Although Congress did not pass HR 5522, “The Combustible Dust Explosion and Fire Protection Act of 2008,” before adjourning last fall, management of hazardous dusts is still a prominent concern for regulators, insurers and industry. This column presents a brief review of some dust-explosion terminology and concepts and is addressed to safety or design personnel who are tasked with meeting current (or new) dust-explosion standards.
There are five elements that must come together in order for a dust explosion to occur:
1. Dust that is combustible
2. Suspension of a sufficient concentration of dust into a cloud
3. Confinement of the cloud
4. Sufficient oxidizer concentration in the suspending medium
5. Source of ignition
Identifying Combustible Dusts
While common organic dusts (e.g., grain, flour, sugar, sawdust, cosmetics) would not typically be present in plants that utilize high-temperature furnaces, IH readers should be aware of other explosive dust hazards that are common to foundries and other metal-processing facilities. These include aluminum, chromium, iron, magnesium, zinc and, often, metallurgical coke. All of these dusts are particularly dangerous in “hybrid” mixtures that contain fuel gas or vapor together with the dust in air. Metal dusts can be released into the workspace from machining or surface-finishing operations, attrition of larger blocks of materials during handling and high-temperature processes that require metal melting and thus generate metal fumes that re-condense into fine particulate.
The most reliable method for assessing whether a certain dust is hazardous is to have it tested. This is particularly true of dusts that are created from attrition of larger particles. The dust hazard is typically more severe for fine dusts with large, specific surface areas. Not surprisingly, a kilogram of 1-cm coal lumps does not pose much of an explosion hazard, but a kilogram of 10-mm coal fines is a very significant hazard. The MEC (minimum explosible concentration) is the most important test to perform, but a testing lab can also measure other parameters that establish correct designs for explosion mitigation (suppression or venting).