Tips for maximizing tube, pipe mill efficiency (Part I)
Making tube or pipe successfully—efficiently—is a matter of optimizing 10,000 details, including equipment maintenance. Sticking to the manufacturer’s recommended preventive maintenance schedule is no mean feat, considering the myriad moving parts in every mill type and every piece of peripheral equipment. Photo: T & H Lemont Inc.
Editor’s note: This is the first installment in a two-part series on optimizing tube or pipe mill operations. Read Part II.
Even under the best of circumstances, making tubular products can be grueling. Mills are complex, they need a lot of scheduled maintenance, and depending on the products they make, the competition is fierce. Many metal tube and pipe producers are under relentless pressure to maximize uptime to maximize revenue, leaving precious little time for routine maintenance.
These days, the industry doesn’t have the best of circumstances. Material costs are sky high and partial deliveries aren’t uncommon. Tube and pipe producers need to maximize uptime and reduce scrap now more than ever, and receiving a partial delivery means making shorter runs. Shorter runs mean more frequent changeovers, which is not an efficient use of time or labor.
“Productive time is extremely valuable right now,” said Mark Prasek, North American tube and pipe sales manager for EFD Induction.
Conversations with industry experts about tips and strategies to get the most out of a mill revealed a few recurring themes:
Running a mill at its maximum efficiency means optimizing dozens of factors, most of which interact with other factors, so it isn’t necessarily easy to optimize a mill’s operations. Sage words from former The Tube & Pipe Journal columnist Bud Graham provide some perspective: “A tube mill is a tool holder.” Keeping this phrase in mind helps to keep it simple. Understanding what each tool does, and how it does it, and how each interacts with the others, is about one-third of the battle. Keeping everything maintained and aligned is another third of it. The final third concerns the operator training program, troubleshooting tactics, and specific operating procedures unique to each tube or pipe producer.
The first consideration in running a mill efficiently doesn’t concern the mill at all. It’s the raw material. Getting the maximum yield from a mill means getting the maximum yield from each coil that feeds the mill. This starts with purchasing decisions.
Coil Length. “Tube and pipe mills thrive when the coil is the longest it can be,” said Nelson Abbey, director of Abbey Products at Fives Bronx Inc. Processing shorter coils means processing more coil ends. Every coil end needs a butt weld, and every butt weld creates scrap.
The difficulty here is that the longest possible coils might come at a premium price; a shorter coil might be available at a better price. The purchasing agent might want to save some money, but this is at odds with the perspective of the folks on the manufacturing floor. Nearly everyone who runs the mill would agree that the price difference would have to be substantial to cover the production losses associated with additional mill stoppages.
Another consideration is the capacity of the uncoiler and any other limitations at the entry end of the mill, Abbey said. Investing in higher-capacity entry equipment to handle larger, heavier coils might be necessary to take advantage of the benefits of buying bigger coils.
The slitter is also a factor, whether slitting is done in-house or if this service is outsourced. Slitters have maximum weights and diameters they can handle, so getting the best match between the coil and the slitter is critical in maximizing yield.
In summary, it’s an interplay among four factors: the dimensions and weights of the coil, the necessary widths of the slit mults, the capability of the slitter, and the capacity of the entry equipment.
Coil Width and Condition. On the shop floor, it goes without saying that the coil has to be the right width and the right gauge for the product to be made, but mistakes happen. Mill operators often can compensate for a strip width that is slightly undersized or oversized, but it’s just a matter of degree. Careful attention to the widths of the slit mults is critical.
The strip’s edge condition also is an overriding concern. A consistent edge presentation, without burrs or any other inconsistencies, is crucial to maintaining a consistent weld along the strip’s length, said Michael Strand, president of T&H Lemont. The initial coiling, uncoiling for slitting, and recoiling also come into play. A coil that hasn’t been processed with care can develop camber, which is problematic. The forming process as developed by roll tooling engineers starts with a flat strip, not a cambered strip.
Tooling Considerations. “Good tooling design maximizes yield,” said Stan Green, general manager for SST Forming Roll Inc. He pointed out that no single tube forming strategy exists, and therefore no single tooling design strategy exists. Roll tooling suppliers vary, the ways they work the tube vary, and therefore their products vary. The yield also varies.
“The surface of a roll has a continuously changing radius, so the rotational velocity of the tooling varies over the face of the tool,” he said. The tube moves through the mill at just one velocity, of course. Because of this, the design influences the yield. A poor design wastes material when the tooling is new and it only gets worse as the tooling wears, he added.
For companies that haven’t held the line on training and maintenance, formulating a strategy to optimize the mill’s efficiency starts with the basics.
Training Mill Operators. The first concern is the staff and how they are trained.
“Regardless of the style of mill and what it produces, all mills have two things in common—operators and operating procedures,” Abbey said. Running a mill as consistently as possible is a matter of providing standardized training and adherence to written procedures, he said. Inconsistencies in training can lead to differences in setup and troubleshooting.
“Running a mill profitably is a matter of training, training, and more training,” Strand said.
To get the most out of a mill, from operator to operator and shift to shift, it’s imperative that every operator use consistent setup and troubleshooting procedures. Any procedural differences usually are matters of misunderstandings, bad habits, shortcuts, and workarounds. Invariably these lead to difficulties in running a mill efficiently. These might be home-grown problems, or they might have been imported when a poorly trained operator was hired from a competitor, but the origin makes no difference. Consistency is the key, including operators who bring experience with them.
“It takes years to develop tube mill operators, and you really can’t rely on a generic, one-size-fits-all program,” Strand said. “Each company needs its own training plan suited to its own mills and its own operation.”
Making a Routine of Routine Maintenance. The second concern is routine maintenance.
“Three keys in operating efficiently are machine maintenance, consumable maintenance, and the alignment,” said Dan Ventura, president of Ventura & Associates. “The machine has a lot of moving parts—whether it’s the mill itself or peripheral equipment at the entry or exit end, or the runout table, or what have you—and routine maintenance is important to keep it in top condition.”
Strand concurs. “Using the preventive maintenance inspection schedule is where it all begins,” he said. “It provides the best chance to run a mill profitably. If a tube producer responds only to emergencies, it’s not in control. It’s at the mercy of the next crisis.”
Proper alignment is just as critical is routine maintenance.
“Every piece of equipment on the mill has to be in alignment,” Ventura said. “Otherwise, the mill fights itself.”
Keeping the roll tooling in top shape likewise is necessary.
“In many cases, as the rolls are pushed beyond their useable service life, they work-harden, and eventually they crack,” Ventura said.
Understanding tooling maintenance sheds a lot of light on the importance of all of the maintenance.
“If the rolls haven’t been kept in good condition with regular maintenance, the day comes that they need emergency maintenance,” Ventura said. If the tools have been neglected, reconditioning them is going to require removing two to three times the amount of material they would otherwise need removed, he said. It’s also going to take longer and cost more.
Investing in spare tools can help prevent emergencies, Strand noted. If tooling is used frequently for long runs, more spares are needed than for tooling used rarely for short runs. The tool function also influences the spare level. Fins can break off of the fin pass tooling, and weld rolls succumb to the heat of the weld box, which are problems that don’t plague forming and sizing rolls.
“Regular maintenance is good for the equipment, and proper alignment is good for the product it makes,” he said. If these are neglected, the mill staff spends increasing amounts of time attempting to compensate. This is time that could be spent making good, saleable product. These two factors are so vital, and so often overlooked or neglected, that in Ventura’s opinion they offer the best opportunities to get the most out of a mill, maximizing throughput and minimizing scrap.
Ventura equates mill and consumable maintenance to automobile maintenance. Nobody would drive a car tens of thousands of miles between oil changes and let the tires go bald. This is going to lead to an expensive solution or a wreck, and the same goes for a poorly maintained mill.
Regular tooling inspections after every run also are necessary, he said. Checking the tooling might reveal a problem such as a hairline crack. Finding such damage immediately after removing the tooling from the mill, rather than finding it right before installing it for the next run, provides the most time to get a replacement tool made.
“Some companies are working right through scheduled shutdowns,” Green said. He understands that it’s hard to abide by a scheduled shutdown in times like these, but he noted that it’s extremely risky. Shipping and trucking companies are so overloaded, or understaffed, or both, that deliveries just aren’t timely these days.
“If something on the mill breaks and you have to order a replacement, what are you going to do to get it delivered?” he asked. Of course, air freight is always an option, but that sends the cost of the shipment spiraling.
Mill and roll maintenance isn’t just a matter of following maintenance schedules, but coordinating maintenance schedules with production schedules.
“To do this effectively, you really have to think long term,” Strand said.
Breadth and depth of experience count in all three areas—operations, troubleshooting, and maintenance. Companies that have just one or two mills to make tubing for their own use usually have few people dedicated to mill and tooling maintenance, said Warren Wheatman, vice president of T&H Lemont’s Tooling Business Unit. Even if the maintenance people are extremely knowledgeable, a small department simply has a shallower reservoir of experience to draw from than a large maintenance department, putting the smaller staff at a disadvantage. If the company doesn’t have an engineering department, the maintenance department has to do its own troubleshooting as well as the repairs.
Strand added that training for both operations and maintenance departments is more important now than ever before. The wave of retirements associated with the aging of the baby boom generation means that much of the tribal knowledge that once kept companies going through thick and thin is being depleted. And while many tube and pipe producers still can rely on consultations and advice from equipment vendors, even this expertise isn’t as plentiful as it once was, and it too is shrinking.
The welding process is just as critical as any other process that takes place when making a tube or a pipe, and the welding machine’s role can’t be overestimated.
Induction Welding. “About two-thirds of our orders are for retrofits these days,” Prasek said. “They’re usually replacing old, problematic welders. Throughput is the primary driver right now.”
Many are behind the eight ball because the raw material shows up late, he said. “Often when the material finally does show up, the welder goes down,” he said. A surprising number of tube and pipe producers are even using machines based on vacuum tube technology, which means they are nursing along machines that are at least 30 years old. Service knowledge on such machines isn’t abundant, and replacement tubes themselves can be hard to find.
The challenge for the tube and pipe producers still using them is how they age. They don’t fail catastrophically, but they degrade slowly. A workaround is to use less weld heat and run the mill slower to compensate, which makes it easy to avoid the capital expense of investing in a new machine. This creates a false sense that everything is just fine.
Investing in a new induction welding power supply can make a big difference in the mill’s electricity consumption, Prasek said. Some states—especially those with large populations and stressed electrical grids—offer a generous tax rebate following the purchase of energy-efficient equipment. A second motivator to invest in a new one is the potential for new production possibilities, he added.
“Often a new welding unit is so much more efficient than an old one that it provides substantially more welding capacity without upgrading the electrical service, saving thousands of dollars,” Prasek said.
Critical also are the alignments of the induction coil and the impeder. A properly selected and mounted induction coil has an optimal location relative to the weld rolls, and it needs a proper and consistent clearance around the tube, said John Holderman, general manager for EHE Consumables. The coil will fail prematurely if not set properly.
An impeder’s job is simple—it impedes current flow, directing it to the strip edges—and like everything else on the mill, positioning is critical, he said. The proper position is right at the weld apex, but this isn’t the only consideration. The mounting is critical. If it’s affixed to a mandrel that isn’t rigid enough to support it, the impeder’s position can shift and it can actually drag on the ID along the bottom of the tube.
Taking advantage of a trend in welding consumable design, the split coil concept, can have a big impact on mill uptime.
“Large-diameter mills have been using split-coil designs for a long time,” Holderman said. “Replacing a single-piece induction coil requires cutting the pipe, replacing the coil, and rethreading the mill,” he said. A split-coil design, which is in two pieces, gets around all that time and effort.
“They have been used on large mills as a matter of necessity, but it took some fancy engineering to apply this principle to small coils,” he said. The manufacturers simply had less to work with. “Small two-piece coils have specialized hardware and cleverly designed clamps,” he said.
Regarding the process of cooling the impeder, tube and pipe producers have two conventional choices: the mill’s central coolant system or a separate, dedicated water system, which can be expensive.
“It’s best to cool the impeder with clean coolant,” Holderman said. To this end, a small investment in a dedicated impeder filtration system for mill coolant can go a long way toward improving an impeder’s longevity.
The mill’s coolant often is used on the impeder, but mill coolant collects metal fines. Despite all efforts to trap the fines in a central filter or capture them with a central magnet system, some get through and find their way to the impeder. This is no place for metal fines.
“They heat up in the induction field and burn themselves into the impeder casing and ferrite, which leads to premature failure and then downtime to change the impeder,” Holderman said. “They also build up on the induction coils and eventually cause damage from arcing there as well.”
Laser Welding. For stainless steel mills, laser welding has a substantial and growing role. Initially these were laser units with CO2 resonators, but those have yielded the field to fiber lasers. This technology has two characteristics that fit in well with tube or pipe mill operations: consistency and reliability.
“A laser source is extremely stable in how it generates and delivers heat. The weld is consistent all day long,” said Kevin Arnold, Midwest sales manager for IPG Photonics. “The systems have the ability to self-regulate to deliver a consistent output.” Another characteristic is its ability to focus the heat, which means this technology can make a successful weld with less heat input than other welding methods. Less heat equates to less distortion. Also, the highly focused heat often allows mills to run faster than with other heat sources, he said, improving throughput.
Perhaps just as important is the ease of setup and maintenance. Gone are the mirrors and complicated focusing system that need to be kept in precise alignment and scrupulously clean when using CO2 technology.
“The laser beam delivery system is an order of magnitude simpler,” he said.
Closely related is reliability. Fewer moving parts means greater uptime. The dependability of today’s laser machines, compared to first lasers used in metal fabrication decades ago, isn’t even comparable, he said.
“In the early days, manufacturers could live with a finicky laser,” he said. “These days, a fiber laser machine is reliable, day in and day out, working in the background, almost invisibly, like a loyal employee,” he said.
And while it’s understood that laser power sources aren’t inexpensive, the prices have dropped substantially over the decades.
“Twenty years ago, a tube or pipe producer would need tens of millions of dollars in annual revenue to afford a laser welding system,” he said. “These days it’s much less, in some cases as little as $1 million per year.”
Power Outages. Strand noted that some businesses have to deal with occasional power cuts, which is a big problem for processing industries.
“It’s usually not a problem in the U.S., but it’s not uncommon in South America,” he said. As populations grow and get ahead of infrastructure development, this is inevitable. Demand for electricity outstrips the supply, and the power utility (or government) has no choice but to institute rotating intermittent power cuts.
Of course, power outages can and do happen in the U.S. Brownouts during peak consumption periods are a fact of life in some states. Prasek and Strand both noted that some companies embrace occasional power cuts. They get a better rate on the power they use in return for the inconvenience of occasional power shutdowns.Editor’s note:Coil Length.Coil Width and Condition.Tooling Considerations.Training Mill Operators.Making a Routine of Routine Maintenance.Induction Welding.Laser Welding.Power Outages.