Power Engineering, Feb. 2017, by Muhammad Saqib Riaz.
http://www.power-eng.com/articles/print/volume-121/issue-2/features/steam-turbine-retrofits.html

"A steam turbine is designed and optimized for a specific set of steam conditions. After long term operations either the steam path components deteriorate or due to changes in steam conditions, a re-optimization of the steam path components is required to regain higher turbine performance. Steam Turbine retrofits are performed to achieve various goals such as improved turbine efficiencies, heat rate or power output. Retrofits are also performed to address turbine reliability and or maintenance issues. Availability of newer technologies and materials for steam path components can be utilized to not only increase steam turbine performance, but also to extend maintenance intervals, and extend life. Retrofits can also accommodate required changes to the thermal cycle based on emerging needs of the power plant. The effectiveness of the retrofit is achieved by utilizing the best available technologies configured to the plant's specific needs."