Welding of tread joints 1

[BJaffa]

A customer has requested the welding of a 3/4" threaded nipple into an SA 105 Sec. VIII-1 coupling nozzle. I seem to remember a Code provision that allowed this if all of exposed threads were "consumed" in the fillet weld. Where might I find this provision.

 

[metengr]

BJaffa;
There is no such statement in the the Code and furthermore, I would recommend against it. Threaded joints are designed as such and seal welding them is poor engineering practice.

 

[dcasto]

If you need a 3/4 " tap, put a weld o let and weld a 3/4" PE x NPT. I found it cheaper to but 12" NPT nipples and cut them in half when out in the field.

 

[grandnobi]

BJaffa,
provided the work you are going to perform is to seal weld a threaded joint on a piping system the code provision you are looking for is:

ASME B31.3 (2006) Par. 328.5.3
Seal welds shall cover all exposed threads

or respectively
ASME B31.1 (2004) Par. 127.4.5
Where seal welding of threaded joints is performed, threads shall be entirely covered by the seal weld.

However, seal welds may be used only to prevent leakage of threaded joints and shall not be considered as contributing any strength to the joints. Since you talked about a "fillet weld" and "consuming threads" I still wonder whether I understood your question correctly.

 

[BJaffa]

Thanks for the response. The intent was more than to seal the joint. I found no similar reference in Sec 1, VIII or NBIC. Also B31.1 or B31.3 would not be a stretch for this application. Our advice to our client will be to replace the coupling/nozzle or live with the threaded joint

 

[justinwest]

I would have to dig around for a specific reference in the books, but it is permissible to seal weld threaded joints.
This is only for sealing. The code does not allow any added strength to be assigned to the joint.

We commonly seal weld 2" NPT joints on temporary NG compression lines using 4000# hammer unions.
Big guys with big hammers can really loosen threaded joints!!

 

[bvi]

There isn't anything wrong with seal welding threaded joints. It used to be done quite frequently with steam systems. But practice has moved towards socket weld joints. See also ASME PCC-2, article 2.3, Repair of Pressure Equipment and Piping, on seal welding threaded joints.