Simulation Study On The Factors Influencing LNG Evaporation And Rollover In Large Storage Tanks

Liquefied natural gas,as the efficient ecotype energy and high-quality transition fuel,can provide a strong guarantee for optimizing the energy consumption structure,improving the atmospheric environment and achieving sustainable development.In recent years,LNG production and trading has become increasingly active globally,with the construction of LNG receiving stations emerging on a large scale.As a major piece of equipment,LNG storage tanks play a crucial role in promoting the infrastructure of natural gas production,supply,storage and sales systems.However,when filling new LNG into tanks or storing it for long periods of time,evaporation,stratification and rollover can occur,which can increase evaporation losses and cause explosions,seriously risking the safe storage of LNG.To this end,the HYSYS software was utilized to analyze the influence of initial filling rate,ambient temperature,thermal conductivity and nitrogen content on the safe storage of LNG in combination with parameters reflecting the evaporation condition such as tank pressure and safe storage time,and the Fluent numerical simulation was employed as a means to reveal the mechanism and analyze the influencing factors of LNG rollover in storage tanks by combining the variation of density field,velocity field and other relevant quantities,so as to study the law of motion and pressure variation in the gas phase space during the rollover process.When the initial filling rate was different,the pressure inside the tank increased with the extension of storage time,but the growth rate varied.There was an optimal filling rate within the working pressure range of the tank.When the initial filling rate was less than or greater than the optimal filling rate,the safe storage time of LNG in the tank will be shortened.When the optimal initial filling rate of 160000 m~3 storage tank was about 78.6%,the maximum safe storage time was about 607 hours.Both the ambient temperature and the thermal conductivity can affect the heat leakage of the storage tank,thereby affecting the pressure variations inside the tank and the safe storage time.However,compared to the influence of ambient temperature,the thermal conductivity had a more significant impact on the static evaporation of LNG inside the tank.The higher the nitrogen content in LNG components,the faster the increased in pressure inside the tank,and the shorter the safe storage time.Based on the simulation results of Fluent software,the rollover process of LNG in the tank was summarized into four stages:rollover stage near the tank wall,rollover transfer stage,rollover recovery stage and the formation of a new stratification stage;the larger the difference in stratification density,the earlier the maximum velocity at the stratification interface and the larger the value.When the total height of the liquid layers was the same but the interlayer heights were different,the greater the height of the upper layer,the later and weaker the rollover occurred;when the total height of the liquid layers was different but the interlayer heights were the same,the greater the stratification height,the later and larger the value of the maximum velocity at the interface,and the more intense the rollover.The rollover of the three-layer structure occurred earlier than that of the two-layer structure,and the rollover lasted longer and was more intense.Increasing the heat leakage from the bottom of the tank will increase the intensity of the rollover and shorten the rollover time.When the heat leakage was the same,the effect of the bottom leakage on the rollover was stronger than the side wall leakage.The motion law of the upper gas phase space during the rollover of LNG in the storage tank can be summarized in four stages,namely the gas generation stage,the gas diffusion stage,the intense pressurization stage and the pressure stabilization stage.The amount of gas escaping from the liquid phase LNG increased under the effect of gravity squeeze and roll-carry,the gas diffused and filled the whole gas phase space,the pressure in the gas phase space increased.The average pressure growth rate in the gas phase space due to rollover was significantly higher than the pressure growth rate during static evaporation.This paper systematically reveals the influencing law of LNG evaporation and rollover,which has important scientific guidance significance and engineering application value for effectively suppressing LNG receiving station destabilization accidents,reducing energy wastage and controlling safe operating loads.