Corrosion-Fatigue Failure of Finned Stainless Steel Economizer Tubes Due to Differential Thermal Expansion

After ten years of satisfactory operation, economizer-tube failures occurred in a large black liquor recovery boiler for a paper mill. The economizer contained 1320 finned tubes. Two fins ran longitudinally for most of the tube length and were attached by fillet welding on one side. The economizer tube leaks occurred at the end of the fin near the bottom of the economizer. A sample from a tube that had not failed showed heavy pitting attack on the inside of the tube, probably due to excess oxygen in the feedwater. Penetrant testing revealed numerous longitudinal cracks on the inside in the area of the fin tip. Cracking at the end of the fin-to-tube fillet weld was noted. The results indicate the failures were due to corrosion fatigue whose stresses were primarily thermally induced. A temporary solution included inspecting all tubes with shear-wave ultrasonics. Tubes with the most severe cracking were ground and repair welded. The square corners of the fins were trimmed back with a gradual taper so that expansion strains would be more gradually transferred to the tube surface. Water chemistry was closely evaluated and monitored, especially with regard to oxygen content.