Integrated Weld Quality Concept—A Holistic Design Approach for Steam Turbine Rotor Weld Joints

The today's energy market requires highly efficient power plants under flexible operating conditions. Especially, the fluctuating availability of renewables demands higher cycling of fossil fired power plants. The need for highly efficient steam turbines is driven by CO₂ reduction programs and depletion of fossil resources. Increased efficiency requires higher steam temperatures up to 630 °C in today's units or even more for future steam power plants. The gap between material properties in the hot and cold running parts of a steam turbine rotor is widened by increased live steam temperatures and the increased demand for flexibility. These technical challenges are accompanied by economic aspects, i.e., the market requirements have to be met at reasonable costs. The welding of steam turbine rotors is one measure to balance required material properties and economical solutions. The rotor is a core component of the steam turbine and its long-term integrity is a key factor for reliable and safe operation of the power plant. An important aspect of weld quality is the determination of permissible size of weld imperfections assessed by fracture mechanics methods. The integrity of rotor weld joints is assured by ultrasonic inspection after the final post weld heat treatment with respect to fracture mechanics allowable flaw sizes. This procedure usually does not take credit from the quality measures applied during monitoring of the welding process. This paper provides an overview of a holistic design approach for steam turbine rotor weld joints comprising the welding process and its improved online monitoring, nondestructive evaluation, material technology, and its fracture mechanics assessment. The corresponding quality measures and their interaction with fracture mechanics design of the weld joint are described. The application of this concept allows to exploit the potentials of weld joints and to assure a safe turbine operation over life time.