Strength weld+light expansion and light expansion+strength weld for tube to tube sheet joint 2

[mechengineer]

Hi Manufacturers of shell and tube heat exchanger,

Strength weld (first)+light expansion (after) and light expansion (first)+strength weld (after) for tube to tube sheet joint, which way is easy and better?

Thanks.

 

[EdStainless]

Typically, there is a light (just to contact) expansion done first and then the weld (strength or seal).
And then you hard expand after the weld.
The first light expansion is to assure a uniform weld gap and centering of the tubes.
You don't want this tight because of issues that could cause with outgassing and expansion during the welding.
The final hard expansion is to provide the mechanical support of the end of the tube.
The hard expansion needs to stay back from the weld (usually 1/4" or a bit more) and also end 1/4" from the back face of the tubesheet.

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P.E. Metallurgy, consulting work welcomed

 

[mechengineer]

@EdStainless,

Thanks for the points of consideration for tube to tube-sheet joint.

The first light expansion is to assure a uniform weld gap and centering of the tubes.

1. The light expansion is just to contact, yes. But there is no gap allowed to against corrosion from shell side medium. My understanding is that the light expansion is just to close the gap between tube and tube hole of TS for preventing from corrosion. Otherwise, it will not require to expand the tube.
2. To avoid the tube damage, whatever light expansion or hard expansion the requirement is necessary that shall be away from the weld (usually 1/4" or a bit more) and also end 1/4" from the back face of the tube sheet.
3. The light expansion above 1, is it any problem that could cause with outgassing during the welding? Or shall it be strength weld first + Light expansion after if it is no gap allowed?
4. In which case that shall require to strength weld (first)+light expansion (after)?

Thanks

 

[weldstan]

Ditto Ed.

 

[EdStainless]

The tube and tubesheet holes should all be clean and dry, so there should be nothing trapped in there.
A light expansion does nothing for corrosion resistance or mechanical support.
Those both require a tube expansion (hard) roll.
The mechanical support is an important item. Every HX that I have seen considered the free span of the tubes (for vibration analysis) to be measured from the back face of the tubesheet.
If you don't hard roll after welding then the tube span is longer by the thickness of the tubesheet which may require different baffle spacing. And the welds will need to tolerate the fatigue from supporting the tube vibration.

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P.E. Metallurgy, consulting work welcomed

 

[mechengineer]

@EdStainless
I think that there is crevice corrosion tendency if without expansion.
Regards

 

[mechengineer]

1. Strength weld first+light expansion after. It is required an additional pneumatic test as per API 660 clause 10.2.1 before light expansion. It is must strictly control the expansion being at least 6 mm (API 660 clause 9.10.2. GB151 is 15 mm) away from welds to avoid the weld damage.
2. light expansion first+strength weld after. It is not possible to check the weld quality independently by leak test. And The tubes and tubesheet holes shall all be clean and dry to avoid outgassing cause any defects in the weld. It may cause the "relaxation" in partial expansion area by welding heat.
I think that users may prefer to option 1 for safety.
API660 clause 10.2.1.
"10.2.1 In the case of welded-and-expanded tube-to-tubesheet joints, the tube weld integrity shall be verified before final expansion of the tubes by a pneumatic test from the shell side at a gauge pressure of between 50 kPa (7.5 psi) and 100 kPa (15 psi), using a soap-water solution to identify leaks."

 

[pv_mkk]

a never ending debate.
Follow TEMA tube to tubesheet joint design and you can get rid of the confusion of "light" "heavy" expand thing.
TEMA only requires seal weld + expand with groove OR strength weld + NO expand.
The rest is depended on your manufacturers' experiences and technique.

I go strength weld + NO expand all day.

 

[mechengineer]

@pv_mkk,

You may end youur detate here before you go through API 660 to understand the clause 7.5.7 and A.4.4.

Regards,

 

[r6155]

Be careful when PWHT is required for shell side.

Regards

 

[EdStainless]

MECH, yes, light expansion, weld, PT from the front and then leak test.
Note, you don't leak test with a soap solution, you buy leak test fluid to use.
ALL soaps contain a lot of Cl and many other chemicals. You never want soap residue on your equipment.

Grooves serve no purpose unless the tubesheets are significantly stronger than the tubes.
If y our mockup doesn't extrude the tube material clear down into the grooves then they aren't helping.
In light wall tubes using roughened holes is the best method.

In crevice corrosion the tighter the crevice the worse the conditions.
A light roll produces a very deep and fairly close crevice. Room for lots of corrosion.
If someone wants to strength weld with no hard roll after ask to see the vibration and fatigue calculation for the tubes and welds assuming that the tubesheet is only as thick as the weld. Because that is what you will have.
I have seen these fail many times.

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

 

[r6155]

If the shell side design pressure is high (external pressure on the tubes) it can be a problem during hydrostatic testing, so hard expansion should be considered.

Regards

 

[mechengineer]

@EdStainless,
Agree with using strength weld + expanded except where work hardening of the tube material shall use strength weld+light expanded. It meets the requirement of API660 7.5.7.2 and A.4.4.2.
Regards

 

[Tkemeny86]

It depends on the application. We use expansion and strength weld if there is vibration/fatigue in the TTS-joints.

Light expansion can work:
- it is simulated with PQR/Mock-Up
- no stress corrosion issue
- and you accept, if a tube haa to be replaced, then this going to be a problem
- common practice for reactive materials (it makes to use gas protection easier on the shell side)