A combustible dust explosion is a serious hazard for a wide variety of industries including manufacturing, processing, metalworking, chemicals, and more. This hazard can materialize in many areas of a plant, but is more likely to occur at the dust collection system. Implementing proper explosion venting in your dust collection system can reduce your risk for a hazardous explosion.
The first step is to identify and determine if your facility has any combustible dust risks present. Click this link to learn more about combustible dust characteristics and the conditions for a dust explosion. If you have identified that your facility is at risk, you should evaluate your options for both prevention and protection.
There are multiple strategies for explosion venting, the best method for your facility will depend on a variety of factors. Start off by asking yourself these questions:
Questions to Consider
• What are the state and/or local regulations for fire safety and explosion venting?
• Is the dust collector setup inside or outside?
• What is the distance of the dust collector from the roof or walls?
• Is the dust collector or vent close to any other structures?
• What’s the cost?
Explosion Venting Strategies
Preventative Maintenance Plan: In conjunction with a protection strategy, every facility should implement a well-designed and operated preventative maintenance plan. A preventative maintenance plan for your dust collection system will help manage the levels of combustible dust. Following a maintenance plan for your baghouse, bin vent, or cartridge collector will help you address any concerns before they create a larger issue.
Explosion Vent or Panels: Explosion vents or panels are designed to rupture at a set pressure (PStat).
When a source of ignition meets a fuel source with sufficient oxygen present, an ignition will occur. As the ignition begins, the pressure inside of the vessel will increase rapidly. Depending on the material’s Kst value, the pressure rise may be slow or extremely rapid. As the ignition progresses, the internal pressure will meet the PStat rating of the explosion panel. The explosion panel will rupture, venting the ignition gasses. The explosion vent provides a relief avenue for the expanding gasses, but the pressure in the vessel will continue to rise until it reaches the Pred pressure. This is the maximum pressure of the ignition event when explosion vents are functioning, so this is usually the pressure rating the vessel is designed to withstand.
Explosion panels can be used with a short length of ducting to allow for interior use without flameless venting.
Although these vents do require replacement once a rupture occurs, they are simple, cost-efficient, and easy to install. Explosion vents are commonly used on baghouses and cartridge dust collectors.
Flameless Venting: Flameless venting can consist of a flame arrestor element, vent panel, and flanged housing. This method is designed to combine the techniques of explosion venting and flame arresting. A flame arrestor element can be installed over a standard explosion vent. When the vent ruptures, the burnt dust and flames enter the flame arrestor element. The element helps to contain the hazardous dust and flames and prevents it from exiting, where it could potentially ignite a secondary explosion or endanger employees. While flameless venting does stop flames from exiting the vessel, there will be extremely hot gasses exiting the flameless vent. When using flameless venting, make sure to pay close attention to the vents proximity to personnel areas. If possible, always aim flameless vents away from regularly occupied areas.
Explosion Latch: Explosion latches work under the same principle as the explosion panels, but they are not single use. When using explosion latches, there will be a network of small, hinged doors along the walls of the baghouse dirty air plenum. The doors have special latches that are designed to release at a set pressure (Pstat), which can be reset if opened. Explosion latches are a more cost effective option, compared to explosion panels, for large a baghouse when a large quantity of vent area is required.
No Return Valve: Protecting the dust collector from over-pressure is essential, but it is equally important to stop an deflagration propagation back to the operator space. To prevent this, a No Return Valve is needed in the inlet duct. This valve is a weighted damper that is held open by the air flow during normal operation, allowing air and dust to pass through to the dust collector. In the event that deflagration occurs in the dust collector, the pressure propagation through the duct work will close the No Return Valve. This prevents the deflagration from reaching any process equipment and also limits the risk of secondary explosions.
Abort Gate: Abort gates are high speed dampers that contain a spring assisted blade and is typically held in place by an electromagnet. They are used to minimize the risk of an explosion by diverting flame, spark, or debris from entering a facility through a return air system. They are activated by a spark detection system that is placed far enough upstream, which would allow time for the gate to be activated. A mechanical barrier will redirect process air to a safe area.
If the baghouse exhaust duct is not being returned to the plant space, an abort gate is not necessary since any deflagration through the clean air duct would remain outdoors. If the exhaust gasses are being returned indoors, an abort gate is required.
Spark Detection & Extinguishing System: This method uses infrared sensors, typically located on the ductwork, to detect sparks or burning material in the ductwork upstream. Before it can travel further downstream and become an ignition source inside the dust collector the extinguishing assembly is activated, and a minimal amount of water is released to eliminate the hazard.
Secondary Event Protection: All of the methods described previously are excellent options for managing primary explosions, but one of the most catastrophic outcomes of a combustible dust explosion is an un-controlled secondary explosion. A secondary explosion occurs when ambient dust outside of the dust collection system is ignited. In many cases, there is a layer of dust on all horizontal surfaces brought on by inadequate dust collection and containment.
When a primary explosion happens, there may be a pressure wave that propagates through the plant. This will “kick up” the layer of ambient dust. If the explosion is not contained in the dust collection system using the methods previously outlined, this ambient dust in the air could come in contact with the primary explosion flame front. This results in an uncontrolled explosion in an occupied space.
To minimize the risk of secondary explosions, the first step should always be to expect perfect performance from your dust collection system. It is not acceptable to have a dust collection system that does not function properly. It is also highly suggested to limit the amount of horizontal surfaces in your plant that cannot be regularly cleaned. Drop ceilings and in-accessible equipment are great examples of places that are rarely cleaned and dust tends to accumulate.
There are many explosion venting strategies that may fit your unique application or facility. It is always recommended to consult your local or state building codes and regulations before choosing your explosion venting strategy. Some areas will have specific regulations for fire safety and environmental safety, and you want to ensure you are meeting those guidelines. To learn more about which option may be right for your facility contact us today to speak with an engineer.
Download our dust collector purchasing guide below to learn other variables you should consider when planning for a new dust collection system.