Analysis And Optimization Of Nitrogen Replacement Process In Large LNG Storage Tanks

Large-scale LNG storage tanks are the main gas storage equipment in natural gas emergency peak shaving storage facilities.Before they are completed and imported into LNG and put into operation,they generally use inert gas(nitrogen)to safely replace the air in the tank to prevent LNG from entering the storage for the first time.The tank and oxygen produce an explosive gas mixture.However,due to the large volume,complex structure,and strong airtightness of the storage tank,the long replacement time,large nitrogen consumption,and high risk factor are faced in the implementation operation.Therefore,it is necessary to conduct further research on the nitrogen replacement process of LNG storage tanks.By exploring and analyzing the diffusion law in the process of nitrogen replacement,understanding the distribution characteristics of the gas concentration field,replacement efficiency and other issues,we can propose a trial run optimization plan that shortens the replacement time and reduces the cost and investment,which will have a certain engineering guiding significance for engineering practice.In order to achieve the research purpose,the thesis takes a 30000 m~3LNG storage tank in a peak shaving storage and distribution station in Shandong Province as the research object,and uses the research methods of theoretical elaboration,mathematical estimation model,numerical simulation and model verification to explore the rule of nitrogen replacement process in the storage tank.Based on the analysis of nitrogen replacement principle of storage tank,the mathematical process function of replacement index(oxygen content and water dew point)with time is established by substituting engineering operation parameters with calculus;With the help of FLUENT software,the turbulent CFD model of nitrogen displacement process in LNG storage tank is established,and the numerical simulation of nitrogen displacement process is completed by using simple algorithm.At the same time,the mathematical estimation model and CFD model are verified by using effective data.The validation results show that the CFD model is in good agreement with the engineering data,and there are some errors between the estimation results of the mathematical model and the engineering data.Based on the effective CFD model,three improved schemes to improve the displacement efficiency are proposed and studied by numerical simulation.The thesis focuses on the nitrogen diffusion characteristics,the variation of oxygen content with the replacement time,and how to improve the replacement efficiency and reduce the nitrogen consumption in the inner tank and vault area with different replacement schemes under the same nitrogen inlet flow rate.The results show that the distribution of nitrogen in the tank is wavy and uneven,and the nitrogen concentration in the central axis area is high,and the nitrogen concentration at the edge of the two walls is low,which is easy to produce"displacement dead angle";In the initial stage,the replacement rate of oxygen increases linearly with the replacement time,but with the increase of replacement time,the increase of replacement efficiency decreases;Under the condition of the same nitrogen inlet flow,when the oxygen volume fraction of the vent is 2%,the replacement time and nitrogen consumption from small to large are the secondary pressure boosting scheme,the scheme of adding vent hole,the scheme of changing the position of the vent pipe,and the engineering example scheme.In this study,it is considered that the effect of displacement optimization with secondary pressure rise is ideal.These conclusions will provide some reference for further optimization of nitrogen replacement process of large LNG storage tank.