204 Y Gong et aL/Engineering Failure Analysis 31(2013)203-210 hy crystallizer PTA device Fig. 1. Flow chart of the refining unit of the Amoco process. COOH COOH +2H +h. mPR (1) CHO Apparently, the pta dryer plays a critical role in purification of the whole manufacturing process. In our incident, such a dryer was indeed applied within a Pta plant with Amoco process in one petrochemical company in Shanghai. As for its con- figuration and parameters, it was a rotating cylinder( 2. 2 r/min) with three arrays of 3 mm-thick heat exchanger tubes inside around the circumference, and the tube diameters were 3, 4, and 5 in respectively. From the inlet of the dryer, the high-tem- perature(135C) steam vapors were conveyed in the tube side, and the wet filter cakes with PTA concentration of 38.7% were transported and heated in the shell side. From the outlet, the nitrogen gas was imported to take away the H,o and other small molecules that were all evaporated from the wet filter cakes lowever in fact, after capacity expanding of the whole Pta plant, severe corrosion occurred on the 316L stainless steel heat exchanger tubes of the Pta dryer not long after beginning of service. Particularly, the corrosion even exhibited a pecu liar morphology, i.e. the upside of the failed tubes was corroded but the downside was intact. In order to vividly depict this interesting appearance, we termed it 'Yin-Yang corrosion, the well-known terminology referenced from the Chinese tradi- tional philosophy. Correspondingly the 'Yin' face was denoted as the corroded area, while the 'Yang face was the intact one Thus, for purpose of investigating the actual causes of this premature failure(designed lifetime was 8 years, failed just after 2 years). a variety of pertinent characterization measures referring to our previous experiences of failure analysis and struc tural integrity evaluation of heat exchanger tubes [2-9] were comprehensively employed for the samples, including matrix materials examination of the tubes, chemical constituents inspection of the process media, and macro/microscopic analysis of the defects. Results showed that the interaction between factors from the process media, the service conditions, and the operation parameters, was the se of this 'Yin-Yang'corrosion. Then, the relevant corrosion mechanisms were dis- cussed and the countermeasures were proposed. This paper actually presents a model case of applying such comprehensive analysis methods for failure analysis in practical engineering, and its achievement will have a reference value for corrosion prevention of heat exchanger tubes operating under similar service conditions. 2. Experimental and results 2.1. Visual observation As revealed in Fig. 2a, corrosion failure only occurred on the second array, i.e. the 4-in. tubes, of all the three arrays of heat exchanger tubes inside the Pta dryer. After sampling, Fig. 2b displayed the external appearance of the above-mentioned 'Yin-Yang, corrosion morphology on one failed tube, and on its surface the horizontal boundary line between the 'Yin and the 'Yang faces could be clearly observed. Further magnified, the corroded'Yin' face was actually covered with densely distributed corrosion pits, seen in Fig. 2C, implying a localized corrosion mechanism. 2.2. Microscopic observation For purpose of obviously comparing the two opic morphologies of its cross-section As show Yin'and"Yang faces of the failed tube, Fig 3 pre een the left'Yin' ented the micro- 3a, a fictitious boundary line could be drawn betw and the right "Yang faces Further magnified, tl face was composed of corrosion concaves and pits, seen in Fig. 3b while contrarily, the'Yang' face was relatively smooth without any significant defects, seen in Fig. 3c.ð1Þ Apparently, the PTA dryer plays a critical role in purification of the whole manufacturing process. In our incident, such a dryer was indeed applied within a PTA plant with Amoco process in one petrochemical company in Shanghai. As for its con- figuration and parameters, it was a rotating cylinder (2.2 r/min) with three arrays of 3 mm-thick heat exchanger tubes inside around the circumference, and the tube diameters were 3, 4, and 5 in. respectively. From the inlet of the dryer, the high-tem￾perature (135 C) steam vapors were conveyed in the tube side, and the wet filter cakes with PTA concentration of 38.7% were transported and heated in the shell side. From the outlet, the nitrogen gas was imported to take away the H2O and other small molecules that were all evaporated from the wet filter cakes. However in fact, after capacity expanding of the whole PTA plant, severe corrosion occurred on the 316L stainless steel heat exchanger tubes of the PTA dryer not long after beginning of service. Particularly, the corrosion even exhibited a pecu￾liar morphology, i.e. the upside of the failed tubes was corroded but the downside was intact. In order to vividly depict this interesting appearance, we termed it ‘Yin-Yang’ corrosion, the well-known terminology referenced from the Chinese tradi￾tional philosophy. Correspondingly, the ‘Yin’ face was denoted as the corroded area, while the ‘Yang’ face was the intact one. Thus, for purpose of investigating the actual causes of this premature failure (designed lifetime was 8 years, failed just after 2 years), a variety of pertinent characterization measures referring to our previous experiences of failure analysis and struc￾tural integrity evaluation of heat exchanger tubes [2–9] were comprehensively employed for the samples, including matrix materials examination of the tubes, chemical constituents inspection of the process media, and macro/microscopic analysis of the defects. Results showed that the interaction between factors from the process media, the service conditions, and the operation parameters, was the main cause of this ‘Yin-Yang’ corrosion. Then, the relevant corrosion mechanisms were dis￾cussed and the countermeasures were proposed. This paper actually presents a model case of applying such comprehensive analysis methods for failure analysis in practical engineering, and its achievement will have a reference value for corrosion prevention of heat exchanger tubes operating under similar service conditions. 2. Experimental and results 2.1. Visual observation As revealed in Fig. 2a, corrosion failure only occurred on the second array, i.e. the 4-in. tubes, of all the three arrays of heat exchanger tubes inside the PTA dryer. After sampling, Fig. 2b displayed the external appearance of the above-mentioned ‘Yin-Yang’ corrosion morphology on one failed tube, and on its surface the horizontal boundary line between the ‘Yin’ and the ‘Yang’ faces could be clearly observed. Further magnified, the corroded ‘Yin’ face was actually covered with densely distributed corrosion pits, seen in Fig. 2c, implying a localized corrosion mechanism. 2.2. Microscopic observation For purpose of obviously comparing the two different ‘Yin’ and ‘Yang’ faces of the failed tube, Fig. 3 presented the micro￾scopic morphologies of its cross-section. As shown in Fig. 3a, a fictitious boundary line could be drawn between the left ‘Yin’ and the right ‘Yang’ faces. Further magnified, the ‘Yin’ face was composed of corrosion concaves and pits, seen in Fig. 3b, while contrarily, the ‘Yang’ face was relatively smooth without any significant defects, seen in Fig. 3c. Fig. 1. Flow chart of the refining unit of the Amoco process. 204 Y. Gong et al. / Engineering Failure Analysis 31 (2013) 203–210