Combustible dusts present both fire and explosion risks. What technologies can help you prevent them?
What is Combustible Dust?
Many manufacturing processes create very small particles of dust that settle on surfaces throughout the plant. Eventually these particles not only create a housekeeping issue, but if the particles are combustible, they can represent a potential fire or explosion hazard. Combustible dusts generally present both fire and explosion risks so it may help to consider the management of these risks separately.
Fire triangle
Fire management strategies traditionally focus on the control or elimination of one of the three key elements necessary for a fire — often represented by the “fire triangle.”
Explosion Pentagon
The explosion pentagon includes two additional elements necessary for an explosion: dispersion of a “Dust Cloud” and “Confinement” of dust. The management or removal of one or more of the elements in the explosion pentagon can reduce the explosion risk.
What is a Dust Explosion and How Does It Begin?
A dust explosion can be defined as: Any solid material that can burn in air will do so with a violence and speed that increases, with increasing degree of subdivision of the material. In other words, when a combustible material is in dust form it has the potential to not only burn but also under the right conditions explode with great force. This is true even of some materials that may not normally be thought of as combustible when in solid form such as food products like wheat flour or metals like iron.
An explosion typically begins when an ignition source enters the dust collector
An explosion typically begins when an ignition source enters the dust collector. This ignition source can come from many things and in most cases is never identified. When a pulse cleaning event occurs, a suspended cloud of combustible dust is present in high concentration within the collector. This completes the five elements of a dust explosion and initiates the explosion.
Many dust explosions that occur in process plants are relatively small, leading to limited damage. However, under the right circumstances, even small explosions can escalate into major incidents. This is most commonly the case when secondary dust explosions happen. The typical scenario is that a small “primary explosion” raises a dust cloud, often from dust deposited over time on plant surfaces, and ignites the resulting dust cloud. This “secondary explosion” takes place where often people are present, placing them in immediate danger. Secondary dust explosions can form a chain reaction that can run through a facility as long as fuel is present, leading to injuries and damage to property.
Mechanism of dust explosions
Even materials that are traditionally thought of as non-flammable, such as aluminum, or slow burning, such as wood, can produce a powerful explosion when finely divided, and can be ignited by even a small spark
Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kg sphere of a material with a density of 1g/cm3 would be about 27 cm across and have a surface area of 0.3 m2. However, if it was broken up into spherical dust particles 50µm in diameter (about the size of flour particles) it would have a surface area of 60 m². This greatly increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material. When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure.
Even materials that are traditionally thought of as non-flammable, such as aluminum, or slow burning, such as wood, can produce a powerful explosion when finely divided, and can be ignited by even a small spark.
Explosive Materials & Equipment
The following materials are prone to dust explosions:
Materials that are prone to dust explosions
• — Coal
• — Fertilizer
• — Cosmetics
• — Pesticides
• — Plastic & plastic resins
• — Wood
• — Charcoal
• — Detergents
• — Foodstuffs (sugar, flour, milk powder, etc.)
• — Ore dusts
• — Metal dusts
• — Graphite
• — Dry industrial chemicals
• — Pigments
• — Cellulose
Typical industrial equipment that requires explosion protection.
• — Dust Collectors
• — Dryers
• — Cyclones
• — Crushers
• — Grinders
• — Silos
• — Pulverisers
• — Conveyors
• — Conveyor ducts
• — Screw conveyors
• — Bucket Elevators
• — Furnaces
• — Hoppers
• — Bins
Dust Collection Systems to Control Combustible Dust Hazards
A properly designed, operated and maintained dust collection system is the great defense against combustible dust hazards in your facility
Many process requirements may make elimination of combustible dust, mist, or fume impractical. However, it may still be very possible to manage the dispersion of dust within your plant by using an appropriate and effective industrial ventilation system including dust collection. A well designed, maintained, and operated industrial ventilation system including good hoods, proper duct sizes, and properly selected collection equipment can provide effective dust control and can therefore help manage the presence of dispersed dust. This not only reduces housekeeping frequency and expense, but could also help you reduce the risk of dust explosions in your facility, particularly the destructive secondary explosions, by helping reduce the presence of dispersed fuel in your facility. A properly designed, operated and maintained dust collection system is the great defense against combustible dust hazards in your facility.
Who Sets Standards for Combustible Dust Safety?
The three key entities involved in combustible dust issues are OSHA, NFPA and AHJ
In general there are three key entities involved in combustible dust issues, each with its own particular area of responsibility: (1) The National Fire Protection Agency (NFPA), (2) OSHA and your (3) local Authority Having Jurisdiction (AHJ). In addition to these agencies, others such as the US Chemical Safety Board (CSB) may assist with investigation of combustible dust accidents and advise on the setting of standards for specific industries.
Combustible Dust Control Strategies
Combating combustible dust hazards effectively in your facility requires dedication and a comprehensive approach. In the following section we will discuss several common combustible dust explosion prevention and protection strategies and technologies as they relate to dust collection systems.
Dust Hazard Analysis Required
NFPA regulations require that a Dust Hazard Analysis (DHA) be performed for all operations that generate, process, handle or store combustible dusts or particulate solids. The standard specifies that the facility owner or operator is responsible for determining if the handled materials are combustible or explosive, and if so, characterizing their properties for the DHA.
NFPA regulations require that a Dust Hazard Analysis (DHA) be performed for all operations that generate, process, handle or store combustible dusts or particulate solids
Hazards associated with combustible dusts and dust collection systems include the following:
- ● Explosion hazards in the right concentrations and conditions.
- ● Ignition sources such as open flames, electrostatic discharge, lift truck activity, moving chains, hot surfaces, and rotating equipment with bearings can ignite accumulated or airborne dust, causing a deflagration.
- ● Downstream through a dust collector’s ducting if not isolated, posing fire, pressure-wave, and noxious-gas hazards.
- ● Dust buildup on floors, elevated surfaces, and in hidden areas can be disturbed by a primary explosion, become airborne, and contribute to a secondary explosion.
- ● Dust buildup inside ducting due to deficient filter performance or poor design can contribute to flame or pressure propagation through the duct and into the workspace.
- ● Metal dusts can have high rates of pressure rise and pressure maximums during a deflagration, causing an improperly designed dust collector to explode and produce shrapnel. • Metal dusts can be reactive with other dust oxides and liquids such as water and produce explosive gases that are highly ignitable.
- ● Metal dust fires are more difficult to extinguish and can be worsened with the use of improper extinguishing agents.
Explosion Protection and Prevention Technologies
There are many types of devices and systems used to comply with NFPA standards for the explosion protection of dust collection systems, but they fall into two general categories: Passive systems react to the event, while active systems detect and react prior to or during the event.
The goal of a passive system (also called protection) is to control a fire or an explosion so as to keep employees safe and minimize equipment damage in the plant. An active system (also called prevention), by contrast, can prevent an explosion from occurring. An active system involves much more costly technology and may require periodic recertification.
Passive Devices (Protection)
- ● Explosion venting: Designed to be the “weak” link of the dust collector vessel, an explosion vent opens when predetermined pressures are reached inside the collector, allowing the excess pressure and flame front to exit to a safe area. It is designed to minimize damage to the collector and prevent it from blowing up in the event of a deflagration, thereby reducing the safety hazard. In addition, a flameless vent extinguishes the flame front exiting the vented area, not allowing it to exit the device. This allows conventional venting to be accomplished indoors where it could otherwise endanger personnel and/ or ignite secondary explosions.
- ● Passive float valve: Designed to be installed in the outlet ducting of a dust collection system, this valve utilizes a mechanical barrier to isolate pressure and flame fronts caused by the explosion from propagating further through the ducting. The mechanical barrier reacts within milliseconds and is closed by the pressure of the explosion.
- ● Back draft damper: A mechanical back draft damper is positioned in the inlet ducting. It utilizes a mechanical barrier that is held open by the process air and is slammed shut by the pressure forces of the explosion. When closed, this barrier isolates pressure and flame fronts from being able to propagate further up the process stream.
- ● Flame front diverters: These devices divert the flame front to the atmosphere and away from the downstream piping. Typically, these devices are used between two different vessels equipped with their own explosion protection systems. The flame front diverter is used to eliminate “flame jet ignition” between the two vessels that could overpower the protection systems installed.
Active Devices (Prevention)
- ● Chemical isolation: Designed to react within milliseconds of detecting an explosion, a chemical suppression system can be installed in either inlet and/or outlet ducting. Typical components include explosion pressure detector(s), flame detector, and a control panel. This system creates a chemical barrier that suppresses the explosion within the ducting and reduces the propagation of flame through the ducting and minimizes pressure increase within connected process equipment.
- ● Chemical suppression: Whereas chemical isolation is used to detect and suppress explosions within the ducting, chemical suppression protects the dust collector itself. It is often used, together with isolation, when it is not possible to safely vent an explosion or where the dust is harmful or toxic. The system detects an explosion hazard within milliseconds and releases a chemical agent to extinguish the flame before an explosion can occur.
- ● Fast acting valve: Designed to close within milliseconds of detecting an explosion, the valve installs in either inlet and/or outlet ducting. It creates a mechanical barrier within the ducting that effectively isolates pressure and flame fronts from either direction, preventing them from propagating further through the process.
- ● High-speed abort gate: The gate is installed in the inlet and /or outlet ducting of a dust collection system and is used to divert possible ignition hazards from entering the collector, preventing a possible explosion from occurring and preventing flame and burning debris from entering the facility through the return air system. A mechanical barrier diverts process air to a safe location. Abort gates are activated by a spark detection system located far enough upstream to allow time for the gate to activate.
Additional Prevention Devices and Strategies
- ● Maintain air velocity safety above the minimum conveying velocity: One of the most common sources of potential fuel for combustible dusts is found in the dust collector ductwork. Accumulated dust in ductwork often provides the fuel for devastating secondary explosions or fires. Maintaining the air speed inside the duct safety above the minimum conveying velocity will prevent material buildup.
- ● Spark arrestor: A mechanical device designed to extinguish spark and embers. They are installed in the ductwork on the inlet side of the dust collector (the dirty air line coming into the unit) upstream from the last pickup but downstream of the collector.
- A few variations exist, but the device causes the embers to burn out by a combination of the following: Turning vanes that cause the dust-laden air to change direction rapidly and forcing it to take a tortuous path that allows more time for the spark to burn out; screens that break up the ember into smaller pieces that then burn out; a housing larger than the ductwork to reduce the air velocity causing particles large enough to burn to fall out of the airstream; forcing the dust-laden air to impact a flat surface, causing the larger particles to fall out. For proper functioning of the spark trap, the length of duct between the spark source and the spark trap should be at least one duct diameter, and between the spark trap and the dust collector should be at least ten times the duct diameter. A shorter distance will prevent the spark trap from working correctly and is not recommended. Finally, be careful not to confuse a knockout box like device or a cyclone dust collector with a spark arrestor. These devices cannot guarantee 100% spark removal.
- ● Special Filter Media: While there is no such thing as a “Fire proof filter” certain filter medias provide a level of added resistance to sparks and embers such as flame retardant coatings. Others use conductive materials within the fabric and can assist with static dissipation.
- ● Sprinklers: Per local building codes you may be required to install traditional water sprinklers inside certain parts of the process, including inside the dust collector incase of fire.
- ● Choice of filter style: For applications with high dust loading, heavy dusts or “sticky” dusts the use of cartridges may create an additional hazard. With horizontally-mounted cartridges, dust becomes trapped in the pleats in the upper third of the filters This trapped dust can burn even if the filter media is fire retardant.
- ● Hopper discharge devices: Normally the choice of a discharge varies depending on the process needs and budgetary concerns. With combustible dust applications however the choice of a discharge device can have a major impact on safe operation. Rotary airlocks rated for combustible dust applications work best. Rotary valves enable quick and continuous emptying of the hopper, eliminating the potential for material in the hopper to serve as fuel for a potential explosion or fire. Discharge to a seal drum or hopper can also work well if they are rated to withstand potential explosion pressures. But these must be constantly replaced in order to prevent material backing up into the hopper. Finally, double dump valves rated for combustible dust applications can prove useful in applications with combustible dust but also larger pieces that could potentially jam rotary valves.
