A turbine disk rim inspection system developed at Southwest Research Institute has been named one of the 100 most significant technical accomplishments of 1994 by R&D Magazine. The ultrasonic inspection system allows the detection of potentially hazardous stress corrosion cracking (SCC) in the blade attachments of turbine disk rims found in the steam generators of many utility power plants. The R&D 100 winners were honored at an awards ceremony September 18 at the National Museum of Science and Industry in Chicago, Illinois.
Institute co-developers Drew L. Goodlin, James F. Crane, Gary J. Hendrix, and Dennis R. Hamlin overcame two problems that have hindered the success of ultrasonic (UT) inspection of blade attachments. First, the blade attachment hooks emit strong signals that obscure the weaker signals caused by SCC. To address this problem, the new system uses custom-designed acoustic lenses, ground at the Institute, to focus the UT beam so precisely it is possible to easily discriminate between the hooks and the SCC signals.
Secondly, the steam turbine industry uses a wide variety of disk shapes and geometries, each of which require individualized lenses and inspection plans. A vital part of the new system is a set of CAD/CAM programs that allow for the efficient analysis, design, and manufacture of the lenses and appropriate inspection plans for each disk.
As power installations age, says Hamlin, problems caused by stress corrosion cracking are being more widely reported in the industry, and plant managers are often forced to make difficult decisions about plant safety while maintaining operational economy.
Crane adds that, The new inspection system allows fast, accurate, and inexpensive examination of the steeple areas of turbine blades, helping utilities avoid expensive repairs and downtime. Regular, cost-efficient inspections will also allow utilities to plan repair and replacement activities in a more predictable fashion, which translates into savings for the consumer.
Field experience, followed up with destructive inspection in the laboratory, confirmed that the system is able to make a complete and accurate inspection by indicating areas of both mild and severe SCC, thus allowing plant managers to schedule repairs to coincide with mandated inspections that occur every five to seven years.
Magnetic particle testing (MT), currently the most widely used examination method for blade attachments, is confined to exposed surfaces of the steeple region and cannot identify cracks in the steeple serrations and roots unless blades are removed, a costly and time-consuming process. In practical terms this means that MT methods can only make spot inspections of up to five percent of the blade attachment area.
A patent has been issued for the turbine disk rim inspection system, which has been successfully used in Japanese and U.S. power plants.
Published in the Fall 1995 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.