316L weld metal buildup to restore base metal on tower 2

[iangineer]

All,

I have a tower made of 316L SS that has some corrosion due to the acid service. The minimum wall thickness due to internal design pressure + hydrostatic head is 0.429". Some areas around the bottom of the tower show thicknesses close to 0.350". An FFS evaluation passes at Level 2 for 5 years.

120" ID
ASME VIII-1

My question:
Can weld metal buildup with Hastelloy be used to bring the wall thickness back up to nominal per code?

My thoughts:
PCC-2, Article 211 specifically says that weld metal buildup can be used for damaged base metal in carbon steel or low alloy steel applications. I believe 316L is not considered low alloy. Weld overlay refers to application of corrosion resistant weld material. My counterpart believes that overlaying the 316L with Hastelloy to bring the wall thickness up to nominal would satisfy the code requirements of bring the pressure boundary back up to proper thickness while upgrading the material with corrosion resistance.

I just wanted to find a code that specifically states that this is a proper repair. In my experience with new equipment design, a weld overlay does not affect the pressure boundary base metal.

Any guidance would be appreciated.

Thank you.

 

[Krausen]

I'm not sure there is anything prohibiting you from restoring your minimum shell thickness using Hastelloy weld metal buildup (WMBU) but you'd be altering your tower design, as opposed to repairing it, & it would require an API510 alteration form & supporting calcs. In your supporting calcs, how would you treat your new shell wall with two different materials through the section? You would also want to coordinate with the AI on this approach too, as there may be issues inspecting a dissimilar metal pressure boundary (316SS w/Hastelloy WMBU) in the future to monitor your required thickness. Typically weld overlay with Hastelloy (or similar highly corrosion-resistant alloy) is for corrosion protection only, and not part of your pressure boundary base metal minimum thickness used in the calcs. It may be easier, faster, & cheaper to WMBU with 316SS to restore the required wall thickness you need & then WMBU with Hastelloy over that for any extra protection.

 

[iangineer]

Following up this thread. The AI was fine with restoring wall thickness with hastelloy. The hastelloy that has been applied in an earlier repair in a different spot has not been attacked by the process, so after the repair, the inspectors will take a baseline UT reading to be able to monitor any future wall loss in the 316L material.
The tmin is 0.429", so I have recommended any wall thickness below 0.400" be built up to tmin so that it will pass a FFS for 10 years.

Side note: the tower has routine external UTs done and has an internal inspection at a maximum of 5 years interval.

 

[Dhurjati Sen]

Kudos to the AI although Mr Krausen was absolutely correct as per code and recommended practice.

Your AI has gone with experience which works quite well without the elaborate paperwork as per API 510.

DHURJATI SEN
Kolkata, India

 

[thruwall]

Correct, me if i'm wrong? This repair is being done internal wall and not outside wall.

 

[thruwall]

The area with corrosion does it not have to be removed and some type of NDT done to the surface before welding? The proper repair would be 316L the same material as the Tower. This would do away with all the questions being asked about code and material and weld overlay, in my opinion

 

[iangineer]

Yes, the repair is being done to the internal wall. The areas being repaired were cleaned and neutralized. NDE done was VT Prep, PT Root/Final. Apparently repairing with overlay to repair the pressure boundary in addition to providing corrosion resistance is common in plants. Our materials engineer agreed. The AI accepted the repair plan.

 

[gtaw]

Wouldn't you have to qualify a new WPS? You changed the P-number. You are no longer welding a P-8 to a P-8, you are welding a P-8 to a P-4X.

You might want to consider determining the Ferrite number using the WRC diagram or the more recent WRC modified by Kotecki and Lippold in or around 2005.



Best regards - Al