Investigation of steam turbine labyrinth seals using an annular air model

Abstract

This thesis describes an experimental project which encompasses the design, fabrication and testing of an annular air flow facility applicable to the evaluation of steam turbine labyrinth seal designs for Westinghouse Electric Corporation. The main objectives of this project were to design and fabricate a test facility to experimentally evaluate seal discharge coefficients for selected stepped labyrinth seal geometries and eccentrically positioned straight-through labyrinth seals as a function of seal exit-to-inlet pressure ratio. Validation of the facility was determined by comparing results of concentrically positioned straight-through labyrinth seals to those obtained from a similar air test conducted by the Westinghouse Corporation in 1974. In all of the seal tests performed, geometric similitude considerations have been fulfilled and Reynolds number has been held to within the same order of magnitude of actual steam turbine conditions. In addition, the Mach number between the air tests and actual steam turbine conditions were similar since the seal pressure ratios and the isentropic exponents of the two fluids are approximately the same. Nine different stepped seal configurations were investigated. These configurations included the combinations of three step heights and three seal-to-step distances. The results of these tests were used to determine the sensitivity of seal discharge coefficient to step height and rotor position for pressure ratios experienced in low pressure steam turbines. In addition, the effect of turbine blade shaft eccentricity on the seal discharge coefficient was also examined.

The seal discharge coefficients of the various seal geometries were obtained by measuring the seal pressure drop and the mass flow rate of the air passing over the seals. The pressure drops were determined by using multiple pressure transducers and a computer controlled data acquisition system. Likewise, the method of determining mass flow rate included the use of this equipment and various sized orifice plates. The results of this project provided the following conclusions and recommendations: 1) The experimental facility used for this study provided discharge coefficients for straight-through seals that were within 10% of those measured by similar air tests conducted by Westinghouse in 197 4. 2) A stepped labyrinth seal with a step height of 0.188 inches (the largest step tested) in conjunction with a seal-to-step distance of 0.62 inches (the nominal distance) provides for the best tested sealing geometry for seal pressure ratios in the vicinity of 0.8. 3) The seal discharge coefficient for the stepped labyrinth seals is reduced for increases in seal step height and reductions in seal-to-step distance in all cases except for the largest step tested (0.188 inches). This step height causes an increase in the discharge coefficient when the seal-to-step distance is either smaller or larger than the nominal distance of 0.62 inches. This occurrence is possibly a result of the effect that the largest step has on the jet of fluid leaving the seal tip.

4) Totally eccentric straight-through labyrinth seals display evidence of decreased discharge coefficients in comparison to concentrically located seals. This result is inconsistent with previously published annular flow data and the uncertainty of this result is discussed. 5) Higher accuracy of future results may be obtained if flow straighteners were installed upstream of the flow measuring orifice plate. 6) The Reynolds numbers could be better matched in future tests if the upstream seal pressure is increased to approximately 80 psia. Yet, a system of two orifice plates would be needed; one to control seal backpressure and the other to measure mass flow rate. Currently, the maximum upstream seal pressure is limited to approximately 30 psia since the orifice plate must maintain a downstream to upstream pressure ratio greater than O. 75. The Westinghouse Labyrinth Seal Testing Facility is currently located on the University of Central Florida (UCF) campus and may be used by Westinghouse and UCF personnel to conduct additional labyrinth seal experiments.

Notes

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Graduation Date

1992

Semester

Spring

Advisor

Gunnerson, Fred

Degree

Master of Science (M.S.)

College

College of Engineering

Department

Mechanical and Aerospace Engineering

Degree Program

Mechanical Engineering

Format

PDF

Pages

151 p.

Language

English

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Identifier

DP0029858

Subjects

Dissertations, Academic -- Engineering; Engineering -- Dissertations, Academic

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