The author of this book has very successfully used a telemetry system for monitoring the vibration displacement and stresses, in the moving blades of a large steam turbine and also on the impellers of a forced-draft (FD) fans, used in large thermal-power plants. These components repeatedly failed in a relatively short time of commercial operation and thus, required detailed investigations. The examination of the fracture surfaces of the steam-turbine blades as well as that of the impellers of the FD-fan under scanning, the electron microscope showed that these failures were caused by fatigue. As explained in the earlier chapters, the operation of the machine elements at resonance or near-resonance condition can result into substantial dynamic stresses (vibratory stresses). Consequently, it was found necessary to measure the natural frequency of the blades and impellers while they rotate at their rated speed and also the total stresses (static plus dynamic) in the components (steam-turbine blades in one case and FD-fan impeller in another case). It may be understood well that the natural frequencies as well as the mode shapes can be determined during the stationary condition of the machines, but the analysis as well as the experience shows that the natural frequencies do change (due to centrifugal-force effects) when the machine rotates. Also, the dynamic-stress measurement during the static test has not got much meaning unless an extensive analysis is carried out to determine the stress-amplification factor at resonance or near-resonance condition.
During the running tests, the steam-turbine blades were appropriately strain gauged at several places on the blades starting from the root portion of the blades. Also for monitoring vibrational displacements / accelerations, accelerometers were mounted around the same location. In a similar fashion, FD-fan impeller was instrumented using strain gauges and accelerometers. For exciting the natural frequency modes, while the bladed discs of steam turbine (and also the impeller of the FD-fan) rotate, an air jet impingement was used as shown in Fig. 7.19, 7.20(a), and 7.20(b).
The jet impingement on the rotating discs is equivalent to providing an impulse periodically on the instrumented moving blades. The test results show the case of steam turbine, one of the natural frequencies, which was very close to the diaphragm or guide blade-passing frequency (no. of diaphragm / guide blades × speed (50 Hz in this case)). To eliminate this problem, the number of diaphragm blades was changed from the existing 50 to 62. The modification proved extremely successful in a sense that even after two decades of operations of the machine, no blade failures have been reported. The tests on FD-fan impellers also showed that near-resonance condition existed. The structural modification significantly altered the natural frequencies of the impeller and thus solved the problem of failures on the FD-fan impeller.