According to OSHA’s definition of combustible dust explosions, any combustible material can burn rapidly when in a finely divided form. If such a dust is suspended in the air in the right concentration, under certain conditions, it can become explosive.
When we know that 70% of material handled in the various process industries contain combustible dust, it is clear why it presents a major safety hazard for process plants. According to the data gathered by the Safety Chemical Board, combustible dust explosions have become more frequent over the past 14 years, causing severe damages to facilities and process industry equipment, as well as taking numerous lives. Some of the most recent dust explosions in America have called regulatory bodies to action. One of these was the titanium explosion in West Virginia, where three employees lost their lives. The other one was a notorious sugar explosion in Georgia that completely devastated the facility, causing the death of fourteen people.
The fact about the latter incident is that it was “entirely preventable,” according to the Chemical Safety Investigation Board’s report. This report also stated that this catastrophic incident would have been avoided if safety management had applied measures to control dust explosion hazards
This devastating sugar explosion prompted the Occupational Health and Safety Administration and the National Fire Protection Association to devise a series of policies and combustible dust safety standards. Process industries need to comply with these regulations to protect their employees and equipment from severe explosions.
Five Factors that Lead to a Dust Explosion
Dust explosions occur when five crucial elements that form a Dust Explosion Pentagon coincide within the processing facility. The factors that facilitate such explosions are:
- Combustible dust particles, i.e., the explosion’s fuel
- Dispersion of dust particles – when accumulated dust is lifted in the air, forming the explosive dust cloud
- Ignition sources capable of igniting combustible dust clouds
- Confined spaces where the pressure builds up, usually leading to the damaged equipment, destroyed facilities, and potential structural collapse.
Dust concentration is also a significant factor that may lead to an explosion. Only specific combustible dust concentrations can cause dust deflagration. If the minimum explosive concentration of dust disperses in the air near an ignition source in an enclosed space, the dangerous explosion occurs.
This dust explosion develops in two stages. The initial stage is a primary dust explosion that happens inside pieces of equipment, like grain elevators and dust collectors. It is not as intense as the second explosion wave. The rapid combustion of dust particles increases the air pressure, creating the pressure wave that can stir dust deposits into the air where they are ignited, causing the secondary explosion.
These secondary explosions swiftly follow the initial one and are more violent and destructive, causing catastrophic damage and loss of life. Dust particle size can contribute to the rapid expansion of fire during the explosion. Fine dust particles are more dangerous because they can easily disperse through the air, and they have a larger surface area, making them highly flammable. The NFPA study showed that an amount of accumulated dust as thick as an average paper clip was sufficient to cause devastating explosions.
The National Fire Protection Agency Combustible Dust Safety Standards
The NFPA devised this standard, which became effective in September 2015, focusing on the measures required to control combustible dust hazards. This standard emphasizes the dust combustibility guidelines and the importance of fire or explosion prevention and protection in the process industry. One of the reasons such explosions are so dangerous is that combustible dust exists in a variety of process industries, presenting a significant safety hazard. Some of these process industries are:
- Agriculture, fertilizer, grain, tobacco, food processing industries
- Wood and paper processing, furniture, textiles, and dies manufacturing facilities
- Petrochemical industry, pharmaceutical components manufacturing
- Plastic processing and recycling facilities
- Coal mining
As stated above, approximately 70% of dusts are explosive. Also, even non-explosive materials can become explosive when mixed with sufficient organic or metal dust concentrations.
Here are some NFPA standards related to the combustible dust hazard control:
- Minimize the fugitive explosive dust and prevent it from escaping processing equipment and ventilation systems. Enclosed tubular drag cable and disc conveying systems, like Cable Conveyors with their completely enclosed tubes, may be of significant help in dust control and removal.
- Install a dust collection system and conduct regular monitoring to make sure it is operating properly.
- Detect and identify all the hidden surfaces containing dangerous dust deposits within the facility.
- Set and maintain a schedule for cleaning all areas prone to combustible dust accumulation and stick to it.
- Use cleaning methods that do not generate dust clouds near ignition sources. Also, avoid compressed air cleaning methods.
- Place pressure relief valves away from combustible dust hazard areas.
- Maintain a comprehensive dust control program with frequent dust hazard inspections, testing, housekeeping, and control initiatives.
Also, try to isolate equipment from other pieces of equipment. When such an explosion happens, it causes flame propagation through the ductwork into other parts of the equipment. Therefore it is essential to isolate explosions so that the flame cannot spread to the adjacent equipment. In case the explosion takes place in an outdoor dust collector, make sure it does not spread through the ductwork into the processing facility and endanger workers.
Additionally, specific operations that generate dust, such as abrasive blasting, grinding, or butting fall under OSHA requirements.
Ignition Sources Control
To keep the ignition sources under control and eliminate this crucial element from the Dust Explosion Pentagon, you need to:
- Employ proper installation, classification, and maintenance of all electrical equipment and wiring. Also, make sure that the equipment complies with the latest electrical safety technology standards.
- Control static electricity by implementing grounding wires or rods.
- Forbid or limit smoking and open flames inside the workplace.
- Limit or isolate sparks or friction sources.
- Limit contact between hot surfaces or heating systems and combustible dust.
- Incorporate spark arrestors or spark traps into all dust collector ductwork.
In case such explosions happen, make sure you have a pre-prepared strategy to alleviate negative effects and control damage caused by this serious safety incident.
- Separate the dust explosion hazard in the most efficient way possible. Distance it from the work environment and isolate it by placing barriers where possible.
- Install deflagration venting.
- Install pressure relief vents on applicable equipment.
- Apply ignition sources detection systems and extinguishing equipment.
- Install an explosion protection system, if possible.
Ensure Appropriate Workers Protection
Even though these dust explosion prevention strategies and standards may reduce dust explosion hazards, they cannot sufficiently protect workers exposed to combustible dust flash fires and explosions.
Some employers still think that wearing helmets, protective gloves, and ear and eye protection is sufficient PPE for workers handling dangerous materials. On the other hand, personal protective equipment like flame-resistant garments and ARC flashing clothing will offer adequate protection in case of dangerous dust explosions.
Since workers are closest to hazards, they need to have adequate explosion safety training to recognize explosion risks and prevent them from happening. Employees should also be encouraged to report possibly risky working conditions.
Combustible dust explosions are dangerous safety incidents that may destroy process plants, resulting in significant loss of human lives and economic consequences. By complying with the described safety standards, you will manage the ignition sources and dust control, ensuring your employees have adequate explosion protection.