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1 Precautions for shutdown operations: Once the shutdown plan is established, it should be carried out in an orderly manner, strictly following the times specified in the plan, the steps involved in the shutdown process, the extent of process changes, as well as the approved sequence diagram for the shutdown operations. The following points should be noted when performing parking operations: (1) The rate of cooling and pressure reduction must be carried out strictly in accordance with the process specifications. In areas with high temperatures, it is necessary to prevent the equipment from leaking due to excessive temperature gradient changes. Chemical processing units often handle flammable, explosive, toxic, and corrosive substances. Leakage of these substances can lead to fires, explosions, poisoning, asphyxiation, corrosion, and burns. (2) All operations carried out during the parking phase must be accurate, and a supervision system should be implemented for critical operations. If necessary, the instructions should be repeated to overcome complacency. When performing each operation, pay attention to whether it meets the intended purpose. For example: the valve opening/closing actions should be slow, etc. (3) When shutting down the unit, all materials in machines, pumps, equipment, and pipelines must be completely removed. It is strictly prohibited to discharge any materials, toxic, or corrosive substances on site, so as to prevent environmental pollution or accidents. Because flammable and combustible liquids pose a high risk of fire and explosion, they generally have a low flash point, vaporize rapidly, and have high vapor pressure; moreover, they easily mix with air to form explosive mixtures that can catch fire or explode when exposed to an open flame. Flammable liquids can form explosive mixtures even at room temperature, making them extremely dangerous. Flammable and toxic materials should be sent to a flare for burning; appropriate safety measures must be taken when discharging the remaining materials. During shutdown operations, all sources of ignition must be eliminated around the equipment. (4) Shutdown procedures for major equipment: 1) Develop a shutdown and material handling plan, which must be approved by the workshop supervisor. Before carrying out the shutdown, provide the operators with technical instructions, informing them of the precautions to take and the safety measures that should be implemented ； 2) During parking operations, the workshop’s technical supervisor must be on site to monitor and direct the process, ensuring everything proceeds in an orderly manner without chaos, and preventing any misoperations ； 3) During the parking process, records of any abnormal situations that occur and the methods taken to address them must be kept at all times ； 4) For critical operations, a supervision system must be implemented. 2 Purging and displacement: The quality of the operations to empty, purge, and drain chemical equipment and pipelines is one of the key factors that determine whether maintenance work can be carried out smoothly as well as ensuring the safety of personnel and equipment. When purging still cannot completely remove the material, steam purging or displacement with an inert gas such as nitrogen is required. 1. Precautions for purging operations (1) When purging, care should be taken in selecting the purging medium. High-temperature pipelines, as well as pipelines and equipment with a flash point below 130°C and devices/pipelines having a low lower explosion limit for the materials contained within them, must not be purged using compressed air. Air can easily mix with such materials to form explosive mixtures; static electricity sparks or other open flames can easily occur during purging, leading to fire and explosion accidents. (2) The valve opening should be small during purging (usually 2 threads). Pause for a moment to allow a small amount of the purge medium to pass through, and pay attention to the flow condition. When steam is used as the purging medium, rubber hoses are sometimes required; these hoses must be secured properly, and their pressure resistance should also be checked. Hoses intended for such temporary purging tasks must not be used with medium-pressure steam. (3) For pipelines equipped with flow meters, in order to prevent excessive flow rates of purge steam as well as the presence of iron slag, rust, and scale inside the pipes from damaging the internal components of the measuring instruments, purge is generally carried out through a bypass line. (4) All pressure gauge valves on the pump outlet pipeline must be closed to prevent water hammer during purging from damaging the pressure gauges. The principle for emptying and replacing the compressor system is to proceed in the order of low pressure, then medium pressure, and finally high pressure. First, one section is emptied completely; if the desired pressure level is not achieved, pressure can be added from the second or third section to aid in emptying that section. This process is repeated sequentially, with the high-pressure gas being discharged into the flare at the end. (5) When purging shell-and-tube heat exchangers and condensers with steam, the process must be carried out in sections, and pressure must be released to prevent liquid from vaporizing and causing overpressure that could damage the equipment. (6) For system pipelines, pipelines with larger diameters should be purged first, followed by those with smaller diameters. The purging sequence is from largest to smallest. (7) During purging, it must be carried out system by system, ensuring that all pipelines (including branch lines) are purged thoroughly, with no dead corners remaining. After purging, the valve on the purging pipeline should be closed first, followed by turning off the steam supply, to prevent the purged medium from flowing back. (8) When emptying and purging a distillation tower system, liquid should first be drained from the top reflux drum and reflux pump, followed by closing the relevant valves; then the liquid in the tower bottom, reboiler, and intermediate reboiler should be drained. The tower pressure should be maintained for a while until all the liquid accumulated on the trays has flowed out, after which the tower bottom should be emptied again to reduce the pressure. After purging the towers, vessels, and heat exchangers, steam must be used to drain the contents from the lowest points until no methanol is present in the steam. Once that is achieved, the steam supply is stopped, and the low-point drain valve is opened to complete the drainage. It is essential to ensure that there is no methanol or explosive gases present in the equipment after it has been opened, in order to guarantee safety during maintenance work. (9) For low-temperature production units, considering that the control of dew point indicators within the restart-up system is very stringent, steam purging is not employed. Instead, chlorine purging should be carried out in sections and centrally; preferably, dried nitrogen should be used for purging and displacement. (10) The purging shall use steam generated by this unit itself; first, it is necessary to check whether the steam contains oil. Oil, steam, water, etc. within the device may mix with each other; once such mixing is detected, steam cannot be used for extinguishing fires or purging. (11) Before purging the pipelines and materials of the equipment, better communication should be maintained between different shifts to prevent accidents such as pressure buildup and roof collapse. (12) The pressure of the purging medium must not be too low, in order to prevent the purging medium from flowing back into the nitrogen pipeline network and affecting the overall system. Generally speaking, larger equipment and vessels should be steamed and washed after the material has been removed from them, such as the three distillation columns, methanol separators, and various heat exchangers. Preventive measures against burns must be taken before steaming. The processing time depends on the size of the equipment, the amount of residues or oil deposits of flammable and toxic substances attached to it, how difficult they are to remove, and the rate of ventilation; it usually ranges from 8 to 24 hours. (13) When emptying and purging materials with low boiling points, it is essential to drain the liquid first and then release the pressure, to prevent excessive vaporization during the discharge of liquid hydrocarbons, which could lead to cold brittle fracture of pipelines and equipment. (14) When equipment made of non-low-temperature materials is used for the emptying and replacement of low-boiling-point materials, a certain level of heating must be maintained; the heating source can be turned off only after the pressure has been reduced. (15) To empty the water heat exchanger in the cooling area, wait until the system has been depressurized, immediately stop the water circulation, and drain the water promptly. Prevent water from flowing back into the cold zone. 3 Equipment Environmental Safety Standards: Through various treatment procedures, the production workshop must analyze the environment surrounding the equipment before it is delivered for maintenance, in order to meet the following standards: (1) When performing maintenance or welding inside the equipment, the oxygen level should be between 19% and 21%, the concentration of flammable gases should be below safe levels, and the concentration of toxic substances should be below the maximum allowable levels ； (2) When performing maintenance on the external surface of the equipment or carrying out hot work, the concentration of combustible gases inside the equipment should be below the safe level ； (3) Wells and trenches at the maintenance site should be cleaned thoroughly and covered with sand seals. There should be no residual pressure, no hot substances, and no sediment inside the equipment and pipelines ； (4) After emptying the equipment and pipelines of materials, flush them with water, then displace the contents with nitrogen and air until the content of combustibles inside the equipment meets the standard, with an oxygen content of 19%–21%. 4. Installation of blind flanges: Between chemical processing units, between units and storage tanks, and between different plants, there are many pipelines that connect together to transport materials. Therefore, when a production unit is shut down for maintenance, and after the contents within the unit are removed and the area is cleaned and flushed with water, blind flanges need to be installed to prevent materials from leaking into the unit being maintained. For example, in 1989, at the reforming unit of a certain aromatic hydrocarbon plant, during a shutdown for loading ceramic balls and catalyst into the reactor, blind flanges were not installed as required to isolate the separation system from the reaction system. As a result, nitrogen from the separation unit entered the reactor system, causing three people to suffocate to death. The following points should be noted when installing or removing blind flanges: (1) The task of installing or removing blind flanges should be carried out by a dedicated person, in accordance with the blind flange diagram for the process flow approved by the process technology department; the flanges should be assigned uniform numbers, and records of their installation or removal should be kept ； (2) The personnel responsible for installing and removing blind flanges should remain relatively stable; generally, the same person is responsible for both installing and removing them ； (3) Personnel responsible for installing blind plates must receive safety training, and safety technical measures must be clearly explained and implemented ； (4) When working at heights, measures such as protection against falls, poisoning, fire, and slips must be taken into account ； (5) When removing the flange bolts, they should be loosened gradually and slowly to prevent the residual pressure or remaining materials in the pipeline from spraying out and causing accidents. The blind plate should be installed at the flange behind the inlet valve; gaskets should be placed on both sides of the blind plate, which should then be tightened with bolts to ensure no leaks occur ； (6) Blind plates must possess sufficient strength; their material and thickness shall meet the technical requirements. In principle, the thickness of the blind plate shall not be less than that of the pipe wall. It should have a handle, and a label shall be attached in a visible location.