Abstracts:Pipelines are the lifeblood of the natural gas market. Compared with the high-pressure long-distance pipeline system and medium-pressure municipal pipeline system, the leak detection of low-pressure gas distribution pipeline system inside buildings that only relies on the combustible gas alarm is relatively backward and passive. Based on the principle of the negative pressure wave method, we proposed a leak detection method combining linear fitting and extreme learning machine leak detection. We designed and built a low-pressure gas pipeline system, which truly reproduces the gas leakage scenes inside buildings. By establishing multi-dimensional sliding windows, combined with a linear fitting method, the original pressure signal can be initially screened. Small leakage and pipe network fluctuations can be effectively identified by the windows. For medium leakage, large leakage, and gas usage that are easy to be confused, the windows will accurately identify the inflection point of the pressure drop for subsequent processing. Aiming at the characteristics of negative pressure waves, we extracted 15-dimensional feature vectors by dividing different stages. After introducing extreme learning machine for training, the detection accuracy of the final model can reach more than 90 %. Compared with support vector machines and back propagation neural networks, the trained extreme learning machine model has higher accuracy and higher speed. The joint leak detection algorithm based on linear fitting and extreme learning machine can greatly reduce the number of gas leakage accidents of low-pressure gas pipeline system through single point pressure monitoring. We applied the artificial intelligence algorithm and negative pressure wave to the low-pressure pipeline system for the first time, and established a new pressure wave processing method, characteristic model and algorithm model, which has a good engineering application prospect and theoretical significance.

Abstracts:The grade P91 and P92 steel are commonly used materials for the components in ultra-supercritical power plants in the temperature range of 590–650 °C. For welded joints, the notch toughness and microstructure stability played an essential role in meeting the requirement of high temperature creep properties. The microstructural stability and notch toughness were strongly affected by the composition of base and filler materials, welding processes and pre and post-weld heat treatments (PWHTs). In the present research, four different combinations of similar/dissimilar welded joints of P91 and P92 steel are prepared using shielded metal arc welding (SMAW). The microstructure of different welded joints has been characterized in the as-welded condition. For the as-welded state, the room temperature tensile properties, flexural strength, microhardness and notch toughness have been studied. The δ-ferrite patches were observed for P92 filler welded joints. It has been concluded that in the weld fusion zone, the higher amount of ferrite stabilizers promotes the formation of the δ ferrite. The tensile strength and flexural strength were increased in P92 filler welds due to the presence of solid solution hardening elements in the P92 filler. The detrimental effect of δ-ferrite patches has been observed on the Charpy impact toughness and microhardness values of the weld fusion zone. The Charpy toughness values in the weld fusion zone were 12 ± 2, 7 ± 2, 8 ± 2 and 5 ± 3 J for P91-P91-P91, P92-P92-P92, P91-P91-P92 and P91-P92-P92 weld joints, respectively, which were lower than the minimum recommended Charpy toughness value (47 J) for ferritic steel welded joints.

Abstracts:Gas Tungsten Arc Welded dissimilar joint of modified 9Cr–1Mo (P91) and SS304H steel plate is prepared using Ni-based ERNiCrCoMo-1 super alloy filler and are post-weld heat treated (PWHT) at 760 °C for 2 h. In particular, joints are investigated for metallurgical and mechanical properties in different heat-affected zones. Furthermore, an attempt has been made to understand the following: (i) Is there any effect of mixing of Ni-based superalloy filler material on the joint? (ii) How does the PWHT process change the metallurgical and mechanical properties of joints and different sections? (iii) Does the PWHT process beneficial as compared to as-welded (AW) joint? Experimental results indicate that improper mixing of filler materials led to the formation of an unmixed zone near the weld metal and HAZ interface. The hardness, residual stress, and impact energy results confirmed the variation in mechanical properties along the weldments. The tensile strength varied from 539 MPa in as-joint to 516 MPa in PWHT, and impact energy varies from 165 J in as-joint to 40 J in PWHT condition. The maximum hardness of 375 H V is observed in P91 CGHAZ. The maximum longitudinal residual stress is estimated as 200 MPa at the top surface of the welded plate, i.e., about 2 mm depth, while the maximum magnitude of the transverse residual stresses is observed as 109 MPa at the same depth of 2 mm. In P91 HAZ, the longitudinal and transverse residual stress magnitudes are found to be in a range of 0 to ˗70 MPa. The SS304H HAZ shows the maximum value of longitudinal and transverse residual stress level at a depth of 3 mm, and it was 144 MPa and 92 MPa, respectively. The PWHT showed a drastic reduction in residual stress value in a different zone of the weldments.

Abstracts:The coupling response of flexible pipe subject to external shear current and internal uniform flow is numerically investigated in this paper. Firstly, without the internal flow being considered, the quantified comparisons of root mean square vibration amplitudes in relation with VIV responses between the numerical results and experimental data were made to verify the efficiency of numerical method. Subsequently, the instantaneous root mean square vibration amplitudes, spanwise waveforms, and 3D time-frequency-energy spectra related to the numerical results were analyzed in detail. It is found that both the external current and internal flow play important roles in determining the vibration modes, and that the vibration intensity is tightly associated with the internal flow velocity. It is worth noting that the increase of internal flow velocity can excite new vibration mode response. The position-depended mono- and multi-modal coexistence, multi-modal dominance, mode switch/shift and fluid-structure interaction are clearly observed along the pipe span, followed by the alternative occurrence of standing and traveling waveforms, due to the combination of vortex-induced vibration and flow-induced vibration.

Abstracts:The present work evaluates solidification cracks in stainless steel welds that have been produced on a laboratory scale by carrying out a detailed EBSD (electron backscattered diffraction) investigation. The welding speed presented a near-linear relationship with the susceptibility to cracking. A wide range of microstructures were generated by varying the welding speed. The cracks were predominantly transgranular type and mainly propagated through grains with lower Taylor factors. These cracks were accompanied by a strong presence of a localized strain field (high local misorientation), which was observed by a higher presence of low angle grain boundaries (LAGBs). A correlation was found between the local in-grain misorientation and the crack sensitivity. With increased thermal tensile strains, certain LAGBs transformed into HAGBs to accommodate the strain and that resulted in the formation of strain-free grains. The elastic stored energy estimated from EBSD also showed an increase with the local misorientation.

Abstracts:In order to solve the problem of insufficient accuracy in predicting the burst pressure of a pipe elbow based on the existing yield criteria, a new yield criterion is established firstly and then used to analyze the plastic failure of a pipe elbow. According to this idea, the mathematical characteristics of the Tresca criterion and the Mises criterion are revealed from the perspective of square shear stress, and a new yield criterion with generality is established by introducing a weighting coefficient δ. The yield criterion can be transformed to various yield criteria with different δ, including the existing yield criteria. On this basis, the plastic failure analysis of a pipe elbow is carried out, and the analytical formula of the burst pressure is derived, in which the strain hardening effect of the material is also considered. Besides, the main parameters that affect the burst pressure are analyzed. It is shown that the theoretical burst pressures calculated by the present yield criterion with different δ can coincide well with the experimental values for different materials, and the burst pressures decrease with the increase of the strain hardening exponent, while increase with the increase of the thickness-diameter ratio. The burst pressure formula established in this paper is of great significance to the structural design and safety assessment of a pipe elbow.

Abstracts:Structural integrity analysis of pipe elbows is of critical significance because they not only withstand the internal or external pressure but also absorb the bending moment in a pipeline system. While the burst capacity of pipe elbows under internal pressure only have been extensively investigated in the literature, the study on the burst capacity of pipe elbows under combined internal pressure and bending moment is still limited. In this paper, the effect of bending on the burst capacity of thin-walled defect-free and local wall-thinning (LWT) elbows subjected to combined internal pressure and bending moment is systematically investigated. A correction factor is defined to quantify the bending effect on the burst capacity; extensive parametric finite element analysis (FEA) is carried out to analyze the dependence of the correction factor on the bending mode and magnitude, elbow size and steel grade, and LWT size and location. The results indicate that as the magnitude of the bending increases, the burst capacity of both the defect-free and LWT elbows generally deceases. For the defect-free elbows, the elbow steel grade has a significant influence on the correction factor while the influence of the elbow geometry is marginal. For the LWT elbows, the correction factor depends markedly on the LWT location and bending mode.

Abstracts:In this paper, the corrosion behaviors of a welded joint of X80 pipeline steel underneath disbonded coating in an acidic soil solution were studied through electrochemical measurements, SEM/EDS and 3D ultra-depth microscopy. The results showed that the corrosion rates of the heat affecting zone (HAZ) specimens were the largest during the experiment, while the smallest corrosion rates always appeared with the base metal (BM) specimens at the same position. The corrosion rate of all the specimens at the opening were always larger than those at the bottom of the disbanded coating during the whole experiment. General corrosion dominated the surface of all specimen at different position, with much nonuniform corrosion inlaying in it. The nonuniform corrosion sensitivity of the HAZ specimens was larger than that of the BM and welded zone (WZ) specimens, whether at the opening or at the bottom. Under the disbonded coating, the nonuniform corrosion sensitivity of the specimens firstly decreased with the distance from opening and increased at the bottom. The maximum corrosion fluctuation of the HAZ specimen was available at the opening.

Abstracts:The existing design codes of pressure vessel like the ASME, EN13445 and GB150 only list some calculation formulas of the wall thickness of the cylindrical, spherical or other shells, but not provide the calculation formula of the geometric deformation of these pressure components. Based on the thermo-elastic theory and non-zero axial strain, the exact theoretical formulas of the radial and axial displacements were derived by the four order differential equation of deflection at the edge zone for a cylindrical vessel and a pipe subjected to the thermo-mechanical loadings, respectively. The proposed theoretical solutions were in good agreement with the finite element (FE) numerical solutions. In addition, the influence of edge effect on the geometric deformation of the pressure components was discussed in detail. The calculation formulas developed in this work can be used to precisely calculate the geometrical deformation of the pressure vessel and pipe, which will make the installation process of the pressure system more reliable.

Abstracts:The finite element method is the most commonly used approach when solving complex practical cylinder-cylinder junctions in pressure vessels. This is used when the geometrical arrangement is out with the permitted scope of the design-by-rule approaches or when detailed stress information is required as in a fatigue assessment. High-stress concentrations occur on the crotch corner for cylinder-cylinder joints, and it is possible to reach solutions for this problem by using both theoretical and numerical solutions. However, those approaches do not fully overlap nor have the same underlying assumptions. As such, an innovative high-fidelity finite element model has been developed to provide a holistic unified approach which can tackle a wide range of problems. In this study, various detailed nozzle design challenges were investigated including single and multiple nozzle combinations, nozzle-cylinder systems with different size ratios, fillet weld applications, limit loads and external loading cases were analyzed. The results obtained are compared with well-respected calculation methods such as WRC537, and a new approach is presented for the analysis of cylinder-cylinder combinations.