Proving code compliance after PWHT

[Loki210]

Recently, a few clients have requested items that we are replacing (nozzles, flanges, etc) be post weld heat treated. However, the vessel wasn't originally post weld heat treated. This is usually requested due to improved corrosion resistance.

Given that "Examples of Alterations" found in the NBIC, part 3.4.4 (k) states "Performing post weld heat treatment where none was originally performed on the pressure retaining item", this would qualify as an alteration. Calculation-wise, there is no difference in mechanical capabilities (thin walled vessels/nozzles), other that a potential drop in MDMT, but that's not what the clients are after.

As such, how do I go about proving code compliance or mechanical integrity of said alteration to my AI? Interestingly enough, several of the AIs we use aren't sure either.

Thank you.

 

[DVWE]

Since you've mentioned NBIC, I'm assuming that's the code you'll be working to.

You treat it pretty much the same way you do a new vessel. Code compliance is proved by providing the required documentation as outlined in NBIC.

Mechanical integrity is proved by one or more of the outlined NDE methods. Hydrotest with some proper NDE methods, depending on the nature of the alteration, such as VT, PT, MT, and UT are typically performed.

And then the R stamp is applied.

If there is further concern about the integrity of the material from the effects of PWHT, I would leave that up to the direction of the client. i.e. "tell us what you would like us to do"

The devil is in the details; she also wears prada.

 

[Christine74]

Are you sure that you're talking about PWHT of the nozzles (typically done at 1150-1175°F for carbon steel), not Normalization heat treatment (typically done at 1600-1650°F for carbon steel)? Normalization heat treatment is sometimes performed in order to protect against cracking at lower temperatures and/or to reduce the MDMT, not necessarily for corrosion resistance.

Continue to discuss this with your AI but I suspect that the NBIC 3.4.4(k) requirement is intended for when the service of the vessel has changed and PWHT of the entire vessel becomes necessary in order to avoid mechanical failure. I personally don't think an Alteration is required if you are only PWHTing a couple of nozzles but verify with your AI.


-Christine