Analysis of failed 1109 HT shift vessel inlet line pressure gauge tapping

A portion (half cut) of the failed 1109 HT shift vessel inlet line pressure gauge tapping was forwarded to the laboratory for analysis. The tapping was of tubular construction made of ASTM A 335 grade steel. The tapping was found to have failed by corrosion at about 10 mm above the socket weld joint. Corrosion had set in on the inner surface of the tube and progressed almost to the outer surface producing a groove of about 6 mm before the tube got sheared off. Semi-quantitative chemical analysis of the corroded region showed presence of chlorine as high as 0.6 wt%. From the analysis, it appears that possibly there was condensation of water vapour at the stagnant zone which in conjunction with chlorine present in the gas resulted in corrosion of the tube leading to failure. There was no abnormality in the tube material