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Up Topic AWS.org / Forum News & Help / Welding After PWHT
- - By BenViana Date 06-07-2019 10:39
Gents ,

Good day .

Currently we are working on a repair of Amine tank . This tank have undergone PWHT . During pre-commisioning, they
put cleaning chemicals and drain the tank  without opening any nozzle on the roof . The tank was sucked , having a big
buckle . We jack the main buckle but residual stress in the end of the dent ( creases ) remained and some measurement
does not pass the API 650 standard when it came to flatness and plumbness.  We also hydrotested the tank and during
the full status it is inside tolerance but when we emptied again it return to out of tolerance position.

Only option now is to put stiffener . My question is , is there a exemption  in welding a PWHT tank wherein  there will be no need
to again perform the PWHT ?

Regards ,

Ben
Parent - By LToca85 (**) Date 06-07-2019 21:20
That happened to us once with an ASME pressure vessel silo. We had to scrap the shell because the inspector stated that there was no way of making sure that the material had the same mechanical strength properties in the creased area or near it.
Parent - By toddler (**) Date 06-11-2019 07:10
Strictly in accordance with governing code which specify the need for PWHT, no welding is allowed after PWHT. Else PWHT will nave to be repeated to relieve any residual stresses that may have been introduced by the new welds.

You may opt however for a localized PWHT  if cost/time is your consideration to avoiding repeat PWHT.

you may consider also the use of welding procedure, qualified with CTOD (not really sure about this, as normally it is used to obviate/waive the need for PWHT on thick materials)https://www.twi-global.com/technical-knowledge/job-knowledge/ctod-testing-076. And I reckon this tank is relatively thin-walled to buckle after draining. If I am the client I might consider already requiring a repeat PWHT due to the jacking operation on the buckle.( or worst, scrap the tank)

my 2 cents
Up Topic AWS.org / Forum News & Help / Welding After PWHT

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