ANALYSIS OF FAILURE MODES RESULTING IN STRESS-CORROSION CRACKING OF 304N TUBING IN A HIGH-PRESSURE HEATER DESUPERHEATER

Santee Cooper (South Carolina Public Service Authority) experienced tw enty-three tube failures in a high pressure feedwater heater that was in service less than three years. The tube failures were located at ba ffles adjacent to both exits of the dual flow desuperheater. Metallurg ical analysis of the failed tubes indicated that stress corrosion crac king of the 304N stainless steel was the primary failure mode (Rudin, 1994; Shifler, 1994). The investigation to determine the factors leadi ng to the onset of stress corrosion cracking included analysis of heat er acceptance tests, the heater manufacturer's proposal and manufactur ing procedures, operational data, eddy current reports, metallurgical reports, and a heater design review for vibration and wet wall potenti al (formation of condensation on the outside diameter (OD) of the tube prior to the desuperheater exit).