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During the inspection and maintenance of toluene floating roof tanks, special attention should be paid to these three key aspects: material flow rate, nitrogen purging, and static electricity elimination. Thorough and meticulous planning along with strict organizational implementation, along with effective control of safety risks, are important prerequisites for ensuring operational safety. A tank area in a certain facility houses a atmospheric pressure storage tank made of toluene, equipped with an internal floating roof; the tank body is made of stainless steel, with a diameter of 4.5 m and a height of 9 m. It uses stainless steel and aluminum internal floating disks, has no nitrogen sealing system, and lacks a vent valve on its roof – only a venting elbow is present ； SIS alarm interlocks for high and low liquid level conditions are provided. The medium inside the tank is toluene (approximately 75 tons of toluene was present in the tank at that time); its density is 0.87, its flash point is 4°C. It is a flammable liquid classified as Category B. Toluene is insoluble in water, and its vapors combined with air can form explosive mixtures; the explosion limit (V/V) is 1.2%–7.0%. The tank farm where the storage tanks are located is classified as a level 2 major hazard source ； This storage tank has been in operation for over 10 years; the carbon steel bolts of the inlet and outlet pipes (DN50, DN80), the bottom drain valve, and the tank manholes are severely corroded. The gaskets at the valve flange connections are also heavily damaged, so it is necessary to empty the tank first before carrying out inspection, maintenance, and replacement work. For inspection, maintenance, and repair, manual cleaning was used; the operation was carried out after all the toluene material at the bottom of the tank was removed. Inevitably, the internal floating disk would sink to the bottom and then rise again. There is still a lack of relevant practical information in the industry regarding how to plan and organize the maintenance work on toluene floating roof storage tanks. What risks arise throughout this process, especially when the floating deck sinks to the bottom and when it rises again? What safety measures should be taken to address these risks? What are the specific operational details required to avoid production safety accidents? The practical experience in ensuring the safe and smooth execution of this maintenance operation has certain reference value. 1 Develop a comprehensive and detailed maintenance plan and obtain approval for it. Due to the maintenance of the storage tanks, the supply of toluene material will be disrupted, which will have an impact on departments such as production, procurement, and safety; therefore, careful planning is necessary, and a detailed plan must be prepared in advance. The maintenance plan should include: the purpose of the maintenance, the timing of the maintenance, the tasks to be carried out, the location where the work will take place, the steps and methods to be followed, the organizational structure responsible for overseeing the work and the allocation of responsibilities, a hazard analysis and risk control measures, other safety measures for the entire process, emergency response plans, procedures for verifying the completion of the work, and attachments. At the same time, the equipment engineering department must also prepare a detailed \"On-site Construction Plan\" as an attachment to the overall plan. Regarding the working duration, depending on the size of the tank, it is advisable to set it at 4 days (2 days for tank cleaning and preparation, 1 day for construction work, and 1 day for inspection and material loading), so as to minimize any impact on production and procurement. In terms of working methods, emphasis is placed on ensuring proper energy isolation, and manual operations are used; no special operations such as temporary power use, hot work, or work in confined spaces are involved. By renting tank trucks, the materials are first loaded into the trucks, then unloaded manually, and finally poured into the storage tank at the end of the operation. In terms of the maintenance steps, task requirements and responsible persons are detailed on a half-day basis to create a schedule. In terms of the organizational command structure, it is necessary to clarify the responsibilities of the chief commander, the expert group, various relevant departments, and the specific persons in charge. In the job hazard analysis, it is necessary to conduct a thorough analysis of the hazards, potential risks, and risk levels at each stage of maintenance operations, and to determine the existing direct control measures and responsible persons. After the work plan is completed, convene a meeting with the members of the organizing and command team sowie relevant personnel to review it, revise and improve it, and then print it out for signature and approval. 2 Thoroughly identify safety risks and formulate countermeasures. Due to the physical and chemical properties of toluene, in the process of the floating roof descending to the bottom and rising again (this is a key point), if risks are not carefully identified and safety measures are not properly implemented, accidents may occur. 2.1 Potential risks associated with tank cleaning operations (transferring material from the tank to a tank truck): ① During transfer, if the flow rate of the material is too high, static electricity can accumulate and not be discharged in time, leading to fires and explosions (explosive gas pockets in the upper space caused by seal ring leaks, or between the internal floating disk and the liquid surface after the disk settles at the bottom of the tank) ； ②The ventilation valve on the inner floating disk is damaged, which causes the inner floating disk to get stuck or collapse during material feeding, leading to fires, explosions, and other incidents ； ③During the material feeding process, the connection point suddenly broke, causing the material to leak and leading to poisoning, fires, and other issues ； ④The contact of the floating disk with the bottom of the tank, as well as friction at the welds, can create a risk of static electricity or even sparks, leading to combustion and explosion ； ⑤Other hazards. Corresponding control measures: ① Introduce nitrogen into the tank (through the vent hole on the tank top) and implement continuous air replacement ； ②Ensure the tank truck is properly grounded to prevent static electricity; check all relevant valves and accessories, and use explosion-proof tools when working ； ③When temporarily taking over the flange removal task, be careful to prevent material from dripping; use a basin to catch any spilled material ； ④During the material pumping process (before the floating disk reaches the bottom), ensure that the flow rate does not exceed 2 m/s (the pump’s flow rate is 17 t/h; the pump outlet pipe has a diameter of DN50, so the theoretical flow rate should not be higher than 14 t/h). Inspect all connections and valves, and address any leaks promptly. Fire-fighting and emergency supplies should be available on site ； ⑤When the floating disc is close to the bottom, reduce the flow rate to below 1 m/s (7 t/h), and closely monitor changes in pump pressure and liquid level; stop immediately if any abnormalities occur ； ⑥When the liquid level drops to the height of the outlet pipe of the storage tank, stop the pump from feeding to prevent vacuum formation, and switch to manual discharge ； ⑦The operators wear anti-static work clothes, safety helmets with guards, and oil-resistant rubber gloves, and use explosion-proof tools for their work ； ⑧On-site control: Unrelated personnel as well as other materials and vehicles are prohibited from entering the tank area for operations ； ⑨Ensure the storage tank is properly grounded against static electricity. 2.2 Potential risks of manual material loading: ① Static electricity in the ton barrels is not effectively discharged, resulting in flash explosions ； ②Excessive material flow velocity leads to the accumulation of static electricity, resulting in fires and explosions ； ③Leakage of pipeline materials led to a fire and explosion. Corresponding control measures: ① Use iron wire for static electricity discharge and metal hoses for discharging from the tank; nitrogen is used for displacement during discharging ； ②Staff should wear anti-static work clothes, safety helmets (with guards), and protective gloves ； ③Take measures to prevent leaks at the site ； ④Implement proper security and isolation measures at the site ； ⑤Control the discharge rate to reduce impact; use a drainage tube. 2.3 Potential risks associated with unloading operations (from tank trucks to storage tanks): ① Excessive material flow rate during feeding can lead to the accumulation of static electricity, which, if not discharged in time, can result in an explosion of the explosive gas mixture between the floating disk and the liquid surface ； ②The inner floating disk gets stuck or collapses when it rises, leading to fires, explosions, etc ； ③During the feeding process, the connection point suddenly broke, resulting in material leakage that caused poisoning, fires, and other issues ； ④Other hazards. Corresponding control measures: ① Ensure proper static grounding of the tank truck, and check the relevant valves and accessories ； ②During the unloading process, inspect all connections and valves; address any leaks promptly, and keep fire-fighting and emergency supplies on site ； ③Closely monitor changes in the pump pressure and tank level; if any abnormalities occur, promptly halt unloading for inspection ； ④Before the floating plate rises to the surface, the flow velocity in the feed pipe must remain no greater than 1 m/s (for a DN50 pipe, approximately 7 t/h). Only after the floating plate has fully risen can the flow velocity be gradually increased to a maximum of 2 m/s at the inlet (for a DN50 pipe, approximately 14 t/h) ； ⑤All hot work and temporary non-explosion-proof electrical operations are prohibited in the area within the fire dike of the tank cluster and within 30 meters of the tanks ； ⑥The tank top is kept under nitrogen purging, and the area below the floating disk remains under nitrogen purging before feeding. 3. Carefully organize and implement on-site construction operations. The Equipment Engineering Department must strictly organize the work in accordance with the personnel assigned, work groupings, and sequence of operations specified in the “On-Site Construction Plan”, and designate a person responsible for on-site construction. For special operations such as hot work and temporary power use involved in construction, the local authorities must, in accordance with the requirements of the \"Safety Specifications for Special Operations in Hazardous Chemical Enterprises\" (GB 30871—2022), obtain the necessary operation permits, carry out risk identification and process risk control, and organize these operations in a thorough manner. Before construction, it is necessary to organize personnel to study the operation plan and conduct on-site inspections. It is necessary to check the readiness of tools, equipment, and materials for use; non-explosion-proof tools must not be brought into the tank area for operations. It is necessary to confirm that the material inside the pipeline has been emptied and replaced with nitrogen, while also physically disconnecting it completely from the pump and other pipelines. During construction, it is necessary to coordinate the various construction teams to prevent accidents caused by concurrent operations. When removing rusted bolts, use rust-removing loosening agents and hydraulic nut cutters; the use of non-explosion-proof tools is strictly prohibited. During this period, loading and unloading of hazardous materials shall not be carried out at the work site. At the end of construction, the site must be inspected and cleaned to ensure that \"all work is done and the site is clean.\" An expert team should be organized to inspect and verify the quality of on-site construction; any deficiencies that do not meet the technical requirements should be addressed through corrective actions ； Before sealing the storage tank, the tank area must arrange for the procedures to confirm its sealing. 4. Ensure thorough implementation of safety measures throughout the entire process. During maintenance operations, the safety and environmental protection department along with the local authorities must work together to ensure that safety measures are implemented properly at every stage. Before starting the work: (1) Regarding the weather conditions for carrying out the work, maintenance tasks should be conducted on overcast days or days with high humidity (preferably above 75%), or during rainy days, with the temperature below 28°C ； Work is prohibited in case of heavy rain, thunderstorms, and extreme weather conditions. (2) The safety and environmental department should organize all workers to receive on-site safety briefings, so that everyone is aware of the physical and chemical properties of toluene as well as the materials contained in the nearby storage tanks and pipelines, along with the relevant safety precautions ； (3) The local authority shall prepare emergency plans and organize relevant personnel from the emergency response team to conduct emergency drills. During the operation: (1) A dedicated person must be assigned for safety supervision at the work site (one from the local area and one from the Safety and Environment Department), who will be responsible for coordinating and overseeing everything on site throughout the process. (2) The safety and environmental department shall inspect the emergency supplies allocated (dry powder fire extinguishers, sand, etc.) ； Two foam fire trucks should be deployed at the site in coordination for emergency support. (3) The work area should be isolated using warning tapes; unauthorized personnel are prohibited from entering or gathering within the isolated area, and all vehicle loading and unloading operations are prohibited. After the operation: Before materials are reintroduced into the storage tank, the chief commander should organize an expert team to conduct a safety review and confirm the conditions for putting the tank back into use. In addition to the aforementioned safety measures, during manual material removal, especially when the liquid level is below the discharge pipe opening, operators must introduce nitrogen into the tank beneath the magnetic flap level gauge to ensure that the space below the floating disk remains filled with nitrogen until feeding begins into the storage tank. 5 Make every effort to ensure good coordination among various departments. Since the tank area where this storage tank is located is classified as a secondary major hazard site, multiple internal and external departments are involved in the entire maintenance process; therefore, it is necessary to strengthen coordination and communication in order to carry out the tasks together. Within the company, the warehousing department needs to coordinate with the PMC department prior to maintenance activities, develop production plans to ensure that materials are prepared in advance without disrupting production, and at the same time keep the liquid level in the storage tanks as low as possible ； Coordination with the production department is necessary to ensure proper material preparation, handling of leftover materials during maintenance periods, and so on ； Coordination with the purchasing department is required: no procurement of this material should take place before or during the maintenance work, and efforts should be made to ensure that the spare parts ordered are delivered as soon as possible ； Coordination with the Safety and Environment Department is required to submit the maintenance plan for upload to the relevant government platforms, as well as to discuss the maintenance procedures and emergency measures ； Coordination with the equipment department is required to inform them of the work plan and urge them to complete the construction plan and all preparatory tasks as soon as possible ； Coordinate with the process department to properly disable and restore the SIS interlocks ； Contact the utility department to ensure the proper operation of the foam fire suppression system and to coordinate support during maintenance work ； It is necessary to report to superiors so that they are aware of the contents and progress of the plan, thereby facilitating the smooth advancement of all tasks. Outside the company, the procurement department must coordinate with suppliers to arrange the timing of material deliveries and the arrival of spare parts. The safety and environmental department must report to the government agencies and coordinate with the fire department in order to deal with any emergencies that may arise. 6 Conclusion: Due to the high potential risks associated with the maintenance of toluene floating roof storage tanks, it is necessary to thoroughly identify these risks during the planning and execution of such operations, and to develop corresponding countermeasures. Additionally, three key points must be addressed during implementation: first, it is essential to control the flow rate of materials at each stage of the operation ； Second, it is necessary to carry out proper nitrogen displacement ； Third, it is necessary to ensure proper static electricity elimination. Through careful planning, strict organization and implementation, and focus on the key points, the risks associated with the maintenance of toluene floating roof tanks can be effectively controlled, ensuring the successful completion of the work.