In-service Welding Safety And Reliability Evaluation Of X70 Pipeline Steel

This paper takes the X70 pipeline steel as study object and deeply analyses the characteristics of heat exchange of the pipe inner wall medium.The in-service welding instantaneous temperature field numerical simulation of X70 pipeline steel is also carried out using the finite element simulation software SYSWELD to analyze the impact of wall thickness,diameter,velocity,pressure,media type on the instantaneous temperature field.Considering the size of the welding molten pool and the metal strength loss in high temperature area,the equivalent welding molten pool(here in referred to as "equivalent molten pool")calculation is conducted using MATLAB.Based on the size of the "equivalent molten pool",this paper calculates in-service welding safe working pressure and analyses the impact of the above factors on the safe working pressure,and then evaluates the in-service welding safety.Researches on the heat exchange of the pipe inner wall medium show that: the heat exchange of gas medium is mainly conducted by means of radiation heat,while the heat exchange of saturated water is mostly belongs to convective heat transfer.Saturated water is obviously better than gas as heat exchange medium and their heat exchange coefficients are not on the same order of magnitude.The heat exchange coefficient of pipe inner wall medium decreases with the increase of the diameter and increases with the increase of flow velocity as well as gas pressure.But for methane media,the influence of the above factors on the heat exchange coefficient is less.Considering the size of the welding molten pool and the metal strength loss in high temperature area,the strength path integral method is used in the equivalence of the welding molten pool.Studies show that: with the wall thickness increases,the high temperature region as well as "the equivalent molten pool" depth increases;when the flow velocity increases,"the equivalent molten pool" downsizes;with gas pressure increases,the size of "the equivalent molten pool " reduces slightly.Viewing the "equivalent molten pool" as volume type defects and based on the "equivalent molten pool",this paper calculates in-service welding safe working pressure and then evaluates the safety.Studies show that: the gas medium pressure and flow velocity have a small impact on the "equivalent molten pool" size and the safe working pressure.Wall thickness strongly affects safe working pressure.With the wall thickness increases,the high temperature region and the "equivalent molten pool" size increases but when the remaining wall thickness increases,the safe working pressure increases.The relationship between the diameter and the safe working pressure is approximately inversely proportional but the "equivalent molten pool" length correction coefficient weakens this effect.Under the conventional air cooling conditions,the safe working pressure is conservative and can be used to evaluate the safety when the media gas is methane.Due to calculation or measurement errors,the various factors of the safety evaluation model are considered as random variables and based on the Monte Carlo method,the in-service welding repair reliability is evaluated.Studies show that: pipe wall thickness,"equivalent molten pool" depths and medium pressure affect the reliability sensitively;diameter,tensile strength,"equivalent molten pool" width and length affect the reliability insensitively.What's more,wall thickness has greater influence on reliability than the depth of the "equivalent molten pool";multiple factors have greater influence on the reliability than a single factor.Improving measurement accuracy and reducing medium pressure can effectively improve the reliability of welding repair.