S-M. Hu et al/Case Studies in Engineering Failure Analysis 3(2015)52-61 Table 1 peration parameters of ammonia evaporators. arameters velocity. P(MPa temp,T(°) temp,T(°C) Tube sid 4019763.6 Shell side 155573 24.7(L)96234(V) Actually, some failure incidents of ammonia heat exchangers have been reported at both abroad and home in the past 1-8. showing that inadequate thermal treatment, stress corrosion cracking and strain aging embrittlement were general failure causes. However, those studies mostly dealt with incidents in extreme process conditions like elevated temperatures and pressures. Whereas the unexpected wall thinning in our case happened under quite different environment as relative low operation temperature(below 10C)and current non-aggressive medium(oDB)in tube side. Owing to diverse situations, it became hard to explain this wall-thinning case by above mechanisms described in literatures. Hence, based on our recent work on failures analysis of various heat-exchange tubes [9-14, systematic investigations were conducted to find out the real cause of this incident, including external appearance, microscopic morphology and chemical compos d then practical prevention measures were also proposed. Overall, the analysis in this paper will provide a reference with significant engineering value to prevent failure recurrence of ammonia s unger sin r operating condition 2. Experiments and results 2.1. Visual observation and sampling Fig 2 shows several tube samples taken from evaporators. Particular attention should be paid to the tube ends where I typical trace of physical expansion exists( Fig. 2(b)). It's common that to achieve a tight-fit joint, the tube end has to be expanded radially at room temperature by hydraulic process and then welded so the joint between tube and tubesheet is permanently strained and secured. while here, the tube ends apparently hadn 't experienced such treatment before weld. So the joints would be prone to cracking when subjected to the fluctuation of operation conditions and thus brought about potential hazards for the In terms of tested dat It tubes by the plant, we chose two failed tubes with severe degradations to study in detail, arked as2and10°( ns shown in Fig 3). 中中中中最 中 Fig. 1 Schematic diagram of the structure of ammonia evaporatorsActually, some failure incidents of ammonia heat exchangers have been reported at both abroad and home in the past [1–8], showing that inadequate thermal treatment, stress corrosion cracking and strain aging embrittlement were general failure causes. However, those studies mostly dealt with incidents in extreme process conditions like elevated temperatures and pressures. Whereas the unexpected wall thinning in our case happened under quite different environment as relative low operation temperature (below 10 8C) and current non-aggressive medium (ODB) in tube side. Owing to diverse situations, it became hard to explain this wall-thinning case by above mechanisms described in literatures. Hence, based on our recent work on failures analysis of various heat-exchange tubes [9–14], systematic investigations were conducted to find out the real cause of this incident, including external appearance, microscopic morphology and chemical composition, and then practical prevention measures were also proposed. Overall, the analysis in this paper will provide a reference with significant engineering value to prevent failure recurrence of ammonia evaporators under similar operating condition. 2. Experiments and results 2.1. Visual observation and sampling Fig. 2 shows several tube samples taken from evaporators. Particular attention should be paid to the tube ends where no typical trace of physical expansion exists (Fig. 2(b)). It’s common that to achieve a tight-fit joint, the tube end has to be expanded radially at room temperature by hydraulic process and then welded, so the joint between tube and tubesheet is permanently strained and secured. While here, the tube ends apparently hadn’t experienced such treatment before weld. So the joints would be prone to cracking when subjected to the fluctuation of operation conditions and thus brought about potential hazards for the whole unit. In terms of tested data about tubes by the plant, we chose two failed tubes with severe degradations to study in detail, marked as 2# and 10# (positions shown in Fig. 3). Table 1 Operation parameters of ammonia evaporators. Parameters Media Pressure, P (MPa) Inlet temp., T (8C) Outlet temp., T (8C) Flow rate, Q (kg/h) Flow velocity, V (m3 /h) Tube side ODB 1.2 11 17 4019763.6 3000.0 Shell side NH3 2.0 8.7 19 15557.3 24.7(L)/9623.4(V) Fig. 1. Schematic diagram of the structure of ammonia evaporators. S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 53