Research On Design Of Thermo-Expanded Piping-Vessel System

Pressure vessels are mostly connected through piping to form an operating system.The piping is often in a certain high-temperature environment due to the medium transported,and the thermo-expanded piping will act on the connected vessel nozzle certain additional load.At present,there are traditional partition method and integrated method for the design of the thermo-expanded piping-vessel system,and stress classification method is often used in the analysis and design of pressure vessel.The limitations of the above design methods make the design results sometimes on the conservative side and sometimes on the unsafe side.In this paper,the design method of the thermo-expanded piping-vessel system and the assessment of vessel strength are studied by theoretical analysis and finite element method.The main research contents and conclusions are as follows:(1)The additional load imposed on the vessel nozzle by the onedimensional thermo-expanded piping is analyzed by theoretical derivation,and the mechanism of the elastic follow-up effect of the piping and its influence on connected vessels are analyzed by theoretical analysis and finite element calculation based on the simplified two-bar model of the thermo-expanded piping-vessel system.It is found that the thermal expansion load of the piping calculated under the fixed support condition is independent of the pipe length,while in the same temperature rise of the piping,the longer the pipe,the greater the load imposed on the vessel nozzle and the closer to the load calculated under the fixed support condition.The follow-up effect of the elastic piping can cause"strain concentration" when plastic deformation is allowed to occur in the vessel.(2)Taking the system composed of the central nozzle of standard elliptical head and one-dimensional thermo-expanded piping as an example,the differences of design results obtained by different design methods under different lengths of external piping are studied by the linear elastic analysis of the traditional partition method and integrated method;The accuracy of design results obtained by different stress classification methods under different lengths of external piping is verified by the elastic-plastic calculation of the integrated model under cyclic loading.The results show that the traditional partition method ignores the flexibility of the vessel nozzle,and the design results are conservative.When the vessel stress caused by the thermal expansion of piping is evaluated according to the primary stress,the design is conservative.When the piping is long and the thermal expansion is large,the design is unsafe according to the secondary stress.(3)Based on the traditional partition method and the integrated method,the load release ratio is newly defined and the load release ratio method for the design of the thermo-expanded piping-vessel system is proposed.The reliability and applicability of this method under different structures and loads are verified by the finite element analysis of a system composed of the typical pressure vessel nozzle and different thermo-expanded piping.(4)In view of the limitations of the stress classification method for the assessment of vessel stress caused by the thermal expansion of piping,the ultimate load and the stabilizing load of the central nozzle of the standard ellipsoidal head under the axial piping load were calculated by the direct method and compared with the allowable nozzle load obtained from the linear elastic analysis to analyze the advantages of the direct method applied to the strength assessment of vessel under the action of thermo-expanded piping.