Henning Almstedt

Robust Methods for Creep Fatigue Analysis of Power Plant Components Under Cyclic Transient Thermal Loading

The aim of this paper is to apply robust mechanisms-based material laws to the analysis of typical high-temperature power plant components during an idealized start-up, hold time and shut-down sequence under a moderate temperature gradient. Among others a robust constitutive model is discussed, which is able to reflect inelastic deformation, hardening/recovery, softening and damage processes at high temperature. The model is applied for a creep analysis of advanced 9–12%CrMoV heat resistant steels and calibrated in particular case against experimental data for 10%CrMoV steel type. For a steam temperature profile transient heat transfer analysis of an idealized steam turbine component is performed providing the temperature field. From the subsequent structural analysis with the inelastic constitutive model local stress and strain state variations are obtained. As an outcome a multi-axial thermo-mechanical fatigue (TMF) loading loop for one or several loading cycles can be generated. They serve as input for a fatigue life assessment based on the generalized damage accumulation rule, whose results come close to reality. In addition, the accuracy of a simplified method which allows a rapid estimation of notch stresses and strains using a notch assessment rule (NAR) [1] based on Neuber approach is examined.Copyright

Integrated Weld Quality Concept—A Holistic Design Approach for Steam Turbine Rotor Weld Joints

The today’s energy market requires highly efficient power plants under flexible operating conditions. Especially, the fluctuating availability of renewables demands higher cycling of fossil fired power plants. The need for highly efficient steam turbines is driven by CO2 reduction programs and depletion of fossil resources. Increased efficiency requires higher steam temperatures up to 630°C in today’s units or even more for future steam power plants. The gap between material properties in the hot and cold running parts of a steam turbine rotor is widened by increased live steam temperatures and the increased demand for flexibility. These technical challenges are accompanied by economic aspects, i.e. the market requirements have to be met at reasonable costs.The welding of steam turbine rotors is one measure to balance required material properties and economical solutions. The rotor is a core component of the steam turbine and its long-term integrity is a key factor for reliable and safe operation of the power plant. An important aspect of weld quality is the determination of permissible size of weld imperfections assessed by fracture mechanics methods. The integrity of rotor weld joints is assured by ultrasonic inspection after the final post weld heat treatment with respect to fracture mechanics allowable flaw sizes. This procedure usually does not take credit from the quality measures applied during monitoring of the welding process.This paper provides an overview of an holistic design approach for steam turbine rotor weld joints comprising the welding process and its improved online monitoring, non-destructive evaluation, material technology, and its fracture mechanics assessment. The corresponding quality measures and their interaction with fracture mechanics design of the weld joint are described. The application of this concept allows to exploit the potentials of weld joints and to assure a safe turbine operation over life time.Copyright

The Role of Rotor Welding Design in Meeting Future Market Requirements

The demand for current and future steam turbine components is driven by higher efficiency but also by higher plant cycling needs and optimized cost balance. An increase in efficiency increases the demand for higher life steam temperatures of up to 620/630°C for today’s units and of even up to 720°C for future steam power plants. The gap between required material properties in the hot and cold running parts of a steam turbine rotor is widened by the increased live steam temperatures and the increased demand for flexibility and adaptability to current and expected future energy market conditions. Besides further material development, welding is one measure to realize such contradictory rotor characteristics. Whereas 720°C is more a future related task, solutions for 560°C / 620°C apply already welded rotors.The paper discusses from a perspective of a steam turbine manufacturer the technical features to enable flexible high efficient rotor components with a focus on advanced welding technologies suitable for different large steam turbine components and what further steps for new welding technologies are under way.Copyright

The Role of Advanced Fracture Mechanics Evaluation Methods for Turbine Components

The demand for higher plant cycling operation and reduced life-cycle costs are the main drivers for the design and assessment of turbine components today. Heavy cyclic loading increases the potential of fully utilizing the fatigue capabilities of the material which might lead to crack initiation and subsequent crack propagation.Fracture mechanics methods and evaluation concepts are widely applied to assess the integrity of components with defects or crack-like findings. The realistic modelling of the failure mechanism plays a key role for the accurate prediction of crack sizes at failure state.A basic treatment of material toughness typically leads to conservative assessments for components with sufficient ductility. A standard approach to describe material behavior with high ductility is to use the start of stable crack extension as a dimensioning parameter for the analysis. By definition a critical condition for a component is reached when the crack driving force is equal to the characteristic material parameter. On the other hand, advanced analysis methods allow determination of the instability point (ductile tearing analysis).This paper will discuss two cases for practical analysis from steam turbine design showing clear advantages for service application by using advanced analysis methods.© 2016 ASME

Setup, Validation and Probabilistic Robustness Estimation of a Model for Prediction of LCF in Steam Turbine Rotors

Flexibility and availability together with fast startup times become more and more important for steam turbine operation. Exact knowledge about the turbine components stresses and lifetime consumption during transient operation is a prerequisite in order to meet these requirements. A transient FE model of an intermediate pressure steam turbine rotor was generated, allowing the prediction of temperature and elastic stress field during turbine startup, load changes and shutdown. Operating data of the steam parameters and of a thermocouple inside the wall of the turbine inner casing were used to indirectly validate the thermal FE model in order to reproduce the measured metal temperatures in a proper accuracy. Subsequently a probabilistic sensitivity study was performed in order to identify the influence of scattering or not well known boundary conditions on the calculated lifetime consumption of the steam turbine rotor during a cold start. This in fact provides information about the accuracy of the prediction. The results of the sensitivity study also help to improve the model accuracy by identifying the boundary conditions with the largest impact on lifetime prediction uncertainty, i.e. the boundary conditions that need further investigation.

Integrated Weld Quality Concept: A Holistic Design Approach for Steam Turbine Rotor Weld Joints

The today’s energy market requires highly efficient power plants under flexible operating conditions. Especially, the fluctuating availability of renewables demands higher cycling of fossil fired power plants. The need for highly efficient steam turbines is driven by CO2 reduction programs and depletion of fossil resources. Increased efficiency requires higher steam temperatures up to 630°C in today’s units or even more for future steam power plants. The gap between material properties in the hot and cold running parts of a steam turbine rotor is widened by increased live steam temperatures and the increased demand for flexibility. These technical challenges are accompanied by economic aspects, i.e. the market requirements have to be met at reasonable costs.The welding of steam turbine rotors is one measure to balance required material properties and economical solutions. The rotor is a core component of the steam turbine and its long-term integrity is a key factor for reliable and safe operation of the power plant. An important aspect of weld quality is the determination of permissible size of weld imperfections assessed by fracture mechanics methods. The integrity of rotor weld joints is assured by ultrasonic inspection after the final post weld heat treatment with respect to fracture mechanics allowable flaw sizes. This procedure usually does not take credit from the quality measures applied during monitoring of the welding process.This paper provides an overview of an holistic design approach for steam turbine rotor weld joints comprising the welding process and its improved online monitoring, non-destructive evaluation, material technology, and its fracture mechanics assessment. The corresponding quality measures and their interaction with fracture mechanics design of the weld joint are described. The application of this concept allows to exploit the potentials of weld joints and to assure a safe turbine operation over life time.Copyright

Application of an Advanced Creep-Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

The demand for steam turbine components is driven by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for life-time prediction. Fracture mechanics methods are usually applied when crack like defects are detected for new rotors but also for rotor components in service. Based on the findings a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime.For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and long-term testing data, an advanced creep-fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth behavior. Furthermore, recent studies show that the crack size for creep crack initiation depends on material ductility and creep strain in the ligament.This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep-fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach — Two-Criteria-Diagram (2CD). For the advanced evaluation concept, the creep crack initiation criterion is combined for analysis with a creep-fatigue crack growth procedure. The benefit of the method especially for ductile material will be highlighted.Copyright

Mechanical Design of Highly Loaded Large Steam Turbines

There are no internationally recognized standards, such as the ASME Boiler and Pressure Vessel Code or European boiler and pipe codes, for the mechanical design of large steam turbine components in combined cycle power plants, steam power plants and nuclear power plants. One reason for this is that the mechanical design of steam turbines is very complex as the steam pressure is only one of many aspects which need to be taken into account. In more than one hundred years of steam turbine history the manufacturers have developed internal mechanical design philosophies based on both experience and research. As the design of steam turbines is pushed to its limits with greater lifetimes, efficiency improvements and higher operating flexibility requested by customers, the validity and accuracy of these design philosophies become more and more important. This paper describes an integral approach for the structural analysis of large steam turbines which combines external design codes, material tests, research on the material behavior in co-operation with universities and experience gained from the existing fleet to derive a substantiated design philosophy. The paper covers the main parameters that need to be taken into account such as pressure, rotational forces and thermal loads and displacements, and identifies the relevant failure mechanisms such as creep fatigue, ductile failure and creep fatigue crack growth. It describes the efforts taken to improve the accuracy for materials already used in power plants today and materials with possible future use such as advanced steels or nickel based alloys.Copyright