Cracking of superheater tube in a captive power plant

Abstract

A steam generator unit in a captive power plant of a heavy water plant reported fish-mouth failure in the secondary super heater bend. The operating pressure of the super heater is 105 kg cm−2 and temperature is 485°C. The super heater tubes made of 2¼Cr–1Mo ferritic steel is exposed to steam at a pH of 9.5–10.5 and conductivity less than 2 µS cm−1 in the inner surface and at the outer surface exposed to flue gas. It has been operational for last 25 years. Layered deposits were observed at the failed region which were iron oxides. The investigations revealed that outer surface which has flue gas environment showed thinning with oxide scale spallation taking place. The SEM studies showed that there was extensive precipitation of molybdenum sulphide and carbides in the microstructure resulting in preferential oxidation along grain boundaries. The excessive outer thinning of tube resulted in fish-mouth failure of the super heater tube. This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).