Phased Array vs Radiography: Examining the Advantages of PAUT
Although it is on the decline, many nondestructive testing (NDT) analysts still use radiography for inspections. However, phased array ultrasound testing (PAUT) is rapidly replacing radiography and other less effective NDT methods as the new standard—and for good reason. When comparing phased array vs radiography side by side, the advantages of choosing PAUT become abundantly clear.
In our comparison below, we focus on pipe welding applications—volumetric inspections concerning defects such as cracks, porosity, and fusion, with a focus on accuracy regarding the location and detailed characteristics of defects.
Phased Array Methods
Phased array ultrasound inspection methods use a probe with multiple small transducers that are pulsed individually to control the focus and sweep of the beam. Software controls the timing of the pulses. The reflected pulses are processed and combined to create an image of any flaws or defects found in the material being inspected. Optimized beam focus and angle enable rapid scanning without moving parts, inspection from multiple angles, and the ability to inspect materials with complex geometries.
Phased Array Setup
Phased array probes and inspection instruments are compact and very portable. The simplest configuration is the probe connected to a handheld inspection instrument. They can run on battery power to facilitate remote or field inspections. Dual battery configurations allow technicians to switch battery sources while performing inspections, thereby extending uptime.
Software configurations can be created for the most frequent types of inspections, materials, and probes. Setup configurations can greatly reduce overall inspection time by allowing the technician to attach the appropriate probe, select the configuration, and immediately begin the inspection.
Portability, simplicity, and flexible configuration are compelling characteristics of phased array inspection devices and techniques.
Phased Array Speed and Accuracy
Phased array provides immediate results regarding the material or components being inspected. Advanced software incorporates full matrix capture (FMC) and total focusing method (TFM) techniques for data acquisition from as many as 64 channels. Powerful algorithms process acquired data to create detailed color images that give the technician detailed information on the location, size, and characteristics of flaws or defects. Phased array can also be combined with the time-of-flight diffraction (TOFD) method to further improve the accuracy of crack measurements.
Because phased array inspections are entirely digital, inspection results are instantaneous, providing immediate feedback to enable faster inspections, confirmation of weld quality or detailed identification of defects. PAUT instruments can easily generate detailed standardized inspection reports that can be printed or saved as digital records to meet compliance requirements. Additional advantages of phased array inspections techniques include:
- High probability of detection (POD) for planar flaws such as fatigue and stress corrosion cracks.
- Ability to assess through-wall depth attributes to accurately determine the location defects.
- Calculation of defect height measurements as an indicator of flaw severity.
- Ability to discriminate among defects stacked throughout the material thickness in a proximate circumferential location.
With the small form factor of phased array devices, inspections can be automated with the aid of a crawler for circumferential or axial weld scanning, as well as custom-engineered mechanical systems for high-volume inspections in manufacturing applications. Phased array provides accurate detection of both surface and subsurface flaws, and is ideal for weld inspections, wall thickness measurements, and corrosion detection.
Phased Array: Health, Safety, and Environment
Unlike radiography, phased array inspection techniques don’t involve hazardous materials or supplies, require additional licensing or certification, or generate waste. Phased array inspections can be conducted safely while other work is being done in close proximity.
Radiographic inspections use either portable X-ray or gamma ray generators to direct a beam at the materials being inspected. A detecting device—an imaging plate (undeveloped film sealed in a cassette) captures the beam as after it penetrates the material. Inspection quality depends on accurate alignment of the beam with the material surface as well as proper exposure time based on the properties of the welded material.
The process produces a two-dimensional image (radiograph) of varying densities according to the amount of radiation penetrating the material and reaching the film. Advancements in image plate technology now provide flexibility to wrap around pipes for the inspection of welds for improved inspection quality. Unlike digital phased array inspections which provide immediate results, the latent negative image on a radiograph must be processed using chemical developer, stop bath, and fixer. Radiographs provide a record of the inspection results. However, images must be scanned if a digital copy is required by compliance regulations.
Radiography: Speed and Accuracy
Depending on location, preparation for radiographic inspection may require permits and clearing the area of other personnel to ensure they are not accidentally exposed to radiation. Assessing the welding area to be inspected, the technician evaluates the best locations for the x-ray generator and image plate so that the generator and image plate are aligned properly.
Optimum inspection quality requires the radiation to be directed to the middle of the section being inspected and be normal to the material surface. Exposure area is depended on the material thickness. Here, standards established by organizations like the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API), as well as the experience and judgment of the technician play a critical role in equipment setup and testing.
The most frequently used inspection methods for round objects include:
- Panoramic single-wall exposure/single-wall view (SWE/SWV) with the radiation source placed in the center of a spherical or cylindrical object and image plates placed on the outer surface;
- Double wall exposure/single wall view DWE/SWV with the source located on the inspection item (the radiation penetrates both walls, but only captures defects located in the wall nearest to the film); and
- Elliptical DWE/SWV exposure with the source offset from the inspection plane and the elliptical image of the weld furthest from the source captured on film.
Once the image has been captured, the negative needs to be developed and interpreted. The trained technician then examines the negative for subtle variations that indicate any defect, its location, and size. Delaminations and planar cracks can be very difficult to detect using radiography, particularly to untrained technicians. Unlike phased array which creates a color image of the area inspected and applies algorithms to identify and visually contrast defects, accurate interpretation of the monochromatic radiographs requires extensive training and experience.
Radiography: Health, Safety, and Environment
Depending on the type, duration, and location of the inspection, radiographic inspections may also require that technicians bring safety equipment such as a Geiger counter, dosimeter, or thermoluminescent dosimeter (TLD) to monitor radiation exposure on site. Competent use of each of these items requires proper training.
Phased Array vs. Radiography
Phased array ultrasound testing is rapidly replacing radiography as the preferred method of testing pipe welds. Portability, convenience, inspection speed, and safety are four compelling advantages of the technology and the technique. In comparing phased array vs radiography, the evidence weighs heavily in favor of PAUT.
Zetec offers top-quality software and instruments that support phased array ultrasound testing to quickly and accurately detect flaws and defects in welded materials. To learn more about the latest PAUT products and innovations, contact Zetec today.
Zetec’s designers are industry-leading experts in ultrasonic and eddy current technologies, and we can help you navigate any of our NDT testing solutions or devices.