Explosions require two additional factors to occur: dust confinement and dust dispersion. Just as with fires, the easiest factor to remove is still the ignition source. Here are a few prevention options along with their possible pros and cons:
• Distance. Creating distance between a spark generation point and accumulated fuel (dust collector) can allow sparks time to extinguish before reaching the combustible dust.
Pros: comparatively low cost; no additional equipment required.
Cons: longer duct runs increase energy requirements and inherently include the potential for dust accumulation in the ducts; uncertain level of ignition source control.
• Passive spark abatement. Devices mounted in the duct create turbulence that accelerates the rate at which sparks extinguish in the airstream.
Pros: comparatively moderate in cost; low pressure losses.
Cons: may have a required installation distance from a dust collector; can become fouled with some dusts.
• Active spark abatement. A spark detector mounted in the duct triggers release of an extinguishing agent downstream in the duct to douse sparks before they reach the dust collector.
Pros: active identification of ignition sources with monitoring and notification capabilities; higher confidence in detection and response.
Cons: comparatively higher in cost with duct length requirements between the sensors and the extinguishing equipment; increased complexity and maintenance.
Because prevention equipment can’t remove 100% of the risk of an event occurring, you always should consider protection strategies as well. These limit damage from a combustion event to a level you determine to be acceptable. Protection equipment generally is specific for either fires or explosions, so mitigation plans often must include strategies for both types of risks. Here are some protection options along with their possible pros and cons:
• Sprinkler system. A temperature rise above a threshold in the dust collector triggers the system to activate to disperse water into the collector.
Pros: integrated with facility’s fire control system; moderate initial cost.
Cons: potentially significant downtime and cleanup if an event triggers a discharge; harder to relocate during process restructuring.
• CO2 fire suppression system. A temperature rise above a threshold in the dust collector initiates release of carbon dioxide into the dust collector. This displaces the oxygen to smother a fire.
Pros: simpler cleanup and can limit damage to dust collector; system often is dedicated to a collector and easier to relocate during process restructuring.
Cons: higher cost; annual system inspection necessary.
• Outside location. If a dust collector can be separated from the process area and located outside with an exclusion zone around it (Figure 2), you may decide that allowing the collector to burn represents a minimal level of risk.
Pros: reduces exposure to the building; additional fire mitigation equipment may not be required.
Cons: without additional mitigation, damage to the collector may result in a longer downtime period for replacement of the dust collector; may not be feasible for every application.
• Explosion vents. A weak panel on the dust collector opens during a deflagration event, directing the resulting gases, flames and debris to a safe location.
Pros: comparatively low cost; passive device.
Cons: discharges a fireball, debris and dust to the surrounding areas; fire risk following the deflagration may exist; longer downtime if fire damage occurs.
• Chemical suppression system. A rapid pressure rise at the start of a deflagration triggers injection of chemical suppressant into the collector, preventing a full deflagration.
Pros: dust containment; reduced damage and associated downtime; reduced risk of a fire following the event.
Cons: comparatively higher cost; annual system inspection.
• Passive isolation. The pressure wave from an explosion mechanically activates a device that closes the duct to block flames from passing back through duct towards the process. Activation should trigger an immediate and automatic shutdown of the protected system.
Pros: minimal equipment setup; comparatively low cost option.
Cons: duct size and dust characteristics can limit application; may require inspections.
• Active isolation. An electronic sensor activates a device to prevent flames or pressure from traveling back through the duct to the process. The device also should trigger an immediate and automatic shutdown of the protected system.
Pros: precise detection; may allow filtered air return to the building.
Cons: higher cost than passive devices; requires system controls.
Succeed At Mitigation
Combustible-dust risk mitigation can seem daunting. However, in simplest terms, it’s based on a series of logical questions: Are materials or dusts in your facility combustible? Where do you have potential nuisance-dust control issues or process hazard concerns? How can you address those hazards? Finally, what actions will allow you to attempt to prevent a combustion event and, if an event occurs despite your prevention efforts, how can you reduce the consequences of fires or explosions. Figure 3 summarizes the decision tree involved.
Answering these questions often is an iterative process, with mitigation strategies driving decisions about dust control system design, and dust control decisions influencing choices in mitigation. Each facility’s dust and process hazard analysis will be unique, so there is no one “right” strategy. The good news is there are mitigation strategies to address the risks, and today’s options allow you to balance your risk strategy with your production needs.
Independent combustible dust mitigation consultants can help you conduct a dust and process hazard analysis, and then recommend a strategy for your facility. For more detail about available mitigation equipment options, contact your dust collector supplier.
KAREN WEAR is market manager, Industrial Air Filtration Product Group, Donaldson Company, Bloomington, Minn. Email her at Karen.Wear@donaldson.com.