Analysis and Summary of Corrosion Repair Solutions for the Tubes in the Ammonia Stripping Tower

1 Introduction to leakage and repair situations: There are over a dozen large and medium-sized urea plants in China that employ the Snam process based on ammonia stripping; the stripping towers are primarily manufactured by the Italian companies FBM and Novibil. Over the past nearly six years since September 1998, multiple leaks in the lining welds have occurred at domestic fertilizer plants. Four companies have had their equipment repaired by our company, and the ammonia stripping towers of these four manufacturers were all manufactured by the Italian company FBM. The details of the various leaks that have occurred are shown in Figures 1 to 4. The first time our company carried out emergency leak repair was in September 1998. The leakage occurred in the lower pipe box, at the ring joint where the manhole lining met the ball head lining (see Figure 1); there was a visible air hole there. After grinding and welding to repair it, the system operated for one and a half years. In March 2000, a second leakage appeared at the same ring joint, near the location of the previous repair. This leakage spot was about 20 mm long and consisted of chain-like air holes resembling cracks. After grinding off the surface layer (by less than 1 mm), white-like pores were found in the weld metal beneath it. At that time, it grew longer and deeper with each grinding session; eventually, the entire weld reached over 200 mm in length. After the weld metal was completely removed, it was repaired through additional welding, and it has been in use to this day (it is now converted to an upper tube box). http://www.nmtech.com.cn/jishuwang/upload/0602061519429212.jpg Figure 1: Leaking points in the distillation column lining. In May 2000, the first leaks occurred almost simultaneously at the second and third fertilizer plants. The leak in the second fertilizer plant was located at the left corner of the gasket cover on the upper tube sheet, while the leak in the third fertilizer plant was found at the corner seam of the tube sheet cover, below the right side of the longitudinal seam; both leaks were relatively small in size. After grinding and repair welding, the third fertilizer plant operated for three years and two months before a second leak occurred in July 2003 at the fillet weld on the right corner of the longitudinal seam cover. The second fertilizer plant was in operation for three years and seven months, and a second leak occurred in December 2003; there were two leakage points – one at the fillet weld on the right corner of the longitudinal seam cover, and the other at the fillet weld on the corner of the cover plate at the junction between the cylinder and the tube sheet (gland ring) on the right side of the longitudinal seam. After grinding and rewelding, it failed again within less than a week, prior to New Year’s Day 2004; there were two leakage points – one located 60 mm to the right of the area where the second ring seam was rewelded, and the other at the corner seam on the opposite side of the area where the first liner cover was rewelded. After grinding and repair welding, it operated for one and a half months before experiencing its fourth leak on February 12, 2004; the leak site was located about 6 mm below the second leak point on the right side of the longitudinal seam cover. After the leak was fixed at the third fertilizer plant in July 2003, it operated for less than 7 months before a third leak occurred in February 2004 at the welds between the upper tube sheet pipes and the tube sheet. Just 1 month and 8 days after the repair (in March 2004), a fourth leak occurred, at the 3 o’clock position at the joint seam where the reinforcement ring (cover plate) of the N1A tube met the lining. In April this year, after the ammonia stripping tower of the third fertilizer plant was cleaned and inverted, a fifth leak occurred at the cover plate of the tube sheet and the fillet welds connecting it, during the process of raising temperature and pressure in preparation to start up the plant. The fourth fertilizer plant experienced its first leak on December 1, 2003, at the fillet weld on the right corner of the longitudinal cover plate; two months and nine days later, on February 10, 2004, a second leak occurred, at a location about 50 mm below the area that had been repaired following the first leak. At that time, it was decided in the research to expand the scope of repair welding, that is, to weld another layer on the fillet welds on both sides of the longitudinal seam cover plate. To facilitate the observation of the deformation of the lining and cover plate after such repair welding, we simply removed the surface contaminants from the welds at the corner joints on both sides of the longitudinal seam cover plate over a height of 800 mm, and then applied a layer of repair welding in each of those areas. After the patch welding, inspection of the cover plate revealed significant deformation. When viewed in the circumferential direction (the width direction of the cover plate), the cover plate changes from arc-shaped to chord-shaped. Since the repair welding was carried out, it has already withstood over three months of operational testing. http://www.nmtech.com.cn/jishuwang/upload/0602061520246409.jpg Figure 2: Past leakage points of E101. As can be seen from the information provided, the four manufacturers experienced a total of 15 leaks, and the locations of these leaks were almost all at the fillet welds of the covers. The fillet welds at the junction of the tubes and the tube sheet were present only in one location (the third leakage point at the third fertilizer plant), and the leakage points were relatively concentrated, mostly in a specific area. The time intervals between leaks varied; the shortest interval was less than a week, from the end of 2003 to the beginning of this year, while the longest interval was only two months and nine days. As a result, frequent leaks occurred on a short-term basis, causing huge economic losses to various fertilizer factories and drawing significant attention from them. 2 Nature of the defects: A total of 13 leaks (in 15 locations) have occurred to date in the tube boxes of the ammonia stripping towers at the 4 fertilizer plants. All of these leakage points were located at the welds, and observations made during repair and grinding indicate that they were all caused by welding porosity resulting from the manufacturing process. Porosity is the most common welding defect in the welding of titanium and titanium alloys. Porosities are of two types: porosities in the weld seam and porosities at the fusion line. There are various reasons for the formation of welding pores, such as: the edges of the cover plate being cut edges, high humidity during welding, low temperatures of the welded parts in winter, and contamination of the surface of the welding groove by substances like water and grease. If these contaminants are not properly removed or dealt with, welding pores will form. Except for the honeycomb-shaped, densely arranged pores in the first fertilizer factory, these pores are all individual; the larger ones have a diameter of 1.2 mm, while the smaller ones are as small as pores and difficult to detect. As is well known, not all pores necessarily lead to leakage; only after certain corrosion occurs, when the pores expand or become exposed on the surface, forming through-pores, will leakage take place. 3 Investigation into the causes of leaks: To date, all leaks have been caused by manufacturing defects – pores – within the welds; no leaks have been observed in the plates, as the probability of such defects occurring in plates is virtually zero (except for cracks). Titanium is highly prone to the formation of welding pores during welding; these pores can occur in various forms – dense or scattered, on the surface or inside the material, penetrating or non-penetrating. PT testing during manufacturing can easily detect fully penetrating pores, whereas non-penetrating pores are difficult to detect during inspection. During manufacturing, there are no suitable detection methods for internal defects in the lining welds, especially those in the corner welds of the lining layer; as a result, the distribution and shape of defects (porosities) in these welds remain unknown. It can only be known after a period of corrosion has taken place. Leakage occurs only when the pore communicates with the root of the weld; otherwise, only a small pit appears. In this way, for the small pores that extend to the root of the weld, the distance between their openings and the weld surface determines the sequence and timing of leaks. Titanium exhibits excellent corrosion resistance in the urea-ammonium system. The corrosion of urea (uniform corrosion) is primarily caused by carbamic acid formed during synthesis and ammonium cyanate generated through the isomerization reaction of urea. Both of these substances are strong reducing agents and possess high corrosivity at high temperatures. However, titanium still exhibits high corrosion resistance in this medium, with an annual corrosion rate of only 0.008–0.012 mm/a. However, the upper tube box serves as the inlet for the material; the flow of material in its liquid phase is turbulent. Both uniform corrosion and erosion corrosion occur there. As production increases and operation operates at an overload, the flow rate of the material speeds up, which inevitably leads to more pronounced erosion corrosion. The local pits and grooves on the surface of the lining are presumably caused by turbulent erosion corrosion. In summary, we believe that the main cause of leakage in the lining welds is defects present in the welds themselves, which lead to leakage once these defects become apparent due to corrosion. As for whether there are other factors at play—such as grinding at the location of leakage, the redistribution of stress during repair welding, and the migration and accumulation of new hydrogen ions or atoms within alpha-titanium—which may cause the defects in the welds to worsen and result in leakage, there is insufficient evidence on this matter, and further investigation is needed in the future. 4 Discussion on repair solutions: Based on the leakage incidents that occurred in the four fertilizer plants, all of them took place in the liquid phase area. With the exception of the first fertilizer plant, where the leakage occurred in the lower tube box, the others had leaks in the liquid phase section of the upper tube box (as the lower tube box leak was detected during the installation of the new tower in the first plant). No leaks have occurred in the lining of the gas phase area yet; therefore, whether it comes to sealing existing leaks or preventing new ones, repairs can be focused within this area. If that is the case, the weld that may leak is in the upper tube box. In this way, there are a total of 6 circumferential seams: those along the longitudinal direction of the tube box and those connecting the tube box to the tube sheet. Among these, two of the circumferential seams – those between the gasket ring and the tube box lining cover plate, and those between the gasket ring and the tube sheet double-layer cover plate – are buttweld seams. The thickness of the weld metal in such seams should be no less than 3 mm; to ensure full penetration, the number of welding passes should also be at least two. Therefore, the likelihood of leakage at these two welds is also low. The remaining ones are the fillet welds at the four covers, and the effective weld thickness of these four fillet welds is only about 2.1 mm. It is more likely to be welded in one go during manufacturing, which increases the likelihood of the occurrence of through defects (porosities). The locations of previous leaks demonstrate that fillet welds are most prone to leakage. Therefore, special attention should be paid to how to repair these four corner seams. Based on the above considerations, the following repair suggestions are put forward for discussion, in order to determine an effective and feasible repair plan for future implementation. 4.1 Local repair combined with an appropriately expanded welding area (the method used for the second emergency repair at the fourth fertilizer plant). This repair method allows the work to be completed in a short time, making it suitable for temporary repairs of leaks. Since the scope of repair is relatively small, the deformation of the lining is also minimal, making its use safer. Although the scope of patch welding has been expanded, it is unknown whether defects that could cause leakage in the short term exist in the areas that have not been patched, and it cannot be guaranteed that no leakage will occur during a maintenance cycle. If there were a method to detect potential defects in the near-surface layer of welds, that is, the location of those defects. If the distance from the weld surface can be measured, the time when another leakage will occur can be estimated based on the normal corrosion rate, allowing for early treatment of defects that are not deeply buried. If this can be achieved, accidental leaks will not occur, giving complete control to the equipment users. We have prepared a plan, welded test plates, and are conducting experiments to explore the possibilities. Of course, to find and master such reliable testing methods, it requires the joint efforts of all parties; only then is it possible to achieve results and apply them in production at an early date, thereby contributing to urea production. 4.2 Remove surface contaminants from all fillet welds in the liquid phase zone (upper tube bank) and then apply another layer of weld metal (perform supplementary welding). Until methods for detecting defects near the surface of welds are available, this approach is a relatively safe way to prevent accidental leaks. However, since the welds resulting from the repair are quite long (about 15 m), deformation of the lining will inevitably occur even when measures are taken to reduce such deformation, and it will definitely be greater than the deformation caused by local repairs. When the lining undergoes significant deformation, it inevitably affects its fit; the consequences of this are unknown, and it remains to be verified in practice whether it can withstand high temperatures and pressures. Of course, the deformation resulting from repairing the lining in this way is greater than that caused by local repairs, but it is much less than the deformation resulting from removing all the weld metal and welding it again. 4.3 Replacing the cover plate of the liquid phase section (upper tube box): When using the method of replacing the cover plate to address sudden leaks in the tube box, its necessity and feasibility should be considered first. If the thickness of the cover plate poses a threat to the safety of equipment operation (in terms of leaks), it is necessary to replace the cover plate; otherwise, there seems to be no need for replacement. When replacing the cover plate, it is also necessary to consider various possible situations regarding the gasket under the cover plate and the corresponding measures that should be taken. The situation that may arise when replacing the cover plate can be complex; for example, it might be necessary to replace the gasket, and at the same time, issues related to the connection between the gasket and the lining plate as well as the leak detection tube must also be addressed. The replacement cover plates must be inserted and removed through the manhole. Since the manholes of the stripping tower are small in diameter and have long passages, only longitudinal cover plates can be inserted and removed as a whole; circumferential cover plates must be divided into several sections in order to enter the tower, which increases the number of welds at the joints between these cover plates. Therefore, this solution should not be adopted unless it is absolutely necessary. In summary, after analyzing the specific structure of the tube sheet in the ammonia stripping tower and the measures that need to be taken in case of through-leakage caused by weld porosity during manufacturing, we believe that at this stage, the first option, namely extending the repair welds appropriately to both sides at the leak site, is suitable for emergency repairs ； The second option, which involves covering the surface of all welds on the upper tube box with a layer, is a practical approach to ensuring a feasible service life (one major overhaul period) ； The third option, namely replacing the upper tube box cover, is a last resort taken when severe corrosion and thinning of the cover affect its functionality, and it should not be used widely. The above is provided solely for the discussion and reference of experts. It is hoped that through discussion, a consensus can be reached to determine a more reasonable and practical solution. http://www.nmtech.com.cn/jishuwang/upload/0602061521128032.jpg Figure 3: Past leakage points of E101. http://www.nmtech.com.cn/jishuwang/upload/0602061521451741.jpg Figure 4: Two leakage occurrences in E101. Last edited by *eqiguo on 2009-2-22 08:32.]

The urea production unit of Zhongyuan Dahua Group Co., Ltd. utilizes the SNAM ammonia stripping process. Since its commissioning in May 1990, the stripping tower E-101 has been in operation for a total of 4,818 days. During the major overhaul in September 2002, although there was uniform thinning of the heat exchange tubes in E-101, the overall corrosion level was not severe； On February 12, 2004, the E-101 was retrofitted with an inverted configuration, and after more than a year of operation, the results were good. Two domestic companies of the same type have experienced corrosion of the stripping towers and perforations in their linings, which led to the shutdown of the entire system and caused significant losses. This article analyzes the causes of corrosion in stripping towers and summarizes the anti-corrosion experiences of our plant, for reference by other similar manufacturers. 1 Structural Features and Operation of the Stripping Tower 1.1 Structural Features The ammonia stripping tower E-101 is an upright shell-and-tube heat exchanger, equipped with 2,574 tubes of diameter φ27×3.5; the total length of these tubes is 5,830 mm, and the heat exchange area is 814 m2. The material used for construction is titanium. The upper and lower tube boxes are lined with 5 mm thick titanium material and contain 1 m3 of stainless steel Pall ring packing. There are 2574 distributors with dimensions of φ28.5×3 and a length of 402 mm; these distributors are made of 25-22-2CrNiMo material. The upper end of each distributor features 2 small holes of φ5 size, while the lower end of the tube has 3 small holes of φ3.2 size arranged tangentially. The structure of the stripping tower is symmetrical on both the upper and lower sides, allowing it to be used in reverse order as well. 1.2 Operation status From its commissioning in May 1990 until the major overhaul in September 2002, the system was shut down 268 times; after each shutdown, the high-pressure seal tower remained shut down for a maximum of 53 hours. During operation of the plant, due to significant fluctuations in the raw natural gas, the maximum load reached 110% while the minimum load was only 30%. The longest continuous operating period was 89 days, and the nickel content in the finished product ranged from 0.04×10‑6 to 0.32×10‑6. Due to the frequent shutdowns of the system and large load fluctuations, overheating can occur at the top of E-101. Under normal conditions, the temperature there is kept at 189°C, with a maximum of 195°C; the temperature at the bottom reaches up to 212°C at its highest and 201°C at its lowest, resulting in a temperature difference of around 17°C between the top and bottom. 2 Corrosion condition of the stripper: Based on previous inspections, the corrosion in the ammonia stripper E-101 is primarily concentrated in the upper part of the equipment. The tube bank, packing, packing bed supports, distributor supports, and pressure grids all show varying degrees of corrosion. The areas where corrosion is most severe and has a significant impact on the safe operation of the equipment are the tubes, which exhibit varying degrees of thinning due to corrosion. By the time of maintenance in September 2002, the average wall thickness of these tubes had reduced to 2.86 mm ； Furthermore, almost the entire outer wall of the upper pipe opening of the tube bundle was covered with a layer of reddish-brown scale; about 10% of the end surfaces of these pipe openings had wave-shaped notches, and the sealing surface where the outer wall of the pipe opening contacted the distributor showed erosion grooves, pits, and holes. To ensure the safe operation of the equipment, 43 tubes with a wall thickness of no more than 1.65 mm were welded shut in 2002, and the equipment underwent end reversal treatment in 2004. 3 Analysis of Corrosion Causes and Protective Measures 3.1 Corrosion at the upper pipe opening The corrosion at the upper pipe opening is mainly caused by inappropriate dimensions of the gasket or poor assembly, as well as deformation and fracture, which result in interstitial corrosion between the stripping tube and the distributor, and between the stripping tube and the PTFE gasket. This is the main cause of corrosion in E-101. Protection measures: Raise the corrosion prevention awareness and skill level of maintenance personnel. When installing or removing the liquid phase distribution tubes in E-101, strictly follow the operating procedures to ensure quality, avoid mechanical damage to the ends of the tube bundles, prevent displacement and deformation of the PTFE rings, thereby reducing erosion and crevice corrosion ； When selecting seal rings for distribution tubes, it is essential to strictly control product quality and choose appropriate seal rings based on the degree of thinning of the tube rows, in order to prevent corrosion in the gaps between the distribution tubes and the seal rings. 3.2 Corrosion caused by improper operation 3.2.1 Over-temperature at the top of the distillation column Frequent start-up and shutdown operations can lead to localized over-temperature conditions, either at the beginning of operation or during emergency shutdowns. In the early stages of operation, due to unstable discharge from the synthesis tower and significant system fluctuations, the liquid seal at the top of the stripping tower is prone to being broken, resulting in an unstable liquid film and localized overheating, which in turn accelerates corrosion ； During the emergency shutdown, E-101 experienced an instantaneous loss of feed, which caused the liquid seal in the liquid distributor to disappear. Meanwhile, the steam on the shell side could not be removed in time, resulting in overheating at the top of E-101 and intensified corrosion. 3.2.2 System NH3/CO2 imbalance exacerbates E-101 corrosion. The SNAM ammonia stripping process is designed with an NH3/CO2 ratio of 3.6, but in order to reduce the recovery load on the subsequent systems and lower steam consumption, this ratio is generally kept around 3.3. A decrease in the NH3/CO2 ratio leads to an increase in side reactions within the system, thereby accelerating the corrosion of E-101. 3.2.3 Insufficient oxygen addition in the system: The designed oxygen addition level (by volume) for the ammonia stripping process is 0.25%–0.35%. In order to reduce the amount of gas discharged from the medium-pressure system and lower consumption, the oxygen content is generally kept at the lower limit of this range. As a result, a proper passivation layer cannot be formed inside the equipment, keeping the titanium in an activated state and accelerating corrosion, which ultimately leads to localized perforations and damage. 3.2.4 The system takes too long to be sealed after parking. SNAM stipulates that the maximum sealing time for the high-pressure section of this system should not exceed 48 hours. In actual production, depending on the requirements of production, the sealing time may exceed 48 hours (for example, on January 13, 2001, the sealing time was 53 hours). The longer the sealing time, the more severe the equipment corrosion becomes. Protective measures: Enforce strict procedural standards, improve the management of high-pressure static equipment, and enhance the corrosion prevention awareness among chemical operators. Under normal production conditions, stable operation is essential to avoid significant fluctuations in the system and prevent overheating at the top of E-101. In the event of overheating at the top, the system should reduce its load as quickly as possible and divert steam for treatment. During startup and shutdown, pay attention to regulating the heating steam on the shell side of E-101. If operation is resumed after major maintenance, it should be carried out in accordance with the temperature and pressure increase curve in E-101; the temperature increase rate must not exceed 12°C/h, the pressure increase rate must not exceed 3.0 MPa/h, and the temperature difference between the top and bottom of the tower must not be greater than 30°C. The oxygen addition rate in the system should be above 0.30% to form a good passivation film inside the equipment. The system NH3/CO2 ratio should be around 3.6, and the H2O/CO2 ratio should be around 0.6. Under normal circumstances, the sealing time of the high-pressure tower shall not exceed 48 hours. 3.3 Other situations: In actual production, the CO2 conversion rate fails to reach the designed 65%, which increases the decomposition load on E-101 and causes it to operate under overload conditions. The increase in material flow rate further exacerbates erosion and corrosion of the tubes. Protective measures: Strengthen training for process personnel to improve their operational skills, thereby maximizing the CO2 conversion rate. To ensure that the liquid is distributed evenly throughout each heat exchange tube, preventing any uneven distribution or flow bias, a thin film is maintained on the wall of each tube, thereby improving the stripping efficiency. The lower end of the E-101 liquid distributor is equipped with 3 φ3.2 holes oriented tangentially. If the diameter of these distribution holes is incorrect, if they are blocked or deformed, it becomes impossible to form a complete and uniform film; this can easily lead to localized overheating in the tubes, resulting in abnormal corrosion and thinning of the tube walls. Protection measures: Improve the quality of maintenance work, carefully inspect the openings in the liquid distributor, and address any blocked or defective openings ； After system maintenance, it should be thoroughly flushed to reduce blockages in the openings of the liquid distributor. 4 Preventive measures: In addition to the above targeted measures, it is also necessary to regularly take samples of the inert gas and condensate on the tube side and shell side of the stripping tower for analysis, in order to enhance monitoring ； Strengthen the inspection of the leak detection tubes on E-101, and address any issues found promptly ； The nickel content in the finished products is analyzed twice a month, and any abnormalities are promptly investigated to determine their causes. The maintenance results of E-101 in 2004 showed that the corrosion rate had decreased significantly, indicating that the aforementioned measures had been effective. This post was last edited by *eqiguo on 2009-2-22 08:37]

Summary of the operation, maintenance, and reversal of the ammonia stripping tower: The urea production unit of Zhongyuan Dahuahua Group Co., Ltd. uses the SNAM ammonia stripping process; from its commissioning in May 1990 to July 2003, the stripping tower had accumulated a total operating time of 94,340 hours. In recent years, as the service life of equipment increases and it ages, the corrosion of the heat exchange tubes in stripping towers has been worsening year by year. Especially during the major overhaul in 2002, the wall thickness of 43 heat exchange tubes was 1.65 mm or less (including 1.65 mm), resulting in a total of 45 tubes becoming blocked. This stripping tower is a vertical falling-film heat exchanger with a symmetrical upper and lower structure, and can be used with the direction reversed. This paper summarizes the operation of the stripping tower and the process of reversing it. 1 Operation of the distillation tower: From its commissioning in May 1990 until the major overhaul in September 2002, the system was shut down 268 times. After each shutdown, the high-pressure section of the tower remained sealed for a maximum of 53 hours. During operation of the unit, due to significant fluctuations in the raw natural gas supply, the maximum load reached 110%, while the minimum load was only 30%. The longest continuous operation period is 89 days, and the Ni content in the finished product ranges from 0.04×10－6 to 0.32×10－6. Due to the frequent shutdowns of the system and large load fluctuations, overheating can occur at the top of the stripping tower. During normal operation, the temperature is kept at 189°C, with a maximum of 195°C; the temperature at the bottom ranges from 201°C to 212°C, resulting in a temperature difference of around 17°C at the top. 2. Condition of the stripping tower: The stripping tower has undergone 8 inspections in total. Based on these inspections, it can be seen that corrosion of the tower is worsening year by year, at an increasing rate. Despite various measures taken, the corrosion problem has not been completely resolved. During the overhauls in 1991, 1993, and 1994, the stripper was in good overall condition, with no significant signs of corrosion observed. During the inspection in 1996, the equipment was in good overall condition; there were no obvious signs of corrosion on the upper and lower tube boxes. However, about 10% of the heat exchange tubes and the outer walls of their ends exhibited corrosion defects caused by varying degrees of compression damage, such as reduced thickness due to corrosion, uneven sealing surfaces, and wavy notches on the end faces of the tubes. Almost the entire outer wall of the pipe outlet is covered with a layer of reddish-brown scale, indicating poor sealing with the distribution pipe. Defect inspections and wall thickness checks were conducted on all 2,574 heat exchange tubes, and no significant defect signals were found. During the major maintenance inspection in 1998, some relatively severe corrosion defects were detected, which posed a threat to the safe operation of the equipment. The surface of the upper tube box lining was in good condition, while the packing showed uniform corrosion, and a small number of Bower rings had thinned out. Obvious mechanical impact marks could be seen on the surface of the titanium lining near the packing bed. In addition, about 11% of the end sealing surfaces of the heat exchange tubes exhibited significant corrosion defects, such as erosion grooves, pits, holes, and end notches; there was no noticeable thinning of the tube walls, and the tube ends of the stripping tubes were replaced for the first time. During the major overhaul in 1999, numerous dense corrosion pits were found in the gas-liquid zones on both sides of the liquid inlet ports. The corrosion condition at the ends of the heat exchange tubes had worsened significantly compared to the 1998 overhaul; a total of 173 tubes with end defects were identified. There was obvious corrosion-induced thinning at the supports of the packing bed, with an uneven surface, and in some cases the connection nuts had almost completely corroded away, compromising the threaded connections. In October, the unit was shut down due to leaks in the fillet welds of pipes No. 39–37, and pipe plugging was carried out. During the inspection in 2001, the corrosion-induced thinning of the heat exchange tubes slowed down compared to the previous cycle, but damage at the tube ends remained severe. During the inspection in 2002, 376 heat exchange tubes were found to be corroded, showing etched grooves on their outer walls, corrosion-induced perforations, as well as V-shaped and wavy notches at the tube ends. The distributor supports are severely corroded; the pressure grids are corroded, welded joints have failed, and deformation has occurred. Pitting corrosion is present on the lining, and the wall thickness of the heat exchange tubes has decreased significantly. To ensure the safe operation of the stripping tower, the following measures were taken: 43 heat exchange tubes with a wall thickness of ≤1.65 mm were plugged (representing 1.67% of the total number; this does not affect the heat exchange efficiency) ； Replace 178 nozzles on the stripping tube. A statistics on the corrosion of heat exchange tubes since 1996 is provided; the overall changes in the wall thickness of these tubes are shown in Table 1, while the variation in the thinning rate of tubes with a wall thickness of less than 1.9 mm (based on the thickness in 2002) is shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload/060207830224191.jpg http://www.nmtech.com.cn/jishuwang/upload/060207831013249.jpg 3. Situation regarding the reversal of the distillation tower: The basis for reversing the distillation tower lies in the wall thickness of the heat exchange tubes; SNAM provided a value of 1.8 mm, whereas the actual minimum wall thickness of the distillation towers in use is much lower than this figure. Given these circumstances, it was decided to reverse the orientation of the distillation towers during the maintenance work in 2004. 3.1 Inspection of the internal components and manholes in the high-pressure tube box: The oil pressure used for removing the head bolts is 80–115 MPa, while the oil pressure used for reinstalling those bolts is 100 MPa. Due to leaks that occurred during pressure testing, the pressure was increased again to 110 MPa, at which point it was found to be satisfactory. There are several small pits on the sealing surface of the lower head; the deepest one is 0.4 mm deep. A depth of 0.40 mm was removed along the 20° conical surface, and the surface was polished with sandpaper until the required roughness was achieved. Inspection of the upper head sealing surface revealed circumferential marks but no radial marks; no action was taken. The other connection sealing surfaces are ground separately according to their specific conditions. After disassembly, it was found that the average corrosion rate of the titanium lining in the high-pressure upper tube box was 0.04 mm/year, with a maximum corrosion rate of 0.10 mm/year; both values fell within the normal range for corrosion. The surface films in the pure gas phase and pure liquid phase regions were grayish-black in color, with a uniform and dense texture, exhibiting the characteristics typical of anatase-type TiO2 formed as a result of uniform corrosion of industrial-grade pure titanium in a urea medium. The gas-liquid interface is distinct, with a large transition zone; the surface film appears gray-black + yellow-green + black, showing uneven coloring. The yellow-green surface film is incomplete, reflecting the characteristics of rutile-type TiO2 formed as a result of uniform corrosion of industrial titanium in a urea medium. The black surface film is quite loose and flakes off in granular form; where it has fallen off, a yellow-green surface film appears, showing typical characteristics of α-Fe2O3 deposition. Corrosion defects at the pipe ends are mainly manifested as corrosion-induced thinning, axial corrosion grooves, corrosion perforations, and end-face notches; in some cases, the pipe ends no longer have a complete sealing surface. No action was taken as the stripping tower needed to be reversed. The entire lining of the lower tube box is covered by a dense surface film (scale), composed of 99% iron oxide and 1% impurities. The surface film is grayish-black in color, dense, and no abnormal corrosion was observed. The film on the inner wall surface of the heat exchange tube is grayish-black + brownish-yellow, with the scale layer being over 1 mm thick. All liquid distributors were replaced with new ones; 2,463 distributors were installed, and all PTFE gaskets as well as grid plates were replaced (the new grid plates were manufactured by Dalian Songhai Petrochemical after repeated verification and modification of the original design drawings). To ensure correct installation, 40 bolt shims were welded on-site, and the distributor together with the pressure grid plate performed well after installation. 3.2 Blockage: The stripping tubes were subjected to eddy current testing by Chongqing Olynt Mechanical and Electrical Technology Co., Ltd.; the thinnest wall thickness was 1.1 mm. There were 65 stripping tubes with a wall thickness of ≤1.60 mm, which led to blockage. Meanwhile, outer wall defects were found in tubes 9–2, which were classified as Grade B tubes and subjected to plugging treatment. A total of 66 pipes were plugged, using argon shielded welding; the purity of the argon was 99.999%, the argon flow rate was 10–11 L/min, and the current was 100 A. The color flaw detection test passed. 3.3 Reversal of the stripping tower: The stripping tower has a diameter of 2190 mm, a height of 15.982 m, an installation elevation of 10 m, and a weight of 107 t. Based on the actual conditions of the equipment and the site, it was decided to use a Dematic 300T truck crane and a KH700-2 crawler crane together to carry out the tasks of moving the equipment, turning it around, and positioning it again. The main crane uses 8 sets of 6×37+1 type U-4mm steel wire ropes. In conjunction with the crane, 4 φ56mm steel wires of the 6×37＋1 type are used. 1) Preparations before lifting: There is a protective cover at the expansion joint of the original stripping tower, as the tower may suffer mechanical damage during transportation and installation. Moreover, the expansion joint can only withstand tensile forces; during installation or repositioning, the expansion joint and the heat exchange tubes may be subjected to torsional, shear, and bending forces. Given that the stripping tower weighs 107 tons, this can result in some irreversible deformation. To protect the expansion joints and heat exchange tubes of the stripping tower, relevant personnel fabricated protection covers for these components before lifting, and installed them prior to reversing the unit’s orientation. Remove the pipes and instrumentation accessories connected to the stripping tower. Seal the equipment outlet with a temporary blind flange. 2) Reversing of the equipment: During lifting, the 300T truck crane is positioned such that its center is 8.7 meters away from the center of the equipment, with a boom length of 25 meters; this allows the equipment to be lifted without placing any load on the supporting steel structure ； Demolish the main load-bearing beam on the west side of the stripping tower, as well as the load-bearing beams on the north and south sides. The load-bearing beams are connected using high-strength bolts and welding; when removing them, care must be taken to avoid damaging the junctions between the main load-bearing beam and the two columns, as well as the junctions between the load-bearing beams on the north and south sides and the main load-bearing beam ； The 300T truck crane lifts the equipment by 800 mm, until the bottom of the equipment is above the height of the crane’s chassis; at this point the radius of rotation is 4 meters, and there is still some space between the stripping tower and the frame columns, so the equipment is moved out ； The equipment is placed on the ground using a 150T crawler crane ； Complete the task in one go by taking advantage of the flexibility of the crawler crane. At this time, due to the obstruction posed by the frame columns to the counterweight of the 300T crane, the 300T crane needs to have its counterweight removed and reinstalled once ； After the equipment is turned around, the 300T truck crane remains as the main crane, with the 150T crane assisting in positioning it. Once the equipment is upright, the 150T crawler crane releases its hook and rotates the equipment by 180°; then the boom of the 300T truck crane is used to position the equipment properly, and the load-bearing beams of the equipment are reinstalled, including the use of high-strength bolts for connections and structural welding ； Adjustment of equipment verticality. Generally, the verticality requirement for static equipment is 1%, but since the ammonia vapor tower is a falling-film heat exchange and separation device, it demands a higher level of verticality; otherwise, it will lead to uneven stripping loads within each heat exchange tube, resulting in an uneven distribution of the liquid film and even dry burning, which severely affects the operation of this device. The original installation requirement for verticality was 5% ； Due to errors in equipment manufacturing, there is a certain discrepancy between the verticality measured using a level on the outer wall and that measured inside the equipment with a plumb bob attached to the tube. Based on practical production considerations, the values obtained through measurements using tubes inside the equipment should be taken as the standard. Four heat exchange tubes are selected from four different directions, and the verticality of the stripping tower is adjusted based on these values, so as to achieve a precision of 2%, which meets the requirements for stable equipment operation ； Recovery of connecting pipes, electrical instruments, and platform trough boxes ； Protect the equipment from corrosion and provide insulation. 3.4 Cleaning of heat exchange tubes and processing of tube ends: After the ammonia stripping tower was reversed, the thickness of the scale layer on the inner wall of the heat exchange tubes was 1 mm, while the amount of material that needed to be removed from the outer wall of the tube ends on each side was only 0.1 mm. If this scale layer is not cleaned, it becomes difficult to determine the center of the tubes during the processing of their outer walls, which can lead to eccentricity. Chemical cleaning can easily cause corrosion to the heat exchange tubes, while mechanical cleaning may damage the tube ends. After analysis, it was decided to use mechanical cleaning; Tianjin Yuandong Cleaning Company was tasked with cleaning the inner surfaces of the heat exchange tubes as well as the tube sheets using rotary nozzles. Deionized water was used for the cleaning process. To ensure safe cleaning of the equipment, the following requirements were set: ① When cleaning the inner surfaces of the tubes, the nozzle should be moved in and out slowly to avoid damaging the tube ends ； ②The gun should be inserted slowly; otherwise, the effect on removing the scale layer will be poor ； ③The cleaning height is 150 mm below the pipe opening. The cleaning water pressure is 200 MPa. To speed up the process, two cleaning units are used simultaneously. After cleaning, it was found at the upper pipe end that: ① Due to manufacturing reasons, the area surrounding the pipe plate at the upper pipe end is higher while the center is lower, giving it a shape similar to the bottom of a pot; the pipe ends around the perimeter are about 1.5 mm higher than those in the center ； ②The height of the surrounding tube ends is 16–16.5 mm, which is lower than the designed size of 17 mm ； ③Multiple fillet welds show undercut at the pipe ends, with the deepest depth reaching 1 mm ； ④The designed outer diameter of the upper pipe end is φ27 mm, while the actual measured outer diameter ranges from φ27 – 0.02 to φ27 – 0.04 mm; this makes it somewhat more difficult to machine the pipe end to a diameter of φ26.8 + 0.10 mm. If, in accordance with the process requirements, levelness is adjusted starting from the end of the central tube, it will result in 1.5 mm of material being removed from most of the tube ends on the surrounding sides. When attempting to machine a 10.5 mm thick sealing step, this process will reach the area where fillet weld undercutting occurs, which severely affects the performance of the upper tube sheet and increases the risk of leaks during production. As a result of research, the following requirements are proposed for the machining of the tube ends in ammonia stripping towers: ① While striving to meet the process requirements, levelness should be adjusted based on a height of 16.2 mm for most of the tube ends on the surrounding sides, ensuring a uniform transition. ②The minimum height of the pipe ends must not be less than 16 mm, and adjacent pipes should be roughly at the same level. ③The height difference between adjacent pipe ends must not exceed 1 mm; otherwise, the pipe ends need to be replaced. The pipe end processing is carried out by Dalian Songhai Petrochemical Maintenance Company. The process achieved an ammonia leakage of 0.25 MPa; maintaining this pressure for 2 hours resulted in a pass. The plant was started successfully after the major repair, and the high-pressure system of the urea unit has been operating stably at a load of 35,000 m3/h to date, meeting the process requirements.

The ammonia stripping tower (E101) is one of the key equipment in the urea production facility of Zhongyuan Dahuahua Group Co., Ltd. It features a symmetrical upper-lower structure, with overall dimensions of (inner diameter × total height) φ2190×13,982; its net weight is 107 tons. The stripping tubes have specifications of (outer diameter × wall thickness) φ27×3.5, are made of titanium, and have a length of 5,830 mm – a total of 2,574 such tubes. From its commissioning in May 1990 until the major overhaul in February 2004, the stripping tower had accumulated 99,052 hours of operation. In recent years, as the service life of the equipment has increased and it has aged, corrosion of the heat exchange tubes in the stripping tower has become more severe year by year. During the overhaul in 2002, the wall thickness of 43 heat exchange tubes was 1.65 mm or less, and a total of 45 tubes were blocked. During its major overhaul in 2004, Zhongyuan Dahua Group Co., Ltd. reversed the orientation of the stripping tower and carried out physical cleaning of the scale layer; this article provides a summary of these efforts, in the hope that it can serve as a reference for plants of similar type. 1 Condition of distillation column maintenance: From its commissioning in May 1990 to February 2004, the system was shut down a total of 304 times. During operation of the unit, due to significant fluctuations in the raw natural gas, the maximum load reached 110% while the minimum load was only 30%. The longest continuous operating period was 89 days, with a Ni content of 0.04 mg/kg in the finished product. Due to the frequent shutdowns of the system and large fluctuations in load, the temperature at the top of the stripping tower E101 is maintained at 189°C during normal operation, with the maximum temperature reaching 195°C℃ ； The bottom temperature reaches up to 212°C, the lowest temperature is 201°C, giving a temperature difference of about 17°C between the top and bottom. From 1991 when it was put into operation until the major overhaul in 2004, a total of 8 inspections were carried out on E101. Based on these inspections, it was observed that corrosion of E101 worsened year by year, with an increasing rate of corrosion. The thinnest wall thickness of the heat exchange tubes was 1.45 mm, with an average thinning rate of 0.16 mm per year. The stripping tubes in the lower part of the stripper suffer little corrosion. Due to the symmetric structure of these tubes, after reversing their orientation, the upper stripping tubes remain almost as good as new, which **helps to extend the equipment’s lifespan. Therefore, the company decided to reverse the orientation of the stripper’s tubes during the major maintenance in 2004. 2 Inversion of the distillation tower: Based on the actual conditions of the equipment and the site, our company decided to remove the equipment from the side, using Hengang Dematic 300t truck cranes and KH700-2 type crawler cranes to carry out the tasks of removing the equipment, inverting it, and reinstalling it in place. ①Crane selection. A 300 t crane is used as the main crane, while a 150 t crawler crane serves as the auxiliary crane. ②Selection of steel wire ropes. The main crane uses 8 φ44 steel wires of the 6×37+1 type, while the auxiliary crane uses 4 φ56 steel wires of the 6×37+1 type. ③Preparations before lifting. The original Ei01 expansion joint was equipped with a protective cover, as the stripping tower might suffer mechanical damage during transportation and installation. Moreover, this expansion joint can only withstand tensile forces; during installation or reversal, the expansion joint and the heat exchange tubes may be subjected to torsional, shear, and bending forces ； Due to the stripping tower’s weight of 107 t, some irreversible deformation will occur. To protect the expansion joints and heat exchange tubes of the stripper, an expansion joint protection cover is fabricated prior to lifting, and it is installed when the pipe is turned upside down. Remove the pipes and instrumentation fittings connected to E101, and seal the equipment ports with temporary blind flanges. ④The inversion of the device. During lifting, the distance from the center of the 300t truck crane to the center of the equipment is 8.7 m, with a boom length of 25 m; the equipment is lifted so that the supporting steel structure remains unloaded ； Demolish the main load-bearing beam on the west side of E101, as well as the load-bearing beams on the north and south sides. The load-bearing beams are connected using high-strength bolts and welding; when removing them, care must be taken to avoid damaging the junctions between the main load-bearing beam and the two columns, as well as the junctions between the load-bearing beams on the north and south sides and the main load-bearing beam ； A 300t truck crane lifted the equipment to 800 mm; once the bottom of the equipment was above the height of the crane’s chassis, the radius of rotation was 4 m. There was still some space between the stripping tower and the frame columns, so the equipment was moved away ； The equipment is placed on the ground with the help of a 150 t crawler crane ； The equipment was turned upside down using a 300t truck crane and a 150t crawler crane, with the flexibility of the crawler crane being utilized to complete the task in one go. At this time, due to the obstruction posed by the frame columns to the counterweight of the 300t crane, the 300t crane needs to have its counterweight removed and reinstalled once ； After the equipment was turned upside down, the 300t truck crane continued to serve as the main crane, while the 150t crawler crane was used to support the rear part of the equipment. Once the equipment was turned upright, the 150t crawler crane disengaged from it; the equipment was then rotated 180°, and the 300t truck crane’s boom was used to position the equipment properly. The load-bearing beams of the equipment were reinstalled, using high-strength bolts for connections and welding for structural reinforcement ； The required verticality for general static equipment is 0.1%, while for the ammonia stripping tower, it is 0.05% during initial installation. Based on practical production considerations, 4 heat exchange tubes are selected from inside the equipment in four different directions, and the verticality of the stripping tower is adjusted using these as a reference, ultimately achieving a verticality of 0.04% to ensure stable operation of the equipment. Reconnect the pipelines and electrical instruments, as well as restore the platform trough boxes; apply anti-corrosion and insulation treatments to the equipment. 3 Cleaning of heat exchange tubes and processing of tube ends: After the ammonia stripping tower was turned upside down, the thickness of the scale layer on the inner wall of the heat exchange tubes was 1 mm, while the amount of material that needed to be removed from the outer wall of the tube ends on each side was only 0.1 mm. If the scale layer is not cleaned, it becomes difficult to determine the center of the tubes during the processing of their outer walls, which can lead to eccentricity. Moreover, chemical cleaning inevitably results in a reduction in the thickness of the tube walls. After comprehensive consideration, it was decided to carry out a physical cleaning of the stripping tower. The cleaning equipment used is the HUSKY cleaning unit from the American company Fluor; its cleaning nozzles are rotary ultra-high pressure spraying tools. The oil pressure acting on the nozzles enables them to rotate at up to 2,000 rpm, and the water jets, which rotate rapidly without any recoil force, can remove any adhering substances or deposits on the surface. The nozzles can rotate 360°. To ensure the safe cleaning of the equipment, the following measures were taken: ① The cleaning was carried out by a professional cleaning company, Tianjin Yuandong Cleaning Company ； ②The cleaning medium is demineralized water ； ③The cleaning water pressure is 200 MPa ； ④When cleaning the inner wall of the tube, move the gun in and out slowly to avoid damaging the tube opening ； ⑤The cleaning height is 150 mm below the pipe opening. To speed up the process, two cleaning machines were used simultaneously for cleaning. After cleaning, the following issues appeared at the upper pipe end: ① Due to manufacturing defects, the tube sheet at the upper pipe end was higher around the edges and lower in the center, forming a shape similar to the bottom of a pot; the pipe ends at the periphery were about 1.5 mm higher than those in the center ； ②The height of the surrounding tube ends is 16–16.5 mm, which is lower than the nominal size of 17 mm ； ③Multiple fillet welds have undercut at the pipe ends, with the deepest depth reaching 1 mm ； ④The nominal outer diameter of the upper pipe end is 27 mm, while the actual measured outer diameter ranges from 27 – 0.02 to 27 – 0.04 mm; this makes it somewhat more difficult to machine the pipe end to a size of 26.8 + 0.10 mm. If, in accordance with the process requirements, levelness is adjusted from the end of the center tube outward, most of the tube ends on the periphery will have 1.5 mm removed. As a result, when machining a 10.5 mm thick sealing step, it will reach the area where fillet weld undercutting occurs, which severely affects the performance of the upper tube sheet and increases the risk of leaks during production. To address this issue, the following requirements are proposed for the processing of the tube ends in the ammonia stripping tower: ① On the premise of meeting the process requirements as much as possible, a uniform transition should be achieved based on the horizontal level of the heights of most of the tube ends around them ； ②The minimum height of the pipe end must not be less than 16 mm, with adjacent pipes being roughly at the same level ； ③The height difference between adjacent pipe ends must not exceed 1 mm; otherwise, the pipe ends need to be replaced. After the pipe ends are processed, an ammonia leakage test is conducted at a pressure of 0.25 MPa; the test is considered successful if the pressure is maintained for 2 hours without any leaks. 4 Conclusion: The distillation tower has been in operation since it was installed upside down, and it is functioning properly to this day. The ammonia stripping tower of Zhongyuan Dahua Group is the largest stripping tower in urea plants that utilize the ammonia stripping process; physical cleaning was also used for the first time on this type of tower. Its successful application will certainly serve as a valuable reference for companies of similar type.

Our company’s new urea plant adopts the ammonia stripping process technology from SNAM in Italy, with the key equipment, namely the urea ammonia stripping tower, being imported. It was tested and put into operation in May 2000, and shut down for maintenance in July 2002 due to a leak in the sealing surface of the manway on the ammonia stripping tower. Inspection revealed that the sealing surface at the tower top was completely damaged, and the lining sealing surface showed extensive ring-shaped corrosion, reaching a depth of 0.2–0.4 mm and a width of 10–20 mm, along with two relatively deep longitudinal grooves. Similar corrosion occurs on the manhole gaskets as well. Since its commissioning, this stripping tower has been carefully inspected every year. During the inspection in June 2001, there were no signs of corrosion on the tower nozzle sealing surface. Such severe corrosion occurred in less than a year. What are the reasons? How can it be solved? This article will explore these issues. 1 Process Flow: This stripping tower is a vertical tubular falling-film heat exchanger. The process flow involves the material discharged from the urea synthesis tower entering through the top head of the stripping tower; after passing through the liquid distributor, it enters the heat exchange tubes tangentially via 3 small holes with a diameter of 3 mm, which are evenly distributed on the nozzles. The material forms a liquid film along the pipe wall and flows downward due to gravity, while the stripping gas flows upward at a certain rate, coming into counter-current contact with the liquid film on the pipe wall to facilitate further decomposition of methylammonium in the urine. Stirring tower operating temperature: less than 210°C at the bottom of the tower, and 190°C at the top; operating pressure is 14.5 MPa. 2 Analysis of leakage causes: (1) The materials for the upper and lower end caps are equivalent to 00Cr25Ni22Mo2N in China; they belong to ultra-low carbon stainless steels with strong resistance to corrosion by ammonium carbamate. These are mature steel grades used in urea production facilities. The gaskets for manholes are made of titanium alloy (SB381Grl), which offers better corrosion resistance. Comprehensive quality assurance documents, such as material analysis reports and corrosion resistance reports, provided from abroad are available. Based on the data in the report, there should be no issue with the material itself. Furthermore, from the time the tower was put into use in 2000 until its disassembly and inspection prior to this maintenance, no significant corrosion was found in the lining layer, which also indicates that the choice of material and its quality are appropriate and satisfactory. (2) Process parameters: The quality of control over process parameters has a significant impact on the corrosion resistance of the material. For example, an excessive H2S content will destroy the passivation film on the surface of the lining, causing the base material to lose the protection provided by this passivation film and leading to activated corrosion, which in turn accelerates the corrosion of stainless steel. Among the process parameters, the operating temperature has a particularly significant impact on the corrosion resistance of lining materials, as various corrosion-resistant materials are extremely sensitive to temperature and each has a temperature limit beyond which their corrosion resistance drops sharply. The temperature limit for the lining 00Cr25Ni22Mo2N is 210°C, while the temperature limit for the titanium alloy (SB381Grl) used in manhole gaskets is higher. By reviewing the process parameters and analysis reports since the vehicle started operating, it can be seen that all process indicators are within the specified ranges. Therefore, corrosion caused by factors related to process parameters can be ruled out. (3) Structure and material selection of the tower head sealing surface: This sealing surface adopts a lens gasket design, with the dimensions of the lens gasket being Φ680×Φ600×50 mm. The sealing surface of the lens gasket is of the SR950 spherical type, while the sealing surface of the tower head lining is conical, at an angle of 20° relative to the end face. The fit between the lens gasket and the sealing surface is that of a sphere and a cone, resulting in linear contact. There is a very narrow annular gap within the contact wire. The corrosion condition of the sealing surface of the tower nozzle lining was observed; the corroded surface was uneven, its structure was loose, and it no longer had a metallic luster. It can be inferred that severe corrosion has occurred, and the possibility of intergranular electrochemical corrosion is high. Crevice electrochemical corrosion occurs in an electrolyte when narrow gaps are formed between different metals; the restricted movement of the electrolyte within these gaps creates concentration cells, leading to localized corrosion. However, crevice electrochemical corrosion can occur only in the electrolyte, while the temperature at the top of the stripping tower is as high as 190°C; the components at the sealing surfaces of the tower top and outlet are in gaseous form. Therefore, crevice corrosion is impossible under normal operating conditions. It should be noted, however, that prior to this shutdown for maintenance, there had already been a slight leak at the seal at the top of the stripping tower for nearly 1 month. In the state of micro-leakage, the condition at the leakage site will change. The gaseous components at the top of the stripping tower contain very small particles with solution components; as these particles leak through tiny gaps, the increased resistance causes them to gradually coalesce into larger particles, thereby forming a solution. Due to the extremely small leakage amount, the solution in the gap is blocked and remains there. The lining 00Cr25Ni22Mo2N and the titanium alloy gasket are made of different metals; their electrode potentials differ, creating a potential difference that provides the conditions for crevice corrosion, thereby leading to severe crevice corrosion. When tiny pits form due to corrosion near the leak site, very small particles in the surrounding area pass through these pits and condense into a liquid state, which further causes more tiny pits to form. This process continues in a circular pattern, spreading outward and resulting in the formation of a ring-shaped crevice corrosion zone; ultimately, deeper longitudinal grooves are created at the leak location. It can be seen that, when there are no issues with the material or process parameters, slight leakage at the tower nozzle seal surface can lead to severe crevice corrosion on seals made of different metals. Furthermore, with regard to the use of the stripping tower, since an oxygen-resistant lining layer is installed at the bottom of the tower, a small amount of oxygen enters the upper head through the tubes; the diffusion of oxygen causes severe pitting corrosion at the gas-liquid interface in the gaps, as well as the formation of activated and passivated cell corrosion, thereby further exacerbating the corrosion. 3 Solutions and Effects 3.1 Solutions First of all, leaks at the tower nozzle sealing surface should be avoided and eliminated promptly. Secondly, the material of the lens pad was changed to the same material as that of the upper head lining layer, so as to make the electrode potentials of the two equal and eliminate the conditions for electrochemical corrosion to occur. To this end, we carried out laser cutting repair on the corroded areas of the seal surface of the tower nozzle lining, and replaced the lens gasket with domestically produced 00Cr25Ni22Mo2N material (the same as that used for the tower nozzle lining), manufactured by the Beijing Iron and Steel Research Institute. 3.2 Effects After being repaired and put into use in September 2002, good results were achieved; it not only effectively solved the corrosion problem of the tower nozzle sealing surface but also enabled the localization of the sealing components (lens gaskets). It saved the company a considerable amount of foreign currency, and it also improved the reliability of the equipment. From October to December 2005, minor leaks reappeared at the top of this stripping tower; more than two months after these leaks occurred, no signs of corrosion were found during maintenance work carried out in January 2006 ； This shows that our analysis of the causes of corrosion and the proposed improvement measures have withstood practical testing, achieving satisfactory results.

The urea production unit of Chongqing Jianfeng Fertilizer Co., Ltd. utilizes the Italian Snam ammonia stripping process, with a design capacity of 1750 t/d. During the major shutdown for maintenance in September 2005, the ammonia stripping tower (300E01) was successfully reversed. However, inspections of the tower revealed severe corrosion and thinning of the tubes. Through analysis, strength calculations, and appropriate corrective actions, this significant potential hazard to the equipment was resolved. 1 Introduction to the structure of the distillation tower equipment: The ammonia steam distillation tower, which is part of the high-pressure equipment in the urea production plant of Chongqing Jianfeng Fertilizer Co., Ltd., was manufactured by the Italian company FBM. The overall dimensions of this equipment are φ1850mm×12860mm ； The total heat exchange area is 670 m2 ； The specifications for the tubes in the stripping tower are φ27mm×3.5mm, with a length of L=5314mm; there are 2370 such tubes in total, and the material of these tubes is ASTM B338 Cr.3 ； The high-pressure tube sheet of the stripping tower is made of titanium-lined explosive composite steel plates; the material used in the pressure-bearing sections is ASTM A508 Gr.3, with a thickness of 100 mm ； The corrosion-resistant layer is made of ASTM B265 Gr.1 (pure titanium), with a designed thickness of 3 mm ； All other internal components of the tower are made of pure austenitic stainless steel 25Cr22Ni2Mo. During design, taking into account the fact that titanium is susceptible to erosion corrosion, the upper stripping tubes are located within a range of 1.5 meters from the tube sheet, where erosion corrosion by rotating liquid against the tube walls as well as corrosion at the tube ends can occur. Therefore, this equipment features a fully symmetric design, allowing it to be used with the tubes oriented in the opposite direction when thinning occurs severely in the upper part of the tubes. 2 Inspection of tube corrosion conditions and cause analysis 2.1 Corrosion inspection of tubes and tube ends 1) During the major shutdown for maintenance of the unit in September 2005, eddy current testing of the tubes in the ammonia stripping tower revealed that the average annual corrosion rate of these tubes was 0.27 mm; in some cases, the corrosion rate was as high as 0.85 mm, which is significantly higher than the previous average corrosion rate of 0.1 mm per year. Among them, a total of 334 tubes were found to have an inner wall thickness of less than 1.90 mm within 1.5 m of the tube end in the upper row of tubes, accounting for 14.12%, as shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/070105901123994.jpg The distribution of the thin-walled tubes across the entire tube sheet is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/070105904027579.jpg 2) Before reversing the ammonia stripping tower, it was found that some of the tube ends in the upper tube sheet exhibited various degrees of tube end deterioration, local perforations, thinning of the inner wall resulting in a flared shape, as well as erosion grooves on the outer wall; these conditions are shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/070105904474545.jpg 2.2 Cause Analysis 1) Reasons for the corrosion and thinning at the upper part of the tubes: ① The rate of tube thinning is closely related to the liquid load on the tubes; the greater the amount of liquid that falls on a single tube, the greater the amount of decomposition at its upper end, and the higher the flow velocity, which in turn leads to more severe erosion corrosion. ②Judging from the distribution of the thinned tubes in Figure 1, it can be seen that the tubes in this area are subjected to the greatest liquid load. ③Impact of system load. As can be seen from the load comparison in Table 2, the average loads in 2004 and 2005 exceeded the design values (especially in 2005, when the average load reached 104%–105% of the design value). It was precisely due to this increased load that the amount of liquid flowing through each tube increased, thereby exacerbating erosion and corrosion. ④The corrosivity of the liquid feed mainly comes from the ammonium methoxide solution; the concentration of ammonium methoxide is highest in the upper part of the stripping column, where the decomposition reaction is most intense. ⑤Compared to 2004, the tube corrosion rate has reached 0.27 mm/year, indicating that the tubes have entered a period of accelerated corrosion. http://www.nmtech.com.cn/jishuwang/upload/070105905534931.jpg 2) Reasons for corrosion at the pipe end: ① When the sealing PTFE sleeve breaks longitudinally during installation, leakage will occur at the break point during operation; the continuous action of liquid flow over time will lead to the formation of corrosion grooves on the outer wall of the pipe end. ②During the installation of the distribution tube, if the PTFE sleeve bends, it will prevent the inner bore of the distribution tube end from being installed in place, creating a gap with the end face of the tube bundle. Liquid material will accumulate in this gap during operation, and over time, oxygen deficiency in this gap can lead to gap corrosion, resulting in corrosion defects such as damage to the tube ends. ③Before entering the distribution tube, the liquid passes through three tangential small holes in the tube, which causes the fluid to rotate preliminarily; this enables a uniform and stable liquid film to be formed within the tubes. It is the rotation of the fluid that generates significant kinetic energy, although the intensity of this rotation decreases as it moves downward along the tubes. As a result of the continuous erosion caused by this rotating fluid, horn-shaped corrosion defects form on the inner walls of the tube ends. 3 Calculation of tube corrosion thinning strength: According to the equipment manufacturer’s data sheet, the design parameters for the tubes are as follows: design pressure of 16.2 MPa for the tube side, and 2.8 MPa for the shell side ； Design temperature 230℃ ； Test pressure: 20.25 MPa for the tube side, 4.2 MPa for the shell side ； Allowable stress at 230°C = 77.2 MPa. http://www.nmtech.com.cn/jishuwang/upload/070105907098102.jpg Where: t—is the wall thickness of the tube, in mm ； P—Design pressure of the tube bundle, MPa ； R—inner radius of the tube, mm ； S—allowable stress of the pipe material at the design temperature, MPa; E—weld joint factor. Therefore, the strength of the tube becomes insufficient once its corrosion-thinned thickness reaches 1.55 mm. However, from σ]t=σs/Ns, we obtain: σs=σ]t×Ns= 77.2×1.5=115.8MPa. Here, σs is the yield stress of the tube material, in MPa ； Ns—safety factor. S=114.24MPa

The urea production unit of Sichuan Tianhua Co., Ltd. uses the Snam ammonia stripping process; it was successfully commissioned in October 1995, and its output has exceeded the designed capacity for several consecutive years. It was operated under extended-cycle overload for 168 days in 2000, and is currently functioning well. The corrosion condition of high-pressure equipment is generally normal, but the corrosion in the titanium stripper is relatively severe, which is a common issue encountered when introducing SNAM ammonia stripping process units. A thorough analysis and summary were conducted on the problem of excessive temperature at the tops of the stripping towers in Sichuan Tianhua Company, Jinxi Plant, and Jianfeng Plant, to identify solutions. 1 Main design parameters and structural features: The stripping tower of Sichuan Tianhua Co., Ltd. was manufactured by the Italian company FBM; it is a falling-film type heat exchanger. The lining and tube materials are made of titanium, while the materials for the distribution tubes and packing are Cr-Ni-Mo 25-22-2. The main design parameters are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/060823958595183.jpg Internal structure of the tube box: After the liquid from the synthesis tower enters E-101, it first passes through a circular liquid separation tank, then flows over a serrated edge before reaching a circular sieve plate. This sieve plate has 1,000 small holes with a diameter of φ10mm, and there are 5 lift pipes with a diameter of φ219mm within the sieve plate. Below the sieve plate, there is packing with a height of 250 mm, dimensions of φ50 mm×50 mm, and a volume of 0.76 m3; beneath this packing are grids, compressed butterfly torsion springs, and distribution pipes. The stripping tubes and linings are made of titanium, which improves the corrosion resistance and the allowable operating temperature of the equipment; its minimum operating load can reach 30%–40%, thereby greatly increasing the operational flexibility of the unit. It features symmetrical upper and lower structures, can be used in reverse orientation, which extends its service life and improves the economic efficiency of the device. A N2 filling point is provided on each steam side at both ends of the shell, allowing N2 to be introduced into the shell. This reduces the heat load on the upper part of the tubes and also slows down corrosion in the upper tube section. The steam inlet is located in the middle of the shell, which facilitates even heat distribution upward and downward and reduces corrosion in the upper pipe section. The fixation of the liquid distribution tube is achieved using butterfly torsion springs; experience has shown that this method is difficult to assemble, yields poor clamping effects, and is easier to disassemble than spot welding. 2 Visual inspection during major overhauls 2.1 Inspection during the 1997 overhaul The distillation column was in operation from October 1995, when it began trial operation, until May 16, 1997, when it was shut down for a major overhaul and the first internal inspection was carried out. 1) The surface of the tube box lining, the cover plate, and the welds are smooth, with no obvious signs of corrosion. There is some scale around the perimeter and above the liquid distribution tray, along with several marks from impacts. 2) The packing is intact; aside from a few manufacturing cracks in the middle section, the distribution pipes are in good condition. The titanium lining of the E-101 inlet pipe is intact, and other components such as the liquid distribution tray are also in good condition. 3) The PTFE sealing rings between the distribution tube and the titanium tube are intact; there are slight signs of erosion at the ends of several stripping tubes. 4) There is scaling inside the tube. During the major overhaul in 1997, thickness inspections were carried out; the thickness of both the tubes and the lining was within normal limits, with no signs of significant corrosion observed. 2.2 Overhaul inspection in 1999: During the 1998 overhaul, the cover of E-101 was not removed for inspection; during the overhaul in August 1999, the cover was removed again for inspection, and obvious changes could be seen on the outside. 1) The surface of the tube box lining, the cover plates, and the weld surfaces are generally smooth; there are layered signs of erosion corrosion around the liquid distribution plate and at its upper part, while there are no obvious changes at the impact sites. 2) The packing is intact, the distribution pipe is intact; all the titanium linings of the E-101 inlet pipes have been washed away, and the other components are in good condition. 3) All the PTFE rings between the distribution pipe and the stripping pipes were shattered (see Figures 1 and 2). 60% of the openings of the stripping pipes were eroded, with some of these openings being completely damaged by erosion; on the outer walls of some of these openings, erosion grooves 0.5–1.00 mm deep appeared (see the first pipe in the upper left corner of Figure 1; the white fragments in Figures 1 and 2 are remnants of the PTFE rings). http://www.nmtech.com.cn/jishuwang/upload/060823959428234.jpg 4) Scaling still occurs inside the tubes; no thickness measurements were taken in 1999 and 2000. 3 Causes of Corrosion and Countermeasures Based on the two inspections carried out to examine the internal condition, the main causes of corrosion of the E-101 titanium material are: 1) The polytetrafluoroethylene ring at the connection between the stripping tube and the distribution tube was of substandard quality. During the major overhaul in 1997, domestic polytetrafluoroethylene rings were replaced. When the equipment was disassembled in 1999, it was found that all of the polytetrafluoroethylene rings had hardened and cracked, resulting in erosion and corrosion at many of the pipe connections; in contrast, the imported polytetrafluoroethylene rings remained intact and elastic even after two years of use. 2) The polytetrafluoro ring assembly is defective. During reinstallation, a thin slice of the PTFE ring is cut off and inserted into the stripping tube to disrupt the liquid film; either the PTFE ring is severed or compressed partially, resulting in erosion corrosion. 3) The distribution pipe and the stripping pipe are not tightly pressed together, resulting in a certain gap between them; at this point, the liquid film is broken, and fluid directly washes against the pipe opening (see location F4 in Figure 3). http://www.nmtech.com.cn/jishuwang/upload/0608231000243733.jpg 4) The dimensions of the polytetrafluoroethylene rings are not satisfactory; rings that are either too large or too small will directly cause erosion at the inlet of the stripping tube. 5) The PTFE gasket used for the internal components of E-101 is of substandard quality, resulting in leaks that can cause erosion and corrosion of the tower wall lining. 6) Operating the E-101 at excessive top and bottom temperatures will accelerate the corrosion of titanium materials. Data indicate that in titanium-based equipment, when the dissolved oxygen level in the liquid phase is below 2 μg/g, it affects the proper formation of the passivation film on the surface of the equipment. Dissolved oxygen in the liquid phase is greatly affected by temperature; when the temperature at the bottom of E-101 exceeds 210°C, the dissolved oxygen level in the liquid phase drops below 2 μg/g, leading to corrosion of the equipment. Comparative experiments were conducted in a 1 mol/L H2SO4 solution on 25-22-2 stainless steel and titanium, showing that as the temperature increased, crevice corrosion in both materials worsened; moreover, the crevice corrosion rate of industrial titanium was significantly higher than that of 25-22-2 stainless steel (see Figure 4). http://www.nmtech.com.cn/jishuwang/upload/0608231001004014.jpg The following measures can be considered for corrosion prevention: 1) Polytetrafluoroethylene rings and gaskets made from imported materials can be used; although there is a difference of a few yuan between imported and domestic materials, repairing a damaged steam stripping tube outlet requires 2000 yuan. 2) Consulted foreign manufacturers regarding the method of installing PTFE rings; during the major overhaul in 1997, it was found that the PTFE rings installed by the manufacturer remained intact even after removal. 3) The poor connection between the distribution pipe and the stripping pipe is mainly caused by improper assembly as well as deformation of the retaining rings and grids; it should be adjusted promptly. 4) In operation, it should be avoided to operate E-101 at excessive top and bottom temperatures. 4 Analysis of the Causes and Countermeasures for Overtemperature at the Top of E-101: Overtemperature occurs occasionally at the top of the stripping tower at Sichuan Tianhua Co., Ltd., while overtemperature problems persist for a long time at the tops of the E101 units in Jinxi and Jianfeng. The reasons are as follows: ① When discharging from the synthesis tower, due to low conversion rates, excessive steam is introduced on the shell side of E-101, resulting in a high stripping load and thus overtemperature at the top. ②After the high-pressure ammonia pump (P-101A/B) tripped, the flow of NH3 was interrupted, resulting in the loss of the liquid seal in the liquid separation tray. Steam on the shell side could not be discharged in time, leading to flooding in the packing section; as a result, the temperature at the top of E-101 rose very quickly. The same as Tianhua Company. ③The upper air outlet holes of the distribution pipes in Jinxi and Jianfeng E-101 are two small holes with a diameter of φ5 mm, which are relatively small (Figure 3, A-A section). According to the material balance sheet, the volumetric flow rate of the gas exiting E-101 is 485.40 m3/h. There are 2,356 stripping tubes in both Jinxi and Jianfeng; therefore, the cross-sectional area of the upper gas outlet is: 2,356×2×1/4×π×(0.005)2 = 0.09247 m2 ； The velocity of the gas through the cross-section is: 485.40÷0.09247÷3600＝1.458 m/s (assuming all the gas flows out through the upper outlet). The Tianhua Company’s distribution pipe has two air outlet holes of φ10mm at its upper part; the total number of stripping pipes is 2476, the velocity of the fluid flowing through the cross-section is 0.347 m/s, and the heat exchange area of E-101 is 700 m2. Jinxi and Jianfeng E-101 have a heat exchange area of 670 m2 each, with little difference between them. But the air flow speeds differ by several times. Since the conversion rate in the ammonia stripping process never reaches the designed value, the gas flow velocity may be higher than the calculated value. As a result, the liquid stream falling through the packing section is affected by the jet-like gas flow, forming misty or bubbling areas within that section, which leads to prolonged overheating due to liquid flooding. Methods to address overheating: 1) When discharging from the synthesis tower, steam should be added to E-101 slowly; if overheating occurs, the steam supply should be reduced appropriately until the overheating is resolved. 2) When overheating occurs due to an NH3 interruption, some steam should be quickly removed from the shell side of E-101; after starting the ammonia pump, an appropriate amount more NH3 should be added to reduce the CO2 load until normal conditions are restored at the top of E-101, after which the load can be gradually increased back to its original level. 3) The air outlet holes at the upper part of the stripping tubes in Jinxi and Jianfeng are too small; enlarging them or replacing the distribution tubes could be considered. It can be seen from the above that by strengthening management in areas such as operation, maintenance, and spare parts, corrosion can be reduced and overheating can be prevented.

1 Overview of the C02 Stripping Tower: The CO2 stripping tower in the urea production unit of Hebei Xuanhua Fertilizer Group Co., Ltd. is a falling film heat exchanger. It has a total height of 11,027 mm, is symmetrical from top to bottom, and can be used in inverted position; its heat exchange area is 180 m2. The heat exchange tubes are zirconium-based bimetallic tubes of type 2RE69 1ZR702, with dimensions of φ26.5×(2+7)×4 900mm, manufactured by Swedish Sandvik; a total of 604 such tubes are used. The tube sheet, upper and lower tube boxes, head, and large cover are all made of stainless steel lined with X2CrNiMo 25-22-2. Its design parameters are as follows: Design pressure for the shell and piping / MPa: 2.7, 16.0; Operating pressure / MPa: 2.1, 14.41; Design temperature / °C: 230, 230. Corrosion conditions in the CO2 stripping tower: This CO2 stripping tower was designed and manufactured by Dalian Iceberg Group Jinzhou Heavy Machinery Co., Ltd., and was put into operation during a major maintenance campaign in June 2001. After one year of operation, the unit experienced leaks due to corrosion in the stripping tower on five occasions between May and September 2002, resulting in continuous interruptions in urea production. The specific parking times and corrosion statistics are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0602061643413979.jpg3 Analysis of corrosion causes: (1) Lack of oxygen protection – Oxygen in the air is used as an oxidizing agent; when there is a certain amount of oxygen present in the ammonium carbamate solution in the CO2 stripping tower, it causes an oxidation reaction on the surface of the tower’s lining, resulting in the formation of a dense and intact passivation layer that protects the lining. However, in actual device production, no oxygen (or air) protection was applied, resulting in increased corrosion. (2) The H2S content in the CO2 feed gas is relatively high. Since H2S is oxidized to SO42- in urine, this reaction consumes a large amount of oxygen from the solution; when the oxygen level drops below a certain threshold, the surface of the lining becomes activated, leading to intense electrochemical reactions with the solution. Therefore, the presence of H2S destroys the passivation film in the stripping tower, exacerbating corrosion at the tube ends (or lining). (3) The actual operating temperature of the CO2 stripping tower is high. The operating temperature has a significant impact on the corrosion rate of the CO2 stripping tower; as the operating temperature increases, the corrosion rate rises and corrosion intensifies. (4) No passivation treatment was carried out after repair. After welding, the weld area and its surrounding areas of stainless steel materials darken due to oxidation; acid washing and passivation can form a new passivation layer on their surface, preventing further oxidation and darkening. However, passivation was not carried out during the previous repairs. 4 Repair measures: (1) For the grooves caused by corrosion in the lower tube box, repair is carried out by combining grinding and welding. (2) Repair the severe pitting that has occurred on the lining. (3) The lining of the liquid outlet pipe was severely corroded and could no longer ensure proper operation; it was removed and replaced with a new welded section. (4) Measure the inner diameter of the heat exchange tube ends; tubes with a wall thickness of less than 1.0 mm should be sealed by inserting plugs and then welding them in place. (5) Measure and record in detail the inner diameter of the tube ends of the unblocked heat exchange tubes. After processing, the inner wall of the pipe end is coated with PTFE sealant, and a titanium inner sleeve is installed; the wall thickness of this titanium inner sleeve is 1.0–1.5 mm, and it fits the heat exchange tube in an interference fit. 5 Repair protection (1) Pickling, passivation, and cleaning. After the repair is complete, remove the slag and oil from the surface. Apply stainless steel pickling and passivating paste evenly to the lower tube box, end caps, weld seams, and the ends of the heat exchange tubes; the coating thickness should be 1–2 mm. Leave it in place for 2 hours, then rinse thoroughly with clean water until the surface is shiny. Oxygen enrichment is used for protection during the operation of the stripping tower; a dedicated compressor is installed to compress the air to 16.0 MPa after two stages of compression (with heating and insulation pipelines), and this air is then fed into the bottom of the stripping tower. (2) Strictly control the H2S content in the raw material gas from the previous process. (3) Strictly control the operating temperature. (4) To prevent the titanium sleeve from falling off and blocking the tube, a mesh tube is installed at the inlet of the liquid outlet tube. 6 Conclusion: Since the stripping tower of the urea production unit at Xuanhua Fertilizer Group Co., Ltd. was repaired and put into operation in October 2002, urea production has not been affected by any corrosion issues related to this equipment. Taking advantage of the plant shutdown period, the stripping tower was inspected twice, and no significant corrosion was found; moreover, none of the titanium linings had come loose, indicating that the unit was operating well.

Introduction: In the 1990s, several of the urea production plants built in our country adopted the ammonia stripping process introduced by the Italian company Snamprogetti. As the unit operated, various problems gradually emerged; the stripping efficiency of the stripper decreased, which led to an increased load on the subsequent systems and higher consumption levels, preventing the unit from operating at high loads. The most prominent issue is that the titanium stripping tower used in high-pressure equipment is not as corrosion-resistant as SNAM claims. The four 400 t/d ammonia stripping urea plants introduced in 1990 with World Bank loans suffered corrosion-induced tube failures in their stripping towers at the Xuanhua, Luoyang, and Yuanping plants one after another, except for the Beishi Chemical Plant which ceased operations for certain reasons; Xuanhua was forced to replace its stripping tower due to an excessive number of blocked tubes. The problems that occur during the operation of the stripping tower and the measures taken to address them play a very important role in ensuring the stable production of urea and extending the lifespan of the plant. 1 Design parameters and structure of the distillation tower: This equipment was manufactured by the Italian company FBM-HUDSON and put into operation in March 1992. Its external dimensions are φ1,210×11,840. The upper and lower hemispherical tube banks are lined with titanium plates 3 mm thick, while the tube sheets are lined with titanium plates 10 mm thick. Since corrosion tends to occur at the upper part of the tubes and at their ends, the overall structure of the equipment is designed to be symmetrical, allowing the manufacturer to invert it at an appropriate time in order to extend its service life. The technical parameters of the stripping tower are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0603281526007101.jpg 2 Corrosion of the tubes in the distillation column and remedial measures 2.1 Macroscopic inspection 2.1.1 Various types of corrosion can be observed on the tubes inside the upper tube sheet; in mild cases, the ends of the tubes have a rough, wavy surface with cracks, and the sealing surfaces at the tube ends show signs of corrosion and pits. In most cases, the wall thickness of the tube ends decreases, taking on a serrated shape. In more severe cases, the tube walls may become perforated or even completely corroded and broken (see Figure 1). http://www.nmtech.com.cn/jishuwang/upload/0603281527037032.jpg 2.1.2 The end faces of the tubes inside the lower tube box are flat, the wall thickness of the tube ends is uniform, and there is no significant corrosion. 2.2 Repair of damaged pipe ends: To address the severe corrosion of the pipe end fittings on the stripping tower, we employed the patented technology for replacing new titanium pipe ends developed by Jinzhou Heavy Machinery Factory of Dalian Iceberg Group to repair those pipe ends. 2.2.1 In accordance with the requirements of Jinzhong Plant, connect the distribution panel and provide an air supply at 0.7 MPa; install axial flow fans at the bottom of the tower to ensure adequate drying and ventilation, thereby reducing the humidity in the construction area. 2.2.2 Use a dedicated pneumatic trolley bed to remove the damaged pipe ends by mechanical turning. The original tube end is 17 mm long; under normal circumstances, 15 mm is cut off, leaving a 2 mm high original tube end. In special cases where the original tube end is severely damaged, it is cut to be level with the tube sheet surface, after which 2 mm of weld metal is added to extend the original tube end. The inner and outer diameters are then machined to the dimensions required for the tubes to be connected. 2.2.3 Align the replacement titanium short tube with the holes of the original tube, fix it by spot welding at 120° intervals, fill it with titanium weld wire using TIG welding; first weld the outer seams at the tube ends, and use special welding equipment to perform fusion welding on the inner seams. 2.2.4 Using a dedicated wind-driven lathe, machine the inner and outer cylindrical surfaces of the pipe end, as well as the pipe end itself, to the dimensions specified in the drawings. After welding, the surface is silver-white; after machining, a 100% dye penetrant inspection is carried out on the welds, and finally, a penetrant test is performed using 0.05 MPa of gaseous ammonia. Following two major overhauls in 1998 and 2000, our company repaired the tube ends of the 254 tubes on the top tray of the stripping tower, thereby preventing the tower from being put into use prematurely due to tube end corrosion and extending its service life. 3 Methods for Treating Fouling in the Tubes of the Stripping Tower 3.1 Fouling inside the tubes During the later stages of the pilot production of urea, as production gradually returned to normal, it was found that the temperature at the bottom of the stripping tower could not be increased, which had a direct impact on production. This led to an increase in production costs and made it difficult to control product quality; thus, the seriousness of fouling inside the tubes became increasingly recognized. In April 1996, upon inspecting the equipment, it was found that the inner walls of the tubes, as well as the inner walls of the upper and lower tube banks, were covered with a scale layer 0.5–0.8 mm thick. This scale was hard and dense, colored brownish-red, and difficult to remove; the scaling on the packing inside the equipment was even more severe. 3.2 Cleaning methods previously used: During the major overhaul in 1996, the company decided to engage a professional cleaning company in China to carry out chemical cleaning. This company used the HL956 cleaning agent for the process, and two consecutive cleaning sessions were conducted, but the results were not satisfactory. Immediately afterwards, high-pressure water jet cleaning was used. The scale removal rate is only about 45%. After restarting the plant following the major repair, the process conditions and the temperature at the bottom of the stripping tower improved slightly, but the effects were not satisfactory. In March 1997, pneumatic pigging machines produced by Shanghai Huyi Pneumatic Tools Factory were used for drilling and descaling. Since a certain gap must exist between the drill bit and the inner wall of the tubes for it to function, it is impossible to completely remove the scale buildup on the inner walls of these tubes. Moreover, mechanical cleaning methods are time-consuming and can easily damage the inner walls of the tubes. As a result of this treatment, the condition of the tubes has improved significantly compared to before, but the results are still not ideal. 3.3 Successful chemical cleaning methods: In August 1998, we asked the Lutianhua Technology Center to carry out chemical cleaning on the stripping tower together with the medium-pressure decomposer and the low-pressure decomposer, all connected in series. The materials selected for the cleaning circulation pipelines and mixing tanks are 1Crl8Ni9Ti stainless steel, while stainless steel pumps are used for the cleaning pumps. The cleaning process is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0603281530131353.jpg3.3.1 Cleaning solution formula: Based on the analysis of the scale sample, the main components are Fe2O3 at 95.04%, Ni at 0.0192%, Cr at 0.0015%, and Ti at 0.52%. These deposits mainly result from the accumulation of corrosion products from the urea system. The technical center of Lutianhua proposed a cleaning solution formula: 11 tons of the specialized cleaner LS-731. http://www.nmtech.com.cn/jishuwang/upload/0603281531235324.jpg 3.3.2 Chemical cleaning: The main steps in this process are degreasing, descaling, passivation, and cleaning. 3.3.2.1 Install the cleaning tanks, pumps, and pipelines as required, conduct leak tests and commissioning to ensure they are satisfactory, and then clean the weld slag and debris inside the pipelines with clean water. 3.3.2.2 Degreasing (alkaline cleaning): A prepared 4% sodium tripolyphosphate solution is used, with the pH adjusted to ≥13 using sodium hydroxide; this solution is used for cyclic cleaning of the system for 3–4 hours, with the aim of removing any small amounts of oil or other contaminants present in the equipment. Then rinse the system with clean water until it reaches neutrality. 3.3.2.3 Descaling cleaning: The system was subjected to cyclic cleaning using a cleaning solution prepared from LS-731 produced by Lutianhua Company. Steam at 0.4 MPa was used to raise the temperature, and the circulation was maintained while the temperature of the cleaning solution was controlled at (110±2)°C. Once the increase in the total iron content in the descaling solution slowed down, the cleaning solution was discharged, and a new solution was prepared for the next cleaning cycle. A total of five cycles of chemical cleaning were carried out, taking approximately 108 hours. After the chemical cleaning was completed, the system was rinsed with clean water to remove any residues. 3.3.2.4 Equipment passivation: A passivation solution is prepared by mixing dilute nitric acid and hydrogen peroxide with deionized water in appropriate proportions, and this solution is circulated through the stripping tower, medium and low pressure decomposers, as well as associated pipelines, to form a passivation film on the cleaned metal surfaces. 3.3.2.5 Cleaning: Wash the system with deionized water 2–3 times until the rinse water is neutral, at which point it is considered satisfactory. 3.3.3 Monitoring of chemical cleaning corrosion: The corrosion rate was determined by using test pieces placed in the upper part of the low-pressure decomposer and within the cleaning solution circulation pipes. The material of these test pieces was the same as that of the equipment to be cleaned (titanium). The actual measured corrosion rate was 0.0035 g/m2·h, which is lower than the value specified in the contract of 0.2 g/m2·h. The formula for calculating the corrosion rate is: K = (W1 – W2) / (F · t), where K represents the corrosion rate in g/(m2·t); W1 is the weight of the test piece before cleaning, in grams; W2 is the weight of the test piece after cleaning, in grams; F is the surface area of the test piece, in m2; and t is the time taken for chemical cleaning. 3.3.4 Cleaning effectiveness: After the cleaning process was completed, the equipment was inspected, and it was found that all the hard deposits on the inner walls of the columns of the stripping tower as well as on the upper and lower tube banks had been removed. The equipment regained its original color, and the packing rings inside the equipment retained their metallic appearance. The scale removal rate was 100%, meeting the expected requirements. By repairing the tube ends of the stripper columns and performing chemical cleaning to remove scaling, the problems that arose during the operation of the stripper were successfully resolved. As a result, the operating temperatures of the equipment increased from 184°C at the upper part and 190°C at the lower part before treatment to 189°C and 204°C respectively. The heat exchange efficiency and stripping efficiency improved significantly, energy consumption decreased, production became more stable, and the output rose from 400 t/d before cleaning to 440 t/d, reaching 110% of the designed production capacity. More importantly, the service life of the equipment was extended. 4 Inversion of the stripping tower 2001-06-23: A tube burst in the urea stripping tower, forcing a shutdown for repairs. Using eddy current testing, it was found that one of the heat exchange tubes had burst due to corrosion that thinned it out near the upper deck. The corrosion occurred within 1 meter of the upper tube end, with the thinnest sections of the wall reaching only 0.85 mm in thickness; there was virtually no corrosion below 1 meter from the upper tube end or at the tube ends of the lower tube box. During this maintenance work, tubes with a wall thickness of less than 1.3 mm were plugged, resulting in a total of 22 tubes being plugged. Based on the usage patterns of the stripping towers, the original design of SNAM’s stripping towers featured a symmetrical structure; after 8 to 10 years of use, these towers were used with their upper and lower parts reversed. In November 2001, our company shut down the urea production line for a week in order to invert the stripping tower. 4.1 Eddy current testing was conducted on the heat exchange tubes; those with a wall thickness of ≤1.2 mm or that were defective were plugged, resulting in 19 tubes being plugged in this instance. 4.2 Use specialized tools to grind and clean the pipe ends of the original lower tube box in order to remove scale. 4.3 Disconnect all pipelines and flanges connected to the stripping tower, as well as remove the floor slabs and beams that interfere with lifting. 4.4 Select five heat exchange tubes and use a plumb line to measure the verticality of the stripping tower before it is inverted; after the tower is inverted, use a plumb line again to adjust its verticality. 4.5 Reset of internal equipment components and auxiliary pipelines. Effects of inverting the stripping tower: There were a total of 41 instances of blockages in the tubes of the stripping tower, with a blockage rate of about 8%. After inverting the tower, the tube openings in the upper tube sheet are now intact, the liquid distributor can be installed in place, and the F4 sealing rings provide good sealing, ensuring uniform distribution of urine and thus maintaining efficient stripping. After the stripping tower was inverted, the production process remained normal and stable, with a daily urea output of 450–460 tons per day, reaching 115% of the designed production capacity. The temperatures at the top and bottom of the stripping tower were 190°C and 204°C respectively, and there were no significant changes in various consumption rates. Furthermore, after the stripper was inverted, the sections of the heat exchange tubes that were severely corroded and had thinned out were moved to the lower part. To determine whether further corrosion and thinning will occur in the lower part of the heat exchange tubes after the inversion, eddy current testing of these tubes will be carried out during the next major maintenance session for the urea plant, along with measurements of the rate of corrosion and thinning in the upper part of the tubes. This will provide a basis for ensuring the safe operation and maintenance of the stripper in the future. 5 Development of urea stripping towers: In the SNAM ammonia stripping process units, the stripping towers suffer from erosion of the upper tubes. This is because although industrial pure titanium possesses better corrosion resistance than the commonly used urea-grade stainless steel, and its operating temperature is 10–15°C higher than that of the latter, it has poor wear resistance, is sensitive to crevice corrosion, and has poor weldability. As a result, severe corrosion occurs in the area around the tube ends of the upper tube box and within 1 meter below them, making it difficult to address problems in these stripping towers. The most successful technology currently in use is the use of bimetallic zirconium tubes for the stripping pipes: the outer tube is made of CrNiMo25-22-2 with a wall thickness of 2 mm, while the inner tube is made of zirconium with a wall thickness of 0.7 mm. Zirconium offers better corrosion resistance than titanium, as well as superior wear resistance; it is therefore widely used in areas subject to severe erosion. Its maximum allowable operating temperature is 20°C higher than that of titanium, reaching 230°C. Xuanhua faced a situation where nearly 120 tubes in its original titanium stripping towers became corroded and blocked, resulting in a blockage rate of over 20%; this severely affected production, forcing the operation load to be maintained at only 90%–95%. Additionally, energy consumption increased significantly. As a result, it was necessary to replace the stripping towers in August 2001. These new towers utilized novel bimetallic zirconium tubes manufactured by Jinzhong Machinery Factory of Dalian Iceberg Group. The cost of these equipment amounted to approximately 4 million yuan. This marked the first time that zirconium stripping towers were manufactured and used in China, and it represented a new direction in the development of such towers.

After the 1990s, multiple ammonia stripping urea production processes were introduced in China. After 8 to 12 years of operation, the ammonia stripping towers in these processes suffered from severe corrosion and leakage issues in their tube sides, which hindered the long-term operation of the plants. In September 2003, Zhongyuan Dahuahua Group Co., Ltd. was the first in China to reverse the orientation of its stripping towers for use. CNOOC Chemical Co., Ltd. completed the reversal of the stripping tower in December 2004, and achieved zero maintenance and safe, long-term operation over the following 3 years. Taking the ammonia stripping tower of CNOOC Chemical Co., Ltd. as an example, this paper analyzes and summarizes the problems that occur during half of its service life, which is particularly important for the safe operation of the equipment in its next service cycle. 1 Structural features of the equipment and main existing problems: When designing the stripper in its early stages, the patent holder Snam Company took into account the high operating temperatures of the ammonia stripping process, as well as the strong corrosive effect of urea/ammonium methylate solutions on metals; therefore, the tube side of the equipment was constructed with a carbon steel lining over industrial pure titanium. The device is a tube-type heat exchanger (with heat exchange tubes of specifications φ27 mm×3.5 mm), and a distribution tube with an external insertion structure is installed at the top of the heat exchange tubes; the device is arranged vertically. According to the design concept provided by Snam, after 8–10 years of use, the equipment can be reversed in direction to ensure that its designed service life exceeds 15 years. Typically, after a ammonia stripping tower is put into operation, severe corrosion problems arise. The main defects include: ① Uniform internal corrosion and thinning within an area of 800 mm at the top of the heat exchange tubes, which generally begins to worsen gradually after 3–4 years ； ② In the 14 mm area at the top of the heat exchange tube (commonly referred to as the tube head), there is fissure and grooving corrosion on the outer wall as well as erosion corrosion on the inner wall; this corrosion progresses rapidly after 5–8 years ； ③ Erosive corrosion of the liquid-phase region in the fillet welds of the upper tube box lining joint plates generally occurs after 8 to 12 years ； ④ Corrosion of the linings in various connections of the upper tube box, as well as corrosion of the gasket lenses of the spare connections due to poor flow ； ⑤ Scaling of titanium-based compounds inside the heat exchange tubes. These problems severely affect the continuous long-term operation of the equipment; especially after 7 years of use, the maintenance intervals become shorter and sudden leakage incidents occur frequently. From March 2003 to February 2004, there were 4 separate leaks over a period of 1 year, which caused the plant to shut down and severely affected its stable operation. 2 Analysis of the causes of defects: 1) The area of the heat exchange tubes located 600–800 mm below the upper tube sheet suffers from severe corrosion; this is the key area to be inspected for corrosion during each major maintenance session. From September 1996 to March 2003, over a period of 6.5 years, the average corrosion rate was 0.24 mm/year; the main mechanisms of corrosion were erosion corrosion and chemical corrosion. The urea/ammonium methoxide solution enters the heat exchange tubes through 3 radially inclined 3.2 mm small holes in the stripping distribution tube at the top of the tubes, causing the liquid to form a spiral downward flow film; the rotation of this fluid generates kinetic energy that scoures the inner walls of the tubes. Titanium has a poor resistance to erosion compared to other metals; as a result, erosion corrosion is quite severe. This type of corrosion becomes more pronounced as the load increases and the flow rate inside the pipe rises. The upper part of the heat exchange tube is the area with the highest operating temperature (210 °C); ammonia, carbon dioxide, and inert gases dissolved in the solution evaporate in this region, causing the corrosion rate to increase significantly, with chemical corrosion being the most severe. In these two corrosion environments, the upper end of the heat exchange tubes becomes the most severely corroded part of the entire equipment. The wall thickness of the heat exchange tubes is the most critical factor determining the actual service life of the equipment; therefore, closely monitoring the corrosion rate in this area serves as an important basis for assessing whether the equipment can be used in reverse direction. 2) For heat exchange tubes with a length of 800 mm or less, friction between the medium and the inner wall of the tube, the upward resistance exerted by the gas, as well as the precipitation of ammonia from the medium, lead to an increase in the urea content in the solution. This results in a significant reduction in the corrosion rate of the heat exchange tubes by the solution. At the same time, a layer composed of titanium, iron, and urea ammonium complexes forms on the inner wall of the heat exchange tubes; the main inorganic components are listed in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/080526945104208.jpg The scale has a rough sandpaper-like appearance and adheres very firmly to the walls of the heat exchange tubes; figures 1 and 2 show comparisons of the scale samples. The scale becomes thicker as one moves downward along the heat exchange tubes, and after 5 to 8 years of operation, it will severely affect the equipment’s heat exchange efficiency. http://www.nmtech.com.cn/jishuwang/upload1/080526945431100.jpg 3) At present, in most ammonia stripping units in China, the tube ends of the stripping towers and the distribution pipes employ an external insertion design, with PTFE-sealed rings used between them. If the quality of these sealed rings is poor or if they are not installed correctly, the PTFE rings are prone to axial fracture and curling. When the medium solution flows into the heat exchange tube from the gap between the fracture site and the heat exchange tube, it will erode the titanium tube ends at that gap. Additionally, the PTFE sealing ring bends, creating a gap between the end face of the tube and the distribution pipe. The lack of flow in this gap during operation results in a lack of oxygen on the surface of the tube end, preventing passivation and leading to rapid chemical corrosion; as a result, the tube end suffers from erosive corrosion. Since the tube head is the junction of the distribution tube and the heat exchange tube, the flow velocity of the spiral falling film is highest there, resulting in severe thinning of the inner wall of the tube head. After 8 years of operation, the inner walls of many pipe ends became severely thinned, and the corrosion conditions are shown in Figures 3 and 4. Some of the corroded pipe ends are as thin as paper sheets, and in severe cases the corrosion can cause those ends to fall off. During each major overhaul, around 100 to 500 pipe ends are typically replaced. 4) After the medium enters the tube sheet, it causes severe erosion between the cofferdam and the tube sheet lining, at the fillet welds of the docking plates that protrude from the lining; local high points lead to erosion due to jet flow. After 8 to 12 years of operation, the fillet welds on both sides of the cover plate become eroded and sunken; the surface becomes smooth to the point that the weld patterns can no longer be distinguished, as shown in Figure 5. Defects such as pores in the welds during manufacturing are exposed to corrosive agents. The original design specified a weld corner height of 3 mm, while in the areas that were severely eroded, the weld corner height was only 1.5 mm. There were two instances of leakage at the fillet welds on both sides of the cover plate on November 30, 2003, and February 8, 2004. http://www.nmtech.com.cn/jishuwang/upload1/080526946557269.jpg 5) Since the equipment is designed to be used in reverse direction after several years, the number and location of all interfaces on the upper and lower pipe boxes are exactly the same. During operation, many of the interfaces on the upper tube box are designated as spare interfaces for reversing direction; they feature an external blind flange structure to keep them temporarily sealed and unused. On March 5, 2003, leaks occurred at the blind flanges of CN2A and N4A. Upon inspection, it was found that the lining of the blind flanges as well as the lens gaskets had suffered corrosion and perforation. The cause of the leaks was attributed to the lack of flow of gas and liquid inside the spare piping, poor external insulation of the equipment, and corrosion resulting from oxygen deficiency in the condensate. These areas are difficult to inspect, which makes corrosion and perforation very likely to occur there. 6) During design, to prevent erosion caused by the medium entering the tube box inlet (N1A), the patent holder adopted a lens gasket structure with long bushings (Figure 6). Since the role of the bushing lens pad was not recognized during installation, a regular lens pad was used in that area, which caused the medium to directly scour the pipe lining, resulting in scouring, corrosion, perforation, and leakage of the pipe lining. March 3, 2003, Stripping tower inlet pipe (N1A)

0 Introduction Urea ammonium solutions are highly corrosive; the higher the concentration and the higher the temperature, the greater the corrosion effect on equipment. The corrosion of metals by these solutions involves both chemical reactions and electrochemical processes. During the urea production process, high-pressure equipment operates in an environment characterized by high temperatures and pressures, as well as a relatively high concentration of ammonium carbamate; therefore, the corrosion of urea production equipment under high pressure is generally more severe than that of equipment in other systems. This article focuses on analyzing the corrosion characteristics and protection measures of urea stripping towers, and also introduces the equipment corrosion control methods during normal operation and tower sealing during shutdowns. 1 Corrosion characteristics of the urea stripping tower. The main form of corrosion in the urea stripping tower is erosion corrosion of the stripping tubes in the upper part of the tower. The upper tube sheet, head, and other internal components of the stripping tower suffer from uniform corrosion, while the lower tube sheet and head of the stripping tower have a dense and hard scale layer on their surfaces; the degree of corrosion of the equipment is not significant. The liquid inlet pipe, distributor, vapor cap, liquid baffle, and other components of the stripping tower exhibit certain levels of erosion corrosion. The area most severely affected by erosion in the stripping tower is located 1.0 to 1.5 meters below the upper tube sheet (including the thickness of the upper tube sheet). The reason for this corrosion is that this area serves as the steam inlet; therefore, based on the temperature distribution along the entire stripping tube, the temperature here is the highest, reaching 220°C. As the temperature rises, the corrosion resistance of titanium decreases ； Secondly, the gas lift effect is strongest at this location; the high temperature causes the gas phase in the solution in contact with the tube wall to evaporate rapidly, thereby creating turbulence that leads to erosion corrosion. The corrosion of the stripping tubes below 2m from the upper tube sheet is minimal. Due to a dense and hard scale layer attached to the surface, it is difficult to accurately monitor the corrosion status of the pipe walls in the sections beneath the scale layer. After 10 years of operation, the stripping column in a certain plant had its scale layer removed using physical methods; the wall thickness of the stripping column was monitored and found to be essentially consistent with the original design thickness. As indicated by the monitoring results, the dense and hard scale layer inside the walls of the stripping tower indeed provides a form of protection for the stripping tubes. Due to its density and hardness, as well as its strong adhesion to the tube walls, this scale layer not only prevents contact corrosion between the medium and the metal surface but also avoids corrosion that occurs beneath the scale. The thinning of the wall thickness of the stripping tube is a key factor affecting the service life of the stripping tower. There are many factors that lead to the thinning of the stripping tube, mainly related to the smoothness of the tube wall, the load on the system, and the degree of corrosion at the ports on the tube bundle. During the first few years of operation of the stripping column’s stripping tubes, the thinning of their wall thickness occurred at a slow pace; this was mainly because the inner surfaces of the tubes were smooth when the equipment first started operating, and erosion of the equipment was not significant ； At the same time, the upper opening of the stripping tube has good sealing, resulting in a uniform distribution of the medium; there are no phenomena such as uneven flow of the medium. During this stage, the corrosion degree of the stripping tube is generally low, with the corrosion rate of the tubes being below 0.1 mm/year. As the smooth surface layer on the inner wall of the stripping tube’s tubes is damaged, corrosion of the tube wall accelerates; during this period, the annual corrosion rate of the tube wall is generally around 0.25–0.35 mm/year. Once the tubes in the stripping column enter a phase of accelerated corrosion, it is necessary to monitor and check changes in the wall thickness of these tubes during each major maintenance session, in order to prevent unnecessary problems from arising. For stripping towers with externally inserted liquid distributors made of titanium, damage to the upper ends of the tower tubes has a significant impact on the erosion of those tubes. For shell and tube exchangers whose tube ends on the stripping tubes are damaged, the annual corrosion rate can reach up to 0.6 mm/year. This is a rather dangerous situation for stripping tubes whose design wall thickness is typically only 3.5 mm. If they are not used or maintained properly, serious accidents such as tube rupture can occur during operation, and such ruptures can cause severe damage to the equipment. Some small and medium-sized fertilizer plants had to scrap their equipment early due to widespread tube failures in the stripping towers. The approximate corrosion conditions at different stages of the stripping tower are shown in Table 1. It can be seen that once damage occurs at the outlet of the stripping tower, the outlet should be replaced promptly and thoroughly repaired in order to extend the service life of the stripping tube. After replacing the pipe end, the annual corrosion rate of the stripping tube can generally be kept at around 0.25–0.35 mm/year. It is easy to understand that the corrosion rate of the tubes in a stripping tower is affected by the load. Since erosion is the main phenomenon occurring inside these tubes, the higher the system load, the greater the load on each stripping tube, and thus the more severe the erosion becomes ； The lower the load, the less erosion. The corrosion in the upper and lower tube banks of the stripping tower is mainly uniform corrosion. The corrosion conditions of the upper and lower tube banks in the stripping tower of a certain plant are shown in Table 2. Note: 1. The thickness of the monitoring lining wall includes the thickness of the surface scale layer. 2. Since the monitoring results take into account the thickness of the scale layer, the calculated annual corrosion rate of the equipment is merely an approximate value for reference. The corrosion of the upper and lower tube banks in the stripping tower is mainly uniform corrosion beneath the scale. In the upper tube sheet of the stripping tower, due to the presence of a gas phase layer, the scale layer is relatively thin, usually only 0.2–0.3 mm. Since the temperature at the upper part is about 10–15°C lower than that at the lower part, corrosion in the upper tube sheet is relatively mild ； The lower tube sheet is submerged in the liquid phase, and the scale layer on the surface of the equipment is relatively thick, typically around 0.5 to 1.5 mm. Since the scale layer in the lower tube sheet is relatively loose, in environments with higher medium temperatures, the under-scale corrosion of the lining in the lower tube sheet is more severe than that in the upper section; the annual corrosion rate is approximately 0.1 to 0.15 mm. The corrosion of the internals in the stripping tower is mainly uniform erosion caused by scouring. The corrosion of the internals in a stripping tower, such as packing, baffles, distributors, liquid distributors, and inlet pipes, varies to some extent; moreover, the focus of attention during use and maintenance differs for each of these components. First, the corrosion issue of the liquid inlet flange inside the stripping tower. In many plants, the inlet flanges in the stripping towers suffer from severe erosion, mainly due to the absence of gaskets or poor sealing. When erosion is detected on the flange surfaces, using soft PTFE tape for sealing generally yields good results. Second, the issue of corrosion and thinning of the Packer ring packing in the stripping tower. After being used for a certain period of time, if the Pall ring packing in the stripping tower becomes significantly thinned and loses much of its strength, it needs to be replaced regularly. It is advisable to use CrNiMo25-22-2 material for the replacement packing, as this can extend its service life. Third, the corrosion issue of the liquid distributor. The diameter of the holes distributed tangentially along the lower part should be checked regularly; if the increase in diameter is significant and exceeds the design specifications, the liquid distributor must be replaced. When replacing the liquid distributor, it is best to replace it entirely at once. If only part of it is replaced, the difference in pore sizes between the holes of the old and new distributors can easily lead to uneven distribution of liquid inside the stripping tube, which affects the efficiency of stripping and accelerates corrosion in some of the tubes. Fourth, the corrosion issue of the pressure grid at the upper part of the liquid distributor. Once the pressure grid at the upper part of the liquid distributor corrodes and thins, its strength is reduced, which in turn affects the degree to which the pressure grid exerts pressure on the liquid distributor. This can lead to looseness or poor sealing between the liquid distributor and the upper ends of the tubes, causing severe erosion of those upper ends and directly affecting the service life of the stripping tubes. Therefore, when corrosion and thinning occur in the pressure grid at the upper part of the liquid distributor, it is necessary to prepare spare parts in advance to prevent pipe mouth corrosion caused by problems with the pressure grid. 2 Factors Affecting Corrosion There are many factors that influence corrosion. From the perspective of process operations, the main factors that have an impact on corrosion in urea stripping towers include temperature, ammonia-to-carbon ratio, water-to-carbon ratio, concentration of ammonium methoxide solution, oxygen content, levels of sulfur and chloride ions, as well as the flow rate of the medium. 2.1 Medium temperature The medium temperature has a significant impact on equipment corrosion. This is because an increase in temperature can raise the corrosion rates of metals in their activated and passivated states, narrowing the passivation zone of stainless steel and accelerating the activation of the material; in other words, it speeds up the oxidation and reduction processes at the cathode and anode, thereby increasing the corrosion rate of the equipment. As the temperature of the medium increases, the rate of chemical reactions accelerates; when the temperature is below 165°C, changes in temperature have a minor impact on the corrosion of stainless steel ； However, when the temperature is between 165 and 200°C, the corrosion rate increases by 3 to 4 times. The operating temperature has a significant impact on equipment corrosion; when the operating temperature exceeds the design temperature, even by just 1–2°C, the rate of equipment corrosion increases markedly. Generally, the degree of corrosion of equipment is determined based on the levels of iron and nickel in the medium; an increase in the content of iron and nickel indicates that corrosion is worsening, and it is necessary to identify the cause promptly so that the values can return to normal as soon as possible. 2.2 Ammonia-to-carbon ratio: An increase in the ammonia-to-carbon ratio helps to reduce equipment corrosion. This is because when this ratio is high, the pH of the system rises, thereby reducing its acidity; as a result, the concentrations and residence times of NH2COO‑ and NCO‑ in the medium are decreased. Thus, the following reactions occur: NH3 + H2O → NH4+ + OH— (1) NH4+ + NH2COO— → NH2COONH4 (2) NH4+ + NCO— → NH4OCN (3) The idea that a high ammonia-to-carbon ratio can reduce equipment corrosion is also confirmed by a comparison between the ammonia stripping and CO2 stripping processes. The ammonia stripping process is designed with a relatively high ammonia-to-carbon ratio of 3.56 ； The ammonia-to-carbon ratio in the CO2 stripping process design is 2.89, and the operating temperatures of the two processes are the same, with the top temperature of the tower not exceeding 188°C. Under normal operation, the corrosion rate of equipment in the CO2 stripping process is generally higher than that in the ammonia stripping process. When shutting down the tower for parking, the shutdown time for the CO2 stripping process generally should not exceed 24 hours, whereas for the ammonia stripping process, the tower can usually be shut down for more than 48 hours. 2.3 Water-carbon ratio: An increase in the water-carbon ratio leads to greater equipment corrosion. This is because when the water-to-carbon ratio in the system increases, the amount of water in the system rises relatively, resulting in a lower solution concentration. This enhances the degree of dissociation of NH4COONH2 and NH4OCN, leading to an increase in the amounts of COONH2– and CNO– ions in the solution; as a result, the corrosivity of the medium toward metals increases. 2.4 Ammonium hydroxide concentration: The higher the concentration of ammonium hydroxide, the greater its corrosiveness. This is because when the concentration of methylammonium solution is high, the amount of COONH2– in the medium increases relatively; COONH2– has strong reducing properties, which cause the passivation film on the metal surface to be continuously damaged, thereby increasing the degree of corrosion of the equipment. 2.5 Oxygen content: The oxygen content in the system is key to the formation of a metal passivation film. If the oxygen concentration in the system falls below the minimum level required to form a passivation film, the oxide film will be destroyed, and the equipment enters a stage of accelerated corrosion due to activation. In some manufacturing plants that use the CO2 stripping process, others add a certain amount of hydrogen peroxide (H2O2) to the system in order to reduce the operating load on the CO2 compressors and increase production capacity. Atomic oxygen released from hydrogen peroxide can directly participate in electrode reactions, which facilitates the formation of a passivation film. When adding hydrogen peroxide, it is necessary to introduce passivated air as well; this is because hydrogen peroxide has poor stability and decomposes rapidly once it enters the equipment, preventing oxygen in the medium from coming into uniform contact with the surface of the equipment and thus failing to achieve the desired outcome. 2.6 Sulfur content: Sulfur possesses strong reducing properties. Whether it is present in the form of organic sulfur (mainly COS) or inorganic sulfur (H2S) in the CO2 gas or air used as raw materials, it undergoes hydrolysis and a series of redox reactions under high temperature and pressure. The end result is the destruction of the metal oxide film, thereby causing severe activated corrosion on the metal surface. Due to the strong reducing nature of sulfur, once its concentration exceeds a certain level, an oxide film cannot form on the metal surface. 2.7 Chloride content: Chloride ions are a major factor contributing to stress corrosion. When a large amount of chloride ions accumulate on the metal surface, stress corrosion can easily occur, leading to cracks or breaks in the equipment. During operation, it is necessary to strictly control the chloride ion content in media such as urea ammonium solution, steam, and washing water, in order to prevent stress corrosion as much as possible. During shutdown, the system strictly prohibits the use of raw water (including domestic water, fire-fighting water, rainwater, recycled water, etc.) to wash urea high-pressure equipment. 2.8 Medium flow velocity: The medium flow velocity is a major factor contributing to erosion corrosion. To reduce this type of corrosion, the pipe diameter should be increased appropriately during design in order to slow down the flow rate of the medium within the equipment. The goal is to prevent the passivation layer on the metal surface from being damaged under high-load conditions. In shell-and-tube exchangers or pipes where gas and liquid coexist, the erosion caused by the medium on the pipe walls is more severe, so these factors must be taken into full consideration during design. 3 Corrosion control during normal production 3.1 Strict control of operating temperature Excessive temperature significantly accelerates corrosion of equipment; the greater the temperature excess, the faster the rate of equipment corrosion increases ； The longer the over-temperature period, the more severe the equipment corrosion. Therefore, in normal production, it is necessary to strictly control the operating temperature of the equipment and avoid overheating as much as possible. The temperature in the titanium urea stripping tower should generally not exceed 207°C. If overheating is detected during system operation, adjustments should be made promptly to keep the temperature within the normal range. 3.2 Strictly control the oxygen addition rate in the system. The oxygen addition rate in the system is key to the formation of a passivation film on the metal surface. Insufficient oxygen supply to the system can result in a poor anti-corrosion effect of the passivation film, leading to oxygen-deficient corrosion ； Excessive oxygen addition to the system leads to increased exhaust gas release, thereby increasing ammonia loss in the system. Therefore, in normal production, it is appropriate to keep the normal indicators at a level slightly above average. During parking, the passivation film is damaged to varying degrees ； At the beginning of system operation, the metal surface is in an activated state, representing a transitional period during which the passivation film is being reformed and gradually restored; during this time, the oxygen consumption of the system increases relatively. Therefore, at the beginning of operation of the equipment, it is appropriate to set the oxygen addition rate in the system at the upper limit of the control criteria; after the equipment has been running for a few hours, the oxygen addition rate can be gradually reduced accordingly. If there is an interruption in the supply of passivation air during operation, and it cannot be restored within a short period of time (usually no more than 10 minutes), an emergency shutdown should be initiated. 3.3 Control of sulfur and chloride content in the system: The control of sulfur content in the system involves monitoring to ensure that the sulfur level in the raw CO2 gas does not exceed the allowable limits. This is particularly important in ammonia-urea production plants that use coal as a raw material. Sulfur and chloride ions cause severe corrosion to equipment; as long as these two elements are present, corrosion of the equipment will occur, and the higher their concentration, the more severe the corrosion on the equipment. When the sulfur mass fraction in the raw CO2 gas exceeds 1.5×10—5, a passivation film cannot be formed in the system, and the equipment will enter a state of accelerated activation corrosion. 3.4 Control of ammonia-to-carbon ratio and water-to-carbon ratio: Operating the system at a high ammonia-to-carbon ratio and a low water-to-carbon ratio helps to reduce equipment corrosion. Therefore, from the perspective of protecting the equipment, in production control the ammonia-to-carbon ratio should be kept as high as possible within the specified limits, while the water-to-carbon ratio should be kept as low as possible within those same limits. 3.5 Corrosion of equipment and its treatment: When the equipment is in normal condition, the urea produced is white and semi-transparent. If the equipment corrodes, the color of urea will change depending on the degree of corrosion. The more severe the corrosion of the equipment, the darker the color of the urea; the trend in color change is: white to light yellow to red-brown to brownish-black. In normal production, if pale yellow or reddish-brown urea appears, it indicates that the passivation film on the equipment has been damaged or cannot be formed at all; the equipment is thus in a state of accelerated corrosion due to activation. If it is difficult to determine the cause in a short time, the machine should be stopped immediately and the tower emptied. Once the cause is identified, raise the temperature again for passivation before restarting the machine. 4 Corrosion control during shutdown and tower sealing: Corrosion control of equipment during shutdown and tower sealing is also important; if operations and maintenance are not proper, the corrosion caused by a single shutdown can be more severe than that resulting from several months of normal operation. Therefore, it is also very important to master the methods and measures to reduce equipment corrosion during parking periods in order to protect urea high-pressure equipment. When parking, to reduce corrosion of the equipment, it is generally necessary to pay attention to the following aspects. 4.1 Control of the ammonia-to-carbon ratio in the system: Before or during shutdown, increasing the amount of ammonia added to the system appropriately to raise the ammonia-to-carbon ratio helps to protect the equipment from corrosion during the shutdown and tower sealing period. When planning to shut down the system, the ammonia level in the feed system can be increased appropriately before shutdown, in order to raise the ammonia-to-carbon ratio during the shutdown period ； During an emergency shutdown, unless the system comes to a stop due to a failure of the high-pressure ammonia pump, it is possible to appropriately extend the time during which the ammonia pump supplies ammonia to the system while shutting it down, thereby increasing the ammonia-to-carbon ratio in the system during that period. 4.2 Control of the system water-carbon ratio: During shutdown periods, the amount of water in the system should be reduced as much as possible to lower the system’s water-carbon ratio. In terms of practical operation, two aspects should be taken into consideration: First, before stopping the system, if parking is planned, the amount of water added to the system can be reduced appropriately, thereby lowering the water-to-carbon ratio in the system ； Secondly, when flushing the equipment and pipelines during shutdown, the flushing time and frequency should be minimized to reduce the amount of water added to the system during the tower sealing period. 4.3 Determination of the tower sealing time Since the system is in different conditions during each shutdown, strictly speaking, the maximum allowable tower sealing time after a shutdown should also vary. The maximum tower sealing time after each shutdown must be determined based on the specific conditions at the time of shutdown, and it is generally between 12 and 48 hours. The specific details are as follows: (1) The system shuts down urgently due to an ammonia shortage. If the ammonia-to-carbon ratio in the system was maintained at the lower limit of the specified range before the shutdown, and no additional ammonia can be added to the system during this shutdown, then the time during which the tower remains sealed should generally not exceed 12 hours. (2) In the event of an emergency shutdown, a certain amount of ammonia can be added to the system when sealing the tower, and provided that the ammonia-to-carbon ratio in the system remains within normal limits before the shutdown, the sealing time should not exceed 24 hours. (3) If shutdown is planned, 2–3 hours before shutdown, gradually bring the ammonia-to-carbon ratio in the system and the air addition rate to their upper limits, while keeping the water-to-carbon ratio at its lower limit. Once the tower is shut down, continue to supply ammonia to the system for a certain period of time; under these conditions, the system can generally remain in this state for about 48 hours. (4) In the event of a shutdown due to an interruption in the passivation air supply, it is generally not advisable to seal the tower; instead, immediate venting of the tower should be carried out to identify the cause, and operation should be resumed after reheating for passivation. (5) If, while the system is in operation, the equipment suffers from severe corrosion of unknown cause, it is not advisable to seal the high-pressure system during a shutdown in such a situation. 5 Conclusion The urea stripping tower is one of the key equipment in urea production. Corrosion is a persistent issue, but its rate is relative. Only by fully understanding and grasping the patterns of corrosion and its influencing factors can we mitigate or prevent unnecessary corrosion. At the same time, pay attention to proper control during normal operation and take care to protect the tower when it is shut down; this will reduce corrosion and extend its service life.