Selection of tube to tube sheet joint 1

[pve5]

Could guys with experience of tube to tube sheet in shell and Tube TEMA heat exchanger joints , review following and comment:

1) Seal weld and expansion in grooved holes
2) strength weld and expansion in plain holes.

In case on horizontal heat exchangers , where the tubes project out side the face of tubesheet, which type of joint will have better performance be better. The pressure is 550 psig @ 750 F with SS 304L tubes.

Thank all very much in advance.

 

[metengr]

The selection of a suitable tube to tubesheet joint configuration is unfortunately not an easy task. It is dependent on specific design conditions and assumptions that are referenced in ASME Section VIII, Div 1, Part UHX. Go to section UHX-15 titled Tube to Tubesheet Welds.

Overall, Part UHX is a difficult section to completely understand unless you are a heat exchanger designer and work with these formulas and assumptions on a daily basis.

My advice, if you really want an answer to your question, you need to discuss your specific application with several reputable heat exchanger companies. The reputable ones can provide answers.

 

[hippocrocopig]

pve5,

You do not mention whether there are any specific considerations in your case (for example two fluids which MUST NOT intermix). So I'll take it that you have a fairly standard set-up: say hydrocarbon one side and steam or water the other. In such a case, with your temperatures and pressure you can expand into two grooves (one if tubesheet thickness is 1" or less).
Properly rolled tubes have very high axial strength which can be demonstrated by a push-out test, and adequate leak integrity. This is the most economic option and your base-line. If you must prevent leakage (eg lethal fluids) you can also seal weld, but this adds cost and not necessarily value.
If additional mechanical strength is required (fixed tubesheet where the tubes experience push-pull loads) you can strength weld. In this case the the rolling is a seal roll to prevent ingress of corrosive material into the back of the tubesheet and you do not need grooves.

In summary, if you have no special considerations, you should always specify rolling into two grooves; but use an experienced shell-and-tube manufacturer and make sure your NDE is proportionate to the job in hand!

hcp

 

[pve5]

hcp

Shell side fluid is H2S, H2 and tube side is feed from reactor. Tube side and shell side temperature are 725 F and 750F. Since it is a horizontal heat exchanger,I estimate the strength weld with light expansion in plain holes will provide better and economical joint.

Thanks!

 

[metengr]

If you would like more information on tube to tubesheet joint design, I would recommend you obtain a copy of the following ASME paper titled, 'Expanded, and Welded-and-Expanded Tube to Tubesheet Joints' by S Yokell. This is one of the better technical papers on the theory and evaluation of expanded versus expanded and welded tube to tubesheet joint designs.

 

[Elvie]

pve5,
for fluid conditions given you may have to do a strength weld and then a heavy expansion of the tubes into at lest in a groove close to back face of the tubesheet.Have Shell,Tubes and Tube sheet H2S proof materials.

 

[roca]

Hope the attached note helps you

The combination of welding and expansion required on each exchanger is normally specified by the client. The following combinations of tube expansion and tube end welding may be adopted
depending on service conditions:
- expanded only;
- strength welded only;
- expanded and seal welded;
- strength welded and lightly expanded;
- strength welded and expanded;
- back face welded.

A strength weld is defined as a weld in which the minimum throat
thickness is not less than the tube wall thickness. A weld having a
smaller throat thickness than this is considered to be a seal weld and its
function is solely to seal the tube and the tubesheet.

For exchanger applications involving non-corrosive and nonpenetrative
services, expanded tube-to-tubesheet joints shall be used.

In many applications, for example condensers and low pressure reboilers, tube
expansion into grooves in the tubesheet without welding is satisfactory and economic

The maximum projection of the tube end shall be 3 mm. For vertical
thermosiphon heat exchangers, there shall be no projection of the tube
ends at the top tubesheet.

For services where total leak tightness is required, welded tube-totubesheet
joints shall be used. The type of welded joint required (i.e.
strength weld or seal weld) shall be specified by the purchaser.

With properly applied strength welds, tube expansion is frequently unnecessary as it
does not significantly add to the mechanical strength of the tube end fixing.

Where the crevice between tube and tubesheet must be minimised (e.g. to avoid
crevice corrosion, or when fretting is anticipated, or when good heat transfer must
be maintained between the tubes and the tubesheet), the tube may be expanded after
welding. This provides intimate contact between the outside diameter of the tubes
and the bore of the tubesheet holes and may be done after welding and leak testing,
but before final pressure testing.

Where light expansion after strength welding is specified (TEMA
RCB7.522), tube hole grooving is not required.

Where the crevice is to be eliminated (e.g. due to high thermal gradients), back face
welding shall be considered. This is an expensive technique because of the handling
of the bundle which is required and the complex welding equipment.
Where the additional security provided by strength welds in combination with tube
expansion into grooves is considered necessary, the sequence of operations and the
technique employed for tube location is important. Porosity can occur in the welds if
the tubes are fully expanded prior to welding and weld cracking may be encountered
with expansion after welding. Generally, expansion after welding is more
satisfactory.

Particular attention shall be given to tube end fixing where any of the following
could occur: thermal shock, thermal cycling or a large difference in thermal
expansion between the tubes and tubesheet.

Failure of tube-to-tubesheet attachments is extremely costly and by no means
uncommon. Selection of the optimum materials for both tubes and tubesheet together
with the correct combination of expansion and welding is essential to ensure
maximum integrity and service reliability

 

[hippocrocopig]

I notice you have free H2 in the shellside. Depending upon partial pressure of the hydrogen relative to tubeside, you might have to consider the TEW (if you impose one) to be in hydrogen service. So DO NOT seal roll behind the weld, as you may trap H2 there, with the potential for the weld to fail. In hydrogen service you may either roll or weld, but don't do both.

HCP

 

[pve5]

hippocrocopig ( Mechanical )

Based on feed back from other users, we would go for strength welding and light expansion in plain holes without grooves.

Thanks all for your valuable comments

 

[hippocrocopig]

OK: as long as you have considered the prospect of hydrogen concentration behind the strength weld.