Thermal Shock Failure Analysis and Remediation Design of an Internal Catalyst Structure

The catalyst structures of interest are internals for four identical cogeneration units which together produce about one half million kilograms per hour of steam for an adjacent refinery and 385 megawatts of electricity. They are structural steel frames made of 304SS with approximate dimensions of 42 feet high by 38 feet wide by 8 feet deep. During start up of the heat recovery steam generator (HRSG) units, hot combustion gases from the gas turbine exhaust flow through the HRSG units producing rapid heating of the interior components. However, because the catalyst structure internals are not pressure containing like the HRSG units, they were designed using structural codes that do not consider thermal loading. Cracking was observed along leading edges of structural members and at the connections of cross bracings. A transient thermal analysis was completed to calculate temperature distributions as a function of time at the critical locations. Thermal stress calculations were then prepared and stress values compared to fatigue curves for the material at temperature. It was found that computed thermal stress values were sufficient to initiate and propagate cracks consistent with those observed. Several remediation options were considered before two were selected and design changes were detailed. The remediation options included removing some of the bracing after a detailed seismic analysis showed that they were not required and adding refractory materials to leading edges to minimize the thermal gradients generated during start up of the HRSG units. Thermocouple data collected at remediation locations and unchanged areas during start up supported the hypothesized failure analysis cause and the benefit of the remediation design changes.