Design Optimization of a Retractable Holder for Compressor Discharge Brush Seal

Abstract

In many industrial gas turbines, a portion of the compressor discharge air is extracted through a secondary flow path to aid the cooling of critical turbine components as well as to supplement purge flow for preventing hot gas ingestion in the first forward turbine bucket wheel space. GE has developed advanced brush seals for controlling the amount of cooling/purge flow passing through this secondary flow path (also called the high pressure packing (HPP) circuit) and has successfully implemented them in the field in a variety of E, F & H class gas turbines. During turbine shutdown, due to a lag in thermal response between the rotor and the stator, interference can result between brush seal bristles and the rotor surface causing significant amounts of wear. This wear can accumulate over several start up / shut down cycles resulting in an increased secondary flow through the HPP circuit and thus a loss in turbine efficiency and power output. In order to alleviate this situation, a seal holder has been designed to passively retract the HPP brush seal, from a low clearance position to a high clearance position, during turbine shut down and thus prevent seal interference/wear. This paper delves into the design and optimization of a retractable seal. An analytical model was developed to predict the seal motion during startup and shutdown of the turbine. Critical geometry and design parameters affecting seal closure and retraction behavior were identified. In addition, criteria for stability of seal motion were developed and the design was optimized to meet these requirements. Seal wear during turbine shutdown is avoided by ensuring that the seal retracts faster than the rate of thermally induced interference. The effect of design variables was minimized to ensure seal closure and retraction behavior does not vary significantly over the operating life of the seal. Model predictions were validated by subscale rig tests performed in the laboratory.