Tube to tube-sheet weld failure & cracking in tubesheet when rectification is done

[namihabib]

DESCRIPTION SHELL SIDE TUBE SIDE
SERVICE CATACARB SOLUTION EFFLUENT GAS-H2, NH3, etc
TEMPERATURE 120C
MATERIAL A285 Gr C clad with ss 304 ASTM A213 TP 304L

We have a semi-lean generator which regenerates catacarb soluton in ammonia plant, the specs of which are given above.
During TA 2015, failure of weld joint of tube-tubesheet weld was observed.
When repair welding is done. More cracks are generated in the tubesheet which propagate to other weld joints of nearby tubes.
Tubesheet material is A240 TP304.
Any idea what might be the reason.
If scc is the cause then it should also effect the tubes but on the contrary, tubesheet is generating cracks and the weld seam is failing.
Chloride ppm in catacarb solution is 50 ppm at 120 degree C.

 

[EdStainless]

Is there any S in this system?
Very small amounts of S mixed with Cl can lead to SCC.
The stresses from the welds (and re-welds) can drive the cracking.

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P.E. Metallurgy, Plymouth Tube

 

[namihabib]

I will have to confirm about the s content.
But my point is scc will effect the tubes as catacarb solution will flow over the tubes.
the weld seam of tube-to-tubesheet is exposed to effluent gas.
kindly view the pic for getting an idea.
furthermore the tubesheet is a double tubesheet

http://files.engineering.com/getfil...-2b21-49b8-b7e0-aed0f2c8e219&file=Capture.JPG