Welding repair of defects in urea synthesis towers

Thread Content

0 Preface Our company uses a full-circulation process involving ammonium carbonate aqueous solutions for urea production. Tower No. 1 was manufactured by Dalian Jinzhou Heavy Machinery Factory; the lining of this tower is 17 mm thick and is made of XCrNiMo18-12(MoD) material. In the presence of oxygen, a dense and tough oxide film can form on the surface of steel, providing strong corrosion resistance to methylammonium solutions and urine; it is covered with 15MnVgC steel plates. The tower has an inner diameter of 1400 mm, a total wall thickness of 120 mm, a total height of 26320 mm, an operating pressure of 21.6 MPa, and an operating temperature of 190°C. The medium used is urea and ammonium carbamate; it was put into operation in July 1989. 　　During an inspection of the tower on May 16, 2003, it was found that the supports of the 8th layer of trays were severely corroded, and the thickness of the supports decreased as one moved upward from the bottom of the tower; the thinnest areas were less than 10 mm thick. The lining was also significantly eroded. A flaw detection inspection was conducted on the surface of the outer cylinder of the urine tower, and defects such as incomplete filling, cratering, and weld beads were found in multiple welds. A small crack approximately 15 mm long and 0.3 mm wide was present at the weld joint between section 6 and section 7. Under these circumstances, the maintenance command decided to thoroughly address the defect. 1 Analysis of the causes of defects 1.1 Corrosion analysis The corrosion reaction mechanism of urea indicates that at high temperatures, urea corrodes due to the isomerization of urea to form ammonium cyanate, which in turn produces free cyanic acid. The reaction equation is as follows: The resulting CNO- has a strong corrosive effect on the oxide layer on the metal surface. The temperature at the bottom of the urea tower is 175–183°C, the temperature in the middle section is 180–188°C, and the temperature at the top is 183–190°C, which meet the conditions for the formation of urea corrosion. During an inspection inside the tower, it was found that the inner surface of the tower was black in color and covered with numerous tiny pinhole-like defects; upon touch, it felt like hard felt, indicating obvious uniform corrosion. The tray supports are exposed to an urea atmosphere; while suffering from uniform corrosion caused by urea, they are also subject to erosive corrosion from the corrosive gas flowing upward, which results in a reduction in the thickness of these supports. 　　1.2 Crack Analysis The results of surface flaw detection on the outer cylinder of the urine treatment tower show that the welding quality of the tower’s welds is poor; the weld pits, spatter, and excess weld height all exceed the specified standards, resulting in inherent defects in the tower. This is also confirmed by the original equipment documentation. The weld seams between section 6 and section 7 of the outer cylinder of the urine tower not only have a residual height that exceeds the specified value by 3.5 mm, but welding residue accumulation in these areas also causes local overheating, which reduces the toughness of the welds. This leads to an increased twist in those areas, resulting in high concentrated stresses. Under appropriate external forces, micro-cracks can easily form in these areas, which may eventually develop into full-scale cracks. 2 Treatment of tray supports 2.1 Welding procedure qualification tests The lining material of the tower is XCrNiMo18-12(MoD), which is equivalent to the Chinese standard steel grade 00Cr18Ni12Mo2Ti. Since the steel plates required by the drawings were not available on the market, we used imported steel plates with similar chemical composition and mechanical properties as substitutes for the bearing materials. Dutch BM310Mo—L stainless steel welding electrodes were selected, and weldability assessment tests simulating actual field conditions were conducted. On the basis of successful testing, a strict welding procedure for on-site welding was established. 　　2.2 On-site welding The welds of the original tray supports are in good condition, but the supports are severely corroded. In line with the principles of minimizing damage inside the urine tower, maintaining its original condition, and ensuring safety and stability, we decided to retain the original supports and weld new supports onto the horizontal circular surface located near them. 　　First, clean and grind the designated welding area until the original metallic luster of the base material is revealed. Then, clean it with acetone or anhydrous alcohol, and finally polish it with a clean white cloth. Use BM310Mo-L welding rod with a diameter of 2.5 mm; the baking temperature should be 150–200°C, with a holding time of 2 hours. During welding, place the electrode inside the insulation sleeve and retrieve it as needed while welding. During welding, the arc is initiated from the arc-starting plate connected to the support (it is strictly prohibited to start the arc on the tower or the support), after which the welder moves quickly to the groove formed by the support and the tower to carry out welding. The welding machine used is a ULF-400 DC-type welder, with a welding current of 75–80A and a welding voltage of 16–18V. The rapid vertical welding method is employed; after welding, the surface slag is removed, and penetrant testing is carried out to meet the standards specified in JB4730-94. 　　2.3 Acid cleaning and passivation First, clean with acetone or anhydrous alcohol to remove oil contamination from the welds and within a range of 50 mm around them, then acid-clean the welds using an acid solution with a total acidity of 34%. The formula for the pickling solution is: 1 kg of nitric acid with a concentration of 65%–68% and a density of 1.43 g/cm3, 0.575 kg of hydrocyanic acid with a density of 1.16–1.18 g/cm3, and 3.3 kg of distilled water. After cleaning with the pickling solution, it is then rinsed thoroughly with large amounts of chemically treated soft water until a phenolphthalein test strip shows that there is no acidity on the weld surface; the Cl- content in the soft water should be less than 2×10-6. 　　After passing the pickling test, a passivation paste is applied to the surface of the welds. The formula for this passivation paste is as follows: 64 kg of nitric acid with a concentration of 65% and a density of 1.43 g/cm3, 0.10 kg of talcum powder, and 40 kg of distilled water. The passivation time is 1 hour. After passivation, it is rinsed with large amounts of chemically softened water until the weld surface is free of acidity. 3 Crack treatment on the tower surface (1) Grind the cracks clean using an angle grinder, and conduct flaw detection with a C-JEA type MT device, with a magnetization time of ≥3 seconds. 　　(2) Welding shall be carried out by welders holding a certificate of competency for the boiler and pressure vessel D2—7J project. 　　(3) SMAW J557/?3.2, polarity DCEP, welding current 120–130A, welding voltage 22–24V. 　　(4) The pre-welding heating temperature is 150–180°C; multi-layer, multi-pass welding is employed, with an interpass temperature of 250°C. After welding, the weld surface is polished to ensure a smooth transition with the base material. 　　(5) Measure the weld temperature using a TM—902C type tracked ceramic electric heater thermometer. Before welding, the circumferential weld is preheated from the outside; the heat retention zone is defined with the weld centerline as a reference, extending 150 mm on each side. Post-weld electric heating tempering: for 15MVR, the heat treatment temperature is 565±15°C. When raising the temperature above 300°C, the rate of temperature increase should be controlled at 60–80°C/h, with a holding time of 1 hour. When lowering the temperature, the rate of temperature decrease should be controlled at 30–50°C/h, and it can cool naturally once the temperature drops below 300°C. 　　After 100% MT testing in accordance with the JB4730—94 standard, it meets the required grade. 4 Hydrostatic test: Water is pumped into the urine tower through the high-pressure water supply pipeline using a condensate pump. Once the tower is filled with water, the blind flanges are sealed, and pressure is applied using a high-pressure cleaning pump. The pressure rise gradients are 5 MPa, 10 MPa, 15 MPa, 19 MPa, and 23 MPa respectively. For each pressure increase step, the pressure must be maintained for 10–15 minutes; meanwhile, thorough inspections of both the upper and lower sections of the tower are carried out. Only if no issues are found can the next stage be proceeded with. When the pressure is increased to 23 MPa, it is held for 15 minutes to check for any leaks. If there are no leaks, no deformation, and no abnormal noises in the tower, the pressure is then reduced to 19.6 MPa and held for a sufficient length of time; the surface of the tower, as well as the areas above and below it, the pipes, flanges, valves, and gasket areas, are inspected. If no signs of sweating, peeling, or leakage are found, the pressure testing is considered successful. 5 Conclusions (1) The urea synthesis tower is the core equipment in the urea production system. The 1# and 2# urea synthesis towers in use at our company have been operating safely and stably for 14 years, which is closely linked to the thorough routine inspections and meticulous equipment maintenance carried out over the years; any potential problems or defects must be addressed as early as they arise. Based on the operational data from our company’s urea towers over the years, it can be seen that when the nickel content in urea is ≤0.07×10-6, the tower operates normally; when the nickel content reaches 0.20×10-6, an alarm is triggered in the tower; and when the nickel content reaches 0.30×10-6, the tower must be shut down urgently. Since the treated No. 1 urea synthesis tower was put into operation on June 20, 2003, the nickel content in the tower has fluctuated within the range of (0.065–0.068)×10-6, demonstrating that the repairs were successful. 　　(2) Practice has shown that the welding process for the XCrNiMo18-12(MoD) material is indeed feasible; it has improved the level of welding repairs for equipment defects in our company, and provides relevant experience and references for other companies in the same industry.