Advanced flexible seals for gas turbine shroud applications

Abstract

INTRODUCTION Modern gas turbines have many shroud and nozzle segments that require high temperature sealing at the segment interfaces. Driven by higher efficiency demands, the need for better seals is inevitable. This work summarizes the evolution of metal cloth shroud seals through the stages of their development at General Electric (GE) Company. Current applications at GE gas turbines, and the achieved benefits are also summarized. The primary advantage of the cloth seal over the rigid seals is to provide flexibility in the. junctions between turbine components and to minimize the wear and leakage by accommodating offsets and misalignments between the components. Starting from rigid metal strips these patented flexible seals are developed through rigorous testing and trial of various sealing concepts. To compare effectiveness of different seal designs a high pressure static seal test setup is developed. Prototype seals are procured as 0.3 m (12 in.) long strips, and back-to-back leakage tests are performed. Flow rates for each sample is converted to equivalent sealing clearances/gaps and normalized with baseline leakage of rigid seal strips. Test results indicate that cloth seals provide up to 65% leakage flow reduction over rigid metal strips. Under offset and misalignment conditions cloth seals leak 77% less than the rigid strips. Application of cloth seals at nozzle, shroud and diaphragm inter-segment locations of GE 7F gas turbines resulted in 0.50% output increase and 0.25% heat rate decrease. Overall, cloth seals offer sizable efficiency improvements over the flat strips without much increase in the cost. As gas turbines are designed to operate at higher efficiency levels and ever increasing tiring temperatures, designers require high temperature, low leakage, and compliant seals to control parasitic leakage flows between turbine components. Advanced sealing applications in gas turbines include the junctions between the stationary and rotating parts in compressor and turbine sections, and between stationary components (nozzles, shrouds, etc.) throughout the internal cooling flow path. Typically, adjacent members have to sustain relative vibratory motion with minimal wear or loss of sealing. In addition, they have to accommodate thermal growth and misalignment. Such interfaces are seen in large assemblies, such as gas turbines, which comprise subcomponents that are packed together to form the entire assembly. Such a breakdown into subsystems is essential for ease of assembly and to accommodate misalignment, thermal growth and vibration during operation. * Copyright