We have a newly (4months) commissioned unit (Gas oil HydroFiner) that experienced cracking failure on one of the exchangers. The exchanger is a low pressure (50 psi) steam generator, boiler feed water on the shell side and gas oil on the tube side. The circuit has two steam generators operating in parallel and only one failed on the shell side. Several cracks > 30 were found transverse to the welds. Cracks were also found on the pipe outlet mostly transverse to the weld and about 2 along the weld. Cracks appear to be brittle fracture. The shell and tube side are made from carbon steel material. Tube side was design at 300 deg C and shell side at 170 Deg C. We suspect that the exchanger (TEMA type NTN) that failed loss boiler feed water flow because of possible inlet control valve closure during operation. My question is what could be the reason for the failure?

David, Thanks for your input. What we found is sodium sulfite being injected to the boiler feed water as oxygen scavenging contributed to the formation of caustic in the exchanger that cracked. After about 95 days in operation (from commissioning) without blowdown the OH reached 3.7ppmw (calculated). With the exchanger tubes (100 out of 352) not submerged in boiler feed water for 95 day, concentration could have reached 15000ppm. What do you think? Did we nailed the cause of the cracking? Thanks and Regards, RAD