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The fundamental way to prevent medium concentration corrosion is to eliminate the conditions that cause corrosion, that is, to eliminate the conditions for furnace water to produce free alkali and acid and the conditions for furnace water to produce local concentration. Due to oxygen corrosion, carbon dioxide corrosion and furnace shutdown corrosion, the iron content of the feed water increases, and these corrosions will promote medium concentration corrosion. In this way, the method to prevent these corrosions also indirectly prevents medium concentration corrosion. Of course, some preventive measures not only have indirect significance, but also directly prevent medium concentration. For example, reducing the carbonate alkalinity of the feed water is not only to prevent carbon dioxide corrosion, but also to eliminate or reduce the free NaOH content of the furnace water, which plays an important role in preventing medium concentration corrosion. In order to avoid repetition, when introducing methods to prevent medium concentration corrosion, all methods that have been covered in previous chapters will only be briefly mentioned, and the focus will be on discussing various methods of treatment in the pot. one. Maintain the good condition of the boiler heating surface. Maintain the good condition of the boiler heating surface, including two aspects.: On the one hand, the surface is clean, and on the other hand, a good holding film is formed on the surface. Only when the surface is clean can a good protective film be formed on the surface. To keep the surface clean, firstly, prevent corrosion of the heating surface during manufacturing, installation, startup and decommissioning. ； Binary prevents metal oxides from depositing on surfaces ； Third, when the heating surface is not clean, chemical cleaning should be performed. two. Reduce the copper and iron content in water supply In order to reduce the copper and iron oxide content in water supply, it is necessary to prevent oxygen corrosion in the water supply system and reduce oxygen corrosion and carbon dioxide corrosion in the condensate water system and hydrophobic system. At the same time, it is necessary to prevent corrosion and corrosion products from being brought into the pot when the equipment is out of service. Measures should be taken to prevent corrosion of the treatment system outside the furnace to reduce the iron content of the feed water. In addition, attention should be paid to preventing corrosion of the condenser copper tubes and heater copper tubes to reduce the copper content of the condensate and feed water. If the condensate water, hydrophobic water and production return water have high copper and iron oxide content, different measures should be taken for treatment according to different parameters and water quality requirements of different types of boilers. For high-pressure and ultra-high-pressure drum furnaces, if the iron content of hydrophobic and production return water is too high, iron removal treatment should be carried out. For once-through furnaces and drum boilers with subcritical parameters and above, because they have high requirements on water quality, the condensate, dewatering and production return water must be iron-removed. three. Adopt a reasonable boiler design and installation plan. In order to prevent medium concentration corrosion, the design and installation of the boiler should be reasonable to ensure that the tube wall temperature and water circulation conditions meet the requirements when the boiler is running. For example, in order to ensure that the water vapor flow rate in the furnace tube meets the requirements, the furnace tube should have a certain slope, and there should be no sharply turning tubes. When welding the furnace tube, the welding joint should not have a convex ring, and the position of the welding joint should be away from the high heat load area. In order to prevent corrosion of copper in the furnace front system and reduce copper deposition in the pot, foreign subcritical and supercritical pressure units tend to replace copper components with steel. For example, the pipe material for Japanese high-pressure heaters is: When the turbine capacity is greater than 250MW, Monel alloy (containing Cu30%, Ni70%, and a small amount of Fe, Mn and other components) and carbon steel are used ； When the turbine capacity is 600MW, carbon steel is used. In the United States, when the boiler pressure is 16.46MPa, the low-pressure heater pipe is made of brass or carbon steel ； When the boiler pressure is 24.01MPa, both the low-pressure heater and the high-pressure heater are made of carbon steel. Of course, using carbon steel only solves the problem of copper deposition, but does not solve the problem of iron oxide deposition. At this point, attention should be paid to taking measures to prevent corrosion of carbon steel, such as appropriately increasing the pH value of the feed water. Four. Maintain the correct operation mode of the boiler. The correct operation mode of the boiler mentioned here mainly refers to keeping the boiler load stable, that is, the boiler load should not fluctuate too much and should not be overloaded for a long time. To keep the boiler load stable, the combustion of the boiler must be stable, the number of starts and stops cannot be too many, and the operation must not exceed the nameplate output. five. Reduce the corrosive components of the feed water. From the perspective of preventing medium concentration corrosion, the carbonate content, Cl- content and pH value of the feed water should be controlled. For this reason, condenser leakage and inorganic acid pollution must be strictly prevented. For large-capacity and high-pressure boilers, foreign countries have strict requirements on the allowable degree of condenser leakage. The standard is: The maximum salt content of the feed water is 0.5 mg/L. If the salt content of the feed water reaches 0.5 to 2 mg/L, and the chemical treatment of the boiler water can ensure that the boiler water quality is within the allowable range, it can be allowed to operate for a short period of time. If the salt content in the feed water exceeds 2 mg/L due to condenser leakage, the machine should be shut down immediately and leak plugging measures should be taken to eliminate condenser leakage. Regarding the Cl- content in the feed water, the UK stipulates that for a steam drum furnace with a pressure of 10.29MPa, Cl- should not be greater than 4mg/L, and for a steam drum furnace with a pressure of 16.07MPa, Cl- should not be greater than 2.5mg/L. If misoperation occurs during the regeneration of water treatment equipment, the regeneration fluid leaks into the water supply system, and the water supply will be contaminated. In this case, immediate measures should be taken to deal with it. six. Choose a reasonable water treatment method in the pot. Because the water supply will inevitably bring some impurities into the pot, chemical treatment in the pot is completely necessary. At present, the methods of processing in the pot include: Phosphate treatment, volatilization treatment, neutral treatment. The first two types of treatment methods are to keep the boiler water in the alkaline range, also known as alkaline treatment. Neutral processing is to keep the temperature in the neutral range. In addition, some boilers use complexing agents and dispersants for in-pot treatment. The various methods of processing in the pot are briefly introduced as follows. 1. Phosphate treatment Phosphate treatment is a widely used treatment method. It can prevent scale, keep the boiler water alkaline, and neutralize the acid generated in the pot due to condenser leakage. There are two methods of phosphate treatment. Ordinary phosphate treatment. The content of PO43- in the boiler water is relatively high, which can prevent scale, keep the boiler water alkaline and neutralize the acid entering the pot. However, the boiler water PO43- is too high. Under the conditions of a high-parameter boiler, Na3PO4 itself may produce free NaOH. In addition, some scholars pointed out that when the purity of the furnace water is very high, even if the phosphate added to the furnace water exists entirely in the form of Na3PO4, when the furnace water is partially concentrated, the concentrated Na3PO4 solution will destroy the protective film. Because Na+ may replace Fe2+ in Fe3O4, and the radius of Na+ is 9.8nm and the radius of Fe2+ is 8.3nm. The radius of Na+ is 18% larger than that of Fe2+. The result of the substitution will distort the Fe3O4 lattice and cause the destruction of the Fe3O4 protective film. If K3PO4 is used instead of Na3PO4, the damage to the protective film will be greater, because the radius of K+ is 13.3nm, which is 60% larger than the ionic radius of Fe+, making the distortion of the crystal lattice more serious. Therefore, for high-parameter boilers, when the feed water hardness is very small (