By Michael Moles Feature Phased Array Inspection of Small-Diameter Pipe Welds New scanner technology combined with focused arrays produced better defect length estimates Traditionally, small-diameter pipes have either not been inspected, or been inspected using radiography. However, radiography has significant limitations: safety and licensing issues, disruption to work schedules, chemical wastes, film storage, and poor detection of planar defects. Some codes permit manual ultrasonic testing, but that method suffers from a lack of recorded data and is highly dependent on the operator’s skills. In recent years, automated ultrasonic testing (AUT) has become commercially viable with the arrival of portable ultrasonic systems (Ref. 1), and now potentially offers both auditable and reliable results for small pipes (Ref. 2). However, the AUT solutions were generally lacking a reliable and low-profile delivery mechanism, and also had significant beam spread in small pipe diameters. Both these problems have been addressed with a new small-diameter pipe inspection system. While little AUT is currently performed on small-diameter pipes, there are potentially many applications: boilers, process piping and product piping in refineries, shipbuilding, pharmaceuticals, nuclear power plants, etc. Codes The arrival of ASME Code Cases 179 (Ref. 3) and 181 (Ref. 4) has permitted AUT of small-diameter pipe girth welds. ASME B31.1 CC 179 is a workmanship-based Code Case. ASME B31.3 does permit manual ultrasonic inspection of pipe welds, but that has the limitations listed previously (slow, operator dependent, no auditable results). As it is fracture mechanics based, B31.3 Code Case 181 needs accurate defect sizing and dimensioning. This is a difficult requirement for small-diameter pipe as the ultrasound beam naturally spreads (defocuses) on entry in the horizontal direction. This leads to defect oversizing, and hence higher repair rates. Phased arrays can focus the beam in an active plane, i.e., the vertical direction in the pipe, which theoretically permits better vertical sizing. However, it is questionable whether the vertical sizing accuracy is adequate for thin walls, but that is another issue not addressed in this article. In summary, there are codes in place, both workmanship or fracture mechanics based, that permit AUT on small pipes, but hardware that can perform an adequate job has been deficient until recently. Inspection Trends / Winter 2011 19 Fig. 1 — The COBRA scanner. Fig. 2 — Two-sided scan being performed on small-diameter vertical pipe.
Inspection Trends - January 2011 - Winter
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