Tube to Tubesheet Strenght welds 3

[KBO_BC]

Dear all,
In ASME BPVC.Vlll.1-2017 Figure UW-20.1, there are acceptable welds. Is there any recommendation if one weld is better than the other or is this depend on the welding process? Do you have any recommendations, which one to use?
Thanks for the help

 

[Krausen]

When specifying tube-to-tubesheet strength welds, I've typically gone with & seen the Fig UW-20.1(b) joint design the most. But it can depend on you're design conditions. This is a groove weld with the tubesheet holes beveled. The size of your groove weld legs is determined under UW-20.6. This is considered a "full strength weld" under code. If you're going to go through the hassle of welding all your tube-to-tubesheet joints, might as well go hell for stout.

The company I work for still requires all tube-to-tubesheet joints to be roller expanded (with two tube hole grooves) even if you're strength welding them. But this is controversial requirement as I understand from working with some exchanger vendors.

 

[Christine74]

Krausen I can think of 4 reasons for performing roller expansion of strength welded tube-to-tubesheet joints:

1) Expansion closes the annular gap between the tube and the tube hole, thereby protecting against crevice corrosion on the shell side of the tubesheet.

2) ASME UHX tubesheet calculations allow for some credit based on the depth of expansion of the tubes, which slightly reduces the required thickness of the tubesheet.

3) Expansion can provide extra protection against tube vibration between the tubesheet and the first baffle.

4) Expansion provides extra protection against interstream leakage through the tube-to-tubesheet joint should one of the strength welds fail.


-Christine

 

[EdStainless]

Christine is right on the money.
I have seen failures directly related to 1, 3, and 4.
Rolling is critical. Just stay in from the weld a bit and back from the inside face of the tubesheet also.
Grooves are a waste of time and effort.
A slightly roughened hole is better.
There are many applications where seal welds are used very successfully.
I have seen very few applications where strength welds were actually needed.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[CuMo]

Have a look at EEMUA Publication 143

https://www.eemua.org/products/publications/digital/eemua-publication-143

https://www.eemua.org/news/identifying-the-appropriate-techniques-for-tube-en

 

[KBO_BC]

Thank you all for this inputs.
As per AMSE Figure UW-20.1, all the welds are acceptable for strength weld. But their are not described in more detail. So I wondering wherever to use only fillet weld or groove weld. Is there an issue that the tube will stand out of the pipe? We are buying heat exchanger from supplier and each does it different.
Some example Data:
Gas: C2H4
Design pressure: 350barg
Design pressure: 200°C
Material: CS
Tube size 3/4"
Tube pitch 25.4
Tube no: 130
Tube to tubesheet specified as , light expanded and strength welded
Thanks again for the help

 

[CuMo]

There is no problem to use either fillet only or groove plus fillet weld provided that the strength requirements of the joint are fullfilled.
The groove + fillet might be preferred because it's normally a two weld run process - root weld run + cap weld run, where the start/stop positions of the two welds are staggered 180 degrees if possible.
After the first run - the tube to tubesheet joint shall normally be examined by the Dye Penetrant Method + a soapy water bubble test for leaks.
If you have a lot of tubes - dye pen on the first run might be a problem because it needs cleaning before the cap weld run is made.

After all - codes permit any of the two methods unless you have process reasons to prefer one or the other methods.
What you should be looking for is welding quality. It is more difficult to deposit weld in the groove sometimes if it is too small (2-3mm).
It is possible that cracks form inside the groove weld which unless you do a weld mock-up in advance and slice and examine it under microscope - would be impossible to reveal.

Regarding the tube end projection - normally no problem on horizontal heat exchangers - 99% of them are normally set up like this.
If it is a vertical exchanger - the tubes edges for the top tubesheet might be set level with the face to facilitate full draining of the top cover/channel.

Also, normally the groove + fillet weld would provide more weld deposit to the tube O/D.
But if your exchanger bundle is free to expand (U-tube or Floating head) and is not restrained at both ends (Fixed tubesheets) the axial loads on the tube ends would typically negligible so you might save some money by not making grooves in the tubesheets.
/Risk assesment of heat exchanger failure can come in handy here/.
For 130 off tubes - I'd go with groove+fillet+light expansion (No grooves) as suggested above. Won't impact the price too much. If you had a bundle with few thousand tubes - that's quite a lot of hours on top.

 

[KBO_BC]

Thank you CuMo, this information are exactly what I was looking for.