Hydrogen-Induced Cracking Failure Analysis of Reducer in Heavy Oil Hydrocracking Cold High-Pressure Separator Bottom Water Discharge Line

Literature Overview

Liu Xiaohui, Zheng Wenlong, and Wang Jianjun from Sinopec Maoming Branch Equipment Monitoring and Research Center and Shanghai Institute of Materials published their failure analysis in Corrosion and Protection (2002, Vol. 23, Issue 1, pp. 30–32). The study investigates cracking in a reducer (大小头) at the bottom water discharge line of a cold high-pressure separator (冷高压分) in a heavy oil hydrocracking (渣油加氢) unit. This is a sour service environment characterized by high hydrogen sulfide (H₂S) content and high operating pressure, conditions known to be extremely challenging for carbon steel materials.

Service Environment and Failure Conditions

The cold high-pressure separator bottom water discharge line in a heavy oil hydrocracking unit operates under the following conditions:

Failure Analysis Findings

The comprehensive analysis of the cracked reducer revealed that the failure mechanism was hydrogen-induced cracking (HIC), also known as blistering or hydrogen blistering, occurring in a sour service environment. The key finding was that even with a carbon steel sulfur content as low as 0.003% (mass fraction), HIC cracking could still occur under the severe conditions of high H₂S content and high pressure.

HIC Mechanism

The hydrogen-induced cracking mechanism in this service involves several steps:

1. H₂S Dissociation: H₂S dissolved in the aqueous phase dissociates, releasing atomic hydrogen (H).
2. Hydrogen Absorption: Atomic hydrogen penetrates the steel matrix through defects, inclusions, and dislocations.
3. Hydrogen Accumulation: Hydrogen atoms accumulate at interfaces between the steel matrix and non-metallic inclusions (primarily manganese sulfide, MnS).
4. Blister Formation: Accumulated hydrogen recombines to form molecular hydrogen (H₂), creating internal pressure that lifts the matrix, forming blisters.
5. Crack Propagation: Adjacent blisters coalesce to form internal cracks, which can link up to form through-thickness cracks.

Role of Sulfur Content

The finding that HIC can occur even at 0.003% sulfur content is significant. While lower sulfur content reduces the number of MnS inclusions (the preferential sites for hydrogen trapping), it does not eliminate the risk entirely. Other inclusions, such as silicates and alumino-silicates, can also serve as hydrogen trapping sites. Additionally, the welding heat-affected zone (HAZ) of the reducer, if it is a welded fitting, may have altered microstructures that are more susceptible to HIC.

Standards and Material Requirements

The failure case has direct implications for material specification in sour service. The relevant standards and their requirements include:

The NACE MR0175 standard specifies several approaches to HIC resistance in carbon steel:

• Clean steel: Sulfur content ≤ 0.003% (as demonstrated in this case to be insufficient alone)
• Thermal treatment: Normalizing or normalizing plus tempering to spheroidize MnS inclusions
• Hydrogen recombination coatings: Surface coatings that catalyze H recombination before hydrogen ingress
• Post-weld heat treatment: To relieve residual stresses and improve microstructure

Engineering Practice Integration

This failure case has several important implications for engineering practice in sour service environments:

1. Material selection: Clean steel alone is not sufficient for HIC resistance in severe sour service. A combination of low sulfur content, appropriate thermal treatment, and possibly surface treatment is required.
2. Welding considerations: The HAZ of welded reducers is particularly susceptible to HIC due to the presence of coarse-grained regions, residual stresses, and potential microstructural variations. Welding procedures should be designed to minimize HAZ susceptibility, and post-weld heat treatment should be considered.
3. Inspection and monitoring: HIC cracks are internal and difficult to detect by conventional external inspection methods. UT testing (particularly phased array UT) is the preferred method for HIC detection. Regular UT surveys of reducers and other critical fittings in sour service are essential.
4. Risk assessment: The failure of a reducer in a bottom water discharge line, while not a catastrophic failure of the main process vessel, indicates that the entire sour service system may be at risk. A comprehensive HIC risk assessment of all carbon steel components in the unit is warranted.

Study Insights and Implications

This failure analysis highlights the complexity of HIC resistance in sour service. The fact that even clean steel (0.003% S) can fail under severe H₂S conditions is a sobering reminder that material selection for sour service requires a holistic approach that considers the entire environment, not just a single material parameter.

From a metallurgical perspective, the HIC mechanism is a textbook example of how hydrogen embrittlement manifests in steel. The role of inclusions as hydrogen trapping sites, the effect of microstructure on hydrogen diffusivity and trapping, and the influence of residual stress on crack propagation are all critical factors that must be considered in material selection and component design.

For the heavy oil hydrocracking industry, this case underscores the need for rigorous material qualification, comprehensive HIC testing (per API 933), and ongoing integrity monitoring of all sour service components. The reducer, as a geometric discontinuity, may also experience higher local stresses that accelerate HIC crack propagation, making it a particularly vulnerable component in the system.