A Fracture Mechanics-Based Approach to Establish Minimum Pressurization Temperature Envelopes

Abstract

Preventing brittle fracture is an essential part of instituting life-cycle management strategies for fixed pressure equipment. Using fracture mechanics principles to establish 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. Components that operate in high-pressure hydrogen environments 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 given 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. Furthermore, taking the appropriate life-cycle management steps, such as establishing MPT envelopes, coupling MPT analysis predictions with targeted inspection, optimizing process operating conditions, and developing a flaw acceptance criteria to mitigate the risk of crack propagation and ultimately, brittle fracture is essential. In this study, a fracture-mechanics based methodology is summarized that is fully documented in upcoming Welding Research Council (WRC) Bulletin 562 [1] 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 summarized 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.