The Creep Damage Simulation Of Ethylene Cracking Tubes Under Multiple Operation Conditions

Ethylene cracking tubes (CFT) are the core parts in petrochemical industry.Premature failures of ethylene cracking tubes which cause huge economical loss andsecurity incidents are often observed. Service related with high temperature andcarburization contributed to the premature failure. Therefore, it is essential to carryout the study of the premature failure of the CFT, and provides technological guidancefor life prediction and maintenance of CFT.It is reported that carburization is the primary factor accounting for the prematurefailure of CFT. The density and thermal expansivity change of CFT caused bycarburization induces stress during the long term of operation. In this paper，severalsamples of CFT base metal and weld which contain different carbon concentrationswere made to detect their density and thermal expansivity. Their phase changes aftercarburization were also studied by optical microscope. The results shows thatcarburization decrease the density of base metal and weld. The thermal expansivity ofbase metal and weld decrease as the carbon concentration rises. The metallographicphase study indicates that the grain boundaries become obscure after carburization.The mathematical model for CFT was established based on the experiment data.Numerical simulation of the stress-strain filed and creep damage on CFT and itsweldment was carried out using Abaqus element analysis software. The resultsdemonstrated that the creep damage caused by internal pressure is far form the criteriafor the premature failure of CFT after the ethylene cracking tubes reaches its designedservice life. The simulation on the creep damage under the multiple conditions revealsthat the inside wall of CFT primary failed after the operation of about27700hours.The creep damage along the CFT is in accordance with the following sequence: theinside wall﹤the outer wall﹤the middle part. The simulation on the creep damage ofthe CFT’ welded joint under the multiple conditions shows that the base metal closedto the material interface firstly failed after servicing17500hours. From the above, thedamage caused by multiple factors is the main cause of the premature failure of CFT.