Ultrasonic Measurement of Overlay Weld Thickness in Hot-Wall Hydrogenation Reactors

Literature Overview

This paper by Zhou Disheng and Wang Qingmei from the Yangzi Petrochemical Company Equipment Inspection Station (published in Nondestructive Testing, Vol. 21, No. 3, 1999) addresses a critical inspection challenge: the ultrasonic measurement of overlay weld thickness in a hot-wall hydrogenation reactor. The reactor shell is fabricated from 12CrMoG10 (equivalent to 2.25Cr-1Mo), with an internal overlay of 1.4541 (X2CrNiMo17-12-2, equivalent to 316 stainless steel). The equipment operates at 400–450°C and 16 MPa in a hydrogen-rich environment, making overlay integrity a paramount safety concern.

Core Technical Points

The Inspection Challenge

Measuring overlay weld thickness in this application presents unique ultrasonic challenges:

Challenge	Description
Material mismatch	Significant acoustic impedance difference between 2.25Cr-1Mo base and 316SS overlay
Coarse grain structure	2.25Cr-1Mo has large austenite grains that cause strong scattering
Curved geometry	The cylindrical shell and dished heads create complex reflection patterns
Hydrogen damage	Long-term hydrogen exposure may cause microvoids or decarburization that affect signal quality
Multiple interfaces	Reflections from both the overlay/base interface and the back wall create signal overlap

Ultrasonic Technique Development

The authors developed a specialized ultrasonic technique to address these challenges:

Transducer selection: A dual-element transducer with a frequency of 5 MHz and a near zone of 30 mm was selected to provide adequate resolution while maintaining sufficient penetration through the thick section.
Couplant optimization: A high-temperature couplant (glycerin-based) was used to ensure reliable coupling on the hot reactor surface.
Signal processing: Time-gain compensation (TGC) was applied to normalize the amplitude response across the measurement range.
Calibration: A stepped-wedge block with known overlay thicknesses was used to calibrate the instrument and establish a measurement curve.

Measurement Methodology

The measurement procedure involves:

Surface preparation: Grinding the external surface of the reactor shell to remove paint, rust, and scale.
Probe positioning: Placing the transducer perpendicular to the shell surface at predetermined intervals.
Signal identification: Distinguishing the overlay/base interface echo from the back-wall echo using time-of-flight analysis.
Thickness calculation: Using the calibrated time-thickness relationship to determine the overlay thickness.
Mapping: Recording measurements at a grid of points to create a thickness map of the overlay.

The accuracy of the technique was validated against destructive test coupons, demonstrating a measurement uncertainty of ±0.3 mm for overlay thicknesses in the range of 3–10 mm.

Engineering Practice Integration

The hot-wall hydrogenation reactor is a critical asset in a hydrocracking unit, where the overlay serves as the primary defense against high-temperature hydrogen attack (HTHA). The integrity of the overlay is directly linked to the safety and reliability of the entire unit.

Inspection Schedule and Acceptance Criteria

Based on the findings of this study, the following inspection protocol is recommended:

Inspection Type	Frequency	Acceptance Criteria
Initial thickness mapping	At commissioning	≥ 90% of specified minimum thickness
Periodic thickness measurement	Every 2 years	No local thinning > 10% of original
Full UT examination	Every 5 years or before major turnaround	No indications > 2 mm in length
Magnetic particle testing	At each inspection	No linear indications in the overlay

Practical Considerations

Several practical issues must be addressed during in-service inspection:

Access constraints: The reactor is often located in a confined area with limited access for inspection equipment.
Surface condition: Corrosion products, scaling, or previous repair welds may interfere with ultrasonic coupling.
Temperature effects: The reactor may be inspected at elevated temperatures, which can affect sound velocity and couplant performance.
Operator skill: The technique requires experienced operators who can distinguish between relevant and spurious signals.

Key Questions and Reflections

A critical question is whether the ultrasonic technique can reliably detect interfacial defects such as lack of fusion or delamination between the overlay and the base metal. The acoustic impedance mismatch at this interface is significant, but the signal may be masked by reflections from the back wall or by scattering from the coarse-grained base metal. Complementary techniques such as phased array ultrasonic testing (PAUT) or thermography may be required for comprehensive interfacial inspection.

Another consideration is the effect of hydrogen damage on the measurement accuracy. Long-term exposure to high-pressure hydrogen can cause microvoid formation and decarburization in the base metal, which may alter the acoustic properties and lead to erroneous thickness readings. Engineers should be aware of this limitation and consider supplementing ultrasonic measurements with other NDT methods when hydrogen damage is suspected.

Study Insights and Implications

This paper highlights the importance of developing tailored NDT techniques for specific inspection challenges. The generic application of standard ultrasonic thickness gauging is insufficient for the complex material combinations and operating conditions encountered in hydrogenation reactors. The systematic approach to technique development—combining transducer optimization, calibration procedures, and validation against destructive testing—provides a robust framework that can be adapted to other challenging inspection scenarios. Engineers should recognize that the accuracy of NDT results is only as good as the technique development and validation process that underpins them, and should invest accordingly in technique qualification for critical assets.