Anti-corrosion treatment for the urea synthesis tower

0 Preface The urea synthesis tower is a key piece of equipment in urea production plants. Since the commissioning of our urea plant, in less than 13 years, we have replaced two urea synthesis towers with a diameter of ￠1400. The main reason is corrosion and leakage of the urea synthesis tower lining, which severely affects normal production. Over the past few years, production has had to be halted for maintenance once every 1 to 1.5 months on average, which not only affects product quality and output improvement but also leads to increased costs ； Thus, it was forced to decide to spend 2 million yuan in 1988 to upgrade a new urea synthesis tower. At the same time, the corrosion leakage and protection issues in the urea synthesis tower also required the company’s close attention. Fortunately, through years of specialized research conducted by the company’s engineering and technical staff, significant progress has been made in preventing corrosion and leakage in urea synthesis towers. To date, the company’s third urea synthesis tower with a diameter of ￠1400 has been in normal operation for over 10 years, providing an essential foundation for the company’s urea production. This article attempts to provide a preliminary analysis and summary of the mechanisms behind corrosion of the linings in urea synthesis towers, the causes of corrosion-related leaks, and the corresponding preventive measures. 1 Mechanism of corrosion of the lining in urea synthesis towers In medium-sized urea production enterprises in China, the linings of urea synthesis towers are generally made from stainless steel plates specifically designed for urea production, with a thickness of 6 mm and composed of 00Cr17Ni14Mo2 or 00Cr17Ni14Mo2Ti (commonly known as 316 L abroad). The operating pressure in the urea synthesis tower is 20 MPa, the operating temperature is 185°C, and the working media include urea, ammonium carbamate, ammonia, carbon dioxide, oxygen, nitrogen, water, steam, and other substances. Based on the pattern of corrosion in the urea synthesis tower lining leading to leakage, there are roughly three types: (1) uniform corrosion of the welds ； (2) Pitting corrosion ； (3) Stress corrosion cracks. The corrosion mechanisms of the above three forms of corrosion are briefly described as follows. 1.1 Uniform corrosion of welds: The cause of uniform corrosion in welds is relatively simple; it is mainly due to improper selection of welding electrodes, or inadequate control of welding processes and parameters during welding, which results in the welds having lower corrosion resistance compared to the base material (00Cr17Ni14Mo2 or 00Cr17Ni14Mo2Ti). When factors such as high H2S content in the raw gas CO2 fed into the tower, insufficient addition of compressed air, low NH3/CO2 ratios, high H2O/CO2 ratios, and excessive temperature control are present, it leads to uniform chemical corrosion in the weld area. 1.2 Pitting corrosion: It is a localized form of corrosion in stainless steel materials. Its formation process generally consists of two stages, namely the induction stage and the growth stage. In the first scenario of the induction period, corrosive anions in the urine, such as S2— and CNO—, adsorb onto the inner surface of the lining, destroying the thinnest and most defective passivation film on the metal surface and thus creating activation sites for corrosion ； In another scenario, sulfur present in the stainless steel itself forms conductive pathways; when the surface of the stainless steel is polarized to a potential corresponding to its passive state, the sulfides tend to dissolve. At this point, the fresh metal is exposed to urine, and it loses its passive state, resulting in the formation of active sites. During the development stage, corrosion reactions occur on the metal surfaces inside and outside the activation site: http://www.nmtech.com.cn/jishuwang/upload/060704828073043.jpg At this time, the rate of electrode reactions is the same inside and outside the activation site. As the process progresses, a lack of dissolved oxygen develops inside the activation site where the cathodic reactions take place, which hinders these reactions. As a result, electrons are forced to move to the outer surface of the pore to carry out the cathodic reactions, causing an imbalance between the cations M+ and N as well as the anion OH– within the metal of the pore. To maintain electrical neutrality, anions with high mobility (such as S2— and CNO—) move toward the pores, increasing their concentrations and thus accelerating the corrosion process. On the other hand, the metal ions generated by corrosion are also hindered from diffusing outward, thus accumulating within the pores where they undergo hydrolysis reactions: http://www.nmtech.com.cn/jishuwang/upload/060704828546764.jpg These hydrolysis reactions increase the concentration of H+ ions and anions such as S2— and CNO— within the pores, resulting in the formation of highly corrosive acids (such as polythionic acids H2SxO6 and isocyanic acid), which accelerates the dissolution of the metal and leads to autocatalysis. As the self-accelerating process continues to cycle, the pH value inside the pores keeps dropping, resulting in intense activated corrosion of the inner surface of the pores, while the outer surface remains in a passivated state. This creates a macroscopic cell with a high potential difference between the inside and outside of the pores, thereby increasing the corrosion current and causing the corrosion inside the pores to enter the activated stage. 1.3 Stress corrosion cracks are cracks that develop in depth in metal materials under the combined action of corrosion and tensile stress in specific corrosive environments. Stress corrosion cracks in the lining of urea synthesis towers occur mainly on the back side of the lining. When steam is used for leak detection in such towers, and if the steam contains a high concentration of Cl− ions, the high-temperature steam induces tensile stresses that cause plastic deformation, resulting in \"slip steps\" which lead to the breakdown of the passive film on the metal surface. At this point, a potential difference is created between the exposed new metal surface and the unbroken passive film; the metal surface acts as the cathode area, while the areas around and at the tips of the cracks serve as the anode areas. Corrosion progresses very slowly on these sides due to the inhibition effect of the cathodic solution containing NI in the lining, whereas the tips experience local stress that causes localized deformation and yielding, thereby accelerating the anodic dissolution of the surface atoms. The corrosion grooves formed as a result of this dissolution develop along the slip lines, perpendicular to the tensile stresses, giving rise to fine cracks. Stress concentration at the tips of these cracks, combined with repeated sliding, further accelerates the dissolution process, leading to the further growth of the cracks. 2 Direct causes of corrosion in the urea synthesis tower lining. There are various factors that lead to corrosion and leakage in the lining of the urea synthesis tower. In summary, there are roughly two main reasons: improper manufacturing of the urea synthesis tower lining, and issues related to the operational procedures and management during the use of the urea synthesis tower. 2.1 Reasons related to equipment manufacturing (1) Lack of emphasis on manufacturing quality or inadequate manufacturing techniques during equipment production. During the manufacturing process, the poor fit between the lining of the urea synthesis tower and the tower wall provides ample room for expansion under operating conditions. Therefore, under an operating pressure of 20 MPa, the urea synthesis tower lining deforms due to insufficient mechanical strength. The stress resulting from deformation reduces the stress-corrosion resistance of the lining layer. (2) Poor welding quality. During the manufacturing process or during maintenance of the urea synthesis tower lining, poor welding quality often results from the use of the wrong type of welding rod or inadequate control of the welding process, leading to uniform corrosion of the lining or even leakage. (3) Liner material issue. During the manufacturing or repair process, if the lining material is not carefully selected and an incorrect material is used, or if the sulfur content in the stainless steel is too high, it can lead to severe corrosion and leakage of the lining. Furthermore, the presence of scratches, dents, inclusions, pinholes, and other defects on the surface of the lining material is also a major cause of leakage. 2.2 Reasons related to process management (1) Excessively high H2S content in carbon dioxide. The process requires that the carbon dioxide entering the tower be free of sulfur

The author analyzes and discusses issues related to the urea synthesis tower in a series of articles; readers are invited to point out any inaccuracies or errors. 1 Leak detection system: The urine tower is composed of a carbon steel shell capable of withstanding high pressures, along with a stainless steel or titanium lining that provides corrosion resistance. To protect the carbon steel shell from corrosion caused by leaks from the lining, the urine tower must be equipped with a comprehensive leak detection system. 1.1 Composition of the leak detection system The leak detection system includes leak detection holes on the tower body ； Leak detection pipeline network ； Analysis and testing of leaks ； Appropriate technical specifications and management. 1.2 Function of the leak detection system: The leak detection system enables the timely detection of leaks in the lining (welds and base metal), preventing the defects from worsening and avoiding catastrophic explosions. 1.3 Leak detection holes [1] Leak detection holes consist of leak detection channels on the back side of the lining, as well as small holes that penetrate through the multi-layered sheet cylinder or the single-layer cylinder, beyond the lining. It is more accurate to call the leak detection hole a signal hole. 1.3.1 Leak detection channels: When corrosion penetrates the lining, the highly corrosive substances that leak out can be quickly discharged through the leak detection holes; therefore, it is necessary to have unobstructed leak detection channels on the back side of the lining. It usually takes the following forms. (1) Longitudinal and circumferential arc-shaped grooves with R=2.5 mm are made on the shell cylinder to serve as leak detection channels. (2) The gaps formed by the incomplete welding of the longitudinal and circumferential seams of the carbon steel blind plates on the outside of the lined inner cylinder serve as leak detection channels. (3) The gap left in the shimming plate serves as a leak detection channel. (4) A bowl-shaped vent gasket with a cross-slot serves as the leak detection channel. 1.3.2 Design of leak detection holes: The structure of leak detection holes varies depending on the lining structure and the manufacturer. Generally, there are the following structural forms. (1) Before lining, drill holes in the carbon steel shell, then insert stainless steel fittings and weld their ends; after the welds are polished, place shims, and finally carry out the lining process. (2) After the tube section has been lined, holes of φ50 mm and φ16 mm are drilled respectively in the lining and the outer shell tube, after which a pipe fitting is inserted and welded to the inner tube. After the weld is ground, it is placed on a backing plate and welded to the lining; small holes with a diameter of φ3 mm are drilled in the backing plate, and finally an overlay plate with a diameter of φ80 mm is welded onto the backing plate. (3) Drill leak detection holes in the surfacing layer, and thread the ends of the holes and connect them with tubes. Regardless of the type of leak detection holes used by the manufacturer, it is necessary to ensure that no leaks from the lining can pass through these holes into the layers between them, and that there is no air leakage between the leak detection holes and the ventilation holes. 1.4 Leak detection piping network: The equipment drawings for urine treatment towers in the engineering design include a \"schematic diagram of the leak detection system\"; the final design drawings provided by the manufacturer show the location and elevation dimensions of the leak detection holes, which are used as a guide for the design and installation of the leak detection piping network. The piping installation for the inlet and outlet pipes of the leak detection hole is shown in Figure 1(a). Steam enters through the leak detection holes (fn) at the upper part of the cylinder section ； It leads out from the leak detection hole (gn) beneath the tube section, running closely along the outside of the tower wall so as to be placed inside the insulation layer. All pipes connected to the leak detection tank must be numbered, corresponding one by one to the numbers of the leak detection holes shown in the equipment diagram, with no omissions allowed. Piping without leak detection holes can lead to serious accidents. For example, in a large urea plant, a plug used to seal one leak detection hole on the lower head of a φ2.8 m urea tower was not removed, which led to leaks in the lining that were not detected in time. As a result, the carbon steel shell corroded and was penetrated, causing the material inside the tower to spill out through the resulting hole. http://www.nmtech.com.cn/jishuwang/upload/061225841127275.jpg Figure 1 Schematic diagram of the steam leak detection pipeline network. The connection method (b) has the following disadvantages: (1) The main steam pipe used for leak detection is prone to containing condensate ； (2) When steam is introduced into GN, corrosive substances can flow back into the steam pipes ； (3) The outlet pipe leads from fn, and the leakage cannot be discharged smoothly. The technical specifications for the urea synthesis tower stipulate that \"the ventilation holes must not become blocked after installation on site.\" 1.5 Analysis and testing: A urine tower with a diameter of φ1.4 m and a volume of 41 m3 has an internal lining surface area of approximately 123 m2, and the length of its welds is over 238 m. There is a high likelihood of leakage of highly corrosive media inside the tower through weld defects and defects in the base material. For example, the lining of the top head on a φ2.8 m urine tower in a certain plant experienced 3 leaks within 45 days [2]. It is evident that regular analysis of the outlet material from the leak detection holes is very important. The analysis frequencies and results should be recorded and archived, and reported promptly to the production team → workshop → branch factory → head office management department. It is not advisable to use the olfactory method to determine whether an exhaust contains ammonia, as people who work for long periods in environments with ammonia levels (with a maximum allowable concentration of 30 mg/m3) become insensitive to low concentrations of ammonia, leading to incorrect judgments. Second, high ammonia concentrations in exported products can cause damage to human sense of smell, eyes, and respiratory tract. Accurate data can be obtained using chemical analysis methods or online analyzers. 1.6 Appropriate technical specifications: It is very important to determine which technical specifications should be used to check for leaks in the lining of urine towers. There are two types of technical specifications seen nowadays. First, Article 279 of the \"Safety Technical Regulations for Fertilizer Production\" issued by the former Ministry of Chemical Industry in February 1983 states that the leak detection holes in urea synthesis towers must be inspected regularly to ensure they remain unobstructed; any blockages should be addressed promptly. If ammonia is detected during leak testing, the operation must be stopped immediately; it is strictly prohibited to continue operating despite the issue in order to increase production. These are the technical regulations that have proven effective to date in large and medium-sized urea plants in the country. Secondly, it was published in the safety management section on page 7 of the China Chemical Industry News dated June 19, 2006, in the 13th line from the bottom of the second row from the left: “The difference in ammonia content at the steam inlet and outlet for leak detection... is less than the 0.01% threshold specified in the Urea Technical Regulations.” The urea technical specifications are an abbreviation for the \"Technical Specifications for the Production of 40,000 tons of urea per year in a fully closed-loop aqueous solution system (Trial Version)\\", which were formulated by the Fertilizer Department of the former Ministry of Chemical Industry in November 1990. If the technical specifications for urea indeed include a target of “0.01%”, then it is doubtful whether this specified value is correct; the analysis is as follows. This “0.01%” criterion uses the difference method, meaning that a difference in ammonia content in the steam entering and leaving the leak detection holes of less than 0.01% is considered acceptable. Imagine, where does the 0.01% increase come from? The only source can be a leak from the lining, which indicates that there is a leak in the urine tower’s lining. Is continuous leakage acceptable? Article 279 provides the answer: it is not allowed. 1.7 Management: Managers must strictly implement the provisions of Article 279; if ammonia is detected during leak inspections, the operation must be stopped immediately for handling. If maximizing output is the goal, operating the urine tower while it is faulty is extremely dangerous. Or, in the pursuit of higher output and profits, the idea of replacing a damaged one with a new one is even more harmful. As we all know, a minor repair of the urine tower lining takes approximately 3 to 5 days, involving processes such as shutting down the system for discharge, cooling down, removing the cover, carrying out the repairs, restoring the system to its normal state, heating it up again, and then restarting the system. If operated while faulty, the liner leakage (urea ammonium solution) causes severe corrosion of the shell-side carbon steel at high temperatures (150–188 °C). According to reference [3], the annual corrosion rate of iron in a urea synthesis medium at 150 °C is 11–140 mm. Taking the average value of 75 mm/year, which is equivalent to 0.205 mm/day, it means that the 6-mm-thick carbon steel blind layer of the shell can be corroded through within 1 month ； In another 2 months, it will be possible to use first-layer laminates with a thickness of 12 mm, and this can be gradually extended to multi-layer laminates; this process is invisible and intangible (of course, ammonia, carbon dioxide, and urea can be detected at the outlets of the leak detection ports). Accidents result in longer repair times. The leak detection hole incident at the lower head of the urine tower mentioned earlier took nearly a month to restore production. According to source [4], there have been numerous cases both domestically and internationally where urine towers failed to shut down in a timely manner due to liner leaks, resulting in severe corrosion and damage to the casing, and even rendering the entire equipment unusable; explosions have also occurred in such instances. If the explosion results in direct economic losses of 7.8 million yuan, then the indirect economic (profit and tax) losses from the disruption to production over 3 months amount to over 30 million yuan. There were also numerous casualties among employees, and those who bear direct responsibility will be subject to corresponding penalties. Therefore, neglecting the safe production of urea and relying on luck will inevitably lead to serious problems sooner or later. 2 Steam leak detection 2.1 Background Information: The two φ1300 mm units introduced into China from 1966 to 1980, as well as the dozens of φ1400 mm urine treatment towers built domestically, had shells made by rolling and welding thick plates, with a loose lining used for insulation. Use fresh steam for leak detection. There is steam from power plants or on-site boiler rooms, as well as steam generated as a by-product of gas production processes. At that time, the lining plates and welding materials used often did not meet the requirements for urea handling; coupled with a low degree of adhesion of the lining and operational issues, this led to frequent bulging and leakage of the lining as well as regular maintenance needs. As a result, most plants adopted the measure of having backup towers, which is why there is now the practice of operating two towers simultaneously. There are also individual plants with poor management that operate while faulty, resulting in the destruction of their urine treatment towers. Steam is very effective for cleaning liner leaks and is still widely used to this day. For this type of urine tower, there have been no reports of the tower’s casing being damaged due to leak detection steam. In the early 1980s, the first domestically produced carbon dioxide stripping urine tower (φ2800 mm) was successfully developed in Nanjing. The thickness of the stainless steel plate forming the inner liner cylinder is 8 mm, while the shell is constructed from multiple layers of carbon steel plates; its total weight is 318 tons. Subsequently, this manufacturing technology was widely applied domestically. The temperature of the material entering at the bottom of the stripping urine tower is relatively high. In the carbon dioxide stripping process, the temperature of the gas and liquid streams fed into the urea tower from the high-pressure ammonium methoxide condenser is 167 ℃; as a result, there are no low-temperature areas on the walls of the urea tower. In the event of any leakage from the lining, it can flow out smoothly in liquid or gaseous form through the leak detection holes, allowing such leaks to be detected. Therefore, steam is not required for leak detection in the urea tower. However, in the urea production process using the full-circulation aqueous solution method, the temperature of the material in the three inlet pipes at the bottom of the urea tower is low: the CO2 gas is at 100–120 °C, liquid ammonia is at around 50 °C, and ammonium methoxide is at around 90 °C. The temperatures in the bottom of the tower and in the first section below it are not high either; if there are leaks in the linings of these areas, solid crystals will form, leading to blockages in the leak detection system. Therefore, using steam leak detection is appropriate. 2.2 Influence of steam quality Since the 1960s, the requirement for steam quality in urea production has been that the chloride ion (Cl－) concentration should not exceed 0.5×10－6 [5]. In the steam from most small and medium-sized urea plants, the Cl– content is as high as (5–10)×10‑6. Steam enters the process condensate through the desorption tower and steam ejector, and then flows back into the urea tower via circulation. The Cl– ions that enter along with the steam cause stress corrosion on the inner side of the lining. Steam contamination entering the leak detection system causes stress corrosion on the outside of the lining, and may even lead to cracking [6] ； On the other hand, it also causes stress corrosion damage to the welds, surfacing layers, or threaded connections of the leak detection tubes; once corrosion penetrates, steam enters the gaps between the multiple layers of sheet metal, leading to stress corrosion in the carbon steel plates as well. One of the characteristics of urea production is high temperature and pressure; in addition, the synthesis products and impurities are highly corrosive to stainless steel materials. Therefore, strict requirements are placed on the quality of both raw materials and utility materials, such as the requirement for the Cl― content in steam. To this end, urea plants need to strengthen management to meet the requirements of industrial production. If substandard raw materials and utility services are used in urea production, a range of safety issues related to equipment and process operations will inevitably arise. 3 Stress Corrosion 3.1 Characteristics of Corrosion and Stress Corrosion The degradation of metals and alloys due to the chemical (or electrochemical) action of external media is called corrosion. A special form of failure that occurs in metals and alloys under the combined action of corrosion and residual stress is known as stress corrosion. The characteristic is that no significant corrosion occurs in the absence of stress, and alone, stress does not cause any particular damage either when there is no corrosive environment; such damage only appears when both factors are present and interact with each other. The characteristic of this type of corrosion damage is the formation of cracks, whose depth is several orders of magnitude greater than their width. In relatively long cracks, the two sides meet closely together, and sometimes they may not be visible to the naked eye. The stress that causes this damage is a stress with a fixed direction, but its magnitude is not necessarily fixed. When metal and alloy components already have cracks, stress corrosion will accelerate the expansion of those cracks. 3.2 Corrosive environments: Anhydrous solid urea placed on a dry carbon steel surface does not undergo any chemical reaction; anhydrous liquid ammonia does not corrode carbon steel storage tanks, and carbon steel can also be used for manufacturing liquid chlorine storage tanks. The reason is that the conditions for a chemical reaction do not exist, that is, there is no corrosive environment. However, once water is present, urea, ammonia, and chlorides undergo chemical reactions with carbon steel, forming new substances that alter the composition and chemical properties of the original materials. For carbon steel, it is corrosion; the presence of water creates a corrosive environment (also known as corrosive conditions). 3.3 Stress corrosion and electrochemical corrosion: Solutions formed by urea, methylamine, ammonia, chloride ions, CO2, etc., in water are conductive media that create complex electrochemical cells on metal surfaces. In the case of cracks, once a chemical corrosion cell forms inside such cracks, the metal at the tip of the crack will dissolve rapidly until the crack breaks. As shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/061225842062059.jpg Figure 2 Schematic diagram of crack stress corrosion [7] 3.4 Is there water in the gap area of the carbon steel multi-layer plate on the side of the urea tower shell? The structure and insulation of the urine tower cylinder are shown in Figure 3. http://www.nmtech.com.cn/jishuwang/upload/061225842527331.jpg Figure 3 Schematic diagram of temperature distribution in the urea synthesis tower; 1—Lining, δ=8mm ； 2—Blind layer thickness δ=6mm ； 3—First layer thickness δ=12mm ； 4—Multi-layer laminate δ=6xn ； 5—Ventilation holes ； ti—Temperature inside the tower ； tf—outside wall temperature of the tower ； ts – Temperature of the insulation layer’s outer surface; 50°C. 3.4.1 The temperature of the exterior wall of the urea tower, tf, is calculated using the heat transfer formulas related to the insulation layer and the tower wall; this temperature is only about 0.15°C lower than the temperature inside the tower, ti. Within the lower head of the urine tower’s low-temperature zone and the first cylinder section counted from the bottom. 3.4.2 Steam condensation conditions in the gap area (1) In the low-temperature section of the tower, the temperature of the medium may be below 100 ℃; the leak detection steam that enters the gap area will result in steam condensate. (2) When the vent hole is blocked and the gap area is not in communication with the ambient atmosphere, the pressure of the leak detection steam that enters the gap area gradually increases. When this pressure exceeds the saturated vapor pressure corresponding to that tiF, vapor condensation also occurs. (3) When the vent hole is in good communication with the ambient atmosphere, the leaked leak detection steam will be discharged through the vent hole; the pressure is slightly higher than atmospheric pressure, and the steam in the gap area is in a superheated state, so no steam condensate is formed. 3.5 Hazards of impurities in steam: The Cl― in steam is actually NaCl and KCl. When chemical soft water is produced by the ammonium ion exchange method, the steam contains NH3, CO2, SO42― (H2SO4), and Cl― (HCl), as well as trace amounts of Ca, Mg, Si salts, and so on. These impurities deposit in solid form in the interstitial areas; when dissolved in vapor condensate, they act as conductive media, forming chemical cells on the metal surface that cause stress corrosion. 3.6 Paths for steam to enter the gap areas: (1) When the leak detection holes are of welded construction, if the lining leaks and the carbon steel blind plate layer as well as the first layer of plates are locally corroded and penetrated, steam enters various gap areas through these penetration holes via the ventilation holes. This situation only occurs at the leak detection holes of the tube section leak detection outlet. (2) When the leak detection hole adopts a threaded structure, high-temperature steam under pressure will pass through the pipe threads and enter the gap area. As is well known, gap corrosion occurs in the meshing parts of threads when threaded connections are used. It is incorrect to use a threaded structure for the leak detection hole in urine towers. 3.7 Circumferential crack stress corrosion: During the wrapping process, the laminates of each section of the urea tower may be damaged by steel wire ropes; as a result of heat effects during the welding of the circumferential joints, as well as internal and external pressures acting on the tower, many circumferential discontinuous cracks can form on the inner and outer surfaces of the laminates. It is difficult to measure the degree of stress corrosion suffered by cracks in low-temperature areas. Based on the fracture pattern, stress corrosion cracking is inferred to be just one \"possible\" cause of explosive fracture. It is a hypothesis. The actual situation requires more in-depth on-site investigations and evidence collection. 4 Countermeasures: (1) If ammonia is detected in the discharge from the leak detection port of the urine tower (or if the amount of ammonia increases), it indicates a leak in the lining; the plant should be stopped immediately, and the procedures specified in Article 279 shall be followed. (2) For urine towers with threaded connection for leak detection, except for the leak detection holes on the lower head that remain open to steam, all the leak detection holes on the cylinder body and the manway covers on the upper head are not open to steam, nor to air (which is very dangerous) or nitrogen. The leak detection inlet pipe is rerouted along the outer wall of the urine tower and laid in Qingdao to lead to the collection platform at the base of the tower. (3) Improve the synthesis process by reverting to the 196 Lutianhua urine tower design, whereby the urine tower consists of a first reactor and a second reactor. CO2, NH3, and ammonium methanate solution undergo the ammonium methanate formation reaction in the first reactor, where the exit temperature reaches around 175 °C, after which they proceed to the second reactor. As a result, the temperature of the medium in the lower head of the second reactor reaches 175 ℃, and the temperature of the entire tower wall is above 175 ℃, which is higher than the melting point of urea at 132.7 ℃. In this way, as mentioned earlier, the leak detection holes on the lower cover of the urine tower, which are connected using threads, can also be kept free of steam. As shown in Figure 4. http://www.nmtech.com.cn/jishuwang/upload/061225843457969.jpg Figure 4 Schematic diagram of the improved synthesis process. The second reactor can be designed as vertical or horizontal; it should be made of urea-grade stainless steel, with sufficient corrosion resistance. [References] [1] Mei Anhua. Production Technology of Small-Scale Urea Plants [M]. Beijing: Beijing Science and Technology Press, 1992, 179. [2] Wu Tianping, Jiang Chengguang. Large Nitrogen Fertilizers, 2000, (3). [3] I. Wilenitz. Equipment Structure Materials in Urea Plants [M]. Translations on Chemical Fertilizers, Volume 6, 1962, 66. [4] Research Institute of Chemical Machinery, Ministry of Chemical Industry. Handbook on Corrosion and Anti-Corrosion [M]. Beijing: Chemical Industry Press, 1991, 64. [5] Fourth Design Institute of the Ministry of Chemical Industry. Operation Manual for Urea Plants with an Annual Production Capacity of 40,000 Tons Using a Fully Circulating Aqueous Solution Process [M]. Wuhan: 1990, 2. [6] Handbook on Nitrogen Fertilizer Process Design·Urea [M], China Wuhan Chemical Engineering Company (Fourth Design Institute of the Ministry of Chemical Industry). Beijing: Chemical Industry Press, 1988, 379. [7] Zuo Jingyi. Stress Corrosion Cracking, Corrosion and Anti-Corrosion [M]. Beijing: China Industrial Press, 1965, 21.

1 Overview The urea synthesis tower is a key piece of equipment in a urea production plant. Its operating conditions are severe, with operational pressures reaching up to 15.1 MPa, and the medium involved is highly corrosive. To ensure that the urea synthesis tower can withstand high pressures and corrosion while also reducing equipment costs, Jianfeng Plant adopted a design featuring a carbon steel shell with a stainless steel lining; the material used for the pressure-bearing shell is 20MnMoNi4.5, while the corrosion-resistant layer is made of SUS 316L (urea grade). The relative positioning of the lining within the cylinder is ensured by welding the root of each weld to the cylinder. To prevent direct welding between the lining and the carbon steel, a transition layer with a width of 60 mm and a thickness of 3 mm is welded at the corresponding positions on the cylinder sections (using ER309L as the welding material), thereby ensuring that the quality of the lining welds and their corrosion resistance are not affected. To check for leaks in the lining during operation, holes with a diameter of 3 mm were made in the surfacing layer ； A circular groove 1 mm deep (i.e., the leak detection channel) leads outside the housing through a leak detection tube; its partial structure is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0605181605302692.jpg On June 27, 2000, during a inspection, it was found that there were slight amounts of yellowish-white crystals in leak detection tube No. 29, located at the lowest section of the urea synthesis tower. According to the leak detection and channel layout diagram (see Figure 2), leak detection tubes No. 29, 30, 59, and 60 are interconnected. Upon checking the other three leak detection tubes in that section, it was discovered that tube No. 60 also had a minor ammonia leak. According to traditional methods, in the event of a leak, it is necessary to shut down the plant, discharge and replace the fluid, inspect the lining, and carry out repairs; however, this shutdown takes at least 5 to 6 days, having a severe impact on production. Given that the amount of leakage at that time was small, and considering the lesson learned at the Jianfeng plant where no leak could be detected after the stripping tower was shut down, careful consideration was given to using medium-pressure flushing water to clean the leak detection channels in order to reduce corrosion. At the same time, measures such as monitoring the trend of leak expansion and measuring the wall thickness around the leak detection pipes were taken to maintain production operations, with good results achieved. While performing flushing for leak detection, we also used the concentration of leaks at various flushing points to determine the location of the leaks, providing a basis for rapid inspection and repair after the plant is shut down. http://www.nmtech.com.cn/jishuwang/upload/0605181606167396.jpg 2 Treatment Measures 2.1 Selection and Basis for Treatment Plans The process medium in the urea synthesis tower is highly corrosive and prone to crystallization; moreover, the tower is lined with a loose lining, allowing the urea-based medium to easily penetrate into the gap between the lining and the cylinder, thereby causing corrosion of the carbon steel cylinder beneath the lining. Consider adopting water flushing or high-pressure medium plugging solutions. The water flushing method involves using pressure flushing water from the urea zone to enter through one or several leak detection ports, and then exit through another one or several leak detection holes, thereby dissolving and removing the leaks ； The high-pressure medium plugging solution involves introducing an inert medium at a pressure slightly higher than that of the synthesis tower, causing it to flow into the tower at the leakage point; this thereby seals the leak and prevents the process medium from continuing to leak out. After comparison, it was decided to use the water flushing method. Compared with the high-pressure medium plugging method, the water flushing method has the following advantages: (1) It is relatively safe and reliable. As can be seen from the structural characteristics of the lining in Figure 1, if there is a leak in the lining welds, the leaked material first enters the interface between the lining and the stainless steel surfacing layer, and then penetrates into the leak detection channel. Since the leak detection channel has a relatively large volume (the lining is in close contact with the cylinder body under pressure), the leaked material is relieved of pressure here and is discharged out of the housing through the leak detection channel and pipes. When the amount of leakage is small, the possibility of it diffusing to the carbon steel section through the transition layer is very low. Therefore, if the leakage can be guided out of the housing in a timely manner, the possibility of the leakage corroding the housing is very low. If the high-pressure medium plugging method is used, a medium at a pressure higher than that of the synthesis tower is injected into the leakage path; if not handled properly, excessive pressure or other factors that result in high backpressure can cause the leakage site to expand or the welds to tear, and may even lead to deformation of the lining, hence it involves certain risks ； (2) By using the flushing method, it is possible to determine the extent of leakage spread by measuring the amount of leakage that is flushed out, which facilitates identifying the appropriate time to shut down the system ； (3) It is possible to determine whether the leakage path is blocked; if it is blocked, the leaked material will seep into the gap between the lining and the cylinder, causing corrosion and leading to serious consequences. At the same time, water flushing itself can prevent blockages ； (4) The location of the leak can be determined with high accuracy, reducing inspection time ； (5) The flushing water can also dilute corrosive substances, thereby reducing their corrosiveness. For safety reasons, while flushing with water, online thickness measurement is also carried out on the carbon steel shell near the transition layer to monitor whether the cylinder has been corroded. 2.2 Treatment measures (1) Water flushing: The water flushing device is shown in Figure 3. The medium-pressure flushing water from the urea zone enters through leak detection ports No. 30, 59, and 60; the pressure is adjusted using valves and pressure gauges installed upstream to maintain a stable flow rate of water. The water that enters passes through the leak detection channels and is discharged via leak detection pipe No. 29. A water cooler is installed at the outlet of pipe No. 29 to cool the washed wastewater to below 40°C, thereby reducing losses of ammonia and carbon dioxide; regular sampling of the wastewater is also carried out for analysis. The purpose of this design is that, as the No. 29 leak detection tube is the closest to the leakage point, it can act as an outlet to expel the leaked material from the housing as quickly as possible, thereby preventing the leakage material from entering the interlayer and causing corrosion of the cylinder ； The remaining 3 openings serve as inlets, which prevent leaks from penetrating into these areas and forming stagnant zones that could cause blockages in the passages ； By analyzing the discharged liquid sample, it is possible to determine the extent of leakage spread and the degree of corrosion. http://www.nmtech.com.cn/jishuwang/upload/0605181606564380.jpg Water flushing procedure: Open the inlet valves of leak detection tubes No. 30, 59, and 60, as well as the outlet valve of leak detection tube No. 29; open the inlet and outlet valves of the water cooler. Activate the medium-pressure flushing water valve, and adjust the flow rate to around 1.2 liters per minute. Take samples every 2 hours to analyze the levels of urea, ammonia, carbon dioxide, and iron in the discharged liquid. Ensure that the chloride ion concentration in the flushing water does not exceed 20×10—6, in order to prevent stress corrosion cracks in the lining welds. The analysis results are shown in Table 1, and it can be seen from Table 1 that the leakage rate increases over time ； That is, the leakage point expands over time, and this expansion is sudden. If the leakage volume increases suddenly from July 21–24 and July 30–August 2, while the iron content remains low and does not increase, it indicates that the flushing is effective and the equipment has not been corroded. http://www.nmtech.com.cn/jishuwang/upload/0605181607372871.jpg (2) Online thickness measurement: A high-temperature probe is used to measure the thickness of the shell at the edge of the transition layer (the thickness measurement points are located about 25 mm away from the leak detection channel, arranged parallel to this channel at intervals of 200 mm; see Figure 2 for the layout). This measurement is carried out once a week, and the changes in thickness are used to determine whether the shell has been corroded. The data measured on June 30 and August 2 are shown in Table 2. As can be seen from Table 2, the thickness hardly changed during the urea synthesis tower leak, indicating that the carbon steel shell was not corroded. http://www.nmtech.com.cn/jishuwang/upload/0605181608091763.jpg3 Determination of the location of leakage points 3.1 Preliminary determination of the leakage point location Based on an analysis of the phenomena, detector No. 29 had the highest leakage rate, while detector No. 60 showed minor leakage; no leakage was detected in the other two detectors. Based on Figure 4, it can be preliminarily inferred that the leak site is most likely to be within the weld seam range of 29–60, but the possibility of it being within 29–59 cannot be ruled out either (for example, if the leak detection channel in the 29–59 range becomes blocked, the leakage may also occur at inspection tube No. 29). 3.2 Determining the location of the leak by changing the flushing direction: Water is introduced into test tube No. 30; the inlet valves for test tubes No. 60 and No. 59 are closed, while the corresponding drain valves are opened. Samples of the water from test tubes No. 60, No. 59, and No. 29 are taken for analysis, and the results are shown in Table 3. As can be seen from the data, water flows out of each leak detection tube, indicating that the leak detection channels are unobstructed. Since leak detection tube No. 60 is closest to tube No. 30, it has the highest amount of water flow, whereas tube No. 59 has the lowest amount of water flow. Since only leak detection tube No. 29 contains urea, analysis based on Figure 4 indicates that the leakage point should be located between leak detection tubes No. 60, 29, and 59. http://www.nmtech.com.cn/jishuwang/upload/0605181608455015.jpg Similarly, the data obtained from analyzing water taken from detectors No. 30, 59, and 29, using water introduced through detector No. 60 for testing, are shown in Table 3. As analyzed from Figure 4, the leak location is between the 60th and 29th leak detection tubes. http://www.nmtech.com.cn/jishuwang/upload/0605181609244805.jpg Based on the above analysis, the leakage point should be between detector No. 60 and No. 29, and it is located near the longitudinal weld of detector No. 29 (as detector No. 29 has the highest leakage rate, while detector No. 60 only shows slight leakage). 4 Treatment effects: (1) The leakage started on June 27, and the unit continued to operate until August 13; it was then shut down for major repairs for other reasons, a process that lasted 1.5 months. (2) During the major repair, an ammonia leakage inspection was carried out on the urea synthesis tower, and the leak was found to be located at the upper end of the longitudinal weld of the lining, about 200 mm away from detector No. 29 (see Figure 1), which is generally consistent with the previous assessment. (3) The equipment has not suffered from corrosion; online thickness measurement and water flushing analysis have confirmed that the cylinder is free from corrosion. As a precaution, the area around the leakage site was inspected during the major repair, and neither tapping the lining with a hammer nor using a magnetic detector revealed any localized corrosion in the carbon steel casing. 5 Conclusion (1) Based on Jianfeng Plant’s experience in dealing with leaks in high-pressure urea processing equipment, when the leak volume is small, it is not advisable to shut down the equipment immediately; otherwise, it will be impossible to locate the source of the leak after shutdown, leading to significant losses. (2) When the leakage from urea high-pressure equipment is small, the water flushing method is an effective way to temporarily maintain operation and determine the appropriate timing for shutdown.

0 General Introduction Corrosion and thinning of the lining in urea synthesis towers are among the main factors that threaten their proper operation, and the primary method for repairing such defects is to replace the lining. For replacing the lining of an entire tower, the construction period is generally long. However, due to production requirements, the maintenance periods available at various manufacturing plants are short. How to replace the lining of the entire tower within these limited maintenance periods in order to resolve the issue once and for all is a concern for all such plants. In response to this, our company has adopted a new method for replacing the entire tower lining; this method was applied in the project of replacing the lining of the urea synthesis tower at Jinxi Natural Gas Chemical Co., Ltd., and achieved very good results. It is introduced as follows. http://www.nmtech.com.cn/jishuwang/upload/0705141500356891.jpg Figure 1: Schematic diagram of the urea tower. 1. General information on the equipment: The urea synthesis tower of Jinxi Natural Gas Chemical Co., Ltd. was manufactured by the Spanish company DELAPENA in 1990. It has a single-layer cylinder structure, as shown in Figure 1, with specifications of Φ2150×78×40300 mm. The cylinder thickness is 78 mm, and the material used is 20MnMoNi45. The cylinder consists of 15 sections from top to bottom; the first section is 2691 mm long, sections 2 to 14 are each 2686 mm long, and the 15th section is also 2691 mm long. The liner material for the cylinder is 316L (mod), with a thickness of 5 mm. The dimensions of the liner for the topmost and bottommost sections are 2683 mm, while the liner dimensions for the 13 intermediate sections are all 2678 mm. The upper and lower end caps are spherical caps; the thickness of the base material is 40 mm, with a radius of R=1085 mm. The material used in both is 20MnMoNi45. The lining material for the upper and lower end caps is 316L (mod), with a thickness of 5 mm. The manhole cover, manhole stub, upper and lower head segments, the lower head plate, and all 15 sections of the cylinder body each form an independent leak detection system. 2 Equipment defects and maintenance plan: This equipment has been in operation since 1992. Inspections have revealed that the lining has thinned significantly due to corrosion, and leaks have occurred, posing a threat to normal production. Therefore, Jinxi Natural Gas Chemical Co., Ltd. decided to replace the entire lining of the urea synthesis tower during the major maintenance in 2006. The lining replacement was carried out by Dalian Songhai Petrochemical Maintenance Engineering Co., Ltd. There are two alternative approaches: one is to cut the head, and the other is to cut the circumferential weld of the cylinder. A comparison of the main conditions for the two options is shown in Table 1. As can be seen from Table 1 on http://www.nmtech.com.cn/jishuwang/upload/0705141501122949.jpg, the approaches of replacing the circumferential welds of the cylinder shell and replacing the head through cutting each have their own advantages and disadvantages. However, due to the short duration of this maintenance period, if the approach of cutting off the head and replacing the lining is adopted, the construction time will be around 45 days. Due to production requirements, such a long maintenance period is not allowed; Jinxī Natural Gas Chemical Co., Ltd. requires that the maintenance time be kept within 38 days ； Another issue is that this urine tower lacks an overall framework; the original framework structure is located at 18 meters height on the tower, and in either case, a framework needs to be constructed. After analysis, the construction plan adopted was to cut the tower at the ring welds of the eighth and ninth section cylinders, place the upper section of the tower upside down on a ground fixture, carry out lining replacement on both sections simultaneously, then restore the upper section of the tower and weld the ring welds together. After passing 100% RT and UT inspections, local stress-relief heat treatment was applied to the ring welds, and finally the internal tray was installed. This solution meets the requirements for construction quality. Its main advantage is that when removing the old lining and installing the new one, one work station is converted into two, allowing work to be carried out simultaneously; this reduces the time required by nearly half. By using this approach, our company was able to complete the maintenance project within 35 days, thus meeting the factory’s requirements. 3 Construction Steps 3.1 Cutting of the upper tower section (1) The workers determine the cutting line for the circumferential weld of the cylinder outside the tower, mark the positions of the holes used for the cutting process, and drill a cutting hole with a diameter of φ8mm; the center line of this hole lies in the same plane as the center line of the cylinder. Orientation marks and inspection lines are marked on the two sections of the cylinder respectively to ensure that the upper section of the tower can be reinstalled in its original orientation. (2) The workers enter the tower, draw circumferential reference lines using the cutting holes as a basis, and use carbon arc gas gouging to remove the welds of the corrosion-resistant layer at the joints of the cylinder lining along these reference lines. They then remove the stainless steel surfacing layer from the circumferential welds of the cylinder until the welds of the carbon steel layer are exposed. (3) With the assistance of a crane, the cylinder is cut along the circumferential weld using a magnetic cutting machine. The upper section of the tower is then lifted by the crane and placed on a stack of sleepers. While Party A prepares for the inversion of the tower, Party B’s workers remove all the internal trays and overflow pipes from the upper section of the tower. Subsequently, Party A’s lifting crew inverts the upper section of the tower and fixes it on the prepared mounting frame, as shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0705141501547491.jpg Figure 2: Schematic diagram after the cylinder is cut. (4) Install the two sections of the tower construction platforms and the top beams, and install and test the lifting equipment used for construction on the tower, such as electric hoists. (5) Party A shall erect wind and rain shelters above and around the tops of the two towers. 3.2 Removing the old lining, clearing the leak detection system, and installing the new lining: (1) Work on both sections of the urine tower simultaneously, removing the old lining from top to bottom. Using the movable platform as a construction platform, an air gouge is employed to cut through the old lining along its longitudinal and circumferential welds, and each section of the old lining is cut into two pieces and lifted out of the tower. When removing the old lining using carbon arc gouging, care must be taken to avoid damaging the carbon steel shell. When removing the lower head lining, first take measures to prevent debris from falling into the pipe. After removal, each material pipe outlet should be covered promptly to prevent debris from falling into the pipes. When removing the upper head, manway cover, and manway spool lining, care must be taken to avoid damaging the manway sealing surface and high-pressure bolts. (2) In this urine tower, each major section of the carbon steel cylinder constitutes a leak detection system; during the construction process, the existing leak detection systems need to be cleared, and the leak detection grooves need to be cleaned and polished ； Grind the longitudinal and circumferential leak detection grooves to make them communicate with the leak detection holes; the distribution of the leak detection grooves remains as in the original design. The dimensions of the leak detection tank are the same as those in the original design. Each leak detection hole is tested with compressed air; those that are blocked are cleared, and the result is confirmed by the factory. (3) Construct the two sections of the urine tower simultaneously, assembling the new lining from top to bottom. During original manufacturing, the lining was directly welded to the surfacing layer of the cylinder; for this replacement with a new lining, the structure remains the same as the original design, and the longitudinal and circumferential weld patterns are shown in Figure 3. To ensure a good fit, the parts are held in place with fixtures before welding, and welding is carried out strictly in accordance with the \"Welding Procedure Specification\". http://www.nmtech.com.cn/jishuwang/upload/0705141502313476.jpg Figure 3: Schematic diagram of the tower section. 3.3 Resetting of the upper tower section: (1) The grooves in the upper and lower sections of the tower are created through mechanical processing; the shape of these grooves is shown in Figure 3. Before groove machining, the groove surface is first surfaced with two layers of J557RH weld metal, about 2 mm per layer. After welding, the groove and root margin are machined according to the requirements shown in the diagram. Preheating is required before welding, and degassing is performed after welding. Welding shall be carried out in accordance with the Welding Procedure Specification. 100% MT inspection shall be conducted on the surfaces before groove surfacing and after processing, and the results must be satisfactory. Groove processing should be based on the inspection line to ensure levelness, thereby guaranteeing the straightness of the upper tower section during repositioning. (2) Remove the electric hoist and its crossbeam, as well as the platform at the top of the inverted section of the tower. Use a crane to lift the upper section of the tower out, and place the inverted section on a pile of sleepers. Once the preparation work for lifting the tower back into position is complete, use a crane to lift the upper section of the tower and align it in its original position. Before fixing the upper and lower sections together, use two theodolites positioned at the base of the tower to measure the straightness of the tower at a 90° angle. Once the straightness is within the standard range, fix the sections in place. Measure again using the theodolites; if the straightness of the upper section is still within the standard range, proceed with preheating the welds ； If the straightness exceeds the standard, cut the solder joints and re-align them before soldering them again. 3.4 Welding, Heat Treatment, and Inspection (1) After the upper section of the tower is fixed in place, preheating and welding are carried out in accordance with the requirements of the Welding Procedure Specification. Welding is completed in one go; hydrogen removal is carried out after welding, followed by 100% RT and 100% UT inspections. If defects are found during weld inspection, a repair plan is formulated based on the specific nature of those defects, so as to repair them until they meet the required standards. (2) After passing the inspection, the weld is preheated and a surfacing layer is applied. Following a PT inspection showing no defects, heat treatment is carried out on the circumferential weld using the same heat treatment process as originally used during manufacturing; the treatment temperature is 600±15°C. After heat treatment, a 100% UT inspection is conducted on the circumferential weld, and a 100% MT inspection is performed on the weld surface. Once no defects are detected, a corrosion-resistant layer is welded on, and a 100% PT inspection is carried out on all the welds of the new lining of the tower. 3.5 Hydrostatic testing and ammonia permeation testing: In accordance with the relevant provisions of the \"Regulations on Safety Supervision of Pressure Vessels\", a hydrostatic test is carried out on the urea synthesis tower, with supervision by a boiler inspection agency. Conduct an ammonia permeation test on the urea synthesis tower in accordance with Method A specified in Appendix A of HG20584-98. After the hydrostatic pressure and ammonia permeation tests are completed, remove all debris from inside the tower ； PT inspection was conducted on all lining welds, meeting Grade I requirements per JB/T4730.5-2005 ； The plate reserved for each tray layer is reinstalled, submitted to the manufacturer for inspection, and the project is completed. 4 Conclusion All quality parameters related to the complete lining replacement of this urea synthesis tower meet the relevant standards. By adopting a method that involves cutting the circumferential welds of the cylinder, inverting the upper section of the tower, and carrying out construction on both sections simultaneously, the maintenance work was completed within 35 days, thus ensuring adherence to the scheduled timeline. Since its operation began in September 2006, the tower has been functioning well without any issues.