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Development of a Scanner and a Control System for an Automated Ultrasonic Testing of Pipe Elbows

It is quite difficult to use conventional scanners for an AUT of pipe elbows, because the curvature of inside radius section is different from that of outside radius section. To solve this problem, a few commercialized scanners use two guide tracks. One is for the guide of longitudinal movements and the other is for that of transverse movements. The scanners of these types have many limitations to be applied to the AUT of piping elbows which have different curvatures like nuclear power plant pipes of which the spaces are narrow among the complex pipe lines. In order to solve these problems, both a new scanner and a new control system are developed for the automated ultrasonic testing of pipe elbows. Instead of a longitudinal guide track and an encoder for guiding and measuring longitudinal movements, we devised a scanner and a control system using surface wave. A surface wave transducer attached to the scanner receives the signal transmitted from another transducer which is fixed on the pipe. The distance between two transducers is calculated immediately, and the current scanner position is generated. This new system can avoid an inaccuracy caused by possible slip of encoders and can remove a curved longitudinal guide track. Introduction An automated ultrasonic testing (AUT) is widely applied to the examination of pressure vessels and pipes to detect and size flaws. In this AUT, the position information of a transducer is very important to locate flaws exactly [1] . In order to obtain position information, guide tracks are usually used with encoders in the AUT systems. We need to inspect erosion and corrosion from pipe elbows which have been reported in the nuclear power plants, typically feed water or main steam pipes [2-5] . Fig. 1 shows two typical AUT systems developed and commercialized recently for the AUT of pipe elbows. Both systems use two guide tracks. One is the circumferential track and the other is the axial track. These tracks need a lot of spaces for their installation among pipe lines. The spaces in the power plant or petrochemical plants are not enough to accommodate guide tracks. Another possible problem is that scanners may be slipped during the scanning due to irregular surfaces or elbow curvatures. Then, the encoder information may include positional errors more or less. This would become more significant problem in the movement of axial direction along with the outside or inside radius section than in the circumferential direction. In order to overcome these kinds of problems, a new scanner was developed which enables us to get position information of transducer in the axial direction along with the outside or inside radius of elbow surfaces without using a guide track and an encoder. To get positional information and control a scanner, we measured a surface distance by using surface wave along with pipe elbows. Position Control Technique Position information is generally obtained from encoders in the conventional AUT of straight pipes or flat surfaces. However, any scanner slip takes place on the pipe, some errors more or less is inevitable in the position information [6] . In this case, flaw locations and its size can not be measured exactly. Slip is a major disadvantage in the scanner with an encoder. Fig. 2 shows a conventional AUT system Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 372-377 doi:10.4028/www.scientific.net/KEM.270-273.372 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:37:44) Title of Publication (to be inserted by the publisher)