API TR 934-F PART 4-2018

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The Effects of Hydrogen for Establishing a Minimum Pressurization Temperature (MPT) for Heavy Wall Steel Reactor Vessels

Published By	Publication Date	Number of Pages
API	2018	112

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Category: API

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Description

Certain types of pressure equipment in refinery service experience increased risk of subcritical crack growth, as well as catastrophic failure during startup and shutdown, due to several service-aging degradation mechanisms. For example, heavy-wall reactor pressure vessels in high-temperature H2 service can be susceptible to hydrogen-assisted cracking near ambient temperature. Atomic hydrogen that dissolves in the reactor wall during elevated temperature operation remains in the steel during a shutdown cycle. Dissolved hydrogen at or near ambient temperature can result in two undesirable outcomes: 1) subcritical hydrogen-assisted crack growth and 2) unstable fast fracture due to a reduction in fracture toughness caused by hydrogen. (Dissolved H can also reduce steel tearing resistance in the resistance curve sense; however, assessment of this form of stable cracking is beyond the scope of this technical report.) The refining industry reduces the likelihood of these outcomes through adherence to a minimum pressurization temperature (MPT). The intent of the MPT is to ensure that the vessel is not subject to above-threshold primary stresses from pressure at temperatures where the material of construction is susceptible to hydrogen cracking.

Developing standard procedures for determining safe MPTs is a high priority for the API Committee on Refinery Equipment (CRE). Two parallel efforts within API have resulted in somewhat conflicting methodologies for specifying MPTs for pressure equipment. The Subcommittee on Corrosion and Materials drafted Recommended Practice 934-F, which covers heavy-wall reactors in high-temperature hydrogen service. The Joint API/ASME Committee on Fitness-for-Service (FFS), which developed and maintains the API 579-1/ASME FFS-1 standard (also known as API 579), has drafted comprehensive rules for determining the MPT in all pressure equipment that operates at elevated temperatures. Welding Research Council (WRC) Bulletin 562 contains the procedures that have been proposed for inclusion in API 579. The WRC 562 methodology differs significantly from the procedures set forth in the draft API 934-F document and supporting-final reports to the 934-F Task Group. The former is based on quantitative fracture mechanics principles pertinent to broad-based FFS assessments, with a particularly strong treatment of H-free fracture toughness, while the latter contains a science-based procedure for specifying the MPT governed by hydrogen.

The CRE has requested that the present authors prepare a technical report to provide guidance on the effects of hydrogen on fast fracture and slow stable crack growth, as they pertain to steel pressure vessels having a degradation of mechanical properties by temper and hydrogen embrittlement. The overarching objective of this report is to establish the technical basis necessary to validate the best practices for quantifying the effect of hydrogen on (a) the minimum pressurization temperature, and (b) fitness-for-service of a specific thick wall hydroprocessing reactor. These recommendations will be in a form that can be consistently adapted by the committees responsible for API 934-F and API 579.

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