Study On The Development Characteristics Of Liquid Film And Droplets In Wet Gas Elbow Pipelines With Low Liquid Loading

The causes of pipeline accidents are complex,and the corrosion of natural gas pipelines is particularly problematic.One of the main reasons is that the condensate water in the pipeline generates droplets and liquid film,which in turn corrode the inner wall of the pipeline.Due to factors such as removal technology and complicated transportation environment,despite the dehydration treatment,there is still a small amount of water and impurities in the pipeline which could corrode pipelines electrochemical,chemical and mass transfer reactions.Liquid droplets,liquid film,condensation of water vapor and localized corrosion in natural gas pipelines with low liquid loading cannot be ignored.This article starts with theoretical analysis,numerical research and experimental measurement.In this paper,the liquid film distribution,droplet development characteristics and water vapor condensation characteristics with low liquid loading are investigated,as well as trajectory and variation of the droplet movement,and the corrosion potential and current distribution of CO₂local corrosion under thin liquid membranes.The main research findings and results are as follows:First of all,low liquidloading transport pipeline was constructed by a test method of similarity criterion.Micrometer calipers and probes were used to detect the instantaneous liquid film distribution and to study the thickness distribution of the liquid-gas pipeline.High speed photography was used to record peeling process of the film and to analyze the critical conditions of liquid film spalling and separation by means of liquid film theory.The results show that,the liquid film in the low-liquid two-phase pipeline fluctuates obviously.The liquid film in the horizontal tube mainly concentrates at the bottom and there exists a thin liquid film on both sides.In the horizontal tube with the same apparent gas velocity,with the apparent liquid velocity increases,the distribution range of the liquid film in the tube becomes larger and the maximum value becomes smaller first and then increases;in the same apparent liquid velocity,the increase of the apparent gas velocity results in the decrease of the thickest value of liquid film.In addition,reducing the surface tension could help improve the level of liquid film thickness and reduce the distribution range.Secondly,according to the actual parameters provided in the production and typical natural gas pipeline model,the Eulerian wall film model was used to simulate the formation and distribution of liquid film,and analyze effect of bending angles on characteristics of the flow field of gas phase and distribution of liquid phase in elbows.The results show that the distributions of gas-liquid two phases are similar in bending pipe with different angles.The turbulence led to the existence of Dean Vortex.The liquid film has obviousnonuniform distribution along the circumference in the cross section of the pipeline.Under the action of gravity,the liquid film in the pipe is mainly distributed in the lower wall.Due to dual-directional extrusion from centrifugation and Dean vortex of elbows,the liquid film will gather in the center of the lower wall.When entering the inclined pipe section,the liquid droplets are pushed by the gas-liquid two-phase rearward and move upward along the upper wall,then finally form a liquid film near the upper wall,while the thickness of the liquid film will reduce due to the impediment of gravity at the outlet of the inclined pipe section.Thirdly,keeping the same parameters and model above,the effects of parameters such as elbow angles,gas-liquid ratio and other parameters on droplet development were studied by using numerical simulation method and PBM.The results show that the different elbow angles have no obvious effect on the droplet distribution and droplet size distribution,which indicate that pipe structure is not the decisive factor for the droplet development characteristics in the pipeline.The significant distribution of different droplet sizes will only generate inside of the pipeline when the liquid content in the inlet is higher than 0.52 g/m³,otherwise the droplet distribution in the pipeline roughly the same particle size.Gas-water two-phase flow will generate vortexes when flowing through the elbows.The flow direction also has an influence on the size distribution of the droplet on the wall,so that the diameters of the droplet along the flow direction on the lower wall surface reduce obviously which on the upper wall surface hold steady.Fourthly,the effects of parameters such as structural characteristics of the elbow pipe on condensation characteristics of water vapor in the pipeline were studied by programming UDF of condensing model.The results show that,the changes of flow field of wet natural gas pipeline caused by irregular flow in different elbows have an effect on the distribution of condensated water,which leading to different degree of corrosion at different locations.The secondary circulation causes nonuniformity of flow velocity in the pipeline,and the faster the flow rate,the greater the condensation amount;distribution of the condensation amount is also directly related to the angle of the pipe elbow,and the larger the angle,the more nonuniform the distribution of the condensation,otherwise the distribution of the condensation is more uniform;no matter how the elbow angle changes,the condensation amount on the outer side of the elbow is significantly higher than that on the inner side;compared to the influence of the elbow structure on the condensation,the wall temperature difference has a significant influence on the pipeline,and the greater the temperature difference,the greater the condensation as well as the influence of the elbow angle on the amount of condensation in the pipe;when the temperature difference within the pipe is small,the influence of the elbow structure on the amount of condensation in the tube is reduced.Finally,based on a wire beam electrode tester,combined with an 11×11 matrix wire electrode,a micro-electrochemical study on the initiation,development of CO₂ localized corrosion under thin liquid film and the laboratory evaluation of commonly used corrosion inhibitor 2-?1-methylfluorenyl?quinoline are carried out.The results show that,under normal temperature conditions?30°C?,the potential distribution of the wire electrode in the simulation pipeline is sensitive with time,the degree of local corrosion is serious,and the severity of corrosion further increases with time;Under high temperature conditions?40°C?,the corrosion current of the wire electrode in the simulation pipeline is stronger,the potential difference is larger,the difference of the potential of some electrode points changes obviously,and the local corrosion degree is more serious;At the same time,the range of potential distribution is further enlarged,and a discontinuous binary distribution with potential concentration areas appears;The wire electrode is more sensitive with time,and the process to reach steady state is less time consuming?In addition,compared with no corrosion inhibitor,after adding the corrosion inhibitor,the pH value increases,the negative deviation occurs from the corrosion potential,the polarization curve shifts to the left,and the corrosion current density decreases.As the temperature and concentration increase,the corrosion inhibition efficiency gradually increases,which effectively slows down the corrosion rate.