents over a 20 year period. (ref 2)
Since their report was published, the CSB has repeatedly requested that OSHA take more action to regulate the safety of operations processing combustible and flammable powders. The 2008 sugar refinery explosion at the Port Wentworth plant of Imperial Sugar should be a warning to a broad range of industries and just how risky and relevant dust explosions are.
Sectors with recorded incidents of combustible dust fires and explosions.
Analysis of over 1100 events the following processes have been found to be the leading causes.
· Dust collection systems
· Powder grinding and pulverizing
· Silo & container filling
· Powder mixing and blending
Electrostatic discharges account for 10% of known primary ignition sources. Even though the majority of combustible dusts have higher minimum ignition energy (MIE) than flammable vapors, the amount of energy available from electrostatic discharges within contained environments will ignite the vast majority of combustible dusts.
Several contributing factors though, need to be present to support the ignition of a combustible dust cloud:
A dispersed dust cloud-oxygen mixture that is above its Minimum Explosion Concentration (MEC).
Physical containment of the dust cloud that will lead to rapid pressure build-up.
A heat source with enough energy to ignite the combustible atmosphere
To prevent uncontrolled electrostatic discharges posing a fire and explosion hazard in powder processing operations, a thorough static audit should be conducted by qualified personnel. The audit should focus on investigating and identifying situations where a charge has the potential to accumulate on conductive and semi-conductive components.
For situations where there is a potential for components within the system to be isolated, dedicated grounding equipment should be installed to monitor and control the release of static electricity.”The American Petroleum Institute (ref 3) recommends that all connecting metal parts of a vacuum collection system are conductive to less than 10 ohms
I’ve only scratched the surface on this topic. Hopefully it has provided a greater appreciation for potentially dangerous applications in your facility and will prompt you to seek the assistance of a professional to insure proper installation of your equipment.
(1) Report No. 2006-H-1 “Combustible Dust Hazard Study”, Chemical Safety Board (2006).
(2) Dust Explosion Scenarios and Case Histories in the CCPS Guidelines for Safe Handling of Powders and Bulk Solids”, Grossel, S.S., Zalosh, R.G., Center for Chemical Process Safety, (2005)
(3) API RP 2003: Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents”, American Petroleum Institute (2008)