Non-Destructive Testing of Welded Steel Penstocks 1

[21121956]

Hello everybody:

Even when I know that it is not good to double post, here I am posting in this Forum making an attempt to obtain some advice regarding this subject.

Reading the attached Excel page, penstock in hydropower plant B has a thickness t = 22 mm; for plant C have a thickness t = 25 mm; for plant D, t = 9,5 mm and for plant E, t = 12 mm to t = 25,4 mm. The diameters are between 2,00 m and 3,50 m.

Hydropower plants A, B, C and D are in commercial operation; plant E is under construction.

The material for these penstocks is steel S355JR that is equivalent to steel A572 Gr60 (P1, Gr2). According to Table UCS-57 of ASME Code Section VIII, Division 1 – 2010, the thickness above which full radiography examination of butt joints is mandatory is 1¼ (32 mm).

I wonder why so different specifications for nondestructive testing were delivered for penstocks of these hydro plants; particularly, the radiographic test.

Am I misunderstanding the code?

Few days ago, I was talking with a Certified Welding Inspector regarding to the radiographic inspection on a trifurcation and he told me that those inspections were very difficult to perform, so instead of that, it has become a common alternative practice to achieve Ultrasonic Test on bi and trifurcations.

Thanks in advance for your comments.

El que no puede andar, se sienta.

 

[weldstan]

Assuming that ASME VIII, Div. 1 is being invoked for construction/inspection of the penstock, why are you asking the question regarding RT when the max penstock thickness < 1 1/4"?

Regarding the penstock NDE requirements provided, you need to ask the Engineer who defined the requirements.

Regarding whether to RT or UT the bi/trifurcations, the effectiveness will be based on weld thickness variation along the length of the weld. Because RT will/should be done incrementally to meet geomatrical unsharpness, it may readily be adapted to perform the inspection.

 

[21121956]

Hello everybody:

weldstan, thanks for your reply.

Regarding with the maximum thickness of the plates for the penstock of all those plants, thickness which is certainly less than 32 mm, that is precisely why I am wondering why the Specifications for NDT indicate the necessity to perform RT.

As for your suggestion to ask the Engineer who defined the requirements, I think it is rhetorical, because I have found this information in documents especially prepared for projects in different countries.

I want to point out that, with Non-Destructive Testing specifications so different, I can not help thinking that they are sometimes (if not always) written by specialists in "copy and paste". Finally, where I come from, the parrots are green wherever you go, and none can change it at their convenience, a code is it, a code.

El que no puede andar, se sienta.

 

[weldstan]

In the USA, historically, RT was the choice for volumetric examination primarily for 2 reasons: the film image record (abvailable for examination by multple examiners) and the ease of obtaining radioactive isotopes (mainly for fieldd applications). Many examination requirements of clients have been based on USA Engineering preferences.

Finally there is legal merit to maintaining the "status quo" should failures occur involving litigation.

 

[Duwe6]

I've gotta agree with your CWI -- RT images of bi- and tri-furcations will be VERY hard to interpret, thus of little real use. They would be useful to show "Due Dilligence" on the part of the contractor and owner if there was litigation of a failure.

HOWEVER, instead of some B.S. 'show-&-tell' film images, take the CWI's professional advice and have these joints UT'd. If there is a legal concern, or if some off-site engineer wishes to 'hold something in his hand' other than just a NDT report, specify Phased Array with data aquisition. This will give you color-coded images that can be sent over the internet, and printed out [in color] to give the off-site folks something to hold and look at. And the raw data files are available for another PA-UT tech to review and verify the accuracy of the field PA-UT tech's 'calls' on acceptability and areas needing repair. This NDT method is accepted by ASME and API construction codes.

 

[gtaw]

When in doubt, ask the engineer that specified the NDT.


Best regards - Al

 

[racookpe1978]

When the Niagara Falls power plants' penstocks failed catastrophically in the early 60's and brought the entire plant down, there were few alternatives to RT. Certainly no continuous UT printouts like described above, RT was about the best available. (RT is harder - almost impossible - if the back of the weld is not available to slide the film or RT source into the exact position.)

As above, get back in touch with the specifying engineer and find out why the RT is required, and - at that time bring up the alternatives. Many, many thousands decisions are made on a "that's what we used to do" basis - which is not always the right choice. Though changing requires good justification and even better research!

 

[21121956]

Hello everybody:

Thanks to everyone, I really appreciate your comments, they left evidence of your vision, your knowledge and your high degree of professionalism.

El que no puede andar, se sienta.