Differential Expansion Sensitivity Studies During Steam Turbine Start-Up

In order to improve the startup flexibility of steam turbines it becomes relevant to analyze their dynamic thermal behavior. In this work, the relative expansion between rotor and casing was studied during cold start conditions. This is an important property to monitor during start-up given that clearances between rotating and stationary components must be controlled in order to avoid rubbing. The investigation was performed using a turbine thermal simplified model from previous work by the authors. The first step during the investigation was to extend and refine the modeling tool in order to include thermo mechanical properties. Then, the range of applicability of the model was validated by a two-fold comparison with a higher order finite element numerical model and measured data of a cold start from an installed turbine. Finally, sensitivity studies were conducted with the aim of identifying the modeling assumptions that have the largest influence in capturing the correct thermal behavior of the turbine. It was found that the assumptions for the bearing oil and inter-casing cavity temperatures have a large influence ranging between ± 25% from the measured values. In addition, the sensitivity studies also involved increasing the initial temperature of the casing in order to reduce the peak of differential expansion. Improvements of up to 30% were accounted to this measure. The studies performed serve as a base towards further understanding the differential expansion during start and establishing future clearance control strategies during turbine transient operation.

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Differential Expansion Sensitivity Studies During Steam Turbine Startup

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4031643 ◽

2015 ◽

Vol 138 (6) ◽

Cited By ~ 6

Author(s):

Monika Topel ◽

Markus Jöcker ◽

Sayantan Paul ◽

Björn Laumert

Keyword(s):

Thermal Behavior ◽

Initial Temperature ◽

Control Strategies ◽

Cold Start ◽

Thermomechanical Properties ◽

Steam Turbines ◽

Relative Expansion ◽

Sensitivity Studies ◽

Differential Expansion ◽

Clearance Control

In order to improve the startup flexibility of steam turbines, it becomes relevant to analyze their dynamic thermal behavior. In this work, the relative expansion between rotor and casing was studied during cold-start conditions. This is an important property to monitor during startup given that clearances between rotating and stationary components must be controlled in order to avoid rubbing. The investigation was performed using a turbine thermal simplified model from previous work by the authors. The first step during the investigation was to extend and refine the modeling tool in order to include thermomechanical properties. Then, the range of applicability of the model was validated by a twofold comparison with a higher order finite element (FE) numerical model and measured data of a cold start from an installed turbine. Finally, sensitivity studies were conducted with the aim of identifying the modeling assumptions that have the largest influence in capturing the correct thermal behavior of the turbine. It was found that the assumptions for the bearing oil and intercasing cavity temperatures have a large influence ranging between ±25% from the measured values. In addition, the sensitivity studies also involved increasing the initial temperature of the casing in order to reduce the peak of differential expansion. Improvements of up to 30% were accounted to this measure. The studies performed serve as a base toward further understanding the differential expansion during start and establishing future clearance control strategies during turbine transient operation.

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

Volume 8: Microturbines, Turbochargers and Small Turbomachines; Steam Turbines ◽

10.1115/gt2017-64281 ◽

2017 ◽

Cited By ~ 2

Author(s):

Monika Topel ◽

Björn Laumert ◽

Åsa Nilsson ◽

Markus Jöcker

Keyword(s):

Electricity Market ◽

Low Cycle Fatigue ◽

Operating Conditions ◽

Limiting Factor ◽

Steam Turbines ◽

Expansion Behavior ◽

Start Up ◽

Schedule Design ◽

Turbine Component ◽

Differential Expansion

Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine 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 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 differential expansion 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 thermo-mechanical model that was used to compare differential expansion 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 differential expansion. 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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Investigation Into the Thermal Limitations of Steam Turbines During Start-Up Operation

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4037664 ◽

2017 ◽

Vol 140 (1) ◽

Cited By ~ 2

Author(s):

Monika Topel ◽

Åsa Nilsson ◽

Markus Jöcker ◽

Björn Laumert

Keyword(s):

Electricity Market ◽

Low Cycle Fatigue ◽

Operating Conditions ◽

Limiting Factor ◽

Steam Turbines ◽

Thermomechanical Model ◽

Start Up ◽

Schedule Design ◽

Differential Expansion ◽

Thermal Phenomena

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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Experimental Investigation Into Thermal Behavior of Steam Turbine Components: Part 3 — Startup and the Impact on LCF Life

Volume 3C: Heat Transfer ◽

10.1115/gt2013-94356 ◽

2013 ◽

Cited By ~ 7

Author(s):

Gabriel Marinescu ◽

Michael Sell ◽

Andreas Ehrsam ◽

Philipp B. Brunner

Keyword(s):

Thermal Behavior ◽

Steam Turbine ◽

Low Cycle Fatigue ◽

Numerical Procedure ◽

Cyclic Fatigue ◽

Low Flow ◽

Steam Turbines ◽

Early Loading ◽

Start Up ◽

The Impact

Steam turbine start-up has a significant impact on the cyclic fatigue life. Modern steam turbines are operated at high temperatures for optimal efficiency, which results in high temperature differences relative to the condition before start-up. To achieve the fastest possible start-up time without reducing the lifetime of the turbine components due to excessive thermal stress, the start-up procedure of cyclic turbines is optimized to follow the specific material low cycle fatigue limit. For such optimization and to ensure reliable operation, it is essential to fully understand the thermal behavior of the components during start-up. This is especially challenging in low flow conditions, i.e. during pre-warming and early loading phase. A two-dimensional numerical procedure is described for the assessment of the thermal regime during start-up. The calculation procedure includes the rotor, casings, valves and main pipes. The concept of the start-up calculation is to replace the convective effect of the steam in the turbine cavity by an equivalent fluid over-conductivity that gives the same thermal effect on metallic parts. This approach allows simulating accurately the effect of steam ingestion during pre-warming phase. The fluid equivalent over-conductivity is calibrated with experimental data. At the end of the paper the impact of ingested steam temperature and mass-flow on the rotor cyclic lifetime is demonstrated. This paper is a continuation of papers [1] and [2].

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