Electric-arc furnaces are complex assemblies, with sub-assemblies aligned to each other to ensure the main functions of the furnace: loading the metals, heating to melt the steel, and pour the melted steel into a ladle. To achieve this, the furnace is composed of 2 main structures: the tilting base and the rotative structure. The tilting base rotates around a horizontal axis to allow the whole structure to pour the melted metal into a ladle. The rotative structure is located on the tilting base and rotates around a vertical axis to open the roof. The rotative structure also supports 3 electrodes mounted on 3 translating columns. These columns translate up and down during the heating process to move the electrodes inside the furnace, through 3 holes in the furnace roof. Electrodes must be well aligned with the furnace center. The furnace itself is composed of 3 major components: the bottom shell, the middle shell, and the roof. The bottom and middle shells are fixed on the tilting base and the roof is attached to the rotative structure, so when the rotative structure rotates, it opens the furnace by the roof to load the furnace with metals.
What are the Alignment Requirements and Conditions?
(Harsh Measurement Conditions)
Of course, all components must be aligned together, but the main requirements are: the 3 parts of the furnace must be centered and leveled, the 3 electrodes must also be centered with the furnace and well located to each other, the tilting base must be leveled when in production position, the rotative structure must be aligned with the furnace, and obviously no collision is allowed. Additional requirements can be: the columns supporting the electrodes must be well located and leveled, pouring spout of the bottom shell and charging door on the middle shell must be well located, bending of the structures must be checked, the location of the whole tilting structure into the mill shop must be controlled.
In these situations, installations can be very old and no reliable references are already set precisely. The references are then to be determined and set to have a reliable coordinate system to locate all the components. Moreover, the environment is really harsh in such facilities, involving high or cold temperatures, dust, vibrations, air flows, rust, very few lines of sight, few inches of dirt on the structures, a lot of people working around with activities such as grinding and welding, long and exhausting shifts, high safety level, and really short time to perform measurements. All can have a large impact on measured data. Fortunately, our teams are experts in such conditions and always take into account the uncertainty factors in their measurements.
A recent project undertaken by Amrikart was to replace the bottom and middle shells of the furnaces during a maintenance shutdown, without changing nor modifying any other components. The new shells were larger and different than the previous ones, and the need was to align them precisely into their environment. Amrikart has proven expertise in high precision 3D metrology. Its teams are experts in measurement, alignment, tooling, and automation.
Four steps for a ‘good at first time’ process
The Amrikart team planned the replacement ahead, and we were able to realize the replacement with a ‘good at first time’ methodology, defined in four steps.
(Using ScanStation and Laser Tracker to scan and measure a new middle shell)
First step was to 3D scan the whole actual environment (with the old furnace) using used a Leica P40 scanstation. This scanstation allows to take very quickly a lot of data, and is very effective for scanning a whole environment and then perform analysis such as collisions. The scans were also used to define all the references and features required for the new shells location. The electrodes and features on the rotative and tilting structures were used as references for the application.
Second step was to 3D scan the new bottom and middle shells. Again the P40 scanstation was used with Leica AT960 laser tracker measurements added at this step, for more accuracy and for its live tracking ability. The tracker is really stable and robust for such environment, it is also really quick to setup and has a fast warm-up time. Amrikart engineers used it to define targets on the new shells that were live tracked during the alignment process.
Third step was to simulate the fit of the new shells within the actual environment. Even if some features were not measurable, being invisible, the Amrikart technicians were able, with the simulation, to check for the collisions and define the rework for both new components and actual environment to fix the majority of collision issues, way before the shut-down. This simulation was very important during the alignments, as it was updated regularly with the real location of the new shells.
Fourth step was the alignment of the new shells itself. During this step, many of the structures and features were removed, revealing the previous missing features which were measured with update to the fit and simulation.
Removing the structures removed also a lot of references and features for the alignment. The only way to check for collision and alignment was by using the simulation. The Leica AT960 laser tracker was used during the shell alignments. It was possible to align the laser tracker on the 3D scan data from Leica P40 scanstation with common features and targets. During all the shell alignment process, the laser tracker tracked the location of the new bottom and middle shells in live, and drove them at the correct location.
The fit simulation was updated continuously to check for collisions with the structures. The optimum location wasn’t achievable because of non-reworkable collisions in structure beams, but it was possible to determine a compromise location that fited all the requirements. Once the shells were located at the right location the structures was fixed in place, and a final check performed and reported before the reassembly of the whole furnace. During the first functional test of the furnace, everything went perfectly well, as predicted by the simulation.
This process in four steps alowed to minimize the timeframe required for the whole maintenance operation. Before 3D measurements, replacement process was ‘try and rework until it is good’ operations, leading to a huge time loss in a lot of deassembly and reassembly operations of the whole structure.
Using 3D measurements methods, a scanstation, a laser tracker, and fit simulation, allowed the complete operation to be ‘good at first time’. Several days were gained when compared to the previous try and rework process with added quality control guaranteeing the right location of all the structures before the final reassembly.
The Amrikart team were certain everything would go well at the functional test since all had been controlled with 3D metrology. When issues arose due to unfixable collisions, the dimmensionnal information was available to find a viable alternative, locate, and fix the structures in configuration. Moreover, the measurement data allowed checking for the other secondary requirements, even after the end of the shutdown and Amrikart are able to plan the next maintenance operations with up-to-date dimensionnal data. All of the 3D data and analysis are archived and available for the next furnace replacements 15-20 years from now.
Author : Jeremy Arpin-Pont