The effect of the steam temperature fluctuations during steady state operation on the remnant life of the superheater header

Ligament cracking in superheater headers is generally understood to be associated with thermal transients that occur during starts. However, recent analysis has shown that under some circumstances stress changes due to the steam temperature fluctuations during continued operation also contribute to crack growth. The correlation of the stress range due to these temperature fluctuations has been established by carrying out a transient thermo-mechanical FE analysis. The contribution to fatigue crack propagation of existing defects has been quantified. Frequent thermal fluctuations at steady state operation were identified as the most influential factor for the remnant life of the header.     

Integrity assessment of a 3D tubeplate using the linear matching method. Part 2: Creep relaxation and reverse plasticity

In an accompanying paper [Int J Pres Ves Pip, 2004], a 3D tubeplate in a typical AGR superheater header was analysed for the shakedown limit, reverse plasticity and ratchet limit based on the application of the linear matching method (LMM). Both the perfectly plastic model and the cyclic hardening model were adopted for the evaluation of the plastic strain range. In the present paper, based on the creep-reverse plasticity model, the accumulated creep strain during a creep dwell at the steady cyclic state is assessed by the LMM. The accumulated creep strains, the creep flow stresses as well as the elastic follow-up factors with differing dwell times are evaluated in detail. The total inelastic strain over the cycle at the steady cyclic state is calculated. By comparing these results with ABAQUS step-by-step inelastic analyses, the applicability of the methods is demonstrated.     

Integrity assessment for a tubeplate using the linear matching method

A series of numerical procedures have been presented recently for the integrity assessment of structures based upon the Linear Matching Method. A typical example of a holed plate has been used to verify these procedures for the evaluation of plastic and creep behaviours of components. In this paper, a more complex 2D tubeplate at the outlet from a typical AGR heat exchanger is analysed for the shakedown limit, reverse plasticity, ratchet limit and creep relaxation based on the application of the Linear Matching Method for a thorough case study. Both a constant material yield stress and a temperature-dependent yield stress are adopted for the evaluation of the ratchet limit. For the evaluation of accumulated creep strains, flow stresses and elastic follow-up factors with differing dwell times at the steady cyclic state, a monotonic creep computation is performed, where the start-of-dwell stress is the rapid cycle creep solution at the beginning of the dwell period. An estimation of the tubeplate lifetime is then obtained by the evaluation of fatigue and creep endurances.     

Integrity assessment of a 3D tubeplate using the linear matching method. Part 1. Shakedown,reverse plasticity and ratchetting

The Linear Matching Method (LMM) has been developed recently for the integrity assessment for the high temperature response of structures. In this article, a complex 3D tubeplate in a typical AGR superheater header is analysed for the shakedown limit, reverse plasticity and ratchet limit based upon the LMM. At this stage, both the perfectly plastic model and the cyclic hardening model are adopted for the evaluation of the plastic strain range. Comparisons of LMM results with other results by ABAQUS step-by-step inelastic analyses for several material models are given. Further cyclic creep-reverse plasticity analyses are presented in an accompanying paper [Int. J. Pressure Vessels Piping (2004)].     

A new monitoring system for piping systems in fossil fired power plants

A CEC-funded project has been performed to tackle the problem of producing an advanced Life Monitoring System (LMS) which would calculate the creep and fatigue damage experienced by high temperature pipework components. Four areas were identified where existing Life Monitoring System technology could be improved:

1. 1. the inclusion of creep relaxation

2. 2. the inclusion of external loads on components

3. 3. a more accurate method of calculating thermal stresses due to temperature transients

4. 4. the inclusion of high cycle fatigue terms.

The creep relaxation problem was solved using stress reduction factors in an analytical in-elastic stress calculation. The stress reduction factors were produced for a number of common geometries and materials by means of non-linear finite element analysis. External loads were catered for by producing influence coefficients from in-elastic analysis of the particular piping system and using them to calculate bending moments at critical positions on the pipework from load and displacement measurements made at the convenient points at the pipework. The thermal stress problem was solved by producing a completely new solution based on Green's Function and Fast Fourier transforms. This allowed the thermal stress in a complex component to be calculated from simple non-intrusive thermocouple measurements made on the outside of the component. The high-cycle fatigue problem was dealt with precalculating the fatigue damage associated with standard transients and adding this damage to cumulative total when a transient occurred.

The site testing provided good practical experience and showed up problems which would not otherwise have been detected.     

基于不同启动方式的1 000 MW超超临界汽轮机高温部件低周疲劳损耗研究

为了解汽轮机寿命损耗情况,对汽轮机高温部件寿命影响因素进行分析。基于低周疲劳理论,建立汽轮机高温部件寿命损耗分析模型,采用定量计算方法分析1 000 MW超超临界机组冷态、热态启动方式下高温部件温度偏差变化情况,计算各边界条件下高温部件等效热应力及寿命损耗,并进行敏感性分析。结果表明：汽轮机低周疲劳寿命损耗率对高温部件温度偏差较为敏感,随着温度偏差的升高,汽轮机寿命损耗率大幅升高;相同的温度偏差出现在不同温度区间时,对汽轮机寿命损耗的影响亦不同,高温区间的温度偏差对寿命损耗率影响较大。汽轮机高温部件寿命评估可以为机组启动期间升温速度控制提供技术支持,降低汽轮机寿命损耗,提高机组运行安全性。     

The linear matching method applied to the high temperature life integrity of structures. Part 1. Assessments involving constant residual stress fields

Design and life assessment procedures for high temperatures are based on ‘expert knowledge’ in structural mechanics and materials science, combined with simplified methods of structural analysis. Of these, R5 is one of the most widely used life assessment methods internationally with procedures based on reference stress techniques and shakedown calculations using linear elastic solutions. These have been augmented by full finite element analysis and, recently, the development of a new programming method, the linear matching method (LMM), that allows a range of direct solutions that include shakedown methods and simplified analysis in excess of shakedown. In this paper, LMM procedures are compared with calculations typical of those employed in R5 for cyclic loading problems when the assumption of a constant residual stress field is appropriate including shakedown and limit analyses, creep rupture analysis and the evaluation of accumulated creep deformation. A typical example of a 3D holed plate subjected to a cyclic thermal load and a constant mechanical load is assessed in detail. These comparisons demonstrate the significant advantages of linear matching methods for a typical case. For a range of cyclic problems when the residual stress field varies during the cycle, which include the evaluation of plastic strain amplitude, ratchet limit and accumulated creep strains during a high temperature dwell periods, the corresponding LMM and R5 procedures are discussed in an accompanying paper.     

冷启动工况下汽轮机转子的疲劳损伤

在对600MW汽轮机高压转子冷启动工况下的温度场、热应力进行了瞬态有限元分析的基础上给出了热应力随启动温升率的变化关系,进而采用转子材料的低周疲劳曲线计算出不同启动温升率下转子的低周疲劳损伤。为现场运行制定安全、经济的启动过程提供了一定的理论依据。     

B＆W 6L60MCE柴油机气缸套非稳态热应力分析

建立过渡工况(起动、停车)气缸套非稳态热应力分析模型以及反映过渡工况特点的边界条件数学模型。对过渡工况下的传热和热应力以及工作循环中的温度波、波动热应力和脉动机械应力的计算结果进行了重点分析。从而对气缸套非稳态热应力做出正确的分析。表明非稳态热应力和脉动机械应力对材料引起的疲劳破坏是气缸套失效的主要原因。     

Linear matching method for creep rupture assessment

The recently developed linear matching method (LMM), which is easily implemented within commercial FE codes, has been successfully used to evaluate elastic and plastic shakedown loads. In this paper, the method is extended to the prediction of the creep rupture life of a structure, based upon a bounding method currently used in the life assessment method R5. The method corresponds to the requirement that, for the operating load history, the structure should shakedown where the yield stress is given by the lesser of the plastic yield stress and a high temperature rupture stress corresponding to a rupture time. A holed plate subjected to cyclic thermal load and constant mechanical load is assessed in detail as a typical example to confirm the applicability of the above procedures. The examples show that the method remains numerically stable, even when the method is inverted.     

采用热流法计算汽轮机转子表面热应力

通过对热量流动的分析,推导出汽轮机转子温度分布的表达式。同时根据温度分布,考虑汽轮机转子表面的应力集中,给出转子表面的热应力计算公式。最后通过典型机组的冷态启动计算,分析了温度场和热应力场的变化,评估低周疲劳损伤,并提出了运行控制的要求。图7参15。     

Preliminary investigation of PWR feedwater line cracking

Circumferential cracks were found in large diameter carbon steel piping that carries feedwater on the secondary side in PWR nuclear power plants. The cracks were confined to a narrow band in the vicinity of the counterbore region of the piping near the weld joining the piping to the feedwater nozzle on the steam generator. In most cases, the cracks were numerous, but small; a typical penetration was less than 20% through the wall. However, in some cases, the cracks were much greater in depth and, in one instance, propagated through the pipe wall, approximately 1·6 cm (0·65 in.). The environment was high temperature water containing less than 1 ppm oxygen.

A metallurgical analysis, in which both light and electron microscopy were utilized, led to the conclusion that the cracks, particularly the large ones, were caused by corrosion fatigue. This conclusion was based upon crack morphology, crack location, the presence of bench marks and striations, fracture appearance, and the presence of corrosion products. The next step in the failure analysis was to determine the source(s) of the cyclic stresses that resulted in the corrosion fatigue.

Stress and fatigue analyses were made in an effort to determine the combinations of loads and cycles necessary to produce the observed phenomena. Environmental effects were taken into account in carrying out the analyses. The results showed that plant specific mechanisms must have been responsible for the rapid fatigue growth. An analytical and experimental programme involving the investigation of thermal fluctuations was planned to determine the precise source of the required combinations of loads and cycles.     

Steady state and transient thermal stress analysis in planar solid oxide fuel cells

Resulting from elevated temperatures the major structural problem foreseen with planar SOFCs is their thermal stress. Due to the brittle nature of ceramic material, operation in or near the material plastic limit can be very critical. Therefore stress levels must always be kept below the tensile and shear limits. The analysis is focused on determination of the stress caused by the difference in thermal expansion coefficients when high temperature gradients occur in the SOFC layers during steady state and transient operation (heat-up, start-up and shut-down). Utilizing an in-house developed tool for assessment of the electrochemical and thermal performance of a bipolar planar cell the input temperature profiles are generated for a finite element analysis code to predict thermal component of the stress. The failure criterion adopted is based on the strength of the cell materials and the principal stresses developed by the thermal loading. To visualize the stress concentration in the fuel cell layers, maximum principal stress is calculated and compared with the yield strength of the SOFC materials found in the literature. The in-house code is capable to predict both steady state and dynamic temperature profiles. Of particular importance is the knowledge gained of the transient stress in the cell, which can be used to establish control parameters during transient operations.     

汽轮机叶片表面状况对疲劳寿命的影响

汽轮机叶片的疲劳操作和疲劳寿命除与交变应力、平均应力、材料的强度和循环性能、运行条件等有关外,在相当大程度上与叶片表面状况（应力集中、加工状况、强化、腐蚀介质、微动磨损等）有关。本文深入研究了叶片表面状况对疲劳寿命的影响,采用改进的局部应力应变法对实际叶片进行的相应计算、对比和分析表明,叶片的疲劳寿命对表面状况很敏感。研究工作对叶片状况、制造、运行及故障分析有着现实的理论和工程应用价值。     

Transient thermal modelling of heat recovery steam generators in combined cycle power plants

Heat recovery steam generator (HRSG) is a major component of a combined cycle power plant (CCPP). This equipment is particularly subject to severe thermal stress especially during cold start‐up period. Hence, it is important to predict the operational parameters of HRSGs such as temperature of steam, water, hot gas and tube metal of heating elements as well as pressure change in drums during transient and steady‐state operation. These parameters may be used for estimating thermal and mechanical stresses which are important in HRSG design and operation. In this paper, the results of a developed thermal model for predicting the working conditions of HRSG elements during transient and steady‐state operations are reported. The model is capable of analysing arbitrary number of pressure levels and any number of elements such as superheater, evaporator, economizer, deaerator, desuperheater, reheater, as well as duct burners. To assess the correct performance of the developed model two kinds of data verification were performed. In the first kind of data verification, the program output was compared with the measured data collected from a cold start‐up of an HRSG at Tehran CCPP. The variations of gas, water/steam and metal temperatures at various sections of HRSG, and pressure in drums were among the studied parameters. Mean differences of about 3.8% for temperature and about 9.2% for pressure were observed in this data comparison. In the second kind of data verification, the steady‐state numerical output of the model was checked with the output of the well‐known commercial software. An average difference of about 1.5% was found between the two latter groups of data. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of strain rate on low cycle fatigue with hold time in 9Cr rotor steel

Abstract

One of the main mechanisms of turbine rotor damage is cyclic thermal stress produced during transition conditions, such as warm-up and shutdown processes. In this paper, the cyclic damage with ramp rate is studied in terms of material properties. To evaluate thermal stress damage during the cyclic operation, experiments were carried out on low cycle fatigue with hold time and a range of strain rates for COST FB2, 9Cr ferritic steel. Initially, the experimental fatigue life was evaluated using the time fraction and ductility exhaustion methods. The two analysis methods are compared with real life cases of different hold times and strain rates, and the accuracy is discussed. Next, to predict the plastic deformation curve for low cycle fatigue with hold time more accurately, an attempt was made to simulate cyclic hysteresis loop behaviour numerically using a time dependent viscoplastic constitutive model, and this was compared with the experimental hysteresis loop curve for each cycle number.     

CRACK ARREST ANALYSIS UNDER CYCLIC THERMAL SHOCK FOR AN INNER-SURFACE CIRCUMFERENTIAL CRACK IN A FINITE-LENGTH THICK-WALLED CYLINDER

This article presents a crack arrest depth analysis under cyclic thermal shock for an inner-surface circumferential crack in a finite-length thick-walled cylinder with rotation-restrained edges. The inside of the cylinder is cooled from a uniform temperature distribution. The effects of heat transfer conditions on the maximum transient stress intensity factor for the problem were investigated with systematical evaluation methods formerly developed. Then, under an assumption of a tentative threshold stress intensity range j K th together with the Paris law, the crack arrest depth under cyclic thermal stress was evaluated. The results suggested the existence of an upper limit for the normalized crack arrest depth, independent of the cylinder material in an engineering sense. Finally, the validity of applying j K max h j K th as a crack arrest criterion under cyclic thermal shock was confirmed by fatigue tests under mechanical loads equivalent to those induced by cyclic thermal shock.     

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.     

Investigation of the temperature oscillations in the cylinder walls of a diesel engine with special reference to the limited cooled case

This work investigates the interesting phenomenon of the temperature (cyclic) oscillations in the combustion chamber walls of a diesel engine. For this purpose, a comprehensive simulation code of the thermodynamic cycle of the engine is developed taking into account both the closed and the open parts of it. The energy and state equations are applied, with appropriate combustion, gas heat transfer, and mass exchange with the atmosphere sub‐models, to yield cylinder pressure, local temperatures and heat release histories as well as various performance parameters of the engine. The model is appropriately coupled to a wall periodic conduction model, which uses the gas temperature variation as boundary condition throughout the engine cycle after being treated by Fourier analysis techniques. It is calibrated against measurements, at various load and speed conditions, from an experimental work carried out on a direct injection (DI), naturally aspirated, four‐stroke, diesel engine located at the authors' laboratory, which has been reported in detail previously. After gaining confidence into the predictive capabilities of the model, it is used to investigate the phenomenon further, thus providing insight into many interesting aspects of transient engine heat transfer, as far as the influence that engine wall material properties have on the values of cyclic temperature swings. These swings can take prohibitive values causing high wall thermal fatigue, when materials of specific technological interest such as thermal insulators (ceramics) are used, and may lead to deterioration in engine performance. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal fatigue on pistons induced by shaped high power laser. Part I: Experimental study of transient temperature field and temperature oscillation

Thermal fatigue behavior is one of the foremost considerations in the design and operation of diesel engines. It is found that thermal fatigue is closely related to the temperature field and temperature fluctuation in the structure. In this paper, spatially shaped high power laser was introduced to simulate thermal loadings on the piston. The incident Gaussian beam was transformed into concentric multi-circular beam of specific intensity distribution with the help of diffractive optical element (DOE), and the transient temperature fields in the piston similar to those under working conditions could be achieved by setting up appropriate loading cycles. Simulation tests for typical thermal loading conditions, i.e., thermal high cycle fatigue (HCF) and thermal shock (or thermal low cycle fatigue, LCF) were carried out. Several important parameters that affect the transient temperature fields and/or temperature oscillations, including controlling mode, intensity distribution of shaped laser, laser power, temporal profile of laser pulse, heating time and cooling time in one thermal cycle, etc., were investigated and discussed. The results show that as a novel method, the shaped high power laser can simulate thermal loadings on pistons efficiently, and it is helpful in the study of thermal fatigue behavior in pistons.