Assessment of increased tube life in boiler convection banks through commercially available tube coatings

The replacement cost of a boiler convection bank due to wear (erosion) is approximately $1 million with a tube life of approximately 10-15 years using current protection technology (tube shields). Computational fluid dynamics (CFD) modelling has been used extensively for redesign of convection banks to reduce tube wear and to predict where tube shields should be located for new convection bank geometries. Tube coatings have been successfully applied in other industries to protect against erosion and can also be applied in areas such as the superheater loops where the higher temperatures can cause problems for tube shields. The erosion performance of commercially available coatings applied by laser and thermal spray (high velocity oxy-fuel (HVOF) and twin-wire arc spray (TWAS)) processes was compared to that of bare tubes and tube shields, in both laboratory scale tests at the CSIRO Clayton facility and factory trials at Mulgrave Mill. The testing was supported by financial analyses at each stage of the project. A WearPro 62 (tungsten carbide) coating applied by laser was the best performer, with an expected coating life of approximately 18 years. The data were extrapolated to predict the coating life for convection banks with different gas velocities. There were also significant issues with early failure of tube shields by mechanisms other than erosion. Such failures are of concern as, once installed, many shielded locations in convection banks are difficult, costly or impossible to access. Further assessment and improvement of the attachment of tube shields to tubes are required.