Denver, CO, United States
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Sabau A.S.,Oak Ridge National Laboratory | Wright I.G.,WrightHT Inc. | Shingledecker J.P.,EPRI
Materials and Corrosion | Year: 2012

This paper provides an introduction to a comprehensive model being developed to predict and control oxide scale exfoliation from the steam-side of superheater and reheater tubes in steam boilers. The model deals with the main phenomena involved in scale growth and failure in steam, and incorporates major variables related to boiler design and operation. The considerations used to calculate oxide growth under the specific constrains of small diameter tubes carrying high-pressure steam and operating with large temperature gradients under temperature and pressure cycling conditions, as well as the evolution of stresses and strains in the scales, are indicated but only a cursory description is given of the details of the analytical treatments. An example is presented of calculations made with the model to predict the extent of blockage expected in a single superheater loop as a function of time and outlet steam temperature under several realistic service conditions. The results suggest that problems due to scale exfoliation would be expected early in the operating life of superheater tubes made from austenitic steel TP347H. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Wright I.G.,WrightHT Inc. | Shingledecker J.P.,EPRI
Materials at High Temperatures | Year: 2015

Interpretation and use of a body of field data for corrosion of superheater and reheater (SH/RH) tubes in coal fired boilers was found to be unsatisfactory without some indication of how the corrosive environment encountered differed when different coals were burned. A new factor intended to represent the relative corrosion potential (RCP) of the coal burned is suggested, based on the accepted mechanism of accelerated corrosion of SH/RH tubes by the development of low melting complex sulphates beneath ash deposits. Initial testing of the RCP concept was encouraging, given the difficulty in some cases of ensuring accurate representation of the actual coal burned during the corrosion exposures. However, some data suggested that new boiler operating modes, such as various approaches to emissions reduction through staging of the combustion process, appeared to be capable of causing significantly more rapid corrosion of SH/ RH tubes than would be expected from the RCP value for the coal. Although increased corrosion with some emissions control systems has been recognised in practice, understanding of the particular corrosion process involved is lacking. Some degree of mechanistic understanding of the key features of this form of corrosion is needed to provide a firm basis for application of concepts such as RCP or development of improved versions. © W. S. Maney & Son Ltd. 2015.

Wright I.G.,WrightHT Inc. | Pint B.A.,Oak Ridge National Laboratory | Shingledecker J.P.,EPRI | Thimsen D.,EPRI
Proceedings of the ASME Turbo Expo | Year: 2013

Proposed open and closed Brayton-type cycle systems employing supercritical CO2 (sCO2) as the working fluid develop hot gas path environments that present challenging requirements of strength and environmental resistance for the materials of construction. The likely materials properties required by different cycle configurations are examined and compared with the capabilities of available materials. In some instances where peak operating conditions in the turbines in closed-cycle systems approach 500°-700°C at 200 bar, or 1150°C at 300 bar for open cycles, the range of available alloys with the requisite temperature-strength and capabilities is limited. The cycles are highly recuperated: hot, lower-pressure turbine exhaust is used to pre-heat high-pressure CO2 entering the external heater or combustor just upstream of the turbine. Obviously, there is a need to match alloy capabilities with the performance requirements of individual components. Where alloys are employed under conditions beyond current experience, or in unusual configurations (the unique properties of sCO2 may impact the design of some components), practical acceptance will depend on successful qualification testing involving close collaboration among equipment manufacturers, materials suppliers, and materials research and development groups. Examination of the range of scenarios published for sCO2 turbine systems suggests that approaches used in the U.S. Advanced, Ultra-Supercritical (A-USC) Steam Boiler program, and some of the data generated for fabrication, joining, mechanical properties, and performance in simulated service environments in that program, will be applicable to some of the components in these systems. An attempt to elaborate and prioritize the materials property and qualification activities required for the successful realization of these sCO2 cycles is presented as a guide to activities needed to facilitate materials selection. Copyright © 2013 by ASME.

Sabau A.S.,Oak Ridge National Laboratory | Wright I.G.,WrightHT Inc. | Shingledecker J.P.,EPRI | Tortorelli P.F.,Oak Ridge National Laboratory
Advances in Materials Technology for Fossil Power Plants - Proceedings from the 7th International Conference | Year: 2014

A model based on a concept of "fraction of exfoliated area" as a function of oxide scale strain energy was developed to predict the extent of exfoliation of steam-side scale from boiler tube superheater loops. As compared with the Armitt diagram, which can be used to predict when scale damage and exfoliation would be likely to occur, a "fraction of exfoliated area" approach provides an estimation of mass of scale released and the fraction of tube likely to be blocked by the exfoliation. This paper show results for the extent of blockage expected in a single bend of a superheater loop was predicted as a function of operating time, bend geometry, and outlet steam temperature under realistic service conditions that include outages. The deposits of exfoliated scale were assumed to be distributed horizontally the tubes bends. Three types of bends were considered: regular bends, short bends, and hairpin bends. The progressive increase in steam and tube temperatures along a single loop of superheater tubing and the ensuing variation of oxide scale thickness are considered. Numerical simulation results for a superheater loop made of TP347H austenitic steel indicated that tube blockage fractions larger than 50% are likely to occur within the first two years of boiler operation (with regularly scheduled outages) for outlet tube temperatures of 540-570°C, which is consistent with practical experience. Higher blockage fractions were predicted for tubes with hairpin bends than for tubes with regular bends, of length that are larger than five internal tube diameters. Finally, the blockage model presented can be used with some confidence to devise operating schedules for managing the consequences of oxide scale exfoliation based on projections of time to some critical blockage fraction for specific boiler operating conditions. Copyright © 2014 Electric Power Research Institute, Inc. Distributed by ASM International®. All rights reserved.

Wright I.G.,WrightHT Inc. | Dooley R.B.,Structural Integrity Associates
Materials at High Temperatures | Year: 2013

Observations of steam-formed scales on type T22 ferritic steel reheater tubes have provided some insights into the scale morphologies associated with exfoliation. Measurements indicated that as the total thickness of the adherent scales increased, the ratio of the thicknesses of the main inner (Fe-Cr spinel) and outer (magnetite) layers increased continuously to a maximum of approximately 2.5. The often-assumed thickness ratio of 1.0 did not persist after a laminated structure of pairs of spinel and magnetite layers was developed in the main inner layer. The development of such multi-laminations appeared to be associated with a decrease in the rate of growth of the main outer, magnetite layer. In this particular set of samples, exfoliation occurred in scales of total thickness greater than approximately 270 μm, and involved separation of a single pair of oxide layers of similar thickness that consisted of the original outer magnetite layer and part of the original spinel layer. The scale lost in subsequent exfoliation events at essentially the same site consisted of a further pair of layers comprising the new magnetite layer re-grown after the initial event, and a further part of the original spinel layer. Exfoliation events did not appear to involve the whole of the spinel layer existing at that time, only the equiaxed region immediately subjacent to the magnetite and a portion of the laminated structure.

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