Research On Pre-cooling Process Of Large Scale LNG Storage Tank

LNG storage tanks are the main storage equipment of LNG plants and LNG receiving stations,and the investment share is relatively large.LNG storage tanks are required to be of high safety generally.Due to the ultra-low temperature characteristics of LNG,a large amount of cryogenic liquid enters the normal temperature storage tank,and there will be sharp and non-uniform temperature drop occurs,which leads to stress concentration of the tank body and sudden pressure rise in the tank,affecting the safety of the receiving station.Therefore,the LNG storage tank must be pre-cooled before being put into use,so that the storage tank can be slowly and evenly lowered from the normal temperature state after the mechanical completion to the low temperature working state.It is difficult and risky to pre-cool the storage tank.Sufficient preparation before pre-cooling operation,reasonable control of cooling rate and estimation of pre-cooling medium usage,and strict implementation of safety measures are an important guarantee for the successful implementation of pre-cooling.Based on the literature review at home and abroad,this paper studies the physical field variation law and main pre-cooling control parameters of large LNG storage tank during pre-cooling process,and verifies them with field measurement data.The main contents are as follows:1.The pre-cooling scheme and pre-commissioning work of the large scale LNG storage tanks of the current LNG receiving station are introduced in detail,and the heat transfer model of the tank top,tank wall and tank bottom were constructed.Based on the basic theory of heat transfer,the LNG storage tank of 8×10~4m~3 was taken as the research object,and the mechanism of the leakage process of the tank was studied.The calculation formulas of the heat leakage at the top of the tank,the side wall of the tank and the bottom of the tank are derived.The heat leakage at the top of the tank,the side wall of the tank and the bottom of the tank were calculated respectively,which provide a basis for further research on the pre-cooling scheme of the tank.2.According to the unsteady characteristics of the pre-cooling process,the energy conservation equation of the open system was established based on the first law of thermodynamics,and the pre-cooling law was analyzed.It was found that the factors affecting the precooling time are the flow rate of pre-cooling medium,the leakage heat of storage tank and the quality of the cooled components.The calculation model of tank temperature and pressure varying with pre-cooling time was deduced.The variation of tank temperature and pressure with time in pre-cooling process was analyzed.The interaction among flow rate of inlet medium,temperature drop rate,pre-cooling time and consumption of pre-cooling medium was discussed.The theoretical calculation was carried out for the pre-cooling process of the liquid nitrogen plus LNG pre-cooling scheme actually adopted by a petrochemical enterprise,and the analysis result was compared with the actual measured data of the enterprise.The error was not more than 7%.Within the scope of actual engineering requirements,the theoretical basis was provided for guiding the selection of precooling spray flow in engineering practice.3.A three-dimensional model of 80,000 m~3LNG storage tank was established.Based on Fluent software and a phase change model of cryogenic liquid vaporization,the temperature field,flow field distribution and temperature time history of the tank during the pre-cooling process of the tank were simulated,and the storage tank during the pre-cooling process was analyzed.Temperature field,flow field and heat transfer process were studied to study the effect of different inlet mass flow rates on temperature drop rate.Comparing the simulation results with the actual measured data of the enterprise,the two are in good agreement.