Materials implications for maintenance of gas turbines

Gas turbines are the essential components in modern gas fired combined cycle and CHP plants, as well as in simple cycle peaking or reserve plants in addition to aircraft engines. In power applications, plants using gas turbines show an attractive combination of cycle efficiency and environmental benefit, with good potential for further development to retain their competitive position. Further increase of gas turbine plants in the share of the power market may be mostly limited at least regionally by access to natural gas of required quantity and price. However, another characteristic feature of gas turbines is their high maintenance cost, mainly due to the short life cycles and considerable spare part cost of the hot path components. Apart from the advances in structural design and cooling techniques, one of the key developments towards efficient gas turbines has required materials that can withstand the severe service conditions of the hot path. These service conditions also limit the life or reconditioning periods of the major components. In theory, new materials and other advances could be applied to extend the component reconditioning and replacement life and to decrease the maintenance cost of gas turbines. In reality much of the materials development has aimed to improve the cycle efficiency, while the design life of coatings and base materials of the hot path has remained nearly constant. This paper reviews some implications of the materials development and the consequences for the maintenance of the hot section of gas turbines. Life limiting damage mechanisms such as creep, thermal fatigue and corrosion/oxidation are greatly accelerated by increasing the peak temperatures. The observed relatively steady lifetimes or reconditioning periods in spite of the increasing temperatures suggest overall success in improving reliability, at least for turbines which are not at their early stages of development.