An explosive situation
- Created: 2018-09-05
Everything you need to know to reduce the risk of combustible dusts
by Trevor Kendal, technical director for ambient systems and VentMapping, RoboVent
Is your facility at risk for a dust explosion? A combination of combustible metalworking dusts, spark-producing processes and inadequate dust control may be an explosion waiting to happen at many fabrication shops.
Dust explosions result in millions of dollars of damages and numerous injuries and fatalities every year. Metal dusts are one of the most common culprits (along with wood dusts and grain dusts). However, there are easy steps that fabricators can take to reduce the risk. The key is understanding how dust explosions occur.
The Dust Explosion Pentagon. All five elements must be present for an explosion to occur. Photo Credit: National Institute for Occupational Safety and Health, public domain.
The "Dust Explosion Pentagon" is closely related to the more well-known "Fire Triangle." All five elements in the Dust Explosion Pentagon must be present before a dust explosion can occur. Those elements are:
- Fuel: The fuel source for combustion is the dust itself. Different types of dust have different potential for combustion. Many metalworking dusts, such as aluminum dust, are highly combustible.
- Oxygen: Fires and explosions require oxygen in addition to fuel; it is the interaction between the fuel source and oxygen in the air that supports combustion. Tightly packed dust in a container is unlikely to explode. When dust floats freely in the air, it is able to interact with oxygen to sustain the reaction.
- Ignition: There must be an ignition source to start a fire or explosion. This is often a spark from fabrication processes, such as welding, cutting or grinding. Ignition can also be caused by static electricity, friction or heat generated by fabrication processes, or an open flame, such as a blowtorch. Under the right conditions, metalworking dusts can self-ignite when static builds up between dust particles in the air.
- Dispersion/concentration: Dust must be concentrated in the air at the right level to present an explosion hazard. If concentrations are too low (below the “lower explosion limit” for the dust type), there will not be enough dust in the air to support combustion at the rate required for an explosion. If concentrations are too high, there will not be enough oxygen in the air to support combustion.
- Containment: Containment prevents dust from dispersing and allows pressure to build up, increasing the likelihood of an explosion.
When these five conditions are met, it results in a deflagration, or rapid combustion event. The speed of the reaction generates extreme air pressure as heated air and CO2 produced during the reaction expand outward. The explosive forces involved can blow out walls and windows, destroy equipment, and injure or kill nearby workers. Sometimes an even larger secondary explosion results when the initial explosion kicks more dust into the air.
Combustible dust explosions are among the most dangerous industrial accidents – but they are entirely preventable.
Many metalworking dusts are highly combustible, but the explosive potential of dust in your facility depends on several factors. These include:
- Chemical composition: Different metals and alloys have different explosive potential. Paints and coatings on the metal can also influence the likelihood of an explosion.
- Particle size: The finer the dust, the greater the risk of explosion. Fine dust has a very high surface area-to-volume ratio compared to larger particles. This higher surface area creates more opportunities for particles to react with oxygen, resulting in rapid combustion once ignited.
- Moisture level: In general, dryer dust is more combustible than dust containing water or oil. Moisture causes fine particles to agglomerate into larger, heavier particles that are more likely to settle out of the air and less likely to cause an explosion.
The explosive power of an airborne dust is measured using two metrics:
- The Pmax value is the maximum pressure that could be produced by a dust cloud explosion – in other words, the size of the potential explosion.
- The Kst value, or dust deflagration index, is the maximum rate of the pressure rise. This depends on the characteristics of the dust, primarily chemical composition and size. Each unique dust sample will have a specific Kst value. The higher the Kst value, the more explosive the dust is considered to be.
Dust explosions are classified by the Kst value into four potential classes. Many metalworking dusts, including aluminum and most aluminum alloys, fall into Class 3, the most dangerous class of combustible dusts.
Many fabrication shops are at high risk for dust explosions due to the nature of their particulate and the many sources of sparks and heat in the fabrication process. Welding, cutting and grinding all generate thousands of sparks per hour – and it only takes a single spark to ignite a dust explosion. Other processes, such as stamping and machining, may generate a lot of heat and friction that could become ignition sources.
Adding to the problem, many shops – especially smaller facilities – do not have effective measures in place to prevent dust from building up in enclosed areas. While their overall air quality may meet OSHA permissible exposure limits (PELs) for workers, high concentrations of dust within sections of ductwork or on the production line may put the facility at risk for an explosion.
Sparks from metalworking processes can set off an explosion when combustible dust is allowed to build up in the air. Heat, open flames and static electricity can also serve as ignition sources.
Fortunately, dust explosions are entirely preventable. Remember that all five conditions in the Dust Explosion Pentagon must be met for an explosion to occur. Remove any of the elements – fuel, oxygen, ignition, dispersion or containment – and an explosion will not have an opportunity to start. Here are some steps that fabricators should take to reduce the risk:
- Have your dust tested to determine the Pmax and Kst values. This will help you determine how likely your dust is to explode and how much damage it could do if an explosion were to occur.
- If your dust is determined to be explosive, implement effective engineering controls to prevent dust from building up in enclosed areas. Dust collection should occur as close to the source as possible. A source capture system keeps dust from building up to dangerous levels in the ambient air of the facility.
- Analyze your production line to determine where dust is produced, where it goes and where it collects. If you have enclosed areas in your production lines (such as an enclosed conveyor belt), pay special attention to how dust builds up inside the enclosure.
- Make sure your dust collector is sized appropriately for the volume of dust you are producing and moves air at a high enough CFM (cubic feet per minute) to keep dust from building up in ductwork, laser cutting enclosures or other enclosed spaces.
- Cutting different materials on the same laser or plasma cutter can create co-mingled dust that may be more dangerous than expected based on the Kst values of the individual dust. If you are comingling materials, be sure that you have testing conducted on the comingled dust. In some cases, it may be advisable to clean out your dust collection system and change filters prior to changing the material type being cut.
- Choose a dust collection system that meets National Fire Prevention Association (NFPA) standards for the collection of combustible dust. The system should have explosion vents that blow out to release excess energy building up in the machine. It may also have an isolation valve to prevent pressure waves from going back through the ductwork into the facility and an airlock between the collection bin or hopper and the dust collector to prevent oxygen from reaching the contained dust.
- Install an effective spark arrestor system to prevent sparks from entering your dust collector or ductwork. This is especially important for processes that produce large numbers of sparks near the area where dust is being collected.
- If your dust is highly explosive and difficult to control, consider changing the layout of your facility to reduce opportunities for dust to collect in enclosed areas. In some cases, physical barriers specifically designed for explosion control can be added to reduce the amount of damage that would occur in the case of an explosion.
- Implement housekeeping procedures that reduce the spread of dust. Use vacuums with filters rated for your dust type rather than brooms and dustpans, which can simply kick dust back up into the air. Make sure employees know how to empty dust collector bins without creating dust clouds. Do not allow excess dust to build up on horizontal surfaces.
Manufacturers are responsible for reducing the risk of dust explosions in their facilities. NFPA has developed detailed standards and guidance to help manufacturers address combustible dust hazards. Manufacturers are also subject to OSHA standards for combustible dusts. OSHA is considering additional rule making based on NFPA's guidance to help manufacturers further reduce their risks.
Dust collectors used for combustible dusts should be fitted with safety features to reduce the risks of explosion and mitigate damages. This collector is fitted with an (A) isolation valve, (B) explosion vent and (C) airlock.
When dealing with combustible dusts, it is best to get expert guidance. A qualified air quality system engineer can help you evaluate your risks and make the right mitigation decisions for your facility. It's a small investment to make to protect your property and your employees.