Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1High-efficiency and High-precision Ultrasonic Testing
Technologies for Complex Shaped Blade Grooves for
Maintenance of Steam Turbine Rotor

MASAKAZU KAMIBAYASHI*1 MIKIYASU URATA*1
TAKURO MASUDA*2

RYOSUKE SUGITA*3

KOJI OKADA*4

ATUSHI NAKAJIMA*5

The low-pressure stage of a steam turbine rotor has the potential risk of blade scattering
caused by stress corrosion cracking in the blade groove where the rotor blade is implanted due tolong-term operation. Mitsubishi Heavy Industries (MHI) Group has developed ultrasonic testing
technologies for axial entry blade grooves with a complicated processing shape that is used todetect and identify minute stress corrosion cracking occurring in the blade groove that can be usedwithout removing the rotor blade. This report presents an overview and inspection examples ofultrasonic testing techniques that can handle special blade groove processing shapes and disk
shapes.

|1. Introduction
Over the years, MHI Group has manufactured and delivered a wide range of steam turbine
equipment that can meet diverse needs, from small single casing turbines for private power
generation to large turbines for nuclear power, as well as variable speed steam turbines forcompressor drives for petrochemical plants. The number of steam turbine equipment units
manufactured and delivered by our group has reached more than 2,500 in 70 countries for power
generation and more than 1,400 in 60 countries for compressor drives.
Steam turbine rotors, which convert high-temperature and high-pressure steam into
high-speed rotating motion, have a structure in which rotor blades for receiving the steam flow andgenerating rotation are implanted in the rotor shaft. Since stress corrosion cracking (SCC) occurs inthe blade grooves of the low-pressure stage of a rotor due to long-term operation, it is important tocarry out maintenance and management through regular non-destructive inspection in order toprevent accidents such as blade scattering. So far, we have developed blade groove inspection
techniques that utilize phased array UT (ultrasonic testing) technique and do not require theremoval of the rotor blade. However, in the case of some axial entry blade grooves that areprocessed in the rotor axial direction, the groove shape and disk shape may be special, so UTcannot be applied and inspection with the rotor blade removed is required. As a result, large-scale
incidental work may be required for the removal and reinstallation of the blade.
Our blade groove UT techniques have been applied to a lot of actual equipment so far, but
we developed inspection techniques that can handle curved blade grooves and complicated disk
shapes in large turbines to expand the application further. These techniques eliminate the need forincidental work to remove and reinstall the rotor blade and can shorten the inspection schedule.

|2. Disk shape and blade groove shape of steam turbine oflow-pressure stage
The shape of the disk that forms the blade groove into which rotor blades are implanted is*1

Service Engineering Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Power. Ltd.

*3

Steam Turbine Department, Service Division, Mitsubishi Power Industries Co. Ltd.

*4

HSE and Quality Management Department, Mitsubishi Heavy Industries Compressor Corporation*5Customer Service Division, Mitsubishi Heavy Industries Compressor Corporation

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optimized according to the customer's plant operation specifications and the rotor bladespecifications, so there are various disk shapes such as a straight type disk, a tapered disk, a disk
with a curvature and a flanged disk as shown in Figure 1. In the case of general UT, it is necessary
to place a sensor on the smooth surface of the disk to detect the blade groove. Therefore, thiscannot be applied to these various disk shapes.
As shown in Figure 2, there are three types of axial entry blade groove shapes, namely
straight type, skewed type and curved type. In order to detect SCC generated in the stressconcentration area of each groove shape, an optimal flaw detection approach that appropriately
controls the ultrasonic beam direction is required.
We have developed ultrasonic testing techniques to handle these various disk cross-sectional
shapes and blade groove processing shapes and the details are described below.

Figure 1 Example of disk cross-sectional shape ofsteam turbine rotor

Figure 2

Processing shape of axial entry blade groove|3. UT technique for axial entry blade grooves
The ultrasonic testing technique for straight- or skewed-shape axial entry blade grooves is themost proven inspection technique to date(1). As shown in Figure 3, this is a technique of detecting
SCC by arranging an array probe on the side surface of the disk and performing a beam scan in thedirection of the disk thickness. In order to detect SCC inclined with respect to the ultrasonic beam
direction with high sensitivity, we designed an array element that optimized the beam focusing
range according to the disk thickness by utilizing ultrasonic simulation, and developed a compact,
high-performance sensor that can be placed in the narrow space between disks. Disks haveapproximately 100 blade grooves or more machined in the axial direction over the entire
circumference, so it is necessary to efficiently inspect all blade grooves. We also developed amanual scanner that can record data of the entire circumference of a disk in just a few minutes totens of minutes with its sensor fixed in the narrow space between disks, and dedicated flaw
detection software that can differentiate SCC with a depth of 1 mm or less from a shape echo withhigh accuracy using software that combines inspection data and 3D-CAD, thereby realizing
highly-efficient inspection.

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In blade grooves, depending on the steam environment, deposit adherence and corrosion pitsthat may initiate SCC, as well as fatigue cracking due to operation and steam conditions may occur.
Over the past 10 years or more, we have accumulated verification data using SCC generated in anacceleration test conducted by heating a blade groove sample in a high-temperature furnace andfrom an actual rotor in which SCC echoes were detected due to long-term operation as shown in
Figure 4. We are working on the identification of harmful SCC by evaluating the correlation ofreflected echoes with reflection sources based on the gained empirical data and by superimposing
inspection data and 3D-CAD.

Figure 3

Phased array UT of axial entry blade groove

Figure 4

Inspection example of axial entry blade groove|4. UT technique for curved axial entry blade grooves
A curved blade groove is processed with a curvature in order to alleviate the centrifugal
stress on the rotor blade due to the high-speed rotation of the turbine rotor. For this blade groove
shape, we developed a flaw detection technique that electronically scans the ultrasonic beam in thedisk circumferential direction with the array divided in the sensor width direction as shown in
Figure 5, unlike the conventional method that scans the beam with the array divided in the sensor
in front and rear direction in order to properly emit ultrasonic waves to detect SCC.
This method can efficiently detect SCC throughout the curved blade groove by recording theflaw detection data of the entire disk thickness for the entire curved blade groove area by
performing mechanical sensor-scanning in the disk radial direction with a dedicated scanner
simultaneously with electronic scanning in the disk circumferential direction. The detection
performance of this technique has been verified with SCC occurring in the long-term operation ofactual equipment and SCC generated by accelerated testing in a high-temperature furnace. This
technique can also efficiently inspect the entire area of various blade grooves with different disk
thicknesses and evaluate them with high accuracy, by combining sensors optimized for shallow
region and deep region according to the thickness of the disk to be inspected.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4Figure 5

UT technique for curved axial entry blade groove|5. UT technique for disks with complicated shapes
In many cases, disks in the low-pressure stage of large turbines have a tapered, curvature, orcurvature flanged shape as shown in Figure 1, so a general flat-shape ultrasonic sensor cannot be
attached to them. Therefore, we have developed an inspection technique that can visualize internal
defects with high accuracy even from a disk surface with an arbitrary curvature or with a tapered
shape by utilizing the adaptive ultrasonic testing technique(2) developed by our company. This
technique applies an ultrasonic acoustic gel between the ultrasonic sensor and the surface of thedisk so that the ultrasonic sensor can adhere to the arbitrarily curved surface or tapered shape andefficiently transmit and receive ultrasonic waves, as shown in Figure 6. In order to make an imageof internal flaws, this technique first utilizes signal processing called FMC/TFM (Full Matrix
Capture/Total Focusing Method) to obtain the spatial coordinates of the surface shape byvisualizing the surface shape with transmitted and received signal data of all elements in the sensor.
It then calculates a propagation path that passes through the surface coordinates and satisfies Snell'slaw to divide the predetermined blade groove area into grids, integrates the peak value of thewaveform time corresponding to each coordinate and performs imaging. This techniqueimplements calculation processing using a GPGPU (General-Purpose Computing on Graphics
Processing Units) and enables real-time flaw detection.
Figure 7 compares the flaw detection waveform of conventional phased array UT and theflaw detection waveform of adaptive UT (FMC/TFM) for SCC occurring in a blade groove of
actual equipment and artificial defects. It is shown that adaptive UT can receive, with all arrayelements, ultrasonic echoes scattered on the irregular surface of SCC due to long-term corrosion
and synthesize them, so the resolution of echoes reflected from groove shapes and artificial defects
is improved, resulting in higher differentiability between those echoes and shape echoes.
Figure 8 shows the actual inspection of a large low-pressure turbine. Even in the narrow
space between disks with a flanged shape near the blade groove, the scanner that records
continuous inspection data in the circumferential direction can perform high-efficiency UTinspection of all the blade grooves using a large ultrasonic sensor incorporating acoustic gel. Inaddition, for T-slot type blade grooves machined in the tangential direction, we have also
developed a scanner that automatically records inspection data with its ultrasonic sensor
automatically travelling in the circumferential direction with reference to the shoulder area where
the blade is implanted as shown in Figure 9. In this way, we have built an inspection system that
improves not only the inspection efficiency, but also the traceability.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
5Figure 6 Principle of adaptive ultrasonic testing for disks withcomplicated shapes in large turbines

Figure 7 Inspection example of disk with complicated shape inlarge turbine

Figure 8 UT of disk with complicated shape in large
turbineFigure 9 Self-propelled scannerfor UT of steam turbine bladegroove

Figure 10 shows a standard inspection schedule of axial entry blade grooves in a single disk
stage. In contrast to conventional inspection involving blade removal, which requires six days forthe entire process, the developed technique eliminates the need for removal work to remove andreinstall the blades, and can significantly shorten the inspection schedule with the rate of one stage
per day. We will continue to strive to improve the discrimination and inspection accuracy of thistechnique.

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Figure 10

Inspection schedule example of turbine rotor blade groove|6. Conclusion
MHI Group has been working on the development of technologies to efficiently detect stresscorrosion cracking that occurs in the blade grooves of the low-pressure stage of a turbine rotor dueto long-term operation of the turbine equipment. This paper described ultrasonic testing
technologies that can handle various groove shapes and disk shapes for axial entry type blade
grooves with complicated machined shapes.
These inspection technologies are currently accumulating results in maintenance inspectionsof many steam turbines including small- and medium-sized units, and it is expected that theirapplication will be expanded to the inspection of areas with complicated shapes in high-speed
rotating machines other than steam turbines. We will continue to improve inspection and evaluationsystems and contribute to the maintenance management of rotating machines during long-term
operation.

References(1)(2)

Urata et al., Ultrasonic Inspection Technique for Axial Entry Type Blade Groove of Steam Turbine, The
Thermal and Nuclear Power Journal, Vol.68 No.3 (2017) p.37-40
Hayashi et al., Adaptive Ultrasonic Testing Technology for Imaging Inside of Product with ComplicatedShape Surface, Mitsubishi Heavy Industries Technical Review, Vol.56 No.1 (2007) p.1-5