Ultrasonic Thickness Measurement of Stainless Steel Cladding Layer on Hydrotreating Reactors

Literature Overview

The paper published in Petrochemical Equipment (2010, Vol. 39, No. 1, pp. 71-72) by Zhang Tao, Zhou Honglan, and Sun Dan from the Dongying Branch of the Shandong Special Equipment Inspection and Research Institute addresses a specific and challenging inspection problem: the measurement of stainless steel cladding layer thickness on hot-wall hydrotreating reactors. These reactors operate under high-temperature hydrogen service conditions, where the austenitic stainless steel cladding layer serves as the primary corrosion-resistant barrier protecting the carbon or low-alloy steel base material from hydrogen attack and sulfidation. Accurate assessment of cladding thickness is essential for determining remaining service life and scheduling maintenance or replacement activities.

The Measurement Challenge

Hot-wall hydrotreating reactors present unique challenges for ultrasonic thickness measurement of the stainless steel cladding layer. The cladding is typically applied by either explosion cladding or overlay welding (TIG or SAW) and has a thickness of 3-6 mm. The base material is usually 16MnR, 18MnMoNbR, or similar low-alloy steels with a wall thickness of 60-120 mm. The fundamental difficulty lies in the impedance mismatch between the stainless steel cladding and the base material, which causes significant signal attenuation and multiple echoes that obscure the cladding thickness measurement. Additionally, the hot-wall construction means the cladding layer is on the inner surface of the vessel, and access for measurement is limited to the outer surface through the entire wall thickness.

Comparison of Measurement Approaches

The authors proposed a simplified measurement method that utilizes a high-frequency single-element ultrasonic probe with appropriate time-gain compensation settings. The key insight is that by selecting a probe frequency in the range of 5-10 MHz and applying TGC to compensate for the high attenuation in the austenitic stainless steel (which has coarse grain structure causing significant scattering), the cladding layer echo can be reliably distinguished from the back-wall echo.

Technical Methodology

The proposed method involves the following steps: first, the ultrasonic instrument is calibrated on a reference block with known cladding thickness to establish the time-gain compensation curve. The reference block should have similar material composition and microstructure to the actual reactor cladding. Second, the probe is coupled to the outer surface of the reactor wall using a high-viscosity couplant to minimize air gaps. Third, the time-gain compensation is set to flatten the amplitude of the cladding layer echo across the measurement range. Fourth, the cladding thickness is calculated from the time-of-flight of the first echo, using the known longitudinal wave velocity in the stainless steel cladding (typically 5790 m/s for austenitic grades such as 309 or 347).

A critical aspect of this method is the identification of the cladding echo among multiple reflections. The first echo corresponds to the cladding layer, the second to the base material, and subsequent echoes are multiples. The authors demonstrated that with proper TGC settings and probe selection, the cladding echo can be identified with a measurement uncertainty of ±0.2 mm, which is acceptable for determining remaining cladding thickness against minimum allowable values.

Engineering Practice and Quality Control

In practice, the measurement program should follow a systematic grid pattern across the reactor shell, with measurement points spaced at 100-200 mm intervals. Areas of concern, such as nozzle penetrations, weld seams, and regions of known corrosion, should be measured at higher density. The measured cladding thickness data should be compared against the original as-built thickness and the minimum allowable thickness specified in the applicable design code (typically ASME Section VIII Division 1 or the Chinese GB/T 150).

The hydrostatic pressure test data should also be considered in conjunction with the thickness measurements. If the cladding layer has been significantly thinned by corrosion, the integrity of the cladding-to-base bond must also be evaluated, as interfacial corrosion can lead to delamination. The ultrasonic method described in this paper primarily addresses thickness measurement; separate bond testing methods such as eddy current or acoustic emission are needed to assess cladding integrity.

Study Insights and Reflections

This paper demonstrates a practical solution to a common inspection challenge in the petrochemical industry. The simplicity of the proposed method is its greatest strength: it requires only standard ultrasonic equipment with high-frequency probes and does not need specialized instrumentation or access to both sides of the vessel. However, the method's effectiveness depends heavily on the operator's skill in identifying the correct echo and setting appropriate TGC curves. Training and qualification of inspection personnel are therefore critical to the reliability of the measurement results. The findings also underscore the importance of maintaining detailed as-built records of cladding thickness during fabrication, as these records provide the baseline against which in-service measurements are compared. Without accurate baseline data, the significance of measured thickness reductions cannot be properly assessed.