Guidelines to achieve successful gas turbine part refurbishment By Stephen R. Reid, P.E., TG Advisers Inc.
Gas turbine hot-section hardware requires part refurbishment after a specified number of operating hours, unit stop-start cycles, or a combination of the two. The repair processes required to achieve an additional interval of service present significant challenges. The variability in incoming part condition, process variation and the human element are key factors to understand and monitor in order to achieve successful part refurbishment.
Key gas turbine repair processes: • Inspection • Coating removal • Joining • Machining • Coating application Failure mechanisms for hot-section hardware include oxidation, creep, cracking and dimensional distortion. It is vital that incoming inspections be robust to properly define repair scope and identify unrepairable hardware from the onset. For most hardware, it is expected that coatings will be depleted or spalled after a service interval. Traditionally, ceramic coatings are removed through an abrasive grit blast and metallic bond coats are removed through acid stripping. Common issues encountered are excessive base metal removal, inter-granular acid attack and incomplete removal. Cracking from thermal mechanical fatigue is often present during refurbishment. This repair has become increasingly challenging with the widespread use of nickel-based superalloys across the industry. The joining processes most commonly performed for crack repair are welding and brazing. Some factors impacting the quality are joining material selection, surface cleanliness, proper process
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Figure 1. Sample gas turbine Stage 1 blade prior to repair; photo courtesy of “Gas Turbine Journal”
inputs and part pre-heat, and pre- and post-process heat treatments. Material build-up and machining, such as tip or hardface repair on rotating blades, is often required during part refurbishment. With part creep and distortion it becomes increasingly difficult to achieve desired dimensional results. It is important that fixturing and machining processes are robust and have the ability to adapt to the often significant variation in part condition. Thermal spray processes are utilized to reapply coating systems to hardware. Most hardware receives coating prior to final machining during initial manufacturing. The presence of such features as cooling holes and seal slots makes recoating more challenging than the initial application. Part geometry, pre-heat temperature, surface cleanliness and roughness, gas and powder properties, and coating gun condition are some of the factors that influence coating quality.
Recommended guidelines to improve repair quality The following steps are recommended to improve the quality of part refurbishment.
SOHRE TURBOMACHINERY® INC. 128 Main Street, P.O. Box 1099 Monson Massachusetts, USA 01057-1099 30 ENERGY-TECH.com PH: 413.267.0590 • 800.207.2195 • FX: 413.267.0592 email@example.com • www.sohreturbo.com
Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants