Rule of Thumb for cutoff Temp./Press of expansion(by rolling) in Tube-to-Tubesheet Joints 1

[JEP109]

Does anyone know of or have a rule of thumb for what the cutoff Temperature and/or Pressure for expansion(by rolling) tubes into tubesheets is? Any literature to reference would also help apart from the valuable years of experience all you fellow engineers may have in the industry.

Also when would it be recommended to roll and weld the tube-to-tubesheet joints?

Anything helps! Thanks!

 

[chicopee]

I am not sure that I understand your question because tube ends are either expanded (and beaded, if required)mechanically or welded.

 

[JEP109]

chicopee,

Thank you for your response.

I have heard anecdotally and read in some heat exchanger literature that expanding and welding the tubes to the tubesheet is not uncommon. With that being said, the designer also has to decide whether the expansion is going to take place before welding or after the welding is complete. In short, the literature does not give any suggestions as to when to use this method, they do state that with experience one will learn these things. That is why I am asking for any suggestions that anyone may have in regards to this topic.

Thanks again for your response.

 

[chicopee]

If you expand, then either flare or bead the tube ends, the welding afterward would be to maintain the tube ends from leaking and that welding method is known as seal welding which is a superficial weld and is not considered strength welding. If the tube ends are to undergo strength welding then there should not be any need to expand the tube ends. If the tube sheet holes have enlarged lets say during a retubing job, ferrules are inserted inside the new tube ends for expansion afterward welding, flaring or beading takes place. Be careful about the term welding as you have to differentiate between strength or seal welding.

 

[JEP109]

chicopee,

Yes, you are correct sir. Although I would like to say that I am assuming that welding either before or after expanding would only be a seal weld and not a full strength weld. Like you said if it is a full strength weld there is no need for post-weld expansion.

My question is when would it be recommended to stop expansion and (a.)move on to seal welding and expansion or (b.)strength welding?

Thanks.

 

[chicopee]

You do not seal weld before expanding the tube ends. For strength welding, there should be no need to expand the tube ends unless something abnormal exist.

 

[EdStainless]

I have seen many failures of strength welded tubes with no expansion. The reason is that in a thick tubesheet (8" or more) the tube free span is now actually longer than the space between the back of the tubesheet and the first support plate. As the tube vibrates it is hitting against the back edge of the tubesheet. You will get a fatigue failure there.
The tubes should at least be expanded to contact near the back of the tubesheet.

The real issues when selecting is what joint strength can be generated. In high strength materials and thick tubesheets you can generate rolled joints that are as strong as the tubes. As alloys get softer (usually non-ferrous), have lower modulus (again, usually nonferrous), and tubesheets get thinner joint strengths can become problematic.
You see seal welding used in cases where joint strength is marginal (thin walled Ti tubes in Ti tubesheets) as well as the more traditional applications where leaks are fatal (acid coolers).

Tube expansion is a science unto it self. Roller expanders come in different styles, and then there is hydraulic expansion.

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Plymouth Tube

 

[JEP109]

Thanks for the feedback EdStainless...

In your experience have you ever dealt with any heat exchangers that handle hydrogen(~75 mol%) at pressure in the 2000+ psi range and temperatures up to 800°F?
I ask this because I have a couple of BEU type Heat Exchangers, and I am trying to choose between strength welding alone, which will take care of the leak tightness, or seal-welding with rolled expansion, in this case I think it would be better to do post-weld expansion, any thoughts? I feel as if we should full strength weld, but from what I just read it seems like they might fail and that would be detrimental. Any comments that you may have will help. Thanks in advance.

 

[EdStainless]

With this temp I presume that these are u-bend units.
With fairly thick tube sheets.
I would think that the best practice would be to:
1. light contact roll near face of tube sheet (within 1/8", but no closer, everything must be clean and dry)
2. Autogenous seal weld (semi-sutomatic)
3. Perform weld QA, PT and/or leak test.
4. Full roll expand in steps, starting 1/2" behind the weld (you don't want to stress the welds) and working to within 1/4" of the back sheet of the tube face.
The expansions should be qualified by mack-up testing prior to fabrication.
The rolling should all be done using torque limits.

This approach lets you use tight pitch tube spacings and address both concerns (strength and sealing).
Since the weld rolled joints should provide both strength and seal, the welds are just backup.

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Plymouth Tube