Abstract
Research on Flow Accelerated Corrosion (FAC) tends to be focused on the influence of water chemistry or material choices on the overall risk of pipe and tube wall thinning. Indeed, choosing a higher chromium (low alloy) material, e.g., P/T 11 or 22, tends to reduce wear rates to acceptable levels over the lifetime of the boiler, regardless of water chemistry regime. Some horizontal gas path Heat Recovery Steam Generator (HRSG) designs use low alloy material in Low-Pressure Evaporator (LP Evaporator) upper tube bends as these are considered to be at higher risk due to the geometry factor. Plants originally designed with carbon steel bends and suffering from FAC failures have replaced them with low alloy materials. Sometimes this was done with a so-called semi-harp replacement, including new bends and tube stubs, and this has been a popular solution. Although this prevents FAC in the tube bend there are several instances where it appears to continue to develop in the upstream straight carbon steel tubes below the low alloy section. This has resulted in failures on several units. This paper presents results from investigations into “straight tube FAC” including comparisons between units, computational fluid dynamics (CFD) simulations, and failure analyses. The case studies are from plants in the EMEA region, and all failure areas are located in the straight part of the tube, in the carbon steel region, upstream of low alloy tube sections. A FAC risk assessment would not show any particular risk of failures in this area given the low geometry factors. The results of the phenomena are discussed and recommendations for managing semi harp replacements are given.
