TOFD instead of RT 3

[rapid1348]

Dear Experts,

I would like you to shed light on the possibility of using TOFD instead of RT in manufacturing a thick wall high poressure vessel.As far as I am concerned TOFD can be used but has some weaknesses like insensitiveness to surface defects up to 1/2".
What is your recommendation.Is it a good technique to be used while manufacturing a vessel or it is better to be used for NDT on the equipments being in the operation for a while?

Cheers

Reza

 

[SnTMan]

TOFD?

 

[metengr]

TOFD is an acronym for time of flight diffraction using ultrasonic testing. It is a sophisticated ultrasonic testing method that requires a computer data acquisition system.

rapid1348 ; My recommendation to you is to follow the requirements in the ASME Boiler and Pressure Vessel Code, Section VIII on unfired pressure vessels. All of the design rules pertaining to material, welding/fabrication, and examination requirements are provided to assure a safe operating vessel.

I have used TOFD on in-service vessel and piping examinations to look for sub-surface defects like creep cavitation damage and weld defects (lack of fusion).

 

[eyec]

TOFD vs RT?

sorry, RT is superior for what it does. TOFD is a new toy to play with when you have a lot of time on your hands (after you have taken all of your shots, read them and confirmed integrity).

 

[redrain63]

eyec,
please see ASME code case 2235-6 about Ut instead RT,

 

[rapid1348]

Dear Friends

Thanks a lot for your valuable posts.As a matter of fact, we are going to substitute TOFD with RT in an Ammonia Unitized chiller where thickness is round 105mm.The vendor has a good practice in this regard and has sufficient expertise but due to some limitations of TOFD like , insensitiveness to flaws in the depts of upto 1/2" and also cracks falling parallel to the surface of inspection can not be identified and as Metengr said it needs a computer software for data analysing, we are going to ask the vendor to do RT on the joints and not to accept TOFD.It looks to be useful for the in service equipments to be checked and I think it is better to have RT results for future refrences.
What is your idea?

Cheers

 

[bvi]

TOFD can be and is used as a substitution of RT. It is superior in that it can fully characterize and size defects that are found. It is permitted as a substitute for RT in the ASME Boiler and Pressure Vessel Code via Code Case 2235.

However, the use of TOFD requires substantisl skill and experience. You cannot go out and purchase equipment and start doing it. I believe at least a couple years of training and experience are appropriate. Note that the Code Case requires performance demonstration.

 

[pvesseleng]

Best way to read Code Case 2235-6 its very clear on cade case.
There isno ncessary to think which is more superior, which is not if Code allows you can do that.

 

[eyec]

riccio, thanks for the reference. i have not read that case yet.

bvi, you get a star for your elaboration. however, RT can also characterize & size defects.

in a nutshell, if Code allows then it becomes a management choice.

 

[JohnRTD]

We did lots of test to compare RT with tofd (time of flight diffraction) and we found that tofd has a POD of more then 90% (RT 60%). But TOFD has also it limitations a wall thickness below 1/3 of an inch RT has better results. For carbon steel it is the ideal replacement for RT in the feature. But for alloy steel before testing we have to do some tests.

 

[rapid1348]

Dear JohnRTD

Thanks for your comment , but could you please elaborate on "for alloy steel before testing we have to do some tests. "?
It would be appreciated if you kindly explain more about the tests.

 

[bvi]

TOFD is so sensitive that it can pick up grain boundaries in some materials, so with some materials, such as austenitic stainless steel, the results may not be as good. I believe that is what JohnRTD was alluding to requiring needing verification for specific alloys.