Influence of steam-side oxide scales on the creep life of a boiler superheater tube (original) (raw)
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Oxidation of Metals, 2010
This effort is concerned with developing a quantitative description of the exfoliation behavior of oxide scales grown inside steam tubes in a pressure boiler. Consideration of the development of stress/strain in growing oxides has included expansion mismatch-induced strains during thermal cycling as well as inelastic mechanical effects from oxide/alloy creep phenomena and volume change from oxide growth. The magnitude of the parameters used has been closely matched to actual boiler operating practice. The creep model used was validated against published data. Representation of oxide growth-induced strain was found to be a difficult challenge because the processes involved are not fully understood. In addition to the traditional uniaxial (radial) and dilatational models, 'lateral' growth models are discussed in the context of experimentally-derived criteria, such as the level of elastic strains involved in oxide exfoliation. It was found that strain variation in the oxide cannot be neglected.
Mechanical thermal stresses and creep analysis of boiler tubes
2016
The boiler tubes are operated continuously at high temperature and pressure. Hence at high pressure for forces acting on the boiler tubes will be high. This research paper focuses on the analysis of one of the long term effect of continued application of high pressure on boiler tubes which causes creeping. By utilizing finite element modelling software, AUTODESK INVENTOR the effect of pressure with the increase in temperature distribution across the steam generator tube was evaluated. The increase of heat transfer rate across the wall caused the oxide scale thickness to grow more rapidly than normal condition. The thermal conductivity in the boiler tubes, life of boiler tubes and creep damage is also analyzed in this research paper. The AUTODESK INVENTOR result is analyzed to determine the main and interactive effects of operating conditions. The effect of steam on boilers and creep damage in comparison with temperature were researched. Optimum condition identification in order to m...
Critical Failure Strain of Oxide Scale in Boiler Austenitic Tubes
2018
Article history: Received 8 March 2018 Received in revised form 12 April 2018 Accepted 13 April 2018 Available online 16 April 2018 Exfoliation of oxide scales is known to be associated with the stress and strain developed during the oxide growth. This paper presents a simple procedure to estimate the oxide scale growths and critical failure strains in boiler austenitic tubes over a period of time. In this work, a classical heat flow formula and the relationship between the Larson Miller Parameter (LMP) and the scale thickness were utilized. An approach called Advance Oxide Scale Failure Diagram (A-OSFD) was adopted. The oxide scale failure diagram would provide a general guidance to the power plant engineers to estimate the critical strains.The technique may be used as a supplementary condition monitoring tool for oxide scale growths and to evaluate the critical failure strains.
Investigation on Wall Thinning and Creep Damage in Boiler Tube due to Scale Formation
Advanced Materials Research, 2012
The boiler tubes are operated continuously at high temperature and pressure. During operation, scales are formed in boiler tube due to tube geometries, flue gas and steam temperature. The remaining wall thickness decreases due to the formation of scale which eventually causes failure of the boiler tubes. In this investigation an iterative technique was used to determine the temperature distribution across the tube with the operating time. The operating time was considered up to 160,000 hours. The remaining life of the steam generator tube was found by finding hoop stress and Larson Miller Parameter from the Larson Miller Parameter curve for SA213-T22 material. By utilizing finite element modelling software, ANSYS 9/ANSYS 11 the temperature distribution across the steam generator tube was evaluated. The increase of heat transfer rate across the wall caused the oxide scale thickness to grow more rapidly than normal condition. It was also observed that due to formation of scale the thermal conductivity in the boiler tubes was affected and the remaining life of boiler tubes was decreased and accelerated creep damage. The ANSYS result was analyzed by Minitab 16 to determine the main and interactive effects of operating conditions. Steam temperature was influencing most the wall thinning and creep damage in comparison to the flue gas temperature. The interactive effects of both the parameters were also prominent. Moreover, the optimum operating condition was identified in order to maximizing the remnant life of the tubes while minimizing the creep rupture damage.
High Temperature Materials and Processes, 2008
The elastic, inelastic deformation and stress distribution pattern of a seventeen years service exposed primary superheater tube in a 120 MW boiler of a thermal power plant have been evaluated by using both analytical and numerical techniques. The commercial finite element computer code ANSYS was used for stress analysis. The methodology is a valuable design tool for development of new boiler tubes as well as for considering the effects of numerous operating variables on creep life. The health of the tube was also assessed based on microstructure, hardness and a few conventional creep tests carried out at 500°C at various stress levels (40-177 MPa). Results revealed that there was not much variation in the microstructure and hardness of the service exposed tubes compared to the virgin material. Creep deformation behavior of the service exposed and virgin tubes of the same material at 40 MPa reveal that deterioration of the creep properties fall within the 20% scatter band, which is well within the specified limits of ASTM standard. The service exposed primary superheater tube is thus in a good state of health.
An Analysis of Creep Phenomena in the Power Boiler Superheaters
Metals, 2018
Higher temperatures of the power boiler superheater operation may lead to high strains caused by the creep phenomenon. This paper presents a determination of the maximum allowable operating temperature limited by the creep phenomenon for steam superheater SH3. The calculations are carried out first on the basis of applicable European standards. Then, calculations are performed based on conducted creep tests, a proposed creep model, and a finite element method (FEM) model. A detailed creep phenomenon analysis based on the conducted creep tests shows that stresses that determine the creep process are mainly caused by pressure. Normal stresses resulting from bending are mainly the effect of thermal expansion. These stresses undergo significant relaxation because of creep. The creep phenomenon analysis explains the equations of the European standards. The presented calculations enable estimation of a safe value of the operating temperature which is constant over time. The estimated time...
Microstructural Degradation of Boiler Tubes Due to the Presence of Internal Oxide Layer
Journal of Materials Research and Technology, 2012
Overheating due to the formation of the internal oxide layer has a large in uence on the life of boiler components. The aim of this study consisted in assessing the effect of the internal oxide layer on the microstructural degradation in boiler tubes. Three samples of 2.25 Cr-1Mo superheater steel tubes were used. The methodology of the study compared the microstructures of the samples received by optical, scanning electron, and transmission electron microscopy. Tensile and hardness tests were performed. At the end of the tests it was proved the deleterious effect of the internal oxide layer on the life of the component. From the measurements of the internal oxide thickness, the remaining lives of the pipes operating under creep regime were estimated with a methodology established in the literature.
Prediction of oxide scale growth in superheater and reheater tubes
Corrosion Science, 2009
In this paper a procedure on how to estimate the oxide scale growth in superheater and reheater tube utilizing the empirical formulae and the finite element modeling is proposed. An iterative procedure consisting of empirical formulae and numerical simulation is used to determine scale thickness as both temperature and time increase. Results of the scale thickness over period of time for two different design temperatures of the steam and different heat transfer parameters are presented. The procedures may provide better estimation on the oxide scale growth, provided that all the heat transfer parameters are well specified.
Prediction of Oxide Scale Exfoliation in Steam Tubes
Numerical simulation results are presented for the prediction of the likelihood of oxide scale exfoliation from superheater tubes. The scenarios considered involved alloys T22, TP347H, and TP347HFG subjected to a simplified operating cycle in a power plant generating supercritical steam. The states of stress and strain of the oxides grown in steam were based solely on modeling the various phenomena experienced by superheater tubes during boiler operation, current understanding of the oxidation behavior of each alloy in steam, and consideration of operating parameters such as heat flux, tube dimensions, and boiler duty cycle. Interpretation of the evolution of strain in these scales, and the approach to conditions where scale failure (hence exfoliation) is expected, makes use of the type of 'Exfoliation Diagrams' that incorporate various cracking and exfoliation criteria appropriate for the system considered. In these diagrams, the strain accumulation with time in an oxide is represented by a 'strain trajectory' derived from the net strain resulting from oxide growth, differences in coefficients of thermal expansion among the components, and relaxation due to creep. It was found that an oxide growing on a tube subjected to routine boiler load cycling conditions attained relatively low values of net strain, indicating that oxide failure would not be expected to occur during normal boiler operation. However, during a boiler shutdown event, strains sufficient to exceed the scale failure criteria were developed after times reasonably in accord with plant experience, with the scales on the ferritic steel failing in tension, and those on the austenitic steels in compression. The results presented illustrate that using this approach to track the state of strain in the oxide scale through all phases of boiler operation, including transitions from full-to-low load and shutdown events, offers the possibility of identifying the phase(s) of boiler operation during which oxide failure is most likely to occur.
Proceedings of the Asian Pacific Conference on Fracture and Strength and International Conference on Advanced Technology in Experimental Mechanics
A small punch creep testing technique using miniaturized specimen with a dimension of 1O ×.1O× O.3i mm has been demonstrated fbr evaluating change in creep property ofhigh temperature components due to long-term service operation, This technique was applied to SUS 316 HTB secondary superheater boiler tube actually used for 100,600 h in a fbssil power plant. The overall shapes of the creep curves deterrnined by the small punch creep test were similar to those obtained from a conventional uniaxial creep test. That is, they exhibited clearly three creep stages, The extent ofsecondary creep stage and rupture time t, decreased with a increase oftesting load level. The creep rupture life ofthe service-exposed tube was shorter than that ofthe unexposed tube atatemperature of650 ℃ and at high load leve]s, i.e. 338 N and 408 N, However, te the contrary, the former became longer than the latter at relatively low load levels, i.e. 234 N and 2g6 N, The difference in rupture life between them had a tendency to increase with a decrease of testing load level. These small punch creep behaviors ofSUS 316 HTB superheater boi]er tubes have been examined from ametallographicpointofview. Kby Pt2)rds : Small punch creep test, Creep, Material deterioration, Long-term service operation, Fossil boiler, Superheater tube, SUS 316 HTB