Steam-side Oxide Scale Research Articles

Measurement of Accumulated Height of Exfoliated Oxide Scales in Austenitic Boiler Tubes Using Pulsed Eddy Current Testing

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.

Numerical simulation and life prediction on T22 boiler tubes with steam-side and fire-side oxide scale

In this study based on Larsen-Miller empirical formula, two simulation models of T22 boiler tube of a super-heater are analyzed by Finite Element Method (FEM). Model one only considers the steam-side oxide scale, and model two considers both the steam-side and the fire-side oxide scale. The results show that the average temperature and Von Mises stresses of the tube increase gradually due to the existence of the steam-side and fire-side oxide scale, in which the effect of fire-side oxide scale is more remarkable. Based on the simulation results, Kachanov-Rabotnov creep damage model was employed to assess the creep life of the tube. It was found out that at the initial stage of the service time the fire-side oxide scale can protect the tube and delay the creep damage obviously, and the critical creep damage of the tube is only about 0.5 before failure occurs.

Vpliv škaje na parni strani cevi izmenjevalnika toplote na dobo trajanja cevi zaradi lezenja

Vpliv škaje na parni strani cevi izmenjevalnika toplote na dobo trajanja cevi zaradi lezenja

Influence of temperature on the oxide spallation of T91 alloy superheater tubes in power plant

The spallation of steam-side oxide scale on boiler tubes can seriously influence the safety of power plants. In this work, the failure behavior of oxide scale formed on T91 alloy superheater tubes was investigated using a uniaxial tension test at room temperature and at elevated temperature (300°C). Acoustic emission (AE) equipment was employed to monitor the failure evolution of oxide scale. The AE features associated with the metallographic investigation were adopted to detect the onset of each oxide failure type. The experimental results showed that oxide spallation occurred only at room temperature. For the elevated temperature test, no oxide spallation was found even at the maximum strain of 2%. An elastic-plastic finite element model was developed to understand the influence of temperature on the oxide failure. Results of the simulation revealed that the material property was one of key factors influencing the oxide spallation. Since the substrate yield strength decreased with increasing temperature, oxide spallation could be prevented at elevated temperature in the field.

Thermal Stress Development in Steam-Side Oxide Scales in High Temperature Components of Boilers under Thermal Shock

Failure of steam-side oxide scales in high temperature components of boilers such as superheater and reheater tubes has significant effect on the safety of thermal power plants. Finite element analysis is carried out to investigate the failure issues of steam-side oxide scales in boiler tubes during thermal shock processes. The effects of different steam temperature variation durations on oxide scale failure issues have been analyzed. According to the results, appropriately lengthening variation duration can contribute to relieving oxide scale failure issues, and are subsequently proposed.

Acoustic emission analysis on tensile failure of steam-side oxide scales formed on T22 alloy superheater tubes

Failure of steam-side oxide scales on boiler tubes can seriously influence the safety of coal-fired power plants. Uniaxial tensile tests employing acoustic emission (AE) monitoring were performed, in this work, to investigate the failure behavior of steam-side oxide scales on T22 alloy boiler superheater tubes. The characteristic frequency spectra of the captured AE signals were obtained by performing fast Fourier transform. Three distinct peak frequency bands, 100-170, 175-250, and 280-390 kHz, encountered in different testing stages were identified in the frequency spectra, which were confirmed to, respectively, correspond to substrate plastic deformation, oxide vertical cracking, and oxide spalling with the aid of scanning electronic microscopy observations, and can thus be used for distinguishing different oxide failure mechanisms. Finally, the critical cracking strain of the oxide scale and the interfacial shear strength of the oxide/substrate interface were estimated, which are the critical parameters urgently desired for modeling the failure behavior of steam-side oxide scales on boiler tubes of coal-fired power plants.

Simulating the implications of oxide scale formations in austenitic steels of ultra-supercritical fossil power plants

Formation of steam-side oxide scale that can cause major failures in boilers is a large concern in fossil-fueled power plant operation. In this work, we attempt to simulate the implications of oxide scale formation and subsequently assumed exfoliation in the given length of an austenitic tube of ultra-supercritical (USC) fossil power plants under different presumed service conditions. The incremental procedures used to determine the average metal temperature and scale thickness over a period of time in superheater/reheater tubes at a specified steam temperature are presented. A number of simulation results for both formation and subsequent exfoliation of the oxide scales are presented and discussed. Based on the simulation results, the estimated scale growth and presumed exfoliation rate in austenitic steels at 650°C seemed to be allowable. However, to run USC power plants with steam condition of around 650°C, more detailed studies are required.

Estimation of residual life of boiler tubes using steamside oxide scale thickness

In thermal power plants, remaining-life-estimation of boiler tubes is required at regular intervals for a safer and a better functionality of boilers. In this paper, a new method is proposed for the residual life estimation of service exposed boiler tubes using Non-Destructive Ultrasonic Oxide scale thickness measurements, average metal temperature and creep master curve. While steady state conduction heat transfer equations are solved to calculate the average metal temperature, creep master curve is generated from short term stress rupture data of rupture life less than 5000 h on a virgin material. In the present study, the residual life of T22 (2.25Cr-1Mo) service exposed Platen Superheater tube is estimated using two master creep curves, i.e. Larson-Miller Parametric (LMP) method of standard ASME T22 creep data and Wilshire approach of short term stress rupture data of T22. As the residual life is calculated from fundamental conduction heat transfer theory and creep rupture data, the proposed method can be applied for different grades of boiler materials.

On the failure of steam-side oxide scales in high temperature components of boilers during unsteady thermal processes

Failure of steam-side oxide scales in high temperature components of boilers such as superheater and reheater tubes has significant effect on the safety of thermal power plants. Taking into account the influence that temperature has on physical properties of metal substrate and oxide scale, transient heat transfer model as well as multi-layered hollow cylinder stress model is successively developed in this work. Employing the Advanced Oxide Scale Failure Diagram (AOSFD) as failure criterion, a systematic analysis model for the failure behavior of steam-side oxide scales in T92 superheater tubes during unsteady thermal processes is established. Focusing on different steam temperature variation processes, the effects of steam-side oxide scale thickness as well as different steam temperature variation modes and variation durations on oxide scale failure issues have been analyzed. Besides, critical steam temperature drops leading to failure of oxide scales when steam temperature decreases linearly have also been investigated. Based on the analysis results, feasible measures such as selecting reasonable steam temperature variation mode and appropriately lengthening variation duration are subsequently proposed. The analysis results as well as the suggested measures in this work can provide valuable guidance for the solution of steam-side oxide scale failure issues in thermal power plants.

Life Assessment of Superheater/Reheater Tubes in Fossil Boilers

Creep rupture failure of superheater (SH)/reheater(RH) tubes is a major cause of forced outages of power boilers. A methodology developed recently by EPRI and its contractors has helped utilities make more informed run/replace decisions for tubes by judiciously combining calculational, nondestructive, and destructive evaluations. In this methodology, the tubes/tube assemblies at risk are identified by ultrasonically measuring the thickest steamside oxide scale and thinnest wall thickness in the tubes. The remaining life of each tube/tube assembly is predicted using a computer code known as TUBELIFE, thus achieving a further level of focus on the tubes/assemblies in the “highest risk” category. Sacrificial tube samples are then removed from the select locations and subjected to laboratory metallurgical evaluation and isostress rupture testing to refine the remaining life estimates. Research has further refined this methodology by validating the ultrasonic technique for scale measurement, identifying the appropriate stress formula and oxide growth laws and evaluating the limitations of creep damage summation rules and isostress rupture test procedures. This paper provides an overview of the research in the field, and establishes a road map for assessing the remaining life of SH/RH tubes.

Dissimilar Metal Weld and Boiler Creep Damage Evaluation for Plant Life Extension

The basic causes of dissimilar metal weld (DMW) failures in boilers are now well understood. Guidelines have emerged for achieving significantly improved DMW life, in terms of operational parameters, weld filler metals, and weld designs. Analytical, as well as experimental, nondestructive techniques have been developed that enable assessment of the condition of DMWs in service so that intelligent run-replace decisions can be made. Techniques for assessing the remaining life of superheater/reheater (SH/RH) tubes are under development. The techniques utilize information on steamside oxide scale growth and dimensional changes on the fireside due to corrosion to compile the temperature–stress profile of operation in the tubes. This information is then combined with standard creep data to estimate the remaining life of the tubes more accurately than was possible heretofore. The potential of a replication technique that allows nondestructive monitoring of creep damage in heavy section pipes has been successfully demonstrated at several utility sites. It has been possible to detect early stages of incipient creep damage and to recommend specific courses of action to affected utilities. Laboratory and field studies currently in progress are expected to result in metallographic and miniature test specimen techniques that would further augment the capabilities of the replication technique, and render it into a more quantitative tool for life assessment of heavy section components.