A large utility turbine comprises of HP turbine, IP turbine, and LP turbine on a common shaft driving an AC generator, experienced repeated failures of one of the stages of IP turbine. The stage consists of blades provided with a lacing wire to increase the natural frequency from 147 HZ to 153HZ of the free-standing blade to 160 HZ to 165 HZ so that the third-harmonic excitation (around 150 HZ) does not cause high vibrations of the blades and consequent failures. At a particular site, a few of these blades failed during the service causing damages to the diaphragm blades. The failed blades were replaced with new blades and the lacing wire was rebraced. The damaged diaphragms were repaired at site by grinding (dressing) and the machine was put back into service. The particular stage failed once again after about 4–6 weeks and was again replaced. The phenomenon of blade failure once again occurred after another 6 weeks of operation.
The author of this book had an opportunity to investigate the failures, which occurred for the third time. The visit to site showed the diaphragm blades which failed in the first instance of failure were hand-dressed using a portable grinder and during this activity the guide blades were dressed by removing the material of guide blades such that guide blade surface becomes smooth. Apparently, the maintenance staff were not aware of the fact that such dressing would result into a variable pitch of diaphragm blades and would give rise to a substantial excitation of forces at 1X, 2X, 3X…DIF, 2 DIF, and so on. Also, it was found that the workmanship of brazing the lacing wire to the blades was sub-standard in a sense that the natural frequency of the unbrazed blade (147Hz) went up to 152 Hz to153 Hz after brazing the lacing wire. Since the set, at times, had run at a grid frequency of 50.5 Hz–51Hz in the night hours, the moving blades had seen the resonant condition due to (1) higher-grid frequency, (2) closeness of natural frequency to 3X frequency perturbation force, and (3) high excitation at 3X frequency due to large variation in the pitch of diaphragm blades.
It must always be remembered that a high-speed machinery such as steam turbine/gas turbine requires a very high degree of accuracy in manufacture—especially in the blade pitches, blade roots, and fixing of devices such as lacing wire and shroud bands; and that is the reason they are so expensive.
In this particular case, the customer was suggested to replace the repaired diaphragm by a new diaphragm and improve upon the quality of brazing of the lacing wire. No failure at this stage has been reported after implementing the suggested corrective action plan.
A vibration engineer must keep himself aware of various perturbation forces experienced by the machine under investigation. We have discussed a few typical perturbation forces such as mechanical, hydraulic / aerodynamic forces experienced by machinery such as pumps, turbines, compressors, fans, generators, and so on. We shall now discuss the electrical perturbation forces.