Sealed Bulk Bag Discharge System Contains a Chemical Dust Hazard

The key feature of the redesigned operation was the installation of a sealed system for unloading bulk bags of the chemical powder and filling the canisters

The Texas A&M University System is famous for its agricultural and engineering programs but less well known is the fact that one of its agencies has the country's largest training facility for firefighters. The 120-acre Brayton Fire Training Field is operated by the Texas Engineering Extension Service (TEEX) and includes full-scale buildings, a chemical complex, a petroleum refinery unit, a loading terminal, a ship, and an aircraft fuselage for realistic firefighting drills.

Each year, TEEX trains more than 80,000 firefighters and emergency response personnel, who come from all parts of the U.S. and more than 45 other countries. In the course of their training, the students have to fight a variety of fires, using various types of equipment, such as hand lines (hoses) and hand-held fire extinguishers that contain sodium-bicarbonate-based (NaHCO₃) chemical powder.

The Brayton Fire Training Field is currently in the midst of a $30 million infrastructure upgrade. One of the first projects to be completed was the installation of an improved refilling operation for portable fire extinguishers. NaHCO₃ is a fine powder of 5-20 microns, similar to talcum. It is easily airborne when handled or transferred from one container to another, creating a potential dust hazard.

The key feature of the redesigned operation was the installation of a sealed system for unloading bulk bags of the chemical powder and filling the canisters. The new system, from Flexicon Corp., Bethlehem, PA, has eliminated the potential for dusting associated with the previous unloading method and is much more efficient, says Ron Peddy, TEEX's program manager for environmental safety and field services. The system was installed by Robert K. Wilson & Associates, a manufacturers' representative located in Houston, TX.

Prior to installing the new system, the powder was received in 400-lb drums. When a drum was needed for the refilling operation, it was hand-rolled to a 9,600-lb hopper that fed the refilling line. A drum dumper was used to empty the drum into a 600-lb capacity hopper, from which the powder was augured up into the large hopper. From the hopper, hoses were used to refill the canisters as they moved along a roller conveyor. Powder was transferred to the hoppers in the open air, and the refilling process was performed outside under an awning to prevent dust inhalation, says Peddy.

In the new configuration, the whole operation has been consolidated indoors in a single building. Chemical powder is received in 2,200-lb bulk bags, which are stored in the building. A bulk bag discharge frame is located above a floor hopper at the head of the U-shaped roller conveyor. The powder is transferred from the bag to the hopper and into the canisters without being exposed to the environment.

Both the bulk-bag discharge frame and the floor hopper were supplied by Flexicon. The Model BFC bulk-bag discharger incorporates a cantilevered I-beam, electric hoist, and trolley. Bags are brought to the discharge frame by a forklift and lifted by the hoist, which has a spreader bar that attaches to the bag's four corner straps. The bag is positioned above the hopper.

Powder is discharged from the bulk bag through a patented Spout-Lock clamp ring that creates a dust-tight seal between the spout and the equipment and a patented Tele-Tube telescoping tube that promotes product flow and complete discharge by exerting continual downward tension on the bag as it empties and elongates.

To make the connection, the clamp ring is raised pneumatically toward the bag spout with the clamp ring in the open position. The spout is pulled over the rim of the tube, and the ring is locked in place over it. The bottom end of the telescoping tube is connected to a collar in the lid of the sealed hopper.

Once the clamp ring is connected, the pneumatic pressure that raised the tube is released, causing the assembly to exert downward tension on the spout. The drawstring on the bag spout is then untied to allow the powder to discharge through the spout. Continuous downward tension keeps the spout taut at all times and helps maintain a steady flow of powder by preventing excess material in the spout from bulging outward and creating dead pockets or falling inward and restricting the flow.

Powder transfer is further enhanced by means of Flexicon's Flow-Flexer bag activators. These are two pneumatically driven plates that rhythmically raise and lower opposite bottom edges of the bag to direct material into the outlet spout. As the bag empties, the stroke of the plates increases to form the bag into a steep V shape and promote total evacuation. An adjustable timer controls the frequency of the strokes.

Canisters are refilled from the floor hopper via hoses, using an air-driven venturi system. This is the same method as was used before, says Donnie Calhoun, field-support foreman, who is in charge of the refilling operation. However, a key difference is that the floor hopper has a capacity of only 200 lbs versus 9,600 lbs for the previously used hopper. In the new arrangement, the bulk bag essentially fills the role of the supply hopper, and the floor hopper is a sealed transfer unit that continually feeds the refill line.

Besides being dust-free, the new unloading system is more efficient than the previous one, says Calhoun. "It used to take about four hours to fill a hopper from the drums, using the auger," he says. "Now, it takes only about five minutes to set up a bag on the discharge frame and start feeding powder."

The Brayton Fire Training Field uses about 13,000 hand-held fire extinguishers per year — each of 20 or 30 lbs. The refill line's weekly volume averages a modest 250 canisters. Most of the extinguishers use pressurized carbon dioxide as a carrier gas for the NaHCO₃, but some use nitrogen. The latter include wheel-mounted extinguishers, of which about 100 are used annually.

Canisters are moved manually along the roller conveyor from station to station. First, residual gas and powder are exhausted from each extinguisher; next, the carbon dioxide cartridge is removed and replaced by a fresh one; then, the canister cap is unscrewed and the hose nozzle is purged with compressed air. At this point, a canister is ready for refill. The container is fitted with a specially designed fill head that creates a vacuum for powder transfer. This is attached to a venturi-driven fill hose from the hopper. Three 8-foot fill hoses are attached to the hopper — one for extinguishers equipped with carbon dioxide cartridges, one for those that use stored-pressure nitrogen, and one for wheel-mounted units. Extinguishers that use nitrogen are first filled, and then N₂ is injected into the canister through an adapter.

The major innovation on the conveyor line is the installation of a dust-collection system. Dust generated by the cleaning operations is removed via overhead hoods and collected on wide-pleated, spun-bond polyester cartridge filters, which are periodically cleaned by back-flushing.

Peddy says, "The bulk-bag unloading equipment is efficient and reliable and has made our job easier by allowing us to move the whole operation into one building." Under the former arrangement, drums of chemical powder were stored in one building, carbon dioxide was stored in another, and the powder-transfer and refill operations were partially outside.

Moving the refill line indoors had another benefit in that it liberated the work from the vagaries of the weather. Peddy explains that refilling must conform to standards set by the National Fire Protection Association and at times it was difficult to meet the standards with the outdoor operation. "For example, we couldn't fill on days when it was raining," he says, "but now we do everything in a building that has a humidity-control system."

More information is available from Flexicon Corp., by calling 610-814-2400, sending an e-mail to [email protected], or visiting www.flexicon.com.