Mitsubishi Heavy Industries Technical Review Vol. 52 No. 4 (December 2015)
22Development of Reliability Verification Technologyfor Gas Turbine Rotor Blades Using an Actual Unit(High-temperature Non-contact Blade Vibration Measurement Technology)
KAZUHIRO TAMURA*1

MASAKI ONO*2

SHUNSUKE TORII*3

TETSUYA SHIMMYO*4

AKIO KONDOU*5

As an economical and clean thermal power generating facility, gas turbine combined cycle
power generation is attracting attention, and the market is expected to expand across the world on
a long-term basis. While developing the world’s first 1,600°C-class Type-J gas turbine for better
performance, Mitsubishi Hitachi Power Systems, Ltd. (MHPS) also proceeded with research forthe development of high-temperature special measurement technology that can be applied under
actual operating conditions, with a view to obtaining technological evaluation data on
newly-developed components and using it as feedback in design. This report introduces our
high-temperature non-contact blade vibration measurement technology, which enables theexamination of the turbine’s first-stage rotor blade vibrations. This technology is applicable to thehigh-temperature (max. 1,600°C) environment of actual gas turbines.

|1. Introduction
Attaining higher temperatures in gas turbines is the key to realizing better performance andefficiency of gas turbines. In the course of improving gas turbine efficiency, MHPS released thelarge-capacity 1,150°C-class gas turbine (M701D) in the 1980s, and one with a turbine inlet
temperature of 1,350°C (M501F) in 1989. In 1997, the applicable turbine inlet temperature wasincreased to 1,500°C (M501G). In the latest gas turbines (M501J and M701J) that were developed
recently, the turbine inlet temperature reached 1,600°C. Such elevation in the temperature can leadto increased thermal stress and lowered fatigue strength of the first-stage rotor blades of turbines,
thus resulting in increased vibration intensity. Moreover, recent designs of turbine rotor blades tend
to have a high aspect ratio for the realization of better turbine performance, resulting in lower
rigidity and as a result, increasing the vibration intensity even more. Therefore, through testing at apower generation demonstration facility, MHPS conducted special vibration measurements ofturbine rotor blades under actual operating conditions (high temperature and high pressure) andverified their reliability.
Conventionally, the dominant method for measuring the vibrations of gas turbine rotor
blades employed a telemeter system by attaching the strain gauges directly to the blades. However,while the use of telemeters has the advantage of very high accuracy in measurement, themeasurement cost is high. Furthermore, as the obtained vibration data are only from thesegauge-attached blades, it is impossible to take the variability caused by turbine blades into
consideration for design. Therefore, an adequate margin for vibration intensity is included in thedesign. To design turbine blades with higher efficiency, we have developed a high-temperature
non-contact blade vibration measurement technology that enables the measurement of the vibration
characteristics of all turbine blades under actual operating conditions at a measurement cost lower
than the telemeter system. It is therefore a variation data-based assessment and realizes
measurement at a turbine inlet temperature of 1,600°C.
*1 Vibration Research Department, Research & Innovation Center, Technology & Innovation Headquarters*2 Chief Staff Manager, Vibration Research Department, Research & Innovation Center, Technology & Innovation Headquarters*3 Chief Staff Manager, Gas Turbine Technology Development Department, Turbine Technology Development Integration Division, Mitsubishi Hitachi
Power Systems, Ltd.
*4 Gas Turbine Technology Development Department, Turbine Technology Development Integration Division, Mitsubishi Hitachi Power Systems, Ltd.
*5 System Technology Development Department, ICT Solution Headquarters

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This report summarizes this newly-developed non-contact blade vibration measurement
technology and presents the measurement results when it is applied to an actual gas turbine.

|2. Measurement technology for gas turbine rotor blade vibrations
under actual operating conditions
The first-stage rotor blades of turbines, which are the critical component to convert fluidenergy into mechanical energy, are used in an extremely severe operating environment of hightemperature and high load to produce better thermal efficiency. Damage to turbine rotor blades ismostly caused by vibration-related factors such as high-cycle fatigue and corrosion fatigue.
Therefore, when developing a new turbine rotor blade, MHPS conducts verification testing throughwhich vibration data on turbine rotor blades are collected to confirm the soundness of turbine
blades and design validity.
2.1 Measurement principle of non-contact blade vibration measurement technology
Non-contact blade vibration measurement technology is a technique to analyze the vibrationsof turbine rotor blades by detecting the timing of a blade passing by the sensors (optical,capacitance, eddy current, etc.) attached to the inner circumference of the casing. When a turbine
rotor blade passes the tip of a sensor, it produces a blade-passing pulse signal as shown in Figure 1.
If a turbine 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, turbine rotor blade vibrations can be estimated by obtaining the time difference between
these two intervals with/without blade vibrations. 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 capacity to measure the vibration response of all blades. This
technology has been used for quality verification tests of our turbine rotor blades and theirreliability demonstration tests under actual operation conditions.

Figure 1 Principle of non-contact blade vibration measurement technology

2.2

Sensor probes used in the measurement of gas turbine rotor blade vibrations

Various types of sensors (e.g., optical, capacitance, and eddy current, etc.) are used in thenon-contact measurement of blade vibrations. When the clearance between the tip of a turbine
blade and the casing is large (for example, during the start-up of a gas turbine), capacitance or eddy
current sensors have difficulty in obtaining signals with a good SN ratio, which affects theirmeasurement of blade vibrations. As a result, we have decided to use optical sensors. However,there is a risk of having the tip of a sensor smeared because of combustion gas or sensor signals
weakened because of deteriorated fibers. To prevent signal weakening, cooling air is supplied tothe sensors. Figure 2 shows sensor probes attached to the actual gas turbine.

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Figure 2

2.3

Sensor probes attached to the gas turbine casing

Data processing in non-contact measurement of blade vibrations

Depending on the type of data processing, non-contact blade vibration measurement
technology can be divided into two types: (1) the method in which a small number of sensors areattached to the stationary side and the time difference of a blade passing between the sensors ismeasured to calculate vibration amplitude (hereafter referred to as the few-probe method) and (2)the multi-probe method in which many sensors are, as noted, also attached to the stationary sideand the timing of a blade passing each sensor is measured to obtain the oscillatory waveforms of ablade.
In the few-probe method, at least a single sensor is required to analyze turbine blade
amplitude. It is suitable to monitor (for abnormality detection) the vibrations of turbine blades forwhich sufficient design data was obtained and verification was completed. While the response ofpass-through resonance can be measured and evaluated when the rotation speed varies (e.g., duringacceleration), it is impossible to accurately measure the synchronous vibration response such as theresonant response when the rotation speed is constant. There are limitations in estimating thevibration frequency even in the case of non-synchronous vibrations. Therefore, since every possible
vibratory phenomenon can occur in demonstration tests of newly-developed blades, it is difficult to
say that the few-probe method is sufficient to manage vibration measurement and monitoring insuch testing.

Figure 3

Typical results of non-contact blade vibration measurement (multi-probe method)

On the other hand, in the multi-probe method, the results can be obtained in the form ofoscillatory waveforms and the amplitude of either synchronous or non-synchronous vibrations ofturbine blades can be measured accurately. The evaluation of turbine blade frequency is also
relatively easy. It is therefore desirable to adopt the multi-probe method in demonstration tests ofgas turbines with newly-developed blades. Figure 3 shows an example of a case of non-contact
blade vibration measurements (multi-probe method).

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The measurement results with the use of a single sensor probe have already been reported inthe MHI Technical Review (Vol. 51, No. 1 (2014)). This report presents the results of measurementby multi-probe method, which was conducted with the aim of improving the measurement
accuracy.

|3. Results of non-contact measurement of blade vibrations using anactual gas turbine
Non-contact blade vibration measurement technology was applied during test operation ofthe M501J gas turbine at the power generation demonstration facility of our Takasago Machinery
Works (T-point). This section presents the results of the turbine’s first-stage rotor blade vibration
measurement in which optical sensors were used. As mentioned earlier, when using optical sensors
during gas-turbine load operation, the measurement of blade vibrations may become impossible
because of signal weakening due to smeared sensor tips. The preventive measures we employedinclude the use of an improved laser source and the air cooling of sensors.
The blade vibrations were also measured consecutively over several days under the high-load
operating conditions of gas turbines (turbine inlet temperature: 1,500°C to 1,600°C). The results areshown in Figure 4, which indicate that there is no substantial weakening of sensor signals despite
the presence of some fluctuations due to the change in clearance between the turbine blade and thecasing. Such retaining of signals at a certain level represents the capability of continuously
measuring the first-stage rotor blades of gas turbines.
Figure 5 shows the resonant response data results of the turbine’s first-stage rotor bladeswith the increasing rotation speed of the turbine, which were obtained by non-contact bladevibration measurement (multi-probe method). As indicated in the figure, the response ofpass-through resonance is clearly measureable. The frequency analysis of blade vibrations duringresonant response has also become possible.

Figure 4

Change in sensor signal output over time

Figure 5 Results of non-contact blade vibration measurement of the turbine’s
first-stage rotor blades (multi-probe method)

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Figure 6 presents the vibration response results of non-contact blade vibration measurement(multi-probe method) on the turbine’s first-stage rotor blades under varying load operation
conditions. As a reference, the same measurement was conducted using strain gauges (telemetersystem) on a different day. The results show that the values of strain gauge estimation are more orless within the variation range of the non-contact blade vibration measurement results, despite theconsiderable fluctuation resulting from the measurement of the vibration response of all blades. For
non-contact blade vibration measurement technology, therefore, the vibration measurement
accuracy during load operation has been confirmed.

Figure 6

Accuracy verification results of blade vibration measurement during load operation|4. Conclusion
As the performance and output of gas turbines are improved, the vibration intensity of
turbine rotor blades increases, making the design more demanding. To attain better reliability of
turbine rotor blades, it is very important to properly interpret the data on turbine rotor bladevibrations in the high-temperature environment of actual units and utilize it in design. MHPS hasdeveloped a high-temperature non-contact blade vibration measurement technology that can beapplied at a turbine inlet temperature of 1,600°C. It has been employed in the measurement of theturbine blade vibrations of our latest M701J gas turbine, thus contributing to the the verification of
the reliability of our new products.

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