| The widespread use of cryogenic liquids such as liquid oxygen,liquid nitrogen,liquid argon,and liquefied natural gas has promoted the design and manufacture of cryogenic vessels towards the direction of large-scale and diversification.As one of the key components of the pressure vessels,cryogenic pipelines are subjected to loads such as medium pressure,temperature,and gravity during service.Meanwhile,due to the displacement constraints of the thermal insulation perlite and pipe clamps,the pipeline stiffness increases,the deformation is limited and stress changes becomes more complex.Once th e stress concentration occurs,it will cause structural strength failure of the pipeline,and lead to large accidents such as fires.A 500 m~3 spherical tank with liquid oxygen as the medium is taken as the analysis object,and the structural strength of the interspace pipeline during service is investigated by numerical simulation.The dangerous working conditions and dangerous parts of the pipeline during service are determined,and the effects of some factors such as thermal insulation perlite,number and position of pipe clamps on the structural strength of the pipeline are studied.The main content are as follows.(1)The stress overload of bottom piping limited by the spherical tank interspace under different working conditions is studied.Based on the structural characteristics of the bottom pipeline,the optimization design method of pipeline flexibility is proposed.The bellows expansion joint is used as the elastic compensation device of the pipeline system to improve the pipeline flexibility and compensate the pipeline deformation.Meanwhile,transition joints are used to optimize the welding structure of the pipeline.By calculating the deformation and stress changes of the pipeline under three working conditions of liquid filling,liquid storage and liquid discharge,it’s found that compared with the situation before optimization,the maximum reduction in the stress extreme value of the pipeline elbow is 75.24%,33.05%and 44.27%respectively.The stress extreme value of the pipeline transition join t is reduced by 61.93%,30.85%and 39.85%,respectively.The structural optimization method effectively reduces the thermal stress of the pipeline and improves the safety margin of pipeline structure.(2)The influence of the number and location of pipe clamps on the stress and deformation of the interspace top pipeline is explored.The results show that the maximum displacement deformation of pipeline decreases with the increase of the number of pipe clamps,while the extreme value of pipeline stress increases.The pipe clamp near the root of the pipeline directly affects the maximum displacement deformation of the pipeline,and the pipe clamp near the elbow directly affects the stress level of the pipeline.In the case that the position of the pipe clamps at both ends of the pipeline is determined,the equal interval arrangement of the pipe clamps in the middle section of the pipeline can more stably fix the pipeline and reduce the maximum displacement deformation.Based on the influence of the number and position of pipe clamps on the structural strength of the pipeline,a reasonable arrangement of pipe clamps is proposed as follows:the number of pipe clamps is 4,the angle between the pipe clamps at both ends of the pipeline and the central axis of the spherical tank is 30°,and the pipe clamps in the middle section of the pipeline are arranged at equal intervals.(3)Based on the Janssen hypothesis and the theory of granular mechanics,the influence mechanism of thermal insulation perlite on the stress a nd deformation of interspace pipeline is revealed.An optimized structural model for the top and bottom pipelines of spherical tank with thermal insulation perlite is established,and the influence of thermal insulation perlite on the structural strength,the reaction force and moment of force of the pipeline root is analyzed.The results show that the stress extreme values of the pipeline model with thermal insulation perlite at the top under the liquid filling and liquid discharge conditions are increased by 110.17%and 125.58%respectively compared to the model without thermal insulation perlite,and the maximum displacement deformation of the pipeline is decreased by 89.85%and 86.85%,respectively.Compared with the model without thermal insulation perlite,the stress extreme values of optimized insulation perlite model of the bottom piping are increased by 160.78%,57.84%and 21.65%under the liquid filling,liquid storage and liquid discharge working conditions respectively,and the maximum displacement deformation is decreased by 0.58%、22.78%and 27.36%,respectively.Meanwhile,considering the insulation perlite,the existence of the insulation perlite can effectively reduce the deformation of the pipeline.However,due to its displacement constraint on the pipeline,the secondary stress in the discontinuous position of the structure increases sharply,which enhances the stiffness of the pipeline and results in a decrease in structural flexibility.As a result,it leads to a significant increase in the reaction force and torque at the root of the pipeline compared to the model without thermal insulation perlite.(4)By adjusting the number and position of the pipe clamps,the variation patterns of various frequencies in the pipeline are studied by using modal analysis.The results show that the inherent frequency of the same order modal of the pipeline increases with the augment of the number of pipe clamps and modal order.When the internal pressure load of the pipeline and the number of pipe clamps increase,the increase of the inherent frequency of the pipeline becomes more significant.There is a difference in the impact of the position of the pipe clamps at both two ends of the pipeline on the inherent frequency.In contrast,adjusting the position of the pipe clamps near the root of the pipeline can better change the inherent frequency of the pipeline.(5)Taking the reasonable arrangement scheme of pipe clamps for the top pipeline of the spherical tank as an example,the models with and without thermal insulation perlite are established and modal analysis is carried out.The results show that the vibration mode amplitude of the pipeline in the model without thermal insulation perlite relatively large in the low and middle order modes,while the amplitude of the pipeline vibration mode change gradually weakens in the high order modes.The vibration mode amplitude of the pipeline in the model with thermal insulation perlite is relatively small,and the maximum displacement and deformation of the pipeline is reduced significantly compared with that of the model without thermal insulation perlite.Due to the increase in pipeline stiffness caused by the insulation perlite,the inherent frequency of the pipeline in the model with thermal insulation perlite i s significantly higher than that in the model without thermal insulation perlite in the same mode.Under the action of seismic loads,compared to the model without thermal insulation perlite,the stress extreme values of the pipeline in the model with thermal insulation perlite under the filling and discharge conditions are reduced by 57.97%and 62.07%,and the maximum displacement deformation are reduced by 90.96%and 90.00%respectively.The existence of thermal insulation perlite effectively reduces the stress and deformation of the pipeline at the top of the spherical tank under the action of seismic load. |
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