Explosion, flash fire, structural damage, personal injury/death, process interruption, business loss, insurance cost. Major dust categories include: agricultural, carbonaceous, metals, plastics, and other varied chemicals.
Potential combustible dusts result from processing, storage, cleaning, and handling materials that have a potential for rapid (explosive) dust oxidation. Example processes include: coal mining, construction, demolition, factories, mills, grain handling equipment, storage silos, animal handling/processing, food processing, pharmaceutical factories, textile processes, plastics processes, specific metals processing, and other chemical production processes. We recommend that any process dusts be tested for combustibility, including for the changing process conditions (ex: dry summer process/storage locations). (Note: Safety data sheets for many manufactured or marketed materials may not include explosive dust information due to lack of product testing, since the products are not usually designed for use in dust form.)
Factors Leading to Explosions
The five key factors needed for a dust explosion include: fuel, oxygen, ignition, dispersion, and confinement.
- Fuel for combustible dust ignitions can be from more than 140 known substances/groups of materials, which can sustain rapid dust oxidation in certain sizes and properties. Both particle size and moisture content are factors affecting dust ignition potential. (Note: Small amounts of dust as little as 1/16-inch in depth on just 5% of surfaces in a structure can ignite with enough explosive pressure to destroy the walls and ceilings. Additionally, extremely small “float” dust is very susceptible to ignition and spread in minimal surface depths as low as 1/200-inch.)
- Oxygen is adequately available in standard atmospheric conditions for most structures or areas.
- Ignition sources are widespread, and include process equipment friction heat/sparks, electrical equipment/motor sparks, static electrical discharge sparks from ungrounded ventilation ducts/vacuum hoses, non-explosion-rated vacuum motors, non-permissible electrical equipment (coal mines, etc.), open fires/flames, smoking, stoves, kilns, hot work (welding/cutting/grinding), and spontaneous ignition in combustible materials piles (damp coal dust drying on beams and adsorbing heat).
- Dispersion is either primary or secondary in nature. The first ignition of dispersed (suspended) dust is known as a primary ignition. A secondary dust ignition occurs when the air pressure wave and flames from a primary dust ignition proceed to disturb, suspend, and ignite settled dust in surrounding areas and rooms. A typical coal mine, factory, or grain silo explosion can include a triple explosion, consisting of primary, close secondary, and extended secondary explosions, with the pressures, heat, and damage highest in the distant coal tunnels, factory areas, or grain silos. (Note: Undisturbed dusts can hold sparks in a smoldering status, such as with a coal dust pile spontaneous ignition, and not burn explosively. If the smoldering dust is slightly disturbed and suspended in air, however, the dispersed dust particles react in a rapid flash [explosive] manner.)
- Confinement of explosive dust clouds results in pressure buildup and explosions with significant damage. The explosive forces vary by dust material type and are rated Kst1 (charcoal, sugar, zinc, etc.), Kst2 (wood, plastics, etc.), or Kst3 (aluminum, magnesium, etc.). (Note: Without confined tunnels, walls and ceilings, dust ignitions are lower pressure buildup deflagrations, or flash fires. Flash fires, however, can quickly spread indoors or outdoors to any connected confined areas, resulting in secondary (confined) dust explosions. Deflagrations have similar energy release in terms of flash heat, but without the same confined pressure buildup.)
Combustible Dust Controls
We recommend the following to control combustible dust accumulations: education, housekeeping, collection systems, explosion venting systems, electrical and grounding controls, structural design factors, and inerting materials.
- Education of all personnel at the location is important. Several work and maintenance activities can create or disturb combustible dusts or lead to ignition sources. Not all materials are immediately thought of as having combustible potential by most people. Procedures for control must be understood and followed. (Note: There are numerous NFPA, ANSI, OSHA, MSHA, and other standards related to risk identification and prevention of dust explosions for general and specific applications).
- Housekeeping is the primary control for combustible dusts. It includes periodic inspections of all surfaces, both open and hidden, to ensure that accumulations are kept below depth levels that could become dispersed and ignited. Cleaning procedures are critical to avoid dispersion of dusts. If possible, wet cleaning methods are recommended, such as in coal handling facilities.
- Collection systems must be engineered to collect and safely remove dusts to designated outdoor collection containers. This includes using combustible dust rated floor vacuums.
- Explosion venting systems are needed on all engineered collection systems. In addition, ignition-detection explosion suppression systems (engineered) may be required for critical equipment and locations.
- Electrical and grounding controls are required that meet regulatory codes for combustible dusts. High- or low-volume vacuum and dust collection hoses must be grounded to eliminate static electrical discharge sparks.
- Structural design should be included in building design. Horizontal beams and light fixtures can be covered with angled, tent-like covers to control float dust buildup. Surfaces can be covered with slick paint or material, rather than coarse or unpainted surfaces that adhere dusts.
- Inerting materials, such as rock dust for coal mine surfaces (powdered limestone), can be placed in some locations to control the spread of flame in dispersed combustible dust. They act as self-contained fire extinguishing powder that prevent the primary and secondary dust ignitions. (Note: Coal mine rock dust quantity requires a minimum of 80% in the total combined dust mix with the coal dust. Frequent additional rock dust layers are added to control new accumulations of float coal dust on top of the previously mixed dust.)
Process Safety Management
Controlling combustible dusts requires engineering, training, operations, and maintenance controls, much the same as with chemical process safety management. The results are similar in that combustible molecules, in larger dust form, are understood and controlled.