s/s 347 heat exchanger

[Adriaan Stoltz]

Good day experts

 

Upon pressure testing of a 347 s/s heat exchanger (tubes and tube sheet) we found numerous miniscule leaks through the tube-to-tubesheet welds as well as from the inner bore of the tube in the HAZ.  The HE is 37 years old, not much operational history except that it is currently operating at app. 200 deg C at a pressure of 500kPa ( cooling water shellside) and 1200kPa (heat transfer oil tube side)

Tubesheet 40mm thick.

Tubes 3/8’’ od  1.2mm thick

 

My questions:

1.       What could be the cause of weld and tube failures?

2.       Preferred procedure for repair?

 

Your input is greatly appreciated.

 

Regards.

[pgoswami]

Hi Adrian,

 

If the exchanger is out of service for greater part of the year, cooling water on the shell side would act as a trap, in the tight crevices of Tube to tube sheet. If so pitting and resultant pinhole leaks are very much possible in T-TS welds . It would be more appropriate to get the more operating details, chemistry of cooling water, chloride content or other halogen contents. 347 and other 300 series alloys could pit in stagnated cooing water with even > 100 ppm of chlorides. 37 years of service history is quite for an exchanger.

 

If the clients insist of re-hab of his exchanger based on the sound  integrity of rest of the exchanger components, the repair of tube to tube sheet welds is a feasible idea. Best option would be to scoop out the old weld re-weld  by manual GTAW or automatic GTAW process.

 

If you could provide more design/operating details, then it may an interesting case for failure analysis.

 

Thanks.

  

 

Pradip Goswami, P.Eng,IWE
Independent Welding & Metallurgical Engineering Specialist & Consultant.
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

pgos...@quickclic.net

pradip....@gmail.com

 

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[Adriaan Stoltz]

Good day Pradip and honourable members.

I hear your advice. Many thanks.

The H/E is in constant service though, we brought it out of service for the 3 yearly statutory inspection. During the previous statutory inspection 3 years ago, the hydrotest was successfull with no leaks. Could the degradation possibly be attributed to thermal fatigue, considering the age of the vessel? 

Secondly, when doing the repair, would you consider pre-heat to be essential? We attempted to repair one of the defective welds but found the adjacent tube to tube sheet weld to crack too. We did not apply a pre- heat though. Could a pre-heat of 150 deg and heat input control possibly solve the problem?

Your advice is higly regarded.

Many thanx!

Sent from my Samsung Galaxy smartphone.

[pgoswami]

Hi Adrian,

 

Preheating will not needed to repair these welds. However some local preheating would be advisable to drive away the moisture and make the weld joints dry, so as to produce good quality welds. Presence of moisture could cause porosities in the welds.

 

It would be advisable to cut a boat sample form one or two the T-TS welds and send the same to the labs for failure analysis to find out the root cause of failures. These points may subsequently be plugged. If your client wants to retain the exchanger in operation for more sustained period of time then the root cause determination would definitely help.

 

Appreciate your thoughts and future course of actions.

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