Min. Girth & Branch Weld Spacing

[Krausen]

Curious forum's opinion on minimum girth weld & branch weld spacing requirements on new piping & modified existing piping (B31.3 code)?

Most companies have best practices/standards that define minimum weld spacing clearances for girth & branch welds for new piping. However, I'm not 100% sure this is a B31.3 code requirement (similar to Section VIII for pressure vessel design).

I have some applications with both carbon steel (P-1) & stainless steel (P-8) piping where modifications are being proposed on existing piping to weld new small-bore branch welds onto existing girth welds. Mostly for installing new low point drains & high point vents in tight spaces.
The existing piping is modern carbon steel(A106-B pipe)& modern stainless steel(A312-TP316 pipe).

The piping would typically not be designed new this way, but the question has come up on if we can still modify the existing piping this way to retrofit these small bore branch welds.

 

[DekDee]

Krausen,
I have just had a huge fight with our project engineers over this exact subject. (I am Quality Lead)
They tell me minimum 50 mm (2") between weld toes.
I ask them, please let me know which code / standard references this ?
Silence !
Someone has plucked a number out of the air 30-40 years ago and put it in their project specs and that has been passed on from project to project without any factual supporting data.
If the distance between HAZ's (which seems to be everyones concern) was deemed critical by the code commitees would not one of the major piping codes have addressed this over the past 30-40 years ?
A Welding Engineer (who I hold in very high regard) responded to my question with this - Every pass of a multi-pass weld has its own HAZ that overlap each other so what is the difference if you have two welds side by side with overlapping HAZ's - there is none.
I must qualify this with we were discussing basic A106 Gr B - high alloy material, different story.

Sorry for the rant,
Cheers,
Shane

 

[mk3223]

See the links below if any help…

https://shipbuildingknowledge.wordp...ance-between-two-circumferential-pipe-joints/

https://www.littlepeng.com/single-p...elds-as-per-international-codes-and-standards

 

[1503-44]

Note that any minimum distance should also be based on weld type and size and geometry. Obviously 2 x 1/4" fillet welds will not fit within a distance of 2mm and full penetration welds may not be suitable for a welder to get the rod to contact all points within the weld, if another fitting is in the way. Adequate welding geometry may be more of a factor than any other. You should set your general minimum requirement considering those factors first, perhaps allowing closer distances for specific cases only when necessary after obtaining authorization. That is apparently what Shell DEP and SA standards have done. The best philosophy IMO.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Andrew ONeill]

as per DekDee, it's not in the piping codes. The Australian code (virtually a carbon copy of B31.x codes), has a vague reference to it.

My only take on where it is useful is to make sure draftspersons / designers use the rule, so that they don't put branches and attachments too close together, making fabrication and / or site welding difficult from a fitup and welding access point of view.



Andrew O'Neill
Specialist Mechanical Engineer
Australia

 

[Piper Mike]

Krausen

The answer to your question is yes, you can modify the existing piping this way to retrofit adding a small bore fitting to the piping wall exterior.

This is normal refinery service, A106 Pipe.

The codes could not define any dimensions between welds because there are too many variables involved.
Following the accepted Codes and PFI spacing guidelines allows you to proceed with engineering and fabrication design, with no further engineering analysis or stress relieving required.

Your retrofit only requires that your engineer confirms the conditions of the design.

Mike