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Circumferential Scanning in Ultrasonic Inspection

Conventional contact ultrasonic inspection is typically performed on relatively flat surfaces. Indications for common configurations such as welded plates and T and Y connections can be plotted and located with ease. In many cases newer ultrasonic units will even do the calculations for the inspector as long as accurate information for wedge angle, wall thickness, wedge delay, and in some cases, the outside diameter (OD) and inside diameter (ID) is input into the ultrasonic unit. However, on those occasions when inspection is performed in the circumferential direction, consideration should be given to what the sound beam is actually doing in the test specimen. When conducting circumferential scans, it is important to understand that the sound path for one leg of sound is going to be increased based on the ratio of the inside diameter to the outside diameter. If the refracted (inspection) angle selected is too great, a vee path will not exist and a complete inspection of the cross section will be impossible to achieve. Figure 1 shows the differences between a flat scan and a circumferential scan using the same refracted angle. As can be seen, both the sound path and the surface distance are greater for a curved part than for a flat part of the same thickness.

References
  1. 1. Clark, B. and R. Stanley. Section 20, “Ultrasonic Testing Applications in the Chemical Industries.” Nondestructive Testing Handbook, second edition: Vol. 7, Ultrasonic Testing. Columbus, OH: American Society for Nondestructive Testing (1991): p 714.
  2. API RP 2X, Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians. Washington, DC: American Petroleum Institute (1996).
  3. ASME Boiler and Pressure Vessel Code: Section V, Nondestructive Examination. New York, NY: ASME International (2009).
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