Tuesday, January 7, 2014
The Right Tool For Removing Combustible Dust
Sometimes, we overlook the obvious and simple solutions to our plant problems. A simple solution to hazardous dust explosions in a dry bulk solids plant may be effective housekeeping. This simple solution is not a modern insight. In 1922, a National Fire Protection Association manual titled Dust Explosions cited the need for a vacuum cleaner which could withstand the challenges of an industrial environment.
The dangers of dust explosions are still a plant concern 90 years later. This 1922 book explains that sweeping dust with brooms or using compressed air to blow it off surfaces can lead to the dust being suspended in the air. That warning remains true today. The suspended dust could ignite and cause a primary explosion or settle back on floors and hard surfaces, thereby creating the opportunity for a secondary explosion.
Primary Dust Explosion
A primary dust explosion requires three components: fuel (combustible dust), an ignition source and oxygen. This combination is referred to as the fire triangle. Without all three, the fire cannot occur. Then if two other elements, a contained environment (a room) and a dust cloud, are introduced to the fire triangle, this may create an explosion pentagon: All five elements must be present simultaneously for an explosion to occur. Effective housekeeping will assist in eliminating two of the components, dust and an ignition source.
In an open environment such as an outside building, igniting a dust cloud would create a brief flame flash. However, in a contained environment, like a dust collector, the dust cloud’s fast burning creates a rapid pressure rise inside the vessel which could consequently damage or destroy the equipment.
Secondary Dust Explosion
If you touch a flame to a layer of dust resting on a table in an enclosed room, the dust will ignite, but won’t explode. Conversely, if you throw that same dust into the air, thereby creating a dust cloud, then touch a flame to it, the dust will explode.
This scenario is an excellent example of what happens in a secondary dust explosion. When the primary dust explosion’s force dislodges fugitive dust from the plant’s surfaces, this produces more dust clouds and thereby creates a chain-reaction that can cause additional explosions.
Choosing the Right Vacuum Cleaning System
Even when your plant takes every precaution to eliminate ignition sources that can cause fires, you must go further to prevent dust explosions. Your plant must also initiate a safe housekeeping plan to remove fugitive dust.
Choosing the right vacuum cleaning system is the first step in effective housekeeping. In the 21st century, we know that alleviating the ignition source is a major step in creating a safer environment in plants. This means that standard shop vacs are not a viable solution for plant housekeeping. Like most non-industrial vacuum cleaners, a shop vac is susceptible to ignition. One of the most common sources of ignition with vacuum cleaning is static electricity. Particles flowing in one direction through a plastic vacuum hose can create a volatile static electricity charge, plus static electricity can build up in the particles themselves. This is why many plants handling the removal of combustible dust with non-industrial vacuum cleaners, such as shop vacs, have been cited by OSHA.
Conversely, industrial portable and central vacuum cleaning systems for the removal of combustible dust must be rated for use in Class II Division 2 areas (a National Electrical Code hazardous area designation). This rating means that not only is the vacuum cleaning system designed so that no particles can contact any source of ignition in the system, but also that the system is redundantly grounded to eliminate any possibility of a static-ignited dust explosion. A vacuum cleaning system powered by plant compressed air is in itself safe because it has no motor and therefore no moving parts or electrical current.
The current NFPA guidelines leave some room for interpretation when it comes to safely cleaning up combustible dusts. Even with a small quantity of combustible dust, it’s best to be cautious. By using a portable or central vacuum cleaning system designed for combustible dust collection, you can safely and effectively clean up dust in your plant, eliminating the fuel needed for an explosion. In fact, using a system designed for safely capturing fugitive dusts is now the preferred housekeeping method for removing dust in dry bulk solids plants per OSHA and NFPA standards.
Protecting your workers, equipment and building from combustible dust hazards is as easy as just having a good housekeeping plan in operation.
Portions of this article appeared in Powder and Bulk Engineering.