Defining the Hazard
According to OSHA, any combustible material can burn rapidly when in a finely divided form. If such dust is suspended in the air in the right concentration, under certain conditions, it can explode. The elements needed for a dust-related explosion include:
1. Fuel (combustible dust)
2. Heat (ignition source)
3. Oxidizer (oxygen in air)
(these three elements are commonly referred to as the “fire triangle”)
4. Dispersion of dust particles in sufficient quantity and concentration
5. Confinement of the dust cloud
(these two elements combined with the first three create an “explosion pentagon” (1))
OSHA has identified dozens of industries that have the potential to be affected by combustible dust. Among them are manufacturing, food production, and plastics and furniture manufacturing. Even materials that do not burn in larger pieces (such as aluminum or iron) can explode in dust form, given the right conditions. These industries should take special precautions to guard against combustible dust (2). Additionally, there is an emerging trend for companies handling coal requiring flame resistant clothing for employees working in areas subject to accumulations of combustible dust.
In 2008, Imperial Sugar Company in Savannah, GA experienced a combustible dust accident. An explosion, likely caused by an overheated bearing on a conveyor beneath the sugar silos, ignited sugar dust spreading sugar dust explosions into the finished sugar packaging area of the plant. The Imperial Sugar accident killed 14 people and injured over 40 (3). In 2010, Imperial Sugar Company agreed to pay $6 million in fines over safety violations (4). As a result of this accident, Imperial Sugar Co. now requires all employees and visitors to the manufacturing areas to wear FR clothing (5).
Prevention & Mitigation
The Imperial Sugar Company accident is an example of a tragic, but completely preventable situation. Multi-step mitigation processes are necessary to ensure that workers are adequately protected from combustible dust because of the complexity of this hazard.
Step 1: Dust Control – NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids, is a comprehensive guide on controlling dust to prevent explosions. This standard includes information on avoiding dust explosions through both facility design and housekeeping. Among others, some basic recommendations include:
- Cleaning dust residue at regular intervals
- Minimizing the escape of dust from process equipment or ventilation systems
- Using dust collection systems and filters
- Utilizing dust collection systems and filters (1)
Step 2: Ventilation – OSHA ventilation standard 29 CFR 1910.94 contains ventilation requirements for certain type of operations which involve dusts, including combustible dusts. Proper ventilation systems ensure that dust doesn’t accumulate.
Step 3: Regular Cleaning – General OSHA requirement 29 CFR 1910.22(a)(1) requires employers to keep work places and other areas clean, which includes the removal of dust. Dust accumulation can be prevented through effective housekeeping measures. The general rule of thumb is to not let dust accumulate to more than the thickness of a dime.
Step 4: Ignition Control – NFPA 654 provides comprehensive guidance on the control of ignition sources to prevent explosions. The following are some recommendations, among others:
- Control static electricity, including bonding of equipment to ground
- Control mechanical sparks and friction
- Control smoking, open flames and sparks
- Use appropriate electrical equipment and wiring methods (1)
*It is important not to confuse Class II equipment with Class 1 equipment. Class II equipment addresses dust hazards while Class 1 addresses gas, vapor and liquid hazards.
Where coal-handling operations may produce a combustible atmosphere from flammable dust, employers covered by the Electric Power Generation, Transmission and Distribution standard must eliminate or safely control ignition sources under 29 CFR 1910.269(v)(11)(xii) (1).
Step 5: Damage Control/Protection Methods – NFPA 654 provides comprehensive guidance to minimize the danger and damage from an explosion. Some suggested protection methods include, but are not limited to:
- Separation of the hazard (isolation with distance)
- Segregation of the hazard (isolation with a barrier)
- Pressure relief venting for equipment
- Explosion protection systems (1)
The last line of defense when a combustible dust event occurs is appropriate PPE. FR clothing, as a secondary protective method, may be effective in preventing or mitigating the severe burn injuries that may occur when a combustible dust explosion happens. (3)
In fact, NFPA 2112, Standard on Flame Resistant Garments for Protection of Industrial Personnel Against Flash Fire, specifically includes dust as a fuel source. NFPA 2112 essentially evaluates protective performance of FR fabrics in flash fires, while its sister standard NFPA 2113 outlines specification, care and use of garments. These two standards should be referenced when implementing the final layer of combustible dust hazard mitigation. Many facilities that experience a combustible dust explosion subsequently put their workers in flame resistant clothing.
Industry Standards & New Guidance from OSHA
One result of combustible dust that should be taken into consideration is the risk of flash fire. In Tyndale’s experience with FR for flash fires, OSHA doesn’t like to name specific standards exclusively in their rule making. However, the underlying performance test for flash fires (which include combustible dust hazards) is ASTM F1930. ASTM F1930 is the Standard Test Method for Evaluation of Flame Resistant Clothing for Protection Against Flash Fire Simulations Using an Instrumented Mannequin, and measures the average predicted body burn of a fabric based on laboratory flash fire simulation.
A garment that meets NFPA 2112 will require less than 50 percent predicted body burn. Tyndale refers to garments that meet this performance threshold as “Flash Fire Rated.” Some garments that don’t meet NFPA 2112 are Flash Fire Rated, and all garments that meet NFPA 2112 are also Flash Fire Rated. However, NFPA 2112 has several limitations in terms of higher cost, limited garment design options, and widespread availability of options. These limitations do not necessarily result in greater protection.
When companies lack direct experience with or knowledge of the combustible dust hazards they face, laboratory testing, published test results or particle size should be used to classify themselves under the new hazard communication standard according to an OSHA memo released December 31st.
Companies are expected to comply with most provisions of the rule by June 2015. OSHA’s standard interpretation memo is designed to guide agency inspectors in deciding whether products are classified properly for combustible dust hazards. Companies can use test data for similar materials if there isn’t any data available for the substance that they are trying to classify for dust hazards.
“At the end of the day, OSHA still doesn’t have a definition of combustible dust, and employers are still expected to identify combustible dust hazards and train their employees accordingly (6).”
For more information on industry standards associated with combustible dust, please visit Tyndale’s FR Safety Library.
References for this post were accessed January 2014: