PAUT: Phased array ultrasound for weld inspection

PAUT (Phased Array Ultrasonic Testing) has established itself as one of the most accurate and efficient techniques in Non-Destructive Testing applied to weld inspection. Its ability to reveal defects in welds through volumetric scanning with full digital traceability makes it an indispensable tool for maintaining reliable and safe assets.

In the energy and industrial sectors, where mechanical integrity is a priority, ultrasonic phased array allows for fault detection, optimization of maintenance times, and compliance with international standards ASME Section V and API 1104, bringing precision, safety, and efficiency to industrial inspection programs.

Mechanical integrity, key to reliability

The success of any energy or petrochemical operation depends on the ability to maintain its assets in optimal condition. Equipment such as pipes, vessels, exchangers, and boilers are subjected to pressures, temperatures, and cycles that require exhaustive monitoring of their structural condition.

International organizations (ASME, API, and ASNT) define standards that regulate the quality and traceability of inspections, seeking to prevent incidents and ensure safety. In this environment, PAUT technology emerges as an advanced solution for monitoring the internal condition of critical materials, offering reliable real-time information and strengthening mechanical integrity strategies.

From operational challenge to technical solution

The modern energy industry operates on an interconnected network of refineries, petrochemical plants, and transportation systems. Each facility depends on assets (pipes, vessels, exchangers, tanks) subjected to temperature, pressure, and load cycles. In this chain, welding is the point where stress, metallurgy, and manufacturing converge; a critical area where discontinuities that escape the eye can emerge.

Mechanical integrity balances production and safety under international standards (ASME, API, ASNT). To maintain this balance, the advanced Non-Destructive Testing ecosystem provides technical evidence without altering the material. This is where ultrasonic phasing makes the difference: it allows the interior of the welded joint to be observed with high resolution, speed, and digital recording.

PAUT: Phased Array Ultrasonic Testing

What is it?

Phased array ultrasound is a non-destructive testing technique that uses a set of electronically controlled piezoelectric elements within a single probe. The principle involves emitting ultrasonic waves into the material to identify defects in welds such as cracks, porosity, inclusions, or lack of fusion.

How does it work?

Each element of the transducer is activated synchronously by controlled delays, generating ultrasonic beams that are electronically focused and directed toward different areas of the material. This modulation allows for electronic scanning without moving the probe, covering large volumes and obtaining high-resolution 2D and 3D images.

The result is an accurate reading of the interior of the weld bead, where the location and size of each indication are determined with millimeter precision.

What is it used for?

Phase-array UT is widely used in the naval, aeronautical, oil, energy, and nuclear sectors, where asset integrity is critical. Its main field of application is the inspection of weld joints in pipes, pressure vessels, and storage tanks, both during manufacture and maintenance.

It is also used for the early detection of cracks, including HIC (Hydrogen Induced Cracking), SOHIC (Stress Oriented Hydrogen Induced Cracking), and SCC (Stress Corrosion Cracking), as well as in the inspection of composite materials and corrosion mapping on metal components.

Its ability to measure defects with millimeter precision makes it a key tool for calculating the remaining useful life of equipment and structures subjected to fatigue, pressure, or corrosion, directly contributing to the reliability and operational safety of the plant.

Advantages over conventional ultrasound

Phased Array Ultrasonic Testing combines precision, coverage, and safety, positioning itself as one of the most effective techniques in industrial ultrasound. Its benefits include:

1. Coverage and speed: Allows large areas to be scanned in a short time, while maintaining superior resolution. Example: When inspecting a pipeline, an automated ultrasonic phased array system covered more than 10 meters of weld in less than an hour, reducing scheduled downtime.
2. Volumetric accuracy: By emitting ultrasonic beams from multiple angles, it generates detailed cross-sectional images that increase the likelihood of detecting internal defects. Example: During the inspection of a root weld in a storage tank, the ultrasonic phased array identified a lateral crack measuring just 1.5 mm that would have gone undetected with conventional UT.
3. Repeatability and traceability: Results can be accurately reproduced and stored digitally, facilitating historical comparisons. Example: In an annual integrity program, repeating an ultrasonic phased array scan confirmed the stability of a repaired weld, with no evidence of defect propagation.
4. Geometric adaptability: Its ability to direct the beam without moving the probe allows for the evaluation of hard-to-reach areas and irregular geometries. Example: A phased array ultrasound scan performed on a 90° pipe elbow detected internal porosity without dismantling the section, taking advantage of the electronic focusing of the beam.
5. Operational safety: Eliminates the use of radiation and reduces personnel exposure to hazardous environments. Example: At a petrochemical plant, replacing radiography with ultrasonic phased array allowed inspections to be performed without evacuating adjacent areas, keeping production active.

This technological integration provides high-resolution internal images, reduces downtime, and offers complete traceability, establishing it as the most effective technique for ensuring the structural integrity and reliability of industrial assets.

Most relevant disadvantages or limitations

1. Highly specialized: Requires certified personnel trained in digital analysis, acoustics, and phased calibration. Example: Level II or III engineers or technicians, qualified under ASNT or ISO 9712, are necessary to properly configure the equipment and analyze the results.
2. Significant initial investment: Specialized equipment, scanners, and software are more expensive than conventional UT. Example: The purchase of a phased array ultrasound system with TFM imaging software can exceed $30,000, although its productivity reduces inspection costs during major shutdowns.
3. Demanding calibration and configuration: Requires precise calibrations with reference blocks representative of the actual component. Example: To inspect a nozzle weld, it was necessary to manufacture a calibration block with geometry identical to the fitting, ensuring the validity of the scan.
4. Surface condition: Surfaces with paint, rust, or excessive roughness affect ultrasound transmission. Example: On a pipe with an epoxy coating, partial cleaning was required to improve coupling and obtain a clear signal.
5. Restrictions in coarse or attenuating materials: Acoustic dispersion in coarse-grained materials or austenitic steels can reduce inspection depth. Example: In a stainless steel pressure vessel, signal loss required supplementing the test with TOFD to ensure complete detection of discontinuities.
6. Big data management: The method generates large amounts of digital information that requires appropriate software and storage. Example: A phased array ultrasound campaign with 500 scans generated more than 40 GB of data, which had to be integrated into a central predictive maintenance database.

These limitations do not reduce the effectiveness of phased array ultrasound, but they underscore the need for technical planning, competent personnel, and advanced digital tools to maximize its potential.

What welding defects can PAUT detect?

The method allows for the identification of a wide variety of defects in weld joints:

• Lack of lateral or root fusion.
• Isolated or clustered porosity.
• Slag inclusions or metal inclusions.
• Thermal or fatigue cracks.
• Laminations in the base material or in the heat-affected zone.

Thanks to the volumetric coverage of phased array ultrasound, inspectors obtain information on the location, size, and orientation of each indication, facilitating accurate decisions on whether to accept or repair the weld.

PAUT implementation rules

The use of this ultrasonic inspection technology in industrial inspection is governed by an international regulatory framework that ensures consistent and verifiable results:

• ASME Section V – Article 4: defines procedures for ultrasonic testing of welds, equipment qualification, and personnel qualification.
• API 1104: establishes specific requirements for inspecting welded joints in hydrocarbon transport pipelines.
• ISO 13588 / EN ISO 17640: provide guidelines for the application of phased array ultrasound and the evaluation of results.

Compliance with these standards ensures traceability and acceptance by regulatory authorities, contractors, and integrity auditors.

Expert perspective from ASNT 2024

During the ASNT 2024 conference, Eddyfi Technologies publicly highlighted PAUT’s progress in advanced inspection. Paul Hillman, Solutions Development Expert, put it clearly:

“PAUT technology is enabling higher resolution and the ability to detect small defects, even those that are difficult to find with other techniques, such as guided waves.”

Below is the full video of this speech, recorded by Inspenet:

This international perspective reinforces the role of phased array ultrasound in the evaluation of critical welds and paves the way for understanding the true scope of the case study presented below.

PAUT application: Eddyfi Technologies case study

The true value of Phased Array Ultrasonic Testing is revealed when theory is confronted with the challenges of the real industrial environment. An outstanding example is the work of Eddyfi, an Inspenet partner company, which applied this advanced technology to solve a critical problem in the inspection of stainless steel and dissimilar metal welds: one of the most demanding tests for any ultrasound technique.

These joints, where materials such as carbon steel, Inconel®, or austenitic stainless steel are combined, present metallurgical and acoustic differences that scatter the ultrasonic beam and reduce detection sensitivity in conventional methods.

To overcome this obstacle, Eddyfi integrated Full Matrix Capture (FMC), Total Focusing Method (TFM), and Plane Wave Imaging (PWI) into its Emerald PAUT platform, achieving high-resolution three-dimensional images and a notable increase in the probability of detection (POD) of cracks and internal discontinuities.

The study demonstrated that the phased array ultrasound system can not only adapt to complex materials, but also accelerate the inspection process by up to 35%, reducing operating costs and eliminating the need for radiographic techniques.

Thanks to the digital reconstruction of the welded volume, the results were visualized in real time and with complete traceability, complying with ASME Section V, API 1104, and ISO 13588 standards.

This case study consolidates Phased Array Ultrasonic Testing as a tool capable of transforming the inspection of complex weld joints into a precise, efficient, and fully digital process, demonstrating how technology makes mechanical integrity a verifiable reality.

Read the full case study: “Advanced inspection of welds in stainless steels and dissimilar metals with PAUT” – Eddyfi Technologies.

Editor’s note: Although conventional Phased Array Ultrasonic Testing may have limitations in coarse-grained or attenuating materials, such as austenitic stainless steels, due to the dispersion and loss of ultrasonic beam energy, this case study from Eddyfi Technologies does not contradict this condition: it surpasses it.

By integrating advanced technologies such as Full Matrix Capture (FMC), Total Focusing Method (TFM), and Plane Wave Imaging (PWI), Eddyfi demonstrated that it is possible to maintain high resolution, sensitivity, and inspection depth even in metallurgically complex joints.

This result confirms that advanced Phased Array Ultrasonic Testing represents the natural evolution of the method, expanding its range of application in difficult materials and consolidating it as a key tool for modern mechanical integrity.

PAUT implementation in mechanical integrity programs

Integrating UT by phased array into mechanical integrity programs requires a structured approach:

1. Critical asset identification and weld prioritization based on operational risk.
2. Proper NDT method selection, combining ultrasonic phased array testing with TOFD or TFM depending on the material type.
3. Personnel qualification in accordance with ASNT or ISO 9712 standards.
4. Equipment configuration and calibration using representative blocks.
5. Automation and digitization to improve repeatability and data analysis.
6. Results management, feeding databases for predictive maintenance.

This approach allows inspection to evolve from a reactive action to a predictive tool, maximizing the reliability of industrial assets.

Comparison: PAUT versus other methods

Technique	Key benefits	Limitations
UT with phased array	Volumetric inspection, radiation-free, immediate results, digital recording	Requires specialized personnel and precise calibration
Conventional UT	Economical and versatile	Lower coverage and resolution in complex welds
X-ray	Good contrast for inclusions and porosity	Use of radiation, long exposure times, not suitable for plants in service, high risk.

The choice of method depends on the geometry, the type of defect expected, and the access conditions, but in most industrial scenarios, phased array UT offers the ideal balance between accuracy, safety, and productivity.

Conclusions

PAUT redefines modern weld inspection. Its ability to detect defects in welds with high resolution, without radiation, and in accordance with ASME Section V and API 1104 guidelines, makes it the most reliable technique for ensuring the mechanical integrity of critical assets.

Driven by technologies such as those developed by Eddyfi Technologies, industrial phased array ultrasound continues to establish itself as one of the most efficient, accurate, and sustainable advanced non-destructive testing methods in the industrial sector.

From the energy industry to welding seams, phased array ultrasound represents the perfect convergence of innovation, precision, and sustainability in industrial asset management.

“Optimize your inspections: adopt PAUT and increase the reliability of your assets.”

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This article is part of the editorial line of Inspenet, official media partner of global events such as GASTECH, API, AMPP, SLOM, and others.

References

1. Z. Gustiani & H. Uzun. “Review of Phased Array Ultrasonic Testing of Weld Joints.” Journal of Marine and Engineering Technology, vol. 4(2), 2024, pp. 77-92
2. N. Jose & H. Azari. Implementation of Phased Array Ultrasonic Testing (PAUT) for Bridge Welds. Technical Report FHWA-HRT-24-010, U.S. Department of Transportation, Febrero 2024
3. E. Nicolson, E. Mohseni, D. Lines, K. M. M. Tant, G. Pierce & C. N. MacLeod. “Towards an in-process ultrasonic phased array inspection method for narrow-gap welds.” NDT & E International, vol. 144, 2024.