Managing Explosive Dusts
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.
Metal dusts can be released into the workspace from machining or surface-finishing operations, attrition of larger blocks of materials during handling and melting processes that generate metal fumes that form fine particulate.
The most reliable method for assessing whether a certain dust is hazardous is to have it tested. 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-µm 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).
The consensus standard for combustible-dust fire safety in general industry is NFPA 654, “Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids,” published by the National Fire Protection Association (NFPA). This standard has already been adopted into law in many jurisdictions and was referenced in HR 5522. Other NFPA standards apply to specific dusts or industries, such as NFPA 484, which applies to combustible metals.
According to Dr. Timothy Myers, senior managing engineer at Exponent and principal member of the NFPA 484 technical committee on Combustible Metals, Metal Powders and Metal Dusts, “The most important factor in reducing the likelihood of a dust explosion is identifying the dust-explosion hazard. Standards for the mitigation of dust-explosion hazards have existed for decades, but many facilities do not recognize a hazard exists.”
Prevention vs. Protection
As with most safety hazards, there are two main pathways for reduction of risk – prevention, which constitutes a reduction in likelihood that an event will occur, and protection, which provides means for a reduction in the severity of an event if one is triggered. Methods of preventing dust explosions include:
- Preventing releases of dusts from an enclosure into the workspace
- Regular housekeeping to remove dust accumulations
- Elimination of ignition sources (including static-electrical charge accumulation) in areas susceptible to dust releases
- Use of inert-gas atmospheres in processing or storage vessels
- Explosion-suppression systems
- Robust vessel construction and fast-acting isolation valves
- Explosion-relief panels and discharge ducts
- Isolation of dust-hazard areas from one another to prevent propagation and from personnel, where possible
Unfortunately, the greatest dust hazard in many manufacturing facilities is the lack of follow-through on required procedures for good housekeeping. NFPA 654 states that the accumulation of as little as 1/32 inch (0.8 mm) of combustible dust on a horizontal surface is enough to create an explosive mixture if the accumulation gets dispersed into a cloud.
A latent hazard sometimes becomes a serious danger when a worker elects to use compressed air to hasten a cleanup effort. Unknowingly, the worker suspends the dust into the air, where it may reach its minimum explosible concentration and can find an ignition source. Use of brooms or listed vacuum systems designed for handling combustible dusts is recommended. IH