Gas Turbine Transition Duct Design Considerations

Abstract

The gas turbine twin shaft variant requires an aerodynamic coupling between its high-pressure gas generator module and low-pressure power module. These two modules are spaced apart for aerodynamic and mechanical reasons. Typically, this is achieved by having a transition duct (TD) in between these two modules which smoothly allows the gas flow from HP turbine to LP turbine. To fulfill this purpose, it includes a radial inner shell and a radial outer shell defining a flow passage and radial struts which connects inner shell and outer shell. Cross-section of these struts are airfoil shaped to facilitate smooth flow of flue gas through TD. In addition, TD allows flow passage for cooling medium, support seals to prevent leakage at various interface locations and holds thermo-well for temperature measurement. Considering thermal and mechanical loads on the TD for standard operating conditions, Low cycle fatigue, creep and oxidation calculations are performed to ensure durability of TD design and necessary design changes are made to fulfill product requirement. Suitable material is selected, and manufacturing method finalized based on the design analysis. While standard offerings of gas turbine fulfill majority of requirements, there are some off-design or customized requirements which require detailed study. To fulfill these types of customized requirements, standard offerings of gas turbine are further analyzed for changed thermal boundary condition. These types of customizations facilitate running gas turbine with different options with a clear understanding of how it impacts durability. Through these studies, necessary maintenance factors are established to arrive at revised maintenance intervals based on severity of change. Low cycle fatigue, creep and oxidation calculations are performed with changed boundary condition also and maintenance factor is determined. This paper is intended to describe the steps followed in determining durability, establish maintenance factor and provide service recommendations.