Abstract: The economizer represents a vertical finned hairpin tube design which utilizes combustion flue gases to heat up Regeneration Gas in two coils. Numerous cracks were observed on the welded regions of the pair of 5” Sch. 80 outlet nozzle welds and their reinforcement pad (repad) welds, after only 3 years in service. The addition of a repad was a modification from the original design based on recommendations made by consultants following similar previous failures on identical economizers. The findings indicated that the root cause of failure was attributed to a combination of poor engineering design, and incomplete joint penetration of the nozzle attachment welds, which facilitated an unusual acid dew-point corrosion mechanism (Nitrate SCC) to manifest itself in the relatively cool plenum chamber. This was the first time such a failure was reported and was considered unique. Piping stress, flexibility analyses and thermal distortion stresses were also considered.
Authors: Riza Khan and Kavir Ramdass
Keywords: Failure Analysis, Weld Failure, Stress Corrosion Cracking (SCC), Nozzles, LNG