Much of the world’s Steam Turbine Generator fleet was commissioned more than 40 years ago.
These turbine generator sets have had many in-service issues leading to poor reliability and, in some cases, requiring full turbine and generator replacements. EPRI surveys indicate that most turbine issues have been related to low-pressure turbines. Discovery of issues during planned shutdown inspections such as rotor shaft cracking, stress corrosion cracking (SCC) of dovetails and blade reliability concerns can be addressed without the costly replacement of the full rotor/blade components. This paper will review and provide an actual case study of methods used to repair LP rotor shaft end cracking.
Low pressure rotor shaft end cracking
There have been more than 30 documented cases of LP rotor shaft end cracking, predominantly on fossil units. It is common for cracking to occur at the base radius formed by the exit side of the last stage disc and the small gland shaft end diameter. Root Cause Analyses (RCAs) completed on these cracks
Typical vibration symptoms
As the shaft end crack propagates, it will eventually become evident in increasing vibration levels over time. Vibration spectra typically exhibit increasing 1x and 2x running speed levels. The greatest magnitude of vibration is usually close to the shaft end that is cracked. As magnitudes increase, there is an increase in shaft end deflection due to compromised stiffness and increasing vibration levels, which can cause additional damage such as gland and blade path rubs. As a result, in some spectrums analyzed by the writer, higher order multiples of running speed also were evident, which are typical of a rub. Increasing 1x vibration is caused by the off center unbalance caused by the asymmetric deflection of the cracked rotor as the crack propagates. The classical 2x vibration found in cracked shafts is often dominant and can equal or exceed the 1x component of vibration. This is not always the case, and in the experience of the writer may only be 50 percent or less of the 1 times running speed spectrum levels. It is believed that this difference was due to the high shaft stiffness and short shaft ends of Low Pressure rotor designs. In addition, some changes in rotor critical speeds (up to 150 rpm) were evident as the crack grew in length. This background is noted to support operational and maintenance decisions that might be needed in new troubleshooting efforts.
Purchasing a new rotor is normally schedule prohibitive based on the lead time for large LP forgings. Typical lead times for new “replacement in kind” LP rotors average 18 months or more.
Therefore the decision process is normally focused on repair options. Proven repair options for larger LP rotors include either bolting on a new shaft end or welding a new shaft forging to the main rotor body. With advancements in welding technology on NiCrMoV LP rotor steels, most repairs during the last 20 years have focused on this option. Earlier in the Westinghouse fleet repair history, which suffered a number of shaft end cracks in the so called “J” groove, bolted on designs were the only repair option available from the original equipment supplier. This design option involved machining a new shaft end with a bolting flange that enabled attachment with tapped holes in the last stage exit disc face. These tapped holes can significantly reduce the on-off cycling capability of the unit, which has become more of an issue with changing merchant plant duty cycle requirements.