Soft Sensor for Oxide Scales on the Steam Side of Superheater Tubes under Uneven Circumferential Load (original) (raw)
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Role Of Oxide Scale Thickness Measurements In Boiler Conditions Assessment
2011
Oxide scale thickness measurements are used in assessing the life of different components operating at high temperature environment. Such measurements provide an approximation for the temperature inside components such as reheater and superheater tubes. A number of failures were encountered in one of the boilers in one of Kuwaiti power plants. These failure were mainly in the first row of the primary super heater tubes, therefore, the specialized engineer decide to replace them during the annual shutdown. As a tool for failure analysis, oxide scale thickness measurement were used to investigate the temperature distribution in these tubes. In this paper, the oxide scale thickness of these tubes were measured and used for analysis. The measurements provide an illustration of the distribution of heat transfer of the primary superheater tubes in the boiler system. Remarks and analysis about the design of the boiler are also provided.
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.
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.
Influence of steam-side oxide scales on the creep life of a boiler superheater tube
Materiali in Tehnologije, 2018
Creep life is a limiting factor in the case of boiler tubes operating above the creep temperature and, for a given material, it is generally determined by the actual operating temperature and stress. Standard approaches to the calculation of a tube creep life mainly take into account the stress caused by pressure loading and the mean temperature in the tube wall. However, the estimation of the temperature and stress may be difficult for water-tube boilers because oxide scales tend to form on the inner surfaces of the tubes, exposed to steam and indirectly affecting the resulting creep life. As the scales increase the thermal resistance of a tube wall and, consequently, the wall temperature, the creep life is reduced. Moreover, the presence of oxide scales leads to a higher hydraulic resistance of a tube, which can cause further increase in the temperature in some tubes of the bundle if the oxide-scale growth rate is not uniform. Additionally, the elements increasing the creep streng...
Estimation of Steam-Side Scale Growth in Ferritic Alloy Boiler Tubes of Coal-Fired Power Plant
Proceedings of the 1st International Conference on Contemporary Risk Studies, ICONIC-RS 2022, 31 March-1 April 2022, South Jakarta, DKI Jakarta, Indonesia
Coal remains to be considered as one of the future major energy sources. However, the formation of steam-side oxide scale as a result of the oxidation process during the steam power plant operation has been widely known to be a serious main root cause, leading to some risks/consequences. This paper presents a simple incremental method to determine the oxide scale growth in the ferritic alloy boiler (reheater and superheater) tubes for a given period of time making use of a classical heat flow formula and the correlation of the oxide scale thickness versus the Larson Miller Parameter (LMP). The ferritic alloy tubes have been used worldwide, especially in the supercritical steam power plants. The presented method may be used as a supplementary monitoring tool for estimating the growth of steam-side oxide scales in boiler tubes.
Russian Journal of Nondestructive Testing, 2020
Austenitic boiler tubes are key components in modern coal-fired power plants but often suffer from blockage or even rupture caused by the accumulation of steam-side oxide scales. Existing nondestructive testing methods are somewhat deficient to quantify the accumulated height of exfoliated oxide scales. Pulsed eddy current (PEC) technique has unique merits such as deep penetration depth and diverse signal features, offering a great possibility to address this issue. This work serves as an exploratory study for applying PEC technique to measure the scales accumulation. Finite element modeling and experimental work are conducted to validate the feasibility and effectiveness of PEC measurement under different circumstances. The PEC signal peak shows a linear relationship with the accumulated height and thereby is used as the signal feature for the quantitative measurement of accumulated height. A flexible sensor consisting of a replaceable flat cable with connectors at the ends is designed, following by the analyses of sensor offset and liftoff effects. To distinguish the unevenness of accumulation, the sensor-coil array scheme is proposed and the output array signals successfully characterize the profile of inner accumulation. Finally, a referencing strategy is explained to eliminate the interference due to magnetism transformation of the tube wall.
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.
STEAM-SIDE OXIDE SCALE EXFOLIATION BEHAVIOR IN SUPERHEATERS AND REHEATERS
2011
Advances in materials for power plants include not only new materials with higher-temperature capabilities, but also the use of current materials at increasingly higher temperatures. This latter activity builds on extensive experience of the performance of the various alloys, and provides a basis for identifying changes in alloy behavior with increasing temperature as well as understanding the factors that ultimately determine the maximum use temperatures of the different alloy classes. This paper presents results from an effort to model the exfoliation processes of steam-side oxide scales in a manner that describes as accurately as possible the evolution of strains in oxides growing inside small-diameter tubes subjected to large thermal gradients and to thermal transients typical of normal steam boiler operation. One way of portraying the results of such calculations is by plotting the evolving strains in a given oxide scale on an 'Exfoliation Diagram' (of the type pioneered by Manning et al. of the British Central Electricity Research Laboratory) to determine the earliest time at which the trajectory of these strains intersects a criterion for scale failure. Understanding of how such 'strain trajectories' differ among different alloys and are affected by the major variables associated with boiler operation has the potential to suggest boiler operating strategies to manage scale exfoliation, as well as to highlight the mode of scale failure and the limitations of each alloy. Preliminary results are presented of the strain trajectories calculated for alloys T22, T91, and TP347 subjected to the conditions experienced by superheaters under assumed boiler operating scenarios. For all three alloys the earliest predicted scale failures were associated with the increased strains developed during a boiler shut-down event; indeed, in the cases considered it appeared unlikely that scale failure would occur in any practically meaningful time due to strains accumulated during operation in a load-following mode in the absence of a shut down. The accuracy of the algorithms used for the kinetics of oxide growth appeared to be a very important consideration, especially for alloy TP347 for which large effects on oxide growth rate are known to occur with changes in alloy grain size and surface cold work.
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.