The Key Technology Research In The Pre-commissioning Of LNG Tank

The LNG tanks serve as the main storage device in the industry chain of liquefied natural gas and the storage temperature of LNG is as low as-162℃,so pre-cooling processing would be necessary for tank system before production.The pre-cooling of LNG tanks and large-diameter unloading lines is the main part of production of LNG storage tanks.Reasonable selection of cooling medium and well controlling of temperature drop rate and pressure during the pre-cooling process are the key issues for pre-cooling operation.On the basis of theoretical analysis,numerical simulation and example verification,this paper studies the variation of physical field as well as the key controlling parameters of LNG tanks and large-diameter unloading lines during the pre-cooling process,the main contents and conclusions are as follows(1)Study the production process of LNG tanks and then propose the pre-cooling technologies as well as corresponding technical requirements respectively for LNG tanks and large-diameter unloading lines.Two pre-cooling methods which are most commonly used--the LNG pre-cooling and liquid nitrogen pre-cooling--are compared and analyzed for their merits and faults,and each method has its best applicable condition.The LNG tanks are recommended to adopting LNG as the pre-cooling medium when the volume of tank is larger than 5×10~4m~3 and adopt the liquid nitrogen when the volume is less than 5×10~4m~3;the large-diameter unloading lines give priority to liquid nitrogen method.(2)The paper analyzed thermal properties LNG tank by means of heat transfer using theory and established the LNG mathematical model of pre-cooling tank.According to the applicable conditions of the method of pre-cooling,paper chose the pre-cooling process of 16 × 10~4m~3 LNG storage tank using LNG and 1×10~4m~3 LNG tank using liquid nitrogen as the research objects,calculated consumption of pre-cooling medium,analyzed wall temperature of tank and the temperature drop rate change with the pre-cooling time.Conclusions are as follows: the main influence of pre-cooling time is tank initial temperature(ambient temperature),so giving priority to the pre-cooling operation in winter;with the decrease of tank temperature,the drop rate of tank wall temperature will also reduce,thus the effect of pre-cooling becomes bad.By reducing pre-cooling medium temperature and increasing the flow of liquid nitrogen can improve the pre-cooling efficiency.Comparing with actual data of pre-cooling process,since the theoretical models have some simplified hypothesis and man-made or natural loss of filed and other reasons,the calculation results have some errors comparing with real data.Then paper gives an idea of using the numerical simulation and theoretical analysis to analyze pre-cooling process.(3)The 3D simulation pre-cooling model of 1×10~4m~3 LNG tank is built to study the changing rules of time,cooling medium feed flow rate,temperature field and pressure field.The simulation results show that the velocity of inlet and outlet is fast with intense flow disturbance,and the temperature of BOG vent is lower because the BOG vent is similar to the structure of orifice jet,resulting throttle expansion decalescence phenomenon.The temperature of tank bottom and tank center decreases fast,and it should be mainly monitored in the pre-cooling process.The temperature of the concrete outside wall maintains 300 K,so the cold insulation property can meet the designed requirement.In the pre-cooling process,there is no obvious pressure gradient,and the pressure surge is small in the vertical with no overpressure risk.it can get the optimal precooling condition and develop a safe and efficient LNG tank precooling plan,through simulating the precooling process in different feed flow rate and feed flow temperature.What’s more,the simulation results are consistent with the actual data,proving the accuracy and the reliability of the simulation model.(4)A large-diameter discharging pipe mode is built to optimize the pre-cooling process of Shandong LNG discharging pipeline.The process is divided into several stages due to the temperature nodes.The trend of the temperature drop is studied under different inlet flow rates and temperatures of liquid to determine the optimal condition.During the pre-cooling process,when the thermal equilibrium is reached,the temperature of the outer wall of the insulting layer decreases to 289 K and remains unchanged.The largest temperature drop happens at the inlet of the pipeline where should be monitored closely.The pressure gradient is small along the pipe and the pressure can meet the requirement of the pre-cooling process.Improper temperature drop rate which exceeds 10K/h is found in the in-situ operation.The optimized process mode is more efficient and safer with a better control of temperature drop.