Curved arrays for improved horizontal sizing in small pipe welds

In the past, most small diameter pipe welds were radiographed for defects. However, radiography has significant limitations: safety and licensing issues, disruption to work schedules, chemical wastes, and poor detection of planar defects. Manual ultrasonics was permitted by some codes, but suffers from lack of recorded data and dependence on the operator’s skills. Automated ultrasonic testing (AUT) has only become commercially viable since the arrival of portable ultrasonic systems(1), and now potentially offers both auditable and reliable results(2). The arrival of ASME B31.3 Code Cases 179(3) and 181(4) has permitted AUT of small diameter pipe girth welds. Code Case 181 in particular needs accurate defect sizing and dimensioning, which is a difficult requirement for small diameter pipe as the ultrasound beam naturally spreads (defocuses) on entry. This will lead to defect oversizing, and hence high reject rates. Phased arrays can focus the beam in an active plane (the axial direction in the pipe), which technically permits better vertical sizing. However, until recently only matrix arrays had the capability of focusing the beam in the horizontal (circumferential) plane, and even their capability is limited. Matrix and curved arrays offer practical solutions for focusing in both axial and circumferential directions. Initially, modelling was performed, which showed that only two curvatures were required to cover essentially all small pipe diameters, independent of wall thickness. Matrix arrays did not offer many benefits over curved arrays, and would be more expensive and complex to implement. The larger radius curved array was manufactured and tested on known reflectors and compared with a standard flat (unfocused) array. Curved arrays have been developed previously for larger pipe diameters(5). This paper describes the results of these tests and shows that curved arrays offer significantly improved sizing in practice. These arrays can be implemented with no extra hardware or software. Modelling