When it comes to refurbishing a rotor in a power plant, delivery time and shipping costs play a major role. Because of its nearby service center, Sulzer was able to make an unbeatable offer to an Indonesian customer and even bring a retired rotor back into operation.
Indonesia contains large oil reserves, e.g., at the Duri Field in Sumatra. There, a cogeneration power plant produces electricity and steam, which are used for oil recovery. The plant contains three Siemens Westinghouse 501D5A combustion turbine generators. One of the rotors passed its service limit of 100000 hours with 102696 actual running hours, and the operator wanted to know if the rotor lifetime could be extended. Since the delivery time was critical, the client selected Sulzer Indonesia to carry out the project. Having Sulzer handle the job would reduce lead time and cost over shipping it to the OEM facility. The power plant awarded Sulzer Indonesia a contract to do a lifetime assessment and a refurbishment of the rotor. The Sulzer team in Indonesia received full support from the Sulzer service center in Houston, which already had experience with this rotor model.
During the rotor inspection, the compressor and turbine section were completely disassembled for a thorough inspection of the individual parts. In general, no indications of cracking were found on the rotor. However, several compressor blades had defects from foreign object damage (FOD) and the turbine had to undergo blade replacement.
Tailored welding solution
Based on information from the client, the other W501DA rotors in the plant had vibration issues, which caused a big difference in thermal expansion between air separator and the torque tube. To avoid this issue on the disassembled rotor, the engineers needed to increase the axial interference by adding length to the air separator to compensate for the difference in thermal growth expansion by welding process.
One of the most critical aspects of turbine welding is the development and testing of the welding procedures. These take place prior to the repair being carried out. The first step of a proper weld repair is evaluating a sample of the rotor body material. The specimen usually is examined and approved by a third-party accredited test facility. The tests of bending behavior, macro hardness, chemical composition of the weld metal, and dye penetration are also performed to validate the process in accordance with the standards of the American Society of Mechanical Engineers (ASME). In the project described, all requirements were fulfilled and certified.
The selected weld procedure for the air separator conformed to the ASTM standard A-471 (Specification for Vacuum-Treated Alloy Steel Forgings for Turbine Rotor Disks and Wheels). To weld repair the air separator axis, the engineers used a new semiautomatic tungsten inert gas (TIG) weld machine on an adjustable-speed rotary table. This process has great advantages because it can provide the highest level of quality and consistency for critical repairs. The new equipment will allow Sulzer Indonesia to expand its TIG welding capabilities—also for the repair of future types of turbo-machinery components.
For the project described, further weld repair was required because the bearing journal of the compressor section showed unacceptable rubs marks. The customer wanted to keep the rotor interchangeable with all units, so the bearing journal had to be restored to the original size through weld repair. Because this rotor is categorized as a long and flexible rotor, this job was challenging. The engineers used submerged-arc welding according to a certified welding procedure specification (WPS). The bearing journal was restored without any quality issues, and the rotor remained interchangeable with its sister units.
Extending rotor lifetime
The hot sections of gas turbines are subject to the most severe conditions, e.g., high temperatures involving complex deformations and interactions throughout the turbine lifetime including creep, temper embrittlement, fatigue, oxidation/corrosion, aging, and other phenomena. That is why gas turbine (hot-section) components only survive a limited time. The weakest points of the design determine the remaining lifetime of the rotor. In order to assess and extend rotor lifetime, Sulzer carefully assessed the rotor using Eddy Current testing, phased-array ultrasonic examination, finite element analysis, and metallography analysis.
Metallographic techniques correlate changes in the microstructure and the onset of incipient creep damage, such as cavitation at grain boundaries. Replication techniques are used on areas that are subjected to the higher temperatures and stresses, commonly on hot gas components, such as the turbine wheel and the air separator. Creep is the most important issue in determining the remaining life of the turbine rotor. Creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part. In the metallographic analysis of this project, the microstructures of the turbine wheels and air separator showed some coalescence cavities, indicating that material degradation due to creep had taken place. Coalescence cavities are characterized by wide black lines, particularly at the grain boundaries. Based on this metallurgical examination, the remaining life of the rotor was estimated at 100000 operating hours. In other words, Sulzer’s refurbishment had doubled the lifetime of the rotor.
Sulzer Indonesia successfully completed the entire scope of work, including disassembly-reassembly, lifetime assessment, compressor and turbine blade replacement, weld repair of the journal, and low-speed balancing. After the project completion, the rotor that would have had to be replaced has now been returned into service for another 100000 hours. The unit was immediately put back into commercial operation, reached the base load without any vibration issues, and has been running since then without any major problems.