Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)10

Non-contact Vibration Measurement of the Rotor Bladesthat Play a Pivotal Role in the Reliability of Gas Turbines

KAZUHIRO TAMURA*1

MASAKI ONO*2

SHUNSUKE TORII*3

KAZUKI MORIMOTO*3

The commercial gas turbines of Mitsubishi Heavy Industries Ltd. (MHI) are not only highefficiency, but also designed based on the data accumulated through component tests and fieldtests, placing importance also on product reliability to assure our clients the safer and securer useof our products. In particular, the first-stage rotor blades of gas turbines are used in an extremely
severe environment, that is, one with high temperature, strong centrifugal force and the greatest
load. Therefore, it is important to demonstrate the validity of their vibration characteristics under
actual operating conditions, from the viewpoint of blade damage prevention. In order to obtain
more data on vibrations of the first-stage rotor blades of gas turbines in the environment of actual
use, Mitsubishi Heavy Industries, Ltd. (MHI) has developed a non-contact blade vibration
measurement technology that can be applied under high temperature gas conditions.

|1. Introduction
In commercial gas turbines, one of the most important components to achieve a higher
thermal efficiency is the rotor blades. As these rotor blades are used under high temperature andhigh load conditions, the most frequently caused damage is fatigue failure or corrosion fatigue
resulting from vibrations. Therefore, the requirements for gas turbine rotor blades include not only
high performance but also sufficient strength against vibrations. It is crucial to accurately predict
their vibration characteristics at the design stage. We have been working to develop moresophisticated blade vibration design technology for gas turbine. As a means of achieving this goal,
it is essential to measure the vibration response of rotor blades under actual operating conditions
and obtain blade vibration characteristics data to use it as feedback in design. Our design is basedon the vibration response data of gas turbine rotor blades obtained by rotational vibration tests (airexcitation) in a vacuum chamber and field tests at the demonstration combined-cycle power plant(T-point). Conventionally, the dominant method for measuring the vibrations of gas turbine rotor
blades employed a telemeter system by attaching the strain gages directly to the blades. However,while use of telemeters has the advantage of very high accuracy in measurement, the obtained
vibration data are only from these gage-attached blades, consequently demanding enormous cost
and time if many rotor blades are to be measured. Therefore, we have improved the conventional
non-contact blade vibration measurement method (which has been used to measure vibrations ofgas turbine compressor rotor blades or steam turbine final-stage blades) and developed atechnology that enables the measurement of minute vibrations of gas turbine rotor blades in ahigh-temperature combustion gas environment. Thus, more vibration data of turbine rotor blades
have become obtainable.
This report presents a summary of the developed non-contact blade vibration measurement
technology and the results of field tests conducted at the T-point.
*1

Takasago Research & Development Center, Technology & Innovation Headquarters*2

Chief Staff Manager, Takasago Research & Development Center, Technology & Innovation Headquarters*3Takasago Gas Turbine Engineering Department, Gas Turbine Products Headquarters, Power Systems DivisionMitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)11|2. Characteristics of the non-contact blade vibration measurement
technology
The non-contact blade vibration measurement technology is a technique to analyze vibrationsof turbine rotor blades by detecting the timing of a blade passing the sensors (optical, capacitance,eddy current or other) attached to the inner circumference of the casing. When a turbine rotor bladepasses the tip of a sensor, it produces a blade-passing pulse signal as shown in Figure 1. If aturbine rotor blade does not vibrate, these pulse signals occur at a constant time interval fromreference signals. However, when a turbine rotor blade vibrates, these intervals fluctuate.
Therefore, a turbine rotor blade vibration can be estimated by obtaining the time difference
between these two intervals with/without blade vibrations. Furthermore, because a vibrating turbine
rotor blade passes the sensors being fixed on the casing (which is stationary), the vibrating
direction of a rotor blade has a significant effect on the measurement accuracy. In other words, theconditions for measurement are most preferable when the vibrating direction of a rotor blade
matches the direction it rotates, but these two directions do not always coincide. Before theinterpretation of the results, the real amplitude has to be recalculated according to the equation
given in Figure 2.
The non-contact blade vibration measurement technology is characterized by (1) there being
no need for rotor modification because it is a non-contact measurement and (2) the capability ofmeasuring the vibration response of all blades. This technology has been used for quality
verification tests of our turbine rotor blades and their reliability demonstration tests under actual
operating conditions.

Figure 1 The principle of non-contact blade vibration measurement

Figure 2 Amplitude obtained by the non-contact blade vibration measurement

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)12|3. History of the development of non-contact blade vibration
measurement technologies
Figure 3 summarizes MHI’s history of developing non-contact blade vibration measurement
technologies. As our engagement in the development of this technology originated from theMoonlight Project, it started under the technical guidance of the National Aerospace Laboratory(NAL; currently, Japan Aerospace Exploration Agency or JAXA). Thereafter, we have madeoriginal improvements to realize the current standards.

Figure 3
History of the development of non-contact blade vibration
measurement technologies

Initially, the non-contact blade vibration measurement technology was adopted for vibration
measurement of compressor blades in gas turbines, and optical sensors were employed for the bladesensing. The improvements such as the development of a laser sensor to achieve better opticalsensitivity allowed the non-contact measurement technology to be used for rotational vibration tests
of integral shroud blades at the final stage of steam turbines. To enable the measurement with actualunits, we also developed analytic algorithms such as the improved few-probe method in which fewer
sensors are required to measure the vibrations of turbine rotor blades, thus making it also applicable todemonstration tests of steam turbines under actual load conditions (Figures 4 and 5). At present,
various blade vibration analysis technologies are available, including the multi-probe method, thefew-probe method (with single or two probes) and the improved few-probe method. We select ananalysis technology that agrees with the measurement purpose, what is measured, and the restrictions
such as the number of attachable sensors.

Figure 4 An example of non-contact blade vibration measurements in theactual load condition test on steam turbines

The turbine blades, which have been the target for measurement, exhibit large vibrationamplitudes and the sensors used are placed in a relatively low-temperature environment. However,when applying the non-contact measurement technology to the on-site vibration measurement ofrotor blades in gas turbines, the following issues have to be addressed:

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)13(1) Requirement of better measurement resolution because of small vibration amplitudes andhigh rigidity of gas turbine rotor blades;(2) Measurement in the combustion gas environment(3) Establishment of a sensor cooling method.
We have enabled blade vibrations in actual gas turbines to be measured, by selecting thesensors suitable for the on-site vibration measurement of gas turbine rotor blades and establishing asensor cooling method and a signal processing technology. Figure 5 outlines our non-contact bladevibration measurement system.

Figure 5

Non-contact blade vibration measurement system|4. Non-contact blade vibration measurement of gas turbine rotor
blades under actual operating conditions
Figure 6 is the exterior view of the demonstration combined-cycle power plant (T-point)
located within the MHI Takasago Machinery Works. At T-point, the first prototype of our latest
gas turbine M501J has been test-operated since February 2011 with the ongoing technical research
to realize higher performance in M501J.

Figure 6 Demonstration combined-cycle power plant at the MHI Takasago
Machinery Works (T-point)

In order to improve the design technology for gas turbine rotor blades, we needed vibration
characteristics data of rotor blades in gas turbines under actual operating conditions, which wereused as feedback in the design. By employing the non-contact blade vibration measurement
technology, we have succeeded in measuring the vibrations of first-stage rotor blades of a gas
turbine. The use of this technology for on-site measurement can result in cost reductions and aconsiderably shortened preparation period for measurement, compared with the telemetermeasurement with strain gages.

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)14

The vibration measurement of gas turbine rotor blades in an environment with a gas
temperature of 1,600°C has been enabled by the adoption of air cooling and high-temperature laser
sensors with excellent frequency characteristics for detection of the timings of turbine rotor blade
passage (Figure 7). The smallest possible high-temperature laser sensors were selected to attainbetter cooling efficiency with use of less air. The axial relative positioning of a sensor and a turbine
rotor blade was determined based on the pre-conducted analysis on the vibration modes of turbine
rotor blades to allow multiple mode measurements. As an example, Figure 8 gives the non-contact
blade vibration measurement results of the first-stage rotor blades in the M501J gas turbine underactual operating conditions. Such on-site measurements of vibration response of gas turbine rotor
blades make it possible to identify the natural frequency of each blade, whereby it becomes
possible to assess the blade vibration design while taking dispersed vibration characteristics of eachgas turbine rotor blade into consideration.

Figure 7 Non-contact blade vibration
measurement of gas turbine blade
under actual operating condition

Figure 8 An example of the non-contact
blade vibration measurement results offirst-stage rotor blades in the M501J gasturbine|5. Conclusion
Rotor blades in gas turbines have a considerable impact on the reliability of gas turbines. We,therefore, examine the vibration characteristics of gas turbine rotor blades of actual units inoperation, thereby, improving the blade vibration design technology for gas turbines. In order toobtain more vibration data on rotor blades in gas turbines, MHI worked on the development of
non-contact blade vibration measurement technology and successfully conducted turbine rotor
blade measurements of actual units in operation. We will further improve the reliability ofnon-contact blade vibration measurement technology to increase its applications, contributing toeven better reliability of our rotary machinery products.

References
1. Hohenberg, R., Detection and Study of Compressor-blade Vibration, Experimental Mechanics, Vol. 7 Issue
6 (1967) pp. 19-24
2. Endoh, M. et al., Noncontact Measurement of Rotating Blade Vibration, (1983), 1, Tokyo, IGTC
3. Umemura, S. et al., Vibration Monitoring by Optical Method for the High Pressure Compressor of GT, 18th
Int. Congress on Combustion Engines, (1989), 199
4. Shibata, M. et al., Development of Noncontact Blade Vibration Measuring System, Mitsubishi Heavy
Industries Technical Review Vol. 37 No. 2 (2000) pp. 106-109
5. Kaneko, Y. et al., Development of Non-contact Measurement Method of Turbomachinery Blade Vibration:
Improved Few-probe Method，Transactions of the Japan Society of Mechanical Engineers Vol. 67 No. 658(2001-6) pp. 150-156
6. Kaneko, Y. et al., Measurement and analysis of random vibration of steam turbine low pressure end blade,IFToMM (2002)