The application of brush seals to steam turbine generators

Waite, Jason S. (1999) The application of brush seals to steam turbine generators. Doctoral thesis, University of Northumbria at Newcastle.

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Abstract

This thesis addresses three aspects of the development of brush seals for blade tip sealing purposes in steam turbine generators associated with tribological aspects, experimental leakage testing and computational fluid dynamic modelling of brush seals.
An elaborate materials selection/screening process was carried out to identify the candidate brush seal bristle materials for the application of brush seals for steam turbines. 25 candidate bristle materials from 4 superalloy groups were tested against an uncoated steam turbine rotor steel in a purpose built (pin on wheel) high temperature wear rig at 540C. Pre and post test hardness, coefficient of friction, and weight change were recorded. The wear scars produced were examined using a scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDAX).
The results demonstrated that iron oxides from the rotor steel were responsible for forming a wear resistant surface glaze on the contacting surfaces of the tribopair which,
once formed, prevented further damage or extensive material removal from the tribopair. The tribopair surface wear scars appeared to be very similar under examination by SEM
and EDAX for each test. No clear relationship between specific material properties and wear resistance was found for the materials tested. Haynes 25 the current gas turbine brush seal bristle material, was one of the materials recommended for further testing.
Back to back experimental leakage testing of conventional single labyrinth fins, single thickness and double thickness brush seals was carried out on a purpose built test rig. The rig and tests were designed to test large diameter (330 mm) brush seals under selected flow conditions encountered in steam turbines. A range of interferences and clearances were tested (-0.5, -0.2, 0, +0.5 and +0.9 mm). Shaft rotation (0-3000 rpm), shaft offset (0-0.9 mm), shaft eccentricity (0.5 mm peak to peak), shaft starting torque, and vibrational characteristics were variables that were explored.
The extensive range of test parameters was specifically aimed at generating a more comprehensive and realistic profile of brush seal operation for steam turbine application. The primary concern was to understand the changes in leakage characteristics of brush seals under changing operating conditions and due to changes in seal geometry. Examination of the extensive results provided evidence that can be compared to results from other studies. The leakage rates for single thickness and double thickness 330mm brush seals in these tests were approximately 5% and 3% respectively compared to the leakage rate through a single fin at the same clearance.
Computational Fluid Dynamics techniques were used to model brush seals in 2-dimensional form in radial and axial cross-section. Idealised bristle packing patterns were modelled and the results produced were used to calculate porosity coefficients. A further model was used to study bristle pack and backing plate clearance variations using different
sets of porosity coefficients.
Plots from both model types allowed detailed examination of flow characteristics. The results and conclusions drawn can be used to improve the design and performance of future brush seals. It was found that non-Darcian porosity coefficients applied to an area of porous media/distributed resistance could be successfully used to model the leakage
through the bristle pack. Calculated coefficients from the Bristle Packing Pattern Models were found to provide poor results when used in axial cross sectional/porous media
modelling. Empirically detennined porosity coefficients provided the best CFD results but the models were not sufficiently robust to predict seal performance for significant changes in geometry or conditions.

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