Failure Analysis On Low-finned Copper Tube Of Oil-cooler

Oil cooler is widely applied in the industrial production, the rapid development of rolling low-finned tube are increasingly used in heat exchangers and other equipment in recent years Because of poor corrosion resistance, it may easily cause premature in complex conditions, and it is easily cause to serious accidence. Therefore, the in-depth failure analysis for the different failure of low-finned heat exchange is necessary to the design improvements of products. It will be of great practical significance for the protection of individual life and property.The author took a failed low-finned copper tube as object of this paper, which is used by an enterprise. After the observation of the fractured low-finned tube through macro, the outward expansion deformation and rupture was found, and the crack extend along the axial. There was also significantly thinning of the inner wall of the tube, some dirt attached the inner wall can also be found. First we simulate different pressure conditions for the low-finned tube by the finite element analysis software ANSYS. It can be shown that the maximum stress areas are located in the inner wall surface of finned tubes. The inner wall of the tube is the area where bears the maximum stress while it can still perform well under the high pressure. After a comprehensive analysis in the path analysis method under the “extreme conditions”, it indicates that hoop stress are greater than the axial over all the defined paths, which can determine that the fracture crack occurred along the axial extension. In order to further explore the corrosion mechanism, the reason from the environmental conditions of the tube need to be found or otherwise.Then, a research on corrosion mechanism of heat transfer tubes was conducted, and its failure analysis is conducted by a series of experiments on the physical and chemical properties. Macro and micro morphology, chemical composition analysis, microstructure observation, hardness testing, test of recirculating cooling water and other means of whole analysis were used. Through the correlation means, the test for low finned tube materials, a detection for the circulating cooling water and an observation for break morphology by characterization tests was conducted, then got an overall reason for the finned tube thinning. The fin-tube materials meet the requirement of intensity criteria, fin-tube rupture is not caused by unqualified material through analysis. The circulating cooling water test results shows that the unqualified water quality is the main reason for fin-tube corrosion. The dissolved oxygen and chlorine ion in cooling water caused pipe wall corrosion and produced a copper-based corrosion fouling layer, Ca, Na, K, Mg and other elements as well as flocky precipitate is further exacerbated by the extent of fouling the inner wall surface. At the same time, combining with a higher content of anions such as Cl- and SO42- accelerates the speed of crevice corrosion. It can be proved by the test of finned tube wall attachments and section corrosion products.Finally, a comprehensive analysis and discussion in terms of failure cause of the finned tubes was conducted, then the corrosion mechanism why the finned tube failure occurred can be proved, the finned tube was conducted under dynamic complex role by corrosion, scaling, erosion due to the unqualified circulating water, in making that the tube wall thinning until its fracture occurs for insufficient strength under pressure. This thesis also proposed some precautionary measures according to the main factors of finned tube in failure.