Welded joints in pressure vessels are classified into five categories: A, B, C, D, and E. Among these, the joint connecting non-pressure-bearing components to pressure-bearing components is designated as an E-class welded joint. E-class welded joints are commonly found in welds between backing plates and vessel walls, as well as in the connections between nozzles with reinforcing rings and the vessel shell.
In structures such as the connection between the skirt and the head, lap fillet welds are used.
During the start-up, shutdown, and operation of in-service pressure vessels, transient temperature changes and fluctuations in operating conditions can induce significant thermal stresses. When lap corner welds contain welding defects such as lack of fusion and incomplete penetration, these defects can lead to
Stress concentration can lead to damage to the base metal of the vessel shell, resulting in medium leakage and even incidents such as fire and explosion. Therefore, nondestructive testing of lap fillet welds is of great importance.
Nondestructive Testing of Lap Fillet Welds
Through comprehensive investigations into the welding procedures employed during the fabrication of lap corner welds in pressure vessels, as well as the service conditions under which these welds operate, it has been determined that the primary defects that commonly occur during construction include lack of fusion, incomplete penetration, porosity, and slag inclusion.
During service, pressure vessels may also develop fatigue cracks, which can subsequently lead to cracking of the vessel shell’s base metal.
Currently, penetrant testing and magnetic particle testing are the primary methods used to inspect the surface quality of lap corner welds. As for internal defects, the significant thickness variation poses certain challenges for radiographic inspection. Even with the adoption of enhanced
Methods such as adjusting the irradiation energy, using dual-film systems, and compensating for material thickness are employed to reduce the difficulty of inspection; however, these approaches still face challenges, including difficulty in ensuring film sensitivity, low inspection efficiency, and the need for radiation protection during testing. In the routine ultrasonic inspection of lap corner welds,
During time measurement, due to its unique structure, it is prone to misjudgment or failure to detect critical defects.
Therefore, at present there is still no comprehensive and reliable method for detecting internal defects in lap corner welds of in-service pressure vessels. With the rapid development of phased-array ultrasonic testing technology, its application is becoming increasingly widespread; this technology possesses
It offers numerous advantages: a single probe can detect cracks and other defects in various locations and orientations simply by being positioned at a fixed location; it enables simulation of the structural configurations of all types of welds and models the coverage and propagation of the acoustic field within the weld; inspection of joints
When presented in the form of an ultrasonic map, such data provide comprehensive information for defect localization, quantification, characterization, and classification; therefore, phased-array ultrasonic testing can be employed to inspect internal defects in lap corner welds of in-service pressure vessels.
Phased Array Ultrasonic Testing of Lap Corner Welds
Artificial Defect Test Block
To perform phased-array ultrasonic testing of lap corner welds and validate the inspection procedure, technicians designed reference test blocks based on the geometry of the lap corner weld, backing plate, and base metal, and introduced artificial defects into these test blocks to simulate conditions that may be encountered in actual service.
The typical natural defects are shown in Figure 1, along with the dimensions of the reference test blocks and artificial defects. 
Figure 1: Schematic Diagram of Comparison Test Blocks and Artificial Defect Sizes
A 2-mm-diameter through-hole was machined at the center of the simulated weld zone in Test Block 1 to simulate porosity and slag inclusion defects in the weld; four artificial grooves, each 0.5 mm wide and 2 mm deep, were machined along the fusion line in the simulated weld zone of Test Block 2 to simulate stress-induced cracking caused by lack-of-fusion or incomplete penetration defects; and four artificial grooves, each 0.5 mm wide and 2 mm deep, were machined in the base metal of Test Block 3 to simulate stress-induced cracking in the base metal.
Test Results and Analysis
The ISONIC 2010 portable phased-array ultrasonic testing system, equipped with weld-seam simulation and imaging capabilities, is selected. This system can simulate the geometric configurations of various test specimens or display their actual structures in real time. During inspection, operators can leverage this functionality to quickly and conveniently identify and evaluate defect indications within the workpiece. As a result, the likelihood of misinterpretation and false judgments is significantly reduced, thereby ensuring the authenticity, validity, and reliability of the inspection results.
The simulation of the weld joint geometry is shown in Figure 2. By setting parameters such as weld width, surface reinforcement height, and root gap, the system accurately depicts the actual geometric configuration of the workpiece and the weld position.
In Figure 2, the area circled in red represents the simulated lap corner weld region; during inspection, focusing solely on the echo signals from this region can prevent misinterpretation caused by geometric-reflection artifacts.
Figure 2: Simulation of the weld joint structure by the system
The system’s sound-field coverage settings are shown in Figure 3. The system also allows for rapid configuration of the probe’s excitation and reception array aperture, incidence angle, focal depth, scan-angle range, and scan-angle step.
Adjust the inspection parameters and probe position to ensure that the sound field fully covers the inspection area.
Figure 3: Sound Field Coverage Settings for the System
The phased-array inspection system was configured according to process parameters that ensure full coverage of the sound field, and phased-array ultrasonic testing was performed on artificial defects embedded in test blocks. The S-shaped waveforms and morphologies of these artificial defects are shown in Figure 4. It can be observed that defects at all locations within the lap corner welds are effectively detected, and the weld-simulation function allows for a detailed visualization of the horizontal position and burial depth of the defects.
Figure 4 S-shaped display and morphology of artificial defects in the test block
Analysis of the inspection data reveals that phased-array ultrasonic testing provides relatively accurate defect localization, with horizontal positioning errors ranging from −1.0 to 1.0 mm and burial-depth measurement accuracy within ±1.6 mm; however, the measured defect dimensions tend to be overestimated.
During the operation of a certain adsorber, the volatile-gas monitor on the south side of the platform triggered an alarm, resulting in an on-site gas-leak incident. Upon inspection, leakage from the surrounding pipelines and from the adsorber itself was ruled out. The inspection team suspected that the leak was caused by cracking in the lap fillet weld connecting the platform support pad to the adsorber, so nondestructive testing was performed on this weld. First, penetrant testing was used to examine the surface defects of the lap fillet weld, but no defects were found. However, since the adsorber contains a large amount of catalyst, opening the vessel for inspection would render the catalyst ineffective; therefore,
The inspection personnel employed phased-array ultrasonic testing to examine the internal defects of the lap corner welds and detected a through-wall crack in the weld metal that penetrated the absorber base material, as shown in Figure 5.
Figure 5: Defect Atlas of a Lap Fillet Weld
Analysis of the defect map reveals that through-thickness cracks are present in the base metal of the adsorber beneath the backing plate, with detected lengths of 101 mm on the outer surface and 58 mm on the inner surface. The cracks exhibit an “S”-shaped propagation pattern through the thickness of the base metal, with their openings on the outer surface located at the root of the lap fillet weld and their openings on the inner surface directly below the toe of the same lap fillet weld.
To verify the accuracy of the phased-array ultrasonic testing results, the inspection personnel ultimately decided to cut open the vessel and perform fluorescent magnetic-particle inspection on the inner surface. A crack measuring 59 mm in length was detected at the corresponding location on the inner surface. The workshop subsequently scrapped the vessel and conducted further inspection of the defect area; the results are shown in Figure 6, including the crack’s internal opening length, its orientation within the base material, and the conditions of both the inner and outer surfaces.
The location and other parameters all meet the criteria for crack evaluation in phased-array ultrasonic testing, ensuring accurate test results. 。
Figure 6: Fluorescent Magnetic Particle Inspection and Penetrant Testing Results at the Defect
Through comprehensive laboratory trials of phased-array ultrasonic testing procedures, the critical process parameters were determined, and the feasibility of the testing procedure was verified. This method effectively addresses the challenge of detecting internal defects in lap fillet welds of in-service pressure vessels, thereby enhancing the efficiency and reliability of such inspections.
