This article presents an overview of technical aspects of the first F-class gas turbine life-extended rotors. Power Services, a GE Power Business, has shipped from the Greenville, S.C., facility its two F-class extended-life rotors, building upon a foundation of experience gained by executing more than 20 E-class rotor life extensions (RLEs). Experience has shown that additional features that require inspection, including rabbet fillets on wheels other than on stage one, cannot be inspected without a complete rotor disassembly. Each rotor is uniquely characterised by the combination of its particular configuration and operational history. Expanding the value of an RLE upgrade also requires that it be executed at the right point in the rotor’s life. For the two rotors evaluated to date, there have been no unexpected issues uncovered from the part inspections as compared to analytical predictions.
Power Services, a GE Power business, recently shipped from the Greenville, S.C., facility its first two F-class gas turbine life-extended rotors, building upon a foundation of experience gained by executing more than 20 E-class rotor life-extensions (RLEs).
Rotor Life Extension Solution
The GE F-class gas turbine rotor service interval is defined by an envelope of 5,000 factored fired starts (FFS) and 144,000 factored fired hours (FFH) of operation (see GER3620 and TIL 1576). Operation beyond this service interval results in increased risk to the rotor structure. Operators are presented with a choice of two strategies to effectively deal with this increased risk profile: risk-measurement and risk-management.
Risk-measurement is characterized by an inspection strategy that varies in scope, from a superficial inspection without disassembly, to a more-rigorous disassembly with full inspection, but in all cases without any replacement of higher-risk components. Upon return to service, this RLE approach is typified by a monitoring regimen wherein degradation of the rotor condition serves as a proxy for risk. Under this approach, some independent service providers (ISPs) offer life-extension advice based on inspecting a single feature only, such as the forward rabbet fillet of the first-stage turbine wheel. Experience has shown that additional features require inspection, but these features that require inspection, including rabbet fillets on wheels other than on stage one, cannot be inspected without a complete rotor disassembly. Requirements for follow-up inspections
such as those that require additional major inspections and rotor swaps
limit risk-measurement strategies based on inspect/repair and re-use. It also forces operators to accept higher risk via exposure to non-inspectable flaws in higher-risk components. In fact, since most critical failure modes are associated with non-inspectable flaws, this is a major limitation of the risk-measurement approach. GE's RLE provides benefits that exceed those provided through a risk-measurement scope.
OEMs possess domain knowledge such as engineering, manufacturing, and material data that enable a more-comprehensive risk management strategy. GE uses a tiered risk-management approach that incorporates a portfolio of three available options: replacement-inkind, performance upgrades, and life-extension. These can be applied individually or in concert to best suit an operator's specific strategic objectives. Each rotor is uniquely characterized by the combination of its particular configuration and operational history. Based on a thorough analysis of this history, components deemed low-risk are thoroughly inspected and returned to service. Medium-risk components are thoroughly inspected and repaired, to increase their fatigue/creep life. Higher-risk components are replaced during the re-build process, since this is the most important method for providing lower operating risk during extended operation. During reassembly, cold section blade clocking is reset for improved compressor durability, consistent with US patent 8,439,626. GE's F-Class RLE portfolio offering provides for one or two maintenance-interval (Ml) extensions on hours, and one Ml extension on starts. This is equivalent to as many as 96,000 additional FFH, or up to 2,400 additional FFS, without requiring any follow-on inspection, by virtue of the replacement of higher-risk components. Figure 2 demonstrates GE's approach to designing RLEs.
GE's RLE Upgrade Approach
Expanding the value of an RLE upgrade also requires that it is executed at the right point in the rotor's life. As stated above, the medium-risk components can have their fatigue/creep life- extended through a repair process. However, fatigue-damaged areas can increase in size overtime, and can exceed repair limits if operated past GER3620 service limits. RLE upgrades should therefore be performed between 96,000 and 144,000 FFH, or 2,500 and 5,000 FFS, in order to ensure that the upgraded rotor can attain an ultimate 240,000 total FFH or 7,400 total FFS.
The Greenville service shop has the advantage of collaboration with the new-make manufacturing team to ensure that current assembly/ disassembly techniques and tooling are applied. This co-located collaboration has been especially beneficial in the development of the inspection and re-build portion of the life-extension process for the initial F-class rotors. GE applies normal shop methods such as Fluorescent Penetrant and Magnetic Particle Inspection (MPI), and Wheel Bore UT Inspections, augmented by Eddy Current Inspections for selected features. The compressor rotor is analyzed by UT examination for selected wheel bores, combined with MPI. Replicas are taken in certain locations to check surface integrity, in combination with hardness checks. Lessons-learned in Greenville are migrated to satellite service shops located globally.
GE's F-class gas turbine rotor experience began in Greenville in 1988 with the first F-class rotor assemblies and has accrued over the next 30 years with many rotor repairs, part replacements, and re-builds, resulting in company- proprietary tooling designs and processes. In anticipation of an aging F-class fleet, GE engineering and manufacturing have collaborated to offer rotor life-extension services for the F-class. This experiential learning has yielded the development of an effective rotor disassembly sequence, proprietary heating and cooling methods for rotor disassembly, and the evolution of disk-inspection and repair-procedures. Supplementing this manufacturing and design experience is, for a typical F-class gas turbine, an average of more than 10 years of monitoring and diagnostic operational data. This includes transient and steady-state temperature data that have been validated with base-load engine testing to generate boundary conditions for predictions of rotor thermal behavior. GE believes that this understanding of thermal transients is key to assessing rotor life, and that these analyses in support of RLEs form vital inputs in selecting part- replacement and repair strategy. This analytical approach is specific to each unit, as it is a function of that unit's specific configuration and operational history. For the two rotors evaluated to date, there have been no unexpected issues uncovered from the part inspections as compared to analytical predictions.
The first two F-class extended-life rotors shipped from Greenville were completely disassembled, with all components inspected, and repaired or replaced as appropriate. The GE process is based on unit-specific operational experience, validated analysis, and learned-out manufacturing techniques. This process is a differentiator that customers are welcome to observe in Greenville or in other global locations, in preparation for extending the life of their GE E or F-class gas turbine rotor, or any similar rotor technology.