The Research In LNG Receiving Station Large Diameter Pipe Precooling

When the LNG receiving station puts into operation,if low temperature liquid LNG（111K under atmospheric pressure）flows into unloading pipe directly,it will cause extremely temperature drop of pipeline in a short time.The large temperature drop rate will cause large thermal stress,which will damage the pipe,valve or the joint parts.Thus precooling the unloading pipeline before the LNG enters is the premise to ensure the commissioning safely.However the researches on precooling of large diameter pipe domestic and oversea are few,what’s more these researches don’t put gravity and buoyancy effect into consideration.On the basis of theoretical analysis,numerical simulation and example verification,this paper studies the temperature field of large-diameter unloading lines during the pre-cooling process and the influence of relative parameters on the heat transfer process,the main contents and conclusions are as follows:（1）Analyze the precooling heat transfer process theoretically and establish a mathematical model.By compiling program,the temperature distribution is calculated.The calculation results have certain error,compared with the Hainan LNG precooling data,which cannot reveal the temperature difference of the top and bottom pipe wall.（2）Establishing a 3D pipe model,this paper simulates the large diameter pipe precooling process,with consideration of gravity and buoyancy effect by applying simulation softwareGrFluent.According to the results of Gr/Re² and Petukhov equation,the heat transfer pattern of precooling is combined heat convection.At the cross section,nitrogen with large density and low temperature gathers at the middle-lower part.There is apparently temperature difference between top and bottom pipe wall.Temperature decreases along the direction of gravity and the lowest temperature is at the lower part.During the precooling process,the inlet temperature drop is large than the other parts.The coefficient of heat transfer of the inlet is large and then decreases to a constant in fully developed area.There is a secondary flow in the cross section.Nitrogen flow upward near the pipe wall.The velocity distribution in direction of gravity is not symmetry,and the nitrogen velocity at the bottom is larger than that of the top.Near the top pipe wall,the velocity gradient is large.（3）Applying control variable method,the influence of different parameters on the pipeline precooling is analyzed.Based on the influence results,an optimum precooling method is proposed.The results indicate that the lower inlet temperature and larger mass flow rate,the faster the pipe temperature decreases and larger the top and bottom temperature difference is.Under the condition of same mass flow rate,temperature drop rate is inverse proportion to pressure.The temperature difference of top and bottom pipe wall of small diameter pipe is lower than that of large diameter pipe.（4）Utilizing dimensionless analysis method,Nusselt number is a function of Renoldnumber,Planck Number and Grashof Number.Based on the simulation results,this paper fits a correlation about Nusselt Number:Nu（28）3.25143e^-15Re^-0.77705Pr^-119.88877Gr^0.38676.（5）A BOG precooling model is established.Compare BOG precooling model with nitrogen precooling model in 4 terms,precooling rate,top and bottom temperature difference,economic aspect and risks.Nitrogen precooling method is proposed before LNG receiving station putting into operation.（6）The precooling of pipeline with thermal bend is simulated.The temperature distribution is studied.The results indicate that the disturbance increase at the bend and the heat transfer is enhanced in the bend cross section,as a result the top and bottom temperature difference is smaller than that of straight pipe.Thus increasing the disturbance in the pipeline can reduce the temperature difference.