In upgrade projects for pulp mills, design plays a crucial role in ensuring the maintenance shutdown stays on schedule. During maintenance shutdowns, improvement modifications are often made, and precise measurements are essential for their planning. Sometimes, up-to-date documentation is not available, necessitating on-site measurements. The targets for measurement at a pulp mill are often confined, tall, or have challenging or manually impossible-to-measure surfaces. In many cases, laser scanning is the only viable option.
In batch cooking, large pressure vessels, often tens of meters tall and several meters in diameter, are used. In this project, laser scanning was utilized for the replacement of screens in a pulp digester by measuring the diameter and shape of the pressure shell for both the cylindrical portion and the spherical top end, where new screens and supporting structures were installed.
Measurement data was collected both before the actual maintenance shutdown and during the shutdown itself. Before the shutdown, laser scanning was carried out through an access hatch by attaching the scanner to a 5-meter-long pole. This measurement helped determine not only the locations of the screen zones but also the diameter and shape of the cylindrical portion of the vessel where the screens were to be installed. With this information, the screens and supporting structures were designed to fit snugly into the vessel shell, and any deformations in the vessel's nominal dimensions during the test were identified.
Typically, the screens installed at the top of this type of digester are installed at approximately a 45° angle, as in this example case. This, combined with the spherical end, poses a challenge for designing the clearance spaces, making it crucial to determine the shape of the digester shell for the design of supports and screens.
The scanning of the spherical end was carried out during the maintenance shutdown after the removal of the old screens and support structures. Two scanning points were sufficient enough to determine the surface shape of the shell, and enough data was collected to define the starting point for reference points. 3D scanning is superior for measuring spherical surfaces. The measurement data can be collected in three dimensions and quickly, which was one of the important criteria in our example case.
After the laser scanning, we were able to provide a measurement report to the installation team within just a few hours of data collection. Based on the report and the determination of the starting point, the installation team could begin installing the support structures. Following the initial scanning of the first boilers, a scan of the upper ends of all ten boilers included in the project was ordered. Andritz's project manager and installation supervisor, Ville Ahokas, expressed satisfaction with the outcome:
The supports installed in areas pre-scanned in advance also fit perfectly into place. Cost savings were achieved as no allowances needed to be left for the supports, and the parts fit into place exactly as they were delivered from the machine shop.
