Methods to Have Tighter Clearance in Gas Turbine: Turbine Section

Typically Gas turbine clearance between stator and rotor parts are governed by radial clearance which determines the performance, cooling flow requirements, part performance etc. These clearances have to be optimized to meet the product requirements. Too tight the clearance at assembly condition causes excessive rubbing during starting or shutdown of the gas turbine which could cause excessive heat generation and damage the rotating and statoric parts. In some case, rubbing can cause tip liberations and damages to the flow path causing aero dynamic losses. Similarly, if rotor tip clearance is large at assembly condition causes aerodynamic losses. Tip clearance should be kept as minimum as possible to have proper balance between Tip rubs at low speed and aero dynamic loss. In this paper describes the experience of Baker Hughes, wherein different design options to have optimized clearance in Low pressure turbine section of Gas turbine is discussed. Typically in Low pressure turbine wherein the turbine airfoils are provided with tip rails and shrouds are brazed with honeycombs. These are allowed to rub during the startup of Gas turbine engine and provide a tighter clearance at Steady state operating condition. In this paper, to have tighter clearance in operating condition few methods of incorporating axial clearance in addition to radial tip clearance is discussed. By incorporating tighter axial clearance, it is found to be evident that performance estimated has improved compared to having only radial tip clearance. This paper also describes how the honeycomb design has to be for axial clearance control and the methods for rubbing the tip shroud with honeycomb. This paper highlights the worm chart developed for a rotor blade and stator shroud configuration and highlight the benefit of axial clearance control mechanism. This paper also covers the different designs of heat dissipation methods in the tip rails of a bucket while it is rubbing with honeycomb. In this paper, analytical case study was shown that, when axial clearance is made tighter, it reduces the leakage flows and improves Low pressure turbine stage efficiency significantly