The hazards of FeS spontaneous combustion during the maintenance of packed towers and its prevention

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Hazards and prevention of FeS spontaneous combustion during packed tower maintenance: 1. Inject water vapor into the tower; 2. Open the manhole after the vaporizer is qualified； 3. Closely monitor the temperature at various points of the tower ； 4. Use FeS flame retardant ； 5. Inspection and cleaning inside the tower. We have always used the FZC passivator to carry out cyclic cleaning of containers, towers, pipelines, and heat exchangers containing hydrogen sulfide for a certain period of time; after opening the equipment, the problem of spontaneous combustion caused by hydrogen sulfide was completely eliminated ； The hazards and prevention of FeS spontaneous combustion during the maintenance of packed towers. In processing industries that use natural gas, oil, and other materials as raw inputs, packed towers are widely used; however, as the proportion of sulfur-rich raw materials increases, the problem of sulfur corrosion is becoming increasingly severe. Sulfur corrosion products mainly appear in the form of FeS. During the shutdown and maintenance period of equipment, if no effective preventive measures are taken, FeS will oxidize or even burn rapidly when exposed to air. Therefore, removing FeS has become the first essential safety step in shutting down and maintaining processing equipment for high-sulfur raw materials. 　　 1 Mechanism of FeS formation in a packed tower 　　 FeS is a dark brown or black solid that is insoluble in water, with a density of 4.74 g/cm3 and a melting point of 1193°C. 　　Sulfur in oils is roughly divided into two categories: active sulfur and inactive sulfur. 　　Active sulfur includes elemental active sulfur (S), hydrogen sulfide (H2S), and thiols (RSH). Its characteristic is that it can react directly with metals to form metal sulfides. Above 200°C, dry hydrogen gas can react directly with iron to form FeS. The formation rate is highest between 360~390°C, and it decreases to become insignificant around 450°C. At 350–400°C, elemental sulfur readily combines directly with iron to form FeS. At this temperature, H2S can undergo decomposition: 　　H2S→S+H2 　　The active sulfur produced by this decomposition reacts very strongly with iron. 　　Above 200°C, thiol groups can also react directly with iron: RCH2CH2SH + Fe == RCHCH2 + FeS + H2. Inactive sulfur compounds include sulfides, disulfides, cyclic sulfides, thiophenes, polysulfides, and others. Its characteristic is that it cannot react directly with iron; instead, it decomposes when heated to produce active sulfur, which then reacts with iron according to the rules mentioned above. The decomposition of different sulfides at various temperatures results in varying degrees of sulfur corrosion. 　　Complex sulfides begin to decompose at 115–120°C, producing H2; the decomposition is more intense between 120–210°C, reaches its strongest level at 350–400°C, and is essentially complete at 480°C. 　　Within the process system, as long as sulfur is present, ferrous sulfide will inevitably be formed, and its formation is influenced by the temperature and flow rate of the medium, as well as the sulfur content and its form of existence. The composition and properties of ferrous sulfide also have a significant impact on its continuous formation. If the resulting ferrous sulfide has a loose structure and provides no protection for steel, it accelerates the formation of ferrous sulfide. 2. The hazards of FeS in packed towers The distribution of FeS in process equipment generally follows this pattern: the higher the sulfur content in the medium, the more FeS corrosion products are formed. However, even in equipment where the sulfur content is only a few parts per million, spontaneous combustion of FeS can occur when the equipment is opened. The reason is not the high sulfur content in the medium, but rather the fine FeS corrosion products that are carried downstream from the upstream area along with the material, and accumulate and deposit in areas where the flow velocity is relatively low. In tower equipment, the flow rate of the material inside is low, and the corrosion products of ferrous sulfide tend to deposit in certain areas; this is especially true for packed towers, where the packing not only serves a distillation function but also has an efficient filtering effect, allowing ferrous sulfide carried in from upstream to be easily trapped. At the same time, metal fillers have a large specific surface area, resulting in a large contact area with the material; even if the sulfur content in the material is low, it can still cause corrosion of the fillers. Due to the low flow rate of the material inside the packed tower, ferrous sulfide formed by corrosion on the surface of the packing is difficult to be carried away by the material. In this way, in a packed tower that operates under high load, for long periods of time, or with multiple consecutive cycles, a certain amount of ferrous sulfide will accumulate inside the tower. 　　Since the ferrous sulfide in tower equipment is not a pure substance, it mixes with coke powder, oil residues, etc. to form scale, and its structure is generally loose. When ferrous sulfide is oxidized in humid air, ferrous ions are oxidized to ferric ions, and sulfur with a valence of -2 is oxidized to sulfur with a valence of +4, releasing a large amount of heat. Due to the local temperature increase, the oxidation of iron sulfide in the surrounding area is accelerated, triggering a chain reaction. If carbon and heavy oil are present in the fouling, they will burn rapidly under the action of ferrous sulfide, releasing more heat. This spontaneous combustion phenomenon can easily lead to fire and explosion accidents. For example, in September 2003, Workshop 1 of a certain alkylbenzene plant was preparing to replace the internal components and packing of the C-405 packed tower during a major maintenance campaign. After removing oil from the tower and installing blind flanges, steam purging was carried out from September 13 to September 16. At 15:00 on September 17, the manhole in the tower was opened for ventilation ahead of the handover of the work site. However, ferrous sulfide inside the tower caught fire spontaneously upon contact with air. Around 22:00, the on-site staff noticed redness in the middle part of the tower, realized that a fire had broken out inside, and immediately called for help to extinguish it. During the firefighting efforts, at around 23:00, the middle part of the tower softened and broke, causing the upper section to tilt and fall onto the open area surrounding the structure. 　　 3 Prevention of spontaneous combustion of FeS during the maintenance of packing towers The presence of FeS, contact with oxygen in the air, and a certain temperature are the three factors that lead to spontaneous combustion of FeS during equipment maintenance. To prevent spontaneous combustion of FeS during equipment maintenance, at least one of these factors must be eliminated. 　　a) Desulfurize and dehydrate the feedstock before it enters the tower, to prevent the formation of FeS. Current desulfurization processes can effectively remove sulfides from the feedstock, with advanced treatment enabling the removal of over 99% of sulfur. This prevents sulfur corrosion and the formation of FeS at its source. Of course, the problem of sulfur corrosion still exists in the desulfurization process system. 　　Hydrogen sulfide formed inside the tower causes corrosion, damaging the tower body, internal components, and packing. However, carbon steel is essentially unaffected by anhydrous hydrogen sulfide at temperatures below 250°C, but significant corrosion occurs in its presence of water. Before the feedstock enters the tower, a \"one-desalination and three-injection\" process centered on electrodesalination and dehydration is employed. The salt content, water content of the desalted crude oil, as well as the pH, Cl- and Fe levels in the effluents from the initial distillation column, atmospheric distillation column, and vacuum distillation column are analyzed in a timely manner and strictly controlled. This reduces the corrosion of the tower equipment caused by the feedstock, thereby decreasing the amount of FeS formed in the equipment. As a result, the frequency of replacing the packing is reduced, and the risk of spontaneous combustion during maintenance is also diminished. 　　b) Control the oxygen content during maintenance to prevent spontaneous combustion of FeS. The oxidation of ferrous sulfide requires the presence of oxygen; therefore, before shutting down the equipment for maintenance, inert gases are used to displace the combustible gases inside the tower, so that the ferrous sulfide powder does not come into contact with oxygen in the air and undergo an oxidation reaction. But this method causes inconvenience in operation. When working inside the tower, it is not allowed to open both the upper and lower manholes at the same time; only the manhole at the location where work is being done should be opened. Otherwise, convection of air will occur, resulting in an **increase in the oxygen level** inside the tower. 　　c) Strict temperature control during maintenance The main factor affecting the oxidation of ferrous sulfide is temperature; therefore, after shutting down the system and performing steam purging, it is absolutely not permissible to open the manholes immediately for work. The temperature inside the tower must be reduced below room temperature, and necessary measures must be taken to prevent spontaneous combustion of ferrous sulfide before the manholes can be opened. In the case of packed towers, due to the large volume and high heat capacity of the packing, natural cooling is slow in the absence of ventilation. Cooling water can be injected from the top of the tower to increase the cooling rate, which makes the process safer. 　　d) Use of passivators to eliminate the activity of FeS. Ferrous sulfide passivators are high-efficiency chemical cleaning agents composed of chelating agents and corrosion inhibitors that have a strong chelating effect on FeS; they can effectively remove inorganic deposits such as FeS and Fe2O3 that accumulate on equipment, thereby preventing FeS from causing self-ignition and damaging the packing or equipment. It features no deposition on equipment, low corrosivity to equipment, no special impact on the environment, stable properties, being non-toxic and harmless, as well as being safe and convenient to use. The safest method to eliminate the spontaneous combustion of FeS is to carry out chemical passivation cleaning. Its activity is eliminated through passivation, thereby preventing spontaneous combustion. A Brief Analysis of Spontaneous Combustion Incidents of Ferrous Sulfide During Maintenance of Petrochemical Plants and Corresponding Countermeasures 1 Introduction Petrochemical enterprises are production facilities prone to fire and explosion, and fires and explosions caused by the spontaneous combustion of substances occur from time to time. The process of spontaneous combustion of materials is generally hidden and often not easily detected by people; spontaneous combustion accidents can sometimes be very difficult to predict. The vast majority of spontaneous combustion incidents occur during the shutdown and maintenance of production units: Case 1: On April 25, 2004, a catalytic unit had to shut down completely due to a problem with the power supply system, and the shutdown was carried out following emergency shutdown procedures. Due to the shutdown of the blower, the furnace in the acidic water stripping system went out of operation. The operators promptly cut off the gas supply, and the remaining gas in the stripping tower was discharged through the furnace’s chimney. Approximately 4–5 hours after the shutdown, thick smoke was observed coming from the chimney emitting the furnace exhaust; monitoring instruments showed a sharp rise in the temperature of the flue. The plant operators immediately closed the valve at the inlet of the acidic gas furnace and introduced temporary steam into the furnace for purging ； Due to timely detection and proper handling, which prevented further escalation of the incident, subsequent inspections revealed severe deformation in the carbon steel chimney at a height of 10 meters above the ground. 　　Case 2: On May 2, 2001, during maintenance work in the catalysis unit of a petrochemical plant, the distillation system was purged and the equipment was opened for venting. At 2 p.m. the next day, thick smoke was observed coming from the manhole of the oil-gas separator at the top of the fractionation tower; shortly after, a flash explosion occurred, accompanied by an irritating odor, which was identified as sulfur dioxide gas. The workers in the workshop immediately poured water into that tank to cool it down, thereby preventing the situation from worsening and avoiding significant losses. 　　Table 1: Sulfur content levels of crude oil at home and abroad 　　 Crude oil name | Sulfur content (%) | Year Daqing crude oil: 0.097 | 1996.8 Shengli crude oil: 0.771 | 1996.11 Huizhou crude oil: 0.031 | 1991.2 Liaohe crude oil: 0.173 | 1984.5 Lufeng crude oil: 0.112 | 1998.7 Oman crude oil: 1.089 | 1997.10 Iranian light crude oil: 1.63 | 1998.4 Iraqi Basra crude oil: 2.11 | 1997.12 Saudi light crude oil: 2.03 | 1998.3 Saudi medium crude oil: 2.56 | 1994.5 Saudi heavy crude oil: 3.09 | 1994.8 Dubai crude oil: 1.66 | 1994.2 According to relevant statistics, China’s annual growth rate in oil consumption is 4%, while the annual growth rate of domestic crude oil production is less than 1%; the shortfall must be covered by imported crude oil. As can be seen from Table 1, the sulfur content in imported crude oil from foreign countries is relatively high; especially for crude oil from the Middle East, it is above 1%, with the highest value reaching 3.09%. Processing high-sulfur crude oil not only poses new challenges in product quality control and environmental protection, but also creates safety risks in daily production and maintenance processes due to increased equipment corrosion. In particular, spontaneous combustion incidents caused by ferrous sulfide during plant maintenance are difficult to predict and occur quite frequently. Therefore, it is crucial for safe production to know how to avoid and properly handle such spontaneous combustion incidents. 　　2 Causes of the formation of ferrous sulfide and its spontaneous combustion mechanism 2.1 Causes of the formation of ferrous sulfide (1) Formation of ferrous sulfide through electrochemical corrosion reactions Over 80% of the sulfur in crude oil is present in atmospheric vacuum residue; these sulfides have complex structures, and under high temperatures, especially in the presence of catalysts, they readily decompose to form hydrogen sulfide and smaller molecular thiol compounds. In the presence of water, these hydrogen sulfides and thiol compounds have a significant corrosive effect on iron-based equipment. The reaction processes are as follows:
H2S → H+ + HS-
HS- → H+ + S2-
This is an electrochemical corrosion process:
Anodic reaction: Fe → Fe2+ + 2e-
Cathodic reaction: 2H+ + 2e- → H2 (penetrating into the steel)
Fe2+ reacts with S2- and HS- to form FeS↓:
Fe2+ + S2- → FeS↓
Fe2+ + HS- → FeS↓ + H+
Additionally, sulfur can react directly with iron to form ferrous sulfide: Fe + S → FeS↓
The ferrous sulfide formed has a loose structure and adheres evenly to the inner walls of equipment and pipes. 　　(2) Atmospheric corrosion reactions produce ferrous sulfide. Due to prolonged shutdown of the facility, the internal components are exposed to air for long periods, which leads to atmospheric corrosion and the formation of rust. Since rust is difficult to remove completely, it reacts with hydrogen sulfide during the production process to form ferrous sulfide. 　　The reactions are as follows: Fe + O2 + H2O → Fe2O3•H2O; Fe2O3•H2O + H2S → FeS↓ + H2O. These reactions proceed relatively easily, and due to prolonged shutdowns, equipment with inadequate corrosion protection tends to produce ferrous sulfide. 　　2.2 Mechanism and phenomena of spontaneous combustion of ferrous sulfide (1) Mechanism of spontaneous combustion of ferrous sulfide When ferrous sulfide and other iron sulfides are exposed to heat or light in the air, the following reactions occur: FeS + 3/2 O2 = FeO + SO2 + 49 KJ; 2FeO + 1/2 O2 = Fe2O3 + 271 KJ; FeS2 + O2 = FeS + SO2 + 222 KJ; Fe2S3 + 3/2 O2 = Fe2O3 + 3S + 586 KJ. (2) Phenomena of spontaneous combustion of ferrous sulfide During the spontaneous combustion of ferrous sulfide, if there is no combustible material present to support the reaction, white SO2 gas is produced, which is often mistaken for water vapor and has a pungent odor ； A large amount of heat is released at the same time. When other flammable materials (such as oils) are present in the vicinity, thick smoke will be generated, leading to fires and explosions. 　　3 Factors Affecting the Formation Rate of Ferrous Sulfide As can be understood from the mechanism of ferrous sulfide formation, in daily production, the formation of ferrous sulfide is a chemical corrosion reaction that occurs as iron reacts with active sulfides. Therefore, controlling the chemical corrosion reaction is a key means to limit the formation of ferrous sulfide. As long as we identify the areas of the production equipment prone to sulfur corrosion and take effective measures based on the characteristics of each area, we can reduce the amount of ferrous sulfide generated, thereby preventing the occurrence of spontaneous combustion incidents caused by ferrous sulfide. Factors such as the sulfur content of the oil, temperature, the presence of water, and Cl- are important factors that influence the rate of this electrochemical corrosion reaction. 　　3.1 Distribution patterns of sulfur during crude oil processing Sulfur chemical corrosion occurs only in the presence of sulfur ； The areas with high sulfur content in the oil are the most prone to corrosion. Therefore, analyzing the sulfur distribution in crude oil during the processing stage is instructive for controlling the formation of ferrous sulfide. 　　 　　 　　Table 2 Sulfur distribution in fractions after atmospheric distillation
| No. | Crude oil name | Crude oil type | Sulfur content in crude oil, % | Fraction range | Sulfur content in fraction, % | Percentage of total sulfur in crude oil |
|-----|----------------|----------------|-------------------------------|----------------|------------------------------|----------------------------------------|
| 1 | Oman crude oil | Intermediate-base | 1.16 | HK~200°C | 0.03 | 0.32 |
| | | | | 200~300°C | 0.10 | 1.43 |
| | | | | 300~350°C | 0.48 | 8.7 |
| | | | | >350°C | 1.97 | 89.6 |
| 2 | Zhongyuan crude oil | Paraffin-base | 0.65 | HK~200°C | 0.04 | 0.92 |
| | | | | 200~300°C | 0.13 | 3.6 |
| | | | | 300~350°C | 0.29 | 6.3 |
| | | | | >350°C | 1.09 | 89.1 |
| 3 | Tarim crude oil | Paraffin-base | 0.71 | HK~200°C | 0.04 | 1.32 |
| | | | | 200~300°C | 0.15 | 4.7 |
| | | | | 300~350°C | 0.47 | 8.3 |
| | | | | >350°C | 1.47 | 85.3 |
| 4 | Shengli crude oil | Intermediate-base | 0.84 | HK~200°C | 0.02 | 0.12 |
| | | | | 200~300°C | 0.08 | 1.2 |
| | | | | 300~350°C | 0.32 | 3.5 |
| | | | | >350°C | 1.08 | 95.2 |
| 5 | Changqing crude oil | Paraffin-base | 0.14 | HK~200°C | 0.03 | 3.0 |
| | | | | 200~300°C | 0.04 | 4.2 |
| | | | | 300~350°C | 0.08 | 5.0 |
| | | | | >350°C | 0.24 | 87.8 |
| 6 | Tahe crude oil | Intermediate-base | 1.43 | HK~200°C | 0.01 | 0.12 |
| | | | | 200~300°C | 0.08 | 1.0 |
| | | | | 300~350°C | 0.66 | 4.9 |
| | | | | >350°C | 2.07 | 94.0 |

Table 3 Sulfur distribution in products after catalytic cracking
| No. | 1 | 2 |
|-----|---|---|
| Sulfur content in feedstock oil, % | 0.80 | 0.92 |
| Product names | Dry gas, LPG, gasoline, diesel, oil slurry, coke | Dry gas, LPG, gasoline, diesel, oil slurry, coke |
| Product yields | 3.28, 12.43, 33.29, 32.55, 6.06, 12.39 | 3.78, 14.72, 35.13, 29.46, 4.85, 12.06 |
| Sulfur content in products | 8.19, 0.15, 0.08, 0.56, 1.46, 1.66 | 8.06, 0.05, 0.11, 0.79, 1.54, 2.21 |
| Sulfur distribution in this process | 33.2, 2.3, 5.7, 22.5, 10.9, 25.4, 32.5, 0.8, 5.6, 24.8, 7.9, 28.4 | 20.4, 5.4, 13.58, 29.7, 7.34, 33.46, 26.6, 3.48, 3.63, 36.8, 17.1, 12.4 |

(1) As can be seen from Table 2, after atmospheric distillation of crude oil, 85% of the sulfur is concentrated in the fractions above 350°C, i.e., in the atmospheric residue. Therefore, equipment through which this atmospheric residue flows is highly susceptible to sulfur corrosion ； In actual production, the internal components of the vacuum tower and the heat exchangers in the vacuum unit are the areas where ferrous sulfide is most likely to form. (2) As can be seen from Table 3, approximately 70% of the sulfur enters the distillation, absorption, and stabilization system along with the reaction oil and gas ； Nearly 30% of the sulfur is present in coke and is discharged with the regenerated flue gas. Therefore, the distillation column top condensation system, the condensed oil filling of the absorption and stabilization system as well as the reboiler, and the diesel extraction system are areas where ferrous sulfide is likely to form. (3) Acidic water treatment systems with high sulfur content, as well as equipment through which acidic water flows, are also locations prone to sulfur corrosion. 3.2 High-temperature sulfur corrosion: The sulfur corrosion reaction is a chemical corrosion reaction, and an increase in temperature accelerates the reaction rate. Therefore, high-temperature sulfur corrosion tends to occur more easily in the atmospheric pressure tower bottom and normal residue heat exchange units, the vacuum unit, and the catalytic cracking diesel extraction system, where the logistics temperature is high. 3.3 The presence of water and Cl‑ can promote sulfur corrosion in equipment. According to the reaction mechanism for the formation of ferrous sulfide, the presence of water accelerates chemical corrosion. When Cl‑ is present, corrosion occurs even at lower temperatures through the following reactions: Fe + 2HCl → FeCl2 + H2↑; FeCl2 + H2S → FeS↓ + 2HCl; Fe + H2S → FeS↓ + H2↑; FeS + 2HCl → FeCl2 + H2S. In the condensation systems at the top of atmospheric pressure towers, such as the tower top, oil-gas vaporization lines, water coolers, and reflux tanks, low-temperature H2S-HCl-H2O corrosion is likely to occur. 4 Case Analysis Case 1 Accident Analysis: ○1 Prolonged power outages caused the fans to stop operating, resulting in the shutdown of the incinerator; acidic gases with high H2S concentrations were then emitted through the chimney ； Since the furnace temperature (around 900°C) remains high, hydrogen sulfide in the acidic gas, after being preheated, reacts with carbon steel to form ferrous sulfide. ○2 Due to the strong winds at that time, the speed at which air entered the furnace and chimney increased. As the amount of acidic gases decreased and the oxygen content rose, ferrous sulfide reacted spontaneously with oxygen, releasing a large amount of heat; this ultimately caused the iron chimney to become severely deformed. ○During maintenance, it was found that a large area around the deformed section of the chimney had thinned out and was severely corroded. Analysis suggests that during startup/shutdown processes as well as normal operational fluctuations, hydrogen sulfide gas that had not been converted into SO2 was present in the flue gases, leading to the continuous formation of ferrous sulfide. Furthermore, the deformed section is not insulated, which may lead to dew point corrosion and cause the pipe to thin out. ○4. Having learned from the lessons of this incident, the workshop replaced the chimney’s material from carbon steel to a more corrosion-resistant steel, and improved the chimney’s insulation. Additionally, a steam line is installed on the fan outlet pipeline; when the incinerator is turned off, it can be used to purge unreacted gases, thereby reducing the likelihood of the formation of ferrous sulfide. Case 2 Accident Analysis: ○1 Upon entering the tank for inspection, a thick layer of rust-like substance was found deposited at the bottom of the tank; tests revealed a high content of ferrous sulfide. ②Due to the prolonged downtime, the internal components of the equipment remain exposed to the air for long periods, which leads to atmospheric corrosion and the formation of rust ； Cleaning before starting up operations makes it difficult to remove this substance; during production, rust and hydrogen sulfide react to form ferrous sulfide. During the next shutdown for purging, the layer of ferrous sulfide is removed as a result of the purging process, and it enters the oil-water separation tank with the air flow, where it settles down. ③Due to the hot weather with temperatures reaching 30°C, the accumulation of heat causes the oil film and moisture on the surface of \"rust\" to evaporate, exposing it to direct contact with air. This ultimately leads to the spontaneous ignition of the dry ferrous sulfide, which in turn ignites the gas and causes a flash explosion. 5 Countermeasures for Preventing Spontaneous Combustion of Ferrous Sulfide 5.1 Controlling the Formation of Ferrous Sulfide at its Source The formation of ferrous sulfide is a process of equipment corrosion; it is necessary to take measures from various aspects to reduce sulfur-induced corrosion of the equipment. (1) Start from the process perspective to reduce sulfur corrosion of equipment and control the formation of ferrous sulfide. ○1 Strengthen the \"one removal and four injections\" system in atmospheric pressure devices to suppress corrosion. Based on the actual conditions of the crude oil, an effective demulsifier is selected to optimize the electrodialysis process and increase the removal rate of inorganic salts (such as MgCl2 and CaCl2), thereby reducing the Cl‑ content at the top of the tower. Use corrosion inhibitors suitable for high-sulfur feedstocks to reduce the corrosion rate. Increase the ammonia injection amount appropriately to reduce sulfur corrosion. ○2 The residue hydrogenation conversion process is used to reduce the sulfur content of atmospheric residue. Catalytic cracking units have high requirements regarding the sulfur content in residual oil. When processing crude oils with high sulfur content, residue hydrogenation conversion technology can be employed to reduce the levels of sulfur, gums, nitrogen, and other substances in the residue, which helps to minimize corrosion of the catalytic equipment while producing high-quality products. ○3 A corrosion inhibitor was tentatively added at the top of the distillation tower to form a protective film on the surface of the steel, thereby preventing corrosion. (2) Take measures on the equipment side to prevent the formation of ferrous sulfide. ○1. Areas prone to sulfur corrosion should be replaced with corrosion-resistant steel. Taking costs into account, select corrosion-resistant steel with a good cost-performance ratio; for example, choose aluminized steel, which offers a reasonable price and corrosion resistance comparable to that of the expensive 316L steel. ○2. The spray coating isolation technique is employed: corrosion-resistant metals are sprayed onto or corrosion-resistant materials are applied to the inner surfaces of equipment prone to corrosion in order to achieve isolation and corrosion protection. However, in the production process, if the flow rate of the oil passing through the equipment and pipelines is high, or if the wear-prone parts in the equipment are not suitable for the use of spray coating isolation technology. ○3 Strengthen anti-corrosion protection during the shutdown period. For units that have been out of operation for a long time, measures such as sealing them with blind plates and injecting nitrogen to displace air should be taken to prevent atmospheric corrosion. (3) Strengthen daily operational management: Enhance the management of operations at relevant positions to prevent the continuous formation of ferrous sulfide due to improper handling. 5.2 Use chemical treatment methods to eliminate ferrous sulfide. In materials such as pressure reduction tower packing and acidic water stripping trays, areas prone to the formation of ferrous sulfide can be treated using chemical methods. (1) Pickling: Dilute hydrochloric acid can be used for cleaning in order to eliminate the presence of ferrous sulfide, but this process releases hydrogen sulfide gas; therefore, an additional hydrogen sulfide inhibitor is required to convert and remove this gas. (2) Chelant treatment: Special highly acidic chelants are very effective at dissolving sulfide precipitates without producing hydrogen sulfide gas, but they are relatively expensive in practice. (3) Oxidation treatment: Sulfides can be oxidized using the oxidant potassium permanganate, which offers the advantages of safety in use and ease of implementation. 5.3 Precautions during shutdown for maintenance (1) Prepare a plan to prevent spontaneous combustion of ferrous sulfide before shutting down. Before parking, formulate a plan to address the spontaneous combustion of ferrous sulfide based on the characteristics of the device itself and past experience; once a spontaneous combustion incident occurs, take immediate action to prevent the spread of the incident and minimize economic losses. (2) During the purging and cleaning of the equipment, special attention must be paid to dead zones such as elbows and corners, and care should be taken to drain condensate from low points in order to ensure the quality of the purging process and prevent the presence of residual oil and gas. Thereby preventing explosions and the spread of fires caused by the spontaneous combustion of ferrous sulfide. (3) The equipment can be opened only when it has returned to room temperature. Before entering, rinse it with clean water to ensure that the internal components are moist; the removed ferrous sulfide should be placed in bags, wetted, and taken out of the equipment, after which it should be buried deep as soon as possible. (4) Strengthen inspections. During maintenance, especially in environments with high temperatures, inspections must be intensified to detect issues promptly and address them right away.