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|NDT.net Issue - 2004-11 - Articles ||NDT.net Issue: 2004-11|
Publication: 16th World Conference on NDT - 2004 - Montreal (Canada) (WCNDT 2004)
Session: Petrochemical Industry
Time of flight diffraction (TOFD), an advanced non-destructive testing technique for inspection of welds for heavy walled pressure vesselsR Murugaiyan
Manager (Inspection), Kochi Refineries Ltd ,KRL, Cochin, Kerala, India
Abstract: Time Of Flight Diffraction (TOFD) is an advanced automated computerized UT based NDT technique, used for in-service inspection of welds for heavy walled pressure vessels. TOFD system is capable to scan, store and evaluate flaw indications in terms of height, length and position with greater accuracy and is suitable for weld thickness ranging from 1" to 16".
TOFD technique is based on diffraction of ultrasonic waves on the tips of discontinuities instead of reflection on the interface of discontinuities. When ultrasound is incident at linear discontinuities such as crack diffraction takes place at its extremities in addition to the usual reflected wave. The diffracted energy is emitted over a wide angular range, presumed to originate at the extremities of the flaw.
Scanning is done externally parallel to weld axis using longitudinal probes with incidence angle range of 45° to 70° and when flaw is detected scanning in perpendicular direction for better evaluation of flaw. Signals from the upper and lower tips of the flaw are displayed as D-Scan image.
Recently, the author had witnessed TOFD conducted by M/s. Japan Steel Works in India at Indian Oil Corporation, Gujarat Refinery for 2 Nos. in-service hydro-cracker reactor of 223 mm thickness after 10 years service.
ASME had approved TOFD for ultrasonic examination of welds and in lieu of RT for thickness over 4". British Standards Institute had formulated Standard BS: 7706 - 1993 for TOFD.
TOFD benefits include real time weld integrity assessment, higher accuracy in flaw detection/sizing and digitized storage of TOFD records. Insensitiveness to surface defects up to ?" deep, high cost and the need of experienced operator are limitations of TOFD.
Keywords: Ultrasonic Testing (UT) (4099), time-of-flight diffraction (TOFD) (131), pressure vessel (117), weld (404),