NDE Before or After PWHT? 1

[chayes]

A client is fabricating a vessel which requires PWHT. They plan to UT the welds of the vessel to satisfy the NDE requirements prior to PWHT. Is it required to repeat the NDE after PWHT, and if anyone knows the specific section of the code (Section VIII, Section V?) that addresses this, it would be extremely helpful.

 

[metengr]

chayes;
This topic has been discussed before, and the answer is a final NDT needs to be performed in accordance with code requirements. You can perform a search to find past threads on this topic.

When the final NDT is performed is up to the manufacturer, unless otherwise specified as being after PWHT by the owner. A final NDT can be performed before PWHT with credit taken as a final NDT –the Code does not prohibit this. The ASME B31 Pressure Piping Codes do require NDT after PWHT for certain alloys.

My recommendation is to perform NDT after PWHT.

 

[Carburize]

Agree fully with metengr - depending on the size and shape of the vessel and the uniformity of the PWHT fairly severe thermal stresses can be set-up during this "final" operation which have been observed on numerous occasions to cause cracking.

 

[davefitz]

I have heard from other code experts the same recommendation as provided by Meteng, but there are some disadvantages to NDT only following PWHT. For P91, if a defect is found following PWHT, one would need to weld repair and PWHT a second time- yet each PWHT increases the aging of the parent material. For alloys as sensitve as P91, NDT before and afer PWHT may be the better approach. If a defect is found prior to the first PWHT, the weld repair would be effectted prior to PWHT and only one PWHT would occur. Also, this method implies the part would need to be cooled to below 100C for NDT, which aids in ensuring complete martensitic transformation prior to PWHT.

 

[JoeTank]

Nice catch davefitz. While in the employ of a tank and vessel fabricator, we chose to do the NDE, especially MT or PT, prior to PWHT. No reason to PWHT a vessel that already has a defect! And then we did it again after PWHT because cracks are sometimes caused during PWHT. It was a risk management thing... learned the hard way.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[ijzer]

For ASME VIII -1 Code NDE may be performed before PWHT , see Interpretations No 42 , VIII-1-98-09.
This does not mean that it is sometimes wise to do it after PWHT also.

 

[codeeng]

ijzer
Now you've started something!

 

[weldtek]

Agree with ijzer, there's no, one situation fits all. In some cases NDE after PWHT makes sense, but, as a fabricator it doesn't make sense to perform NDE after PWHT without having done it prior to PWHT.
Continuing this thought, neither would it make sense to change inspection procedures after PWHT.

 

[metengr]

All valid comments above. However, Codes and Standards only provide minimum requirements to assure safe design, nothing more. My comment above regarding NDT after PWHT refers to what I call a FINAL NDT, which is something I recommend as part of our engineering specification. Depending on the type of material, welding process and application, multiple NDT operations could be performed at our discretion.

The Code does not prohibit one from performing multiple NDT operations, only one is needed to satisfy Code requirements, and I suspect it will remain that way for some time. I see no reason for change.

 

[pvesseleng]

I agree with metengr, code case 2235-6 states that under some circustances you can use UT in lieue of RT.

 

[GSDGoBucks]

I guess one question I would ask... is this driven because of metallurgy or is this driven due to process (i.e., aggressive environmental service)?

Do agree what Steve stated concerning NDE prior to and after PWHT. The additional costs to perform NDE could be a plus if a deficiency is found prior to PWHT especially if the fabrication vessel has a tight schedule.