Investigation On Precooling Scheme Of C-type LNG Cargo Tanks

LNG cargo tanks need to be pre-cooled to slow down before they are officially loaded with LNG.This is because LNG is an ultra-low temperature cargo of about-162 oC.For structural safety reasons,it cannot be directly loaded with the cargo tank at room temperature.Otherwise,safety problems may occur.In this paper,the pre-cooling scheme for tank is designed from the perspective of local temperature stress in the pre-cooling process of LNG tanks.The main research object is independent C-type LNG tanks.The finite element software Ansys is used to realize the thermal stress coupling analysis of the tanks,including two steps.One is the thermal analysis of cargo tanks to find the temperature distribution in the cargo tanks.Another is the analysis of the thermal stress of the cargo tanks to find the temperature stress under the temperature distribution.Firstly,the thermal analysis model of tanks was established to simulate the temperature field distribution in the precooling process of tanks.At the beginning of precooling,spraying LNG from each spray point into the tank resulted in local low temperatures in the tank.As the spray point continued to spray LNG into the cargo tank,the low temperature area of the cargo tank gradually spread until the entire cargo tank cooled to a safe temperature.At the beginning of pre-cooling,LNG was sprayed at various spray points in the tank,resulting in local low temperatures in the tank.As the spray point was continuously sprayed with LNG,the low temperature area of the cargo tank gradually spread until the entire cargo tank was cooled to safety temperature.Then,the temperature field results at various times during the process are extracted and converted into a temperature body load and applied to the thermal stress analysis model of the tank.Then the temperature stress distribution of the tanks at various times in the pre-cooling process would be solved.Based on the requirements of the specification for the stress of cargo tanks,the temperature stress of the cargo tanks in the pre-cooling process can be monitored within the safe range.In the thermal analysis section of the cargo tank,a fluid domain model in the cargo tank must be established to simulate the process in which the flowing gas in the tank transfers the low temperature at the spraying point to other positions in the bulkhead.So it is necessary to apply a suitable heat transfer coefficient to this part of the fluid domain model.Since this part involves the process of flow and heat transfer and phase transition,which is difficult to simulate in Ansys,while there is no suitable empirical formula for reference,Fluent was used to simulate the flow and heat transfer process in the precooling of the fluid area in the cargo tank to get this suitable heat transfer coefficient.The values of the heat fluxes of the cargo tanks in the precooling stage with the average temperature of the cargo tanks were extracted and converted into the equivalent heat transfer coefficients for thermal analysis in the finite element fluid domain according to the principle of heat transfer.The pre-cooling thermal stress for every time in the pre-cooling process is calculated according to the above-described process for the designed spray scheme.It is found that the cooling rate decreased quicker in the beginning than in the late period of pre-cooling process.By studying the change law of the thermal stress in the precooling stage of the cargo tank,it is found that the tanks usually have the highest thermal stress when the local low temperature distribution non-uniformly in the early stage of precooling.As the LNG spray continues to spread in the low-temperature region of the cargo tank,the local stress caused by local cryogenic temperatures in the cargo tank will gradually decrease.The selection of spray area is very important for the spray tank design of cargo tanks.The number of spray spots will be too much if the spray area is too small.Then the layout of the spray pipe system becomes complicated while the flow of the pre-cooling medium will be limited,and the pre-cooling speed of the cargo tank is reduced.But the excessive temperature stress on the structure may occur in terms of excessive spray area is designed.Therefore,it is necessary to design a spray scheme based on the structure that can withstand the maximum spray area,so as to ensure the fastest spray efficiency within the allowable range of structural strength and improve the rationality of the design of the spray pipe scheme.Therefore,it is necessary to calculate the maximum local spray area of the cargo tank before designing the spray hole layout solution in the cargo tank.Based on the maximum local spray area,an initial spray scheme can be designed according to the Saint-Venant principle.On the basis of the initial spray scheme,the spray arrangement is optimized by gradually making the spray points dense.The more the spray points are distributed,the less the precooling time of the cargo tanks is.When the spray points are applied dense enough while the thermal stress meets the requirements,the pre-cooling time calculated under the spraying plan is taken as the tank in this design.The fastest pre-cooling time that can be achieved.By using the principle of heat transfer to calculate the mass flow of LNG under these spray schemes,a reference for the actual pre-cooling operation of tanks can be provided.In accordance with the above process,a pre-cooling scheme has been designed for the cargo tanks with different compartment volumes,providing a reference scheme for the precooling operation of LNG tanks for various compartment volumes.In addition,since the spray area determines the initial temperature distribution of the cargo tank,which will directly affect the distribution of the thermal stress of the cargo tanks,it is necessary to control the spray area.Therefore,the maximum spray area that the cargo tanks of each compartment can be tolerated was calculated to provide reference for pre-cooling spray area control in precooling practical operation.