Repair options for leakage in a regenerator (Heat exchanger)

[namihabib]

SHELL SIDE SERVICE: CATACARB SOLUTION
TUBE SIDE: EFFLUENT GAS-H2, NH3, etc
TEMPERATURE catacarb 120C
shell MATERIAL: A285 Gr C clad with ss 304
Tube side material: 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, also tubes were leaking from near the expanded joint.
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.
Chloride ppm in catacarb solution is 50 ppm at 120 degree C.
Also how to repair the two types of leakage as welding is not effective and plugging may not rectify the leakage from the rolled joint space between tube & tubesheet.
refer to the drawing.
the tubesheet is double and the weep holes have been plugged o get rid of leakages

 

[EdStainless]

Have you ordered the replacement bundle?
Once you have leaking between and plug the weep holes it is all over.

There are lots of ways to mess up tube to tubesheet joints.
Rolling too near to the back face of the TS or too near to the weld will cause cracking. Welding without proper rolling will put all of the load on the welds and cause cracking also.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Would have thought SS304 would be a poor choice in this case where chloride SCC would occur at 120degC. Why not go for an all welded plate heat exchanger made out of AL6XN or SMO254 or similar with adequate PREN for this service, since you have clean fluids on both sides ?

 

[namihabib]

[highlight #CC0000]Have you ordered the replacement bundle?[/highlight]
We have two such heat exchangers currently in operation. We have ordered one new bundle with modified design i.e. having a single tube-sheet. But our concern is that similar problems maybe seen in the second Heat exchanger which was not inspected in our last TA’15.
As a preventive measure, what could be our way forward for the second heat exchanger? We intend to go for repair options.

[highlight #EF2929]“Once you have leaking between and plug the weep holes it is all over.”[/highlight]
Can you elaborate this point? I mean we can remove the plug from the weep hole and would like to go for an option for repair of the leakages.

 

[namihabib]

Dear georgeverghese,
I think plate type heat exchanger would not be a good choice as we have effluent gas at one side. furthermore catacarb crytallization maybe a problem.
Is it easier to clean plate type HX or shell & tube HX?

 

[georgeverghese]

Agree, it is not possible to clean a PHE in the usual way you can with a shell and tube HE. But if you can minimize the risk of fouling the plates, or have 2x100% units, with one taken offline for flushing with some solvent or otherwise, this would be a good way to go. There are also welded PHE with wide plate option if you see some advantages with this.

Another option to consider would be a spiral HE manufactured by Alfa Laval.

Else you may have to spend a lot more money for a more corrosion resistant S/T HE.

A less corrosive solvent than catacarb would be what many modern plants have : activated MDEA - doped with piperazine.

 

[namihabib]

Dear georgeverghese,
My company will be reluctant to go for such a change. I will explore the possibility though.
Is changing the material of present HX the only option to get rid of SCC?
any options for the repair of leakages that we are encountering?

 

[EdStainless]

In order to really plug these you need to remove the tube, so that you can plug the inner tubesheet.
These designs are not meant to have those vents plugged, you are crating stresses that were not anticipated.

Material change is the only way to get away from SCC. What are other people doing? Are these being built in 2205? That might be an option for both tubes and tubesheets, using thinner tubes.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Am not an expert on chloride SCC, but it does look like the SS304 / 304L you've got now begs justification from your regen unit plant designer with the 50ppm chloride at 120degC. Talk to this catacarb unit supplier for their opinion on all welded duplex 2205 (or higher PREN) PHE if the design press / temp limits for these are within the range of mechanical design for this HX.

 

[namihabib]

Thank you so much -EdStainless & georgeverghese- for your replies.
I have forwarded my recommendations of re-evaluating the design & material & checking the process parameters.
As it clearly indicates that SCC is the problem

 

[georgeverghese]

I'm not familiar with all the chemicals in this catacarb solution - could it be that chloride is not supposed to be present ? That may explain why SS304 was chosen for the material of construction on this HX. If so, what is the source of these chlorides ? Poor quality makeup water or a leakage from a catacarb-cooling water HX ?

 

[namihabib]

I have been pondering over this as well.
maybe you are right & chlorides should not be present. Will send my queries to process department for clarifications.
nice direction of thinking!!!!!

 

[namihabib]

The licenser of Catacarb Solution allows 200-400 PPM Chloride in the solution without risk of Stress Corrosion Cracking where as we have very nominal Chloride in comparison (<50 PPM).

According to experience, the inhibited CATACARB solution has greatly reduced the chance of occurrence, even with a normal chloride level of 200-400 ppm. However, the chloride content should still be tested occasionally and monitored. To minimize chloride contamination, all makeup chemicals should have low chlorides, normally less than 100 ppm.

this is the reply that i got

 

[georgeverghese]

This is now a topic for corrosion engineers to comment :

a)What is the operating pH range for the catacarb solution where this failure occured - the pH would have been low if this is the CO2 rich catacarb solution. From what I read, a pH of less than 10 is required for CL SCC

b)Is the licensor quoted permitted 200-400ppm chloride level relevant for non sensitized SS304 or for materials that have become sensitized, also what is the pH for which this permitted level is allowed?

 

[namihabib]

Yes, I also read it in API RP 581....working on collecting pH data.
presently new catacarb solution data is as under:
CATACARB CATALYST 400H (10% AQ. SOLUTION): 11-12
CATACARB INHIBITOR 922AH (5% AQ. SOLUTION): 12.1
CATACARB DEFOAMER WBUH: 7

But i dont have the pH of catacarb after absorption of co2. working on that!!!
Is the licensor quoted permitted 200-400ppm chloride level relevant for non sensitized SS304 or for materials that have become sensitized
how sensitization is a problem here?
i dont think sensitization temperatures are achieved

 

[EdStainless]

The tubes may not have been well annealed in the first place, and hence the sensitization issues.


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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

It may help if you also asked the catacarb licensor if they have any concerns (crystallisation?) with the use of a 2205 duplex (or higher PREN) all welded plate heat exchanger in this application.

 

[namihabib]

pH comes out to be 9.7. Seems like SCC is the culprit