| As an important auxiliary of the ultra-supercritical thermal power unit, circulating water pump (CWP) plays a crucial role in ensuring the thermal power unit operates safely and efficiently. Once the CWP were failure, the efficiency of the whole thermal power unit would be seriously influenced, great economic losses would be suffered and even safety accidents would be engendered. Since the surrounding of the CWP is always natural seawater which usually contains high contents of salts, chloride ions and sediment particles, it requires high performance such as excellent corrosion resistance and good mechanical behaviors for the material used in the CWP. Duplex stainless steel (DSS) has been extensively used in many industries and fields such as petroleum engineering, chemistry engineering, paper and pulp engineering, food industry, military and marine industry for the last decades because of its large number of excellent properties. DSS is also one of the best choices for the material used in CWP.As one of the largest thermal power plants in the east of China, Jiaxing power plant phase Ⅲ has two 1000MW ultra-supercritical thermal power units, both of which are equipped with three CWPs. These CWPs were made of 2205 DSS, which was used as the material of CWPs in thermal power plants for the first time in China.However, in this event, after only ten months’operation, substantially less than design lifetime of 30 years, a number of severe fractures occurred on the CWPs, causing substantial economic losses as well as inconvenience for local people. If the root cause of this premature failure were not figured out and effective countermeasures and suggestions were not proposed, greater losses would be suffered and more serious safety accidents would be engendered.Hence, fundamental theories of failure analysis and a variety of characterization methods were comprehensively utilized to investigate this failure accident from the following aspects. To start with, the chemical compositions, the metallographic structures, the mechanical properties of the in-service CWP material were inspected. What’s more, the ferrite/austenite ratio of the butt weld was obtained by electron backscatter diffraction (EBSD). It reveals that the base material used in these CWPs was qualified, which was in accordance with the requirements of the UNS31803 grade DSS, but the weld joint had a higher content of carbon, together with an unbalanced ferrite/austenite ratio, which should be an important cause to decrease the toughness of the weld joint. Then a comprehensive investigation was carried out on the failed CWPs. Microscopic morphologies and chemical compositions of the fracture were observed and detected by three-dimensional optical microscope (3D-OM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS). Analysis results reveal that many lack of penetrations (LOPs) in the butt weld between the flanges induced by improper process decreased remarkably the strength and fatigue resistance performance of the weld joint, acting as the root cause to the severe fractures of CWPs. The LOPs and solidification cracks founded in the weld joints were another reason to decrease the strength of the shell.Finally, effective countermeasures and suggestions were proposed, which provided instructive significance in the fracture prevention for the CWP, even other equipments running under similar conditions in engineering practice. After adopting these quick, simplified, and effective failure analysis method and its countermeasures, recurrences of similar failures were successfully prevented, economic losses were greatly reduced, and safe operation of the CWP was ensured. |
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