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Advanced Non-Destructive Inspection for Turbine Generator Components
Osamu Iwai, Yoji Ikeuchi, Katsumi Maruyama,
Shigeru Harada, Motoharu Goto, Kazufumi Ohmatsu
Toshiba Corporation, Keihin Product Operations
2-4 Suehiro-cho, Tsurumi-ku, Yokohama, 230-0045, Japan
Many components in the power generation today are taking advantage of these technologies to
greatly reduce inspection cost and improve the reliability and so on. As computer aided inspection technology have evolved greatly in the last few years, so it is possible to acquire high quality condition assessment data while minimizing the risk and cost. This paper presents advanced inspection systems that are inspected turbine generator retaining rings and brazing joints, and the other application for the actual generators.
1. Retaining Rings Inspection System
18Mn-5Cr stainless steel retaining ring has a high sensitivity for stress corrosion cracking (SCC) .
SCC tends to grow under the frequent start and stop and the influence of stored moisture.
Although retaining ring inspection generally performed by PT is highly depending on the inspector
skill, the influence of skill was large. Therefore, we needed to develop the automated UT/ET
inspection system for retaining rings.
The schematic design of the inspection system shows Figure 1. The PC-based eddy current and
ultrasonic instrument is operated to acquire UT/ET signals, and displayed signals as A-scans, B-scans and C-scans. A two-dimensional scanner is controlled by the instrument to move
rotational and axial direction. This scanner can move the half of the length round of the ring at one setting. The UT/ET probe is pressed on the surface by spring wire so that stable contact can be obtained in spite of changing of mechanical clearance.
Fig 1: |
Figure 2 shows the result of automated scanning detectability using the UT/ET test system. The
EDM notch in size 3mm, 5mm, and 10mm long can be detected clearly and each defect size can
be estimated by C-scan image. B-scan image gives easy interpretation for flaw signal.
2. Brazing Joints Inspection System
For stator / rotor copper coils on turbine generator, to keep the conductivity and the insulation
from the other coils or components are highly significant. Therefore inspected parts that are used many brazing joints are required high reliability. TOSHIBA also developed advanced UT method
and system for the brazed parts.
Figure 3 shows the inspection part of brazing joint for generator coils. An immersion method is
applied to detect smaller defects and the whole brazed parts using by two-dimensional scanner.
The probe locations and false echo from the bad joints are displayed real-time.
Figure 4 shows the C-scan image to detect the small artificial defects (flat bottom holes(FBH's),
slit) and bad joints. The dimension and the locations of indications can also be calculated by this system to display scales overlaid C-scan.
Fig 3: Brazing Joint Parts|
3. Other Components Applications
The phased array UT system is applied the inspection for the generator rotor wedges. It is difficult to focus ultrasonic beam to the target region and to scan conventional UT probe that have various focal points because the wedge surface is very narrow. Figure 5 shows the C-scan image of 0.5mm to 1.0mm depth EDM notches and no defects. Phased array UT is very helpful for the
rotor wedge inspections.
The progress of computer technology has contributed the various improvements of eddy current
and ultrasonic inspection field. Such sophisticated inspection systems are essential for the
inspection of generator rotors that need high reliability. This developed systems have expanded its field application.
- O.Iwai, Y.Ikeuchi, K.Maruyama, et al.: "Development of UT/ET Inspection System for Retaining
Rings", presented at the 6th EPRI Turbine/Generator Workshop, St.Louis, August 1999
- O.Iwai, M.Goto: "Application of the Portable Phased Array UT System for Turbine Components",
presented at the 4th EPRI Turbine/Generator Workshop, Milwaukee, August 1995