Leak seal welding for TTS expansion joint

[Metallurgistrbi]

Hi,

We have a Heat exchanger with Tubesheet SB171 and same tube.

The TTS joint is mechanical.

However client wants to seal weld the same.

Iam worried about the thermal expansion failure due to seal weld. Is there any guidelines available regarding thermal expansion failure due to weld in TTS joint?

Thanks for all your feedback.

 

[ShabsRBI]

It is recommended to follow an approved WPS for performing the seal welding. Ensure that the correct welding consumables are used as defined in WPS. It is a hard task seal welding the TTS joints as due to thermal expansion the TTS joints in the proximity tubes tend to crack and then it becomes a chase to perfectly seal all of them without any cracks.

 

[weldstan]

Too little information. What is the alloy designation? SB-171 includes many different copper alloys.

 

[EdStainless]

You seal weld before you roll the joints.
In reality most shops would refuse to weld Cu alloy tubes.
The tubesheet spec should require higher hardness than the tubes.
If you are going to weld then everything must be clean and dry.
The tubes are often lightly rolled, just to contact in order to keep them in position.
Then they are welded.
And then you test the welds (penetrant and leak test).
And then you roll the tubes in making sure not to roll too close to the welds or to the back of the tubesheet.
All of these steps require validated procedures.

 

[JoshRanch]

Hi,

We have a Heat exchanger with Tubesheet SB171 and same tube.

The TTS joint is mechanical.

However client wants to seal weld the same.

I am worried about the thermal expansion failure due to seal weld. Is there any guidelines available regarding thermal expansion failure due to weld in TTS joint?

Thanks for all your feedback.

is your concern about welding copper nickel material particularly?

If tubes are closely packed together then seal welding is usually required. I don't have a copy of TEMA but clients ask for it.

The weld will protect the mechanical joint created by the expansion process and assist with heat dissipation across the tube sheet.

 

[Krausen]

You seal weld before you roll the joints.

Many major client specs require the exact opposite of this. Seal/Strength weld your TTS joints first, PT (Liquid Penetrant) test them all, then roll them. PT (Liquid Penetrant) test them all again after rolling.

 

[r6155]

Leak test for seal weld only (avoid PT).

 

[ShabsRBI]

As mentioned by @EdStainless the following rules are followed.

1. Expansion of tubes are done by contact expansion (1%-3%) of tube expansion.
2. Expansion from front of Shell side is done leaving 6-8mm.
3. Expansion to the back of tube sheet is done leaving approximately 3mm.

 

[r6155]

Application Tube Wall Reduction*
Nonferrous tubes in surface condensers 3% to 4%
Steel tubes in heat exchangers 5% to 10%
Soft copper and aluminum tubes in heat exchangers 8% to 12%
Boiler tubes 12% to 14%
*After metal-to-metal contact of the tube outer diameter with the tubesheet

I suggest a Mock up

 

[EdStainless]

AWR is a lousy estimate of tube expansion.
It all depends on the actual hole size, tube OD, and tube wall.
Do you really want to measure all of those?
Large projects that I have worked on always used the rolling torque to control the tube expansion.
They build a mockup, measure all of the exact sizes. roll tubes to various torques, and then do a push out test.
Then they use a torque that gives then 85-90% of max joint strength.
The expansion tool is looked at after every tube.
If there is damage or debris they swap it out.
At every tool change of 50 tubes WECF they recheck the rolling torque.

 

[ShabsRBI]

For Carbon steel 3/4" tubes with 2.11mm thickness the prescribed Torque is approximately 7Nm.

 

[r6155]

@ ShabsRBI
We are talking about SB171, not carbon steel.
The torque value is defined during fabrication..

 

[weldstan]

Whereas the OP has not stated the actual alloys under SB-171 to be welded and I have seen Cu-Ni alloy tubes and auminum bronze tubesheets and other dissimilar Cu alloys used in HX, the seal weld can be problematic. One thing we can all agree is that a mockup is mandated.