Zhuojin Pipe Fitting Co., LtdFailure Analysis of LMPH Module Inlet Elbows in Ethylene Cracking Furnace Convection Section
Literature Overview
The paper by Chen Binghua, published in Petrochemical Corrosion and Protection in 2023, presents a detailed failure analysis of inlet elbows in the Low Margin Process Heat (LMPH) module of the convection section of Ethylene Cracking Furnace No. 2 at a petrochemical company. The failure was attributed to stress corrosion cracking (SCC) caused by both sulfide and chloride species. The study employed macroscopic examination, chemical composition analysis, corrosion product analysis, and scanning electron microscopy (SEM) to determine the root cause and proposed material upgrade and welding process improvement as remedial measures.
Technical Background
Ethylene cracking furnaces are among the most demanding equipment in the petrochemical industry. The convection section, which recovers heat from the flue gas, operates at temperatures ranging from 300°C to 600°C depending on the location within the section. The LMPH module is designed to extract low-grade heat from the flue gas, typically at the lower temperature end of the convection section. The inlet elbows of this module are subjected to a combination of thermal cycling, mechanical stress, and corrosive species from the process gas and flue gas.
The operating environment in this location is particularly aggressive:
- Sulfur-containing compounds from the feedstock decompose and deposit as sulfide species on the tube surfaces
- Chloride ions from catalyst residues or process additives can accumulate in condensate
- Thermal cycling during startup and shutdown produces cyclic mechanical stresses
- Condensation of acidic species at cooler sections creates a highly corrosive environment
Failure Analysis Methodology and Findings
The investigation followed a systematic approach:
Macroscopic Examination
The failed elbows exhibited characteristic cracking patterns consistent with stress corrosion cracking. The cracks were observed to initiate at the weld heat-affected zone (HAZ) and propagate in a direction perpendicular to the maximum tensile stress. The cracking was predominantly intergranular, indicating that the microstructural features of the HAZ played a significant role in crack initiation and propagation.
Chemical Composition Analysis
The analysis confirmed that the base material met the specified chemical composition. However, the HAZ showed localized enrichment of sulfur and chlorine, indicating selective corrosion of the microstructural features in this region. The weld metal composition was also analyzed and found to be within acceptable limits.
Corrosion Product Analysis
X-ray diffraction (XRD) analysis of the corrosion products identified iron sulfide phases (FeS, Fe₃S₄) and iron chloride phases (FeCl₂, FeCl₃). The presence of both sulfide and chloride corrosion products confirms that both sulfide stress corrosion cracking (SSCC) and chloride stress corrosion cracking (ClSCC) mechanisms were active.
Scanning Electron Microscopy
SEM examination revealed the following features:
- Intergranular cracking with secondary branching, characteristic of SCC
- Presence of corrosion products within the crack voids
- Evidence of hydrogen embrittlement at the crack tips
- Preferential attack along grain boundaries in the HAZ
| Analysis Method | Key Finding |
|---|---|
| Macroscopic examination | Cracks initiated at HAZ, perpendicular to max stress |
| Chemical composition | S and Cl enrichment in HAZ region |
| Corrosion product analysis | FeS, Fe₃S₄, FeCl₂, FeCl₃ identified |
| SEM | Intergranular cracking with secondary branching |
| EDS | Sulfur and chlorine concentrated at crack sites |
Root Cause Determination
The failure was attributed to the combined action of sulfide and chloride stress corrosion cracking. The mechanism can be described as follows:
- Stress establishment: Residual stresses from welding, combined with thermal cycling stresses and mechanical loads from the piping system, created a tensile stress state at the HAZ.
- Corrosive environment formation: Sulfur and chloride species from the process gas deposited on the internal surface of the elbows. During periods of reduced temperature or shutdown, condensation of these species created a highly corrosive liquid film.
- Crack initiation: The combination of tensile stress and corrosive environment initiated intergranular cracks at susceptible microstructural features in the HAZ, particularly at prior austenite grain boundaries and carbide-matrix interfaces.
- Crack propagation: The cracks propagated in an intergranular fashion, with secondary branching characteristic of SCC. The crack growth rate was accelerated by the cyclic nature of the thermal and mechanical loading.
Remedial Measures and Recommendations
The author proposed the following measures:
- Material upgrade: Replace the existing material with a more resistant grade, such as a duplex stainless steel or a nickel-based alloy with improved SCC resistance.
- Welding process improvement: Reduce residual stresses by optimizing welding parameters, including lower heat input, increased preheat temperature, and post-weld stress relief treatment.
- Corrosion protection: Implement enhanced corrosion protection measures, including improved insulation to prevent condensation, periodic chemical cleaning, and corrosion inhibitor injection.
- Monitoring and inspection: Establish a regular inspection program using eddy current testing and ultrasonic testing to detect early-stage SCC before failure occurs.
Engineering Practice Implications
This failure case is instructive for several reasons. First, it demonstrates that even in equipment designed for specific service conditions, unexpected combinations of environmental factors can lead to premature failure. Second, it highlights the importance of considering both sulfide and chloride SCC mechanisms when evaluating the integrity of equipment in petrochemical service. Third, it underscores the critical role of the HAZ as a preferential site for SCC initiation, due to its altered microstructure and residual stress state.
In my experience, this type of failure is not uncommon in ethylene cracking furnace convection sections, particularly in areas where condensation can occur. The key to prevention lies in a comprehensive approach that addresses all contributing factors: material selection, welding quality, corrosion protection, and inspection. The failure analysis methodology presented in this paper—combining macroscopic examination, chemical analysis, corrosion product identification, and microstructural characterization—is the gold standard for SCC investigations and should be followed in all similar cases.
Study Reflections
This paper is a valuable contribution to the field of petrochemical equipment integrity. The systematic approach to failure analysis, the clear identification of the dual SCC mechanism, and the practical remedial recommendations make it highly useful for practicing engineers. The case also serves as a reminder that material selection for high-temperature service must consider not only mechanical properties and corrosion resistance but also susceptibility to environmental cracking mechanisms. The combination of sulfide and chloride SCC is particularly insidious because the two mechanisms can interact synergistically, with sulfide-induced cracking providing pathways for chloride ingress and vice versa. Engineers designing and maintaining ethylene cracking furnace systems should take this case as a cautionary example and ensure that all contributing factors are addressed through a systematic integrity management program.