Reducing boiler maintenance costs with improved materials for air-heater tubes

Corrosion of boiler air-heater tubes costs the industry several million dollars each year in repairs, reduced boiler steam output and reduced boiler efficiency. There have been many cases where the reduced boiler steam output caused by leaking air-heater tubes has reduced factory crushing rates and electricity export. Corrosion of air-heater tubes can, in many cases, be minimised by improving the gas and air flow distributions with turning vanes and ductwork redesign, but in nearly all cases, there are some tubes on the cold air side of air-heaters that are still susceptible to dew-point corrosion. Using improved tube materials in parts of boiler air-heaters that are susceptible to dew-point corrosion will significantly extend air-heater life. Available materials and coatings were reviewed, and laboratory- and factory-scale trials, metal temperature measurements, dew-point calculations and a financial analysis-based ranking of commercially available tube materials were undertaken. S-TEN 1 had a similar corrosion performance to SS304 stainless steel. Both the S-TEN 1 and SS304 stainless steel tubes had significantly greater resistance to dew-point corrosion than the carbon-steel tubes typically used in the air-heaters of Australian sugar factory boilers. The good performance of S-TEN 1 in this project is not consistent with the poor performance of S-TEN 1 (no better than carbon-steel) in earlier trials carried out by Isis Mill. The conditions experienced by the trial tubes in this project were not as severe as those experienced by the tubes in the earlier Isis Mill trials, and this appears to be the main reason for the current improved performance of the S-TEN 1 tubes. This requires further investigation.