The corrosion of the fluid to the pipes could damage the pipe structure, and then affect the safe operation of the whole pipeline system in the nuclear power plant. In order to assess the influence of the corrosion flaws on pipe integrity, it is necessary to perform the stress analysis and calculate the remaining strength of the pipes containing flaws. In this paper, the corrosion flaws in the seawater cooling pipes are considered as the volumetric type flaws. The remaining strength of the pipes containing the flaws has been calculated, the minimum required wall thickness of the pipes has been obtained, and the flaws have been assessed according to SY/T 6477 Standard. The finite element method has been used to analyze the pipes containing corrosion flaws. Stress distribution of the pipes has been obtained under the pressure and nozzle loading. Based on the RCC-M Standard, the strength assessment has been conducted. Results showed that the stress calculated from SY/T 6477 Standard is close to that from the finite element method, and it can be considered that the assessment method summarized from SY/T 6477 Standard is suitable for the current pipes containing flaws under the nozzle loadings. Therefore, this method can be used to assess the corrosion flaws of the seawater cooling pipes rapidly, and provide advices for the monitoring and repair of the pipes containing flaws in nuclear power plants.
A new precipitation strengthening NiCoCr-based superalloy with favorable mechanical performance and corrosion resistance was designed for ultra-supercritical power generation equipment. The degradation of mechanical properties and steam corrosion at high temperatures put forward higher requirements for alternative alloy materials; however, when the superalloy is processed to form complex shaped components through advanced additive manufacturing techniques such as laser metal deposition (LMD), hot cracks are prone to appear. This study proposed that microcracks in LMD alloys could be alleviated with powder decorated by Y2O3 nanoparticles. The results show that adding 0.5 wt.% Y2O3 can refine grains significantly. The increase in grain boundaries makes the residual thermal stress more uniform to reduces the risk of hot cracking. In addition, the addition of Y2O3 nanoparticles enhanced the ultimate tensile strength of the superalloy at room temperature by 18.3% compared to original superalloy. The corrosion resistance was also improved with 0.5 wt.% Y2O3, which was attributed to the reduction of defects and the addition of inert nanoparticles.
The heat supply network bleed valve is located in the steam pipe of the steam turbine heater. When conveying the steam medium of high temperature and high pressure, it impacts the valve disc and pipe wall, resulting in flow-induced vibration and noise of the pipe, resulting in excessive pipeline vibration, which is not conducive to the normal operation of the heat supply network and stable steam supply. In this paper, by using large eddy simulation turbulence theory to study bleed valve disc under different rotation angle operating characteristics, pipe flow field regularity of distribution, and the pressure pulsation of measuring points, the Q criterion is applied to analyze the locomotion mechanism of vortex structure in the downstream flow field of the valve disc, the use of common noise prediction method to calculate the noise of the heat supply network structure features, analysis of the inherent law of flow-induced vibration. It is found that with the increase of the rotation angle of the valve disc, there is an obvious negative pressure center area at the back of the valve disc, and the vortex is formed under the action of the adverse pressure gradient. The decrease in vortex size makes the flow efficiency at the back of the valve increase significantly. Large eddy simulation results show that the fluid generates a double vortex structure similar to the dean vortex behind the valve, and is eventually broken and separated due to the dissipation of viscous turbulent kinetic energy. The smaller the rotation angle is, the more seriously the dissipation vortex blocks the flow area near the pipe wall. The noise characteristics of the flow field in the heat supply network present a dipole state and the overall broadband characteristics. The energy of the flow field is concentrated in the low and middle frequency, and there is a peak of noise SPL. Simulation results showed that when the valve disc rotation angle is less than 30°, the vortex structure will seriously block the pipeline flow area, accounting for more than 90%, and the pipeline flow efficiency will decrease significantly with strong pulsation and noise. The fundamental frequency of the noise in the heat supply network shows a frequency doubling relationship with the natural frequency of the structure, and the rotation angle of the valve disc is negative correlation significantly with the magnitude and position of the fundamental frequency of the noise. Both of them can induce the flow-induced vibration and noise of the heat supply network, which complies with the field measurement data.
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