Southwest Research Institute® (SwRI®) News

Turbine Disk-Rim Ultrasonic Inspection System

San Antonio, Texas -- September 1, 1995 -- A turbine disk rim inspection system developed by engineers at Southwest Research Institute® (SwRI®) has been named one of the 100 most significant technical accomplishments of 1995 by R&D Magazine. The ultrasonic (UT) inspection system allows accurate and cost-effective detection of potentially hazardous stress corrosion cracking (SCC) in the blade attachments of turbine disk rims, components found in the steam generators of many utility power plants. Principal investigator James F. Crane and Dennis R. Hamlin, and the other R&D 100 winners will be honored at an awards ceremony September 18 at the National Museum of Science and Industry in Chicago, Illinois.

"SCC can lead to catastrophic failure," says Hamlin. "As utility installations age, SCC is more widely reported, and has created the need for more frequent inspections. At the same time, privatization of the power generation industry in the U.S. is forcing utilities to operate in an increasingly competitive market that may reduce spending for modernization, thus encouraging the use of existing equipment as long as is practical."

"The new inspection system allows fast, accurate, and inexpensive examination of the steeple areas of turbine blades, helping utilities avoid costly repairs and downtime and plan their repair and replacement schedules in a more orderly way," adds Crane. "This translates into improved plant safety as well as savings for the consumer."

Two problems have hampered the success of conventional UT blade inspections to date. Firstly, the blade attachment hooks themselves emit strong enough signals to obscure the weaker signals caused by SCC. To overcome this, the new system uses custom-designed acoustic lenses, machined at the Institute, to focus the UT beam so precisely that it is possible to easily discriminate between the hook and SCC signals.

Secondly, the steam turbine industry uses a wide variety of disk shapes and geometries, each of which requires individualized lenses and inspection plans. A vital part of the new system is a set of CAD/CAM programs that allow efficient analysis, design, and manufacture of the lenses to address these particular problems and optimize inspection plans for each disk.

For example, in planning for an inspection, after the rotor is placed on roller for a 360 scan, computer modeling is used to simulate the disk configuration and the optimum path of the UT beam as it passes through the rim and steeple areas. This provides a convenient and economical method to evaluate different inspection strategies. Software is then used to design the lenses that takes into account the geometry and acoustic properties of the disk, as well as UT requirements such as approach direction, desired focal point, and transducer size and frequency. These data are in turn used by a computer-controlled milling machine to precisely fabricate the need acoustic lenses. Precise design entails the use of only one or two transducers and up to four interchangeable lenses, thus minimizing costs.

Field testing, followed by destructive assay in the lab, has confirmed that the system is capable of making a complete surface and subsurface disk inspection that accurately indicates areas of both mild and severe SCC at a fraction of the expense of other current inspection techniques.

Magnetic particle testing (MT), currently the most widely used examination method, is restricted to inspection of exposed surfaces and cannot identify subsurface cracks within the steeple serrations and roots without costly and time-consuming blade removal. MT examination methods are therefore forced to depend on random blade removal and random spot inspections that examine only up to five percent of the blade attachment region. using the new SwRI-developed system, 100 percent of the blade attachments can be inspected more quickly and at a lower cost.

U.S. patent No. 4,502,331 has been issued for the system, which has been successfully used in both Japanese and U.S. power plants.

For more information about the Turbine Disk-Rim Ultrasonic Inspection System, contact Joe Fohn, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas, 78228-0510, Phone (210) 522-4630, Fax (210) 522-3547.

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