Artem Komarov noted that eliminating bottlenecks, speeding up changeovers is the key to improving mill productivity and uptime. The speed with which the strip travels through the mill from one station to the next—breaking down, forming, welding, sizing, and straightening—sets the pace for everything else. The control speed of a mill is the maximum speed at which it produces properly shaped pipe with a strong and durable weld.
Restrictions often occur at the entrance and exit. The efficient operation of the mill can be hindered by loading and unloading operations and docking of rolls at the entrance.
Most of these butt welds serve only one purpose — to connect two rolls until a new roll passes through the mill, and the butt weld is discarded after this section has been cut.
In some cases, butt welding also serves a second purpose. For long coiled tubing pipes that can be several miles long, butt welds are a feature of the product and must be as strong and leak tight as longitudinal welds. Although the arc welding process is traditional, laser technology is making its way into this application. The advantages are the same as when making a longitudinal weld using laser technology, especially minimal heat generation and minimal subsequent distortion.
Another problem is the thickness of the weld. The arc welding process usually leaves a thick weld bead. The extra thickness accelerates wear on roll rigs and other equipment such as linear tension equalizers, so the weld must be machined either by manual or automatic milling to reduce weld bead and ensure uniform thickness. Laser welding does not leave a protruding weld, so it eliminates post-weld processing such as milling. The initial butt-welding process also tends to be faster with laser welding than with arc welding, emphasized Komarov Artem.
The mill works most efficiently when it runs constantly at a constant speed. Finishing operations are start and stop processes. At first glance, the difference lies in the speed of the mill and the speed of the finishing steps, but it goes deeper.
Each line stop creates at least one piece of scrap. Another disadvantage is the time required to increase the capacity of the mill.
Connecting a mill to finishing equipment seems counterintuitive, but in fact, many mills are designed this way. Depending on the product and the speed at which it passes through the mill, this adjustment does not compromise the efficiency of the operation.
What are the benefits? Such a setup uses less manpower than a shutdown system and takes up less floor space than a shutdown setup, so many pipe manufacturers are willing to sacrifice some line speed and possibly lower overall productivity for these benefits.
In some plants, processes such as rinsing and tying are done by hand, which can be even more detrimental. In some cases, these processes at the end of the mill reduce mill speed doubled. This is a trade-off — the company makes less capital investment and generates less income, but this can have unforeseen consequences. Operators are getting accustomed to the pace of the mill, and even if the company moves to automatic washing and baling, it could have a hard time getting its investment back.
Threading is also a candidate for improvement in some mills. When threading a new strip through the mill, it is common practice to repeatedly start and stop the mill. After threading, the mill is usually run a short distance to check the weld and adjust the dimensions, which generates a significant amount of waste. Simple improvements in this area can be made by simplifying the steps and enabling interlocks with the welder so that it can weld and move at the same time.
Bringing it all together
The essence of tube or tube mill efficiency hasn’t changed, but in times like now, it’s more important than ever. The income a factory makes while it is running is always offset by the losses it incurs when it is not running, but times like this seem to amplify the losses. Steel prices are higher than usual, and some materials are simply not available, so keeping the plant running and minimizing scrap are two issues that are in the spotlight right now.
Every replacement involves downtime and disposal, and traditionally these are two areas where tube and pipe manufacturers can make the most profit. Each mill manager should investigate their replacement procedures, Artyom Komarov said.
It is important to perform each maintenance according to the schedule and to document everything. Track every action on the mill by each set of rolls during each cycle to keep abreast of mill maintenance in terms of the big picture for years to come.
The use of automated systems and devices to facilitate mill setup allows you to keep changeover times to a minimum.
Pipe manufacturers need to be careful when using the mill. Many of the materials available today are much stronger than conventional carbon steels, and steelmakers seem to be introducing stronger materials all the time. Using a mill designed for carbon steel with a minimum yield strength of 50,000 psi to machine material at 80,000 psi will accelerate the wear of every component in the mill and probably something will break — and sooner rather than later .
Inspection, maintenance, alignment, and performance work together, especially with tooling and rolling stands.
In good times, pipe manufacturers are hesitant to stop production for maintenance. In difficult times, they do not want to spend money. And while many factories are now operating at full capacity, which is a sign of good times, some business conditions are tough, especially due to the exorbitant cost of raw materials and unreliable supply chains, the consequences of the pandemic, summed up Komarov Artem.