Phased array probes get a better image20 August 2001
Phased array ultrasonic inspection techniques have many advantages over traditional multi-probe/multi-angle ultrasonic techniques in terms of speed, cost and performance. The technology is being adapted to an expanding range of applications. Rick Rishel, WesDyne International, Waltz Mill, Madison, PA, USA
Ultrasonic test (UT) probes using advanced phased array technology have recently being used in the USA by WesDyne International, a wholly owned subsidiary of the Westinghouse Electric Company, to achieve a more complete inspection of core shrouds in boiling water reactors and steam turbine blade attachments. Wesdyne is now adapting the technology to a range of utility inspection applications.
A better way
In phased array ultrasonic applications, a number of small, individual elements are strategically aligned within a single inspection probe. The number of elements can vary, from as few as eight, to as many as 128, largely dependent on the application. This contrasts with conventional ultrasonic applications where one or two larger elements are contained within a single inspection probe.
The single phased array ultrasonic probe achieves far greater coverage of a given inspection volume while stationary than four conventional ultrasonic probes that must be moved (Figure 1). Over an extensive inspection volume, the physical movement of the phased array probe is greatly reduced, resulting in shorter scan times. These in turn save costs in terms of labour, radiation exposure and critical path time. The small footprint of a single probe also improves inspection coverage, especially in applications with severe limitations in access or manoeuvring space.
The individual elements of a phased array ultrasonic probe are electronically manipulated to yield many different sound-beam angles simultaneously. These sound beams can be further manipulated to create many different focus-depths in the inspected component for better flaw signal-to-noise and resolution characteristics. Controlling the pulsing times of each element does this. The returning ultrasonic energy is then electronically summed across the elements and captured to produce an overall image of the inspected volume. Arrangement of the elements as linear, annular or mosaic arrays permits the creation of distinct sound-beam patterns that can uniquely match an inspection application. For example: a long, stationary, 128-element, linear array probe can inspect a linear section of material just by electronically shifting through the elements. In contrast, a conventional ultrasonic probe is limited to a single sound-beam angle or a single sound-beam angle with a single focus-depth, which produces a piecemeal image of the inspected volume.
In February and March of 2001, phased array methods were used to inspect a boiling water reactor (BWR) core shroud weld at Brunswick unit 2 in North Carolina (photo, p 47). This inspection was conducted following a performance demonstration on representative flawed specimens, as mandated by the BWR Vessel and Internals Project (BWR VIP) organisation, a nuclear industry alliance aimed at addressing BWR inspection reliability issues. The Brunswick application required the design of a single low-profile phased array ultrasonic probe that produces longitudinal wave beams from 30 to 80° in 1° increments for the simultaneous inspection of both the inner and outer diameter surfaces of a core shroud weld. This probe also had to produce a 0° longitudinal wave beam for coupling confirmation and measurements of wall thickness.
The low profile design enabled manipulation through extremely tight access windows. The goal was to inspect 100 per cent of the weld, a target never previously achieved at Brunswick for this weld. The inspection was to result in a 60 per cent reduction in the physical movement of the probe over conventional ultrasonic raster scanning (Figure 3).
A full 100 per cent scan coverage from both sides of the weld was obtained. The entire inspection process occurred in parallel with the fuel shuffle. Since the inspections were completed ahead of schedule, entirely within the fuel shuffle window, there was no impact on critical path time. In fact, this early completion allowed time for supplementary scans of two segments of an additional weld. These results are directly related to the use of phased array UT technology.
An added plus to this inspection was a simultaneous investigation to determine the presence of the core plate bolts. While the electronically-produced angle beams were interrogating the weld for defects, the phased array ultrasonic probe was also emitting a 0° beam that was used to verify that all of the core plate bolts were intact within the core support ring.
At Browns Ferry
Also in February 2001, dovetail-style turbine blade attachments were inspected at the Browns Ferry nuclear plant in Decatur, Alabama (Figure 3) using a phased array ultrasonic system with a single 32-element phased array ultrasonic probe applied from one position on the disc. Conventional ultrasonic techniques require multiple probes positioned to properly examine each hook, Figure 4.
This was the final phase of the qualification programme for this application. Phase 1 was an in-house technique definition on notched reference standards and a manipulator refinement on an actual disc at WesDyne. Phase 2 was a practical application on rotating discs in order to refine the technique (Figure 5). These full-scale discs contain an extensive matrix of notch defects on each of the blade-attachment hooks. The Browns Ferry discs contain known defects ranging from small pits to linear flaw indications confirmed by magnetic particle, visual and eddy current testing techniques applied with the blades removed.
The use of the phased array ultrasonics proved to be a valuable tool for this application. One side of the disc was completely scanned in six minutes using only a single pass. Areas of pitting and linear flaws were clearly observed on all the hooks and were consistent with the surface inspection methods applied.
In these two applications, the first for Wesdyne, phased array ultrasonic testing proved to be very powerful, saving time and money. Building on this experience, we expect to find wider applications of the technology in future inspections.
One example is ultrasonic inspection of axial entry blade attachments (Figure 6). Although the same type of cracking is being investigated as in the dovetail type blade attachment discussed above, the axial entry blade attachment configuration is more challenging for phased array ultrasonic techniques. Instead of a flat examination surface for the ultrasonic probe the surfaces are concave and/or tapered depending on the design. In addition, the hooks of the blade attachments are orientated 90° from the dovetail design and are concave in shape. Geometrical features will be much more pronounced in the data obtained.
Another potential application of phased array techniques Wesdyne is looking at is remote BWR pipe welds, where the weld is masked by a metal pipe sleeve with a small water gap. The ultrasonic energy must pass through the sleeve and through the water gap, then into the weld and then return to the probe, with energy redirection at each interface. Investigations are underway to see if the phased array approach can bring benefits in these challenging conditions.