Phased Array Ultrasonic Testing of Overlay-Welded Cracks in Hydrogenation Reactor Raised Bosses

Literature Overview

The paper by Chen Yongqiang, Xiao Xiong, Yu Jian, Wang Yulin, and Chen Liyi, published in NDT International (Vol. 36, No. 12, 2014, pp. 12-14), reports on the application of phased array ultrasonic testing (PAUT) for detecting cracks in overlay-welded layers on raised bosses (studs) of hydrogenation reactors. The authors successfully demonstrated the feasibility of PAUT for this application, achieving a minimum detectable crack height of 2 mm on test blocks and 3 mm on actual components, with a crack height measurement error of less than 1.0 mm. The technique is shown to be applicable to raised bosses of various specifications, making it a versatile tool for the inspection of hydrogenation reactor internals.

Background and Technical Context

Hydrogenation reactors are critical pieces of equipment in petroleum refining, where they are used to catalytically remove sulfur, nitrogen, and other impurities from hydrocarbon feeds under high temperature and high hydrogen partial pressure conditions. The raised bosses (studs) on the reactor shell serve as mounting points for internal components such as catalyst baskets, distributors, and support structures. These bosses are typically made of low-alloy steel and are often protected by an overlay-welded layer of austenitic stainless steel or nickel-based alloy to resist the corrosive and erosive effects of the hydrogen-containing environment.

The overlay-welded layer on raised bosses is susceptible to several types of damage:

• Hydrogen-induced cracking (HIC): Atomic hydrogen diffuses into the overlay layer and accumulates at inclusions or interfaces, leading to blister formation and cracking.
• Stress corrosion cracking (SCC): The combination of tensile residual stress from welding and the corrosive hydrogen environment can lead to intergranular or transgranular cracking in the overlay layer.
• Fatigue cracking: Cyclic thermal and mechanical loading during reactor start-up and shutdown can initiate cracks at stress concentration sites such as weld toes.
• Welding defects: Poorly controlled welding procedures can result in lack of fusion, porosity, or cracks in the overlay weld itself.

Traditional inspection methods such as RT and conventional UT have limitations for detecting these cracks in the overlay layer of raised bosses. RT is difficult to apply due to the geometry of the raised boss and the limited access. Conventional UT with a single-element probe may miss cracks that are not oriented favorably relative to the beam path. PAUT, with its ability to steer and focus ultrasonic beams electronically, offers a significant advantage for this application.

PAUT Technology and Implementation

Phased array ultrasonic testing uses an array of multiple transducer elements that can be fired sequentially with controlled time delays to create a steerable and focusable ultrasonic beam. This capability allows for:

• Beam steering: The beam can be directed at various angles to interrogate different regions of the weld and overlay layer.
• Beam focusing: The beam can be focused at specific depths to improve signal-to-noise ratio and resolution.
• Multiple scan configurations: Different scan configurations (e.g., linear scan, stepped scan, serpentine scan) can be used to cover the entire inspection area efficiently.

The authors describe the following implementation details:

Test Block Design

A test block was designed to simulate the geometry and material conditions of the actual raised boss. The test block incorporated reference cracks of known dimensions to calibrate the PAUT system and establish detection limits. The test block was essential for:

• Establishing the relationship between crack height and PAUT signal amplitude
• Determining the minimum detectable crack height
• Validating the scan strategy and beam configuration

Scan Configuration

The PAUT scan was performed using a linear phased array probe with a 16-element or 32-element configuration. The probe frequency was typically 5 MHz or 10 MHz, depending on the thickness of the overlay layer. The scan was performed with the probe in contact with the overlay surface, using a water or gel couplant. The scan path was planned to cover the entire overlay-welded area of the raised boss, with appropriate overlap between successive scan lines.

Signal Processing and Interpretation

The raw PAUT data was processed using beamforming algorithms to produce S-scan (depth-angle scan) and C-scan (plan-view scan) images. Defects appeared as distinct signals in the S-scan, with the crack height determined from the difference in arrival times of the diffractions from the top and bottom of the crack. The C-scan provided a plan-view map of defect locations, facilitating the assessment of defect distribution and extent.

Performance Characteristics

The following table summarizes the key performance characteristics reported in the paper:

Comparison with Conventional UT

The following table compares PAUT with conventional single-element UT for this application:

Engineering Practice Considerations

The application of PAUT to hydrogenation reactor raised boss overlay welds has several important implications for engineering practice:

Pre-Inspection Preparation

Before performing the PAUT inspection, the following steps should be taken:

1. Surface preparation: The overlay surface must be cleaned to remove paint, scale, and other surface contaminants that could interfere with acoustic coupling. Grinding or polishing may be necessary to achieve a smooth surface.
2. Geometry assessment: The exact geometry of the raised boss and overlay layer must be known to plan the scan path and select the appropriate probe configuration.
3. Calibration block preparation: A calibration block with reference defects representative of the expected damage types should be prepared and used for system calibration.
4. Equipment verification: The PAUT equipment should be verified using a reference block to ensure that the beam angle, focal depth, and gain settings are correct.

Scan Strategy

The scan strategy should be designed to ensure complete coverage of the overlay-welded area. Key considerations include:

• Scan line spacing: The spacing between successive scan lines should be less than the probe element length to ensure full coverage.
• Scan angle range: The beam should be steered through a range of angles that covers the expected defect orientations. For cracks in the overlay layer, angles from 0° to 60° may be required.
• Focal depth: The focal depth should be set to the expected depth of the defect. If the defect depth is uncertain, multiple focal depths should be used.
• Scan speed: The scan speed should be slow enough to ensure adequate data acquisition but fast enough to be practical.

Defect Evaluation

The evaluation of detected cracks should follow a systematic approach:

1. Signal identification: Distinguish true defect signals from noise, layer reflections, and other artifacts.
2. Defect sizing: Measure the crack height using the time-difference method. For planar cracks, the crack length can be estimated from the extent of the signal in the C-scan.
3. Defect location: Determine the depth and planar position of the crack relative to the overlay surface.
4. Defect characterization: Assess the orientation and morphology of the crack based on the signal behavior across multiple beam angles.
5. Acceptance/rejection decision: Compare the measured defect dimensions with the acceptance criteria specified in the applicable standard or procedure.

Study Insights and Reflections

This paper represents a significant step forward in the inspection of hydrogenation reactor internals. The successful application of PAUT to overlay-welded raised boss cracks demonstrates the versatility and power of phased array technology for complex inspection tasks. The achieved detection limits (2 mm on test blocks, 3 mm on components) and measurement accuracy (< 1.0 mm error) are satisfactory for most engineering applications, where the acceptance criteria for overlay layer cracks are typically based on crack height thresholds.

However, several limitations and challenges should be acknowledged. First, the minimum detectable crack height on actual components (3 mm) is larger than on test blocks (2 mm), which is expected due to the increased complexity of the actual geometry and material conditions. In practice, this means that very small cracks may be missed, and the inspection should be supplemented with other methods such as magnetic particle testing (MT) for surface-breaking cracks. Second, the measurement error of less than 1.0 mm, while acceptable for most applications, may be insufficient for damage assessment and remaining life prediction, where higher accuracy is required. Third, the technique requires skilled operators who are capable of interpreting complex PAUT data and distinguishing true defects from artifacts.

From a broader perspective, the application of PAUT to this problem highlights the importance of advancing NDT technology to meet the challenges of modern industrial equipment. Hydrogenation reactors operate under increasingly severe conditions, and the integrity of their internal components is critical for safe and reliable operation. The ability to detect and size cracks in overlay-welded layers with high accuracy and confidence is essential for implementing risk-based inspection (RBI) strategies and extending the service life of these critical assets.

In conclusion, the paper by Chen Yongqiang and colleagues provides a practical and validated approach to the inspection of overlay-welded cracks in hydrogenation reactor raised bosses using PAUT. The technique offers significant advantages over conventional UT in terms of detection capability, sizing accuracy, and data visualization. Engineers and inspectors should consider PAUT as a primary inspection method for this application, while maintaining awareness of its limitations and supplementing it with other NDT methods as necessary.