Methods to Have Tighter Clearance in Gas Turbine: Turbine Section

Abstract

Typically, Gas turbine clearance between stator and rotor parts are governed by radial clearance which determines performance, cooling flow requirements, part performance etc. These clearances must be optimized to meet product requirements. Clearance which are kept too tight at assembly condition causes excessive rubbing during starting or shutdown of gas turbine which results in excessive heat generation and damage to rotating and statoric parts. In some cases, rubbing could cause tip liberations and damages to flow path which results in aero dynamic losses. Similarly, if rotor tip clearance is large at assembly condition results in aerodynamic losses. While designing component design at interface locations, 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 which are considered to arrive at optimized clearance in low pressure turbine section of Gas turbine have been discussed. Typically, in low pressure turbine wherein the turbine airfoils were provided with tip rails and shrouds are brazed with honeycombs. These were allowed to rub during 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 are discussed. By incorporating tighter axial clearance, it was found to be evident that performance estimated has improved compared to having only radial tip clearance. This paper also describes design considerations for honeycomb pertaining to axial clearance control and the methods for rubbing tip shroud with honeycomb. This paper highlights worm chart developed for a rotor blade and stator shroud configuration and highlight benefit of axial clearance control mechanism. This paper also covers different designs of heat dissipation methods in tip rails of a bucket while it is rubbing with honeycomb. In this paper, based analytical case study proves that, when axial clearance was made tighter, it reduces leakage flows and improves Low pressure turbine stage efficiency significantly.