Analysis On The Welding Crack Of The Second Fccu Regenerator In The Refinery Plant

This paper presents discussion on whether the welding cracking on the second FCCU regenerator in the Refinery Plant of a large Petro-Chemical Corporation affects its safe operation or not. Through analysis on operation condition, material composition, mechanical properties, metallurgical tests and fracture observation, it was found that the cause of the failure was due to the stress corrosion cracking (SCC)of the regenerator. Measures such as to raise the wall temperature were proposed to avoid the condensation of acid containing flue gas and to prohibit the further spreading of the existing cracking effectively. According to the Code for Pressure VesselDefects Assessment (CVDA-1984), stress analysis and safety assessment was presented for the regenerator post overhaul with the consideration of actual conditions that there might initiate the possible cracking.It is concluded that the cracking on the regenerator was caused by stress corrosion cracking owing to the following three factors. Firstly, corrosive environment of acid containing flue gas and its condensation; Secondly, corresponding enhancement of medium and material to corrosion; Thirdly, the existence of tensile stress in the structure material.It is known from the operation process that there exist flue gases containing harmful compounds which may condense to form corrosive fluid in certain condition. Based on the analysis of chemical composition of regenerated fluid gas, there exist NOx , S0x2 and Cl in the condensed fluid. As the inside wall temperature of the regenerator is lower than dew point temperature of the harmful gases, such gases are easily condensed to create the right environment for the cracking initiation and development.The analysis and test of chemical composition of material and mechanical properties before and after the failure. Metallurgical examinations show there are no obvious changes in metallurgical structure before and after the failure. If there were no the cracking, which spread and lead to cracks, the material ' would be within the range of acceptable application.From the examinations and analysis of fracture figure of cracking corrosive products, it was observed that there were no obvious plastic on fracture faces. The cracking is intergranular and containing NO3- in the corrosive products. NO3 is a kind of anion which may cause low carbon steel stress corrosion cracking easily.The results of stress analysis and calculation show that, the maximum stress lies in the inside face of the connection between transition cylinder and upper cylinder with the value of'217. 328Mpa. According to theCode for Pressure Vessel Defects Assessment (CVDA-1984) and stress intensity factor method of linear elastic fracturemechanics analysis with the consideration of the actual conditions, SCC is the main cause of the failure of the regenerator. The results show the cracking on the regenerator after repair is allowable, and the regenerator can be used safely. This is in accordance with the actual conditions of regenerator.