Elbow Design Stops Feed-Line Blowouts
EP Editorial Staff | September 20, 2018
Changing from conventional to deflection elbows on pneumatic lines solves wear problems for injection molder.
The Banjo Corp. (banjorcorp.com) site in Crawfordsville, IN, manufactures a variety of industrial and agricultural liquid-handling products for customers around the world. A unit of Idex Corp., Lake Forest, IL (idexcorp.com), Banjo also operates plants in Brazil and the Netherlands. The dedicated production facility in Crawfordsville runs 36 injection-molding machines that produce plastic items ranging from quarter-inch pipe couplings to large-format centrifugal pump bodies.
The polypropylene that Banjo molds is custom-compounded for maximum durability. According to maintenance-department manager Derek Thompson, the resin is almost indestructible. “You could,” he said, “use a pipe wrench to connect some of our finished fittings and never damage them.” This toughness comes from the 20% or 40% fiber-reinforced polypropylene with which the parts are molded. But the fiber reinforcement increases pellet abrasiveness to the point where they were wearing through the elbows of the pneumatic conveying lines, causing cracks, holes and, on occasion, blowouts.
One particularly memorable spill occurred with a conventional elbow on a line from an outside silo to the injection machines. The result was a 1-in. layer of resin that blanketed part of a roof. “We had to sweep it up, empty the pellets into a bucket, and rope the bucket down to the ground,” Thompson recalled. “It took several men four or five hours to clean it up.”
The silo experience underscored the fact that conventional sweep elbows, including those with ceramic, porcelain, and other types of hard linings, are weak points in pneumatic conveying systems that move large volumes of abrasive pellets. The silo cleanup also activated a search for elbow alternatives. Thompson’s research led him to Smart Elbow deflection elbows from HammerTek, Bethlehem, PA (hammertek.com). They turned out to be even more durable than porcelain-lined options. “We bought the first two in 2009,” he noted, “and they lasted eight years before needing replacement.” Banjo has since installed a total of 24 deflection elbows and associated couplings.
CUSHIONING RESIN FLOW
Unlike conventional sweep elbows, wherein material hits the elbow wall to change direction, the Smart Elbow design features a spherical vortex chamber that protrudes from the 90-deg. angle of the elbow (see diagram below). When pellets enter the
elbow, a portion of the flow is automatically diverted into the chamber. There, it forms a loosely packed ball of material that rotates slowly in the direction of flow.
The rotating mass prevents impact from the resin on the elbow wall and cushions and deflects the flow around the bend. Instead of exiting at the outermost radius of the elbow as with conventional sweep elbows, material exits the deflection elbow in a laminar, non-turbulent pattern spread evenly across the outflow. Furthermore, as Thompson pointed out, less resin flow turbulence reduces conveying noise. As soon as the material infeed stops, the pellets evacuate the vortex chamber and are conveyed downstream.
FROM SILOS TO MACHINES
The 24 deflection elbows in Crawfordsville are cast of ductile iron and connect to pneumatic lines using a coupling and gasket designed to last the life of the elbow. Eighteen are 4-in.-dia. elbows on pneumatic pipes that run from four silos into the main plant. Four 2.25-in.-dia. elbows are installed on inside feed lines to the injection-molding machines. Two 6-in.-dia. elbows are on lines that transport granulator scrap to a storage area.
While Banjo’s main silo stands 60-ft.-high, three others are 30-ft. high. A 4-in. pneumatic conveying line descends the side of each silo, then makes a 90-deg. connection, and runs horizontally for approximately 100 ft. Each line then makes another 90-deg. drop to connect with four indoor surge bins, from which four 2.25-in.-dia. pneumatic lines distribute the resin pellets to the feeding banks, and on to the injection-molding-machine hoppers.
The tallest silo contains 40% reinforced polypropylene. Two of the shorter ones hold 20% glass-filled PP pellets. The other 30-ft. silo holds “white ball material,” i.e., neat, or unreinforced, polypropylene. Although this resin is much less abrasive, the HammerTek elbows were still installed on its feeding line.
The 60-ft. silo holds four truckloads of resin, each weighing 40,000 lb. Thompson said as much as two truckloads a day can be delivered. Transferring resin from truck to silo takes four hours with a vacuum pump that runs off the truck’s power.
RETURN ON INVESTMENT
Banjo’s Crawfordsville facility now keeps its injection-molding operation running at high throughput rates without interruption. The benefits generated by the change in elbows appear to have been considerable. As Thompson characterized them, compared with conventional sweep elbows, the reduced maintenance costs, reduced downtime, and fewer replacements associated with 90-deg. deflection elbows delivered a return on investment of approximately six months. EP