Temperature distribution of LNG unloading pipelines during nitrogen pre-cooling

LNG (liquefied natural gas) is usually pre-cooled before it flows into normal temperature unloading pipelines. In the process of low-temperature nitrogen pre-cooling, the temperature difference between the top and the bottom of the pipeline is large. If the temperature difference is too large, the pipeline will be upwarped. In this paper, a 3D nitrogen pre-cooling model for LNG unloading pipelines was established by using Fluent. Then, the temperature distribution of unloading pipelines during nitrogen pre-cooling was simulated by decreasing the inlet temperature of nitrogen step by step. And finally, a series of studies were performed on the causes and influence factors of top-bottom temperature difference of unloading pipelines. The results show that, during pre-cooling, the temperature gradient is higher near the pipe wall, but lower inside the pipeline. Attention shall be paid to the natural convection inside the pipeline during pre-cooling. After its heat exchange with the pipelines, the nitrogen rises in temperature and drops in density. Under the effect of buoyancy, the nitrogen flows upward. And consequently, the temperature at the top is higher than that at the bottom. The top-bottom temperature difference is influenced by the following factors, such as pre-cooling time, mass flow and nitrogen temperature. The temperature difference increases at first and then decreases. The larger the mass flow is and the lower the nitrogen inlet temperature is, the larger the temperature difference appears. In order to prevent the top-bottom temperature difference from rising too high and keep the pre-cooling rate below 10 K/h, the temperature is decreased gradually to 123 K by means of step pre-cooling.