Research On Transportation And Security Of LNG Vacuum Multilayer Insulation Pipe

With the amount increase of imported liquefied natural gas(LNG),the LNG transportation researchment is needed after being unloaded.Compared with pipe transportation after gasificationm,pipe transportation in liquid state directly has the advantages as followed.Firstly,LNG pipe transportation can remain cold energy of LNG,which can be better used further.Secondly,the function of peak shaving in city pipeline can be played.Thirdly,liquid pipeline power required in liquid pipe is much less than the gas pipe under the same mass flow rate.However,the reliable insulation form and the scientific pipeline model is the key to ensure the smooth transmission and security in the LNG pipeline;the method of theory analysis,numerical simulation and experimental research was adopted in the paper to study LNG pipeline transportation and safety.The mainly contents were completed in this paper are as followed research contents:(1)Thermodynamics properties of LNG were calculated.The influence of thermodynamic and tranfer properties were fully considered during LNG transportation.The phase equilibrium was calculated by programing using LKP equation.The changes regular of density,specific heat,thermal conductivity and viscosity with temperature under different pressures were further discussed respectively.The results showed that: the density,specific heat capacity,viscosity and thermal conductivity were affected by pressure slightly and the responsed sensitively with temperature in the liquid phase region.The density,specific heat capacity,viscosity and thermal conductivity showed high volatility near the critical point.The influence of relevant thermodynamic parameters affected by pressure became apparent,and the influence affected by temperature decreased.(2)The insulation form can be used for LNG pipeline are accumulation insulation,vacuum insulation and vacuum multilayer insulation.The heat transfer process was analyzied and consist of thermal resistance was discussed.The most suitable form wad determined.The vacuum multilayer insulation is better than vacuum insulation,and vacuum insulation is better than accumulation insulation in the adiabatic performance.If vacuum insulation was used,the insulation performance will decreace greatly once vacuum loss happened during the LNG transportation.The vacuum multilayer insulation pipeline was adopted in LNG pipeline,because it has low requirement on vacuum and high insulation performance.Vacuum multilay insulation pipe was anlyzied in cylinder coordinate in aspect of heat transfer based on two dimensional steady state assumptions.The radiation between reflective screens,the heat conduction of gas molecule and the solid conduction of fibers are included in the heat transfer.The temperature distribution and heat flux of vacuum multilayer insulation pipe were obtained by programming under different vacuum degree.(3)The phase states were defined in given components,included subcooled state,density state and supercritical state.The math model of LNG pipeline was established,which was simplized to model of vacuum multilayer insulation pipe.The temperature field was two dimensional axial and radial steady field and the boundary conditions of fluid field and temperature field.The temperature change in the direction of LNG fluiding was obtained,which agreed with the expermential results well.Because the temperature increase was not obvious,the pressure was controlled to be higher than the saturated pressure or the temperature to be lower than the saturated temperature was the two necessary conditions in subcooled state transportation.The no gasification distance was calculated by programming,and the factors affected on the distance are inlet pressure,inlet temperature and flow rate.The pressure is higher and temperature is lower,the distance is longer.(4)The crack damage on inter or outer pipe can both lead vacuum multilayer insulation pipe vacuum loss.Then the insulation performance decrease and subcooled state damage.The heat transfer process after inter or outer pipe damage was discussed respectively.The natural convection heat transfer in circle space was anlyzied based on assumption and the heat transfer model was established.The outer wall temperature,the amount of layers,the conductivity coeiffient of layers and the density of layers are the factors influented heat flux after vacuum loss.The results showed: the heat flux will be higher,if the outer wall temperature is higher,the amount of layers is less,the density and the conductivity coeiffient is bigger.(5)The vacuum loss process of LNG vacuum multilayer insulation pipe was studied experimentally.The pressure,wall temperature and heat loss were tested in circle space under different layer amount.On condition that micro crack propagation was also studied in micro aspect by molecular dynamics approach.The results agreed with the experiment well.It provides a strong guarantee for the LNG long-distance pipeline transportation of smooth and thermal safety.