We performed failure analysis on a cracked heat exchanger for NASA. A number of cracks were found around the bolt holes of the inlet and outlet tubesheets of a heat exchanger at NASA Glenn Research Center. We performed a stress analysis on a proposed plan by NASA for repairing the inlet and outlet of the heat exchanger cracked tubesheets, which involved removal of all the flange bolt holes and welding the heads directly to the tubesheets. Different head to tubesheet connections were proposed for the inlet and outlet sides. A finite element analysis was used to simulate the thermal and structural responses of the heat exchanger tubesheets after completion of the repair, when subjected to operating conditions.
ANSYS was used to perform the thermal and structural analysis. Two detailed three-dimensional models of the inlet and outlet tubesheets were constructed. Each was modelled separately, each one including the tubesheet, tubes, head and the shell. Due to the symmetry of the tubes in the tubesheets, only a 30-degree segment of each tubesheet was modeled. Sufficient length of the tubes, head and the shell were included in the models to account for their stiffness.
It was determined that although the heat exchanger was originally designed and constructed to ASME Section VII Division 1 of the ASME Code, the resulting stresses did not meet the allowable stress limits of the Code. We made design modifications to enable the heat exchanger to meet the Code.