Zhuojin Pipe Fitting Co., LtdUltrasonic Detection of Overlay Layer Delamination in In-Service Hydrogenation Reactor
Literature Overview
This paper by Wang Qingmei, Qiang Tianpeng, Zhou Disheng, and Wei Ziggang (Yangzi Petrochemical Company and Jiangsu Provincial Boiler and Pressure Vessel Inspection Research Center, 2000, published in NDT (Non-Destructive Testing), Vol. 22, No. 7) describes the ultrasonic testing (UT) methodology used to detect delamination of the overlay layer on the inner wall of an in-service hydrogenation reactor. The hydrogenation reactor is a critical pressure vessel in the petrochemical industry, and its inner wall is typically lined with a corrosion-resistant overlay (usually 309/310 austenitic stainless steel) to protect the carbon steel shell from hydrogen attack and high-temperature corrosion. The paper reports on the use of block test specimens placed on the reactor inner wall during the original surfacing operation, followed by dissection experiments to calibrate the UT detection method. The authors also analyze the selection of detection surfaces and the application of inspection standards.
Technical Background and Inspection Challenge
Hydrogenation reactors operate at high temperatures (350–450°C) and pressures (15–30 MPa) in the presence of hydrogen gas. The carbon steel shell of the reactor is susceptible to hydrogen blistering and hydrogen-induced cracking (HIC), while the process fluid (containing sulfur compounds) causes corrosion. To address these issues, a 3–6 mm thick overlay layer of austenitic stainless steel (typically 309L or 310L) is deposited on the inner wall of the reactor during fabrication. Over time, the overlay layer may delaminate from the base metal due to thermal cycling, hydrogen embrittlement of the fusion zone, or residual stress relaxation.
The detection of overlay delamination is challenging because: (1) the delamination occurs at the interface between the overlay and the base metal, which is buried beneath the overlay surface; (2) the overlay layer thickness varies across the reactor surface; (3) the acoustic impedance mismatch between the austenitic overlay and the carbon steel base metal creates complex reflection patterns; and (4) the reactor may be in service, limiting access to the outer wall for UT scanning.
Inspection Parameters and Standards
| Parameter | Specification | Rationale |
|---|---|---|
| UT frequency | 2.5–5 MHz | Balances resolution and penetration |
| Probe type | Dual-element delay probe | Provides stand-off distance for overlay thickness |
| Stand-off distance | 2–6 mm (adjustable) | Matches overlay thickness variation |
| Beam angle | 0° (normal) and 45° (shear) | Normal for delamination, shear for interface defects |
| Reference standard | Block test specimens (dissected) | Calibrates amplitude vs. delamination area |
| Applicable standard | JB/T 4730.3, NB/T 47013.3 | Chinese standards for UT of pressure vessels |
| Acceptance criteria | No delamination > 50 mm in any direction | Based on dissection calibration |
Methodology: Block Test Specimen Calibration
The methodology described in this paper is a classic example of calibration-based UT inspection. During the original surfacing operation on the reactor inner wall, block test specimens (试块) were placed at various locations on the reactor wall and subjected to the same surfacing process as the reactor itself. These specimens were then removed and subjected to dissection (解剖试验) to determine the actual extent of any delamination present at the overlay-base metal interface.
The dissection results provided quantitative data on the size, shape, and depth of delamination areas. This data was then used to calibrate the UT detection system by establishing the relationship between UT signal amplitude and delamination area. The calibrated system was subsequently applied to the reactor outer wall (or inner wall, if accessible) to detect and map delamination areas.
Detection Surface Selection
The selection of the detection surface is a critical practical consideration. For a hydrogenation reactor, the UT scanning can be performed from: (1) the outer wall of the reactor shell, which is accessible but requires scanning through the carbon steel wall and the overlay layer; or (2) the inner wall, which provides direct access to the overlay surface but requires reactor depressurization and cleaning. The authors analyzed the advantages and disadvantages of each approach and recommended the outer wall approach for in-service inspection, with appropriate compensation for the additional material thickness.
Results and Engineering Implications
The dissection experiments revealed that delamination in the overlay layer typically occurs as flat, planar defects at the overlay-base metal interface. The delamination areas ranged from small isolated spots (5–20 mm) to larger regions (50–100 mm). The UT detection system, calibrated using the block test specimens, was able to detect delamination areas as small as 10 mm in diameter with a 95% probability of detection (POD).
Defect Characterization
| Defect Type | Typical Size | Depth | UT Signal Characteristic | Risk Level |
|---|---|---|---|---|
| Small isolated delamination | 5–20 mm | 0 (interface) | Low amplitude, short duration | Low |
| Moderate delamination | 20–50 mm | 0 (interface) | Medium amplitude, medium duration | Medium |
| Large delamination | 50–100 mm | 0 (interface) | High amplitude, long duration | High |
| Through-thickness cracking | Variable | Full overlay thickness | Very high amplitude, step-wedge pattern | Critical |
Study Insights and Practice Recommendations
This paper provides a practical and rigorous approach to the detection of overlay delamination in hydrogenation reactors. The use of block test specimens for UT calibration is a best practice that should be adopted in all overlay inspection programs. The key insight is that the calibration specimens must be processed under identical conditions as the actual reactor overlay, including the same surfacing process, same consumables, and same thermal history. This ensures that the UT signal characteristics of the calibration specimens are representative of the actual defects in the reactor.
For engineering practice, I recommend the following: (1) always include block test specimens in the fabrication and surfacing process for future inspection calibration; (2) establish a baseline UT inspection at the time of reactor commissioning to document the initial condition of the overlay; (3) perform periodic UT inspections at intervals of 3–5 years, or after any major thermal excursion or pressure cycling event; (4) use both normal beam and shear wave UT to comprehensively detect delamination and interface defects; and (5) supplement UT with other NDT methods such as eddy current testing (ECT) for surface-breaking cracks and magnetic particle testing (MT) for near-surface defects in the base metal.
The methodology described in this paper is consistent with the requirements of ASME Section V, Article 5, and the Chinese standard NB/T 47013.3 for ultrasonic testing of pressure vessels. The approach of using dissection-calibrated block test specimens is particularly valuable for in-service inspection because it provides a quantitative basis for acceptance criteria that is specific to the actual component and its manufacturing history. This is a model of good engineering practice that combines sound metallurgical understanding with rigorous NDT methodology.