Cladding on tubesheet 1

[atedionyahoo]

I recently am working on a s&T heat exch. in which both tubesheet faces are cladded. The tubes are strength welded to tubesheet. As per tema RB-7.6, te tubeside clad thk. shall be 1/8" min while the shellside clad thk. shall be 3/8" min.
The presumption is that the weld prevents the tubeside fluid to penetrate the tube-tubesheet area and cause corrosion. The question is that what can prevents the shellside fluid to penetrate the tube-tubeshhet area. In fact, what is the advantage of considering 3/8" thk. on shellside face?
Please bear in mind that the tubes are not expanded and are strength welded.
Any remark would be appreciated.

 

[SnTMan]

atedionyahoo, 3/8" clad on the shellside does two things:

1) allows machining of the shellside gasket surface without cutting thru the clad
2) allows rolling of the tubes into the clad to seal the tubehole

If your shellside process is such that the tubesheet needs cladded I would definitely roll the tubes.

Regards,

Mike

 

[atedionyahoo]

Thanks. I am wondering why 1/8'' for tubeside and 3/8'' for shellside. If the reason is machining the gasket face, there shall be no difference in clad thickness for both shellside and tubeside (1/8'' seams to be sufficient). But if the reason is lightly expanding the tubes, 1.does it mean that tema implies full-length expanding in case of cladding is mandatory, even if tubes are strength welded.
And then, 2. shall expanding be done without grooves, because joint are strength welded?
And finally, how to expand for full length if tubesheet is too long?
Many thanks.

 

[SnTMan]

I don't know why 1/8" for tubeside, but I suppose for some constructions it could make sense, such as tubesheet integral with the channel.

TEMA does not require grooves for strength welded tubes, see RB- & C-7.44, does not require full depth expanding, see RCB-7.51. I don't think TEMA mandates any particluar depth of expansion other than this. However, each design must be evaluated on its particulars.

Joints in thick tubesheets can be expanded in steps, can be hydraulically or even explosively expanded, although this last is usually reserved for thick tube walls, so far as I know.

It might be possible for you to expand only a small length at the shellside to seal the hole.

Regards,

Mike

 

[atedionyahoo]

Many thanks SnTMan. I do agree with you. I suppose that the only way to seal the joints is to slightly expand the tubes at the shellside.
Dear SnTMan, thanks for your advices. It really was helpful. I mean it.
Regards.

 

[SnTMan]

atedionyahoo, thanks and good luck.

 

[rmw]

Rolling tubes isn't a precise art and is dependent on the skills of the operator. Since it is not good practice to roll past 1/8" from the end of the tubesheet, it would seem to me that the 3/8" gives an adequate margin for inaccuracy at the shell side of the tube roll.

If the tubesheet is thicker than the mandrel length, the tube is rolled in steps.

DO NOT roll past the edge of the shell side tubesheet face.

rmw

 

[TJOrlowski]

Rolling tubes isn't a precise science, it is an art and is dependent on the skills of the operator

Corrected.

-TJ Orlowski

 

[rmw]

Appreciated.

rmw

 

[OSIRIS7]

First of all: cladding on the tube side? How are you going to seal (weld) the tubes to the cladding?

Because of my experience I can suggest 2 methods of cladding:
1) explosion cladding (preferable)
2) turn inserts in the threaded holes in the tube sheet en weld the cladding to it.

Before inserting the tubes, but after the exploclad, a very small undercut of the tubesheet material has to be made to avoid sucking up tube sheet material in the tube to clad weld.

The tubes are preferably rolled in a chambered tubesheet.