Failure Analysis of Elbow Rupture in Prehydrotreating Unit Piping

Literature Overview

The study by Li Hua and colleagues, published in Physical Testing and Chemical Analysis in 2021, presents a comprehensive failure investigation of a pipeline elbow rupture in the prehydrotreating unit of a catalytic reforming facility. The research, funded by the Guangzhou Municipal Market Supervision and Administration Bureau, employs a multi-methodological approach combining macroscopic examination, wall thickness measurement, chemical composition analysis, mechanical property testing, metallographic examination, fractographic analysis, and X-ray diffraction (XRD) to establish the root cause of the failure. The investigation was conducted by the Guangzhou Special Pressure Equipment Inspection and Research Institute, a recognized authority in pressure equipment failure analysis.

Failure Background and Operating Conditions

The prehydrotreating unit is a critical section of catalytic reforming operations where hydrocarbon feedstocks are treated with hydrogen to remove impurities such as sulfur, nitrogen, and metals before the reforming reactor. The operating environment is characterized by:

The rupture occurred at the outer arc of the elbow, which is the region subjected to the highest centrifugal forces during fluid flow and consequently experiences the most severe erosion-corrosion interaction.

Methodology and Technical Findings

Macroscopic Examination and Wall Thickness Measurement

The macroscopic examination revealed a localized rupture at the outer arc of the elbow. Wall thickness measurements around the circumference of the elbow showed significant non-uniform thinning:

The severe and localized thinning pattern is characteristic of erosion-corrosion rather than uniform chemical corrosion. The outer arc of a 90° elbow experiences the highest fluid velocity and the greatest centrifugal force acting on the fluid, which drives the fluid against the outer wall and accelerates the removal of protective corrosion products.

Chemical Composition and Microstructural Analysis

The chemical composition analysis confirmed that the base material met the specified requirements for the grade of carbon steel used in the piping system. The mechanical property tests revealed that the tensile strength and yield strength of the remaining material at the failure zone were within acceptable limits, indicating that the rupture was not due to material degradation or overheating.

Metallographic examination of the cross-section at the failure zone revealed:

• A distinct corrosion layer on the inner surface, approximately 0.5-1.2 mm thick
• A transition zone between the corrosion layer and the unaffected base metal
• The corrosion layer consisted of iron oxides, sulfides, and chlorides as confirmed by XRD analysis
• The microstructure of the unaffected base metal showed normal ferrite-pearlite morphology

Fractographic Analysis

Scanning electron microscopy (SEM) examination of the fracture surface revealed:

• A mixed fracture mode combining ductile and brittle features
• The outer portion of the fracture surface (corresponding to the outer arc of the elbow) showed features consistent with overload fracture due to insufficient remaining wall thickness
• The inner portion showed evidence of corrosion product spallation and surface initiation
• No evidence of hydrogen embrittlement or stress corrosion cracking was observed

XRD Analysis of Corrosion Products

X-ray diffraction analysis identified the following corrosion products:

Root Cause Determination

The investigation concluded that the elbow failure resulted from a synergistic erosion-corrosion mechanism:

1. Primary corrosion: The acidic aqueous environment (HCl + H2S + NH3 + H2O) caused continuous electrochemical corrosion of the carbon steel surface.
2. Erosion mechanism: The high-velocity fluid flow at the outer arc of the elbow mechanically removed the protective corrosion product layer before it could mature into a stable, protective film.
3. Synergistic interaction: The removal of corrosion products exposed fresh metal to the corrosive environment, accelerating the corrosion rate. The corrosion products that remained were loose and non-adherent, providing no protection against further attack.
4. Wall thinning: The combined effect of corrosion and erosion caused severe wall thinning at the outer arc, reducing the effective wall thickness to approximately 2.1 mm from the original 8.0 mm.
5. Final rupture: When the remaining wall thickness could no longer sustain the internal pressure, a localized rupture occurred at the thinnest point.

Engineering Implications and Preventive Measures

This failure case has significant implications for the design and operation of prehydrotreating unit piping systems:

Design Considerations

• Material selection: Higher-grade materials such as duplex stainless steel (e.g., UNS S31803) or nickel-based alloys should be considered for elbows in prehydrotreating service where H2S and HCl coexist in aqueous environments.
• Corrosion allowance: The standard corrosion allowance of 1.5-3 mm is grossly inadequate for this service. A corrosion allowance of 6-8 mm or more should be specified, or alternatively, the design should incorporate periodic wall thickness monitoring and planned replacement.
• Bend radius: Increasing the bend radius from 1.5D to 3D or greater reduces the fluid velocity at the outer arc and can significantly reduce erosion-corrosion rates.

Operational Measures

• Flow velocity control: Limiting the fluid velocity in elbows to below 2 m/s for two-phase flow conditions can reduce erosion-corrosion damage.
• Flow pattern management: Ensuring proper flow regime (avoiding slug flow) through proper pipe routing and elevation changes can reduce erosion at elbows.
• Corrosion inhibitor injection: Continuous injection of corrosion inhibitors designed for acidic H2S environments can slow the corrosion rate.
• Wall thickness monitoring: Implementing a systematic ultrasonic thickness measurement program with increasing frequency at known erosion-prone locations.

Inspection Strategy

A recommended inspection strategy for prehydrotreating unit elbows includes:

Study Insights and Implications

This failure analysis provides a textbook example of how environmental factors, flow dynamics, and material behavior interact to produce unexpected and severe damage in pressure piping systems. The key lesson is that the design of process piping elbows must not be treated as a simple pressure vessel calculation; the dynamic interaction between fluid flow and material corrosion must be explicitly considered in the design phase. The synergistic erosion-corrosion mechanism identified in this study is particularly insidious because it produces localized damage that may not be detected by routine inspection unless specific attention is paid to the outer arc of elbows in aggressive service. Engineers designing piping for prehydrotreating and similar services should adopt a risk-based approach that incorporates material selection, geometric optimization, operational controls, and systematic inspection to manage the inherent corrosion-erosion risks.