Leaks on threaded fittings. 4

[ronracer]

Is anyone familiar with the code implications of welding over threads as a means of leak control? It obviuosly limits reuse but I have seen it done. I have an application with a SS tank with 2" inspection ports that are a problem. The pressure is 550PSI and the temperature cycles from 20°C to 220°C.

Thanks for any help.

 

[ctopher]

I have seen it about 12 yrs ago, but don't remember much, except the weld was big and deep. I believe there was a pin installed and welded thru it also. It was for a pressure valve (steam). Sorry if not much help.

 

[unclesyd]

We have always welded threaded connection if not needed for disassembly. We try to limit the use of threaded connections if at all possible.
If you do have to weld the threads make sure you cover any threads, don’t leave a notch. Make the transition as smooth as possible. The AI has always concurred though at times he has suggested a complete rework of the connection eliminating the threads.

SS threads don’t like high pressure if there is a temperature cycle as in your case

 

[jte]

ronracer-

We require all threaded connections to be seal welded where I work. There are a few issues to be aware of with B31.3, such as noting that the seal weld shall not be considered a strength weld, no thread compound should be used on joints intended to be seal welded, etc. Look up "seal weld" in the index for specifics. As long as you stay away from high pressure piping (sections K...), which you are, you should be ok.

If you have a situation such as a threaded half coupling nozzle on a heat exchanger with an extended body valve screwed into it then you could get into an argument regarding the seal weld and Section VIII vessel repairs/alterations. Our perspective, seconded by our local AI, is that the joined threads are the sealing surface which define the Section VIII - B31.3 interface. The threads (and edge of the coupling) which are being seal welded are considered to be in the domain of B31.3.

jt

 

[jte]

As unclesyd mentioned, all of the threads shall be covered with weld. This is not merely a suggestion - B31.3 328.5.3 states that "Seal welding shall be done by a qualified welder. Seal welds shall cover all exposed threads."

As I mentioned in my previous post, its worthwhile to go to the index and read the noted sections... Thats part of why I like to participate in this forum - it forces me to go back and research and understand more about things which we just consider a standard procedure.

jt

 

[jay165]

JTE

From your 2 posts you seem to be implying that seal welding threads is REQUIRED by B31.3. What the code says is that if you're GOING to seal weld threads, then all threads have to be sealed.

 

[unclesyd]

Unless the code had changed in the last couple of years, the cover all threads referred to the threads on the fitting you are seal welding. This was to prevent the creation of an undesirable mechanical notch.

 

[jte]

jay165-

I did not mean to imply that seal welding is required. I agree that B31.3 simply puts some guidelines on seal welding if that is what you choose to do.

jt