Corrosion and control of the urea double-evaporation heater

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Corrosion and Control of the Urea Double Evaporation Heater 1.1 Equipment Overview? The double evaporation heater (abbreviated as double-evaporation heater) is a key device in the urea evaporation system; its function is to further evaporate and concentrate the urine with a concentration of 95% from the first stage to 99.7%, thereby meeting the requirements of the urea granulation process. It is a typical shell-and-tube heat exchanger, and its technical specifications are shown in Table 1. The upper and lower tube sheets of this heater are made of 1Cr18Ni9Ti, with a thickness of 26 mm ； The tube material is 0Cr18Ni12Mo2Ti, with specifications of φ25×2mm and a length of 2.5m; there are 99 tubes in total ； The tube sheet and tubes are welded using strength welding, with electrode grade A207 ； The shell material is Q235-A, with a thickness of δ=6mm ； The material of the outlet pipe at the back of the shrimp-shaped component is 1Cr18Ni9Ti, with specifications of φ377×5mm ； The shell steam inlet pipe is 250 mm from the upper tube sheet. Table 1: Technical specifications of the double-evaporation heater. http://www.nmtech.com.cn/jishuwang/upload/0602161054522543.jpg? During operation, urine with a concentration of 95% and a temperature of around 120°C enters the tubes from the bottom of the heater. At the boiling point, it is heated by the steam outside the tubes (under normal conditions, the steam temperature is between 220–250°C to ensure an adequate temperature difference for heat transfer), causing the water in the urine to vaporize rapidly. Driven by the upward flow of steam, the urine rises rapidly along the inner wall of the tubes in the form of a thin film. The urine ejected at high speed from the top of the tube enters the secondary vaporization separator through the outlet tube located on the back of the shrimp (hereinafter referred to as the outlet tube), where vapor-liquid separation takes place, resulting in urine with a concentration of 99.7%. 1.2 Condition of equipment corrosion? According to statistics, from 1995 to 2002, Hunan Jinxin Chemical Company used and replaced a total of 7 such two-stage heaters over those 7 years; the longest interval between replacements was 1 year and 7 months, while the shortest was 1 year. At the same time, 5 heater outlet tubes were replaced. The reason for the replacement was severe leakage caused by equipment corrosion, which affected the operation of the urea production system. The basic situation of corrosion in the double-evaporation heater is as follows. (1) Corrosion of the upper tube sheet and tube sheet welds. According to the maintenance records, the reason for replacing the secondary vapor heater each time was corrosion and leakage caused by pores and microcracks in the weld areas where the upper tube sheet met the tube bundles, and each unit was repaired multiple times before its annual major overhaul. For the 3 heaters replaced before 1998, weld leakage occurred only in the local tube welds each time, whereas the situation was more severe for the 4 heaters replaced after 1998; almost every time repairs were carried out, 15%–20% or even 30% of the welds were found to be leaking. When using condensate to detect leaks, the leakage site appears to be gushing like a fountain; the weld surface is rough, with severe pitting, pinholes, and cracks. ? (2) Corrosion of the upper tube sheet. The tube sheet material in contact with the urine medium is 1Cr18Ni9Ti, and it experiences significant corrosion-induced thinning. The heater installed during the major overhaul in 1998 was replaced again during the major overhaul in 1999, and thickness testing of the upper tube sheet revealed a corrosion rate as high as 2 mm/year. ? (3) Corrosion of the urine outlet tube. From 1995 to 2000, the material of the 3 replaced outlet pipes was 1Cr18Ni9Ti, with δ=9mm. According to the thickness measurement checks, the corrosion rate is also astonishing. For example, when the export pipe used in 1997 was replaced for thickness measurement in 2000, the thinnest portion was 4.2 mm thick, with a corrosion rate as high as 1.79 mm/year; whereas the \"Process Design Manual for Nitrogen Fertilizers (Urea Volume)\」 recommends a corrosion rate of only 0.15 mm/year for the second-stage evaporator. There is clearly a huge difference between these two values. Furthermore, leakage at the four welds of the outlet pipe was also severe; holes often appeared in these welds. Upon inspection of the replaced outlet pipe, its inner surface appeared silver-gray and shiny due to urine erosion, and the welds had poor formation, with obvious slag inclusions and under-welding. ? (4) Tube corrosion at the steam inlet pipe. During the inspection, it was also found that some of the tubes within 200 mm of the upper tube sheet, on the side near the steam inlet, showed thinning and cracking. For example, on September 18, 2001, the moisture content of the finished urea was too high; during a temporary shutdown of the system to identify leaks, maintenance work was carried out on the second vaporizer, and it was found that two of its tubes had ruptured, allowing large amounts of steam to mix into the urea solution. This type of situation has occurred 4 times in total. ? Since the above-mentioned corrosion and leakage issues frequently occur after each heater has been replaced and operates for up to 7 months, and the shortest interval between maintenance tasks is just one month, these problems not only lead to a decrease in the vacuum level in the second stage but also cause a significant increase in moisture content in the urea product (for example, in August 2000, due to a leak in the secondary vaporizer, the quality rate of first-grade urea was only 76%). Additionally, each short-term shutdown for repairs takes approximately 3–5 hours, resulting in a loss of 50–85 tons of urea production. Frequent corrosion-related leaks result in significant economic losses; it is estimated that the annual economic loss due to leaks in the secondary vapor heaters amounts to 150,000 yuan, and this figure rises to 300,000 yuan when the cost of replacing the equipment each year is taken into account. ? 2 Analysis of corrosion causes? 2.1 Improper material selection – the tube sheet and outlet pipe lack corrosion resistance? In the original design, the tubes of the secondary vapor heater that were in contact with the highly corrosive urine medium were made of 316 steel, namely 0Cr18Ni12Mo2Ti; however, for economic reasons, the upper and lower tube sheets as well as the outlet pipe of this equipment were made of 1Cr18Ni9Ti. In 316 steel (0Cr18Ni12Mo2Ti), Mo and Ni enhance the steel’s corrosion resistance. A content of 1.8% to 2.5% Mo in the steel improves its resistance to corrosion in urine-based media; in particular, it boosts resistance to pitting and crevice corrosion, a benefit that is especially significant in reducing media such as urine. Studies have shown that adding 2% Mo to stainless steel can reduce the tendency of the steel to become sensitized, as well as protect it from corrosion caused by low oxygen levels in urine. 316 steel has a Ni content that is 3% to 5% higher than that of 18-8 steel; such a high level of Ni enables the steel to be single-phase austenitic, preventing the formation of too many galvanic cells in the multi-phase structure and thus avoiding a decrease in its stability ； At the same time, it can reduce the solubility of carbon in austenite, thereby enhancing resistance to intergranular corrosion. Therefore, 316 steel is a suitable material for resisting the highly corrosive medium of urea. Compared with 316 steel, 1Cr18Ni9Ti has a higher carbon content; aside from the disadvantages associated with Mo and Ni, this high carbon content makes it highly susceptible to intergranular corrosion under long-term operation at high temperatures, resulting in severe corrosion of the tube sheet and its welds, as well as the urine outlet pipe. Therefore, it is inappropriate to use 1Cr18Ni9Ti steel, which has poor corrosion resistance, for the tube sheet and outlet tubes. ? 2.2 The operating conditions are harsh, resulting in severe corrosion at the joints between the upper tube sheet and the tubes. The most common problem with double-effect heaters is leakage at these joints; in addition to factors such as material quality, poor operating conditions also play an important role. Due to the special nature of the welded structure between the tubes and the tube sheet, as well as the excessive penetration at the tube sheet joints during welding, urine flowing rapidly along the tube walls expands suddenly at the outlet, which can cause erosion of the edge welds. When the welding process control for the welds between the upper tube sheet and the tubes is poor, resulting in high temperature difference stresses, microcracks can easily form on the surface of these welds. In addition, the oxygen content in urine in secondary evaporators is relatively low (around 0.45×10-6). When the oxygen trapped within these cracks is exhausted and no fresh oxygen is supplied, crevice corrosion can occur. The temperature difference at the upper part of the tube bundle is significant; since the steam inlet is located at the upper part of the tube side, the water in the urine near the top of the tubes will boil and turn into steam. This steam rises upward, resulting in intense vapor-liquid turbulence at the outlet of the tube bundle together with the urine flowing along the tube walls. Moreover, with a narrow distance between the tubes (only 7 mm), turbulent interactions between the ammonia liquid and steam flows also occur easily. At this point, interference shock phenomena occur in the stagnation region of the upper tube sheet weld. Furthermore, the high temperature at the tube sheet (200–300°C) combined with a low oxygen content in the medium leads to stress corrosion at the welds between the upper tube sheet and the tubes, under conditions of high temperature and low oxygen. ? In contrast, at the lower tube sheet, the temperature difference for heat transfer due to condensate is small, so the aforementioned corrosion does not occur there. ? 2.3 High production load, severe erosion and corrosion of the outlet pipe? The design capacity of our company’s urea production unit is 110 kt/a. Since the late 1990s, annual production has exceeded the designed capacity. In 2000, the system produced a total of 137,145 tons of urea, with a maximum daily output of 512 tons and an average daily output of 405 tons. Due to the equipment operating under high load for extended periods, resulting in high input-output volumes and high evaporation gas velocities, severe erosion and corrosion occur in the outlet pipes and welds. Additionally, since the material used is 1Cr18Ni9Ti, which has low corrosion resistance, the wall thickness of the outlet pipes decreases significantly, and leaks at the welds occur frequently. 2.4 Poor manufacturing quality and poor welding quality? The four circumferential welds on the back of the export tube shrimp are cut using gas gouging during fabrication, resulting in a large heat-affected zone. Moreover, during assembly welding, gaps at the weld joints are inevitably of varying sizes and may be misaligned; coupled with the lack of diligence on the part of the welder, slag inclusions and incomplete welding can occur, leading to leaks at the welds during use. During the welding of the tubes and tube sheets, poor control over the welding process led to excessive wire energy input, resulting in poor welding quality. Defects such as pores, slag inclusions, undercuts, and lack of fusion that occur during welding can all accelerate local corrosion. Furthermore, the improper selection of welding materials results in poor corrosion resistance of A207 electrodes in certain environments. ? 2.5 Improper location of the steam inlet? The cracking of a few tubes within 250 mm below the upper tube sheet is also caused by various factors. In addition to erosion corrosion and poor welding quality (poor metallurgical quality of some tubes with high impurity content), another important reason is the high temperature at this surface, which results in the highest urine concentration and the lowest partial pressure in the gas phase; this leads to a reduction in wall thickness near the outlets of certain tubes, thereby causing cracks. In the original design, the steam inlet was only 250 mm away from the upper tube sheet, and the steam temperature was generally above 200°C. According to available data, the operating temperature for 316 steel should not exceed 195°C. Under low-oxygen conditions, the temperature should not exceed 186°C; especially at 195–200°C, the corrosion rate is three times that at normal temperatures. It can be seen that tube rupture and gasket weld leakage are related to the improper selection of the steam inlet location as well as excessively high operating temperatures. ? 2.6 Poor process operation: Unstable production, uneven output, fluctuating loads, large variations in the system’s process parameters, and uneven utilization of equipment all lead to a wide range in corrosion rates, which are difficult to control; this is also one of the factors contributing to equipment corrosion. ? 3 Preventive measures against corrosion? 3.1 Use of materials resistant to urine corrosion? To enhance the resistance of the secondary vaporizer to corrosion by urea, all components that come into contact with this highly corrosive substance are made of 316 steel. Meanwhile, to further improve its resistance to intergranular corrosion, 316L steel with a lower carbon content, namely 00Cr18Ni12Mo2Ti, was used. In this way, **the corrosion resistance of the equipment is improved. ? 3.2 Rational structural design? (1) Change the outlet pipe design from a shrimp-back bent pipe to a blind plate tee (see Figure 1). This not only facilitates the detection of defects but also transforms the four shrimp-back bend welds, which are difficult to weld, into a single weld, while increasing the plate thickness from 5mm to 8mm. Under the premise of meeting process requirements, the structure has been simplified and its corrosion resistance has been improved. http://www.nmtech.com.cn/jishuwang/upload/0602161055395273.jpg Figure 1 Improvement to the structure of the urine outlet tube: (2) Increasing the length of the tubes extending outward. In the original design, the length of the tubes extending beyond the tube sheet was specified as 2 mm in accordance with GB151-89 \"Steel Shell and Tube Heat Exchangers\". In this specific environment, such a structure has extremely weak resistance to erosion corrosion, impact corrosion, and crevice corrosion at the welds. To this end, the tube extension length is modified to: 25 mm for the upper tube sheet and 10 mm for the lower tube sheet. This special structural design can **reduce or mitigate the defects caused by erosion, impact, and crevice corrosion. ? (3) The steam inlet position is lowered. The tube section at the shell-side steam inlet and the upper tube sheet experience the highest heat loads; they are the two key areas where tube ruptures and leaks in the welds of the upper tube sheet can occur. Corrosion is most severe in these areas. By changing the location of the steam inlet, the heat load can be distributed more evenly throughout the tube side, thereby eliminating or reducing localized overheating. At present, there is a lack of sufficient experimental research on the exact location of the steam inlet. Based on experience, the inlet should be located in the lower part of the tube side, at a height of 0.5 m above the condensate outlet. To this end, in the updated design, the steam inlet pipe was changed from 250 mm to 950 mm from the upper tube sheet, and a perforated plate distributor was added at the steam inlet to reduce the erosion of the tubes by the steam. ? 3.3 Careful fabrication with strict control over welding quality? The secondary vaporizer is manufactured strictly in accordance with the design specifications, an appropriate welding process is selected, and strict supervision and inspection are implemented during fabrication. For the welds of the outlet tee and the welding between the tube sheet and the tubes, the highly corrosion-resistant imported ultra-low carbon electrode BM310M-L from the Netherlands is used as a substitute for welding material A207. High standards are required regarding the quality of the groove preparation; during welding, heat input must be strictly controlled and careful welding techniques must be employed to improve the quality of the welds. ? 3.4 Strict Quality Control of Process Operations? (1) Strictly control all process parameters. Minimize the shell-side steam pressure as much as possible while meeting the process specifications. ? (2) Strive to achieve balanced production, keeping the daily output at around 390 tons, and minimize the time spent under high load in order to reduce the effects of erosion corrosion and other types of corrosion. ? (3) Ensure an adequate oxygen supply to the system in order to increase the concentration in the urine flowing into the secondary vaporizer. It enhances the corrosion resistance of the tube sheet welds. ? 4 Conclusion? Corrosion of urea double-evaporation heaters is a common problem in affiliated units engaged in medium-nitrogen fertilizers. The urea double-evaporation heater, designed, manufactured, and installed in accordance with the corrosion control measures proposed in this paper, has withstood over three years of operational testing, demonstrating extremely effective corrosion resistance: no leaks have occurred to date at the welds of the tubes in the upper tube sheet or at the welds of the outlet tee ； No significant thinning is observed in the columns, tube sheets, and outlet tubes ； No tube ruptures were observed in the tubes at the lower part of the upper tube sheet either ； The expected service life of this type of equipment is over 5 years. Effective control of corrosion in the urea double-evaporation heater creates important conditions for ensuring the quality of urea products and enabling the long-term operation of urea production systems. This has been highly recognized by users, and it also serves as a reference for other manufacturers in the same industry.