Repair of cracks in SS 304 tubesheet? 1

[aliim991]

Greetings

We have a SS 304 tubesheet found during Turaround suffering from huge number of stress corrosion cracking SCC (craze cracks); due to chloride or sulfur..

we tried to grind some for up to 2mm deep, however crack is still there. Tubesheet thickness is 160mm. Our concern, if we continue grindings, it may damage the TTS welds as well and end up with huge problem.

We try to do PAUT to know the crack depth, however, we need your technical opening in this case whether to leave it with risk and apply some barrier coating to minimize growth rate and exposure to service condition?

Can we do any kind of FFS? Long term plan to replace it however we need it to run fir 4 years

 

[Dhurjati Sen]

Dear aliim991,

If there are 2 mm cracks on the tube-sheet due to chloride SCC, then the SS 304 tubes would have vanished !!!

Since, it seems they haven't, better leave those cracks alone and box-up the heat exchanger.

However, carry out a failure analysis that would help you choose the future plan of action properly.

Regards.


DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[aliim991]

thanks Dhurjati for your reply,

I think due to large number of welding on tubesheet, it get sensitized and that makes it more vulnerable to SCC than tubes. based on surface DPT inspection, these cracks are reaching TTS welds.

Regards,

 

[Inspectengr]

Have u performed any pneumatic test or soap film test. Or any ammonia etc test.


To identify which of these have thru opening.

Rectify those. Rest cracks try to identify depth. For record and monitoring.

 

[r6155]

1)You should keep looking for how deep the defect is.
2)Can you send us pictures?
3)Try with Acoustic Examination Test.

Regards

 

[aliim991]

Yes we will be doing peneumatic leak test from shell side. Below photo showing crack location and shape. It is all over tubesheet

 

[r6155]

I can see excessive fillet weld.

Regards

 

[weldstan]

Cracks appear to be in the HAZ. Do further grinding to see depth which may well follow the HAZ to the tube hole. If so, machine weld out to sound base metal and reweld. Because of the thickness of the tube sheet, rapid cooling during welding creates high residual tensile stress in the HAZ.

 

[weldstan]

Are these the only cracks found, near the edge where tube sheet ligaments are the longest?

 

[aliim991]

Thanks all for your responces!

Cracks can be found all over the tubesheet and in between tubes. The concern if we continue grinding it may reach TTS welds where it will be beyond repair within SD window.

We are evaluating applying thermal spray coating ( nickle based material) which been tried in many site for similar cases; however crack will remain there. Long term to be replaced..

 

[chicopee]

From the picture it seems that the tube ends are welded and the cracks are at the toes of the welds. Is that right in my observation?

 

You cannot rely on visual examination or surface inspection to establish the cracking mode(s), which is vital to establishing the root cause, and without which knowledge you will be unable to prevent/mitigate in future.

Send samples to a met lab for investigation by a competent metallurgist.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."