Integrated Approach to Gas Turbine Rotor Condition Assessment and Life Management

Turbine manufacturers place limits on the service life of gas turbine (GT) rotors/discs based on either the number of hours of operation or the number of start-stop cycles. A significant number of gas turbine rotors are either condemned or slated for replacement during a future outage. Some turbines experience premature cracking which results in the replacement of select rotor components. Examples of such cases are GE Frame 7EA compressor disc cracking, Frame 7FA/9FA turbine disc air-feed slot and post cracking, and Frame 6001B turbine disc rabbet cracking Many Alstom 11N and Siemens W-501 rotors and discs are also replaced based on design life limitations. This experience prompted EPRI, sponsored by gas turbine owners to conduct projects in this area. Under this program, TurboMet International and AccTTech,LLC conducted metallurgical evaluation of cracked discs to understand the crack initiation and propagation mechanisms, detailed structural engineering analysis to understand the root cause of cracking and developed solutions; and to provide recommendations to turbine owners to mitigate such failures. Condition and remaining life analysis of several turbine models was conducted using rigorous engineering analysis to provide objective technical recommendations to turbine users to safely extend the life of the rotors. This collective experience has result in guidelines for safe reinspection intervals to mitigate future risk. In order to obtain pertinent material properties needed for such detailed engineering analysis, retired rotors and discs were obtained from both compressor and turbine sections. Nondestructive examinations (NDE) and materials testing were conducted to assess component condition and mechanical properties such as tensile, fracture toughness, crack growth, creep, low-cycle fatigue, etc. This paper provides an overview of an integrated rotor condition and life assessment approach including several examples of component evaluations.