Pneumatic conveying pipe bend with raised-diamond interior pattern works like a gem.
After trying many pipe bend styles, a plastic resin manufacturer finds one that's lightweight, durable, and that reduces plastic pellet degradation.
Quantum's USI Division, Morris, Ill., manufactures low-density polyethylene (LDPE) and linear LDPE (LLDPE) resins. The company first manufactures ethylene, then synthesizes the ethylene into plastic and extrudes the plastic into 1/8-inch by 1/8 inch pellets. The finished pellets are fed through a rotary airlock into the first leg of a dilute-phase pressure pneumatic conveyor that's approximately 1,100 feet long overall. The conveyor has 8-inch pipes and uses 3,000 scfm of air supplied by a positive-displacement blower.
The conveyor's first leg moves the pellets approximately 500 feet to a bottom-fill blending silo, which blends the pellets to ensure product uniformity. During blending, a warm air purge applied continuously to the blender's bottom degasses the pellets by driving off any residual ethylene. After blending and degassing, another leg of the pneumatic conveyor moves the pellets approximately 300 feet to a unit that uses counterflowing air to remove fines and streamers (fine strings of plastic). The next leg then moves the pellets approximately 300 feet, where more fines and streamers are removed before the pellets are loaded into railcars.
Pneumatic conveying pipe bends wear out in 6 months
Since the Morris plant's 1971 startup, supervisor Wayne Mace has tried several different types of pipe bends in the plant's pneumatic conveyor. The goal has been to minimize pneumatic conveying pressure drop, streamers, and bend weight while maximizing durability. The original pipe bends had a long bend radius of 12 line diameters (8 feet) to minimize pressure drop. To prevent streamers, each bend's interior was sand blasted to present a rough surface so conveyed pellets would roll along the interior. (Along a smooth surface, conveyed pellets slide, causing friction and heat that partially melt the pellets' surface; the melted plastic then flows from the pellets, forming streamers.)
The pipe bends were fabricated from Schedule 10 (about 1/8-inch thick) Type 6063T6 aluminum with a Brinell hardness of 73. Despite the long bend radius, each pipe bend was lightweight, so a worker could position the bend during installation. However, the bends wore through in 6 months on average.
Plant tries many pipe bend styles
To get a more durable pipe bend, in 1972 the plant switched to stainless steel bends with the same long radius as the previous aluminum bends. The stainless steel bends were harder and more durable, but their increased weight meant that either a lifting device (a come-along) or a cherry picker was required for installation. Installing the heavier bends created a potential safety hazard. In addition, the bend interiors were difficult to roughen through sand or shot blasting. As a result, the plant had to frequently sand or shot blast the bend interiors to prevent streamers.
The same year, the plant replaced some of the stainless steel pipe bends with stainless steel offset-tee bends.. The bends prevented pellet degradation by creating a cushion of pellets in each bend's offset tee. The bends were durable but tended to add pressure drop to the pneumatic conveyor. As a result, the plant could install only a limited number of the bends or risk a pneumatic conveyor plug.
In 1985, the plant began manufacturing LLDPE. To compensate for the conveying changes brought on by conveying LLDPE, the plant replaced some of its offset-tee bends on a trial basis, installing offset-tee bends with rounded ends designed to better cushion material and reduce bend wear without adding too much pressure drop. However, the bends weren't self cleaning and were susceptible to cross-contamination of product.
In 1989, the plant tried pipe bends fabricated from Type 6061T0 aluminum, each with a bend radius of 3 line diameters (2 feet). The aluminum had a Brinell hardness of only 30 so the aluminum would bend to the tighter radius. To compensate for the softness, the bends were made of Schedule 40 material (about 1/2 thick). The bend interiors were shot blasted to prevent streamers. However, the bends lasted only 4 months on average.
In 1990, the plant tried a pipe bend with a 3-line-diameter (2-foot) bend radius made of Type 356-T6 cast aluminum, which is lightweight and has a Brinell hardness of 85. The bends showed very little wear after a year's service, but, as with the stainless steel, the harder surface required frequent sand or shot blasting to prevent streamers.
Pipe bend with raised-diamond interior pattern reduces streamer formation
In 1991, the plant tried another Type 356-T6 cast aluminum pipe bend with a 2-foot bend radius. The bend's interior surface was cast in a raised-diamond pattern to present an irregular surface that would reduce streamers. The raised-diamond pattern was cast so that conveyed pellets would impact the diamonds' long sides. In operation, the raised-diamond pattern imparts a rolling motion on conveyed pellets and also creates eddy currents that cushion conveyed pellets.
New bend reduces previous conveying problems
An inspection of the pipe bend after handling more than 200,000 pounds of plastic pellets revealed minimal wear. Quantum's USI Division has since been switching exclusively to the bends, through attrition.
"Presently, we have more than 50 of the pipe bends installed," Mace said. "It's an ongoing project; as the other bends wear out, we replace them with the new ones. None of the new bends have worn out yet." The bends' increased durability and light weight have reduced maintenance and replacement costs. The raised-diamond interior pattern reduces streamer formation, improving product quality. The bends also clean out completely between conveying runs, elimination the risk of cross-contamination. In addition, the bends don't introduce unacceptable pressure drop into the conveyor, reducing the risk of a conveyor plug.
The pipe bends' overall results mean that other Quantum USI Division plants may soon switch. "The success of these pipe bends has been relayed to all the plants within the company," Mace said.
-Reprinted from Powder and Bulk Engineering