Welding Tips and Tricks

I am a piping stress person. As per my calculation requirements at certain places in a plant I have following requirement:-

1. I have 40 " SS 304 pipe with 9.525 mm thickness. As per calculations to avoid overstresses in a piping system I need TEES of higher thickness . Say 12.7 mm , 15 mm. 20mm etc. I propose to weld a pipe of same thickness as TEE to the TEE. This higher thickness pipe will be welded at other end to 9.52 mm thick pipe.I want to know whether it is possible to weld 9.52 mm pipe to 12.7 mm pipe in a butt joint?

2. What is the maximum thickness to which a 9.52 mm pipe can be butt welded and also what is the minimum smaller thickness to which this pipe can be welded ?

3. Whether it it possible to weld a 12.7 mm thick TEE to 9.52 mm thick pipe or we need a companion pipe of 12.7 mm thickness to be welded to TEE since TEE edges can not be prepared for welding.

I need an expert opinion to finalise some very critical piping in an LNG plant.

Thanks in advance.
Regards,
Sunil

Sunil,

First, welcome to the forum!

I've done this recently in hydrogen service for a MAWP of 8800 psi. Forgive me for not taking the time to convert to bar or KPa. In this case, I was joining 3/4" XXS to a valve on one side, and 3/4" sch. 160 on the other. The difference in wall thickness was approx. 2.4mm. I used 3" (aprox. 75mm) stubs of the XXS, and used a tapered spiral-flute reamer to gradually transition the XXS stubs to the wall thickness of the sch. 160. The (very picky) engineers approved this method.

Alternatively, you can use a large reamer or even a transitioning stone to bevel the inside of the tees (about 60*) to match the inside diameter of the pipe you're welding to it. This is common practice, and what I would have done in the above example, if it had been permitted.

Also, your post is not in the best place to get answers. Since this is LNG, I'm sure this is a TIG (GTAW) weld, and I'll relocate your post to our TIG-specific forum.

Steve S

I have seen sched40 pipe welded to sched80 and been in service to 2.1MPa working pressure for decades without issue. I had a welding test one time where I was given dissimilar coupons. When I drew this to the attention of the examiners I was told to just get on with it. I used the die grinder to prep the inside of the thicker pipe it was not too pretty but the root looked good. The coupon was subjected to destructive testing and passed. So I know many things will work including what Steve says above.

Most welding inspectors will tell you to get a short piece of pipe same size as your tee to use as a transition piece. One end you machine inside to match your pipe for about 25mm and then a 45* step back to stock size to prevent stress concentration. If you have time, stock material and machining capability, this would be the utopian method.

I find this topic really interesting because stress in welded pipes is something that affects my work but don't know enough about which is why I tend to do things by the book as much as possible. I had one 8" chilled water job in 304 sanitary tubing, the header end near the pumps with multiple tees in close proximity kept splitting at the tees no matter what I did. The way I fixed it was to remake the header out of sched 40 LCS, flanged to fit in the 304 line and hot dip galvanised. Customer raised his eye brows but I told him to cope. No more probs. If I knew more about designing for stress I could probably have stayed with 304.