Study On Structural Strength Of Main Cryogenic Heat Exchanger In Bog Reliquefaction System

Tube and shell heat exchanger is the most important heat transfer equipment which widely used in the petroleum and chemical industies. The U-tube heat exchanger is the key equipment in a multiuse reliquifaction cargo ship. Under low temperature, the structure strength is influenced by thermal stress. The structure analysis of its important parts has great significance for the equipment safety operation.Combined the design standards and the finite element analysis method, the tube-tubesheet-flanges-shell 3-D finite element model is established in the ANSYS software, and the loads and constrants are set according to the practical conditons. On basis of this model, the temperature field analysis and the stress analysis under starting up, operating and shutdown conditions are done.The results of temperature field analysis indicates: the temperature gradient of the tubesheet perforated region closed to shellside is very high, but from the tubeside surface, the temperature of most tubesheet parts in the axial direction is closed to the tubeside fluid temperature, and the temperature gradient change of this region is small; the temperature of the unperforated annulus of the tubesheet changes linearly in the tubesheet axial direction.The temperature distribution of the tubes are influenced by the tubesheet.The temperatue change is gradual for the tubes closed to the perforated tubesheet edge, but the temperature gradient at the region connected to the tubesheet close to the shellside is very high. For the shellside flange, at the limited regions close to tubesheet the temperature change is steep, and the outside fillet region is another high gradient.The structural strength analysis under 7 conditions indicates the most dangerous condition is when temperature load and the shellside pressure load act on the exchanger. The temperature load leads to huge thermal stress is the determinant factor of the model stress distribution. The shellside pressure amplified the stress condition and the tubeside weakened the stress condition on the model slightly, but generally the pressure loads influence little to the structural stress when exerted individually. As long as the temperature load exists on the model, the maximum stress intensity of tubes occurs on the inside surface at the connected section of tube and shellside tubesheet, the stress intensity at flange fillet is relatively high. The pressure loads weakened the thermal stress of flanges.The stress intensity assessment shows that the dual function of temperature and shellside pressure makes the stress intensity of tubesheet exceeds triple allowable stress, which means the structure strength not safe at this condition. In order to improve the deteriorated condition, the optimized tubesheet is of 90% of its original thickness, and the stress assessment satisfied the standards.