Crack Failure Analysis of Hydrogenation Heat Exchanger Shell-Side Outlet Elbow - Technical Study Note

Literature Overview

This paper by Chai Keliang, Peng Qiong, Guo Zhijun, and Zhong Yanping, published in Petrochemical Equipment (Volume 32, Issue 2, 2003, pages 15-17), presents a detailed failure analysis of a hydrogenation heat exchanger shell-side outlet elbow. The authors are affiliated with PetroChina Lanzhou Petrochemical Refinery and the Lanzhou Petroleum Machinery Research Institute. The case involves a 26 mm long transverse through-thickness crack found at the weld joint between the elbow and the first reducing tee flange, with additional transverse cracks discovered on the inner wall near the weld zone after disassembly.

Background and Operating Conditions

Hydrogenation units in petroleum refineries operate under conditions that are particularly challenging for materials and weldments. The process involves high temperatures, high pressures, and the presence of hydrogen and hydrogen sulfide. These conditions create a unique environment where multiple degradation mechanisms can act simultaneously, including hydrogen attack, sulfide stress cracking (SSC), and wet hydrogen sulfide stress corrosion cracking (wet H2S SSC).

The E30 1B heat exchanger in question is a critical piece of equipment in the hydrogenation unit. The shell-side outlet elbow connects the heat exchanger shell to the downstream piping and is subject to thermal cycling, pressure fluctuations, and the aggressive chemical environment of the process stream.

Failure Description and Examination

The initial failure was identified as a transverse through-thickness crack at the weld joint between the elbow and the first reducing tee flange. The crack was 26 mm in length and penetrated completely through the wall thickness. Upon disassembly of the elbow, additional transverse cracks were found on the inner wall near the weld zone. These cracks were numerous and distributed in the heat-affected zone (HAZ) and weld metal region.

Root Cause Analysis

The failure analysis confirmed that the cracks originated as welding delayed cracks and subsequently propagated through wet hydrogen sulfide stress corrosion cracking (wet H2S SSC). This two-stage failure mechanism is particularly insidious because the initial welding delayed cracks may be small and difficult to detect, and the subsequent SSC propagation can occur rapidly under operating conditions.

Stage 1: Welding Delayed Cracks

Welding delayed cracks are a form of hydrogen-induced cracking that occurs in the heat-affected zone (HAZ) of welds, typically hours to days after welding. The mechanism involves the diffusion of hydrogen atoms into the weld zone during and after welding, where they accumulate at microstructural features such as grain boundaries, martensite lath boundaries, and inclusion-matrix interfaces. When the hydrogen concentration reaches a critical level, it causes localized decohesion and crack initiation.

The factors contributing to welding delayed cracks in this case include:

• High Hydrogen Content: The welding process introduced hydrogen into the weld zone, either from the welding consumable, the base metal, or the surrounding atmosphere.
• Hard Microstructure: The HAZ microstructure likely contained hard, martensitic phases that are susceptible to hydrogen cracking.
• Residual Stress: The welding residual stress provided the tensile stress component necessary for crack initiation and propagation.
• Susceptible Base Metal: The base metal composition may have contained elements that promote hard phase formation in the HAZ.

Stage 2: Wet H2S Stress Corrosion Cracking

Once the initial welding delayed cracks were formed, they served as initiation sites for wet H2S SSC. The wet H2S environment in the hydrogenation unit provided the aggressive chemical conditions necessary for stress corrosion cracking. The mechanism of wet H2S SSC involves:

1. Sulfide Scale Formation: H2S reacts with the steel surface to form iron sulfide (FeS) scales.
2. Hydrogen Generation: The reduction of H2S produces atomic hydrogen, which diffuses into the steel.
3. Hydrogen Embrittlement: The accumulated hydrogen weakens the metal and promotes crack propagation.
4. Stress Concentration: The existing welding delayed cracks provide stress concentration sites that accelerate SSC propagation.

The transverse orientation of the cracks is consistent with the tensile stress field in the elbow wall, which is predominantly hoop stress due to internal pressure and bending stress from thermal and mechanical loads.

Metallurgical Analysis

The metallurgical examination of the failed elbow would have involved several key analyses:

Engineering Countermeasures and Prevention

Based on the failure analysis, several countermeasures can be recommended to prevent similar failures:

1. Welding Procedure Optimization:

• Use low-hydrogen welding consumables to minimize hydrogen ingress.
• Implement preheat and interpass temperature control to reduce hydrogen diffusion and residual stress.
• Apply post-weld heat treatment (PWHT) to relieve residual stress and soften hard phases in the HAZ.
• Consider using welding procedures that produce lower hardness HAZ microstructures.

1. Material Selection:

• Select base metals with lower susceptibility to hydrogen cracking, such as materials with lower carbon equivalent (CE) values.
• Consider using corrosion-resistant alloys for critical components in wet H2S service.
• Ensure that the base metal composition meets NACE MR0175/ISO 15156 requirements for wet H2S service.

1. Inspection and Monitoring:

• Implement mandatory post-weld inspection using magnetic particle testing (MT) or ultrasonic testing (UT) to detect welding delayed cracks.
• Conduct periodic in-service inspection of weld joints in wet H2S service using appropriate NDT methods.
• Monitor hydrogen content in the weld zone during manufacturing and after service exposure.

1. Design Considerations:

• Minimize stress concentrations in the design of elbows and weld joints.
• Use fillet-welded or full-penetration welds with proper geometry to reduce stress concentration.
• Consider using long-radius elbows to reduce bending stress.

1. Environmental Control:

• Implement H2S removal or mitigation strategies in the process stream.
• Apply corrosion inhibitors to reduce the aggressive effects of H2S.
• Control operating conditions to minimize the severity of the wet H2S environment.

Reflections and Lessons Learned

This failure case illustrates the complex interplay between manufacturing defects and environmental degradation in critical process equipment. The two-stage failure mechanism, where welding delayed cracks serve as initiation sites for wet H2S SSC, highlights the importance of addressing both manufacturing quality and environmental control in preventing failures.

The case also underscores the importance of proper welding procedures and post-weld heat treatment in hydrogen service. The presence of hard, martensitic phases in the HAZ, combined with residual stress and hydrogen, creates a perfect storm for cracking. The subsequent exposure to wet H2S then accelerates the failure through stress corrosion cracking.

For engineers involved in the design, manufacturing, and operation of hydrogenation units, this case study serves as a powerful reminder that multiple failure mechanisms can act synergistically to cause equipment failure. A comprehensive approach that addresses material selection, welding quality, inspection, and environmental control is essential for ensuring the integrity of critical equipment in aggressive service environments.

Reference Value

This failure analysis provides valuable lessons for the petroleum refining industry, particularly for engineers involved in the maintenance and integrity management of hydrogenation units. The detailed investigation methodology and the identification of the two-stage failure mechanism offer a template for similar failure analyses. The recommended countermeasures are directly applicable to preventing similar failures in other hydrogenation units and can be incorporated into integrity management programs and welding procedure specifications.