Seal welding of threaded connections

[Fizza]

Section I (Rules for the construction of Power Boilers) also permits the seal welding at some places like PG-31, PG-59.1.1.4 etc.
Intersting in Section is that you may use seal welding of threaded fittings after final hydro test if PW-54.3 is satisfied.

[Fizza]

Now please see from VIII-1
UG-43(e) in VIII-1 says that other means of sealing, if provided, then other than standard taper threads may be used in threaded connections. Seal welding is one of the "Other means" as referenced in UG-43(e).
Expanded connections are prohibited for flamable and/or noxious gases and liquids in Section VIII-1 unless the connections are seal welded as per if they are not seal welded as per UG-43(f)(5)(b).
UG-46(i) also reads about the seal welding for threaded opening (used for inspection and cleaning) and closed with plug or cap.
See also UG-34 for seal welds.
See also intermediate head connections in Fig. UW-13.1(f).
There are also many other paragraphs in VIII-1 about seal welding.

[octane]

Thanks for such extended help.
Please explain what's the reason code says to cover all the threads with seal welding? And how a qualification of seal welding welder can be done? Any guideline provided by the code?

[mechcolor]

Nice explanation Nasir.
Many a times, people do this seal welding and assume that now they are much more safe than just a threaded connection. I mean they tend to make seal weld a strength weld (full sized fillet). They apply welding of at least three pass fillet / socket weld over the threaded part considering this weld as pressure retaining.
What would you say on this?

[Fizza]

Octane,
you asked
"Please explain what's the reason code says to cover all the threads with seal welding? "

Please read 2nd paragraph of Foreword of any ASME BPVC Code book. It reads " The Comitte's function is establish the rules of safety............." so covering the threads with seal welding is a rule of safety.
Qualification for seal welding is not especially done but the welder shall be qualified.

Mechacolor:

If proper WPS is followed, then blind welding is avoided. You are true that welders do like this way as you noted.

[octane]

Nasir, I understand what you have referred to but sometimes, covering all the threads seem impractical that's why I asked this question. And how can we qualify a welder for seal welding? What tests should be conducted? Dye penetrant or MT check only seem applicable in this case.
Appreciate your value addition.

[mechcolor]

Octane
When you seal weld the threaded connections you weld the notched surfaces of threaded part, while doing this if you would leave some of the threads not welded then that specific area would become a stress concentration point. In order to avoid such situation, it is recommended by code that you must cover all the threaded portion while performing seal welding.

Regarding tests on seal welded area, you are right that only the basic checks are applicable and those are enough to ensure that the quality. In addition to this, a thorough visual inspection would also help that how the welder is making the seal weld. What amperage, voltage is being maintained by him, is there any undercut or not. These type of basic inspections along with penetrant testing would ensure the quality of work.

Welder must be qualified for the filler material he has to work with, the material he has to perform welds and the position.

[arcpro]

Very useful discussion.

[salzx64]

Nasir! nice elaborated response.

Just to add one thing more and that is for Threaded Connections of insertion type of instruments/controls etc above 495 deg C or above 103.4 bar pressure Seal weld as per B31.1 (114.2.3) becomes mandatory.

Regards,
Salz

[Fizza]

Dear Salzx,

I fully agree with you, the case as you noted, seal welding becomes mandatory as per 114.2.3