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Maintenance and upkeep: Conducting proper maintenance on pressure vessels and strengthening their repair efforts is a very important and indispensable task for ensuring their safe and economical operation. Main contents: 1) The safety devices of pressure vessels (safety valves, pressure gauges, pressure relief holes, and explosion-proof membranes) should be reliable, sensitive, and accurate, and shall be inspected and calibrated regularly. 2) The anti-corrosion measures for pressure vessels should be regularly inspected to ensure they remain intact. At the same time, measures must be taken to prevent any “leakage, seepage, dripping, or escaping” from the pressure vessels and related connecting pipes. 3) The fasteners and seals of pressure vessels should be regularly inspected to ensure they are intact and reliable ； Reduce and eliminate vibration in pressure vessels. 4) Check the static grounding of pressure vessels to ensure that the grounding system is complete and in good condition. 5) Pressure vessels that have been taken out of service or sealed should also be regularly maintained. Note: The maintenance of pressure vessels must comply with the regulations established by the **special equipment regulatory authorities. During maintenance, special attention should be paid to the following issues. (1) Purging and displacement: After a pressure vessel in use is shut down, purging and displacement must be carried out in accordance with the specified procedures and time limits. A systematic and comprehensive purge and displacement procedure table must be established for all pipelines, and the purging and displacement must be carried out item by item in strict accordance with the specified procedures. For flammable, explosive, and toxic media, especially pressure vessels with high viscosity and complex structures due to scaling on their inner walls, the flow rate, velocity, and duration of purging must be sufficient to ensure thorough cleaning. When purging with steam, steam condensate accumulates inside the pressure vessel pipes; therefore, after purging with steam, it is necessary to use compressed air for further purging in order to drain the water from the lowest points. If personnel need to enter a pressure vessel that has been purged with nitrogen for maintenance work, it must first be purged with air to remove all the nitrogen, and entry is only permitted once the gas analysis shows that the conditions are satisfactory. (2) Installation of blind flanges: Between pressure vessels, or between pressure vessels and compressors, as well as between pumps or other equipment, there are many pipes that are interconnected. For a petrochemical complex, in order to ensure safe operation, a set of systems that are shut down for maintenance must be isolated from the numerous pipelines connected to them using blind flanges. Without blind plates, relying solely on valves is not sufficient. Due to factors such as long-term exposure to media erosion, corrosion, scaling, or the accumulation of impurities, it is difficult to ensure that the valves remain airtight; otherwise, if flammable and explosive materials manage to pass through them, they can cause explosion and fire accidents when exposed to welding or other heating processes during construction ； If toxic or asphyxiating substances leak in, the personnel working inside the pressure vessel may be poisoned or asphyxiated to death. In short, all material pipelines that could cause fires, explosions, or harm to people should be isolated using blind flanges. (3) Use of fire during maintenance and construction: The use of fire during maintenance and construction must be approved. The principle of “three no-fires” must be observed: that is, no fire shall be lit without approval ； No welding will take place unless fire prevention measures are implemented ； Do not start the fire if the guardian is not present. The use of fire must take place at clearly designated locations and times. Incidents of fires and casualties occur from time to time due to the failure to comply with fire safety regulations. Therefore, the fire management system must be strictly enforced. (4) Dismantling and sealing inspection of pressure vessels and equipment: When repair and operation personnel arrive at the site, the first task they undertake is to open the manholes of the pressure vessels, remove the manhole covers as well as the flanges of pipelines, pumps, etc. Safe operation must be given priority during disassembly work; the slightest carelessness can lead to accidents and cause personal injury. When closing the manhole, due to a lack of careful inspection, safety helmets, rags, gloves, tools, screws, and other items were left inside the equipment, causing blockages once operations resumed. 1) When opening the manholes of tower-type equipment, it is necessary to open them in sequence from top to bottom. When closing the manhole, it should be closed sequentially from bottom to top. Before closing the manhole, at least 2 persons (one from the construction team and one from the production team) should jointly inspect it; it can be closed only after confirming that there are no items left inside. 2) Whenever a pressure vessel’s bottom manhole or access hatch is to be opened, the low-point drain should be opened first. Attention should be paid to any false indications caused by blockages; only after confirming that there are no issues can the bottom manhole or access hatch be opened, and it should not be done with people in the vicinity. 3) When removing the pump, the inlet and outlet valves should be closed, the bottom drain valve should be opened, and a warning sign should be placed on the power switch to prevent accidental power supply; the pump may be removed only after confirmation. 4) When removing pipeline flanges, it is necessary to first drain and relieve pressure, especially for pipelines carrying corrosive media such as acids and alkalis. Do not remove all the bolts after loosening them, to prevent the pipes from sagging and causing injury. 5) When tightening bolts on manholes, flanges, etc., gaskets should be selected based on the operating temperature and pressure. The tightening bolts should be done symmetrically, so that the bolts and washers are subjected to even stress, thereby ensuring a tight seal with no leaks. (5) Entering the interior of a pressure vessel for work: When a pressure vessel that has contained or been used for harmful gases or liquids is opened, it is not possible to enter it immediately for work. First, samples must be taken to analyze the levels of oxygen as well as toxic and flammable gases. Only after the analysis shows satisfactory results, and after all necessary preparations and clean-ups have been carried out in accordance with relevant regulations, can one enter the vessel. Entry is only permitted after obtaining the permit for work inside the pressure vessel, and a dedicated person should be assigned to supervise the outside of the pressure vessel. (6) In systems for lifting and hoisting pressure vessels, some of these vessels are large in size, with lifting loads reaching nearly 100 tons ； Some pressure vessels are installed at high elevations, with heights of several dozen meters ； During maintenance, a valve or a section of pipeline needs to be lifted. Due to the limited conditions at the maintenance site, carrying out lifting operations presents many difficulties. To ensure safe maintenance, it is essential to follow the relevant regulations regarding lifting tasks. For work at heights, it is necessary to comply with the regulations regarding lifting operations; each lifting operator must possess certain professional technical knowledge and must pass relevant training and examinations before being allowed to carry out lifting tasks ； For each lifting device, the lifting load must be accurately calculated, with an appropriate safety factor applied; lifting beyond the rated capacity is strictly prohibited. (7) Working at heights. Working at heights is primarily aimed at ensuring the safety of workers and preventing falls from heights. Besides providing operators with the necessary working conditions (such as scaffolding), it is crucial that they abide by relevant regulations pertaining to working at heights. (8) Electrical safety 1) For pressure vessels whose working medium is flammable or explosive, explosion-proof lighting fixtures must be used. 2) When operators enter pressure vessels for maintenance, lighting fixtures must use a voltage of 12–36V to ensure safe operation. 3) When operators use handheld power tools, measures must be taken to install electric shock protectors. (9) Maintenance quality: When maintaining pressure vessel systems, operators must follow the process requirements strictly, on-site supervisors must exercise strict oversight and conduct thorough inspections to ensure the quality of the maintenance work. (10) Other precautions: 1) No holes shall be made in the pressure vessel arbitrarily. 2) A proper welding process must be established during repair. 3) After maintenance, necessary inspections must be carried out and thorough cleaning should be done. 4) No repairs or tightening operations shall be carried out on the main pressure-bearing components when there is pressure inside the pressure vessel. 5) After the maintenance is completed, store the filled-out records in the archive. Repair and modification: In addition to the considerations for repairing and modifying pressure vessels mentioned earlier, the following actions are also required. 1) For lined pressure vessels, if there are defects such as cracks, pores, or inclusions in the lining, repair welding or partial replacement of the lining can be carried out. Before patch welding, the defective area should be ground and then subjected to penetrant testing ； Penetrant testing should also be performed after patch welding. For the repair of linings with special corrosion resistance requirements, special inspections for designated items shall also be carried out in accordance with the design specifications. 2) For pressure vessels with relatively low pressure and severe local corrosion, patching or replacement of the cylinder sections and heads can be employed; the replaced cylinder sections or heads shall be manufactured and inspected in accordance with current standards. 3) If the sealing surface of a pressure vessel is damaged, it can be repaired by grinding and patch welding; metal sealing elements used in high-pressure vessels can be repaired by grinding. 4) Repair of tube-to-tube sheet joints in heat exchange vessels. When defects such as cracks appear in the tube-to-tube sheet weld joints of heat exchange vessels, or when the tubes need to be replaced due to corrosion, the weld metal in the joint can be removed by reaming or grinding, followed by rewelding. 4. During maintenance of mobile pressure vessels, tasks such as replacing safety accessories and repainting may be required; the maintenance work on such vessels must be carried out in accordance with relevant regulations.