How to properly maintain the boiler during shutdown period？
Industrial boilers are commonly used in electric power, chemical industry, light industry and other industries, and are more widely used in the lives of enterprises and institutions. When the boiler is out of use, a large amount of air will flow into the boiler’s water system. Although the boiler has discharged water, there is a water film on its metal surface, and oxygen will be dissolved in it, resulting in saturation, which leads to Oxygen erosion. When there is salt scale on the metal surface of the boiler, which can be dissolved in the water film, this corrosion will be more serious. Practice shows that severe corrosion in boilers is mostly formed during the shutdown process and continues to develop during use. Therefore, taking correct protective measures during the shutdown process is of great significance to prevent boiler corrosion, ensure safe operation, and extend the service life of the boiler.
There are many methods to prevent boiler shutdown corrosion, which can be divided into two categories: dry method and wet method.
1. Dry method
1. Desiccant method
Desiccant technology means that after the boiler is stopped, when the water temperature drops to 100~120°C, all the water will be discharged, and the waste heat in the furnace will be used to dry the metal surface; at the same time, the scale precipitated in the boiler water system will be removed , water slag and other substances are discharged. Desiccant is then injected into the boiler to keep its surface dry to avoid corrosion. Commonly used desiccants include: CaCl2, CaO, and silica gel.
Placement of desiccant: Divide the medicine into several porcelain plates and place them on different boilers. At this time, all soda and water valves must be closed to prevent the inflow of outside air.
Disadvantages: This method is only hygroscopic. It must be inspected after adding the desiccant. Always pay attention to the deliquescence of the medicine. If deliquescence occurs, replace it in time.
2. Drying method
This method is to drain the water when the boiler water temperature drops to 100~120°C when the boiler is shut down. When the water is exhausted, use the residual heat in the furnace to simmer or introduce hot air into the furnace to dry the inner surface of the boiler.
Disadvantages: This method is only suitable for temporary protection of boilers during maintenance.
3. Hydrogen charging method
The nitrogen charging method is to charge hydrogen into the boiler water system and maintain a certain positive pressure to prevent air from entering. Since hydrogen is very inactive and non-corrosive, it can prevent boiler shutdown corrosion.
The method is: before shutting down the furnace, connect the nitrogen filling pipeline. When the pressure in the furnace drops to 0.5 gauge, the hydrogen cylinder begins to send nitrogen to the boiler drum and economizer through temporary pipelines. Requirements: (1) Nitrogen purity should be above 99%. (2) When an empty furnace is filled with nitrogen; the nitrogen pressure in the furnace should be above 0.5 gauge pressure. (3) When filling with nitrogen, all valves in the pot water system should be closed and should be tight to prevent leakage. (4) During the nitrogen charging protection period, the pressure of hydrogen in the water system and the tightness of the boiler must be constantly monitored. If excessive nitrogen consumption is found, the leakage should be found and eliminated immediately.
Disadvantages: You need to pay strict attention to hydrogen leakage problems, check on time every day, and deal with problems in a timely manner. This method is only suitable for the protection of boilers that are out of service for a short period of time.
4. Ammonia filling method
The ammonia filling method is to fill the entire volume of the boiler with ammonia gas after the boiler is shut down and water is released. Ammonia dissolves in the water film on the metal surface, forming a corrosion-resistant protective film on the metal surface. Ammonia can also reduce the solubility of oxygen in the water film and prevent corrosion by dissolved oxygen.
Disadvantages: When using the ammonia filling method, the copper parts should be removed to maintain the ammonia pressure in the boiler.
5. Coating method
After the boiler is out of service, drain the water, remove dirt, and dry the metal surface. Then evenly apply a layer of anti-corrosion paint on the metal surface to prevent out-of-service corrosion of the boiler. Anti-corrosion paint is generally made of black lead powder and engine oil in a certain proportion. When coating, it is required that all parts that can be contacted must be evenly coated.
Disadvantages: This method is effective and suitable for long-term furnace shutdown maintenance; however, it is difficult to operate in practice and is not easy to paint at corners, welds, and pipe walls that are prone to corrosion, so it is only suitable for theoretical protection.
2. Wet method
1. Alkaline solution method:
This method uses the method of adding alkali to fill the boiler with water with a pH value of above 10. Form a corrosion-resistant protective film on the metal surface to prevent dissolved oxygen from corroding the metal. The alkali solution used is NaOH, Na3PO4 or a mixture of the two.
Disadvantages: Care needs to be taken to maintain a uniform alkali concentration in the solution, frequently monitor the boiler pH value, and pay attention to the formation of derived scale.
2. Sodium sulfite protection method
Sodium sulfite is a reducing agent that reacts with dissolved oxygen in water to form sodium sulfate. This prevents metal surfaces from being corroded by dissolved oxygen. In addition, the protection method of mixed solution of trisodium phosphate and sodium nitrite can also be used. This method is based on the fact that this mixed liquid can form a protective film on the metal surface to prevent metal corrosion.
Disadvantages: When using this wet protection method, the solution should be drained cleanly and thoroughly cleaned before starting the saw furnace, and water should be added again.
3. Heat method
This method is used when the shutdown time is within 10 days. The method is to install a water tank above the steam drum and connect it to the steam drum with a pipe. After the boiler is deactivated, it is filled with deoxygenated water, and most of the water tank is filled with water. The water tank is heated by external steam, so that the water in the water tank always maintains a boiling state.
Disadvantage: The disadvantage of this method is that it requires an external steam source to supply steam.
4. Protection method for stopping (backup) use of film-forming amines
This method is to add organic amine film-forming agents to the thermal system when the boiler pressure and temperature drop to appropriate conditions during the shutdown of the unit. The agents circulate with the steam and water, and the agent molecules are tightly adsorbed on the metal surface and oriented sequentially. The arrangement forms a molecular protective layer with a “shielding effect” to prevent the migration of charges and corrosive substances (oxygen, carbon dioxide, moisture) on the metal surface to achieve the purpose of preventing metal corrosion.
Disadvantages: The main component of this agent is high-purity linear alkanes and vertical film-forming amines based on octadecylamine. Compared with other agents, it is more expensive and troublesome to administer.
The above maintenance methods are easier to operate in daily use and are used by most factories and enterprises. However, in the actual operation process, the choice of maintenance methods is also very different due to different reasons and times for shutting down the furnace. In actual operation, the selection of maintenance methods generally follows the following points:
1. If the furnace is shut down for more than three months, the desiccant method in the dry method should be used.
2. If the furnace is shut down for 1-3 months, the alkali solution method or sodium nitrite method can be used.
3. After the boiler stops running, if it can be started within 24 hours, the pressure maintaining method can be used. This method can also be used for boilers that operate intermittently or are out of service within a week. But the pressure in the furnace must be higher than atmospheric pressure. If the pressure is found to drop slightly, a fire must be started to increase the pressure in time.
4. When the boiler is stopped due to maintenance, the drying method can be used. If there is no need to release water, the pressure maintaining method can be used. If the boiler after maintenance cannot be put into operation in time. Corresponding protection measures should be adopted according to the length of credit period.
5. When using wet protection, it is best to keep the temperature in the boiler room above 10°C and not lower than 0°C to avoid freezing damage to the equipment.