Time Of Flight Diffraction (TOFD)
This ultrasonic inspection is commonly performed on welds and weld overlays, piping, pressure vessels, clad material, storage tanks and structural steel. As fabricated heavy wall vessels and piping can be thoroughly assessed for fabrication flaws with TOFD.
- To verify compliance with fabrication requirements for piping and vessels in accordance to standards such as:
- ASME Section I,
- Code Case 189,or
- ASME VIII Division 1 and 2
- For in-service defect monitoring
- For detecting defects during the manufacturing process
- High degree of repeatability
- Position and size data for every flaw can be compared for repeat scans of the same area to track flaw growth or corrosion rates both generally and for individual pits
- Not limited by defect orientation
- Precise defect sizing capability
- Sensitive to all kinds of defects
- Portable, battery operated testing device
- On-line, volume inspection
- Very fast scanning
- Set-up independent of weld configuration
- Amplitude insensitive, acoustical coupling less critical
- Weld must be accessible from both sides
- There is a dead zone for detection near the surface of the material
- Gathers large volumes of information which takes experienced technicians to interpret
Time of flight diffraction (TOFD) inspection is an ultrasonic technique which allows for the location and sizing of defects. Two probes are used in a transmitter-receiver arrangement, with the transmitter introducing ultrasonic sound waves at an angle into the material, and the receiver picking up diffracted signals from any defects present. The amplitude of the signal is not displayed, but the position of the signals on the time scale is shown. This enables the technician to determine the defect location, length, and defect height.
The scanners and UT systems can perform both conventional pulse-echo (angle beam) and Time of Flight Diffraction (TOFD) scans simultaneously if required. Data is recorded in blocks (scans) anywhere from 12 inch to 210 inch in length. Successive scans began at the endpoint of the previous scan. To ensure complete coverage of the examination volume, a data point and depth reading can be taken every 0.04 inch in the axial (Y) scan direction before indexing by 0.200 inch in the circumferential or longitudinal (X) direction to begin the next scan stroke. This overlap of the search unit while indexing ensures that small flaws can be detected reliably.