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Summary of Operations, Maintenance, and Reversal Activities for the Ammonia Stripping Tower: The company’s urea production plant 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 the equipment 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 maintenance 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 was 89 days, and the Ni content in the finished product was 0.04×100.32×10^(-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 obvious signs of corrosion observed. During the inspection in 1996, the equipment was in good overall condition; there were no signs of significant 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, with no significant defect signals detected. During the major overhaul inspection in 1998, some relatively severe corrosion defects were found, 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 Böhr ring elements 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 beds, resulting in an uneven surface, and some of the connection nuts had almost completely corroded away, compromising the threaded connections. In October, the unit was shut down due to a leak in the fillet weld of pipe No. 337, 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 erosion 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, 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 those tubes with a wall thickness of less than 1.9 nlnn (based on the thickness in 2002) is shown in Table 2. 3. Situation regarding reversal of the tower: The basis for reversing the distillation tower is the wall thickness of the heat exchange tubes. The data provided by SNAM is 1.8 tnnl, whereas the actual minimum wall thickness of the distillation towers in use is much lower than this value. In light of these circumstances, it was decided to reverse the distillation tower during the major overhaul in 2(I)4. 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 acceptable. There are several small pits on the sealing surface of the lower head; the deepest one is 0.4 mm deep. A portion of 0.40 mm was removed from this area using a 20-degree cone tool, and the surface was then polished with sandpaper until it reached the required roughness level. 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, reflecting the characteristics 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, or black, showing uneven coloring. The yellow-green surface film is incomplete, reflecting the characteristics of rutile-type TiO2 that result from uniform corrosion of industrial titanium in a urea medium. The black surface film is quite loose and falls off in granular form; where it has fallen off, a yellow-green surface film appears, showing typical characteristics of A.O3 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 layer) composed of 99% iron oxide and 1% impurities. The surface film is grayish-black in color, dense, and no abnormal corrosion was detected. The film on the inner wall surface of the heat exchange tubes is grayish-black or 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 as well as 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 tnln. There were 65 stripping tubes with a wall thickness of ≤1.60 mm, which led to blockage. At the same time, the outer wall of tube No. 2 showed defects, and it was classified as a Grade B tube and was treated by plugging. 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 100A. 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 Dematic300T truck crane and a KH7 type 2 crawler crane together to carry out the tasks of moving the equipment, turning it around, and repositioning it. The main crane uses 8 QT4MM steel wires of the 6×37 1 type. In conjunction with the crane, 4 steel wire ropes of type 6×37 1, 6-strand, 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 stripper, the 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 pipe openings with temporary blind flanges. 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; the equipment is lifted so that the supporting steel structure remains unloaded ； Demolish the main load-bearing beam on the west side of the stripper and 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 should be taken to avoid damaging the joints between the main load-bearing beam and the two columns, as well as the joints 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 utilizing the flexibility of the crawler crane. At this time, since the frame columns block the counterweight of the 300T crane, the 300T crane needs to have its counterweight removed and reinstalled once ； After the equipment was turned around, the 300T truck crane continued to serve as the main crane, with the 150T crane assisting in positioning it. Once the equipment was upright, the 150T crawler crane released its hook and rotated the equipment by 180 degrees; then the boom of the 300T truck crane was used to position the equipment properly. The load-bearing beams of the equipment were reinstalled, involving the use of high-strength bolts for connections as well as structural welding ； Adjustment of equipment verticality. The required verticality for general static equipment is 19’00, but the ammonia vapor tower, as a falling-film heat exchange and separation device, 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 required verticality during initial installation was 0.5‰ ； 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 measurement 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 using these values until it reaches 0.2‰, thereby meeting 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 cleaning. 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 ； ② The gun should be inserted slowly; otherwise, the effect on removing the scale layer will be poor ； ③ The cleaning height is 150 mm below the pipe opening. The cleaning water pressure is 200 MPa. To speed up the process, two cleaning units were 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 ； ② The height of the surrounding pipe ends is 16–16.5 mm, which is lower than the designed dimension of 17 mm ； ③ Multiple fillet welds show undercut at the pipe ends, with the deepest depth reaching 1 mm ； ④ The designed outer diameter of the upper pipe end is 7 mm, while the actual measured outer diameter of the pipe 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 6.8 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 around it. When then attempting to machine a 10.5 mm thick sealing step, this process will affect the areas where fillet welds are present, severely impacting the performance of the upper tube sheet and increasing 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 around it, ensuring a uniform transition. ② The minimum height of the pipe ends must not be less than 16 mm, with adjacent pipes being roughly at the same level. ③ The 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. During the process, an ammonia leakage of 0.25 MPa was allowed, and the test was considered successful after holding the pressure for 2 hours. 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.