Knowledge

Time Of Flight Diffraction (TOFD)

Reliable Welding Inspections with high Detection Rate :


The ability to accurately size and monitor pre-existing or growing in-service discontinuities becomes more and more important in chemical and refining plants especially when a risk based inspection (RBI) approach is used. With the industry’s growing need for more reliable, simpler, accurate and cost effective techniques, Time of Flight Diffraction (TOFD), which was originally developed for crack-sizing in nuclear applications, established itself as a widely used tool for flaws detection in welds.


This unique ultrasonic technique has big advantages in speed, detection of defects and is the most accurate defect sizing technique in general use. One of the important advantages of using TOFD for weld inspection is the absence of radiation.


Furthermore, in several validation projects TOFD has proved itself to be a technique, which combines a high detection rate with a very high reliability in pre-service and in-service inspections.


IS RELIABILITY IMPORTANT FOR YOU?


Advanced Inspection Technique :


Time of Flight Diffraction (TOFD) does not rely on the reflectivity of the flaw but uses the diffracted sound initiating from the flaw tips. This means it can detect flaws irrespective of their type or orientation.


In a TOFD system, a pair of probes sits 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 principle has several advantages compared to the pulse-echo technique and/or radiography, such as:

Detection of gas, lack of fusion and binding defects, slag inclusions and cracks independent of defect orientation

Very accurate sizing of defects (±0.5 - 1.0 mm)
Nearly independent of fluctuation in surface conditions
Digital data storage (evaluation afterwards/high reproducibility)
High inspection rates achievable
No interruptions or delays in other operations because of lack of radiation
Inspection rate is nearly independent of the wall thickness of the object under examination


As a "stand alone" inspection TOFD has the highest reliability with respect to defect detection and excluding falsecalls. Besides, TOFD has turned out to be the most accurate and effective NDT technique in determining defect height.


The TOFD technique is a fully computerized system able to scan, store, and evaluate indications in terms of height, length, and position with a grade of accuracy never achieved by other ultrasonic techniques. The TOFD inspection system can be operated on a 12 Volts battery as well as on 110 to 240 Volts AC and is very compact, making it suitable for inspections at almost every location.



Application :


Two primary groups of applications can be distinguished Pre-service inspections

- Weld inspection of vessels, pipelines and plates In-service inspections
- Weld inspection of vessels, pipelines and plates
- Detection of corrosion underneath reinforcement rings of nozzles
- Cladding thickness measurement
- Determining depth of cracks


Non-parallelscan:


Perpendicularto probe beam direction
Most frequently used for weld inspection
Detection
Initial sizing
High speed inspection
Limitations

  • Defect depth measurement only accurate when probes are symmetrically positioned with regard to defect
  • Uncertainty on lateral position of defect results in height sizing error


Parallel Scan:


Parallel to probe beam direction
Accurate sizing and positioning
Limitation

Weld inspection: weld cap often reduces or makes impossible the extent of the scan.

So it should be removed!!!



Single zone



Multiple zone




Advantages:

  • Detection of lack of fusion and binding defects, slag inclusions and cracks independent of defect orientation
  • Very accurate sizing of defects (±0.5 - 1.0 mm)
  • Nearly independent of fluctuation in surface conditions
  • Digital data storage (evaluation afterwards/high reproducibility)
  • High inspection rates achievable
  • No interruptions or delays in other operations because of lack of radiation
  • Inspection rate is nearly independent of the wall thickness of the object under


Application:

  • Weld, forge, wrought parts
  • Weld inspection of vessels, pipelines and plates
  • Detection of corrosion underneath reinforcement rings of nozzles
  • Cladding thickness measurement
  • Monitoring crack propagation
  • Inspection rate is nearly independent of the wall thickness of the object under