The NFPA’s Colonna admits that 1/32 of an inch — about the thickness of a mechanical pencil lead — sounds miniscule, but if that amount covers the entire working surface of a facility, it could create a dust cloud with the ideal concentration to form a combustible dust explosion.
“It is a very small amount of dust, yet it represents that marginal threshold above which you can be certain there is enough dust present that if it gets thrown into the air and becomes suspended and concentrated, you have a significant and legitimate hazard,” says Colonna.
According to Colonna, a provision in NFPA 654 that will be adopted next year includes a formalized hierarchy of safe housekeeping measures:
1. Vacuum dust with an intrinsically safe vacuum that is bonded and grounded, so it doesn’t become an ignition source. It may be vented or ducted to a dust collector.
2. Where the vacuum cannot reach, conduct a water wash down or carefully sweep with a broom in a manner that does not stir up dust.
3. Finally, if the previous two measures are not effective, cleaning with compressed air is permissible in small areas with operating equipment shut down.
Even with effective dust collectors and diligent housekeeping, the threat of an explosion may still be present if ignition sources are not properly dealt with.
Generally speaking, the first four factors of the explosion pentagon may co-exist without causing an explosion, but the final factor — ignition — is all the catalyst needed to set one off.
Common ignition sources include sparks generated by milling processes that are grinding or abrasive, or heat from friction-causing equipment. Colonna notes that upkeep and maintenance of equipment plays an important role in minimizing ignition sources.
“In grain handling you have bucket elevators, conveyors, and other pieces of equipment that that are propelled by bearings and gears,” says Colonna. “When maintenance fails, the equipment can seize up, causing either overheating or friction, which generates sparks and can become an ignition source.”
Spark detection and extinguishing systems were introduced to the U.S. market in the late 1970s, according to Allen Wagoner, president of Greensboro, NC-based FLAMEX, Inc.
Spark detectors are high-speed infrared sensing devices usually mounted on material handling ducts where dust is pneumatically conveyed.
“If a spark is detected in the ductwork, the spark detection system can be used to redirect the airflow or activate suppression by water or an extinguishing agent to eliminate the ignition source,” says Wagoner.
Bob Barnum, vice president of sales for Grecon, Tigard, OR, notes that many agricultural companies choose diversion or chemical suppression over water suppression.
“The typical milling application ends by sending the product into a storage silo,” says Barnum. “Using a fine mist of water to extinguish the spark could damage the stored grain and form mold or mycotoxins, so chemical suppression or diversion techniques are commonly used in agriculture.”
Suppression, isolation and venting
In addition to managing dust and eliminating ignition sources, precautions including suppression, isolation and venting can help ensure a safe outcome should an explosion occur.
With suppression, when sensitive flame or pressure sensors detect a hazard, they release a chemical — usually sodium bicarbonate — to quench the fireball and reduce the pressure buildup before exploding.
Meanwhile, explosion isolation, a provision in NFPA 654, requires barriers positioned throughout the processing system to prevent an explosion from traveling upstream or downstream.
Chemical suppressants may be released at one process to quell the initial blast, or can be installed throughout the ductwork to act as an isolation method.
“If you invest in a sensor and the control panels, it’s a very small incremental step to do both the suppression and isolation chemically, rather than try to do them independently through mechanical means,” says Boston.
Mechanical isolation devices, like slide gates that quickly slam shut to cut off nearby areas, are permitted, but Fenwal’s Grandaw notes that mechanical isolation has limitations.