We were asked to investigate the root cause of cracked tube to tubesheet welds on a chemical reactor. This included the startup, controlled heatup and cool-down processes on the reactor with respect to the potential for creating leaks through the tube to tubesheet welds.
Finite element analysis was used to simulate the (transient) thermal and structural responses of the reactor tubes, tubesheets, perforated plate rim, and tube-to-tubesheet welds during the controlled heat-up and cool-down process.
The rate of 8 deg. F/hr was first used for heat-up and cool-down for both air and salt temperatures. The heat-up and cool-down processes, as outlined below, were analyzed in thermal analyses. The thermal results were then used at various time steps to calculate stresses in the tube-to-tubesheet weldments. These stresses were not sufficient to cause leak paths in the weldments. Once acceptable stresses were achieved at a rate of 8 deg. F/hr, the rate was progressively increased and evaluated until a rate of 14 deg. F/hr was reached, and the stresses remained well within acceptable limits even for weldments with considerable porosity.
ANSYS was used to perform transient thermal and structural analyses. To analyze the reactor, two finite element models were constructed. The first model was used for thermal interaction of the perforated areas and the rim of the tubesheet. The thermal analysis from this model provided time varying temperature information between the perforated part of the tubesheet and the average tubesheet rim.
The second model is a three-dimensional representation of the tube, tubesheet, and weld. Due to the repetitive pattern of the tubes in the tubesheet, only a 30-degree segment of the tubesheet was modeled. Symmetry planes were imposed on the model at 0 and 30 degrees to account for the rest of the tubesheet assembly. Sufficient length of the tube, tubesheet, and weld were included in the model to account for their stiffness and overall contribution to the system. The effects of the thermal lag of the tubesheet rim compared to the perforated portion of the tubesheet was taken into consideration using the effective flexibility obtained from the first FEA model.
The controlled heat-up process was modeled from an ambient temperature of 70 F. Conservatively, heated air at 203 F was inserted into the tubes within half a minute. This insertion of preheated air covers a thermal transient that the reactor could experience at this step in the process. The air temperature was then ramped from 203 F to 400 F at a given rate. Next the salt was introduced into the reactor at 400 F. The salt and air temperature are then given an hour to stabilize at 400 F. Finally, the salt temperature was ramped from 400 F to 608 F at a given rate.
The Von Mises stresses and stresses in the radial direction from the center of the tube were determined for each analysis. These stresses are important in evaluating the potential for creating leak paths through the weldments.
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