Christopher Edward Wolfe

Fundamental Design Issues of Brush Seals for Industrial Applications

Advanced seals have been applied to numerous turbine machines over the last decade to improve the performance and output. Industrial experiences have shown that significant benefits can be attained if the seals are designed and applied properly. On the other hand, penalties can be expected if brush seals are not designed correctly. In recent years, attempts have been made to apply brush seals to more challenging locations with high speed (>400 m/s), high temperature (>650 °C), and discontinuous contact surfaces, such as blade tips in a turbine. Various failure modes of a brush seal can be activated under these conditions. It becomes crucial to understand the physical behavior of a brush seal under the operating conditions, and to be capable of quantifying seal life and performance as functions of both operating parameters and seal design parameters. Design criteria are required for different failure modes such as stress, fatigue, creep, wear, oxidation etc. This paper illustrates some of the most important brush seal design criteria and the trade-off of different design approaches.

Improved Steam Turbine Leakage Control With A Brush Seal Design.

Brush seal designs offer improved leakage control over conventional labyrinth seals by applying a compliant bristle with very tight clearance over the rotating shaft. Originally developed for the aircraft engine industry, brush seals have also been applied in land-based gas turbines. More recently, brush seals have been applied in steam turbines. The steam turbine environment adds some unique design considerations that must be addressed to assure a robust and effective brush seal design, and to minimize the impact on the steam turbine as a system. This paper discusses the performance benefits of the brush seal and the design considerations important to a robust design in a steam turbine. The paper also addresses the system characteristics important to 33 IMPROVED STEAM TURBINE LEAKAGE CONTROL WITH A BRUSH SEAL DESIGN

Design and Testing Methodology for Motion & Vibration Characterization of Advanced Seals

Advanced seals like brush seals, retractable labyrinth seals, leaf seals and film-riding seals are flexible seals capable of accommodating radial and axial motions of the rotor relative to the stator in turbomachinery applications like gas turbines, steam turbines and aircraft engines. In addition to determining the leakage performance of these advanced seals, it is important from a seal-rotor interaction perspective to characterize the seal radial motion relative to the rotor and from a seal reliability perspective to characterize seal vibrations caused by the surrounding air flow. In this paper, we present the development of a high-pressure non-rotating test fixture to study aerostatic performance of a film-riding seal. We present test data for airflow actuated seal radial motion relative to a stationary rotor, flow-induced seal vibrations, and pressure fields around the seal and compare these test data with simple predictive models. We demonstrate the utility of this test fixture as a design tool for characterizing aerostatic performance of advanced seals and as a method for validating mathematical models for predicting seal performance.

Experimental Test Results for a Fiber Bragg Grating-based Flow Sensor:

A fiber optic-based mass flow sensor has been developed that uses fiber Bragg gratings to deduce flow velocity. Flow velocity, local temperature, pressure measurements (which all can be extracted using fiber Bragg gratings), and geometric information can be combined to determine mass flow. A range of concepts have each been investigated and compared using the same “design of experiment” for each sensor. The most promising concept has been further developed into a prototype. The working prototype successfully demonstrated a thermally insensitive sensor design that has the capability to track flow velocities. The sensor design is incorporated directly with a structural beam element to magnify the strain effect while simultaneously compensating for thermally induced wavelength shifts in the sensor response. Further testing has been performed using three flexible beams at different angular positions, demonstrating that flow angles can be measured using a similar approach to that used for three-hole pneumatic probes. As a final test, the sensor has been tested in a shock tube, demonstrating superior performance to steady pneumatic measurements, which rely on tubing to reach the measurement location.

Design Optimization of a Retractable Holder for Compressor Discharge Brush Seal

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.Copyright

Experimental Test Results for a Fiber Bragg Grating-Based Flow Sensor

A fiber optic-based mass flow sensor has been developed that uses fiber Bragg gratings to deduce flow velocity. Flow velocity, local temperature, pressure measurements (that all can be extracted using fiber Bragg gratings) and geometric information can be combined to determine mass flow. A range of concepts have been investigated and compared using the same “design of experiment” for each sensor. The most promising concept has been further developed into a prototype. The working prototype successfully demonstrated a thermally insensitive sensor design that has the capability to track flow velocities. The sensor design is incorporated directly with a structural beam element to magnify the strain effect while simultaneously compensating for thermally-induced wavelength shifts in the sensor response. Further testing has been performed using three flexible beams at different angular positions showing that flow angles can be measured similar to the approach used for 3-hole pneumatic probes. As a final test, the sensor has been tested in a shock tube demonstrating superior performance compared to steady pneumatic measurements which rely on tubing to reach the measurement location.Copyright