Failure analysis of AISI 321 tubes of heat exchanger

• AISI 321 tubes of tube shell heat exchanger failed by SCC in high chloride water.
• The cracks initiated in crevices formed in the tube-tube sheet contact.
• An Eddy current inspection procedure was developed to detect and measure the cracks.
• The replacement of the AISI austenitic stainless steel by superduplex UNS S32750 steel is recommended.

The present study is about the failures of tubes from a tube shell heat exchange in one of the compression stages used to cool and purify the gas produced in an off-shore platform. Conventional hydrostatic tests did not detect leaking in the heat exchanger, but these tests were conducted at room temperature, i.e., very distant to the service temperature (163 °C). However, tightness tests with helium gas have detected leaking. Some tubes were taken off for analysis and the liquid penetrating test detected cracks in crevices formed between the tube and the tube sheet. The cracks were found to be typical stress corrosion cracks with branched pattern. A non-destructive test with Eddy current probes was developed to give high percentage of detection. The work was conclusive about the main cause of failures. Due to the increase of chloride content of the water processed to concentrations higher than 1000 mg/L over the years, the substitution of AISI 321 tubes by superduplex steel is recommended.