The Influence Of Flow Field On Single-Phase Flow-Accelerated Corrosion At High Temperature

The pipeline rupture accidents caused by flow-accelerated corrosion(FAC)have never stopped worldwide,which not only caused significant economic losses,but also caused casualties,and thus,the research around FAC should be further studied.The main factors affecting FAC are water chemistry conditions,pipe composition,and hydrodynamics of fluid flow.The research on FAC can be divided into two research directions.The first research direction is aimed at the first two factors.The second research direction is aimed at the third factor.The mechanism of hydrodynamics affecting FAC was studied by the flow characteristics such as secondary flow instability and turbulent structure evolution of single-phase or two-phase flow.In this work,the combination of theoretical models and experimental data was used to study the effect mechanism of the flow field on the FAC,and the focus was on the flow pattern formed in the local area of eblow and downstream of orifice.The main contents are as follows:(1)There are few circulating loop FAC test devices carried out in the laboratory.Most of the existing circulating loop FAC devices are only suitable for normal temperature and pressure conditions.The circulating water tank and circulating pipes were made ofplexiglass tube,which could not meet the pressure and temperature requirements of the test.In addition,the existing FAC rate data using ultrasonic detection technology to measure the wall thickness on site requires the huge effort and resources,and often has large measurement errors.In this work,a high-temperature circulation loop test system reflecting the actual flow and a high-temperature array corrosion electrode that can be directly embedded in the pipe wall were designed.The electrode surface was completely consistent with the inner surface of the pipe to avoid the problem that the traditional probe-shaped sensor would be interfered with the flow field.Moreover,the local FAC rate at different positions could be obtained online,which could not only compare the difference of FAC rate at different positions,but also present the change of FAC rate at a specific position.(2)There are few experimental studies on the FAC rate of the eblow under high-temperature conditions,and there is also a lack of systematicness.The validity of the explanation of the FAC mechanism under normal temperature conditions used to evaluate high-temperature FAC should be verified.Taking these problems into consideration,the distribution of FAC rate under different pH,different flow rates,different temperatures,and different materials were studied using array electrode technique.The results show that the minimum charge transfer resistance and maximum corrosion current of the array electrodes under high-temperature conditions are located at the outermost side of the eblow,and the most severe area of FAC is at the outermost side,revealing that there is a galvanic corrosion presented on the surface of the array electrodes under high-temperature conditions.In this galvanic corrosion system,the inward bend is considered as the cathode where the FAC rate is suppressed,and the outward bend is considered as the anode where the FAC rate is accelerated.(3)The hydrodynamics parameters reflecting the distribution of FAC rate has not yet been agreed.Turbulent kinetic energy,turbulence intensity,shear stress and radial components have been suggested.Therefore,the relationship between the local FAC rate and the hydrodynamic parameters of eblow under high-temperature conditions was studied by combining the array electrode technique and computational fluid dynamics simulation.Furthermore,the relationship between the hydrodynamic parameters and the FAC was established by comparing the experimental data and simulation results.As a result,the radial velocity as the indicator parameter can be used to characterize the FAC rate of eblow,revealing that the radial velocity can be used as the standard to measure the FAC rate(4)Most computational fluid dynamics models need further experimental verification.The selection of the turbulence model,the division of the grid and the setting of the boundary conditions would have a great influence on the simulation calculation results,especially on the local flow field with complicated flow patterns.In addition,it is difficult to obtain accurate calculation results using unverified computational fluid dynamics models.Therefore,the indicator parameters were used to determine the prediction scheme for reducing the FAC rate and the predicted models were verified by experimental data.On this basis,three prediction methods that can characterize the FAC rate of the eblow under high-temperature conditions were proposed,which were verified by experimental data.Additionally,the proposed three prediction methods were used to analyze the FAC rate of downstream flow of orifice with different ratios to verify the versatility of the prediction methods.The research results can be applied to the design optimization,operation monitoring and maintenance strategy of single-phase and two-phase flow transportation pipelines in chemical industry,nuclear power,thermal power and other industries.