Comments on the present practice of fatigue analysis of nuclear power plant components

In the evaluation of present operating experience with respect to the safety assessment of nuclear power plant (NPP) components, the following conclusions can be drawn. (1) Thermally induced loads such as local temperature stratification (large temperature differences in pipe cross-sections under laminar flow conditions) and fluctuations have been underestimated in the former thermal-hydraulic analysis. (2) Environmental influences are not covered adequately in the design codes. (3) The freedom of the stress analyst in grouping the specified cyclic loadings, as well as the degree of conservatism in calculating the stresses, results in a large variation of the calculated usage factor; the estimated life usage fractions for the different local regions are conservative but not necessarily representative. The goal of the fatigue analyses for NPPs should be (a) to identify the local areas where significant fatigue loadings are expected to occur, as well as (b) to show that the component is protected against fatigue failure. At present the emphasis in performing and reviewing fatigue analyses is on the licensing steps before the NPP is put into operation. The designer must evaluate the fatigue loading pattern at an early stage in order to optimize the design. With respect to the lifetime assessment of the plant, it is the opinion of the authors that only a simplified procedure should be required for the operating license. To assess the residual fatigue life at any time during operation, it makes much more sense to follow the plant operational history as it develops. To follow this approach, it is necessary to manage and process the plant operating data in such a way that it can be used for a detailed fatigue evaluation. Computerized systems which can meet these requirements are commercially available and used for viable parts in conventional power plants. The advantages of this procedure are (i) a more representative number to account for the actual fatigue load and life fraction used; (ii) a better selection of local areas for recurring in-service inspection; (iii) a more reliable data base for the application of sophisticated fatigue analyses to assess damage mechanisms including environmental and load-history effects. As a result, an increase in plant performance and safety is expected.     

Wavelet transform-based stress-time history editing of horizontal axis wind turbine blades

Full scale blade fatigue testing is required to verify that the horizontal axis wind turbine (HAWT) blades posses the strength and service life specified in the design. Unfortunately, fatigue tests must be run for a long time period, which has led blade testing laboratories to seek ways of accelerating fatigue testing time and reducing the costs of tests. The objective of this article is to develop a novel method called a WT-based fatigue damage part extracting method. Based on wavelet transform (WT), this method extracts fatigue damage parts from the stress-time history and generates the edited stress-time history with shorter time length. Also, this article proposes a concept of applying accumulative power spectral density (AccPSD) to identify fatigue damage events contained in the stress-time history of HAWT blades. Wavelet functions used in this study are Morl, Meyr, Dmey, Mexh and DB30. It has been found that Mexh wavelet with an AccPSD level of 9000 Energy/Hz provides the edited stress-time history having a maximum reduction of 20.77% in length with respect to the original length, whilst fatigue damage per repetition can be retained almost the same as the original one. In addition, an existing method, time correlated fatigue damage (TCFD), is used to validate the effectiveness of a WT-based fatigue damage part extracting method. The results suggest that not only does the WT improve the accuracy of fatigue damage retained, but also it provides a shorter length of the edited stress-time history. To conclude, WT is suggested as an alternative technique in fatigue durability study, especially for the field of wind turbine engineering.     

电站锅炉用P91钢蠕变/疲劳交互作用的试验研究

对电站锅炉用P91的母材及焊接接头的蠕变/疲劳交互作用进行试验,使用改进的延性耗损法对试验结果进行分析和处理,得出了P91钢的蠕变/疲劳交互作用特性及交互作用状态下的寿命估算方法.结果表明,P91钢在蠕变/疲劳共同作用下呈负交互作用;在P91的电站锅炉构件寿命估算时,疲劳因素可忽略.     

On the operation strategy of steam power plants working at variable load: Technical and economic issues

The context of the deregulated energy market leads to high competitiveness among producers and requires suitable strategies in plants and systems management: strongly irregular and discontinuous operation is required in order to meet the user demand and produce energy mainly during peak hours, when the electricity price is higher. This operation strategy is generally asked of all power plants, not only those traditionally devoted to load regulation and peak request, but also those originally designed to cover the base load (steam power plants, for example). As a consequence, greater income is ensured in the short term, but a reduction in the lifetime of the most critical components is likely to occur, due to creep and thermo-mechanical fatigue loadings. This will cause additional costs associated with unplanned maintenance and unavailability of the plant if a failure occurs.This paper presents a procedure aimed at evaluating this extra cost related to flexible operation, and at assisting the management decision about power plants’ operation and maintenance scheduling. The procedure, on the basis of the historical data, predicts the residual life of the most critical components, considering the effects of creep, thermo-mechanical fatigue, welding, corrosion and oxidation. It also permits one to choose different future strategies for plant management and evaluate the residual life and the economic effects for each of them. An example of application to a real steam power plant will also be presented.     

基于试验与仿真相结合的动应力混合模拟

将已测试位置动应力试验数据用于结构上不可测试位置的动应力时间历程模拟,提出一种试验与仿真相结合的动应力混合模拟新方法。该方法也可用于载荷谱无法准确获取结构或新修改结构的动应力时间历程数据模拟,克服了传统动应力仿真算法要求对结构真实服役环境下载荷时间历程准确获取的限制条件,解决了试验无法直接测量位置动应力时间历程获取的问题,给出了一种基于原服役结构测试数据预测新修改结构疲劳寿命的方法。基于应变模态理论推导了具体实现公式,实例验证了该方法的有效性。研制了相应的动应力混合模拟软件,结合线路动应力试验数据,进行机车车辆转向架未测试位置的疲劳寿命预测和评估。     

300MW机组锅炉汽包寿命在线监测系统的研究

采用三维有限元理论,对300MW机组锅炉汽包在内压作用下的应力场进行了分析,所得到的理论应力集中系数比TDR301的推荐值大17.3％,并用热弹性理论计算准稳态下汽包的热应力。在此基础上对TRD301给出的疲劳寿命计算方法提出了改进建议,开发了一套300MW机组锅炉汽包寿命在线监测系统。该系统利用计算机、智能化数据采集和通信转换对锅炉汽包运行全过程实行温度、压力和寿命的管理。经在某电厂1021t/h锅炉上运行,效果良好。达到了对锅炉汽包寿命进行在线监测和指导锅炉运行的目的。     

Assessment of fatigue life of engineering structures

The possibility to extend the life of any existing plant beyond the original design demands a number of theoretical and experimental analyses to be done. Due to generally limited information regarding loading history and low availability of specimens, sampled from plant components in the actual damage state, a number of analytical as well as experimental problems arise. One of the prospective sources of investigation or/and recalculation is the cumulative fatigue damage area. In this paper a new energy based method to fatigue life calculations is discussed.     

Emerging technology for component life assessment

Operating plant component damage and failure experience is reviewed. Loading conditions such as thermal stratification and striping, turbulent flow and flow-induced vibrations are often found to limit useful life, even though such loadings were typically not considered when the components were designed. High cycle thermal and mechanical fatigue are identified as important damage mechanisms. A new method of correlating fatigue data and extrapolating to the very high cycle regime is described.

The results of environmental degradation testing during the past fifteen years have shown that such effects are much more deleterious than previously assumed. Therefore environmental and aging effects must be taken into account in evaluating the reliability and dependability of components for extended periods of operation. Since most of the available data on environmental effects focus on measured crack growth rates, methods of developing improved fatigue life evaluation methods which include environmental effects on crack growth rates are now being developed.

Fatigue tests on polished specimens are characterized by nominal stress amplitudes over yield, where linear elastic fracture mechanics (da/dn vs. ΔK) methods, such as those used in the ASME Code, are not valid. The small plastic zone corrections used in the Code do not account for the plastic crack-driving energies encountered in low-cycle fatigue testing. J-integral solutions are used herein to evaluate the growth of cracks in these specimens. This approach can be shown to correlate the growth of cracks over the entire range of loading from elastic to grossly plastic conditions in widely different geometries and sizes, including the growth of very short cracks for materials of major interest in pressure vessels and piping. It can be used to correct S-N fatigue life evaluation, curves for known differences in crack growth rates whether they are due to corrosion-assisted fatigue or other variables.

Environmental effects on the crack initiation phase of fatigue failure can be directly incorporated into S-N life evaluation curves. Once the crack propagation effects are included, the resulting improved S-N curves provide a means for plant operators to evaluate the current condition of these components and systems, taking into account the cumulative damage from operating transients and cycles which the plant has experienced. The safe residual life can then be evaluated using the S-N curves to include cumulative damage for the anticipated future period of operation. This plant life evaluation approach is applicable even where in-service inspections are not feasible. It provides a sound quantitative basis for making repair/replacement decisions.     

Matching experimental and numerical data of dynamic power train loads by modeling of defects

Defects on wind turbines such as power train misalignments or blade pitch angle deviations are dealt with. These defects cause additional dynamic excitations and thus can reduce the fatigue life of wind turbine components. In order to improve the reliability of dynamic load computations and related fatigue dimensioning of wind turbines, a highly discretized simulation model that incorporates potential system defects is set up. A sensitivity analysis of the impact of system defects on power train dynamics is performed. Experimental measurements of gearbox orbital paths and of the corresponding torque arm loads could be reproduced with good correlation when the simulation model was complemented by power train misalignments and by blade pitch angle deviations. Comparisons of experimental and numerical data are presented in time and frequency domains. Feasible consequences about the impact of alignment defects on the resulting fatigue damage are presented. Copyright © 2014 John Wiley & Sons, Ltd.     

Design and coordination of a capacitor and on‐load tap changer system for voltage control in a wind power plant of doubly fed induction generator wind turbines

Larger percentages of wind power penetration into the grid translate to more demanding requirements coming from grid codes; for example, voltage support at the point of connection has been introduced recently by several grid codes from around the world, thus making it important to analyse this control. Voltage control is actuated by reactive power injection, and for a wind power plant of doubly fed generator turbines, reactive power capability can be a challenge, which typically is overcome by installing reactive power compensators. The integration and the interaction between all these reactive power sources and the on‐load tap changer of the main substation transformer need to be analysed and taken into account in the control design. In this paper, a novel coordination and control strategy for capacitor banks and on‐load tap changer for a wind power plant is introduced. The capacitor banks are controlled in such way that the steady‐state usage of the converters for reactive power injection is driven below to a maximum desired value of 0.1 pu. Additionally, the control transients because of the capacitor bank switching are minimized by using a suitable control structure. The tap changer control is coordinated with the plant control to decrease the impact of the capacitors reactive power in the line drop calculation, thus reducing the amount of tap operations and improving the accuracy of the line drop voltage estimation. The coordination of the central controller with the plant components is analysed and tested through electromagnetic transient program simulations. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of the effects of the operation strategy of a steam power plant on the residual life of its devices

In the deregulated market scenario wider power generation flexibility with respect to the past is needed; on the other hand, frequent changes of the operating conditions may reduce the life of the most critical components, such as steam heaters or turbine blades. Fatigue failures produced by cyclic thermal and/or mechanical stresses will be considered in this work. The estimation is based on creep and fatigue failure models and is applied at the component level. In particular, in this paper evaluation of the impact of thermo-mechanical fatigue in the superheater pipes of an actual coal power plant will be carried out to estimate its residual life. Then, this evaluation at the device level will be translated into plant level assessment.     

Life improvement in high temperature components for advanced power engineering

Advanced power engineering rely on technological solutions allowing the design, construction and operation of power plants according to the state of the art, extending the limits of available materials for higher cycle efficiencies, for improved reliability and availability of the systems, and assuring in the meanwhile longer service lives.

Elevated temperature components, high pressure and high energy sections might be considered relevant with respect to time behaviour of the plant, since load-bearing structural materials are mainly subjected to metallurgical ageing, creep processes due to temperature exposure, thermomechanical and low cycle fatigue induced by load histories, as well as crack initiation and growth. Life improvement objective has to be fully considered and assumes a twofold application: i) at the design and construction stage, with the introduction of new materials and/or adopting better reference data and validated extrapolation procedures; ii) for existing plants, managing proper life extension methodologies integrated with plant rejuvenation, refurbishment or repowering decisions.

In the following, attention will be devoted to still open questions which are susceptible to contribute to the goal of safe and reliable, high efficiency thermal power plants, as residual life evaluation in components under multiaxial stress state, on-line plant monitoring, non destructive techniques and expert systems for residual life assessment and in situ repair/treatment techniques.     

用于非比例载荷下疲劳分析的最大最小循环计数法

研究了最新ASME-Ⅷ-2规范推荐的用于非比例载荷下循环计数的最大最小循环计数法。首先通过定义最大最小循环计数法在应力-应变迟滞回线上的循环路径,赋予了最大最小循环计数法合理的物理意义。然后结合von-Mises损伤准则和Miner线性损伤累积理论,编写了非比例加载下使用最大最小循环计数法进行疲劳循环计数的实用程序,并成功应用该程序实现了对某核电站振动小支管疲劳寿命的预测。对比了非比例载荷下最大最小循环计数法和传统雨流计数法的循环统计结果,表明非比例载荷下最大最小循环计数法统计的等效应力幅值约为雨流法的1.085倍,循环次数约为雨流法的1.478倍。     

Reconciliation of fatigue usage factors for class-1 components considering plant operation history

The evaluation of metal fatigue for the period of extended operation requires a time-limited aging analysis (TLAA) and is an important task for Nuclear Power Station. Both cumulative usage factors of components and the actual numbers of design transient cycles have to be reviewed. The factors of environment are also imposed on metal fatigue by NRC as a multiplier. In reviewing the cycles of a generic BWR pressure vessel design transients, the number of cycles of some design transients have no sufficient margin for current license basis even for period of extended operation. The operating margins of technical specification, operating cycles of design transients and cumulative usage factors of components, are subjected to severe challenge under the review of license renewal. The objective of this study aims to relieve such limits by using design transients reconciliation based on existing plant operating history. The predicted fatigue usage factors of Feed water nozzle of the reactor vessel are shown as an example using the adjusting cycles of design transients.     

电站汽轮机研制与生产的可靠性技术研究

给出了电站汽轮机的10个可靠性特征量的计算公式或术语以及汽轮机强度与振动的17个设计判据。介绍了电站汽轮机关键部件寿命评定技术研究的新进展和汽轮机关键部件寿命设计技术规程。分析了超临界汽轮机、超超临界汽轮机、空冷汽轮机、燃气轮机联合循环汽轮机和核电汽轮机结构可靠性设计的技术特点。介绍了电站汽轮机研制和生产过程的可靠性通用大纲,内容包括可靠性工作的5项总要求和18项详细要求。参14     

基于名义应力的焊接结构疲劳强度评定方法研究

分析和研究了我国钢结构设计规范、英国钢结构疲劳设计和评定规程以及国际焊接学会焊接接头及构件疲劳设计标准中基于名义应力的焊接结构疲劳强度评定方法。各规范或标准均使用经典的疲劳理论作为疲劳计算依据,评定方法简单易用,且提供的疲劳强度数据以实验室疲劳试验为基础,应用于工程实际问题具有较高的可靠性。合理应用这些数据和方法于机车车辆焊接承载结构的疲劳强度分析,对提高机车车辆的安全可靠性具有十分现实的意义。     

General review of developments in Acoustic Emission methods

The ability of Acoustic Emission (AE) to detect, with high sensitivity, the mechanical energy released by flow evolution, may be used to get information about crack growth, onset of new defects and their dynamic behaviour, during the first hydrotest and later requalification testing of nuclear components and continuous monitoring of power plant operation.

In this review, the most recent developments in AE for nuclear component monitoring are examined. Suitable AE equipment for logging high rates of AE data, real-time source location and parametric analysis are here discussed.

In recent years many AE processes (deformation, slow crack growth, fatigue processes, intergranular stress corrosion cracking, leak monitoring) have been studied and considerable effort has been made in preparing guidelines, recommended practices and standard procedures based on AE.

In this paper all these aspects are considered and the main results are reported.     

Application of secondary and residual stresses to the assessment of the structural integrity of nuclear power-generating plant

Magnox nuclear power stations were built in the 1960s to design codes that, in general, required weldments to be subject to a post-weld heat treatment to remove residual stresses. Implicit in this was the assumption that the heat treatment reduced the stresses significantly such that as stated in the codes “stresses caused by fabrication and welding are practically annulled”. However, it has since been realised that the stresses remaining, although small, could still be significant when incorporated into the subsequently developed failure avoidance methodologies such as R6. Moreover, either at the time of construction or during the operating life, repairs are undertaken to remove manufacturing or service-induced defects. These repairs can be put into service with or without a post-weld heat treatment. As a consequence of a paucity of data for the two- and three-dimensional distribution of the magnitude of these stresses, extremely conservative values of stresses have been adopted to ensure that the plant is secure against the design intent throughout the service life.     

Modelling crack growth for creep and fatigue loading

Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts. In particular the creep parameter C* has become widely used together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilise experimental data. For high temperature components, which are subjected to cyclic (fatigue) as well as creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which the growth of pre-existing cracks grow by creep and fatigue is difficult to quantify. In this paper the response of Type 316L stainless steel is examined. This material progressively strain hardens under reversed cyclic loading, and the creep behaviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fatigue crack growth. Using the maps a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components.     

Dynamical measurement system for wind turbine fatigue load

A measurement system specially used in wind turbine fatigue load assessment is developed based on Labview platform and Control Area Network (CAN). By applying CAN bus communication technology, the system can perform data automatic acquisition, data stable transmission and data real-time monitoring. By adopting the technology of virtual instrument modular design, the system is designed to analyze the wind turbine mechanical load levels against wind and power, equivalent loads and lifetime fatigue loads, etc. Considering the effects of small load strengthening, low amplitude load damaging and multilevel load interaction, a novel fatigue lifetime prediction model is proposed to obtain more accurate and reliable prediction of blade fatigue life. With the developed measurement system, the in-field load measurements are performed and the results showed the system has satisfactory accuracy and good adaption, convenient operation, high integration, low cost and great practicality to load measurement of large wind turbine. And based on the proposed model the fatigue life of WT blade can be estimated more trustworthily and reliably.