Length of Tube Expansion for Heat Exchanger Tube Sheet design.

[DK44]

Dears.

1. For Tube sheet Design to ASME Sec VIII Div 1 UHX, Length of Tube Expansion in Tube Sheet Holes (ltx) is an input parameter. This influences the Thickness requirement of Tube Sheet. As I understand, we have basically two types of Tube To Tube sheet joints:

a) Expanded Joints (with or with out Tube Hole grooves), where the Expansion is solely responsible for the Strength (holding power of Tubes) and Leak tightness of the joint.

b) Strength welded joints (generally associated with Light / Contact Expansion of Tubes in Tube Holes), where the welding is solely responsible for Strength (holding power of Tubes) as well as Leak Tightness of the joint. The additional Light / Contact expansion being chiefly meant for avoiding crevice corrosion between Tube and Hole as well as to achieve good Heat Transfer from Tube to Tube sheet especially for higher thickness Tube Sheets.

2. ASME defines Tube Expansion differently at Appendix A and UHX.
a) Appendix A considers expansion (at nomenclature for fe) as achieving holding power of Tube with Tube sheet Hole with or without welding.

b) Where as UHX considerers (at nomenclature for ltx) Expansion as to achieve contact with Tube Hole.

3. It gives an impression that,
a) for Strength welded joints (even with Light / Contact Expansion),ltx shall be set to zero. This gives higher thickness of Tube sheet. But if it is set to the length of Contact Expansion specified in the job, Tube sheet Thickness would be lesser and optimum. Vendors and Clients can differ in this matter.

b) for Expanded only joints, ltx to be specified according the specified length of expansion which of course leads to optimum Thickness of Tube Sheet.

4. Request guidance in these circumstances as to how to use the expansion length parameters properly.

 

[EdStainless]

What materials?
What pressures?
What temperature and how much cycling?
What is the risk of a leak?

You cannot expand to near the edges of the tubesheet, the common setback is 1/4"-3/8".
Tubing in steam surface condensers is roll expanded only. There is 75psi water on one side and full vacuum on the other.
The tubesheets are often 1 1/4" thick so the expansion portion is only 3/4" maximum. And the tubes are thin, commonly 0.028" wall.

The other option is to use expansion for the mechanical holding but use a light autogenous seal weld for sealing. This is done with thinner walls or softer material (Stainless or Ti tubes that are 0.016"-0.022" thick).

Grooves almost never improve pullout loads or sealing. A rough hole ID works much better (think of dozens of very small grooves).

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P.E. Metallurgy, consulting work welcomed

 

[DK44]

Thank you EdStainless.
The question is not about the methods of fixing Tubes in Tube sheet Holes, rather it is about the consideration of Length of Expansion in the Code Design of Tube sheet as detailed in my question.

 

[SnTMan]

DK44, I am reading from an older (2017) Edition, so take that into account:

Definition of ltx only specifies that contact be made between tube and tubesheet. It does not mandate any particular tube wall reduction (see below also). Therefore I would say:

Your 3. a) it would be justified to use actual expansion length for ltx. If there were no expansion whatsoever I'd set ltx to zero

3. b) agreed. This would also apply to seal welded, expanded joints.

As an aside, it appears to me that the quantity ltx is only used in effective tubesheet properites calculations of UHX-11. Allowable joint loads are calculated per other Code rules. See for example UHX-13.5.9 (b)(2), Lmax.

Appdendix A likewise apparently does not require any particular wall reduction in the caclulation of Lmax. Apparently a contact roll is as good as a hard roll.

It is possible interpretations exist on the subject.

Regards,

Mike


The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

I presume that these issues are why we see cases where tubes are expanded to contact, welded, and then they go back and hard expand them.
Another aspect of this (again that does not apply to your questions) is related to the unsupported span of the tubes and vibration. While a strength weld with little or no expansion may work for the tubesheet design, if it is a thick tubesheet you may have just made the first tube sapn too long.
Just pointing out that there are reasons other than tubesheet thickness to consider how this is done.

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

 

[DK44]

Thank you SnTMan and EdStainless.

Finally the question remains as :
In the Design of Tube Sheet to ASME Sec VIII Div1, UHX, given a Tube to Tube sheet joint with Strength welding + Contact Expansion (the contact Expansion meant for just closing the gap between Tube Hole and Tube OD without Tube wall Thinning and not having any Tube holding power),is ltx to be considered as Zero or the specified length of Contact Expansion?

This arises due to difference in the definition of Tube Expansion at Appendix A and UHX as explained in my 2(a)and (b) above and the Tube Sheet Thickness being influenced by the consideration of ltx.

 

[SnTMan]

As I said regarding ltx, apparently a contact roll is as good as a hard roll.

I have nothing to add...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DK44]

Thank you SnTman.
Hence it is the Change in Tube ID (increase)by Contact or Hard Expansion is the consideration rather than holding power with hard expansion. As such I feel that App A definition of Tube Expansion shall be in line with UHX definition. Hope you agree with me.

However, such change / increase in Tube ID can be larger in case of Hard Expansion and smaller in case of Contact / Light Expansion. Can ltx criteria be same in both cases?