Damage Control: Wet H2S Damage Assessment

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

As discussed in the previous installment of Damage Control, wet H₂S damage mechanisms, including hydrogen blistering, hydrogen induced cracking (HIC), stress-oriented hydrogen induced cracking (SOHIC), and sulfide stress cracking (SSC) incite many equipment reliability, maintenance, and safety issues across numerous process units in the oil refining and related industries. Furthermore, several different inspection methods (visual, surface, and volumetric) are often utilized to proactively identify and characterize these damage mechanisms in critical pressure equipment. Once damage has been detected and accurately described (e.g., location, size, extent, orientation, etc.), knowing how to evaluate the observed damage and qualify it using fitness-for-service (FFS) techniques without the need for costly equipment repair or replacement can be invaluable (lost production can also be minimized). This issue of Damage Control offers a practical perspective on how to assess the different forms of wet H₂S damage using modern FFS and computational analysis techniques, with the ultimate goal of justifying the continued safe operation of damaged pressure vessels, piping, and associated components.

Historical Overview of Wet H₂S Damage FFS Methods

Part 7 of API 579-1/ASME FFS-1, Fitness-For-Service (API 579) addresses the evaluation of hydrogen damage in equipment ^[1]. High-temperature hydrogen attack is not covered in Part 7, and the assessment procedures are limited to carbon and low-alloy equipment operating below 400°F (204°C). Some of the background information related to the development of the Part 7 assessment procedures is summarized herein ^[2-4]. Specifically, Part 7 covers assessment of hydrogen blisters, HIC, and SOHIC, and addresses two different types of failure modes associated with these damage mechanisms as follows:

• Protection against Plastic Collapse:
• Characterized as gross plastic deformation (yielding) and eventual rupture/loss of containment.
• Failure due to insufficient material strength to accommodate internal pressure loading or other sustained supplemental (primary) loads such as dead weight.
• Can be evaluated using closed-form calculations or computational methods such as elastic stress analysis, limit load analysis, or non-linear elastic-plastic analysis.
• Protection against Brittle Fracture:
• Defined as sudden unstable crack propagation that usually exhibits little-to-no ductility or plastic deformation.
• Analysis anchored in the use of the Failure Assessment Diagram (FAD) in Part 9 of API 579 (see Figure 1) – considers stress (applied and residual), crack geometry (length, depth, and orientation), and material fracture toughness ^{[1, 5–7]}.
• Often limits evaluation of hydrogen damage per Part 7 of API 579, especially for equipment that was not subject to post weld heat treatment (PWHT).

The first edition of API 579 (published in 2000) included rules for hydrogen blistering but did not offer guidance related to the evaluation of HIC damage ^{[2, 8]}. Furthermore, at that time in the industry (early 2000s), it was relatively common for engineers and practitioners to assume any areas containing HIC were 100% damaged; that is, the material was assumed to have no load carrying capacity. Given this, utilizing the local thin area (LTA) rules offered in Part 5 of API 579 (2000 Edition) represented a means to conservatively evaluate protection against plastic collapse. Practical experience suggests employing this assumption is likely overly conservative in many cases. This operating experience in conjunction with laboratory testing carried out by the Materials Properties Council (MPC) led to industry support of new FFS rules, where damaged regions are credited with some remaining strength ^[9]. To this end, the 2007 Edition of API 579 included updated guidance for the evaluation of HIC that also required fracture mechanics-based crack-like flaw assessments, as well as strength checks, to quantify protection against plastic collapse. This methodology is the basis of the current FFS procedures in Part 7 of API 579 ^[1].

Currently, Part 7 of API 579 does not specifically address SSC damage (discrete singular cracks); however, the fracture mechanics principles of Part 9 can be leveraged to determine critical flaw sizes in equipment at specific locations (e.g., seam welds or attachment welds) ^[1]. These critical flaw sizes can then be used as an acceptance criterion for crack-like flaws. Note that careful consideration needs to be given to potential crack growth rates associated with SSC or HIC/SOHIC. Since there are many unknowns associated with SCC and HIC/SOHIC damage progression, often due to non-steady state conditions, scrupulous inspection plans are often required in conjunction with a detailed FFS assessment performed by an experienced engineer. Additionally, evaluation of laminations (not in hydrogen-charging service) is covered in Part 13 of API 579 ^[1]. Laminations are defined as planes of non-fusion in the interior of a steel plate that result from the steel manufacturing process and exist on one or more planes; however, they cause no bulging of the metal surface, have no cracking in the through thickness direction, and are not linked ^[1]. While laminations may serve as damage initiation sites for hydrogen blistering or HIC (notably in older steels), in the absence of these forms of wet H₂S damage or hydrogen charging, they are usually not problematic from an equipment reliability standpoint, especially if they appear remote from major structural discontinuities (e.g., nozzles and other structural attachments) and exhibit no through-wall geometry or crack-like flaw propagation.