Upgrading a seamless mill can improve yield, quality
December 2, 2008
Many seamless tube producers in North America use cross-roll piercing mills built in the 1950s that were based on designs from the1930s. While it would be advantageous to replace such aged equipment, that isn't always necessary. A minor equipment upgrade can do wonders. Improving the bar steadiers—the devices that hold the mandrel the steady as the pipe exits the mill—can greatly improve the mill's output and reduce the pipe's wall thickness variation.
Many products seem to be in a continuous state of revolution. Equipment steeped in electronics, such as computers and communications gear, are two examples. It seems that the basic practices and equipment of these industries go through major upheavals almost daily. At the root of this phenomenon is the relentless advance in integrated circuits, which double in performance approximately every 18 months.
Other industries change slowly. For example, in North America a substantial amount of the equipment used for producing seamless pipe is decades old. And while quite a bit of this equipment performs well, it can be improved substantially by making minor, incremental changes. It doesn't need a sweeping upgrade. In other words, by taking advantage of just a few new technologies, seamless pipe mill operators can coax their machinery to evolve into a more productive state. One strategy involves upgrading the bar steadiers, which are found on the outlet tables on cross-roll piercing mills.
The first seamless pipe produced on a cross-roll piercing mill in North America
was made by National Tube Co., Ellwood City, Pa., in 1895. This new process was needed to supply the demand for high-quality seamless tubes for bicycle frames. In the years that followed, many market changes necessitated equipment upgrades, but by 1950 the changes slowed and eventually stopped. Many North American seamless pipe mills in use today were built in the 1950s based on designs developed in the 1930s.
Myriad seamless pipe production processes are in use these days—elongation, stretching, straightening, cold drawing, and so on. Regardless of the process, the first step is usually a matter of poking a hole through a red-hot solid billet. The quality of the piercing process has a major effect on the downstream processes and the finished product.
During the primary piercing operation, when the red-hot steel billet is rolled on the cross-roll piercing mill, the pipe "grows" out of the mill over a very hard plug held in place between the mill rolls by a long, water-cooled mandrel. The length of the mandrel effectively determines the maximum length of the pipe, so for maximum productivity, the mandrel is as long as possible.
For practical purposes, the plug's OD determines the pipe's ID, and the OD of the mandrel is less than both. In other words, a substantial clearance separates the pipe's ID and the mandrel's OD. The pipe and mandrel rotate together between 200 and 800 revolutions per minute (RPM). It looks like a red-hot sewer pipe rattling around a flagpole. The motion is violent and can appear to be a catastrophic failure in the making.
The machines that control the movement of the mandrels and the pipe as it exits the piercing mill are called bar steadiers. These machines have three rolls in a movable steel frame that clamp down on the mandrel and hold it in place while the pipe and mandrel rotate down the mill. As the pipe grows, the bar steadier rolls must open partially in sequence to allow the pipe to grow past each steadier.
When the entire pipe has been pierced, the mandrel and plug are pulled out of the pipe, the top roll of each steadier opens completely, and the pipe is removed. Then the mandrel moves back into position and the steadiers clamp back down on the mandrel to await the next pipe. Most mills have several steadiers (See Figure 1 and Figure 2).
Manufacturing specifications tend to become more stringent as technologies improve. The specifications for seamless pipe's wall thickness variation are one example. Holding the mandrel as firmly as possible during the piercing process is the key to decreasing the mandrel plug's motion and therefore reducing this variation. Early piercing mills had no support for the mandrel bar and supported only the exiting shell.
The first design for mandrel support was a split pivot guide commonly referred to as a "lemon squeezer." Each exit table had several lemon squeezers that would provide minimal support for the mandrel and pivot apart to open for the exiting shell. Lemon squeezers had inserts that had to be changed manually for different mandrel diameters.
The next design used four rollers on a common pivoting bracket. These steadiers were manually adjusted for different mandrel diameters, so inserts were not required. The next and latest design was the aforementioned three-roller apparatus, which uses electrically actuated stops for adjustment and easier maintenance.
A traditional three-roller bar steadier employs simple directional hydraulics to move the rolls around the mandrels and mechanical stops to define the clearances. These mechanical stops are adjusted by jack screws, drive shafts, and an electric motor. They can be adjusted to different mandrel diameters, but the design isn't easy to use or accurate. Usually the adjustment is either too tight (which results in excessive roll and mandrel wear) or too loose (which allows mandrel and plug movement during piercing). This type of system adjusts all the steadiers simultaneously with one motor; therefore, tuning each individual steadier for optimum operation is difficult. The jack screw-drive shaft arrangement between the steadiers complicates removal and replacement of individual steadiers.
Several cross-roll piercing mills have undergone this type of upgrade, including mills operated by The Timken Co., Canton, Ohio, and Michigan Seamless Tube, South Lyon, Mich. The scope of these projects was straightforward: Minimize the changes to the mechanical design while improving accuracy, reliability, and ease of maintenance. Two additional design considerations were functional separation and quick installation. To the greatest extent possible, the new equipment would operate as a stand-alone system and have minimal interaction with existing systems on the mill. Installation would be as short as possible to have the smallest effect on the mill's downtime.
The mechanical and hydraulic upgrades were similar for each project. The biggest differences between the projects were the electrical and automation systems.
Mechanical and Hydraulic. The new systems used the basic three-roll mechanical design with minor changes to the bearings and bushings. Because the mechanical design already employed a mechanical linkage to keep all three rolls evenly spaced around the passline, the upgrade replaced the jack screw-adjustable stop with a hydraulic cylinder with closed-loop position control. A new hydraulic cylinder with a position transducer, a high-performance proportional valve, and a digital controller were incorporated to control the rolls' position. The upgrade didn't affect the second hydraulic cylinder and directional valve arrangement, which opens the top roll only at the end of piercing. The original mechanical stop adjustment system was removed.
A new hydraulic power unit provided oil to the bar steadiers. This minimized the potential start-up and operating problems that would occur from operating a new position-controlled hydraulic system with an existing hydraulic power unit and piping system. A new hydraulic system minimized contamination problems.
The hydraulic power unit and valve stand were arranged to prevent long, interconnecting pipe runs and use hoses for the shorter runs and final connections. Because the pipe runs must be cleaned after they are plumbed and welded, short runs are preferred. Long runs must be cleaned in place, which is more expensive than sending short runs out for cleaning.
Electrical and Automation. The modifications and additions to the electrical and automation systems were based on the customer's requirements and familiarity (or comfort level) with new technology. A typical system is supplied as a complete package, with a dedicated PLC and motor control center (MCC) for the hydraulic power unit. The new PLC has a multiple-axis, dedicated motion control card to control the new bar steadiers. The interface with the existing automation system was bus wiring. Other projects used existing PLCs and MCCs but added dedicated motion control cards.
One project upgraded to five single-axis, dedicated digital position controllers, one for each individual bar steadier, with each controller interfacing to the main mill controller via an RS-232 serial link and relay logic.
Designing a system as a stand-alone unit as much as possible increases the likelihood of a smooth start-up because it facilitates testing the system before shipping it and installing it. It also reduces the downtime needed for the installation. Every minute of testing and troubleshooting before shipping the unit is one less minute of downtime after installation.
Each mill that has installed the new bar steadier system has reported improved tube quality as well as increased production. Tuning each bar steadier individually to the proper setting allows proper mandrel settings without subjecting the steadiers to unnecessary loads. The electrohydraulic control systems as well as the hydraulic power units have been operating with few problems.
These projects were not even close to the effort needed to create a new mill. In fact, they weren't even considered major modifications. Developing a design that blended together the best aspects of older mechanical designs with new control technologies allowed these projects to bear fruit with minimal investment or risk.
Everyone would like to have a new mill, but the reality is that purchasing a new mill is a rare event. This does not mean that the production practices that exist today—which, in many cases, haven't changed in decades—cannot evolve to the point that the output catches up to modern standards. A small, evolutionary step such as an upgrade is not a big change, but it can produce a substantial improvement.