PAN,

The TOFD is the best alternative for the inspection of the welds to RT and conventional UT, specifically designed for thick plates. For thin plates apply all the other methods, RT, UT, MPT, DPT, etc....
Find below an excerpt from one of the websites discussing the applicability issue of the TOFD;- try typing TOFD in your browser.

"While the Time of Flight Diffraction inspection technique is quite simple and straightforward, there are limitations of the technique. Even as the technique detects defects in weld, its surface and near-surface sensitivity is reduced by the width of the lateral wave signal. The lateral wave signal blanks the near surface defects. The surface and near-surface sensitivity can be improved by reducing the pulse width of the lateral wave signal. The same applies for inspection of welds in thin plates, e.g. less than 10 mm, where the time difference between the lateral wave and LL wave is significantly reduced. The simplest way to increase the time difference between the lateral wave signal and the LL wave signal is to use transducers with the small pulse widths. To improve the examination, the following parameters have to be optimized and incorporated into the procedure.

Transducer frequency: Higher transducer frequency reduces the pulse width thereby increasing the aperture between the lateral wave signal and LL signal. However, higher frequencies reduce ultrasound penetration.
Transducer diameter:  Smaller transducer diameter reduces pulse width. However, smaller transducer means lower sensitivity or lower signal to noise ratio.
Increased damping: Increased transducer damping reduces pulse width. Typical pulse width for TOFD transducers should be approximately 1-½ cycle. However, increased damping reduces sensitivity.
Probe separation distance: Smaller probe separation will increase the time difference between the lateral wave signal and the LL wave signal. Probe separation is normally set to obtain beam intersection at 2/3 t to t."
Cheers
gr2vessels

PAN

Your major concern should be understanding the specific requirements of the Code to which you will be working and the associated acceptance criteria.

All the Code Cases require production of a qualification block made from project material containing intended reflectors (top and bottom surface notches and a mid-wall reflector) of specified dimensions. The chosen technique parameters suitability is demonstrated by the ability to clearly detect these reflectors when scanned in the same way as will be used for production welds.

If a TOFD method optimised for the smallest feasible top and bottom surface "dead zones" cannot clearly detect the top and/or bottom artifical reflectors then you will need to consider the use of pulse-echo probes or some other NDE method (e.g. Eddy Current)for those regions.

Consultation with a TOFD-experienced NDE company with an appropriately qualified Level III (e.g. EN 473 and not ASNT as, at present, there is no TOFD element in the latter exam)will help minimise wasted time and money on getting the technique right.

B31 Case 181 is the appropriate Code Case for pressure piping and Table 1 is the acceptance criteria for welds of thickness less than 1 inch.

Nigel Armstrong
Karachaganak Petroleum
Kazakhstan

Nigel,

Summarily dismissing an ASNT level III is to dismiss hundreds of ASNT UT level III's who have extensive TOFD experience including myself. It doesn't matter as much which certification scheme your under, as much as it does knowing what your doing through extensive training and experience. I have meet good and bad ones in my career from around the world (including EN473).

Regards,
usndeIII

usndtIII

My apology for appearing to be offhand with ASNT level III -it was not my intent to denigrate any certification scheme and I most certainly do not want to debate trhe relative merits of central and employer certification.

However there are EN473 Level II's and III's specifically qualified in the ToFD method - I should have been more specific in denoting the qualification. Even an inexperienced contractor would still be wise to ask for the written pratice and CV's of any  practitioners, whether EN 473 UT Level 3 ToFD or ASNT UT Level III in order to gauge work depth of experience on similar projects. However many ASNT UT Level III's will NOT have had specific training or experience with the method and the customer would need to be aware of how the practitioner is trained and qualified. Is ACCP qualification for TOFD  available in the USA?

Nigel Armstrong
Karachaganak Petroleum
Kazakhstan