Study Of Large-Scale LNG Storage Tank Stratification And Rollover

Liquid Natural Gas(LNG)is a liquid obtained by cooling natural gas to-162°C under normal pressure and removing water,carbon dioxide,and heavy hydrocarbons by adsorptive separation.It is stored in a cryogenic storage tank.The volume of liquefied natural gas is only1/600 of the volume of gaseous natural gas of the same substance,the weight is about 45%of the same volume of water.Nowadays,LNG has been used in various fields such as urban gas storage and peak regulation,residential gas and industrial gas,and its role in production and life is also increasingly important.LNG storage and transportation technologies are also rapidly developing,and the related technologies of LNG are also Occupy a more important position in the natural gas industry.Large LNG storage tanks have high storage efficiency,high overall utilization of resources(land,personnel),and lower construction costs than small tanks of the same size,is the preferred choice for the reserve of natural gas,it can be used in urban air storage peak shaving,LNG receiving terminals,and LNG liquefaction plants.Because of the extensive use of large-scale LNG,tank construction and safety issues need to be strengthened,stratification and tumbling in tanks can lead to various accidents.Therefore,it is of theoretical significance and guidance to study the process and influence factors of stratified tumbling in tanks.Study of LNG stratification and rolling process in this paper,the physical model of tumbling was selected from the LNG two-tiered and three-tiered buildings with the same height in the storage tank..Mathematical models were established using fluid mechanics and heat transfer and CFD software was used to simulate the tumbling process of LNG in storage tanks,analyze the change of density field and velocity field in LNG tumbling process under two layered and three layered states.According to the results,the rolling process is divided into three stages:the rolling of the tank wall,the transfer of the tumbling surface from the wall to the center,and the formation of a new delamination.Obtained by simulation analysis of the effect of initial density difference change on the rolling,the effect can be divided into two range:when the difference in the initial test density is small,the tumbling between the two layers starts later and the tumbling process is slow and long-lasting;when the initial density difference is large,the tumbling between the layers starts earlier and the mixing is completed very quickly,the tumbling process is very intense.With the same tank diameter and volume,the greater the difference in liquid density between two adjacent layers,the earlier the rolling occurs,and the rapid and violent tumbling progresses,and the threat to the tank is greater.Analyze each layered feature that may affect the rollover and draw conclusions.The more layers are,the more severe the rollover is,and the longer the rollover time is.Under the same density difference,the layering height increases,the contact area between the LNG liquid layers and the inner wall of the storage tank increases,so the viscous force increases,the interface between adjacent layers is more stable,and rolling does not occur easily.,so the tank is in a more secure environment.And in the 160,000m~3 LNG tank with a diameter of 80m,the critical density difference for safe storage is 3kg/m~3,4kg/m~3,and 5kg/m~3 when the LNG layered height is 1m,2m,and 3m.Through the numerical simulation,it is concluded that the layering characteristics affect the occurrence of tumbling.Finally,it proposes methods to eliminate stratification and the measures to prevent rollover and provide references for further research on LNG storage and transportation.