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- since 1996 -
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Technical Discussions
Ramu Chakka
NDT Inspector,
International Inspection Centre , Kuwait, Joined Sep 2010, 1

Ramu Chakka

NDT Inspector,
International Inspection Centre ,
Kuwait,
Joined Sep 2010
1
09:15 Sep-28-2010
TOFD for piping applications.

How accurate is TOFD in piping industry for thickness ( 5mm to 20mm)
Pipe dia. 2 inch to 16 inch.
How many scans are required with what angles?
How good is TOFD for this application?

 
 Reply 
 
gandhidasan
NDT Inspector,
StenaTech , Singapore, Joined Jun 2010, 32

gandhidasan

NDT Inspector,
StenaTech ,
Singapore,
Joined Jun 2010
32
14:00 Sep-28-2010
Re: TOFD for piping applications.
In Reply to Ramu Chakka at 09:15 Sep-28-2010 (Opening).

Dear Ramu,
TOFD is very good Technique for Sizing , Practically -/+ 1mm Accuracy is possible. For 5 to 20mm 10 to 15Mhz 70degree Probe is ideal. From my Experience one line Non Parallel scan is enough, If you need more accuracy one more parallel scan is very good for more accurate sizing(Parallel scan need flush grind on weld), TOFD is a one of very good technique.

 
 Reply 
 
andrew cunningham
NDT Inspector
Canada, Joined Jun 2008, 238

andrew cunningham

NDT Inspector
Canada,
Joined Jun 2008
238
16:28 Sep-28-2010
Re: TOFD for piping applications.
In Reply to Ramu Chakka at 09:15 Sep-28-2010 (Opening).

Hi Ramu
I have personally seen it totally miss a near through wall pit on a pipe line at BP in the UK. It was a well-qualified inspector not wet behind the ears scanned and interpreted his own results.
So treat it as any NDT method. “ Only as good as the tech on a good day”.
It can let you down if you believe in it blindly.
Crosscheck with another method when method when needed.


Andrew Cunningham

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
17:35 Sep-28-2010
Re: TOFD for piping applications.
In Reply to andrew cunningham at 16:28 Sep-28-2010 .

Andrew, in addition to your caution I would like to add more. The initial quesiton asked about accuracy on wall 5-20mm and diameters 2-16 inch. I would caution against using TOFD on welds less than about 7-8mm thick, especially pipe weldes where excess root metal and mismatch conditions exist. Added to these issues are the regular ring time issues. Even if you could get a true 15MHz pulse of 1.5 cycle into the test piece the Lateral ring time of 3mm and the backwall ring time of 0.5mm would reduce your effective region of assessment to 4.5mm. Add to this the problem of getting the probes close enough to the weld since pipe welds will not likely have the caps ground flush for UT inspection and you require about 28-30mm PCS for the 70° L mode. Signal processing can help in some cases but your system would need to be well equipped to process the data suitably.
Even if you found a flaw in the region after the lateral wave it would need to be greater than about 1.1mm to even start to size it using the normal tip diffraction principles. And even though I used 15MHz as the nominal frequency, the fact will remain that the wedge and metal will reduce the frequency content and your test will more likely be using a centre frequency nearer 8-10MHz.
EN 14751 makes allowance for TOFD on wall 6mm and over but it is not always reasonable or practical to use TOFD at the lower end of the allowed range.

 
 Reply 
 
Vicky
NDT Inspector, Blue Star Limited
IE , India, Joined May 2009, 29

Vicky

NDT Inspector, Blue Star Limited
IE ,
India,
Joined May 2009
29
08:18 Sep-29-2010
Re: TOFD for piping applications.
In Reply to Ed Ginzel at 17:35 Sep-28-2010 .

hello friends, As usual Ed has provided an perfectly reasonable answer for this query as well very informative to all. I would also suggest use of phased array rather then tofd in such cases.

regards

Vicky

 
 Reply 
 
nigel armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

nigel armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
11:45 Oct-18-2010
Re: TOFD for piping applications.
In Reply to andrew cunningham at 16:28 Sep-28-2010 .

Sorry for the late continuation - which method eventually discovered the through wall pit, Andy? Did the "not wet behind the ears" inspector adjust his technique parameters and so confirm the existence of the pit, i.e. was it down to wrong set-up? Though if its nearly through wall perhaps there were no accessible tips for diffraction?

 
 Reply 
 
Massimo Carminati
Consultant, AUT specialist
IMG Ultrasuoni Srl, Italy, Joined Apr 2007, 691

Massimo Carminati

Consultant, AUT specialist
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
691
08:05 Oct-20-2010
Re: TOFD for piping applications.
In Reply to Ramu Chakka at 09:15 Sep-28-2010 (Opening).

It is difficult to add something to Eds' analysis; in order to cover Lateral wave dead zone, you can "simply" add 2 "well designed" creeping wave probes; the main problem is the geometrical "defocusing effect" of small diameters pipes, and it is necessary to have different probe design for smaller pipes (focus and squint angles, crystal size etc..) . I would highlight the mismatch influence; the mismatch allowed by the construction code may be too high to permit a good TOFD inspection. In reply to Andrew, a through wall pin would have been properly detected by the creeping wave probes (both!).

The "simple" solution means:

1) 3 channels vs. 1 channel
2) probe design capabilities or a good special probes supplier
3) more complex scanner (4 probes instead of two)

but crosscheck evaluation is direct right after scanning and I believe this is absolutely a must when in site weld testing is performed.

 
 Reply 
 
Mal Hill
United Kingdom, Joined Jun 2012, 14

Mal Hill

United Kingdom,
Joined Jun 2012
14
13:28 Jul-28-2014
Re: TOFD for piping applications.
In Reply to andrew cunningham at 16:28 Sep-28-2010 .

Hi ANdy
I'd like to join Nigel in kindly asking the problems behind the missed pitting.I have witnessed TOFD scanning longitudinally on 8mm piping and seen accuracy to a couple of tenths of a mm ,crosschecked with an A scan thickness Gauge.I'm thinking your application was less than 5mm wall-For thin material the people I witnessed used snub nosed 70 and 75 degree special design probes by Phoenix. They were higher frequency and had a closer emmision point//smaller dimensions, allowing a bit of manipulation with the PCS for better calibration and results?

 
 Reply 
 

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