Subject:study On The Crack Ductility Growth Of Impurity Supercritical CO₂ Pipeline

Supercritical CO₂ is very suitable for long-distance and large-scale pipeline transportation of CO₂ in CCUS technology because its density is similar to liquid,and its viscosity and diffusion coefficient are still close to gas.However,the pipeline cracks under the action of various factors,and the formed cracks will cause the rapid expansion of cracks under the action of high-pressure transportation in the pipe,and eventually cause the fracture and expansion of the gas pipeline.With the development and progress of the metallurgical industry,the brittle growth of pipelines has been gradually curbed.At present,the crack propagation accidents of pipeline steel are mainly concentrated on the ductility of cracks.Therefore,seeking an effective and practical method to predict whether the crack ductility growth of impurity supercritical CO₂ pipelines can be quickly arrested under actual operating conditions has important research significance.This paper takes the impurity-containing supercritical CO₂ pipeline crack ductility growth problem as the research object,and proposes a crack ductility propagation criterion suitable for impurity-containing supercritical CO₂ pipeline.The Bartel hyperbolic method is used to compare the pressure-crack ductility growth rate curve and the pressure-The gas decompression wave curve is used to predict the required toughness of the pipe,so as to judge whether the ductile expansion of the pipe can stop the crack.The determination of the decompression wave velocity requires a clear thermodynamic characteristics of the conveyed fluid,but when there are impurities in the CO₂ flow,it will have a significant impact on the thermodynamic properties of supercritical CO₂ .Changing the phase equilibrium curve will cause two phases to appear during the decompression process.area.In this paper,a calculation model for the decompression wave velocity of supercritical CO₂ containing impurities is established.The pressure calculation software of REFPROP is used to calculate the decompression wave velocity in the single-phase area,and the MATLAB software is used to calculate the decompression wave velocity in the two-phase area.Various state equations are screened.After comparing with the experimental data,it is concluded that the calculation result of the GERG-2008 equation of state is the most accurate,and the average error of the model is 5%,indicating that the model can accurately calculate the decompression wave velocity of the impurity supercritical CO₂ decompression process.Through the calculation model of decompression wave velocity,the influence of initial temperature,initial pressure and impurities on the propagation characteristics of decompression wave of a series of representative CO₂ binary mixtures is studied.The increase of the initial temperature and the decrease of the initial pressure will make the length of the pressure platform gradually shorter,and the platform pressure will gradually increase,which is not conducive to pipeline cracking;the addition of impurities changes the phase boundary of pure CO₂ and increases its saturation pressure,So that the pressure platform in the decompression wave curve is raised,in which H₂and CO have the most significant impact on the pressure platform,and H₂S has the least impact.Among the influencing factors in the above research,the platform pressure value obtained by sensitivity analysis is most sensitive to temperature changes when the gas source composition is unchanged.In the 0.36 million tons/year and 4 million tons/year impurity-containing supercritical CO₂ pipeline transportation schemes of Yanchang Oilfield,Based on the actual gas source composition,operating conditions and pipeline design specifications,according to the impurity-containing supercritical CO₂ cracks ductile growth criterion,combined with the crack arrest criterion,judges whether the currently selected pipe specification can provide sufficient crack arrest toughness,whether it can stop the crack by itself when the pipe crack occurs ductile growth,and proposes the pipe section with a higher risk of crack growth Reasonable improvement plan.