A qualitative risk-based assessment procedure was developed to determine the relative probability of failure for high temperature hydrogen attack (HTHA) of C-1/2Mo steel. It is well documented that C-1/2Mo possesses a variable resistance to HTHA. Over the years, the Nelson curve limits, as published in API RP 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants, have been significantly reduced since originally published. This has created a need for an assessment procedure to evaluate C-1/2 Mo equipment that was designed many years ago when the Nelson curve was at much higher operating conditions than today. This assessment procedure is based on the current ExxonMobil assignment of C-1/2Mo resistance to HTHA on the Nelson curve and the relative dependence of HTHA on temperature, hydrogen partial pressure and time. We use the Pv relationship that appears in literature to define the relative dependence of HTHA on temperature, hydrogen partial pressure and time. This qualitative assignment of probability levels for HTHA was validated against actual HTHA failures of C-1/2Mo in high temperature hydrogen service that have been reported to the API and other internal Company incidents not reported to API. This assessment procedure is used to qualitatively define the probability of failure consistent with the probability levels defined in the ExxonMobil risk matrix which is used to manage all risk based decisions in the Corporation.

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