Technology Research In The Pre-commissioning Of LNG Tank

The LNG storage tank is the key equipment in the LNG receiving station and the storage temperature of the LNG under atmospheric pressure is as low as 111K.Thus,the pre-cooling of tank is a must in the pre-commissioning.This paper first studies the parameters change rules of LNG tanks in pre-cooling,filling and steady storage by experimental methods.Then the precooling process of large LNG storage tank was studied from three aspects:theoretical analysis,numerical simulation and example verification.The main research contents and conclusions are as follows:Taking 195L small cryogenic storage tank as the research object,liquid nitrogen as an experimental medium,the experiment to study the change rule of parameters in pre-cooling,filling and stable storage stages is carried out.Conclusions are as follows:(1)The pre-cooling process is divided into three period,the high temperature constant period,rapid temperature drop period and slow temperature drop in the late pre-cooling period.(2)At the filling stage,there is a temperature rise phenomenon in the tank,and the temperature rise of the tank decreases from the bottom to the top.(3)The evaporation flux fluctuated cyclically,and the trend of fluctuation is opposite to the trend of environmental pressure,and it has nothing to do with the trend of temperature fluctuation.(4)The higher the initial filling rate,the smaller the pressure rise speed,the longer the safe storage time.This paper studies the pre-cooling technologies as well as corresponding technical requirements of LNG tanks,coMPares the LNG pre-cooling and liquid nitrogen pre-cooling,analyzes their economy and security and give the applicable condition of each technology.The paper carried out the heat transfer analysis of 16×10~4m~3 LNG tank pre-cooling process,and set up the pre-cooling mathematical model which was solved by MATLAB.The pre-cooling law and its influencing factors were studied.Conclusions are as follows:(1)At the beginning of pre-cooling process,the pressure of tank and BOG discharge flow increase rapidly;with precooling carried out,LNG spray flow rate increase and growth rate increase,tank pressure decrease and BOG discharge flow rate increase slowly until the late period decrease.(2)The larger temperature drop rate,the less time precooling,larger LNG flow rate,BOG discharge flow rate and pressure of tank.The larger temperature drop rate,the smaller total amount of LNG consumption and total amount of BOG emission,and the influence weaken when the temperature drop rate>-3K/h.(3)The initial pressure of the tank and the set pressure of the BOG discharge have little effect on the precooling process.Thus,it is suggested to use small temperature drop rate in the early period of pre-cooling period,and increase the temperature drop rate in the later period,and control the average temperature drop rate from-2 to-3K/h in the whole pre-cooling process.The initial pressure should be 10~15kPa and the pressure of BOG discharge should be 15kPa.The three-dimensional LNG pre-cooling numerical model of 16×10~4m~3 LNG storage tank was established.The phase transition of LNG was theoretically analyzed and the UDF was compiled for simulation.The numerical simulation results are in good agreement with the Hainan LNG pre-cooling data,which proves the reliability of the model.Study the change law of physical field in precooling process and analyze the effect of spray ring on pre-cooling,the results indicate that:(1)The trend of temperature change in the large tank is consistent with the experimental results of small tank pre-cooling.(2)The temperature field interacts with the flow field in the tank.The half of the tank forms a swirl on either side.(3)Secondary reflux occurs in the central area of the bottom of the tank during the precooling process,slowing down the temperature drop.(4)In the pre-cooling process,the pressure in the lower part of the tank is higher than the upper pressure,and the pressure gradient is obvious.(5)The position,inner diameter and number of nozzles of the spray loop pipe have influence on the pre-cooling process of the tank.For the 16×10~4m~3LNG storage tank,it is suggested that the distance between spray loop pipe and vent should be increased;the diameter of spary ring should be 13m,the number of nozzles should not be too large,the number range is generally18-28.