Treatment of seal plate leakage at the outlet of the synthesis tower/circulating water heat exchanger

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During the energy-saving upgrade project carried out in 2002 to increase the production capacity of its ammonia synthesis plant by 50%, Yunnan Yuntianhua Co., Ltd. made substantial modifications to the synthesis circuit. The synthetic routing path has been merged from its original two parts into a single large system ； At the same time, in order to reduce the pressure drop in the system, no shut-off valve is installed in the circuit ； To reduce leakage points, welding was used as the connection method for all joints. Therefore, once a failure occurs in the synthesis circuit’s equipment, given the large number of devices in this circuit, the scale of the system, and the fact that the gases involved are flammable, explosive, and volatile, performing welding tasks for extended periods poses difficulties in maintaining safe gas conditions at the areas where welding takes place during the processes of gas displacement and maintenance. At 20:00 on December 28, 2006, the personnel on duty detected a leak in the seal plate of the outlet gas/circulating water heat exchanger (124-CA) of the synthesis tower, and decided to shut down the unit for repairs. To reduce the parking time, it was decided to change the method of nitrogen purging: first, a one-pass purging method would be used, after which blind flanges would be installed to isolate the synthesis tower; subsequent purging would then be carried out to address any leaks. Once this process was complete, a short water seal would be created at the outlet pipe of 124-CA to prevent the spread of flammable gases, thereby creating a favorable environment for the construction work. 1 Parking and replacement: At 9:00 on December 30, 2006, the unit began to reduce its load; by 9:58, after one full cycle of the syngas compressor, the synthesis system was shut down. The vent gas was discharged in front of the compressor inlet isolation valve, and the process load was reduced to 10 t/h, while the ammonia compressor continued to operate. The synthesis loop began to depressurize at 10:00, and depressurization was completed at 12:30. Since the new air separation unit was in the startup phase, there was no qualified nitrogen available. It was not until 15:10, when the gas analysis of the nitrogen main pipe showed satisfactory results, that nitrogen purging of the synthesis loop began. Given the large size of the synthesis loop system, the medium in the equipment ahead of 124-CA exists in gaseous form, so there are no dead zones; however, liquid ammonia is present in the equipment behind 124-CA. Using pulse discharge would take a long time and would not yield satisfactory results. After research, it was decided to employ a continuous nitrogen flushing method for purging the system. 1.1 System purging 1) Purging before installing the blind flange: The purpose of nitrogen flushing at this stage is mainly to create favorable conditions for installing the blind flange. The nitrogen purge points are located on the tube side of the heat exchangers at the inlet/outlet of the synthesis tower, as well as after the electric valve at the outlet of the syngas compressor; discharge is carried out at the vent and relief gas release valves in the synthesis loop. At 17:20, sampling was conducted at these vents and release valves, and the results were satisfactory, allowing for the installation of blind flanges. 2) Nitrogen purging during the insertion of the blind plate: During this process, the nitrogen purging point is moved to the drain at the outlet of the synthesis tower, with the gas being discharged through the drain located before the synthesis tower; this helps to protect the catalyst in the synthesis tower during construction. At 21:00, install the blind flanges at the outlet of the synthesis tower and at the electric valves in the circulation section of the syngas compressor. 3) System purging after installing the blind flange. The purpose of nitrogen flushing is to create suitable working conditions for the removal and re-welding of the 124-CA sealing plate. After the blind flanging was completed at 21:00, the nitrogen purge points were changed to the gas side of the reactor outlet gas/boiler water heat exchanger and the shell side of the reactor inlet/outlet gas heat exchangers; purge gas was discharged from the relief valve, the primary ammonia cooler, and the reactor outlet gas/circulation gas heat exchanger. At 22:30, after three fire hazard analyses were conducted and found to be satisfactory at 124-CA, the syngas distribution valve, and the drain of the primary ammonia cooler, preparations were made to establish a water seal at the outlet pipe of 124-CA. 1.2 Water seal 1) Establishing a water seal: A water seal device as shown in Figure 1 is established; since the gas pipeline for 124-CA is close to the ground, this creates favorable conditions for setting up a water seal. First, create a tee. The 1# port of the tee is connected to the bottom drain of the 124-CA outlet; the 2# port is connected to the desalination water pipe via a hose, while its other end is connected to the water seal level storage tank. The height of this storage tank is 1250 mm (this height must not exceed the lower tangent height of the reactor 124-CA, in order to prevent water from the water seal from entering the heat exchanger). http://www.nmtech.com.cn/jishuwang/upload1/0810281701367830.jpg 2) Introducing the water seal: To ensure proper introduction of the water seal, it is first necessary to stop nitrogen flushing in the synthesis circuit and reduce the system pressure to a slight positive level ； Then open valve 1# fully; the opening degree of valve 2# is determined based on the rise in liquid level, so as to gradually establish the water seal level. Once the level rises enough that water begins to overflow from the storage tank, close valve 2# – at this point the water seal is established. After the water seal is put into use, close attention should be paid to its liquid level to prevent combustible gases and ammonia from spreading to the leakage site once the liquid level drops. If the level of the water seal fluid drops, valve 2# can be slightly opened to add deionized water, allowing some water to overflow from the storage tank. To prevent ammonia-containing gases from flowing into the 124-CA from sources such as the high-pressure ammonia separator and damaging the water seal system, the downstream system should be maintained under a slight positive pressure. 3) Inspection method for slight positive pressure: Slightly open the nitrogen filling valve at the drain connection on the tube side of the primary ammonia cooler, and discharge gas through the vent valve and the synthesis circuit vent. Connect a hose to the drain connection at the outlet of the high-pressure ammonia separator; immerse this hose in a bucket containing water, ensuring that the length of the hose submerged in the water is less than 10 mm. Control the amount of nitrogen filled so that bubbles just start to emerge from the hose. This way, the system remains under slight positive pressure, with a pressure of less than 0.1 kPa. The flowchart of 124-CA in the system is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload1/0810281702273513.jpg 2 Maintenance and water seal drainage: On December 30, 2006, at 23:20, the 124-CA head was removed; by 8:00 on December 31, the gas-side gasket of 124-CA had been removed and the welding surfaces polished. After welding was completed at 19:50, the head was reinstalled and the sealing water began to be drained. Since the ammonia compressor is in operation, if moisture enters the downstream system, it can cause the equipment to freeze and become clogged. The water-seal drainage is to be completed at the drain points of 124-CA, and system replacement is carried out only after confirming that the gas is dry at the drain points of the syngas distribution valve and the primary ammonia cooler. At 21:40, install the 124-CA head, and at the same time remove the blind flange at the electric valve of the synthesis gas compressor circulation section. 3 Driving: At 23:00 on December 31, 2006, the replacement of the synthesis circuit was completed successfully; thereafter, work began on removing the blind flange at the outlet of the synthesis tower. This work was finished at 0:40 on January 1, 2007. Immediately afterwards, the electric valve bypass at the outlet of the syngas compressor was opened, as well as the vent valves for the synthesis circuit, in order to keep the pressure in the synthesis tower below 1.4 MPa. Subsequently, operations to warm up the synthesis tower were initiated. To accelerate the warming of the synthesis tower, the methods employed include raising the gas temperature and increasing the gas flow rate. Following this approach, the following method was adopted: the Prissen pretreatment system was started in advance to increase the volume of gas entering the synthesis tower ； To increase the temperature of the boiler water in the heat exchanger for the gas exiting the synthesis tower and the boiler water, the following method is used: reduce the flow rate of the boiler feed water in the heat exchanger for the conversion gas and boiler feed water, thereby raising the water temperature. The temperature difference between the boiler water temperature in the heat exchanger for the gas exiting the synthesis tower and the boiler water (TI-45) and the boiler water temperature in the heat exchanger for the gas exiting the methanation unit (TI-106) should be kept within 50°C. The temperature difference between TI-45 and the temperature of the gas exiting the methanation furnace should also be within 50°C. TI-106 should be as close as possible to the saturation steam temperature of the drum, but still below that temperature. This approach achieves the goal of increasing the temperature of the gas entering the synthesis tower, thereby accelerating the process of warming up the synthesis tower. At 4:05 on January 1, 2007, the warming up of the synthesis tower was completed; pressure increase operations for the synthesis tower were initiated. Leak checks were conducted on 124-CA, the electric valve in the circulation section of the syngas compressor, the electric valve at the outlet of the syngas compressor, and the flange at the outlet of the synthesis tower, with no leaks detected. 5:05 Pressurization and balancing of the synthesis tower ； At 5:15, the heating furnace was started up and ignited, and the temperature rise process of the synthesis tower began ； The ammonia synthesis reaction begins at 7:00, and the level in the flash tank starts to rise ； At 7:30, the cold ammonia product pump is started to supply ammonia to the ammonia tank. 4 Summary: The shutdown time for this maintenance operation was reduced from the originally planned 4.5 days to 46 hours, showing significant results. 1) The maintenance was carried out by replacing the air with nitrogen in one go and establishing a water seal, thereby preventing flammable gases and ammonia in the system downstream of 124-CA from spreading to the area where the work was being done. This approach avoided the difficulties associated with carrying out maintenance in a nitrogen atmosphere, and the absence of pressure at the work site ensured the quality of the welding. 2) Through this practice, a relatively clear concept of micro-positive pressure was established, one that can be quantified with data and verified, rather than being a vague notion. 3) The one-pass nitrogen displacement method used in this parking replacement process is suitable for media environments that are easy to vaporize. 4) Optimizing the process operations while driving also bought time for the rapid resumption of production; therefore, a thorough understanding and study of the operation manual is an effective way to improve skills.