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No More Conveying Line Blowouts
After a successful trial in 1995, the company gradually began applying the wear-resistant cladding to various vulnerable plant process parts, including the cyclone pictured here.
Bowles is a compounding equipment asset coordinator at Solvay Advanced Polymers, Augusta, Ga. Solvay produces high-performance polymers that are used in many demanding applications in the automotive, aerospace, industrial, food service, medical, and electronics industries. One of the company’s products AMODEL polyphthalamide, a semi-crystalline polymer, presents particular problems because of its extremely abrasive nature. Some AMODEL compounds contain a high percentage of glass. The polymer-glass combination provides high strength, stiffness, impact resistance, and heat resistance. These qualities also make the glass-filled 3-millimeter plastic pellets hard on conveying lines and processing and handling equipment. Battling BlowoutsOn their journey from pelletizer to packaging, the pellets travel through hundreds of feet of conveying lines and equipment the plastic material travels from the extruder to a strand conveyor or water bath. From there, it’s fed into the pelletizer, which cuts the strands into pellets. The pellets are pneumatically conveyed into storage bins, which, because of the Solvay unit’s high production rate, are 4 stories tall. This means that the pellets have to be conveyed vertically 120 feet before being gravity-discharged into the storage bins. From the bins, the compounded resin is fed into the packaging area to be bagged or boxed. The pellets travel at up to 5,000 fpm in the dilute-phase pneumatic conveying lines, racing through straight sections and around corners. On their way, their extreme abrasiveness works away at the conveying line piping and equipment and eventually blows out pipes and elbows. In fact, Type 304 stainless steel pipes were blowing out in as little as 1.5 weeks. “Every time you change the conveying line’s geometry – every time a line turns, every time there’s an adapter from one component to another, every time the line is interrupted by a fabricated valve or other equipment – you get wear,” says Bowles, “and we had a lot of wear.” In addition, he says, “We have a lot of plant real estate, and not all of it’s readily visible to our operators. Because of this, our operators had to be constantly on the prowl, looking for blowouts and vulnerable spots on the line.”
Often, those vulnerable spots weren’t recognized in time, and the company was losing more than $200,000 of product a year to undetected blowouts. (One hour of undetected blowout costs the company $4,000 in lost product.) Blowouts not only caused lost product and, consequently, yield reduction, but cost the company in process disruptions and caused safety hazards and housekeeping problems. The company was budgeting $60,000 annually in labor just to repair worn and blown-out piping and equipment. “We were doing repairs almost on a daily basis,” says Mike Clark, a fitter from the company’s pipe department. When blowouts were discovered, operators fixed them quickly to prevent material loss, sometimes using duct tape or temporary patches until someone from the pipe department could come in and do a proper weld repair. However, weld repairs weren’t really a good solution because the weld weakened the surrounding metal, making an even larger area of the piping vulnerable to future blowouts. Eventually the company began using unsightly premade patches that could be clamped on. While these worked for a time, Bowles says, “They gave the message to employees and customers that we accept blowouts as normal. That’s not the message we wanted to send.” Working reactively to blowouts rather than proactively to prevent them didn’t make sense to Bowles. The company began looking for a permanent solution. In addition to preventing excessive wear, it would have to allow operators to easily and quickly clean the conveying line piping and equipment to prevent cross-contamination during color changes. “Because of the extensive plant real estate,” says Bowles, “over time, we’ve had a lot of opportunity to try various solutions.” While several of these helped incrementally, only one was wholly satisfactory. Switching elbowsThe conveying lines were particularly vulnerable to abrasion damage at corners and bends, so the company tried using various elbows, including standard Type 304/316 stainless steel elbows, long-radius bends, pocketback and deflection elbows, ceramic-lined elbows, and glass and glass-lined elbows. Standard elbows only had a 7- to 12 day service life in low-pressure conveying (4 psi), so they were completely unsatisfactory for this application. The long-radius elbows seemed like a good idea. The pellets should travel smoothly around them, making little impact against the piping. However, the company found that the longer turn simply meant there was more cross-sectional area for blowouts. The pellets also degraded in these elbows, leading to more dust, fines, and angel hair and loss of product quality. Other promising elbows were the pocketback and deflection types. Instead of being a smoothly curved elbow, these elbows have a pocket or a widened area at the bend’s outside apex that deflects the material, reducing the pellets’ impact and abrasive contact with the elbow wall. With the pocketback elbow, material travels to the bend, where some material forms a loose, revolving mass in the pocket that deflects the other material passing around the bend and reduces its abrasion to the elbow. The deflection elbow has a widened area where some of the material forms a slow-moving plate that the rest of the material travels along instead of abrading the elbow wall. Both elbow types are often used in abrasive applications. However, while these elbows extended the wear in the AMODEL application, they didn’t prevent blowouts, and their service life was only 9 to 12 months in this application. In addition, because some material would remain in these elbows, they had to be removed for cleanout when changing colors or product grades. Extended process downtime to do this was unacceptable. The company also tried ceramic, glass, and glass-lined elbows. The biggest problem with the ceramic-lined elbows was contamination. The ceramic substrate is made of ceramic beads, and occasionally beads would come off the lining and mingle with the plastic pellets. A 1-millimeter ceramic pellet could short out a customer’s circuits, plug a feed gate in automatic molding equipment, or damage the quality of products molded from the pellets. Since the ceramic beads can’t be detected by a metal detector, there was no way to ensure that the plastic pellets remained ceramic-bead-free. Similar problems happened with glass and glass-lined elbows, which had to be replaced regularly after about 12 months to prevent breakage.
Why not use heavier material for the piping? “We did try this,” says Bowles. “Initially, we used schedule 10 stainless steel, which failed in less than a week. We switched to schedule 40, which lasted about three weeks. We tried Hastelloy and other alloys, but we never got more than about three months out of different metallurgy.”
Modifying conveying methods Blowouts didn’t happen only at elbows. They also happened in straight pipe sections, particularly at transition areas, and abrasion damage occurred in various pieces of equipment, including valves, cyclones, and samplers. So the company considered addressing the abrasion problem holistically: Maybe using a different conveyance method would be the answer. The company explored using dense-phase pressure conveying and spiral and vibratory mechanical conveying. Dense-phase conveying would have had some advantages. It uses a lower conveying velocity than dilute-phase conveying does, and it causes less pellet attrition and fines generation. However, the initial investment involved in switching to this conveying method would have been substantial, the system would have been more complex, and cleanout procedures would have become more difficult. Spiral and vibratory conveyors would also have been expensive and would have had more limited conveying parameters than a pressure system. (The storage bin height, in particular, would have been a problem.) Another issue for these methods was Georgia’s high summer temperatures and humidity. The plastic pellets are extremely hygroscopic, and spiral and vibratory conveyors would have increased the product retention time and exposure to the environmental moisture too much. Bowles says that the pellets can be pneumatically conveyed to the bin tops in 2 seconds, but using a spiral or vibratory conveyor would have taken 15 minutes. “We would have had to do expensive nitrogen purging to get rid of the moisture,” says Bowles. “It definitely wouldn’t have been cost-effective.” Solving the problemFortunately, not every method the company tried was a failure. In fact, one method worked well – infiltration-brazed tungsten carbide cladding, applied by Conforma Clad Inc., New Albany, Ind. The supplier provides severe-wear solutions for metal components used in abrasive, corrosive, and erosive applications. Solvay had extrusion equipment that the wear-solutions supplier had treated with cladding, and its wear was impressive, so in 1995 Bowles decided to try the cladding on some short-radius 45- and 90-degree elbows in 4-, 6-, and 8-inch conveying line pipe. The cladding is an alternative to weld overlay wear-resistant coatings and, when applied correctly to the interior of conveying line piping and to wear surfaces of other equipment, provides increased erosion protection with less added weight, says Chad Juliot, Conforma Clad industrial applications engineer. The cladding is a nonwoven preformed “cloth” infiltrated with densely packed tungsten carbide particles. The brazing process metallurgically bonds the tungsten carbide to the base metal, forming an extremely wear-resistant cladding that can be formed to complex geometries and that wears at a consistent, predictable rate. Since one of the company’s concerns was the unpredictability of pipeline blowouts, this was a definite plus. The cladding is resistant to chipping, cracking, or flaking. It’s typically only 0.03-to 0.06-inch thick and, says Stan Branham, a welder in the company’s pipe department, “It’s so much better than the chrome coating we tried.” The cladding isn’t something that can be applied on the spot to fix an unexpected blowout. Although the supplier does produce preformed replaceable liners for conveying line pipes and other components, in most cases components are sent to the supplier, which applies the cladding to all or part of the component at its plant. The supplier’s engineers evaluate each component and the application in which it will be used to determine the proper tungsten carbide formula for optimum wear protection. Then, using CAD software and digital measuring equipment, they thoroughly measure and document each component to design a cloth that conforms perfectly to the component or the component’s vulnerable areas.
Cladding success From Solvay’s very first experiments cladding the short-radius elbows, it was clear that the process worked. In fact, elbows the company formerly had to replace in days or months have now been in use with no problems for 2 and 3 years and longer. At least one piping section that was part of the original experiment 9 years ago is still in use, according to Bowles. Bowles was so impressed with the cladding that he gradually clad more parts of the system. “When we started to learn how to identify our problems, we worked on the most aggressive first – the pneumatic conveying lines, which lasted only days or weeks. Gradually we caught up with the less aggressive wear too – the gravity lines, which lasted three or four months,” he says. In 1997, the company clad the 30inch downstream piping sections after elbows and turns. In 2001, it clad the diverter valves. From 2001 to 2003, it clad the pelletizer strand guides and the extruder discharge transition chutes. In 2002, it clad the cyclones. In 2003, it clad the gravity systems, special transition pieces, sampler transitions, and other parts. In 2004, cladding additional miscellaneous parts continues. The company also keeps spare clad parts on hand to use when unclad parts blow out or show signs of excessive wear, and Bowles now specifies the cladding on new equipment. He says he’s found that wear-coating on OEM equipment often has limitations, so he specifies this supplier’s cladding. “At first, it was hard to convince company management to spend the extra money to do the cladding,” says Bowles, “but now we’ve got so much data showing how much money in labor, parts, and lost product we’re saving that it’s no problem. It’s part of our standards, and we write it into any new project plan. Payback is generally within four to five months because of our yield improvement and elimination of product loss due to blowouts.” The cladding isn’t inexpensive. Bill McConnell, maintenance department planner, says that it costs about 5 times as much as stainless steel, and Bowles reports that the company has spent more than $140,000 to go back and fix places in the system where other, less satisfactory fixes had previously been used. However, company welders haven’t had to do any work on parts that have the cladding, according to McConnell. Mike Clark adds that they’ve seen an 80 to 90 percent decline in conveying line blowouts. Best of all, says Bowles, “It’s a walkaway solution. Now we don’t have to worry about it or have operators on the constant lookout for blowouts and vulnerable areas.” PBE Note: To find other articles on this topic, go to www.powderbulk.com, click on “Article Index,” and look under the subject heading “Abrasion resistance,” or see Powder and Bulk Engineering’s comprehensive “Index to articles” in the December 2003 issue. Reprinted with permission from Powder and Bulk Engineering, October 2004 |
