Comparison of Blade Tip Timing With Strain Gauge Data for Evaluation of Dynamic Characterization of Last Stage Blade With Interlocked Shroud for Steam Turbine

The tip-timing technology has been widely developed and has become an industry standard in aircraft engine and gas turbine over past decade. The main application of the tip-timing method is to verify safe operation of blades and monitor the health of blades. But tip-timing technology gets rarely used to the last stage blade of steam turbine. Particularly the blade is designed with an integral shroud, snubber and fir-tree root. The article mainly describes the process of identifying the dynamic characterization of last stage blade with an integral shroud and snubber by contactless measurements provided by tip-timing technology. Attention is focused on the comparison of tip-timing results with the results from strain gauge data. Firstly, the frequency response of the bladed blisk is calculated by using Computer-Aided-Engineering (CAE) technologies. Secondly, according to the results of finite element modal calculation, the location of strain gauge is confirmed. The dynamic strain of blade is measured by utilizing telemetry technology. Finally, according to the design features of integral shroud, the tip-timing probe locations must be accurately confirmed in order to acquire the valid data. All probes are positioned along the radial direction of blades. The rotating vibration test of the bladed blisk has been carried out in the high-speed test rig. In order to validate the tip-timing measurement, all the results from the tip-timing, especially the resonant frequencies and damping ratios, are compared with results from the strain gauges with which only a few blades were equipped.