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TOFD - Time of Flight Diffraction Technique

Time of Flight Diffraction is a rapidly increasing technique with unique features.High accuracy defect sizing is combined with high inspection speed. This combination provides a fast and high quality technique for the inspection of welds and other critical areas in installations. Other than most NDT techniques, TOFD is able to measure defect length and through thickness height, which are he key parameters in mechanical strength calculation.

TOFD principal

TOFD inspection applies two ultrasonic probes on either side of the weld. The transmitter probe emits a wide angle beam covering the complete weld volume and heat affected zone in one scan. Thicker components (>50mm) are subdivided in several depth zones. Possible defects diffract the sound beam at the upper and lower defect tips and the receiver probe picks up diffraction signals.

Accurate defect sizing

Defect sizing is based on position mapping of the diffraction signals. The defect height can readily measured by straight forward subtraction of lower and higher defect tip positions. A typical figure for the accuracy that may be achieved in through thickness height measurement is 0.5 mm. Surface breaking defects are discriminated from embedded defects; volumetric defects from planner ones. An important advantage of TOFD, compared to conventional ultrasonics and radiography, is a high probability of detection, virtually independent on it defect orientation.

TOFD weld inspection

The preparation required for TOFD scan is minimal, which makes the technique attractive even when only a small number of welds have to inspected. TOFD may be applied during construction, where time constrains exist. TOFD allows examination directly after welding (up to 200 C) without hold up of production speed; acceptance result are directly available. TOFD is a fast safe and cost effective alternative to radiography.

On stream inspection with TOFD

In contrast with radiography , for TOFD examination only external access to the object is required. In the service stage of process installation and pipework, TOFD may be applied ‘ on stream’ to inspect equipment while it remains in service.‘ 'Fingerprints’ of the object are taken to monitor initially acceptable defects and reveal and progressively monitor service induced defects.

TOFD applications

TOFD is applicable for weld inspection of pipes or plated in a thickness range of 6 mm upward (also heavy wall vessels up to 300 mm can be examined), pipe to plate nozzles, complex joints etc. All types of welding defects including lack of side wall fusion and root penetration can easily be detected and sized. FOR TOFD inspection, the feasibility of coarse –grained materials such as several types of austenitic steel, is usually assessed on a case to case basis by means of test specimen. Varying concentration Ni and Cr alloys as well as duplex materials can be inspected successfully.




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