A procedure was developed to establish the minimum pressurization temperature (MPT) for Cr-Mo reactor vessels greater than 2-inches thick in high temperature hydrogen service. The procedure is consistent with a Level 2 assessment as defined in Part 3 of API RP-579 and accommodates the potential effects of temper embrittlement and hydrogen assisted fracture. Temper embrittlement effects can be accounted for by use of typical fracture data for Cr-Mo steels of similar vintage as used in the construction of the reactor vessel in question or by using data from actual reactor heat samples exposed in the reactor over time. The potential for hydrogen assisted fracture can be evaluated for specific vessel situations by estimating residual hydrogen levels in the vessel steel after shutdown. At very low levels of hydrogen (2 ppm and lower) hydrogen assisted fracture should not be a concern. An envelope for the reactor operating temperature and hydrogen partial pressure conditions is defined below which hydrogen assisted cracking should not be a concern. It should be noted that a more detailed Level 3 assessment as defined in Part 3 of API RP-579 can be performed to further refine the assessment if more material toughness data, flaw size information and/or stress information is available on the vessel. This procedure has been used within our company to safely start-up and shutdown heavy wall Cr-Mo vessels.

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