PSI - Issue 5

Jaroslav Václavík et al. / Procedia Structural Integrity 5 (2017) 1349–1354 Jaroslav Václavík et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 1. Full bridge strain gauge configuration used for torque measurement.

3. Application of torsional monitoring system

3.1. Hardware design

The requirement for installing the system for measuring the torsional vibration originated in cracks, appeared in the last stage of low pressure turbine blades of 4P turbine generator. The goal was to find out, if the torsional amplitudes are high enough to be able to excite the resonant frequencies of the LP blades. For this purpose, the system based on sheer stress measurement according Fig. 1 was used. For HBM resistance 350 Ω strain gauges were glued to the shaft surface (Fig. 3, (5)). The Manner 1-channel telemetry system with PCM technique and ethernet interface was used for this purpose (Fig. 4). The measuring system was installed on the generator shaft just in front of the coupling housing with the LP turbine shaft (Fig. 2, Fig. 4a).

Fig. 2. Position of torsional measuring system on the generator shaft

The 16-bit sensor signal amplifier based on flexsubstrat formable around the shaft (Fig. 3, (1)) was integrated to special four-part split aluminum loop ring of 560 mm internal diameter (2), fastened on the rotor shaft (3) with eight bolts. The measuring amplifier has the sampling rate 3.4 kHz, bandwidth 1 kHz and used input range 4 mV/V. Special rotor loop, dividable for fast mounting, based on epoxy / polyimide material with integrated induction loop (4), was fastened to the lateral side of the aluminum ring. This rotational system was designed for maximum speed 4000 RPM and maximum temperature of 85°C. The power and digital transmission was realized with receiving antenna with flat induction area for large distances and also large axial displacements between the antenna and rotational loop (6). The stationary receiving unit was installed in the housing in turbine room (Fig. 4b), connected with receiving antenna with 20 m long coaxial cable. The receiving unit enables the remote shunt calibration and other control of rotor module (gain, zero, etc.). For this purpose the Manner procedure Telemetry interface can be started on the control computer just before starting the data acquisition, for maintenance purposes or for recalibrations. The data are accessible from analog output, or can be monitored digitally via Ethernet interface. From the analogue output of the receiving unit, the data are sampled with the rate of 3200 s -1 with the measuring unit DVS 813 (producer Ing. Pohl, Czech Republic) (Fig. 5). In parallel, the on-line transferee of the data and pre processing is made using program Monitor to the industrial computer. Here, the data are stored to daily files. Program Monitor enables the processing the data in the time and frequency domain. It starts the processing of the Rainflow matrices from stored data each day. The pre-processed data are stored in daily files, each of 1,8 GB. The digital output of the receiving unit is used for data sampling to the power station generator diagnostic unit, data are stored to hourly files and further advanced evaluation is prepared for the future.