Investigation on the Failure of Desal Condenser Tubes of Jeddah

Cupronickel 90/10 alloy has been used world wide as a standard material in heat recovery tubes. The alloy exhibits excellent service characteristics and can also withstand flow up to 3.3 m / sec of seawater. This alloy (Cupronickel, 90/10) was used in the construction of Jeddah IV heat recovery section of desal condenser tubes. Jeddah IV desal unit has 4 modules, each module consists of 21 stages and was of long tube design. The condenser tubes belonged to heat recovery section of module # 1, namely stages # 4, 5 and 6 showed leakages during operation and the problem was referred to R & D Center. The failed sections of the tubes were subjected to investigation for determining the cause of failure. Methods of metallographic analysis, Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA) were used during investigations. Some of the corrosion product analysis from the failed tubes were characterized by chemical analysis and X-ray Diffraction Analysis (XRDA). The surfaces of the stages 5 and 6 tubes were found covered with dark brown color thick deposits of copper oxide, where as stage 4 tubes were covered with light red thin deposits. In one of the tubes in stage 5, localized perforation due to pitting was observed and the place of perforation was at the junctions of tube and tube support plate. The results of the investigation indicated that the corrosion on the tubes from stage 5 and 6 was due to vapor space corrosion. The perforation of stage 5 was due to acid vapor attack by the non-condensable gases including bromine. The corrosion of stage 4 was generic corrosion of copper alloys during operation. 1 Presented in Second Acquired Experience Symposium on Desalination Plants O&M, SWCC, Al-Jubail, Sept.29-Oct.3, 1997. 1445 To prevent the recurrence of such failure it was recommended to install an effective ventilation system for removal of all non-condensable gas zones and plugging all air in leakages. Possibility of redesigning could also be explored. It was further recommended that the during shut down, the evaporator internal should be kept dry in order to protect the tube metal from general corrosion.