In some cases, simple work practices are sufficient to protect a facility – for example, maintaining high housekeeping standards to prevent dusts from accumulating at the facility, which greatly reduces the likelihood and severity of dangerous secondary explosions. In most cases, however, more detailed additional measures are needed.
Combustible Dust Testing
An essential first step for every industrial facility is to determine the explosive nature of any dusts on-site. The NFPA establishes that “any material that will burn in air” in a solid form can be explosive when in a finely divided form. Material Safety Data Sheets (MSDS) are sometimes sufficient for providing information on the combustibility of a material. But, according to a study performed by the U.S. Chemical Safety Board, a high percentage of MSDS either do not warn of explosion hazards or do not do so clearly and explicitly. Therefore, analytical testing may be necessary to conclusively establish whether a dust is combustible or not.
In order to obtain meaningful test results, it is critical to select and obtain proper dust samples. Contact Lewellyn Technology for guidance through the initial phases of a dust hazard assessment.
A number of factors are at work in helping to determine a dust’s combustibility – thus requiring the need for testing. Some of the parameters that need to be tested for in order to classify a dust include:
- Minimum Ignition Energy (MIE)
- Minimum Ignition Temperature (MIT)
- Minimum Explosible Concentration (MEC)
- Moisture content
- Particle size distribution
- Electrostatic Charging Tendency (ECT)
- Dust deflagration index (Kst)
- Limiting Oxygen Concentration (LOC)
- Maximum explosion pressure (Pmax)
- Maximum rate of pressure rise (dP/dt)max
The results of these tests are used to determine whether or not the dust is a Class II combustible dust. The National Materials Advisory Board (NMAB) 353-3-80, Classification of Combustible Dusts in Accordance with the National Electric Code, defines dusts having an Ignition Sensitivity (IS) greater than or equal to 0.2 or an Explosion Severity (ES) greater than or equal to 0.5 to be appreciable explosion hazards requiring electrical equipment suitable for Class II locations. Ignition Sensitivity is the product of the MIT, MIE and MEC of a given dust normalized to Pittsburgh coal dust. Explosion Severity is the product of the Pmax and the (dP/dt)max normalized to Pittsburgh coal dust. Mathematically, the IS and ES are defined by:
Combustible Dust Hazard Analysis
A detailed combustible dust hazard analysis can be instrumental in reducing the risk of a dust explosion. A Combustible Dust Hazard Analysis is a systematic analytical study of a facility and its processes to identify combustible dust hazards to employees, property, and the public at large. Aspects of a facilities operation, such as individual pieces of process equipment, ducts and dust collection system, are examined individually and aggregately to determine any additional administrative or engineering safeguards that should be implemented to reduce the risk of a combustible dust event and bring the facility into compliance with OSHA’s NEP. Potential ignition sources are studied and methods to eliminate or reduce possible ignition of a dust cloud are examined.
Areas that are deemed to be hazardous locations according to NFPA standards are identified to determine what level of rated electrical equipment is necessary to prevent ignition of a dust cloud. Contact Lewellyn Technology if you would like an estimate to conduct a Combustible Dust Hazard Analysis based on OSHA’s National Emphasis Program and all NFPA standards that may apply to your facility.
Electrical Equipment Provisions
Locations inside a facility are classified based on the characteristics of the combustible dusts present and the likelihood that a hazardous concentration of dust will exist. Areas that are hazardous due to the presence of combustible dusts are referred to as Class II locations. Class II locations are further divided into Division 1 and Division 2. According to NFPA 70, National Electric Code, Article 500, a Class II, Division 1 area is one in which a combustible dust is suspended in the air in sufficient quantities to produce a deflagration under normal operating conditions or where a mechanical failure could produce a dust cloud capable of supporting a deflagration and the failure is also capable of providing an ignition source for the dust cloud. A Class II, Division 2 location is one in which a combustible dust may be suspended in the air in sufficient quantities to produce a deflagration under abnormal operating conditions or where combustible dust accumulations are present and could, as a result of mechanical malfunctions of handling or process equipment, become suspended in the air or where combustible dust accumulations in, on or around electrical equipment could potentially interfere with the safe dissipation of heat or could be ignitable by abnormal operation or failure of the electrical equipment.
Class II, Division 1 locations must be equipped with electrical equipment that is dust ignitionproof. This type of equipment is enclosed in such a manner that prevents the entry of dust into the enclosure and does not allow arcs, sparks or heat generated inside the enclosure to leave the enclosure and possibly ignite dust on or around the enclosure.
Class II, Division 2 locations must be equipped with electrical equipment that is dusttight. This type of equipment is enclosed in such a way as to prevent the entry of dust into the enclosure.
An alternative to the Class/Division classification is the Zone classification method outlined in ATEX 95, Equipment Directive. Locations that are hazardous due to the presence of combustible dusts or ignitable fibers or flyings are classified as Zone 20, 21 or 22 according to NFPA 70, Article 506. A Zone 20 location is one where combustible dusts are present in sufficient quantities to be hazardous either continuously or for long periods of time. A Zone 21 location is one where combustible dusts are likely to occasionally exist under normal operating conditions in quantities sufficient to be hazardous. A Zone 22 location is one where combustible dusts are not likely to exist in hazardous quantities under normal operation and, if they do occur, will only exist for a short period of time.
Zone 20 locations must be equipped with electrical equipment that does not permit dust to enter the equipment and does not allow arcs, sparks or heat to ignite dust on or around the equipment. Zone 21 locations must be equipped with electrical equipment that does not allow dust to enter the enclosure and does not allow the surface to become hot enough to ignite a dust layer or cloud. Zone 22 locations must be equipped with electrical equipment that is dusttight.
Each room, area or section of a facility is classified individually based on the specific environment. A single room can have multiple areas with differing classification requirements. Facilities must maintain drawings that show the hazardous (classified) locations. Lewellyn Technology’s engineers can identify the classified locations at your facility and develop the required drawings.
Preventative Measures
There are two basic types of preventative measures, administrative measures and engineering controls. Administrative procedures include detailed housekeeping, hot work and lockout/tagout procedures that address the hazards of performing maintenance work in areas where combustible dusts are present. Proactive housekeeping practices, coupled with routine inspections of equipment and dust accumulation levels, can greatly reduce the hazard level in a facility. Vigilant maintenance of equipment with an emphasis on fugitive dust emissions can dramatically reduce the dust accumulation levels, and thus the hazard level, in a facility. Employers must also inform workers of the hazards associated with combustible dusts present, as outlined in OSHA’s Hazard Communication requirements.
Personal Protective Equipment (PPE)
The Tentative Interim Amendment (TIA) to NFPA 654, Standard for the Prevention of Fires and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, mandates that operating and maintenance procedures must address personal protective equipment (PPE) including flame-resistant garments in accordance with the workplace hazard assessment required by NFPA 2113. This is due to the specific hazards of a flash fire. Flash fires burn extremely fast, usually lasting less than three (3) seconds. Non-flame resistant garments can be ignited by the flash fire and continue to burn after the initial fire has burned out. The body area beneath non-flame resistant clothing is often burned more severely than exposed skin. The survival rate of personnel exposed to a flash fire drops significantly as the percentage of the total body area affected increases. Flame resistant clothing can drastically reduce the effect of a flash fire. Personnel that operate in dusty environments may also need to wear conductive footwear and grounding straps to prevent the buildup of static electricity. Contact Lewellyn Technology for assistance in determining what level of PPE is required for your workers.
Fugitive Dust Control
The greatest hazard associated with combustible dust comes from the threat of secondary explosions. Secondary explosions occur when a primary explosion, often inside process equipment or in an isolated area, sends pressure waves through a facility that dislodges fine dust that has accumulated on floors, walls, and overhead surfaces. This fine dust then forms a cloud that spreads into a large area. If this dust cloud is ignited, a large, potentially devastating flash fire or explosion could occur. NFPA guidelines state that a dust layer 1/32 inch (0.79 mm) thick spread over just 5% of the floor area of a facility is sufficient to pose a combustible dust hazard. Dust that settles on rafters and piping above the floor can account for covering as much as 10% of the floor area of a building. Cleaning these overhead and relatively inaccessible areas can be difficult and even dangerous. Instead of removing accumulations of combustible dust, preventing the escape of fugitive dust from process equipment is the safest, most effective method of reducing the risk of a combustible dust explosion.
Lewellyn Technology has had great success in helping facilities to mitigate fugitive dust accumulations while reducing the need for labor intensive housekeeping. We have done this by assisting facilities to evaluate and modify process equipment to reduce dust emissions. We can assist with mechanical design that can minimize and hopefully eliminate fugitive dust from process equipment. Some of the equipment we have worked with includes:
- Pneumatic and mechanical conveying systems
- Sifters and screens
- Bins and silos
- Material feeders
- Dryers and coolers
- Grinders and hammermills
- Bag loading and unloading
- Truck and railcar loading and unloading
If you would like assistance with reducing the dust accumulations in your facility, please feel free to contact Lewellyn Technology.