I. Advantages of PAUT in the Inspection of Portable Tank Containers

Compared with conventional ultrasonic and radiographic testing techniques, PAUT offers the following advantages:

1) High inspection efficiency: during inspection, only a single, simple non-parallel scan of the circumferential or longitudinal welds of the portable tank container is required, eliminating the need for back-and-forth movement to complete the weld inspection.

2) Possesses superior imaging capabilities, image resolution, and data analysis abilities, enabling real-time display; during the inspection process, welds can be analyzed and evaluated, and the results can be printed, saved to disk, and permanently archived.

3) Automated welding inspection for portable tank containers features high sensitivity, a high defect detection rate, and simple, accurate defect quantification and localization.

4) Compared with radiography, green-ray inspection is safer, does not interfere with production, and offers a higher detection rate for area-type defects.

II. Scope of Application of PAUT in the Inspection of Portable Tank Containers

In the weld inspection of portable tank containers, PAUT technology can fully replace conventional ultrasonic testing.

In addition, it is suitable for carbon steels and low-alloy steels, as well as for materials such as aluminum and its alloys, austenitic stainless steels, ferrite–austenite duplex stainless steels, titanium alloys, and for the inspection of their welded joints.

According to ASTM E2700-09, the applicable base-material thickness range is 9 mm to 200 mm; with verified effectiveness, the standard may also be used for inspecting thinner or thicker workpieces.

ISO/DIS 13588:2010 specifies that PAUT is suitable for semi-automatic or fully automatic inspection of full-penetration welded joints in metals with a thickness of ≥6 mm.

The 2017 Application Guide for Time-of-Flight Diffraction (TOFD) and Phased Array Ultrasonic Testing (PAUT) stipulates that PAUT is suitable for inspecting full-penetration welds in base materials with a thickness of ≥6 mm, including flat-to-flat butt joints, pipe-to-pipe butt joints, T-joints, and corner joints. It is also applicable to longitudinal welds in cylindrical shells with an outer diameter greater than 250 mm or an outer-to-inner-diameter ratio exceeding 0.7, circumferential welds in cylindrical shells with an outer diameter greater than 200 mm, and intersection welds on curved surfaces of various sizes.

In conjunction with the aforementioned international phased-array standards and classification society rules, PAUT inspection covers all commonly used materials, base-metal thicknesses, and joint configurations employed in the fabrication of portable tank containers.

III. Classification Society Requirements for Weld Inspection of Portable Tank Containers
According to the IMDG Code, the structure and design of the tank shell of a portable tank shall comply with the requirements of pressure vessel codes approved by the competent authority.

Regulatory authorities generally do not establish independent inspection and testing organizations; instead, they authorize classification societies to carry out the approval and inspection of portable tank containers. The following sections describe the nondestructive testing requirements for pressure vessels as stipulated by the China Classification Society and Lloyd’s Register, respectively.

1) The China Classification Society stipulates the following non-destructive testing requirements for pressure vessels:

Note: ① For cylindrical components with an outer diameter ≤170 mm, 25% of the header-to-header girth welds shall be subject to radiographic inspection; for other pipe-to-pipe girth welds, a random 10% radiographic inspection shall be conducted.

2) Lloyd’s Register’s non-destructive testing requirements for pressure vessels are as follows:

Note: ① Nondestructive testing methods: For volumetric nondestructive testing, radiographic testing may be used; when the base material thickness is ≥8 mm, ultrasonic testing may also be employed. For surface nondestructive testing, magnetic particle testing is recommended for ferromagnetic metals, while liquid penetrant testing is recommended for nonferromagnetic metals.

Class societies have varying NDT requirements for pressure vessels; however, with the advancement of ultrasonic phased-array technology and the growing acceptance of this technique by owners of portable tank containers, ultrasonic phased-array inspection, owing to its superior detection capabilities compared with radiographic testing, is increasingly replacing radiography in field inspections.

IV. Application Requirements for PAUT in the Inspection of Portable Tank Containers
Based on a comprehensive review of ASTM standards and the nondestructive testing requirements of classification societies, the following requirements apply to the implementation of PAUT-based NDT for mobile tank containers.

1) Testing Institutions and Personnel

Organizations conducting PAUT inspections shall possess certification recognized by the relevant classification society. Personnel performing such inspections must hold nondestructive testing qualification certificates issued or recognized by the classification society in order to carry out inspection tasks corresponding to their qualification level.

2) Testing equipment and instruments

The equipment host shall be accompanied by a calibration report issued by a manufacturer-authorized or nationally accredited metrology authority, and the report must be within its specified validity period. The horizontal linearity, vertical linearity, and attenuator linearity of the instruments and equipment shall be periodically verified and documented, with the maximum verification interval not exceeding one year.

The instrument shall be equipped with at least ultrasonic wave transmission, reception, amplification, data acquisition, recording, display, and analysis functions. The system shall provide sufficient gain, which shall be continuously adjustable with a step size no greater than 1 dB.

The equipment shall have a nominal frequency range of 1 MHz to 10 MHz for both transmission and reception, with a digital sampling rate of at least five times the probe’s nominal frequency. The instrument must be capable of storing all acquired raw data in an immutable format.

The transducer used shall be accompanied by a factory test report and a certificate of conformity. The number, size, and center frequency of the transducer’s piezoelectric elements shall be matched to the equipment being used or selected in accordance with the manufacturer’s recommendations. The curvature of the wedge profile selected shall be consistent with the curvature of the tank shell to ensure good contact between the transducer and the shell.

In addition, the scanning device shall ensure that the probe moves smoothly along the pre-set scan path; for automated or semi-automated scans, the probe’s movement must be synchronized with the encoder counts. The couplant shall maintain stable and reliable ultrasonic properties over the specified operating temperature range and shall be identical to the couplant used during calibration of the inspection system.

3) Design requirements for testing procedures and processes

Prior to inspection, a specific inspection procedure for the pressure-containing shell of portable tank containers shall be prepared, and such procedure must be submitted to the classification society for approval. If the inspection procedure is modified, the inspection procedure document shall be revised and resubmitted for approval.

The procedure shall, at a minimum, cover the following elements: determination of the inspection objective (inspection task, location, and level); applicable standards and regulations; scope of inspection (weld geometry, specifications, material, wall thickness, welding process, etc.); inspection equipment and instruments, as well as calibration, test blocks, and focusing rules; inspection procedures (probe configuration, scanning method, supplementary inspection methods for transverse defects, etc.); surface preparation requirements prior to inspection; data acquisition; procedure test report; analysis and interpretation of inspection data; defect evaluation; and re-inspection requirements.
   
In addition, inspection procedure cards shall be prepared in accordance with the approved inspection process documentation for different types of welds (circumferential welds, transverse welds, and fillet welds), and inspection personnel shall conduct inspections on the specific structures in accordance with the requirements specified in these procedure cards.

The inspection procedure card shall, at a minimum, include the following: applicable standards, acceptance criteria, timing of inspection, inspection coverage ratio, and surface requirements; model numbers and manufacturing serial numbers of instruments, probes, and test blocks; names of wedges, scanning devices, and couplant; scanning surface, probe parameters and arrangement, instrument sensitivity settings, scan increment, area segmentation and beam coverage, scanning method, scanning speed, and, where required, the inspection plan for transverse defects; inspection marking requirements and inspection operating procedures; and data recording requirements.

4) Surface Preparation Standards for Inspection

The area over which the inspection probe is moved shall be ground smooth and free of weld spatter, metal chips, oil, and other contaminants that could impede normal probe movement or attenuate sound energy transmission. If the inspection surface exhibits significant depressions, these shall be repaired by weld overlay and the repaired area subsequently ground flush with the adjacent base material to ensure proper coupling.

5) Requirements for the timing of testing

Nondestructive testing shall be carried out only after the portable tank container has passed inspection for its shape, dimensions, and visual quality. If heat treatment is required, such testing shall be performed only after the heat treatment has been completed. In any case, nondestructive testing shall be conducted no sooner than 24 hours after welding is finished. For high-strength steels with a minimum yield strength of 395 N/mm² or greater, nondestructive testing of welds shall be performed no earlier than 48 hours after welding.

6) Determination of the detection range

The width of the inspection area shall encompass both the weld and the heat-affected zone. The width of the heat-affected zone shall be determined either by the actual measured data approved for the relevant procedure or by a 10-mm-wide zone on each side of the fusion line, whichever is greater. When performing segmented scanning along the length of a butt weld, there shall be at least a 20-mm overlap between adjacent scan segments; for circumferential welds, the termination point of each scan shall extend beyond the starting point by at least 20 mm.

7) Calibration of testing equipment

Prior to testing, it is necessary to verify the operability of each phased-array probe element to ensure that every element is capable of both transmitting and receiving ultrasonic waves. In addition, the performance of each transmit/receive module and the electrical conductivity of each channel’s cable must be confirmed.

For all phased-array probes, if more than 10% of the elements are defective, the probe shall not be used for this inspection.

After acoustic velocity and wedge delay calibration, the instrument’s depth and horizontal positioning accuracy for a 3-mm hole in the test block shall not exceed 1 mm. Following phased-array sensitivity TCG calibration, the beam-to-beam consistency error shall not exceed ±5% of the screen span.

The equipment shall be recalibrated whenever there are changes in the probe or wedge, probe cable type or length, ultrasonic instrument, inspection personnel, couplant, power supply type, focusing method, scanning method, software version, or other relevant parameters.

System verification and confirmation shall be performed at the start and end of each shift; if the continuous working period is prolonged, verification shall also be conducted every four hours.

8) Data Interpretation

Personnel performing data evaluation shall hold Level II or Level III qualifications. All inspection data shall be recorded, and any indications with a reflector amplitude exceeding 20% of the TCG shall be evaluated and documented. Defect records shall include information on defect type, length, depth, height, and location. All critical defects, such as cracks, lack of fusion, and incomplete penetration, shall be classified as nonconforming.

9) Test Report

All recordable information obtained during phased-array ultrasonic testing shall be documented in the inspection report to facilitate subsequent verification and review. The inspection report shall include, but is not limited to, the following: verification of the ultrasonic equipment, settings of test parameters, details of the welds inspected, description of the testing procedure, defect evaluation results, repair information, and details of the inspection personnel.

10) Repair and re-inspection of repaired portable tank containers

For welds that have undergone 100% phased-array ultrasonic testing, if any unacceptable defects are detected, the defective area shall be thoroughly cleaned and repaired by re-welding. The repaired area must then be re-inspected, and the original acceptance criteria shall be applied for assessment until the weld is deemed acceptable.

For welds subjected to localized phased-array inspection, if unacceptable defects are detected, the inspection length shall be extended at both ends of the defect, or two additional segments of the same weld shall be selected for inspection, with each extended or additional segment being no less than 250 mm in length. If the results remain non-compliant, the weld shall be cleaned and re-welded together with the corresponding test weld specimen, or the entire weld shall undergo 100% inspection.

A single unsuccessful repair of a detected defect may be permitted to be followed by another repair; however, the subsequent repair must involve an expanded removal of the base metal within the original heat-affected zone.

V. Conclusion
Ultrasonic phased-array technology, a rapidly advancing nondestructive testing technique in recent years, offers numerous advantages over conventional ultrasonic and radiographic testing.

However, at present, the relevant national standards and specifications for ultrasonic phased-array inspection and evaluation methods are relatively outdated, which to some extent has constrained the widespread adoption and application of this technology within China.

This paper synthesizes selected foreign ASTM and ISO requirements for phased-array technology, as well as the standards of two classification societies, and provides recommendations for manufacturers seeking approval from a classification society to employ phased-array technology for nondestructive testing of the shell and piping of portable tank containers. These recommendations cover aspects such as test procedures and test process design, key considerations during test execution, and the preparation of test reports.

This website reprints content originally published by our editors for the purpose of disseminating additional information; such reprinting does not imply endorsement of the views expressed or responsibility for the accuracy of the content. If the content involves issues related to intellectual property, copyright, or other matters, please contact us within 30 days, and we will promptly remove the relevant material.