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

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.