Design of Heat Shield for Compressor Case of Industrial Gas Turbine to Meet Clearance Requirement

Abstract

Abstract In Gas Turbine design key important role was to establish proper clearance between rotating and statoric parts during operating conditions which controls the performance, cooling flow requirements, part performance etc., These clearances must be optimized to meet product requirements. Too tight clearance at assembly condition causes excessive rubbing during starting or shutdown of gas turbine which could cause excessive heat generation and damage rotating and statoric parts. In some case, rubbing can cause tip liberations and damages to flow path causing aero dynamic losses. Similarly, if clearance is large at assembly condition causes aerodynamic losses. In this paper describes the experience of Baker Hughes, in design of compressor case wherein different design options in casing design with and without considering external features / components are considered to have adequate clearance between rotating and statoric parts. It also describes the Heat shield design iterations which was provided on a compressor case to establish proper thermal response during transient operating conditions. This helps in providing adequate clearance without causing excessive rubs or too large clearance avoiding aerodynamic losses. During development of heat shield design, challenges encountered considering clearance, manufacturing aspects, assembly feasibility and part life capabilities like low cycle fatigue, high cycle fatigue requirements are discussed in the paper. Also heat shield was subjected to high thermal gradient due to temperature difference, this makes heat shield to have constrained growth. This restriction in growth provides huge stresses beyond the material limit causing it to fail before product requirement time period. To avoid constrained growth, this paper describes how the heat shield was connected to casing by different means are mentioned. It also describes the impact on frequency margin if there is not adequate fixity in heat shield design. Some of the design parameters like circumferential & axial ribs and intermittent stiffeners and its influence on stress by comparing against yield and on frequency margin with reference to potential driver are also discussed in this paper. It also incorporates the methods to control intersegment leakages and design features to avoid interface interference. Feasibility study of heat shield design was done using finite element modeling techniques using ANSYS tool and its best practices would also be dealt in this paper.