On The Failure Of Steam-Side Oxide Scales On High Temperature Steam Generation Components

Failure of steam-side oxide scales formed on inner surface of boiler tubes, such as cracking, lifting and exfoliation, may occur during normal operation periods of power plants. The current research efforts are mainly focused on steam oxidation mechanism of alloys, while for the failure of steam-side oxide scales, the investigation works are limited. This condition can not only meet the desire for improved reliability and economy, but also affect the development of plants with higher steam parameters and larger capacity. In this work, focusing on failure of steam-side oxide scales formed on high temperature steam generation components, research efforts which employed the methods of analytical modeling, numerical simulation and experimental investigation, were performed respectively from the following five aspects.(1) An evaluation model for the creep rupture life (CRL) of T91 alloy boiler tubes, taking into account the influence of steam-side oxide growth on tube temperature and stress, was established. To improve the prediction accuracy, the newest creep testing data presented by the European Creep Collaborative Committee (ECCC) were adopted. The steam-side oxide scale thickness was correlated with the residual CRL of the boiler tubes, which enabled the estimation of tubes’residual CRL simply through measuring the steam-side oxide scale thickness during the maintenance periods of the plants. The correlations between the CRL and steam temperature and steam mass flow were also established respectively, with which the allowed highest steam temperature and lowest steam mass flow can be determined. The calculation results as well as the obtained conclusions can benefit the design optimization, tube replacement, and maintenance of the high temperature heating surfaces of boilers. As compared with traditional oxide growth models, the present model has considered the variation of oxide growth temperature with the oxide growth, enabling a more accurate prediction of oxide growth.(2) Uniaxial tensile tests employing acoustic emission (AE) monitoring were performed in this work to investigate the failure behavior of steam-side oxide scales formed on T22 alloy boiler tubes. The characteristic frequency spectra of the captured AE signals were obtained by performing fast Fourier transform (FFT). Three distinct peak frequency bands encountered in different testing stages were identified in the FFT frequency spectra, which were confirmed to respectively correspond to substrate plastic deformation, oxide vertical cracking, and delamination of oxide/substrate interface with the aid of scanning electronic microscopy (SEM) observations, and can thus be used for distinguishing different oxide failure mechanisms. As compared with other traditional methods such as in situ SEM inspection or AE methods relying on the variation of signal amplitude or number of AE events, the present method can not only avoid the troubles induced by observation delay and shelter effect, but also eliminate the influece of signal attenuation or external disturbance. Finally, the critical cracking strain of the oxide scale was estimated, which is the critical parameter urgently desired for modeling the failure behavior of steam-side oxide scales on boiler tubes of coal-fired power plants.(3) A novel method for evaluating the shear strength of oxide scale/substrate interface was proposed. This method established a stress model to obtain the shear stress distribution along the oxide scale/substrate interface, while adopted the identification method employing AE frequency spectra to determine the failure moment of oxide scale/substrate interface. As compared to previous stress models, the present model did not rely on any proactively designated shear stress distribution along the oxide scale/substrate interface, and utilized the "elastic-linear hardening" assumption, while not the traditional "elastic" or "elastic-plastic" assumption, to simulate the plastic deformation of metal substrate.(4) Considering the influence of temperature on coefficients of thermal expansion of metal and oxide scales, a multi-layered hollow cylinder steady stress model was developed. Different stress components in the steam-side oxide scales of T22, T91 and TP347H alloy superheater tubes were then calculated, focusing on the plants’ load-reducing processes. The calculation results indicated a potential for the occurrence of distinct oxide scale failure modes that may be activated by large hoop stress or axial stress components. The obtained conclusions can provide valuable reference for determining more reasonable load-reducing strategies.(5) An analysis model for the failure of steam-side oxide scales formed on T92 alloy boiler tubes during unsteady thermal processes was established. This model, considering the influence of temperature on the coefficients of thermal expansion of both metal substrate and oxide scale, employed the advanced oxide scale failure diagram recently proposed by Electric Power Research Institute as the failure criterion. Since the stochastic nature of related geometrical, mechanical, and thermophysical parameters may hinder the usual deterministic investigation from obtaining a reliable evaluation, probabilistic failure evaluation which was not based on bivalent logic was thus performed. Focusing on the unsteady thermal processes induced by the variation of steam temperature, effects of oxide scale thickness as well as different steam temperature variation processes on the cracking probability of steam-side oxide scales were analyzed. It was found that the cracking probability of thicker steam-side oxide scales was higher, while through reasonably selecting steam temperature variation mode and appropriately lengthening variation duration, lower failure probability can be achieved. Feasible measures were also presented with the aspiration to solve the steam-side oxide scale failure issues.It is believed that, apart from guiding the solution of oxide scale failure issues encountered in coal-fired power plants, this work can also benefit the nuclear power plants, metallurgical industry, as well as chemical engineering areas.