Failure Analysis Of Heat Exchanger Tubes And Residual Stress Analysis Of Tube To Tube Sheet Joints

In the oil refinery industries, there are many manufacturing processes which need to cool hot fluid, heat cold fluid, gasify or condense. Heat exchanger is the important equipment not only ensures the technological conditions of these processes, but also saves energy by realization of the secondary energy utilization and heat recovery. With the high speed development of modern industry, the application fields of the heat exchange equipment become more and more wide. While heat exchanger is also required more reliable and longer service life. The increasingly harsh conditions of heat exchanger in various fields make it no end of malfunctions, in which the damage and joint failure caused by corrosive medium, high temperature and strong pressure are the most common failure modes, especially in refinery industries. In order to prevent heat exchanger from premature failure, the failure analysis should be paid more attention and research. Therefore, this thesis carries out failure analysis of the heat exchange tube’s corrosion problems and the residual stress on the tube to tube sheet joints.This paper begins with analyzing the leakage phenomenon of the heat exchange tube. The methods of microstructure and morphology observation and chemical composition analysis of the leaking heat exchange tube are employed to identify the failure mode and to analysis failure causes. It is concluded that the failure mode of the tube is stress corrosion cracking that is caused by the corrosive Cl- and S2- in the corrosion products which inside the tube. The cause of corrosion failure of the bending pipe is a combined action of internal stress and corrosion products such as FeCl2, Fe (OH)3 and FeS. In the meantime, an electrochemical experiment under ideal conditions of laboratory is conducted to simulate the corrosion of 20G steel in a water medium. The controlling variables method is adopted to test the relationship between corrosion rate and the medium’s temperature, pH value and concentration of Cl-, to explore how these factors impact the corrosion behavior of 20G steel, and to analyze the corrosion mechanism of 20G steel.Both the finite element analysis (FEA) which is used to calculate and X-ray diffraction method which is used to measure are applied to analyze residual stress of the tube to tube sheet joint. First, field of weld temperature and residual stress in the welding process of the joint are predicted by the thermal stress analysis module of FEA software ABAQUS. The simulation results of stress and strain shows that the maximum radial stress appears on the heat affected zone (HAZ) between the adjacent weld in the surface of the tube sheet. And this is the main factors lead to crack on the surface of the tube sheet. The maximum value of hoop stress is located in the root of the weld, close to the gaps between tube and tube sheet. The gap of weld root is directly contact with medium, hence the hoop stress is the main factor of stress corrosion cracking and primary cause of the failure connection of heat exchange tube to tube sheet. Tube to tube sheet joint has a series of welds in a general way, all the welds which have been completed undergo heat treatment in the process of welding heating of the next weld. It is the same for the joint which has eight welds in this thesis. The stress value has fallen dramatically because of this reason. The welding residual stress of the welded tube is less than the adjacent weld that is welded subsequently. This item is very favorable for lowering the sensitivity of stress corrosion cracking.Then the residual stress is measured. The results show that the large value of residual stress in the joint is produced by local high thermal gradient. Therefore, the overall heat treatment after welding is an effective way to remove welding residual stress for those complex welding structures which have a large number of short welds. The variation trend of FEM prediction result is consistent with the measured value, so the finite element model established in this paper can be used for residual stress analysis of tube to tube sheet joint.