CLADDING TECHNOLOGY SHANXI CO., LTDUltrasonic Thickness Measurement of Stainless Steel Overlay on Hydrogenation Reactors A Technical Study Note
Literature Overview
This 2010 publication by Zhang Tao, Zhou Honglan, and Sun Dan from the Dongying Branch of the Shandong Provincial Special Equipment Inspection and Research Institute addresses the challenge of ultrasonic thickness measurement of stainless steel weld overlay layers on hydrogenation reactors. Hydrogenation reactors are critical pressure vessels in petrochemical plants that operate at high temperatures and pressures in the presence of hydrogen. To prevent hydrogen attack of the carbon steel base material, a stainless steel overlay layer is applied to the inner surface. The integrity and thickness of this overlay layer must be periodically inspected to ensure continued protection against hydrogen damage.
Technical Challenges of Ultrasonic Measurement
The ultrasonic measurement of weld overlay layers presents unique challenges that distinguish it from conventional thickness measurement of homogeneous materials. The primary difficulties include the presence of multiple material interfaces, the potential for signal attenuation and scattering at the bond line, and the possibility of internal defects such as lack of fusion or porosity within the overlay layer. The acoustic impedance mismatch between the carbon steel base and the stainless steel overlay creates partial reflection of the ultrasonic pulse, which can complicate the interpretation of the thickness measurement.
| Challenge | Impact on Measurement | Mitigation Approach |
|---|---|---|
| Multi-layer structure | Multiple echoes and signal complexity | Use of high-frequency transducer and gated measurement |
| Bond line defects | Signal attenuation and reduced back-wall echo | Use of contact coupling and careful gate positioning |
| Grain structure variation | Signal scattering and noise | Use of focused transducer and signal averaging |
| Surface roughness | Reduced coupling efficiency | Use of high-viscosity couplant and surface preparation |
Measurement Methodology
The study describes a method using a dual-element ultrasonic transducer operating at a frequency of 5 MHz or higher. The dual-element design provides both transmitting and receiving functions with a fixed delay, which helps in distinguishing the overlay thickness echo from the back-wall echo. The measurement is performed in pulse-echo mode with the transducer positioned on the outer surface of the reactor shell, through the carbon steel base material.
The measurement procedure involves calibrating the instrument using a reference block with a known thickness of stainless steel over carbon steel. The time delay between the initial pulse and the back-wall echo is used to calculate the total thickness, and the time delay between the overlay-to-base interface echo and the back-wall echo is used to determine the overlay thickness. The sound velocity in the stainless steel overlay is typically 5,790 meters per second for austenitic grades such as 304 and 316L, while the sound velocity in the carbon steel base is approximately 5,920 meters per second.
| Material | Longitudinal Wave Velocity (m/s) | Typical Thickness Range |
|---|---|---|
| Carbon steel (base) | 5,900-5,950 | 20-60 mm |
| 304/316L stainless steel (overlay) | 5,780-5,800 | 3-10 mm |
| Inconel 625 (overlay) | 5,530-5,550 | 3-8 mm |
Interpretation of Results and Quality Assessment
The measured overlay thickness must be compared with the design specification to determine whether the overlay is still adequate for hydrogen service. According to the API 934 standard for hydrogen service, the minimum required overlay thickness depends on the hydrogen partial pressure and operating temperature. For example, at a hydrogen partial pressure of 10 megapascals and a temperature of 400 degrees Celsius, a minimum overlay thickness of 6 millimeters of 304 or 316L stainless steel may be required. If the measured thickness falls below this minimum, the reactor must be taken out of service for overlay repair.
The study also discusses the importance of measuring the overlay thickness at multiple locations around the circumference and along the length of the reactor, as localized thinning due to erosion or corrosion may occur at specific locations. A typical inspection grid consists of measurement points at intervals of 150 to 300 millimeters, with additional points at areas of known wear or corrosion.
Study Insights and Reflections
This study addresses a critical inspection challenge in the petrochemical industry where the integrity of the hydrogenation reactor overlay is essential for safe operation. The ultrasonic method described provides a non-destructive means of verifying overlay thickness without requiring reactor decompression or disassembly. The emphasis on proper calibration, transducer selection, and measurement technique is essential for obtaining reliable results. The study reinforces the importance of periodic inspection programs for hydrogen service equipment and provides a practical methodology that can be adopted by inspection organizations. The integration of ultrasonic thickness measurement with other inspection methods such as radiographic testing and magnetic particle testing provides a comprehensive assessment of overlay integrity. This approach aligns with the principles of risk-based inspection and supports the safe and efficient operation of hydrogenation reactors in petrochemical facilities.