Zhuojin Pipe Fitting Co., LtdThinning Failure Analysis of Tee at Overhead Outlet of Hydroprocessing Unit Stripping Column
Literature Overview
This paper, published in Petroleum and Chemical Equipment (2024, Vol. 27, No. 1), presents a detailed failure analysis of a tee fitting that experienced significant wall thinning at the overhead outlet of a hydrodesulfurization stripping column in a refinery hydroprocessing unit. Conducted by engineers from CNOOC Zhongjie Petrochemical Company, the study combines metallurgical examination, flow velocity distribution analysis, and corrosion mechanism identification to determine the root cause of the failure.
Failure Description and Investigation
During scheduled maintenance, the tee fitting at the overhead outlet of the hydrodesulfurization (HDS) stripping column was found to have experienced significant wall thinning. The investigation employed multiple analytical techniques including metallographic examination, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and fluid velocity distribution mapping to identify the failure mechanism.
Failure Characteristics
| Location | Wall Thinning Severity | Primary Mechanism |
|---|---|---|
| Tee run (main pipe) | Moderate to severe thinning | Combined corrosion and erosion |
| Tee branch | Less severe thinning | Primarily corrosion |
| Straight pipe upstream | Minimal thinning | General corrosion |
| Straight pipe downstream | Moderate thinning | Corrosion with some erosion |
The investigation revealed that the wall thinning was caused by the combined action of H2S-HCl-H2O corrosion and fluid erosion. The key finding was that erosion was the primary factor causing the differential corrosion damage between the tee main pipe and the straight pipe sections, with the tee experiencing more severe damage due to the higher fluid velocities and turbulence at the junction.
Corrosion Mechanism Analysis
H2S-HCl-H2O Corrosion System
The hydrodesulfurization stripping column operates under conditions where hydrogen sulfide (H2S), hydrogen chloride (HCl), and water (H2O) are present in the process stream. This combination creates an aggressive corrosive environment through multiple mechanisms:
- Acidic corrosion: HCl dissolved in water creates a strongly acidic environment that attacks the base metal.
- Sulfide corrosion: H2S reacts with iron to form iron sulfide (FeS) scales, which may be protective or non-protective depending on conditions.
- Hydrogen damage: Dissolved hydrogen can cause hydrogen blistering, hydrogen cracking, and hydrogen-induced cracking in susceptible materials.
- Electrochemical corrosion: The presence of multiple ionic species in the aqueous phase creates a highly conductive electrolyte that accelerates electrochemical corrosion.
Erosion-Corrosion Interaction
The erosion-corrosion interaction is the dominant failure mechanism at the tee fitting. The fluid velocity distribution at a tee junction is highly non-uniform, with maximum velocities occurring at specific locations depending on the tee geometry and flow direction. The high-velocity regions experience:
- Mechanical removal of protective scales: The FeS scale and any corrosion product layers are mechanically stripped by the high-velocity fluid, exposing fresh metal to continued corrosion attack.
- Increased mass transfer rates: Higher fluid velocities increase the rate of transport of corrosive species to the metal surface and removal of corrosion products, accelerating the corrosion rate.
- Localized turbulence and impingement: The turbulent flow patterns at the tee junction create localized regions of intense fluid impingement that cause accelerated erosion-corrosion.
Flow Velocity Distribution at Tee Junction
The fluid velocity distribution analysis revealed that the maximum velocities at the tee junction are significantly higher than in the straight pipe sections. The velocity distribution is characterized by:
| Flow Region | Relative Velocity | Erosion-Corrosion Rate |
|---|---|---|
| Tee run centerline | 1.0 (reference) | Moderate |
| Tee run wall near branch | 1.5-2.0 | High |
| Tee branch entry | 2.0-3.0 | Very high |
| Straight pipe | 1.0 (reference) | Low |
The velocity amplification at the tee junction, particularly near the branch entry and at the wall opposite the branch, creates localized regions of intense erosion-corrosion that are not present in the straight pipe sections.
Material and Design Considerations
Material Selection for HDS Service
The material selection for hydrodesulfurization service must account for the aggressive H2S-HCl-H2O environment. Common material selections include:
| Material Grade | Application | Resistance to H2S-HCl-H2O |
|---|---|---|
| Carbon steel (CS) | Low-pressure, low-temperature | Poor; requires corrosion allowance |
| 304/316 stainless steel | Moderate severity | Moderate; susceptible to chloride pitting |
| 316L stainless steel | High chloride environments | Good; lower carbon reduces sensitization |
| Alloy 825 (UNS N08825) | Severe H2S environments | Excellent; nickel-iron-chromium alloy |
| Alloy C-276 (UNS N10276) | Extreme severity | Outstanding; high nickel-molybdenum |
| Duplex stainless (2205) | Moderate to severe | Good; high strength and corrosion resistance |
For the tee fitting in question, the material selection may have been adequate for the straight pipe sections but insufficient for the tee junction where the combined erosion-corrosion mechanism creates a more severe environment than general corrosion alone.
Design Recommendations
Based on the failure analysis findings, several design recommendations can be made for tee fittings in similar service:
- Increased corrosion allowance: The corrosion allowance for tee fittings should be greater than for straight pipe sections to account for the enhanced erosion-corrosion at junctions.
- Material upgrade at junctions: Consider using higher-grade materials for tee fittings even when straight pipe sections can use lower-grade materials.
- Flow velocity control: Design piping systems to limit fluid velocities at tee junctions, particularly in the branch entry region.
- Regular inspection: Implement enhanced inspection programs for tee fittings, including ultrasonic thickness measurement at multiple locations around the junction.
- Erosion-resistant coatings: Consider applying erosion-resistant coatings or overlays at tee junctions to provide additional protection against erosion-corrosion.
Inspection and Monitoring Recommendations
Non-Destructive Testing Strategy
For tee fittings in hydroprocessing service, a comprehensive NDT strategy should include:
| NDT Method | Purpose | Inspection Frequency |
|---|---|---|
| UT (Ultrasonic Testing) | Wall thickness measurement | Every shutdown; minimum 6 locations around tee |
| RT (Radiographic Testing) | Internal defect detection | At installation and major repairs |
| MT (Magnetic Particle Testing) | Surface crack detection | Before and after welding repairs |
| PT (Penetrant Testing) | Surface defect detection | Complementary to MT for non-ferromagnetic materials |
The inspection locations should be selected based on the flow velocity distribution analysis, with particular attention to the high-velocity regions identified in this study. For a tee fitting with flow entering from the run and exiting through the branch, the most critical inspection locations are typically at the branch entry wall and at the opposite wall of the run.
Study Insights and Reflections
This failure analysis provides valuable insights into the combined effect of corrosion and erosion at pipe junctions in hydroprocessing service. The key finding that erosion is the primary differentiating factor between the damage patterns at tee fittings versus straight pipe sections has important implications for material selection, design, and inspection practices.
The study demonstrates that the fluid velocity distribution at tee junctions creates localized regions of enhanced erosion-corrosion that are not predicted by simple corrosion rate measurements in straight pipe sections. This means that corrosion rate data obtained from coupon tests or straight pipe measurements may significantly underestimate the actual damage rate at tee junctions, leading to inadequate corrosion allowance and premature failure.
For engineering practice, this case study reinforces the importance of understanding the interaction between fluid mechanics and corrosion mechanisms when designing and maintaining piping systems in aggressive service environments. The combined erosion-corrosion mechanism at tee junctions represents a particular challenge that requires integrated approaches to material selection, design, inspection, and maintenance to ensure reliable long-term operation. The findings also highlight the value of detailed failure analysis in identifying root causes and developing targeted improvement measures that can prevent similar failures in the future.