Shaft bowing can be a transient or a permanent defect of a turbine shaft and occurs when the localized stress at a shaft section exceeds the permissible yield stress of the material at a given temperature. When the stress exceeds the fracture limit at a specific temperature, cracks and ruptures could also occur. Most of the time the shaft bow is a result of a thermal process caused by localized rubbing or quenching of the external surface of the rotor and will present material metallurgical changes at the defect location. A multitude of reasons could cause a rotor to bow in operation, such as improper start-up process, improper steam path alignment, water intake, casing bowing due to improper draining, extended time operation at low speed during the start-up, shut down when a rubbing occurs, etc. When the bow is minor, the shaft could be “cured” on slow turning (turning gear operation), with steam feed at the glands. When the bow is major, straightening of the shaft would require its removal from the casing.​

There are several methods of shaft straightening, some of them being applicable on site, some of them requiring the rotor to be transported in a repair facility, and a thermal straightening or thermal post straightening process would be always required.​

Several of those methods have been successfully applied alone or in combination, and shafts with deformations up to 0.5 mm eccentricity (1 mm TIR) have been successfully straightened on site and put back in operation after just Low Speed Balancing on site. The straightening activity always starts with the geometrical, metallurgical and event evaluation, in order to determine the straightening chance of success and the risk of repair. Then it is followed by successive thermal processes with proper definition of the soaking parameters.