Using Fracture Mechanics to Establish Minimum Pressurization Temperatures for Pressure Equipment
Preventing brittle fracture is an essential part of establishing life-cycle management strategies for fixed pressure equipment. Using fracture mechanics principles to develop permissible minimum pressurization temperature (MPT) envelopes for components is one way to mitigate the potential for unstable flaw growth. In the refining industry, heavy-walled, low-alloy hydroprocessing reactors are designed to operate at elevated temperatures and high hydrogen partial pressures.
These components require special treatment and necessitate guidance that falls outside the bounds of current pressure vessel construction codes. This operating environment results in two factors that affect the MPT envelope: long-term temper embrittlement and hydrogen embrittlement. Additionally, hydrogen charging can manifest damage in two ways: fast (brittle) fracture due to a reduction in fracture toughness and slow (subcritical) hydrogen-assisted crack growth. When developing a MPT envelope for a typical hydroprocessing component, both failure modes need to be considered, in addition to residual stress effects from weld overlay or cladding. MPT envelopes provide insight into the permissible pressure-temperature combinations for specific locations and for chosen reference flaw sizes. From a reliability standpoint, understanding the risk of brittle fracture associated with heavy-walled reactors for all operating scenarios is crucial.
In this webinar, a fracture-mechanics-based methodology is summarized that is fully documented in Welding Research Council (WRC) Bulletin 562 to determine MPT envelopes for all components (in any service environment) based on fast fracture, with supplemental MPT requirements based on slow fracture for equipment that operates in high-pressure hydrogen environments. Finally, a finite element analysis-based case study of a 2-1/4 Cr – 1 Mo hydrotreater reactor is presented and practical life cycle-management guidance is offered based on analysis results. This example highlights how evaluation of start-up and shut-down procedures for heavy-walled reactors has the potential to save significant time and related cost per unit shut-down cycle, while maintaining an acceptable risk tolerance against subcritical crack growth and brittle fracture.
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