TOFD (Time of Flight Diffraction) By OrientNDT Co.,LTD
Contents
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A. What is TOFD (Time of Flight Diffraction) ?…..2 B. Principle of operation ……………………………3 C. Feature of TOFD………………………………….4 D. External Links…………………………………….5 E. Instruction of TOFD-Scanner……………………6 F. Video clip for TOFD scanning in various fields…9 G. FAQ ……………………………………………….10
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A.What is TOFD (Time of Flight Diffraction) ?
Time of Flight Diffraction (TOFD) method of Ultrasonic inspection is a very sensitive and accurate method for nondestructive testing of welds for defects. TOFD is a computerised system that was invented in the UK in the 1970s for the nuclear industry by Dr. Maurice Silk. The use of TOFD enabled crack sizes to be measured more accurately, so that expensive components could be kept in operation as long as possible with minimal risk of failure.
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B.Principle of operation Measuring the amplitude of reflected signal is a relatively unreliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. Instead of amplitude, TOFD uses the time of flight of an ultrasonic pulse to determine the position of a reflector. In a TOFD system, a pair of probes sit on opposite sides of a weld. One of the probes emits an ultrasonic pulse that is picked up by the probe on the other side. In undamaged pipe, the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall. When a crack is present, there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Using the measured time of flight of the pulse, the depth of a crack tip can be calculated automatically by simple trigonometry.This method is even more reliable than Radiographic testing of a weld.
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C. Feature of TOFD ·
A computerised and automated system for weld inspection.
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Probes are mounted on a buggy that travels along a weld, recording data as it moves.
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Compared to conventional methods of ultrasonic inspection, TOFD is sensitive to cracks and measures their dimensions accurately.
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TOFD has two blind zones where it is not sensitive to defects, hence TOFD ought always to be supplemented by a conventional pulse-echo examination of the near and far walls. The pulse echo probes are commonly mounted on the same buggy as the TOFD probes.
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Requires ultrasound technicians with advanced training.
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D.External Links http://www.ndt.net/article/icem2004/papers/193/193.htm http://www.ndt.net/article/wcndt2004/pdf/petrochemical_industry/208_murugaiyan_corr ected.pdf
http://www.aaende.org.ar/sitio/biblioteca/material/T-127.pdf
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E. Instruction of TOFD-Scanner Portable Inspection Inspection Scanner
Features: o
Compact design for field inspections
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Built-in 30 dB pre-amplifier
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High-resolution scanning for pipes and flat surfaces
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Minimum pipe diameter of 2"
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Magnetic wheels for easy setup
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21" adjustable transducer distance
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Optional coupling kit
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Optional motorized TOFD scanner
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Optional extended cable up to 200'
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Available TOFD transducers up to 15 MHz
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Available probe cables with all kinds of connectors
Description: The pTOFD scanner is a single-axis coordinate scanner that, with the help of the UltraScan 5 system, is used to inspect welds on pipes and flat surfaces. The diameter of the pipe can be as small as 2 inches. The magnetic wheels are ideal
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for quick setup on ferrous surfaces.
A built-in water-proof pre-amplifier can be used for weak signal to provide the high quality signal for UT system. This is very helpful in some difficult testing conditions such as using 15 MHz TOFD probes and inspections on stainless steel. In addition it can be used for pulse/echo mode.
Other accessories include DC motor and controller, extended cables and coupling kits. The coupling kit consists of a couplant compressor, tubing, a Y-connector, and a flow-control valve. The 100-foot and 200-foot extended cables are built with differential line drivers and receivers which ensure the cables' reliable operation without attenuation.
The pTOFD scanner is designed to work with UltraScan 5, an ultrasonic inspection and analysis system. The system includes a portable computer containing an all-in-one pulser/receiver and high speed digitizer card.
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Specifications: Scan Resolution
0.0039 inch (0.1 mm)
Scan Range
2745'
Test Pipe Diameter
greater than or equal to 2" (50 mm)
TOFD Head Length 8" or 22" Pre-Amp Band Width 0.5 MHz to 30 MHz Pre-Amp Gain
30 dB
Temperature Range -40ºC to 60ºC ambience Humidity
20% to condensing Coupling kit TOFD transducers and wedges up to 15 MHz
Options
Transducer cables with all kinds of connectors 100' and 200' extended cable Scanner tracks for non-magnetic pipes
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F.Video clip for TOFD scanning in various fields http://www.youtube.com/watch?v=fOynJUtmWDg http://www.youtube.com/watch?v=WILmUskahNY http://www.youtube.com/watch?v=eNsKe8pjypY http://www.youtube.com/watch?v=J65zyyewf18
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G.FAQ q1) How can I read current position of TOFD sensor when scanning? ,does it have encoder-output ? The scanner comes with a DB25 male connector which will fit into the DB25 female connector on your board. There are two ways to read the encoder values in your software: q2) What kind of power-supply is suitable for TOFD scanner? You don’t need any power supply. The scanner will get the power from pin 5 of the DB25 connector. q3) What is best couplant for it ,for instance GEL or water-ball ,etc? Water is the best. The scanner comes with the wedge holders and there are two water tubes on each of them. You can feed the water through tubes. q4) How long is extention cable possible from sensor to Pulser/receiver ? There is a preamp on the TOFD scanner and it can drive at least 30 meters.
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