Seal welding of threaded connections

[octane]

When is it necessary to seal weld the threaded connections of piping?
Is there any reference available?

[ashfaqanwer]

It depends on the leakage classification. For hazardous gases like ammonia, hydrogen threaded connections are good to be seal welded.
Normally, designers take care of connection design and provide the most suited one for the service type and conditions.

Also before taking the line in-service and during hydrostatic testing, if some threaded connections are leaking and not being rectified by tightening, it's good to seal weld them too.

[ben]

Requirement of seal welding of threaded connections has not been mentioned anywhere.
If you foresee any leakage which would be hazardous for the personnel working nearby, you may go for seal welding of threaded joints.
However, B31.3 mentions that the seal welding must be done on all the exposed threads and by a qualified welder.

[octane]

Thanks...
We have threaded connections of several thermowells in hydrogen service. Should we seal weld them?

[ashfaqanwer]

At our site, we follow the practice of seal welding threaded connections of all thermowells on Synthesis gas service.

[Usman Amin]

Dear OCTANE,

In addition to the above discussion, I would like to add a practical example of my own project. I was executing a project of "Installation of Deluge Valve”. Maximum pipe size was 6" and the complete material was galvanized and joints were threaded. We hvnt used seal welding and threads were tightened with Teflon tape. But during flushing 40 % joints leaked, so we sealed those welds via welding while remaining joints remained as those were. Now the project is in operation and all is well. So I am also of the opinion that seal welding is requirement base. Where the service is critical threaded joints are not used and proper welding is done.

Regards,
UA

[octane]

Thanks everybody

[Fizza]

Octane,
In para. 100.2 ASME B31.1 defines the seal welds as " A weld used on a pipe joint primarily to obtain fluid tightness as opposed to mechanical strength" , so it reinforces the above all statements.
Then see para. 111.4 of ASME B31.1 that supports the above definition and refer to another para. 127.4.5 that says threaded joints, if seal welded, threads shall be entirely covered by seal weld and seal welding shall be done by qualified welders.
So B31.1 does not mandate the seal welding. Hope it helps.

[Fizza]

Octane,
In para. 100.2 ASME B31.1 defines the seal welds as " A weld used on a pipe joint primarily to obtain fluid tightness as opposed to mechanical strength" , so it reinforces the above all statements.
Then see para. 111.4 of ASME B31.1 that supports the above definition and refer to another para. 127.4.5 that says threaded joints, if seal welded, threads shall be entirely covered by seal weld and seal welding shall be done by qualified welders.
So B31.1 does not mandate the seal welding. Hope it helps.

[Fizza]

Octane,

Let me now quote you from B31.3.
Para. 311.2.6 says seal welding may only be used to prevent the leakage not to contribute towards mechanical strength.
Then para. 328.5.3 says seal welding shall cover all the exposed threads and it shall be done by qualified welders.
Also, keep remember always, in ASME Code "shall" is a mandatory language and "may" means neither require nor prohibited.I have used the same language in above sentences. Hope it helps.