Very large differences in tube wall thinning rates by flow-accelerated corrosion (FAC), or in some cases fluid erosion, are observed in the low-pressure (LP) evaporator tubes of certain HRSG designs. The tubes located near the duct wall and occasionally near the gaps between module bundles have more rapid thinning. Tubes in a given row nominally should have very similar process conditions, both on the gas-side and on the waterside. Different wall thinning rates mean that process conditions differ across the tube row.
Computational Fluid Dynamics (CFD) simulations are used to investigate the gas-side process conditions in tube assemblies and to determine the impact of tube location in the row on heat transfer. The impact of differing tube heat fluxes on waterside process conditions is analyzed by thermo-hydraulic simulations using a computer model of the tube assemblies. A correlation between increased thinning rates as a function of tube position and LP evaporator process conditions is identified.
Tube thinning caused by flow-accelerated corrosion (FAC) is one of the most frequent causes of failure in the heat exchanger tubes of heat recovery steam generators (HRSGs), with localized progressive thinning of tube walls eventually leading to rupture and leaks. Any carbon steel tubes that carry water or wet steam are potentially at risk.
The overall mechanisms of FAC, wherein the protective magnetite layer of the base metal is dissolved at an accelerated rate, are relatively well-understood. Key factors controlling magnetite dissolution are temperature, water chemistry and fluid mass transfer rate. Two strategies for avoiding or at least greatly reducing the problem in operating plants are available; the easiest in most cases is to change the water chemistry by raising pH and creating an oxidizing environment. If this is not sufficient or if other factors (such presence of copper-alloy components in steam cycle) prevent this, then it may be necessary to replace carbon steel tube sections encountering wear with a more resistant material, namely low-alloy steel containing some chrome.
Figure 6: Modified Case Temperature distribution between fins [K]