Rotor Lifetime Assessment: A Reference Report

A gas turbine rotor is a large complex structure, made of different stages of discs which are exposed to a range of temperatures and stresses during operation. Depending on the nature of its operation and the standard of its design, various damage mechanisms such as creep-fatigue, oxidation and corrosion act to limit the life of the gas turbine rotor. However, because of its nominally long design life less attention is traditionally paid at intermediate inspections to this highly critical part of the machine. A large numbers of gas turbine rotors are expected to reach the end of their nominal design life in the coming years. Replacing a turbine rotor (or part of it) or sending a rotor for an inspection and overhaul requires some careful advanced assessment and planning. The consumed life of a turbine rotor can be different to that of its peers operating in different units, driven by the operational conditions. Any rotor life model should be able to provide clear recommendations to the plant operator to run, repair or replace a turbine rotor (or part of it). This can only be achieved by an in-depth knowledge of damage mechanisms, materials data, and thermal transient modelling of the rotor combined with an assessment of confidence levels associated with the non-destructive techniques used to inspect it. Initially, this paper focuses on rotor life definition, then it describes the lifetime assessment process in more detail. Some cross-references are made to integrity assessments of turbine rotors in flight engines. Finally, the gaps identified in current practices are discussed in the context of managing the uncertainties and improving the confidence in the rotor life assessment. Given the issues discussed in this paper, it is greatly beneficial to the turbine community to run a collaborative rotor lifing program to establish a generic lifing protocol for gas turbine rotors. Therefore, ETN invites gas turbine owners and operators (including service providers and OEMs) to join this working group, to improve the awareness of the issues surrounding rotor integrity, working towards higher confidence in future gas turbine operation for older machines.