A Theoretical Study Of The Smallhole Leakage Characteristics Of CO₂ Pipelines

Pipeline transportation of CO2 is an indispensable part of carbon capture system, CO2 pipeline failure may lead to the leakage of CO2 into environment. Since CO2 is harmful to human beings and animals, therefore, CO2 leaking may result in serious people and property losses. As a common failure mode, the small-hole leakage need be studied comprehensively. The investigation of the small-hole leakage characteristics will give a good guidance to predict and control the leakage accidents. It is very important to study the flow characteristics and the change of various parameters such as pressure, temperature and leakage rate inside the CO2 long-distance pipeline transportation with small-hole leakage to adopt effective remedial method after the accident before the leakage. Both analytical and numerical methods are used in this paper to study the phenomenon aiming at establishing an analytical prediction equation for small-hole leakage rate calculation. The main contents of this paper are as follows.Firstly, a non-adiabatic leakage mathematical model for leakage rate of the long distance pipeline transportation of CO2 with small hole leakage is established, and the analytical expressions of pressure, temperature and leakage rate are obtained. The influences of the environmental heat effect, initial pressure, initial temperature, leakage diameter and environmental temperature on the gas pressure, temperature and leakage rate during the leakage process are investigated. The results show that the leakage diameter and initial pressure has a greater influence on the parameters during the leakage process, the existence of the environmental heat effect will lead to more obvious temperature variation than the pressure variation. However, on the other hand, it is also found that the environmental heating has little influence on the leakage rate, compared with the adiabatic leakage model, which is much more simple and convenient to use.Secondly, another mathematical model taking the phase change of CO2 condensation into the small-hole leakage process of the CO2 long-distance pipelines is established, the analytical expressions of pressure, temperature and leakage rate are also obtained, the influences of the phase change on the leakage process are studied. The results show that the condensation will produce the same effect as the environmental heating that can inhibit the temperature, but the gas temperature rise little. Besides, the CO2 condensation during leakage process has little influence on the leakage rate, compared with that without non-phase change model, the adiabatic leakage calculation can be applied in the engineering calculation.By using the commercial software FLUENT, small-hole leakage process of the long distance pipeline transportation of CO2 with different heat transfer coefficient is carried out, the change rules of the gas pressure, temperature and leakage with the leakage time and the distribution of the gas pressure, temperature, velocity, Mach number and concentration of CO2 are obtained, the numerical results are compared with the analytical solutions. The results indicate that the errors between the numerical results and analytical results is less than 20%, which verifies the analytical solutions of the small-hole leakage process are reasonable in some degree. In addition, the gradients of the flow parameters such as pressure, temperature, velocity near the leakage hole vary obviously, but the parameter gradients in the tube vary smoothly, which proves that the homogeneous field assumption of the analytical solution is reasonable. Although the flow time in the critical stage of leakage and subcritical stage is nearly equal, the CO2 leakage will be completed during the critical stage, that is to say, the critical stage plays a leading role during the leakage process.