Study On Multilayer Urea Reactor Inspection And Residual Life Assessment Method

Urea reactor is the core equipment of urea synthesis devices. Presently, Most of the domestic urea reactors have multilayer structures. The strength of the thick-walled cylinder was enhanced without increasing wall thickness due to the tight joint of the layers. Therefore, this kind of vessel has been generally considered as very safe equipment and widely used in the oil refining, petrochemical and chemical fertilizer industry.However, the multilayer urea reactor cylinders were connected by heavy welds, defects can be easily initiated during welding and operation due to the use of multilayer welding in the heavy weld structure and abolishment of post-weld heat treatment. In addition, multi-layer structure was generally taken as a single layer during design. Thus, interlayer clearances were often neglected in the manufacturing process, and the layer stress and deformation non-coordination under operation temperature and pressure loads were overlooked accordingly, which leading to layer deformation constrained parts such as heavy weld endure enormous stress concentration under the operating conditions. Also, more than 90 percent of the current domestic fertilizer plants use urea aqueous full recycling production process and the vast majority adopt steam as leak detection media. Steam leakage was often observed at the leak detection system due to its structural causes or outside effects; Steam will fill in the interlayer clearances during the course of leakage and induce stress corrosion cracking of layers. Series cracking may lead to serious urea reactor blasting accidents, which caused heavy casualties and property losses.Nondestructive testing technology such as acoustic emission, ultrasonic andγ-ray had been applied to investigate the layer and heavy weld stress corrosion cracking of urea reactors both at home and abroad. However, the crack detection rate and reliability of these methods need further study. In addition, how to carry out evaluation work of checked out flaws in the in-service vessel, and further assess the safety condition of the vessel is a very urgent problem. The anthor started work from structure and urea production process factors which affected urea reactor corrosion and stress distribution, and investigated urea reactors with different diameters using finite element analysis. The urea reactor stress distribution with interlayer clearances was acquired. On the basis of upper work, dissection was conducted to a serious stress corrosion cracked urea reactor, and the stress corrosion cracking mechanism was studied. Nondestructive testing research and followed site verification was carried out for the lots in-service urea reactor layer and weld crack defects, and acquired a reliable multilayer urea reactor detection method. As for the cracks found during inspection, urea reactor structural integrity assessing method was refined in light of the results of finite element analysis.Multilayer urea reactor structure, urea production process and leak detection process were analyzed. Results showed that due to the urea reactor special structure and the strong corrosive of process media, corrosion may occur at the liner and the external layers of the urea reactor. Internal liner corrosion is caused by the changes of reaction medium and processing technology. External layer corrosion was aroused mainly by steam. Corrosive deposits such as alkaline ions and chloride ions brought by leak steam would concentrate at the interlayer clearances, which would induce corrosion and cracking of outer layers. In addition, the internal reaction medium leakage would generate strong corrosion of external layers, and the steam medium containing chloride ions can also lead to outer side stress corrosion cracking of the liner.Finite element analysis (FEA) of a urea reactor was conducted. Factors of heat stress, material nonlinearity, friction constitutive relations and interlayer clearances as well as heavy weld structures were considered during the analysis. The results showed that under the operating pressure, the internal FEA stress values and Pimshtein formulas calculation results of the cylinder layers had a gradually decreasing trend radially from the inside to the outside surface. The stresses of inner layers were higher than Lame formulas calculation results, whereas outer layer stresses were lower than Lame formulas calculation results. With the increasing of clearances between layers, the internal layer stresses increased, and the stress of outer layers decreased, which lead more external layers to an idle state. Therefore, the cracking possibility of inner layers which endure working pressure increased. As for urea reactors with 0.25 mm gaps, only seven layers endure the inner pressure. When the thickness exceeds more than seven layers, the stresses of layers nearly tend to zero. When considering the impact of temperature, the heavy weld stress generally increased and the maximum stress transfer to the annulus layer. Elastic-plastic analysis of reactors with the clearances of 0.25 mm showed that the stress states of some internal layers had exceeded the material yield limit to a plastic state.Since urea reactor has gaps between layers, the layers showed tension states in the inner side and compression states in the outer side under pressure. The stress states between two sides became larger when nearing the heavy weld, and eventually generated great stress concentrations at the weld toe. The stress states showed strong locality in the weld toe, and decline rapidly when leaving away from it. Under both operation pressure and temperature, the stresses redistributed, and the inner layer stress state relieved, but the weld toe was still the most stress concentrated area. Local yield zone occurred at the weld toe of the liner, the annulus and the first strength layer. The yield zone decreased gradually from inner surface to the outside, and the geometrical shape conform to the actual crack propagation direction found during dissection.The reason of crack initiation of the urea reactor strength layer and the weld were analyzed. Lining corrosion and leakage, strength layer cracking and the explosion type of the urea reactor were summarized. The macro inspection, dissection analysis, microstructure analysis, as well as deposition and corrosion product analysis results showed that the strength layer cracks were caused by stress corrosion cracking. Statistics of crack lengths and cracking positions showed that cracks have the characteristics of initiating at the weld parts. The reasons for urea reactor cracking were studied from the view of material, stress, corrosion environment. The results showed that the cracks were initiated by a variety of factors such as material defects, local stress concentration and leak detection medium condensation. Multi-cylinder deep seam welding produced large widmanstaten organizations, which reduced the material structure uniformity. On the other hand, leak steam condensation in the layer clearances occurred, which produced stress corrosion cracking conditions. Heavy weld happened to stress corrosion and galvanic corrosion under the effect of leak detection liquid concentrated in the crevice of layers. Therefore, urea reactor cracking occurred, and expanded outward along the direction perpendicular to the main stress. The strength layer cracking form of urea reactors have some relation to leak detection steam pressure, leak detection steam contamination condition, leak detection structure and connectivity condition between interlayer clearances and outside environment. The most serious stress crossing cracking would occur when the following conditions happened simultaneously: high leak detection steam pressure, interlayer clearances filling with steam and the clearances were blinded to the outside environment. When Leak steam pressure was relatively low, the steam which accessed into the interlayer clearances did not have the condensation conditions; stress corrosion cracking would occur only in the external layers and weld toes. When the leak detection steam dew point temperature is below the outmost layer temperature, layer cracking will be arrested. When the leak detection steam pressure was relatively high, the leak detection steam which entered the clearances has the condensation conditions. Multi-points corrosion concentration would happen at the weld toe, which would induce stress corrosion cracking of inner layers and welds. In addition, the liner leakage and process system leakage caused by leak detection steam pollution also can cause layer corrosion cracking.The existed defects of the urea reactor were detected and comprehensively evaluated using acoustic emission, ultrasonic phased array examination,γ-ray detection and metal magnetic memory methods. First, comparison analysis of acoustic emission, phased array,γ-ray examination and metal magnetic memory was conducted according to the characteristics of urea reactor inspection. Results indicated that only give full play to the advantages of these detection methods, can the urea reactor integrity detection be completed. Second, acoustic emission examination was conducted to a urea reactor. Results showed that, with increment of the test pressure, apparent acoustic emission location signals come forth in the stress concentration sites especially in the heavy weld toe, which were in consistent with the stress distribution pattern of FEA results. Subsequent dissection results confirmed that the acoustic emission source locations were heavy weld radial expansion cracks. Acoustic emission signal characteristics were statistically analyzed, and defects characteristic parameters were acquired. Heavy weld sections of the urea reactor were imaged using ultrasonic phased array system. Results showed that the ultrasonic phased array detection method can clearly show crack type reflectors and crack latent depth, thus weld toe cracks, weld defects and layer mismatch etc. could be directly detected. The length and depth of cracks could be measured through this method, which could provide data for decision-making of equipment maintenance and defect assessment. This method was especially useful for acoustic emission source location re-inspection acquired by acoustic emission examination.γ- ray testing results showed that theγ-ray crack detection rates were very low to the large tilt angle cracks, only when the acoustic emission signals positioned on layers or weld acoustic emission signal could not efficiently detected,γ-ray re-inspection was needed to be utilized. Metal magnetic memory measurement was used for the testing of the localized stress concentrations; the buried crack detection reliability of urea reactor welds needs further study. Results showed that the use of acoustic emission testing technology combining ultrasound phased array andγ-ray non-destructive testing method had higher reliability and efficiency for urea reactor integrity situation evaluation. Combination of these methods can effectively find out heavy weld defects, and had prosperous advantages for testing multilayer structures similar to multilayer urea reactors.Safety evaluation of urea reactors containing defects was studied. First, cracks in the urea reactor had been simplified on the principle of safety. Cracks in the urea reactors were ruled into two categories: Circumferential cracks with surfaces perpendicular to the axial and axial cracks with surfaces parallel to the axial. Then, the urea reactor crack stress intensity factors were calculated in accordance with the FEA results and actual crack sizes acquired during urea reactor dissection. Therefore, the initial crack size and critical crack size expression formulas of urea reactors were acquired. The urea reactor remaining life could also be estimated. On this basis, urea reactor remaining life curve was given. The remaining life of the urea reactors could be estimated during the acoustic emission testing process in accordance with the acoustic emission source location appearance pressures.In short, urea reactor stress distribution and liable failure location were studied by urea reactor structure and finite element analysis, crack initiation analysis, non-destructive testing method study and safety assessment method investigation. This work provided a complete set of solutions for urea reactor inspection and integrity evaluation to ensure the urea reactor safety operation, and had tremendous economic and social benefits. However, urea reactor failures are closely related to their design, manufacture, installation, usage, inspection, repair and transformation. Each factor could have important impact on the failures of urea reactors. Therefore, it is also necessary to carry out further research in these areas, so as to take corresponding measures to prevent urea reactor accidents.