Research on the Calculation Method of Crack Driving Force of Nitrogen-Containing Supercritical Carbon Dioxide Pipelines

The released supercritical carbon dioxide does not decrease the pressure at the crack top of the pipeline due to phase transition. When mixed with nitrogen, the maintained pressure at the crack top, related to the phase transition, further increases, easily driving long expansion of the cracks. By combining phase transition pressure simulations of supercritical CO2 containing nitrogen with crack driving force estimations, a carbon dioxide pipeline crack driving force model is derived from the Peng-Robinson (PR) equation and phase diagram. The solution method based on the Newton-Raphson and iteration procedures of the model is also proposed. A CO2 pipeline with a diameter of 914 mm and a wall thickness of 25.4 mm containing 5%–10% nitrogen is applied to verify the proposed model. The effects and correlation strength of nitrogen and operation parameters on the driving force are analyzed using the Spearman correlation coefficient method. The results indicate that as the content of N2 impurities increases, the pressure in the pressure reduction wave plateau area significantly increases, which corresponds to an increased driving force. The increase in impurity content, decrease in release pressure, and increase in release temperature of the CO2 pipeline all raise the pressure in the pressure reduction wave plateau, thereby affecting the safe operation of the pipeline. CO2 concentration is strongly correlated with phase transition pressure, whereas release temperature is moderately correlated, and release pressure is weakly correlated. At release conditions of 300 K and 15 MPa, increasing the N2 concentration from 0%–5% raises the phase transition pressure from 4.155–7.624 MPa, an increase of 83.5%.