| Heat exchangers are one of the significant and substantially-used equipments in engineering areas like petrochemical, fossil power, nuclear power, and so on, and the tubes inside them are the most key parts for heat exchanging and transferring. However, owing to the variability of the process media and the complexity of the service conditions, e.g., the interactions between different corrosion mechanisms, corrosion and scaling, corrosion and abrasion, corrosion and erosion, as well as high temperature and high pressure, such heat exchanger tubes are always encountered with premature failures. As a result, the lifetime of the whole equipments would be shortened, great economic losses would be sufferred, and even safety accidents would be engendered. Thus, in order to ensure safe operation of those heat exchangers and avoid recurrence of similar accidents, this thesis will carry out a series of analyses and researches into two aspects. Based on several typical failure incidents, the first part will deeply investigate the premature failure behaviors of the heat exchanger tubes in some important petrochemical plants, and will also study the relevant failure mechanisms and propose the pertinent prevention methods. Achievements of this research will have a reference value for failure prevention of the in-service heat exchangers that operate under similar service conditions. The second part will conduct systematic performance evaluation and analysis on the welded joints, the most-prone-to-failure components, of the novel heat resistant steels that are mainly applied for the heat exchanger tubes like superheater, reheater, etc, in new-generation ultra-supercritical (USC) and supercritical fossil power plants. Results of it will provide practical experimental data for safety and reliability of the under-construction power plants and their heat exchangers. Detailed research contents and results are summarized as follows:1. Failure analysis of heat exchanger tubes in PTA dryersThe stainless steel heat exchanger tubes of the two dryers that were used for purification in one PTA plant were frequently encountered with localized corrosions not long after beginning of service, and some of them were also detected other failure modes involving accelerated thinning of their wall thickness and even bursting. Consequently, in order to ascertain the main causes of these premature failure behaviors, comprehensive investigations into four aspects were then carried out, including matrix materials, process media, service conditions, and maintenance management. According to the analysis results, the overproof chloride ions introduced by the inappropriate operation in alkali washing procedure during routine maintenance were determined as the root cause of those serious localized corrosions as pitting and crevice corrosion. Subsequently, the relevant corrosion failure mechanisms were deeply discussed, and both the effective pertinent countermeasures and the prevention measures were put forward as well. Based on this incident analysis, the initiating factors and prevention methods of potential failures of the heat exchanger steel tubes operating under similar service conditions were commanded, which would play an important role in safe and smooth operation of the whole heat exchangers in engineering applications.2. Failure analysis of quench oil/diluted steam heat exchanger tubes in ethylene plantIn the ethylene manufacturing plant of one petrochemical works, failure defects like localized concaves and even perforation were detected just after several months’ service on the surfaces of most carbon steel tubes of the quench oil/diluted steam heat exchangers that were used for generating diluted steam to supply heat. So as to immediately determine the actual causes of this premature failure, a series of advanced characterization apparatus including3D stereo microscope, SEM, EDS, ion chromatograph, ICP-AES, and pH inspection device, etc were utilized to observe the macro/microscopic morphologies of the defects and analyze the chemical features of the process media. The results showed that the interaction among pitting corrosion, acidic corrosion, stress corrosion cracking because of the frequent sharp fluctuations of the process water pH values that were led by the inappropriate feeding methods during routine operation, was the root cause of the localized defects on tubes’surfaces. Furthermore, aiming to clearly illustrate the combined effects by such complicated corrosion modes, a novel ’acidic/caustic alternating corrosion’ mechanism was innovatedly put forward, and its concrete relationships among defect morphologies, featured sizes, and detailed mechanisms were established.3. Failure analysis of heat exchanger tubes of circulating cooler in high-pressure polyethylene plant For the sake of enhancing the utilization rate of ethylene, circulating coolers that cooled and separated the melted ethylene oligomers and monomers by using room-temperature process water were applied in one high-pressure polyethylene plant. However, since the circulating cooling water, i.e. the process water system was exposed to the environment, it was inevitably polluted and its quality was seriously decreased, which would consequently affect the normal operation of these coolers. Indeed, the carbon steel inner tubes of one circulating cooler with jacket structure were detected severely corroded by such cooling water in the shell side, and their wall thicknesses were also thinned, and one of them even burst at the tee tube near the outlet. Then, on the basis that these failures were undoubtedly induced by the unqualified process water, the dynamic damaging effects combined by dissolved oxygen corrosion, crevice corrosion, and high-speed erosion that were all engendered by the water was discussed in detail. Achievements of this research would have important reference values for studying failure mechanisms of heat exchanger steel tubes that operate under both corrosive and abrasive conditions.4. Properties research of T91welded jointsThe properties at both room and elevated temperatures, and the microscopic structures of the welded joints of one China-made T91ferritic heat resistant steels were comprehensively evaluated, demonstrating that their overall performances were qualified enough for being applied in supercritical fossil power plants and nuclear power plants nowadays. The results would have both practical and economic values in popularizing the domestically-made T91steels, and had also paved the way for studying the similar and/or dissimilar welded joints of T92steels, which have similar chemical compositions and microstructures as T91.5. Research on structures and properties of T92/HR3C dissimilar steels welded jointsComprehensive structural performance evaluation was conducted on the dissimilar steels welded joints between novel T92ferritic and HR3C austenitic heat resistant steels that were mainly applied in the USC fossil power plants, including mirostructures, mechanical properties, creep rupture properties, and residual stresses distribution, etc, and the relationships between them were discussed as well. Among them, the creep rupture data and their linearly fitted equation were the first to be reported at home and abroad. Then, the maximum allowable stress at625℃was accordingly calculated, and the resultant value40MPa was sufficient for serving under current USC conditions. From both the scientific and the economic points of view, results of this study would have significant values in popularizing not only such novel heat resistant steels but also the new-generation USC fossil power plants. |
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