Failure Analysis of a Steam Superheater Tube of a Vacuum Distillation Unit (VDU) in a Petrochemical Industry
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of a Steam Superheater Tube of a Vacuum Distillation Unit (VDU) in a Petrochemical Industry Parikshit Munda . Md. Murtuja Husain . Chandra Veer Singh . Sumanta Bagui
Submitted: 21 November 2019 / in revised form: 24 July 2020 Ó ASM International 2020
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. Keywords Thermal fatigue Corrosion fatigue Superheater tubes
P. Munda Md. M. Husain C. V. Singh S. Bagui (&) Materials Engineering Division, CSIR-National Metallurgical Laboratory, Jamshedpur, Jharkhand 831007, India e-mail: [email protected]
Introduction In the petrochemical industry, heaters (fired furnaces) are extensively used to provide the requisite heat to the feed in an enclosed chamber for various units. These heaters can be classified into three categories based on their service units, i.e., process heaters, reformer heaters and cracker heaters. Process heaters are mainly used to heat the crude oil prior to entry into the atmospheric distillation unit or crude distillation unit (CDU) as well as to heat the atmospheric residues of the CDU prior to entry into the vacuum distillation unit (VDU) for the recovery of light vacuum gas oil (LVGO) and heavy vacuum gas oil (HVGO). A typical heater (fired furnace) is made of three major components: steel structure, refractory lining and heater tubes that carry the service fluid as well as the process feed to be heated in the furnace. Fuel is fired through a single burner or multiple burners and combusted inside the furnace. Heat generated due to the combustion of fuel is utilized to heat the tubes mainly through radiation and/or convection. Heat emitted directly from the flame and indirectly from hot refractory lining to the heater tubes is transferred through radiation. Hot flue gases generated due to combustion contributes primarily to the convection mode of heat transfer to the heater tubes and heater tubes in contact with the refractor
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