Question regarding Sec VIII Div 1 and closest approach welds can have

[jistre]

I have a S&T HXer with a bit of a problem. I have a repad on the channel nozzles that is relatively large for the channel. The end result is that the repad welds and the girth flange welds come within 1/2" of each other. I have several solutions to the situation, but what I don't have is the section of the BPVC that covers how close those two welds can be. I've gone through the 715 pages of Div 1 and have been unable to find the section of the code that covers this problem. For my report, I'd like to quote the code, but as of yet don't have the citation.

Do any of you guys know where this is addressed in the code? I looked under welding procedures and the procedures for shell and tube exchangers and have seen nothing. So, either I'm looking in the wrong place or my eyes have failed me.

Hopefully, one of you has encountered this before and can point me in the right direction. Thanks a bunch, guys!

 

[weldtek]

You won't find it in the Code you reference. Due to design, we often have repads cover round or long seams, so the fillet weld crosses a major seam.
It's pretty common for client specs to address it though.

 

[unclesyd]

As you didn't post the operating parameters here a two options you might want to pursue.

Two things you might do are clip the repad adjacent to the flange weld to give a little more separation or use a long weldneck nozzle (LW) to reduce the required area.

 

[nashof]

I don't find specially strange that a pad covers a seam, as weldtek sayed.
You should revise your client specifications, because it is quite common that include something about it.
For example UOP specifications, which quite a good reference for all petrochemical projects, asks to have 3" between these kind of welds.
If you need yo cross the seam, and you can't find another solution, I would recommend full RT of that weld and something like 6" around it.

 

[vesselfab]

CODE give specific instructions on a pad or clip that covers a seam.

 

[gr2vessels]

Hi jistre
The ASME Code does not specify a minimum distance, because the risk of mis-interpretations and incorrect aplications. Other Codes like EN or even Australian one, do specify minimum distance of 3 times the shell thickness between the weld toes or 40 mm, which ever is smaller.
The typical reason is the potential damage to the material structure the weld could do;- the close proximity of two HAZ could compromise the material specific properties and could be unexpected source of equipment failure. However, this is not a fully valid statement, since welds are rutinely criss-crossing on a pressure vessel, without any damaging consequences, under controlled situations. There are post weld stress relieving heat treatments which are required to minimise the impact of weld induced stresses, including those in close proximity or crossing each other. In your case I can see a major problem, which is related to the reo pad running up on the girth flange hub after crossing the girth butt weld (I assume this is the only type of flange accepting some overlap of the reo plate without gross interference with the flange). I would change the pad to smaller diameter and thicker plate.
cheers,
gr2vessels

 

[unclesyd]

The reason for the LWN is it minimizes welding close to the circumferential weld at the flange.

A thicker pad is ok if in it's self doesn't cause a larger thermal gradient at the nozzle if there is thermal cycling of the Hx. You also have to be careful to not run into the limits of reinforcement.

I would assume that the OP is already using a thick pad to increase the clearance to the flange weld.

It would help to know what type of flange is being used along with the design parameters.