[MW:24158] s/s 347 heat exchanger

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.