Welding Repair Technology For Life Extension Of Ethylene Cracking Furnace Tubes

In the ethylene production unit the cracking furnace is one of the key equipment. Particularly the tubes in the radiant section of the furnace subjected to severe temperature and corrosion condition are the major parts of concern in the maintenance. After serviced for certain time period, the furnace tubes may acquire more damage in the part of higher temperature, which leads to the premature failure of a furnace tube. However, this does not mean that the whole tube should be discarded. If one can identify the most damaged part and repair the tube locally, a significant saving of maintenance cost could be expected.The weldability, repair quality and reliability of a furnace tube bear closed relation to the accumulated damage at microscale. Certain correlation between the hardness and damage could also be found. Nevertheless relocation of damage parts and repeated repairs have been the common practice due to lack of support of quantitative research of the relations. In order to challenge the difficulties, the present thesis designed a weldability test rig for the tubes. Aiming at clarifying the influencing factors on the weldability, considerable welding tests, metallographical analysis, general mechanical tests and creep tests have been performed on the repair-welded tubes removed from the service at different used periods. A rapid assessment method and corresponding repair technology have been proposed. The major conclusions drawn from the research are given as follows:(1) To study the cracking tendency of the welded joints of ageing furnace tube and new furnace tube, the present work designed a weldability test rig for the furnace tubes, which provides an important tool for improving the welding process. Welding tests show the tube cracking mode in practice can be well reproduced. The constraint produced in the test is more severe than that of on-site welding which ensures a conservative assessment.(2) In order to assess the damage condition of the tubes and obtain a correlation with weldability, the present work classified the damage into 8 ranks on the basis of previous research and engineering practice.(3) Metallographical analysis reveals that the cracked tubes after welding test experienced significant damage in service. Internal cavities and microcracks appearedin the damaged grain boundary or defected locations. The fracture surface are generally brittle. The welding tests show that the cracking tendency is increased when the damage rank is higher than 4.(4) A rough correlation between the hardness and damage rank is obtained. The hardness is increased as the damage rank is raised. A hardness smaller than HB223 or HRC15.4 (Damage rank smaller than 4) is essential to ensure the weldability and can be used as a criterion for rapid assessment of weldability in practice.(5) The repaired welds have a higher strength than the parent metal in ambient temperature. A higher instantaneous high temperature strength and a higher creep strength than the parent metal have been obtained which verifies the feasibility of the proposed welding procedure.