Influencing Factors And Working Condition Simulation Of Local Thermal Arching Effect Of LNG Pipeline

LNG pipeline will be deformed under the influence of uneven temperature load and gravity load,the deformation of the pipe will affect the flow of the fluid in the pipe.Low temperature load,deformation and other factors will affect the stress distribution of the pipeline at the same time,resulting in the damage of the strength of the pipeline and the damage of the seal,which seriously endangers the operation safety of the pipeline.In this paper,a simplified physical model of LNG pipeline with flange is established.Using the Workbench platform,two-way data transmission between the fluid dynamics software Fluent and structural mechanics software Transient Structual,the variation law of the fluid thermal and hydraulic parameters and the displacement and stress of the LNG pipeline with flange is obtained when the LNG pipeline flows in single phase and boiling flow in two phase,in this paper,the hot arch phenomenon in pre-cooling process of LNG pipeline is simulated and analyzed.The main research contents and conclusions are as follows:In the cryogenic nitrogen precooling phase,the fluid in the LNG pipeline flows in layers under the action of gravity and buoyancy,the fluid temperature distribution in the pipe is not uniform,as a result,the heat transfer between the top and bottom of the pipeline is inconsistent,resulting in the generation of temperature difference between the top and bottom.The influence of nitrogen inlet temperature drop value,nitrogen flow rate,pipe diameter and other parameters on top and bottom temperature difference was simulated and analyzed by using control variable method,then the influence of different temperature difference between top and bottom on the displacement and stress of pipeline is studied.The analysis results show that when the nitrogen inlet temperature is reduced in a stepped manner,the temperature difference between the top and bottom of the pipeline shows a ’n’shape periodically,the larger the inlet temperature drop,the smaller the flow rate of nitrogen,and the greater the temperature difference between top and bottom.The higher the inlet velocity of nitrogen,the higher the temperature drop rate and the shorter the precooling time.The larger the pipe diameter,the greater the temperature difference between top and bottom.The larger the temperature difference between the top and bottom,the more obvious the phenomenon of thermal arch of the pipeline.When the temperature difference between the top and bottom of the pipeline is about 40 k,the pipeline begins to arch upward.Secondary stress caused by temperature load is the main factor of pipeline failure and seal failure.The temperature distribution on the gasket is consistent with the stress distributionlaw,the inside is greater than the outside,the top is greater than the bottom.In the actual pre-cooling process,in order to avoid the occurrence of hot arch,it is suggested that the top and bottom temperature should be controlled below 40 k.In the pre-cooling process,the temperature drop interval should be increased while the inlet temperature drop value is increased to avoid the existence of the accumulative temperature difference between the pipe wall and the fluid.When liquid nitrogen is in direct contact with LNG pipeline,two-phase flow occurs in the pipeline,accompanied by boiling phase transformation heat and forced convection heat transfer.The fluid flow state in the pipeline is different at different level heights,when the inlet liquid level is low,the mass flow rate of liquid nitrogen is small,and the existence time of each stage of boiling is extremely short,when liquid nitrogen is present,the temperature of the pipe in contact with it has dropped below the boiling point of liquid nitrogen.When the inlet liquid level is high,the mass flow rate of liquid nitrogen is large and the existence time of each stage of boiling is relatively long,when liquid nitrogen is present,it is still in the boiling stage and the pipe temperature is above the boiling point.Therefore,the lower the inlet level,the greater the temperature difference between the top and bottom,and the greater the upward displacement of the pipeline.Compared with the stage of cryogenic nitrogen pre-cooling,although the temperature difference between the top and bottom increased,the arch of the pipeline was not caused,but the contraction deformation of the pipeline was serious.