- Category: Safety
- Created: 2017-11-29
Reducing the risk of dust collector fires and explosions
by Duncan Beaumont, solution director, RoboVent
Your dust collector plays a critical role in keeping factory air clean and safe for workers. But under the right conditions, it can also be one of your biggest safety hazards.
Dust collector fires and explosions can cause tremendous damage to facilities and be fatal for workers caught in the immediate vicinity. These hazards are almost entirely preventable with proper equipment selection, placement and maintenance. Here is what you need to know to protect your workers and your facility.
How fires and explosions start
All fires require three things to start and spread:
A fuel source
An ignition source
Unfortunately, a dust collector can easily provide all three. The filter media inside the collector is an excellent fuel source as are the collected particulates in the filters and containment drums. The airflow created by the blower as it moves dirty air through the filters creates a steady flow of oxygen to sustain a combustion reaction.
All it takes now is a single spark from nearby welding, cutting or grinding operations or even an electrostatic spark from inside the machine to make its way to the filter media to start the reaction. Once a fire starts in a dust collector, it can quickly get out of control if it is not contained immediately.
Highly combustible dusts can also create an explosion hazard. Unlike a fire, which starts small and grows quickly, a dust explosion involves rapid combustion that generates extreme air pressure that can easily blow out walls, destroy nearby equipment and kill people in its vicinity.
In addition to the elements of the Fire Triangle shown in Figure 1, a dust explosion (or deflagration) needs two more elements: the right concentration of a combustible dust and containment of that dust. Together, these five elements are known as the Dust Explosion Pentagon,” as shown in Figure 2.
Many types of dust produced in industrial processes have the potential to become explosive in the right circumstances, including metalworking dusts; flours, sugars and food starches; wood and paper dusts; and plastic dusts. Under the right conditions, some of these dusts can self-ignite, eliminating the need for an external ignition source.
Therefore, all dust collectors, regardless of dust type, should have some basic fire safety elements installed. These may vary depending on the types of processes you are running and the location of the collector.
Dust collector fires can quickly get out of control, causing facility-wide damage if they are not contained.
If your dust collector is located near the source of sparks from welding, cutting, grinding or other applications, it should be outfitted with an effective spark arrestance system. These systems use a variety of approaches to extinguish sparks before they make their way to filter media or prevent them from entering the collector ductwork to begin with. Some of these include:
Baffles and metal mesh filters that are designed to provide a tortuous path for sparks, causing them to bounce off multiple metal surfaces.
Fixed-blade mixing vanes that create strong turbulence in the air stream, another way to strip off the thermal envelope.
Simple plates placed in front of the dust inlet, which are designed to deflect sparks from entering the ductwork.
Dropout boxes, enlarged enclosures designed to allow heavy particles to drop out of the air stream before air hits the filters.
These systems have varying levels of effectiveness, but all of them do allow a small percentage of sparks through. Some may also restrict airflow to the dust collector, lowering system effectiveness and efficiency. Most require regular maintenance to clean out particulates that accumulate in the spark arrestance system.
Another approach, used by RoboVent in the company’s Delta3 spark arrestance system, relies on centrifugal force to extinguish sparks. Sparks and embers are repeatedly driven against the outer wall of the device, stripping off the thermal envelope that surrounds them so that they are rapidly cooled and extinguished. High airflow velocity is maintained on all surfaces of the device to make sure it is self-cleaning.
Dust explosions are among the deadliest and damaging industrial accidents.
Shutdown and damper
While spark arrestance systems focus on the ignition leg of the Fire Triangle, other dust collector features focus on cutting off the oxygen supply. Remember a fire cannot spread without oxygen to fuel combustion.
Dust collectors should be equipped with sensors that can detect smoke or heat inside the collector. These sensors can be used to control failsafe systems that shut down the blower and compressed air cleaning systems to immediately stop airflow through the filters. Damper systems that automatically shut when smoke or heat is detected can further cut off oxygen supply. Without airflow, most fires die down quickly.
Fire suppression systems should be in place for all dust collectors. Facilities should ensure that sprinkler systems, required by law in most places, are working properly and the collector is within range of the sprinklers. Readily available external fire extinguishers should be near every dust collector.
Even better, look into a system with an internal fire suppression system. An internal system is triggered by the same sensor system that shuts off the blower and closes dampers when smoke or heat is detected. Fire retardant chemicals are released inside the dust collector to smother the fire as soon as it is detected.
These systems can quickly get fires under control before external fire suppression systems kick in, reducing damage to the dust collector and the facility and minimizing the cleanup that must be done after a fire. Highly effective systems can limit damage to a single filter rather than allowing a fire to destroy the entire collector.
If your facility generates dust that is combustible, you also need to have strategies in place to minimize the risks of a dust explosion. OSHA’s Process Safety Management (PSM) standard may apply if the dust you generate is on the list of highly hazardous chemicals.
Effective dust control is the most important step; dust will not explode if the concentration in the air is below the lower explosive threshold limit. A Process Hazard Analysis (PHA) may also be required for hazardous processes that fall under the OSHA PSM standard. The PHA may recommend other changes to facility layout, material selection and dust producing processes to reduce containment, eliminate potential ignition sources and reduce the overall combustibility of the dust produced.
When working with combustible dusts, it is also important to ensure that the dust collector itself is equipped with features designed to reduce the chances of an internal explosion and minimize damage should an explosion occur. Manufacturers should look for dust collectors that comply with NFPA standards. Specific features to look for include:
Explosion vents: These panels are designed to blow out and release excess pressure building up inside the machine if an explosion should occur. They should be positioned to direct pressure away from people and structures.
Isolation valves: These valves are located in the ductwork between the dust collector and the building or enclosure. They prevent pressure waves from going back through the ductwork into the facility.
Airlock: The airlock is between the collection bin or hopper and the dust collector. In the event of an explosion within the hopper, the airlock will prevent additional oxygen from reaching the contained dust and keep explosive pressure away from the dust collector.
Dust collector fires and explosions are a serious safety hazard for manufacturers. In addition to looking at the features of the collector itself, manufacturers should take additional steps to reduce the risks. These may include:
Reducing or eliminating ignition sources near the dust collector.
Properly maintaining all fire alarm and suppression systems.
Performing all required dust collector maintenance, including conducting system checks for fire safety systems, cleaning spark arrestance systems when required, emptying containment bins and changing filters when they become saturated.
With careful attention to the dust collector itself and to external factors that can lead to fires and explosions, manufacturers can reduce their risks to nearly zero.