Enhancements in Non-Destructive Testing: Role of Phased Array Ultrasonic Testing (PAUT) in Process Piping Inspection

Process piping plays a critical role in various industries by facilitating the safe transportation of fluids and gases. Compliance with ASME B 31.3 standards is vital to maintain the integrity of process piping, reduce potential hazards, and meet regulatory requirements. 

While Ultrasonic Testing (UT) and Radiographic Testing (RT) have historically been key non-destructive testing (NDT) methods for inspecting process piping, the introduction of Phased Array Ultrasonic Testing (PAUT) has revolutionized inspection processes with its numerous advantages.

When to opt for PAUT over UT and RT

PAUT excels in specific scenarios compared to traditional UT and RT methods. 

The real-time imaging, volumetric scanning capabilities, and offline data interpretation of PAUT make it invaluable for conducting thorough inspections of welds. The precision of PAUT due to its high POD for detecting flaws like cracks, lack of fusion, and porosity ensures with accuracy, and excellent resolution during pre-service and in-service inspections makes it leading choice. 

Moreover, PAUT provides a compelling alternative to RT, particularly in environments where the use of radioactive materials is limited. By eliminating radiographic exposure and associated safety precautions, PAUT offers a safer and more environmentally friendly option. Additionally, its ability to inspect from one side of the material eliminates the need to access both sides of the piping, as required by RT.

Thickness range for PAUT instead of RT

The guidelines provided by ASME B 31.3, Appendix R, recommend utilizing fracture mechanics-based acceptance criteria for ultrasonic examination as an alternative to radiography. This allows to use PAUT in lieu of RT for thickness ≥ 25mm. In this context, PAUT is well-suited, particularly for thickness ranges where RT may be impractical or costly. 

While RT is typically impractical to perform for double wall thicknesses 75 mm or above i.e. single wall thickness > 35 mm (where DWSI technique is used) due to higher exposure time with Ir-192 radioisotope. PAUT can effectively assess thicker sections, extending the range of inspection to thicknesses exceeding this impracticable condition. In case of RT the exposure time increase exponentially whereas for PAUT thickness increase does not affect the inspection time much. This flexibility makes PAUT a preferred choice for evaluating the integrity of thick-walled piping components.

Similarly, in operating plant where the pipe diameters are less than 3” e.g. water wall tubes in power plant, quantity of joints are quiet higher and tubes are so close to each other most of the time the

requirement of taking 2 shots perpendicular to each other is not possible due to tubes alignment. In such cases where quantity is higher and difficulty to meet minimum number of exposure in RT, PAUT could be a great inspection methodology to ensure the quality within the time period and can help plant owner greatly

Considerations for workmanship and fracture mechanics-based acceptance criteria

The selection of acceptance criteria in PAUT inspections is crucial and depends on factors such as material properties, service conditions, and regulatory standards. 

In Phased Array Ultrasonic Testing, two approaches for acceptance criteria are commonly utilized: workmanship-based and fracture mechanics-based criteria.

For workmanship-based acceptance criteria, the decision to accept or reject is based on the amplitude of the signals received. Flaws are categorized as planar or volumetric. Planar flaws, such as cracks, lack of fusion, and lack of penetration, are deemed unacceptable. Volumetric flaws are evaluated based on their amplitude and length relative to the thickness of the material being inspected. Flaws exceeding specified thresholds are rejected, while those within acceptable limits are deemed acceptable.

In contrast, fracture mechanics-based acceptance criteria require a more in-depth characterization of flaws. Flaws are classified as surface or subsurface, and the aspect ratio, which is the ratio between flaw height and length, becomes a decisive factor in acceptance or rejection. Flaws with aspect ratios above a certain threshold may be rejected due to their potential impact on structural integrity, while those below the threshold may be accepted.

These approaches ensure that flaws are rigorously evaluated based on their type, size, and potential implications for the integrity of the inspected material. By employing either of workmanship-based and fracture mechanics-based criteria, phased array ultrasonic testing can effectively identify and assess flaws, contributing to the overall reliability and safety of the inspected components.

Personnel qualification and demonstration requirements

Ensuring the competence of personnel conducting PAUT inspections is essential to maintain result reliability and accuracy. ASME B 31.3 outlines specific qualification requirements for PAUT technicians, including training, examination, and practical experience. Moreover, demonstration requirements validate the personnel's ability to interpret PAUT data accurately and identify relevant indications.

A demonstration test is a critical component of the qualification process, where personnel must demonstrate their ability to perform PAUT inspections accurately and reliably. This test typically involves inspecting a series of test blocks or specimens containing known defects using PAUT equipment. The personnel's performance and technique adequacy is evaluated based on their ability to detect, characterize, and accurately size the defects according to specified acceptance criteria.

Though code/standard provide guidance when to perform demonstration and what are required to demonstrate but as a recommendation whenever the acceptance criteria is fracture-mechanics based, material is other than carbon steel (such as SS, DSS, Cu-NI, AL etc.) it is recommended to perform demonstration to qualify and establish the complete system.

Considerations for pipe diameter, thickness and material

Despite its versatility, PAUT presents challenges in particular scenarios, such as inspecting small-diameter tubes (<1") with thin walls (<3.5 mm). Specialized probes and equipment may be required to achieve adequate coverage and resolution. 

Conversely, when dealing with thick-walled piping (≥100 mm), factors like beam divergence and signal attenuation necessitate adjustments in inspection techniques, special focus probes, multi groups and other parameters. 

Additionally, PAUT can be applied across various materials like carbon steel, stainless steel, cupro-nickel, and aluminium, each needing meticulous calibration to mimic real-world conditions.

Advantages and limitations of PAUT

PAUT offers several benefits over conventional NDT methods i.e. UT or RT, including:

• Real-time imaging and volumetric scanning
• Enhanced sensitivity and resolution
• Versatility in inspecting complex geometries
• Reduced inspection time and cost
• Elimination of radiation exposure

However, PAUT also has limitations, such as:

• Dependency on skilled personnel
• Sensitivity to surface conditions
• Limited penetration depth in certain materials
• Higher initial investment in equipment and training

Conclusion

Phased Array Ultrasonic Testing (PAUT) marks a significant advancement in process piping inspection, providing unmatched capabilities and advantages over traditional NDT methods. 

By leveraging the precision and adaptability of PAUT, industries can ensure the integrity and safety of their piping systems while enhancing efficiency and compliance with regulatory standards. 

As technology progresses, the role of PAUT is poised to expand further, ensuring the reliability and performance of process piping across diverse industrial applications.

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