Ultrasonic Detection Imaging System for Overlay Layer Delamination in Thick-Walled Vessels

Literature Overview

This paper by Zhang Baojun, Zhang Haibo, Yan Zhi, Zhong Zhimin, and Li Wei, published in Machine Tool & Hydraulics (2014, Vol. 42, No. 8, pp. 123-126), presents the development of an ultrasonic detection imaging system specifically designed for identifying overlay layer delamination in thick-walled pressure vessels. The research was conducted jointly by China Nuclear Power Operation Service Co., Ltd. and Shanghai Gaoqiao Petrochemical Equipment Research Institute. The classification codes TG455 (overlay welding) and TP274 (automatic control) reflect the interdisciplinary nature of the work, combining welding metallurgy with non-destructive testing (NDT) and automated systems engineering.

Background and Technical Challenge

Thick-walled pressure vessels are widely used in the petrochemical and power industries, with hydrogenation reactors being a prime example. These vessels operate under extreme conditions:

• High temperature — Typically 350–450°C in hydrogenation service
• High pressure — Operating pressures up to 15–25 MPa
• Corrosive media — Hydrogen-containing environments that attack overlay layers

The overlay layers applied to the internal surfaces of these vessels serve as corrosion-resistant barriers, typically composed of nickel-based or austenitic stainless steel alloys. However, prolonged service in high-temperature hydrogen environments can lead to overlay layer delamination, where the overlay separates from the base metal. This delamination compromises the corrosion protection function and can lead to catastrophic failure if undetected.

The detection challenge is significant because:

1. The overlay layer is relatively thin (typically 3–10 mm) compared to the vessel wall thickness (often 200–400 mm).
2. Delamination may be partial, creating complex interface defects that are difficult to characterize.
3. The thick base metal creates substantial acoustic attenuation and multiple reflections that obscure the overlay-base interface signal.

System Architecture

The developed system comprises three integrated subsystems:

Mechanical Scanning Device

The mechanical scanning device provides precise positioning and movement of the ultrasonic probe across the inspection area. For thick-walled vessels with internal overlay layers, the scanning device must accommodate:

• Large-diameter vessel internals (typically 2–6 m inner diameter)
• Curved surfaces requiring conformal probe contact
• Repositioning capability for complete coverage
• Stable coupling medium application

Control System

The control system orchestrates the scanning sequence, manages data acquisition timing, and coordinates with the imaging software. Key functions include:

• Probe position control and trajectory planning
• Scan speed regulation based on inspection sensitivity requirements
• Real-time monitoring of coupling conditions
• Integration with data processing algorithms

Ultrasonic Detection and Imaging System

The ultrasonic detection subsystem generates and processes ultrasonic signals to identify and characterize overlay layer delamination. The imaging component converts raw ultrasonic data into visual representations that clearly display the extent and location of delamination.

Technical Parameters and Performance

NDT Method Selection Rationale

The selection of ultrasonic testing (UT) for this application is well-justified by the physics of the problem:

• Thickness measurement principle — UT is inherently capable of measuring thickness, making it ideal for detecting where the overlay layer has separated from the base metal.
• Resolution capability — Modern phased array and contact UT techniques can resolve thin overlay layers and their interfaces with the base metal.
• Quantitative assessment — UT provides quantitative data on delamination extent, unlike magnetic particle testing (MT) or penetrant testing (PT) which only indicate surface-breaking defects.

The semi-automatic approach strikes a practical balance between the thoroughness of manual testing and the speed of fully automated systems. For in-service inspection of large vessels, where access is limited and inspection windows are short, semi-automation is often the most practical solution.

Engineering Practice Implications

For inspection engineers and asset integrity managers in the petrochemical and power industries, this system addresses a critical gap in inspection capability:

1. Preventive maintenance — Early detection of overlay delamination allows for scheduled repair before catastrophic failure occurs.
2. Remaining life assessment — Quantitative delamination data supports engineering assessments of remaining service life.
3. Regulatory compliance — Visual documentation of overlay integrity supports compliance with inspection codes and standards.
4. Cost optimization — Targeted repair of delaminated areas is far less expensive than complete vessel replacement or full overlay re-cladding.

Quality Assurance Considerations

The reliability of the detection system depends on several factors that must be controlled during implementation:

• Coupling quality — Consistent coupling between the probe and the vessel surface is essential for signal quality; variations in coupling can produce false indications.
• Probe calibration — Regular calibration against reference standards ensures detection sensitivity remains within specified limits.
• Operator training — Even with semi-automation, qualified operators are needed to interpret results and identify genuine defects versus artifacts.
• Data documentation — Comprehensive recording of inspection parameters and results is essential for trend analysis and regulatory compliance.

Study Insights and Reflections

This paper represents a practical solution to a well-recognized industry challenge. The development of specialized NDT systems for overlay layer integrity assessment is an important area of engineering development, particularly as the industry moves toward longer inspection intervals and more condition-based maintenance strategies. The semi-automatic approach taken here is pragmatic, recognizing that fully automated systems for large in-service vessels face significant practical barriers related to access, surface preparation, and system complexity. The integration of mechanical scanning, control systems, and ultrasonic imaging into a unified platform demonstrates the systems engineering approach necessary for modern NDT equipment development. For engineers responsible for asset integrity management, this type of technology enables more confident decisions about vessel continued use, repair timing, and replacement planning.