Failure Analysis of a Steam Superheater Tube of a Vacuum Distillation Unit (VDU) in a Petrochemical Industry

Abstract

The aim of this investigation is to examine the premature failure of a low-pressure steam superheater tube used in a vacuum distillation unit of a petrochemical industry. Failure of the heater tube occurred in the form of leakages of fluid during the hydrostatic test. Visual observation of the failed tube revealed several fine cracks on the outer surface of the tube and localized corrosion pits/cavities on the inner surface of the tube. Chemical composition, hardness and tensile properties of the failed tube conform to the designated standard of ASTM A335 P11 grade of steel. The microstructure of the tube comprises ferrite–pearlite structure without any remarkable microstructural degradation. SEM micrographs of the leaked surface revealed the presence of fatigue cracks generated because of fluctuating thermal stresses experienced due to frequent shutdowns during plant operation. Fatigue cracks were found to originate from the pits/cavities on the inner surface of the tube. Pits/cavities were formed because of localized corrosion due to the presence of excess dissolved oxygen in the water along with S and Cl.