On-Site Turbine Shaft Straightening
Geometrical measurements and NDT
Horizontal Heat Treatment
Turbine Rotor Straightening
On-Site Straightening Methods:
A. Machine a new rotor axis
Will require a mobile lathe and one or both journals axis will be machined eccentric in successive ways (good results for minor deformations but will require additional machining of the couplings, coupling holes, spigot etc., but the disks axial run-out will remain excessive)
B. Hot Spot Straightening
Straightening by applying localized heating at the rotor surface. This will create a compressive stress at the heating location.
It is able to achieve good results for medium deformations but will require very good control of the straightening parameters.
C. Thermal soaking
Could be done for stress relaxation purposes; a certain straightening will also occur when the thermal process is properly defined and applied.
Vertical and horizontal methods have been applied.
Shaft bowing could be a transient or permanent defect of a turbine shaft and will occur when the localized stress at a shaft section will exceed the permissible yield stress of the material at a given temperature. If 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 or shut down when a rubbing occur, etc. When is minor, a bowed shaft could be “cured” on slow turning (turning gear operation) with steam feed at the glands. When is a major deformation the rotor removal from the casing and shaft straightening might be the only quick fix repair option.
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 will be always required.
Many 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. The straightening activity it is always starting with the geometrical, metallurgical and event evaluation, in order to evaluate the straightening chance of success and the risk of repair and it is always followed by successive thermal processes with proper definition of the soaking parameters.