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

2017 ◽  
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.

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

2017 ◽  
Vol 140 (1) ◽  
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.

Utilization of a Thermo-Mechanical Model Coupled With Multi-Objective Optimization to Enhance the Start-Up Process of Solar Steam Turbines

2018 ◽  
Author(s):  
Monika Topel ◽  
Andrea Vitrano ◽  
Björn Laumert
Keyword(s):  
Steam Turbine ◽  
Solar Power ◽  
Low Cycle Fatigue ◽  
Steam Turbines ◽  
Multi Objective Optimization ◽  
Concentrating Solar Power ◽  
Multi Objective ◽  
Start Up ◽  
Turbine Component ◽  
Start Ups

The need to mitigate the climate change has brought in the last years to a fast rise of renewable technologies. The inherent fluctuations of the solar resource make concentrating solar power technologies an application that demands full flexibility of the steam turbine component. A key aspect of this sought steam turbine flexibility is the capability for fast starts, in order to harvest the solar energy as soon as it is available. However, turbine start-up time is constrained by the risk of low cycle fatigue damage due to thermal stress, which may bring the machine to failure. Given that the thermal limitations related to fatigue are temperature dependent, a transient thermal analysis of the steam turbine during start process is thus necessary in order to improve the start-up operation. This work focuses on the calculation of turbine thermo-mechanical properties and the optimization of different start-up cases in order to identify the best solution in terms of guaranteeing reliable and fast start-ups. In order to achieve this, a finite element thermal model of a turbine installed in a concentrating solar power plant was developed and validated against measured data. Results showed relative errors of temperature evolutions below 2%, making valid the assumptions and simplifications made. Since there is trade-off between start-up speed and turbine lifetime consumption, the model was then implemented within a multi-objective optimization scheme in order to test and design faster start-ups while ensuring safe operation of the machine. Significant improvements came up in terms of start-up time reduction up to 30% less than the standard start-up process.

Optimizing Lifetime Consumption and Increasing Flexibility Using Enhanced Lifetime Assessment Methods With Automated Stress Calculation From Long-Term Operation Data

2013 ◽  
Author(s):  
Jan Vogt ◽  
Thomas Schaaf ◽  
Klaus Helbig
Keyword(s):  
Power Plants ◽  
Low Cycle Fatigue ◽  
Combined Cycle ◽  
Transient Temperature ◽  
Steam Turbines ◽  
Transfer Coefficients ◽  
Term Operation ◽  
Flexible Mode ◽  
Start Up ◽  
Safety Margins

In the past most of the steam turbines were designed as base load machines. Due to new market requirements based on the effect of renewable energies, power plant operators are forced to operate with more frequent start-up events and load changes, resulting in a fundamental higher low cycle fatigue (LCF) lifetime consumption. Traditional methods of lifetime assessment often use representative start-ups, for the calculation of LCF damage, which can provide very conservative results with reasonable safety margins. For a high number of starts these safety margins may result in an overestimation of the LCF damage. At Alstom, an enhanced method for lifetime assessment has been developed, that evaluates the actual lifetime consumption from real operation data in an automated manner and provides much more realistic results. The operation data is used to calculate the transient temperature distribution and heat transfer coefficients along the rotor for each start-stop cycle. The corresponding stress distribution in the rotor is evaluated by means of a Finite-Element-method analysis. Finally the number of remaining cycles is extracted for the most critical locations using material data. In combination with the creep damage the lifetime consumption is evaluated. The entire process is highly automated, but also facilitates easy monitoring through the lifetime engineer by graphic presentation of calculation results. Using this enhanced method of lifetime assessment, the computed lifetime consumption is closer to the actual value, supporting the planning of overhauls and component replacements and minimizing the risk of failure or forced outages. The utilization of remaining lifetime can be optimized in favour of a more flexible mode of operation (e.g. low load operation and fast start-up) or extension of operational lifetime for conventional and combined cycle power plants.

Differential Expansion Sensitivity Studies During Steam Turbine Start-Up

2015 ◽  
Author(s):  
Monika Topel ◽  
Markus Jöcker ◽  
Sayantan Paul ◽  
Björn Laumert
Keyword(s):  
Thermal Behavior ◽  
Initial Temperature ◽  
Control Strategies ◽  
Cold Start ◽  
Steam Turbines ◽  
Relative Expansion ◽  
Start Up ◽  
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 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.

High Temperature Low Cycle Fatigue Behaviour of MarBN at 600 °C

2015 ◽  
Author(s):  
Richard A. Barrett ◽  
Eimear O’Hara ◽  
Padraic E. O’Donoghue ◽  
Sean B. Leen
Keyword(s):  
High Temperature ◽  
Low Cycle Fatigue ◽  
Experimental Testing ◽  
Computational Modelling ◽  
Operating Conditions ◽  
Experimental Program ◽  
Limiting Factor ◽  
Cycle Fatigue ◽  
Viscoplastic Material ◽  
Flexible Plant

The changing face of fossil fuel power generation is such that next generation plants must be capable of operating under (i) flexible conditions to accommodate renewal sources of energy and (ii) higher steam pressures and temperatures to improve plant efficiency. These changes result in increased creep and fatigue degradation of plant components. The key limiting factor to achieving more efficient, flexible plant operation is the development of advanced materials capable of operating under such conditions. MarBN is a new precipitate strengthened 9Cr martensitic steel, with added boron and tungsten, designed to provide enhanced creep strength and precipitate stability at high temperature. Accurate characterisation of this material is necessary so that it can be used under flexible plant operating conditions with high temperature fatigue. This paper presents a combined work program of experimental testing and computational modelling on a cast MarBN material. To characterise and assess the fatigue performance of MarBN, an experimental program of high temperature low cycle fatigue (HTLCF) tests is conducted at a temperature of 600 °C. MarBN is found to give an increased stress range compared to previous P91 steel experiments, as well as considerable cyclic softening. To characterise the constitutive behaviour of the cast MarBN material, a recently developed unified cyclic viscoplastic material model is calibrated and validated across a range of strain-rates and strain-ranges, with good correlation achieved with the measured data throughout.

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

2013 ◽  
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].

Optimization of Reactor’s Start-Up and Shutdown Procedures by Transient Thermo-Mechanical Finite Element Analysis

2018 ◽  
Author(s):  
Sang-Mo Lee ◽  
Ohgeon Kwon ◽  
Vitor Lopes Garcia
Keyword(s):  
Finite Element ◽  
Thermal Stresses ◽  
Structural Integrity ◽  
Low Cycle Fatigue ◽  
Fluid Dynamic ◽  
Flow Behavior ◽  
Operating Conditions ◽  
Element Analysis ◽  
Heating And Cooling ◽  
Start Up

Efficient refinery start-up and shutdown durations are vital in establishing prolonged productivity in refineries operating hydrotreating reactors. The benefits of efficient start up and shutdown cycles are extensive, and include considerable operational and cost reduction. Reduced start-up and shutdown cycles, however, require increased heating and cooling rates, which cause higher temperature gradients throughout the reactor vessel, consequently leading to higher thermal stresses, which may affect damage mechanisms and limit reactor’s life. The equipment’s OEM has defined guidelines for the reactor heating and cooling during start-up and shutdown cycles and any attempt to reduce the start-up and shutdown duration is usually limited by these guidelines. It is therefore necessary to carry out an engineering assessment to determine the effect of changing the start-up and shutdown procedures beyond the OEM guidelines on reactor’s life. Multiple thermo-mechanical Finite Element analyses for a series of different start-up/ shutdown procedures, including the current procedure, were carried out to determine the through-wall thermal gradient and stresses, and identify the most critical locations. In order to estimate convective heat transfer coefficients, Computational Fluid Dynamic (CFD) analysis was utilized to describe the complex fluid flow behavior of the feedstock in the presence of catalysts and internal geometry features. Low Cycle Fatigue (LCF) was adopted as a main damage mechanism to quantify the damage as a result of the changed operating conditions. It was determined that the LCF life calculated in the reactor vessel’s critical damage locations was found to be sufficiently long with respect to the frequency of start/shutdown cycles, even with operating conditions exceeding the OEM limit. Therefore, alternative guidelines were suggested to achieve the time reduction in startup/shutdown operation by increasing ramp rates without compromising structural integrity of the vessel.

A Framework for Life Prediction of 2.25Cr-1Mo Under Creep and Thermomechanical Fatigue

2018 ◽  
Author(s):  
Firat Irmak ◽  
Ali P. Gordon
Keyword(s):  
Life Prediction ◽  
Low Cycle Fatigue ◽  
Thermomechanical Fatigue ◽  
Operating Conditions ◽  
Limiting Factor ◽  
Low Alloy Steels ◽  
Experimental Database ◽  
Damage Maps ◽  
Alloy Steels ◽  
Prediction Approach

Low alloy steels are often utilized in components experiencing decades of usage under aggressive operating conditions. Even though there has been remarkable advancement in the development of modern alloys, however, these materials continue to be applied in boilers, heat exchanger tubes, and throttle valve bodies in both turbomachinery and pressure-vessel/piping applications. These steels display excellent resistance to deformation and damage under creep and/or fatigue at moderate temperatures. For example, the material 2.25Cr-1Mo has exceptional balance of ductility, corrosion resistance, and creep strength under temperatures up to 650□C. Both creep and non-isothermal fatigue conditions have been the limiting factor for most 2.25Cr-1Mo components; therefore, a life prediction approach is constructed with the capability of approximating the number of cycles to failure for conditions where the material is experiencing creep and fatigue with thermal cycling. Parameters for the approach are built on regression fits in comparison with a comprehensive experimental database. This database includes low cycle fatigue (LCF), creep fatigue (CF), and thermomechanical fatigue (TMF) experiments. The cumulative damage approach was utilized for the life prediction model where dominant damage maps can be used to determine primary microstructural mechanism associated with failure. Life calculations are facilitated by the usage of a non-interacting creep-plasticity constitutive model capable of representing not only the temperature- and rate-dependence, but also the history-dependence of the material.

Operation and Maintenance Improvements of Steam Turbines Subject to Frequent Start by Rotor Stress Monitoring

2020 ◽  
Author(s):  
Federico Bucciarelli ◽  
Damaso Checcacci ◽  
Gabriele Girezzi ◽  
Annamaria Signorini
Keyword(s):  
Fatigue Damage ◽  
Thermal Stresses ◽  
Low Cycle Fatigue ◽  
Weather Conditions ◽  
Control Panel ◽  
Steam Turbines ◽  
Stress Monitoring ◽  
Warm Up ◽  
Start Up ◽  
Critical Components

Abstract Steam Turbines operating in Concentrated Solar Plants and Peaking Combined Cycles are subjected to daily thermal stresses, induced by start-ups and load variations, deeply affecting allowed production per day. The extent and number of such thermal stresses is largely depending on the capability, of both plant and operators, to smooth the variations in steam temperature and load resulting from both weather conditions (in CSPs) and grid demand. In this operating scenario, conservative simplified rules are normally applied to determine daily warm-up times duration at starts, to preserve critical components from Low Cycle Fatigue damage; the planned maintenance intervals, as well, have been typically defined on the basis of a specified number of starts and running hours. In this article, the application of an online Rotor Stress Monitoring (RSM) technology, installed in the Steam Turbine User Control Panel, is used to directly determine the fatigue damage cumulated by each Start-Up and variation in operating condition. The results of application of this technology, with respect to standard formulations, are shown for a specific Concentrated Solar Plant across an operating period of four years. It is shown how, using the RSM as a basis for either startup or maintenance scheduling, can result in optimization of start-up times and maintenance intervals both for new units and retro-fit. The applicability of rotor stress direct monitoring and life analysis to higher temperature services is also introduced.

Integrated Approach for Steam Turbine Thermostructural Analysis and Lifetime Prediction at Transient Operations

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
Leonid Moroz ◽  
Glenn Doerksen ◽  
Fernando Romero ◽  
Roman Kochurov ◽  
Boris Frolov
Keyword(s):  
Power Plant ◽  
Steam Turbine ◽  
Thermal Stresses ◽  
Low Cycle Fatigue ◽  
Integrated Approach ◽  
Combined Cycle ◽  
Secondary Flows ◽  
Lifetime Prediction ◽  
Start Up ◽  
Differential Expansion

In order to achieve the highest power plant efficiency, original equipment manufacturers continuously increase turbine working parameters (steam temperatures and pressures), improve components design, and modify start-up cycles to reduce time while providing more frequent start-up events. All these actions result in much higher levels of thermostresses, a lifetime consumption of primary components and an increased demand for accurate thermostructural and low cycle fatigue (LCF) simulations. In this study, some aspects of methodological improvement are analyzed and proposed in the frame of an integrated approach for steam turbine components thermostructural analysis, reliability, and lifetime prediction. The full scope of the engineering tasks includes aero/thermodynamic flow path and secondary flows analysis to determine thermal boundary conditions (BCs), detailed thermal/structural two-dimensional and three-dimensional (3D) finite element (FE) models preparation, components thermal and stress–strain simulation, rotor–casing differential expansion and clearances analysis, and finally, turbine unit lifetime estimation. Special attention is paid to some of the key factors influencing the accuracy of thermal stresses prediction, specifically, the effect of “steam condensation” on thermal BC, the level of detailing for thermal zones definition, thermal contacts, and mesh quality in mechanical models. These aspects have been studied and validated against test data, obtained via a 30 MW steam turbine for combined cycle application based on actual start-up data measured from the power plant. The casing temperatures and rotor–stator differential expansion, measured during the commissioning phase of the turbine, were used for methodology validation. Finally, the evaluation of the steam turbine HPIP rotor lifetime by means of a LCF approach is performed.

Sign in / Sign up

Export Citation Format

Share Document