Papers by Keyword: Boilers

Abstract: Problems with corrosion in boilers and their parts can be solved by the application of different nickel alloys like 622, 625 or 686 by a variety of welding processes. This solution is used mostly in Waste-to-Energy plants or biomass stations burning waste wood, but it can also be found in the recovery boilers used in the paper industry. The most common material grades include low alloyed boiler steels like 16Mo3, 13CrMo4-5 and 10CrMo9-10. When there is a need to increase steam temperature and pressure more complicated alloys to become a natural choice. This paper focuses on the fabrication experience of welding of 7CrMoVTiB10-10 base tube with Inconel 625 weld overlay and presents a welding solution of matching filler metal used for root pass and S Ni 6625 filler metal for a fill-up and the cap performing a full strength weld without the need of overlay peeling and manual tie-in overlay. This method of welding saves a lot of time during execution in the workshop and on-site during installation and assures much better quality in the end. All examinations were performed to allow welding procedure qualification according to ASME and EN standards.

Abstract: We examine the mechanical and physical properties of Sanicro 25 steel that are relevant to the performance requirements of supercritical boilers (e.g., resistance to high-temperature corrosion). Sanicro 25 is mainly used in the construction of power plant components. Materials research has demonstrated the stability of properties at elevated temperatures (700 to 750°C) during long-term use. We conducted tests of corrosion resistance for Sanicro 25 steel, and confirmed its resistance to oxidation in steam and exhaust gas heat .

Abstract: Sanicro 25 stainless steel (X7NiCrWCuCoNbNB25-23-3-3-2) is a preferred material for structural elements of boilers with supercritical and ultracritical parameters. Welded joints are critical sites that are vulnerable to failure. Hand-welded and automatically uniform welds of Sanicro 25 are evaluated in material studies, and the structure and basic mechanical properties of welded joints are examined. The proposed welding technology for Sanicro 25 ensures proper microstructure, and has obtained quality class B certification.

Abstract: This review discusses key papers presented at an EPRI sponsored Workshop on “Scale Growth and Exfoliation in Steam Plant” that was held at the National Physical Laboratory (NPL) in September 2003 [1]. Additionally, some more recent developments on modelling both scale growth and exfoliation are described. Scale exfoliation in the steam circuit of power plant boilers leads to tube blockages and, further downstream in the power plant, to erosion of the steam turbine blading; and this can have serious consequences for plant performance. Factors controlling this behaviour are reviewed. These include the thermochemistry of oxide formation as a function of operating conditions, scale microstructure and scale growth rates. It is well known that the oxidation rate of steels in steam is about an order of magnitude greater than that in air or oxygen, but the mechanism responsible for this increased rate is still unclear. Various hypotheses, which consider transport of volatile species through cracks and pores, diffusion of OH - or protons and direct access of steam to the metal oxide interface, are proposed to account for the increased rates of reaction in steam compared with air. Modelling exfoliation of thick oxide scales is considered in a number of ways. The basis of the original model by Armitt et al [2] has been extended and further developed. A popular approach is to assume that an oxide layer develops through-thickness cracks when a critical tensile stress (the oxide strength) or strain (the oxide strain to failure) is encountered. Another approach applies fracture mechanics principles to defects that are assumed to exist in the oxide layer, although there is great uncertainty regarding the relevant defect size distributions that control behaviour. A third lower bound (and conservative) approach is to consider the energetics of steady state through-thickness cracking that involves the fracture energy for through-thickness cracking and avoids the difficult issue of needing to know the defect size that initiates through-thickness cracking. Additionally, the need to incorporate kinetics of scale growth into the developing exfoliation models is briefly discussed.