NDE before and after PWHT 4

[Downhand]

Most companies require NDE to be performed after PWHT for P1 materials, however, I cannot find this as being a requirement anywhere in B31.3. I find it interesting that B31.3 states that leak testing should take place after PWHT (implying to me that soundness should be checked for for after PWHT),however, no where does it state that NDE on P1 material should be done after PWHT, which is also a check for soundness. Any thoughts?

 

[Ron]

The primary purpose of NDT after PWHT is to check for cracks that might have occurred during PWHT. If the PWHT process is laborious, it would be nice to rule out weld defects before PWHT; however, that is not practicable in many cases since PWHT should be contiguous with the welding and NDT on hot material is tricky.

 

[metengr]

Downhand;
This question has been asked many times before, and the same answer is provided,

ASME Codes/Standards provide minimum requirements for safe long-term operation. For certain base materials, NDT after PWHT is a specific process step that is not addressed because there is no specific concerns regarding sequence. This means all that is required is NDT after welding.

A User can certainly add process hold points for NDT to be done before and after fabrication (meaning welding AND PWHT). For P-No 1 base materials that are readily weldable, I have performed RT after welding and before PWHT and simply used surface NDT after PWHT. Other cases, I have performed both RT before and after PWHT. I like flexibility.

 

[stanweld]

Downhand,
B31.3 specifically does not mandate final NDE after PWHT for carbon steel (P No. 1) pipe. I'm not so sure about your statement, "Most companies require NDE to be performed after PWHT for P1 materials..." It has been my experience that most of the Owners/Engineers that we deal with do not.

 

[weldtek]

I concur with Stanweld. In certain cases our customers require NDE after PWHT, thicker sections, special services, but it's not the norm.
It is the norm that PWHT be performed before leak or hydro testing.

 

[Downhand]

Thank you for confirming that B31.3 specifically does not mandate final NDE after PWHT for carbon steel (P No. 1) pipe. However, it does state that leak testing should take place after PWHT. What is the reason for leak testing after PWHT?

 

[metengr]

PWHT does result in relaxation of residual stresses which can effect piping connections (flanged joints, etc), and to verify no leaks.

 

[stanweld]

Essentially the welding procedure used to make a weld joint per B31.3 is not complete until PWHT has been performed.

In addition to comments of metengr, PWHT may not have been performed as desired; this was common back when exothermal kits and other marginally controled heating methods were used for localized PWHT (before electric resistance heating and modern induction heating methodes were employed).

 

[Downhand]

Although the code doesn't call for it on P1 materials it seems to me that it would be good practice to perform NDE after PWHT. Cracking can occur up to a couple of days after welding so if a joint was welded and then a few hours later NDE was performed and PWHT wasn't performed until a couple of days later, there could be a chance that in the time frame between NDE and PWHT the weld could crack. For this reason it seems as though NDE after PWHT would be a good idea although not a code requirement.