Small tube socket weld gap

[rpcampbell]

Have application where welding 1/4" OD x 1/32" wall 304L tube into counter bored housing (3/16" deep). Is the 1/16" gap applicable for this small of tube, as tube would be approx 1/3 out of bottom of socket. Welding is required to comply with B31.3.

 

[rneill]

Interesting question. Since the gap is required to deal with possible differential thermal expansion between the fitting and the pipe/tube, it seems logical that the correct amount of gap is dependent on the size of the fitting and the distance between the socketweld and the end of the pipe/tube. The greater the distance, the more potential expansion and the greater the necessary gap.

I have never seen any rules suggesting a different gap for anything other than a standard socket weld pipe joint but it does seem logical that a different gap might make sense.

On the other hand, it sounds like this is a one off female connection (custom manufactured) so the joint may be outside the intent of what the code was intended to address (it doesn't sound like this joint meets ASME B16.11 requirements). How are you validating the design of the joint? Can you have this issue (the 1/3 out of the bottom of the socket) validated as part of the engineering design for the joint?

Have you checked the code interpretations to see if this issue might have come up before?

 

[moltenmetal]

You have a custom weld geometry, not a standard socket-weld connection that B31.3 is addressing.

Note once again, as mentioned in plenty of previous threads, B31.3 does NOT set a requirement for gap in such welds post welding, i.e. a gap which must be proven to be present post-welding by radiography or other inspection. Rather, B31.3 requires that a gap be present PRIOR to welding. This gap may shrink away to zero during the welding process.

The amount of gap you'd actually need to address thermal expansion in your geometry is infinitessimal.

Be aware that the welder will need a VERY delicate hand with the TIG torch to obtain a socket weld with adequate parent metal penetration while at the same time leaving the full bore of your 1/4" OD tubing when they're done. Tubes of this size are often automatically orbitally welded (i.e. a fillerless TIG butt weld done by machine).

 

[unclesyd]

We have made hundreds of very similar welds on SS manifold distribution systems using a micro-tig welding system or a torch like the Arc Zone torch. We leave no gap and use square ended tube to eliminate crevices as our working fluid will sour and raise bugs.
One of the secrets to this type welding is have everything clean.
Our operating temperature is 120F but we do clean with purified 300# saturated steam. I can't recall any problems with any of the joints

http://www.arc-zone.com/index.php?main_page=index&cPath=13_18_39