We were asked to perform a failure analysis on 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.
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.