Zhuojin Pipe Fitting Co., LtdBurst Failure Analysis of Process Pipe Elbows in Crude Mixed Amine Systems
Literature Overview
This paper, published in Piping Technology and Equipment (2012, Issue 2, pp. 56-57) by Zheng Sulü and Zheng Shuifa, presents a failure analysis of a process pipe elbow that burst in a crude mixed amine system at a chemical plant. The study combines macroscopic examination, thickness measurement, and chemical composition analysis to identify the root cause of the catastrophic failure. While the paper is concise, it provides a clear diagnostic methodology applicable to similar process piping failures.
Failure Description and Diagnostic Approach
The failure occurred at an elbow in the crude mixed amine process line. Mixed amine systems are commonly used for acid gas removal in natural gas processing and petrochemical applications, handling fluids containing hydrogen sulfide (H2S), carbon dioxide (CO2), and potentially other corrosive species.
| Diagnostic Method | Purpose | Key Finding |
|---|---|---|
| Macroscopic crack examination | Identify crack morphology and initiation site | Crack consistent with wall thinning failure |
| Wall thickness measurement | Quantify material loss | Significant wall thinning observed |
| Chemical composition analysis | Detect compositional changes due to corrosion | Changes in alloying element content at failure zone |
| Service condition review | Understand operating environment | Long-term exposure to corrosive medium |
Root Cause Determination
The analysis identified two synergistic damage mechanisms as the cause of the elbow burst:
- Long-term corrosion of the pipe wall: The amine medium, containing dissolved acid gases, caused sustained chemical and electrochemical attack on the pipe wall, progressively reducing the effective wall thickness below the design minimum.
- Erosion by fluid flow: The high-velocity amine solution flowing through the elbow caused mechanical erosion, particularly at the outer arc (concave side) where the flow impinges, and at the inner arc (convex side) where secondary flow patterns create turbulent recirculation zones.
The combined effect of corrosion and erosion—often termed erosion-corrosion—accelerated wall thinning beyond what either mechanism alone would achieve. The elbow geometry inherently creates adverse flow conditions: the fluid must change direction, generating centrifugal forces, secondary vortices, and regions of flow separation that intensify both mechanical wear and chemical attack.
Engineering Practice Implications
Design Considerations for Amine Service Piping
| Design Parameter | Recommendation | Rationale |
|---|---|---|
| Material selection | Carbon steel with corrosion allowance or stainless steel (e.g., 316L, duplex) | Amine solutions are corrosive to carbon steel |
| Wall thickness | Include adequate corrosion allowance (typically 3-6 mm) | Compensate for long-term wall loss |
| Elbow type | Long-radius (1.5D) elbows preferred over short-radius (1D) | Reduces flow turbulence and erosion rate |
| Flow velocity | Limit to manufacturer recommendations (typically < 2.4 m/s for amine solutions) | Higher velocity increases erosion rate |
| Inspection intervals | More frequent UT thickness monitoring at elbows | Elbows are erosion-corrosion hotspots |
Maintenance and Monitoring Recommendations
- Ultrasonic thickness mapping: Regular UT scanning at elbows, focusing on the outer arc (erosion zone) and inner arc (turbulence zone), with measurements at multiple clock positions around the bend.
- Corrosion coupons: Install corrosion coupons in the amine loop to monitor corrosion rates and validate material selection.
- Flow velocity verification: Periodically verify actual operating velocities against design values, as process changes can inadvertently increase velocity.
- Elbow replacement criteria: Establish a minimum remaining wall thickness threshold (typically 50% of original thickness) for mandatory replacement.
Key Reflections
This failure case underscores a frequently overlooked principle in process piping design: the elbow is not merely a geometric connector but a critical component where flow dynamics and material degradation interact in complex ways. The relatively short service life of the failed elbow suggests that the original design may not have adequately accounted for the combined corrosion-erosion environment, or that the material selection was insufficient for the actual service conditions.
The chemical composition analysis finding—changes in alloying element content at the failure zone—provides an important diagnostic indicator. In similar investigations, such compositional shifts can distinguish between uniform corrosion (where composition changes are relatively uniform) and localized attack (where selective leaching of specific elements occurs). Engineers conducting failure analyses should always include microchemical analysis of the failed zone to identify selective corrosion mechanisms.
This case also highlights the importance of considering the full process fluid composition rather than treating amine solutions as benign. Crude mixed amines contain impurities, degradation products, and dissolved gases that can significantly alter corrosion behavior compared to clean laboratory-grade solutions. The study provides a reliable basis for process piping design and management, helping to prevent recurrence of similar accidents and ensuring safe and economical chemical production.