SS316L tube failure in Heat exchanger

[ryu.changmyong]

Dear ALL,pls let me ask you some technical advice for tube failure in Heat exchanger
as shown on attached Tube failure status.
Since Heat Exchanger operation in 2001 year, for 15 years
Tube failure occurred in drain outlet where located between
Tube sheet and 2ND baffle at 3, 4 pass area particularly
as much as 8~10 tube leakage yearly.
Currently total Tube Plugged q'ty is 127 of 932 tubes.
this is issue why this area damaged rather than other zone.
at the moment, this is related to operating level of condensate water
due to corrosion compound when chemical cleaning.

1. Tube OD 19.05, thk 1.65mm
2. Shell Side Flash Vapor : 0.3% of Terephthalic Acid contained Water,Condensate water pH 3.8
3. Tube side : Reactor Feed
4. Material : Shell-SS304L, Tube-SS316L
5. Tube side Pass ; 4 pass
6. Tube failure major status : Founded Pitting and Crack assumed by SCC (Stress and Corrosion Cracking)
at tube outside surface. For more information, refer to Lab inspection data as attached file.
Lab inspection report for sample tube say as follows.
1. Major damage : Pitting and Stress Corrosion Cracking
2. Tube surface crack into tube inside : 1.1mm and growing slowly
3. Most damage area : It occured at back side of Tube sheet (2ND baffle area)
in 3 pass and 4 Pass zone in a length of 600mm.
Also,Scale cluster founded this area.
4. It founded S,Cl,Na at damage tube as corrosion factor. Chloride 0.75~0.99Wt%

Could you advise me of your technical opinion in this regard.
Ryu Changmyong/P.E

 

[ryu.changmyong]

Dear ALL

Additionally, let me show you more information regarding this issue.
This kind of Heat Exchanger installed similar design condition.
But only this item occured SCC problems at 3 pass and 4 pass area.
I want to know why this SCC occured at 3 pass and 4 pass area only

I guess that tube thickness and chloride gas would be suspected to tube SCC
and Cholide vapor gas gather in this area, if this is
correct,
Does tube side velocity affect that because of 3 and 4 pass velocity is more
high than 1 pass and 2 pass

Operater say,
1) Shell Side Flash Vapor : 0.3% of Terephthalic Acid contained Water,
Condensate water pH 3.8
2) They also found little part of S, Cl, Na component

 

[georgeverghese]

Maybe some kind of corrosion caused by reaction of reactor feed leakage with condensed water - terephtalic acid at some tube to tubesheet joint leak points? Are the tubes just plain roller expanded into the tubesheet or seal welded and stress relieved ?

 

[ryu.changmyong]

Dear georgeverghese
Tube leakage occured at 3 pass and 4 pass over tube bundle from 2 nd baffle not for tubesheet joint.

 

[ryu.changmyong]

Dear ALL

What I want to know is why SCC happen at 3 pass, 4 pass only over 2nd baffle from tubesheet.

 

[hacksaw]

316 SS is definitely prone to chloride SCC, beyond that it is not clear what else may be happening.

One of the frames in you attached seems to indicate vibration orbitals. Are you having excessive tube vibration?

 

[georgeverghese]

Is the location at passes 3and 4 at baffle 2 close to where the normal vapor -liquid interface is on the shellside ? If the condensed phase is rich in dissolved oxygen, this may explain the SCC here at this interface. What is the O2 content in the feed flash vapor? If chloride SCC and chloride pitting are the corrosion mechanisms here, a higher PREN CRA material may be unavoidable if you cannot eliminate chloride and oxygen in the feed.

 

[ryu.changmyong]

Dear georgeverghese and hacksaw
I agree with Tube vibration where located near to Drain nozzle because of two phse flow especially
and it is very sensitivity in a phase changes of condensated flow into Drain outlet.
So what is the way to prevent tube vibration, I guess that additional support plate would be available. Anything else ?

 

[EdStainless]

Better baffling and routing of the flow, you need to reduce the crossflow velocity.
You could look at using stakes for some stiffening between support plates. They would not interfere with flow as much.

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P.E. Metallurgy, Plymouth Tube