F-f. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 E: K2B 2A HF1B 1A G界 NERATOR IEH TURBINE SI ⊥:」 GEN ARRANGEMENT(S-1/20c) T/B ocation of Condenser Fig. 1. Schematic diagram of the arrangement of four condensers in a unit. Working parameters of the condenser Parameters Media Flow rate Q(m/s) Flow velocity Pressure P(kPa) Inlet temperature Outlet temperature Sea water 130.1 Shell side Steam Main steam flow rate 1033 kg/s(3718 T/h) 4-49 Note: The steam temperature in the condenser in operation was 47C(steam drain temperature of low pressure cylinder). Temperature of condensed water around30° Every tube is sustained by 22 perforated support plates made of carbon steel (IS SS400)with thickness of 13 mm and interval distance of 755 mm, and the diameter of the internal borings is 25.6 mm. Two ends of the tubes are welded with titanium clad carbon steel plates(ASTM B265 Gr 1 [ 1+ A515 Gr 65[))whose thickness is 40 mm. It is the titanium steel side that contacts sea water he units started commercial operation in August 2002 with actual runtime of about 8 years. During the 5th overhaul in 2010, part of the tubes were found to have suffered from serious wall thinning and thus temporarily stopped operation. After spection of the failure tubes, it was discovered that the wall thinning mainly happened near the support plates but to var- ious extents and at different positions of the tubes. If the thinned tubes had continued to be used, leaking caused by perfo- ration would have been very likely to happen, which could have brought about catastrophic safety problems. Therefore, were asked by the plant to conduct failure analysis on the abnormal wall thinning of the titanium tubes Previous work on mechanical performance of thin tubes 3, 4 and failure analysis of tubes used in condensers and plants [5,6 has provided some clues to this problem. Our team [7, 8 completed the failure analysis of leakage on titanium tubes ithin heat exchangers in a different phase of the same power plant as in our case. The failure was ascribed to electrochem cal corrosion and mechanical degradation. However, in our case, the thinning dominantly happened on the outer wall of the tubes at quite regular positions, which implied a different story To find out the root cause and mechanism of the thinning failure, we conducted a number of experiments for material, microstructure and chemical composition characterization based on previous successful failure analysis experiences 9-12 2. Experiments and results 2.1. Visual inspection and sampling To find out the cause of abnormal wall thinning of titanium tubes, we investigated condenser 1B of unit 2 under repair on the spot with the focus on the appearance of the support plates, their connection with the tubes and the surface condition of them both in the water inlet chamber. Some obvious defects found are shown in Fig 3. Corrosion extent varied among dif- ferent support plates and the most serious corrosion happened between the tube sheet and the first support plate( ig. 3(a)). oducts deposit was seen on the surface of many tubes near the support plates(ig. 3(a)and(b)).Every tube is sustained by 22 perforated support plates made of carbon steel (JIS SS400) with thickness of 13 mm and interval distance of 755 mm, and the diameter of the internal borings is 25.6 mm. Two ends of the tubes are welded with titanium clad carbon steel plates (ASTM B265 Gr.1 [1] + A515 Gr.65 [2]) whose thickness is 40 mm. It is the titanium steel side that contacts sea water. The units started commercial operation in August 2002 with actual runtime of about 8 years. During the 5th overhaul in 2010, part of the tubes were found to have suffered from serious wall thinning and thus temporarily stopped operation. After inspection of the failure tubes, it was discovered that the wall thinning mainly happened near the support plates but to var￾ious extents and at different positions of the tubes. If the thinned tubes had continued to be used, leaking caused by perfo￾ration would have been very likely to happen, which could have brought about catastrophic safety problems. Therefore, we were asked by the plant to conduct failure analysis on the abnormal wall thinning of the titanium tubes. Previous work on mechanical performance of thin tubes [3,4] and failure analysis of tubes used in condensers and plants [5,6] has provided some clues to this problem. Our team [7,8] completed the failure analysis of leakage on titanium tubes within heat exchangers in a different phase of the same power plant as in our case. The failure was ascribed to electrochem￾ical corrosion and mechanical degradation. However, in our case, the thinning dominantly happened on the outer wall of the tubes at quite regular positions, which implied a different story. To find out the root cause and mechanism of the thinning failure, we conducted a number of experiments for material, microstructure and chemical composition characterization based on previous successful failure analysis experiences [9–12]. 2. Experiments and results 2.1. Visual inspection and sampling To find out the cause of abnormal wall thinning of titanium tubes, we investigated condenser 1B of unit 2 under repair on the spot with the focus on the appearance of the support plates, their connection with the tubes and the surface condition of them both in the water inlet chamber. Some obvious defects found are shown in Fig. 3. Corrosion extent varied among dif￾ferent support plates and the most serious corrosion happened between the tube sheet and the first support plate (Fig. 3(a)). Besides, corrosion products deposit was seen on the surface of many tubes near the support plates (Fig. 3(a) and (b)). Fig. 1. Schematic diagram of the arrangement of four condensers in a unit. Table 1 Working parameters of the condenser. Parameters Media Flow rate Q (m3 /s) Flow velocity V (m/s) Pressure P (kPa) Inlet temperature T (C) Outlet temperature T (C) Tube side Sea water 1,30,100 1.97 – 18.8(note) 27.8 Shell side Steam Main steam flow rate 1033 kg/s (3718 T/h) – 4–4.9 – – Note: The steam temperature in the condenser in operation was 47 C (steam drain temperature of low pressure cylinder). Temperature of condensed water was around 30 C. 30 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41