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Research Papers: Gas Turbines: Turbomachinery

Investigation Into the Thermal Limitations of Steam Turbines During Start-Up Operation

[+] Author and Article Information
Monika Topel

Department of Energy Technology,
Royal Institute of Technology,
Stockholm SE-100 44, Sweden
e-mail: monika.topel@energy.kth.se

Åsa Nilsson

Siemens Industrial Turbomachinery AB,
Finspång SE-612 83, Sweden
e-mail: asa.nilsson@siemens.com

Markus Jöcker

Siemens Industrial Turbomachinery AB,
Finspång SE-612 83, Sweden
e-mail: markus.jocker@siemens.com

Björn Laumert

Department of Energy Technology,
Royal Institute of Technology,
Stockholm SE-100 44, Sweden
e-mail: bjorn.laumert@energy.kth.se

1Corresponding author.

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 3, 2017; final manuscript received July 11, 2017; published online September 19, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(1), 012603 (Sep 19, 2017) (8 pages) Paper No: GTP-17-1252; doi: 10.1115/1.4037664 History: Received July 03, 2017; Revised July 11, 2017

Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine (ST) component, one key aspect of its flexibility is the capability for fast starts. In current practice, turbine start-up limitations are set by consideration of thermal stress and low cycle fatigue. However, the pursuit of faster starts raises the question whether other thermal phenomena can become a limiting factor to the start-up process. Differential expansion (DE) is one of such thermal properties, especially since the design of axial clearances is not included as part of start-up schedule design and because its measurement during operation is often limited or not a possibility at all. The aim of this work is to understand DE behavior with respect to transient operation and to quantify the effect that such operation would have in the design and operation of axial clearances. This was accomplished through the use of a validated thermomechanical model that was used to compare DE behavior for different operating conditions of the machine. These comparisons showed that faster starts do not necessarily imply that wider axial clearances are needed, which means that the thermal flexibility of the studied turbine is not limited by DE. However, for particular locations, it was also obtained that axial rubbing can indeed become a limiting factor in direct relation to start-up operation. The resulting approach presented in this work serves to avoid over-conservative limitations in both design and operation concerning axial clearances.

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References

Figures

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Fig. 1

Steam turbine thermal transient model

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Fig. 2

Turbine modular geometry and corresponding temperature measurement locations

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Fig. 3

Comparison of casing temperature results of thermal model and measured data for CS

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Fig. 4

Comparison of casing temperature results of thermal model and measured data for WS

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Fig. 5

Comparison of differential expansion results of thermal model and measured data for CS

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Fig. 6

Comparison of differential expansion results of thermal model and measured data for WS

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Fig. 7

DE behavior throughout the axial direction of the turbine: comparison of the validated transient starts against the established thermal lower and upper limits for clearance design

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Fig. 8

Differential expansion behavior against time for three seal locations during validated (left) cold and (right) warm starts

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Fig. 10

Available space at the clearances of the modeled turbine for the nominal case against two scenarios of temperature differences between casing and rotor at the beginning of the start

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Fig. 9

Differential expansion margins for clearance design throughout the axial direction of the turbine: comparison of the nominal case against two scenarios of temperature differences between casing and rotor at the beginning of the start

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Fig. 11

Design (left-solid), measured data (left-dotted), and faster (right) cold start schedules

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Fig. 12

Differential expansion behavior against time for three seal locations during (left) nominal and (right) aggressive cold starts

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