Study On Flow Accelerated Corrosion Mass Transfer Process Of Pipeline Near Wall

Flow accelerated corrosion(FAC),also known as Flow accelerated corrosion,is one of the failure methods of carbon steel pipelines in power plants.The oxidation film on the inner surface of carbon steel or low alloy steel pipe will become thin under the action of high speed and high temperature water,which weakens the protective effect on the metal matrix,increases the corrosion rate of metal and leads to the thinning and rupture of the pipe wall.FAC is one of the common failure modes of secondary circuit metal pipelines in thermal power plants and nuclear power plants.Based on the special working environment of power plant thermal system pipeline,it is difficult to carry out monitoring and testing.To establish the prediction model of FAC,numerical simulation method is used to calculate the prediction model variables of FAC,which has become an effective method to predict FAC rate at present.In this paper,the above methods will be used to study the influence of fluid dynamics factors on FAC,and the mass transfer coefficient will be taken as the entry point to analyze the influence of various factors on FAC.Through the flow accelerated corrosion mechanism,characteristics,easy to produce parts and influencing factors of thorough research,determined on the basis of M.I.T model,established the mathematical model of carbon steel in single phase flow in the FAC solubility model and the model of mass transfer,mass transfer model through ion mass transfer type and fick's law was set up,set up the relationship between the mass transfer coefficient and the concentration field.(1)The solubility model described above was used to calculate the solubility of oxidation product film(Fe₃O₄)in solution at different temperatures and p H.Fluent software was used to simulate the flow field distribution and concentration distribution,and the mass transfer coefficient was calculated.A new FAC corrosion rate prediction model was obtained by coupling the solubility and the mass transfer coefficient.FAC rate model was used to calculate the FAC condition of orifice plate.The results show that temperature and p H affect FAC process by affecting the solubility of oxidation product film(Fe₃O₄)and the concentration of Fe²⁺in solution.As the flow velocity increases,the shear stress and turbulent kinetic energy at the wall surface increase,and the mass transfer process is enhanced.It can be proved by the variation law of Fe²⁺concentration that the FAC rate increases.The higher the fluid velocity,the higher the mass transfer rate in the flow field,and the more intense the FAC velocity is.The mass transfer coefficient increases with the increase of the inlet velocity,mainly because the velocity affects the shear force and turbulent kinetic energy of the wall,leading to the increase of the Fe²⁺concentration gradient on the wall.(2)FAC mathematical model and ANSYS software package were used to calculate the FAC rate,and the distribution of flow field and turbulent kinetic energy Fe²⁺in the downstream of time-varying diameter pipes with different velocities and pipe types were obtained.The different diameter ratio E will make the axial position and radial width of the generated vortex appear different.The larger the diameter ratio E is,the closer the vortex appears to the downstream,and the wider its width will be.The smaller the diameter ratio is,the closer the vortex center area is to the wall surface,that is,the larger the pipe diameter ratio is,the closer the position of Fe²⁺minimum concentration appears to the downstream and the closer to the fluid center.The emergence of eddy current will reduce the concentration of Fe²⁺at the wall surface,increase the concentration gradient of Fe²⁺,increase the mass transfer driving force,and strengthen the FAC process.When the diameter ratio E is the same,the axial position and radial size of the vortex are roughly the same,and the turbulence intensity will increase with the increase of the velocity.