CLADDING TECHNOLOGY SHANXI CO., LTDUltrasonic Testing of Overlay Layer Delamination in In-Service Hot-Wall Hydrogenation Reactors
Literature Overview
This 2000 study by Wang Qingmei, Qiang Tianpeng, Zhou Disheng, and Wei Zigang from Yangzi Petrochemical Company and the Jiangsu Boiler and Pressure Vessel Inspection Research Center addresses a critical safety and reliability issue in hydrogenation reactor operation: the detection of overlay layer delamination in in-service hot-wall reactors. Hot-wall hydrogenation reactors, used extensively in petroleum refining and petrochemical production, are typically fabricated from Cr-Mo steel with a nickel-based alloy overlay layer (commonly Inconel 625 or Incoloy 800) to resist hydrogen attack and corrosion. The overlay layer is essential for maintaining the structural integrity and corrosion resistance of the reactor vessel, and its delamination represents a serious integrity threat.
Core Technical Challenge
The detection of overlay layer delamination in in-service reactors presents unique ultrasonic testing (UT) challenges that distinguish this application from conventional UT of pressure vessels:
| Challenge | Description | Impact on UT |
|---|---|---|
| High-temperature exposure | Reactor operates at 350–450°C | Material property changes, possible overlay embrittlement |
| Thick base metal | Cr-Mo steel wall thickness 50–150 mm | Long sound paths, signal attenuation |
| Thin overlay layer | Typically 6–12 mm | Small defect size relative to wall thickness |
| In-service condition | Reactor in operation or recently shut down | Limited access, thermal gradients |
| Complex geometry | Nozzles, heads, weld seams | Beam steering and signal interpretation difficulty |
Overlay Layer Delamination Mechanisms
Delamination of the overlay layer can occur through several mechanisms:
- Thermal cycling fatigue: Repeated heating and cooling during reactor start-up and shutdown cycles causes differential thermal expansion between the Cr-Mo base metal and the nickel-based overlay, generating cyclic stresses at the interface.
- Hydrogen embrittlement: Atomic hydrogen diffuses into the overlay layer and can accumulate at the interface, reducing cohesive strength.
- Welding residual stress: Residual tensile stresses from the overlay welding process can initiate and propagate interfacial cracks.
- Corrosion under the overlay: If the overlay is incomplete or contains defects, corrosion products can accumulate at the interface and reduce bonding strength.
Ultrasonic Testing Methodology
Inspection Technique Selection
The study evaluated multiple UT techniques for overlay layer delamination detection:
| Technique | Frequency | Probe Type | Sensitivity | Limitations |
|---|---|---|---|---|
| Conventional UT | 2.5–5 MHz | 0° or 22.5° normal | Moderate | Poor for thin overlay delamination |
| TOFD | 5–10 MHz | Dual element | Good | Requires access on both sides |
| PAUT | 2.5–10 MHz | Linear array | Excellent | Equipment cost, complex data interpretation |
| Phase-array UT | 2.5–10 MHz | Phased array | Excellent | Requires skilled operators |
For in-service inspection of hot-wall reactors, conventional UT with a normal beam probe at 2.5–5 MHz is the most practical approach, supplemented by TOFD or PAUT for critical areas. The normal beam technique detects delamination by measuring the time-of-flight difference between the back-wall echo and the delamination echo.
Inspection Procedure
The recommended inspection procedure for overlay layer delamination detection includes:
- Surface preparation: Clean the external surface of the reactor to remove insulation, paint, and corrosion products, exposing bare metal.
- Couplant application: Apply a high-temperature couplant suitable for the inspection surface temperature.
- Calibration: Calibrate the UT equipment using a reference block with a known delamination thickness to establish the detection threshold.
- Scanning: Perform systematic scanning of the overlay layer area with a step size of 25 mm or less, ensuring full coverage.
- Signal evaluation: Identify and evaluate indications using amplitude, time-of-flight, and waveform analysis.
- Sizing: Determine the extent of delamination using the 6 dB drop method or TOFD sizing.
Acceptance Criteria
The acceptance criteria for overlay layer delamination are typically defined by the relevant code or standard:
| Code/Standard | Delamination Acceptance |
|---|---|
| ASME VIII Div.1 | No delamination exceeding 25% of overlay thickness |
| GB/T 150 | No delamination exceeding 30% of overlay thickness |
| API 934 | No delamination exceeding 20% of overlay thickness |
| NACE MR0175 | No delamination exceeding 15% of overlay thickness |
Engineering Practice and Case Studies
The Yangzi Petrochemical case described in this study involved a hot-wall hydrogenation reactor that had been in service for approximately 8 years. The reactor was fabricated from 1.25Cr-0.5Mo steel with a 10 mm thick Inconel 625 overlay layer deposited by submerged arc welding (SAW). During a scheduled inspection, UT revealed multiple delamination indications at the overlay layer interface, primarily located near nozzle welds and at the transition between the cylindrical shell and the dished head.
The delamination was found to be most severe in areas where the overlay welding sequence created high residual stresses. The authors recommended a combination of stress relief by localized heating and overlay repair welding to restore the integrity of the affected areas. This case highlights the importance of proper overlay welding sequence design and post-weld stress relief in preventing delamination during service.
Key Insights and Reflections
This study is particularly valuable because it addresses a real-world integrity issue in a critical piece of process equipment. The systematic approach to UT technique selection, calibration, and signal interpretation provides a practical guide for inspectors working on similar equipment. The emphasis on in-service inspection challenges—thermal gradients, limited access, and the need for rapid assessment—reflects the practical constraints of industrial inspection work.
A key insight from this work is that delamination detection requires a combination of UT techniques. Conventional UT provides rapid screening, while TOFD or PAUT offers superior sensitivity and sizing accuracy for critical areas. Engineers should not rely on a single technique but should develop a multi-method inspection strategy tailored to the specific reactor geometry and service history.
The study also underscores the importance of understanding the root causes of delamination. Simply detecting delamination is insufficient; engineers must identify the mechanism responsible and implement corrective measures that address the root cause. In the case of thermal cycling fatigue, for example, modifying the reactor start-up and shutdown procedures to reduce thermal gradients can significantly reduce delamination risk.
Implications for Modern Practice
Today, the UT techniques described in this 2000 study have evolved significantly. Phased array UT with advanced signal processing algorithms has become the preferred method for overlay layer inspection, offering superior imaging capability and defect characterization. However, the fundamental principles—proper calibration, systematic scanning, and multi-method verification—remain unchanged. The study also anticipated the need for in-service monitoring and predictive maintenance, which are now standard practices in the petrochemical industry. Engineers working on hydrogenation reactor integrity today should use this literature as a foundational reference for understanding the metallurgical and inspection challenges associated with overlay layer delamination.