Variable Guide Vanes are common to many of the Gas Turbines. Nozzle Guide Vane (NGV) is a variable nozzle present in the hot gas path of a twin-shaft gas turbine. In addition to guiding the hot gases onto the buckets, the NGV also controls the energy split between the high-pressure turbine and power turbine. The NGV is rotated to the desired position for different machine operability requirements through an actuation system. A twin shaft gas turbine equipped with NGV allows higher operational flexibility and higher efficiency at partial load/speed.
MS3002F is a heavy duty gas turbine developed by GE in 1960’s with water cooling system for the turbine casing and the NGV. Over the period of time, technology has evolved and water cooling system in Gas Turbines has become obsolete, as it leads to uneven cooling of the casing and scaling that cause performance losses, unplanned outages and down-time. To cater the existing fleet with improved reliability and availability, a new product introduction program was launched to design air-cooled system to replace the water-cooled system, within the boundaries of retrofit ability. The air cooling is achieved by extracted secondary flow air; the reduction on performance due to this is compensated by increasing the firing temperature. Owing to the removal of water cooling and increased firing temperature, the turbine casing and hot gas path components including NGV have to be redesigned.
The present paper discusses the redesign of the NGV assembly for new air-cooled configuration. This redesign includes geometry modification, assembly interface definition (contact geometry and coating), and material selection. The key challenge is to redesign the NGV with no cooling (neither water nor extracted secondary flow) while maintaining the steady state clearances of the NGV assembly as those of water-cooled kit. These redesigned components are validated for structural integrity under loads affecting fatigue, creep and oxidation. The verification of the functional integrity of NGV actuation system (that was originally designed for water-cooled system) for all operating conditions relevant to the new air-cooled turbine case kit is also discussed.