Managing wood dust danger
By Ed Chovanec and David Grandaw
March 24, 2014 - Industrial explosions are a constant threat to anyone who handles finely divided combustible dust. Most organic material will burn in a solid form, and if this same material is in a dust form, under the right conditions, it will explode. Combustible dust explosions happen frequently in the processing industry.
By Ed Chovanec and David Grandaw
Industrial explosions are a constant threat to anyone who handles finely divided combustible dust. Most organic material will burn in a solid form, and if this same material is in a dust form, under the right conditions, it will explode. Combustible dust explosions happen frequently in the processing industry. Sometimes these explosions are confined to the process vessel in which they originate in, but more often than not, the initial explosion will result in a secondary explosion with devastating results.
|Active explosion isolation: Explosion isolation devices prevent a deflagration in a process vessel from propagating through a connection to other equipment where it could cause subsequent explosions.
Companies that handle wood dust, including wood pellet plants, must deal with this risk daily. Having a comprehensive plan to prevent an explosion from happening under normal circumstances, and mitigating the effects of the explosion under upset conditions, is critical to the safe operation of any facility that handles a combustible dust.
In order for an explosion to occur, five elements must be present: fuel, an ignition source, an oxidizing agent, confinement, and dispersion of the dust into the airstream.
In wood processing and handling plants, the process equipment most typically associated with dust explosions include dust collectors, cyclones, storage hoppers or silos, mechanical and pneumatic conveying, milling systems, pellet coolers, and bucket elevators. All of these vessels can have wood dust in suspended form, either during normal operations or in an upset condition. Once the wood dust is suspended into a dust cloud, all it takes is an ignition source to initiate the deflagration. The pressure from a deflagration travels at the speed of sound, while the growing fireball initially propagates at a much slower speed.
A typical sequence for a dust explosion includes:
- Ignition of the dust cloud;
- The deflagration pressure results in rupture of the vessel;
- The shock wave from this ruptured vessel liberates dust that has accumulated on horizontal surfaces in the process area, such as atop beams, ducts, conveyors and even light fixtures, causing it to become suspended in the process area;
- The escaping fireball from the initial process vessel ignites the newly-suspended dust in the process area, causing a secondary explosion that can destroy the building;
- Flame propagation occurs through interconnected ducts, chutes or conveyors to connected equipment upstream and/or downstream, with the resulting additional explosions.
Explosion risk management
Managing this explosion risk requires a comprehensive understanding of the normal, abnormal and upset conditions in the processing equipment. The Canadian fire code follows the U.S. National Fire Protection Association (NFPA), which suggests that a Process Hazard Analysis be performed. This is commonly referred to as a PHA, and includes an assessment of the combustible properties of the dust being handled, identifies where a dust cloud can occur in the process, and determines likely ignition sources. The PHA should include an assessment of the consequences of a deflagration should one occur, including both primary and secondary explosion scenarios. Identifying explosion prevention and mitigation steps that should be taken to reduce the risk of an incident is a critical part of a PHA.
Once there is an understanding of the explosion risks present in a process or process vessel, the first line of defence should be explosion prevention. This includes measures to minimize ignition source potential. Typical ignition sources include bearing failure/overheating, electrostatic discharge, tramp metal, and mechanical failure of product or air-moving equipment. Ignition prevention techniques commonly used are strategically-placed magnetic separators, electrostatic bonding and grounding, using the correct electrical equipment for the area classification, and robust hot work procedures.
Spark and ember detection systems are commonly used on dust and pneumatic conveying lines at wood facilities. These systems are designed to detect a burning ember traveling through the ductwork, then inject a water spray downstream to extinguish the ember before it reaches the air-material separator downstream. Diverter valves and abort gates are often used in conjunction with a spark detection system, to either stop airflow or re-direct it to a safe area.
Minimizing the residual dust layers on horizontal surfaces is an important part of any explosion prevention program. Diligent housekeeping to prevent the accumulation of dust outside the process equipment will reduce the risk that an explosion in a process vessel will result in a secondary explosion that destroys the facility.
Ignition control, proper housekeeping of residual dust, continuous training of plant personnel on managing the dust explosion risk, and management of change to address the effects of a process or product change, are all critical to helping prevent an explosion from occurring under normal operating conditions. Unfortunately, abnormal conditions that result in an explosion can occur in any process line. This is why NFPA requires the use of explosion mitigation techniques for vessels subjected to an explosion threat.
NFPA 69 lists a number of different mitigation methods that can be employed to deal with an explosion threat. Some of them, such as containment (building the process vessel strong enough to withstand the explosion pressures), inerting (operating in an oxygen-depleted environment), and dilution (injecting a non-combustible substance to create a mixture that will not explode) are not usually feasible in the wood pellet or process industry. The most common techniques include explosion venting, explosion suppression, and explosion isolation.
Explosion relief venting
Explosion relief venting, one of the most widely used methods for mitigating dust explosions, requires one (or more) explosion relief vent installed on the wall of a process vessel. The vent consists of a membrane that’s constructed of a material weaker than the vessel wall. During a dust explosion’s incipient stage, the vent ruptures and directs the explosion’s overpressure, flame, burnt and unburnt material, and other combustion by-products away from the vessel to a safe location. The explosion relief vent is designed to ensure that the explosion’s pressure rise doesn’t exceed the vessel’s pressure shock resistance. These vents are designed according to procedures in NFPA 68: Standard on explosion protection by deflagration venting. NFPA provides an equation for estimating the vented fireball’s size; from this information, you can calculate the safe distance required in front of the vented vessel to protect workers, equipment, and the building structure from the ejecting fireball.
|Explosion suppression and isolation: Typically, explosion suppression systems include explosion isolation for interconnected ducts through which flame propagation may occur.
Flameless venting protects indoor equipment from dust explosions by combining an explosion relief vent’s weak membrane with a mesh trap that arrests flame and retains particles. Like an explosion relief vent, the flameless vent’s membrane, installed on the process vessel, ruptures during a deflagration. But unlike with a relief vent, the deflagration’s overpressure, flame, and material discharge through the membrane into the mesh trap, which prevents the flame and material from discharging into the surrounding area. Instead, the flameless vent discharges hot gas and overpressure. A safety perimeter must be established around the flameless vent to protect workers from this discharge.
Like the explosion relief vent, the flameless vent is designed according to procedures in NFPA 68. To ensure that the flameless vent can successfully protect the vessel, the ratio of room volume to vessel volume (that is, the ratio of the volume of the room in which the vented vessel is located to the vessel’s volume) must also be kept below the flameless vent manufacturer’s recommendations. Since any dust or dirt blocking the openings in the mesh trap would compromise the vent’s operation, the mesh must also be regularly cleaned to ensure that the surface is free of dust or dirt at all times. Some vent manufacturers offer low-inertia, fire-resistant fabric covers to help keep the mesh surface clean.
It is important to understand that explosion venting only relieves the deflagration pressure from the protected vessel. It does not stop flame from propagating to any interconnected vessel, nor does it address the post-explosion fire in the vented vessel. Other protection measures are needed to deal with these threats.
Explosion suppression systems are often installed in applications where it’s not possible to safely vent an explosion away from process equipment. The system detects an incipient dust explosion very soon after ignition and discharges a chemical extinguishing agent quickly enough into the developing fireball inside the equipment to extinguish the deflagration before a destructive overpressure develops. Major components in a typical explosion suppression system are one or more explosion suppressors, one (or more) explosion pressure detector, one (or more) flame detector, and a control panel. Explosion suppression systems are designed according to NFPA 69: Standard on explosion prevention systems. Typically, explosion suppression systems also include explosion isolation for interconnected ducts through which flame propagation may occur.
The major advantages of explosion suppression are that no flame is ejected from the protected vessel, and the risk of a post-deflagration thermal event is greatly diminished.
Explosion isolation devices prevent a deflagration in a process vessel from propagating through a connection such as a duct, chute, or conveyor to other equipment, where it could cause subsequent explosions. The devices work by mitigating the flame propagation and pressure piling between connected equipment. An isolation device can be active or passive. An active device has detection components, including explosion pressure and/or flame detectors, and a control unit. The detectors detect explosion pressure or a flame and send a signal to the controls to rapidly deploy the device. These active explosion isolation devices are either chemical or mechanical. A chemical isolation device works by rapidly discharging a chemical extinguishing agent, such as sodium bicarbonate, into connecting ductwork to mitigate flame propagation. A mechanical isolation option includes a high-speed gate valve. Milliseconds after the active high-speed gate valve’s detectors sense explosion pressure or flame, the controls rapidly deploy a mechanical barrier — closing the valve’s gate across the connecting ductwork.
In addition to the active isolation options, there are passive isolation means available to mitigate flame propagation. The passive valve, which can have a flap or float, is self-actuated by the airflow from a deflagration so it requires no detectors or controls. This device is typically used to isolate dust-handling equipment with relatively low dust loads.
Dust explosions at wood-handling facilities do not need to happen. Understanding where an explosion may happen in the facility, implementing ignition prevention systems, following rigid housekeeping standards, and proper employee training should all be part of designing and operating a process to minimize the threat of an explosion starting. Employing explosion protection measures such as explosion venting, suppression and isolation, will minimize the risk of an incipient explosion in process equipment from escalating into a catastrophic event in the facility.
Ed Chovanec C.E.T., C.S.P. is the Northeast Regional Manager of Fenwal – IEP Technologies and is located in Burlington, Ontario. David Grandaw is the Vice President of Sales for Fenwal – IEP Technologies in Marlborough, Mass. For more information, go to www.ieptechnologies.com, or email the authors at firstname.lastname@example.org or email@example.com.