September 1, 2010

Ten Years of Experience with FAC in HRSGs

Peter S. Jackson, PhD, PE; David S. Moelling, PE; and Mark Taylor

Future Technology Required

FAC is shifting from a rapid wear phenomenon over large areas of affected HRSG components to a slower wear process with the risk of locally higher rates. In many cases, the higher wear rates are not detectable by wear measurements in accessible locations by relation to gross flow models. The high percentage of problems in two-phase areas of LP, and to a lesser extent intermediate-pressure (IP) evaporator systems, highlighted the need for good flow modeling of these zones and FAC predictive models for two-phase conditions. More detailed fluid modeling design, inspection, and repair tools are required to identify local high-risk locations as well as the impact of operational changes on FAC wear.

Design Technology. Another key need is the mapping of test loop and advanced flow modeling studies into an easy-to-use methodology for better predicting FAC risk in highly localized zones inside the HRSG. Drawing on existing predictive approaches, it might, for example, involve an updated or extended set of geometry factors. These would characterize common structures, such as tube-to-header interfaces in the HRSG flow path. The improved method should give reasonably good predictions of wear of both “global” and “local” FAC, using only design and process data that is readily available to plant operators.

Predictive models for FAC in HRSGs are improving, but further development in the following areas are also needed:

• Two-phase conditions. Better understanding of FAC wear rates in high void fraction flow in LP and IP evaporators is required, including the impact of material removal and transport.
• Local flow disturbances. The local FAC wear condition at the HRSG tube/header interface is now recognized as a significant problem area. Identification of bulk flow and geometric conditions that lead to high FAC wear rates in this area is important.
• Local and off-design flow conditions. Better integration of FAC models with plant simulation models to identify operating modes that increase FAC risk is required.

Inspection Technology. Past efforts at improving technology for HRSG nondestructive examination (NDE) have focused on inspection in the finned tube areas not currently accessible to many NDE techniques. A bigger problem for FAC is poor accessibility of bare tube segments at headers of horizontal gas path HRSGs. Only the outermost tube rows are typically accessible, and often only part of the tube circumference for these. Borescopy can be a useful tool, but access is usually limited or requires cutting into the header or connecting piping. Advances in digital radiography and other techniques offer prospects for improved assessment of FAC damage. New technologies for under fin thickness measurement, digital radiography, and advanced borescopy all will help characterize FAC in HRSGs.

Repair Technology. FAC damage to tubes at the tube/header joint can be a large issue if more than a few tubes are affected. Improved technology for repair welding is under development and early deployment. Design to allow better access for repair is also required.