Correlation Between Liquid Film Distribution And Localized CO₂ Corrosion Behavior In Wet Gas Pipelines With Low Liquid Loading

Phase transition occurs and saturated vapor turns into condensate water when temperature or pressure changes in wet natural gas gathering pipeline.The gas liquid flow with low liquid loading is formed.Pipelines face localized corrosion failure risks by erosion of corrosive gas and water two-phase flow.CO₂ corrosion behavior caused by thin liquid film exists in the top and two sides region of pipeline.The correlation between gas liquid flow and CO₂ corrosion in the pipelines with low liquid loading is a complicated process.The droplets movement in the pipeline,hydrodynamic characteristics of liquid film formation and development and localized CO₂ corrosion under different liquid film thickness,gas liquid flow were studied by experiments,numerical simulation and theoretical analysis.The main works and achievements show:?1?The experimental pipeline with low liquid loading was established according to similarity criteria.A system of measuring transient liquid film thickness was designed with micrometer and probes.Film distribution characteristics in the gas pipeline with low liquid loading were studied.Film stripping process was obtained with high speed photography technology.The critical conditions of film stripping and film separation were analyzed by theoretical means.The results show fluctuation in the film distribution.Liquid film is concentrated on the bottom of horizontal pipe.Thin film exists on the side walls of horizontal pipe.The critical superficial gas velocity of film stripping in the horizontal pipe?d=90mm?is16.63 m/s.Under the same superficial gas velocity,the region occupied by liquid film is extended and the value of thickest film is increased first and then decreased with increasing superficial liquid velocity.Under the same superficial liquid velocity,the thickest values of film thickness are decreased with increasing superficial gas velocity.As the surface tension of liquid is reduced,film thickness is increased and film region is enlarged in the horizontal pipe,and the circumferential equivalent uniform film thickness is increased in the vertical pipe downstream of the elbow.?2?Based on the theory of gas liquid flow in pipeline,mathematical model of gas liquid flow with low liquid loading was established.Eulerian model and Eulerian wall film model were coupled.Gas flow field,film fluctuation characteristic,droplet spatial distribution and variation with time were studied in detail.The flow mechanism was obtained.Boundary layer mesh and enhanced wall function were combined to simulate the flow field near the wall.The results are in good agreement with the experimental results.Affected by the elbow,there is a pair of vortex in the vertical pipe downstream of the elbow.The center of secondary flow gradually moves to the right side along the flow direction.There is a fluctuation in the axial film distribution of the horizontal pipe.The difference between wave peak and wave trough are 87?m(U_sg=16.68m/s,U_sl=0.00699m/s).Droplets are concentrated on the lower region of the horizontal pipe.Centrifugal force in the elbow promotes droplets to move to the outside.Affected by the elbow,droplets are concentrated on the right side of the vertical pipe downstream of the elbow.Droplet concentration near right wall gradually decreases along the flow direction.The area occupied by droplets expands,and the droplets have the intention of moving left.The droplets fluctuation moves periodically over time.?3?The 100 microelectrodes in the wire beam electrode were made from X80 pipeline steel wire.On the basis of multi-channel galvanic corrosion test system,a set of film preparation device and local electrochemical test system in CO₂ environment was invented and improved.Based on the water film thickness distribution in the pipe with low liquid loading,a WBE device was used to measure the electrochemical distribution during the initiation and development of CO₂ corrosion under the static uniform liquid film.The CO₂corrosion kinetics characteristics in the thin liquid film and CO₂ corrosion mechanism under the influence of liquid film thickness were analyzed.The results show that uniform corrosion is developed from highly localized pitting on the surface of X80 pipeline with decreasing the film thickness from 2500?m to 220?m.The total anodic current density increased slightly as water film thickness was decreased from 1500?m to 648?m.The non-protective porous Fe₃C accumulated on the surface.With respect to the thickness of the water film ranging from541?m to 350?m,the anodic current density increased with the decreased water film thickness.When the distance between water gas interface and steel surface decreased,the dissolution of CO₂ from gas phase and the hydration of dissolved CO₂ became significant.The concentration of H₂CO₃ increased and the cathodic reaction was accelerated as the water film thickness decreased.The H⁺,HCO₃^-ions were generated through carbonic acid dissociation.At low water film thickness,the diffusion rate of H⁺,HCO₃^-,H₂CO₃,HAc towards the electrode surface increased as water film thickness was decreased.The peak anodic current density was found at water film thickness of 350?m.When the water film thickness was less than 350?m,the concentration of CO₃^2-increased and local pH increased as well,which increased precipitation rate of FeCO₃.Besides,Fe²⁺has little capacity for migration and hydration because of the high resistance of thin water film;therefore,FeCO₃was easily formed on the steel.?4?The microelectrodes in the curved wire beam electrode were made from X80 pipeline steel wire.Thin film distribution under the thickness gradient was created.Electrochemical research of CO₂ corrosion under static gradient film distribution was carried out by combining the multi-channel galvanic corrosion test system.Under the millimeter liquid film thickness gradient,film thickness has little effect on the corrosion process of microelectrodes.The initiation of local corrosion shows a random distributon and has an aggregation effect.Under the micrometer liquid film thickness gradient,uneven distribution of liquid film leads to the uneven distribution of surface electrochemical parameters.Galvanic current on the X80pipeline steel surface show the gradient distribution.The region with the thinnest film faces the greatest risk of corrosion when the film thickness is greater than 352?m.FeCO₃ corrosion product precipitates on the Fe₃C,showing the inner layer of loose Fe₃C,more FeCO₃precipitation in the outer layer.?5?A set of multiphase flow CO₂ corrosion test device was designed and developed.Electrochemical research of CO₂ corrosion under dynamic liquid film was carried out by combining the wire beam electrode and multi-channel galvanic corrosion test system.Under the millimeter dynamic liquid film thickness gradient,the distribution of anode and cathode shows a random characteristic.There is a certain degree of change in the area of the anode and cathode.The film thickness has little effect on the corrosion process of microelectrodes.Under the micrometer dynamic liquid film thickness gradient,corrosion potential and galvanic current on the X80 pipeline steel surface show the gradient distribution.When4.7m/s<U_sg<11.8m/s,U_sl=0.0071m/s,value of film thickness is in the range of 995?m⁶79?m.With the increase of superficial gas velocity,the effect of flow erosion of Fe₃C layer is more pronounced and corrosion rate slows down.The value of film thickness is in the range of679?m⁴57?m when 11.8m/s<U_sg<14.2m/s,U_sl=0.0071m/s.The enhancement of flow pattern occupies a dominant position.The phenomenon of entrainment intensifies and the thickness of liquid film is gradually thinned with the increase of superficial gas velocity.When the distance between water gas interface and steel surface decreased,the dissolution of CO₂ from gas phase and the hydration of dissolved CO₂ became significant.The concentration of H₂CO₃ increased and the cathodic reaction was accelerated as the water film thickness decreased.The H⁺,HCO₃^-ions were generated through carbonic acid dissociation.At low water film thickness,the diffusion rate of H⁺,HCO₃^-,H₂CO₃,HAc towards the electrode surface increased as water film thickness was decreased.