What are the symptoms of a damaged water seal at the bottom of a pulverized coal boiler? How should it be handled?

When the water seal at the bottom of the boiler is lost or the water level in the slag removal machine is low, a large amount of cold air can enter the furnace from the bottom of the boiler, resulting in unstable combustion or even shutdown of the boiler. To prevent such abnormalities, the following contingency plans have been established: 1. Accident symptoms: 1. Abnormal expansion of the static blades of the boiler exhaust fan. 2. The fan speed was reduced, but the oxygen content in the flue gas increased abnormally. 3. The smoke exhaust temperature increases. 4. The temperatures of the main steam and reheat steam rise abnormally, the flow rate of the desuperheating water increases abnormally, and the wall temperatures of various heating surfaces rise abnormally or even exceed the allowable limits. 5. Large fluctuations in furnace negative pressure lead to deteriorated boiler combustion: (a. A large amount of cold air at ambient temperature passes through the combustion zone at the bottom of the boiler, disrupting the normal combustion process in that area. b. Due to the influx of large amounts of cold air, the oxygen content in the boiler flue gas increases; as a result, the opening degree of the blower vanes must decrease. When the opening degree of these vanes decreases, the secondary air pressure in the second-stage boiler drops **to zero or even lower. The rigidity of the secondary air is too low to maintain a proper level of swirl intensity. C. The temperature of the cold air that leaks into the furnace is nearly 300°C lower than the normal temperature of the secondary air, resulting in a sharp drop in the temperature in the combustion area and making it difficult for the coal powder to ignite). II. Handling: 1. When the above phenomena occur simultaneously, it is necessary to promptly realize that the water seal at the bottom of the furnace has been lost; immediately verify the situation on site and contact the ash removal team to conduct a joint inspection. 2. Use an oil gun to assist combustion; automatically disconnect the intake and exhaust fans, maintain the opening degree of the exhaust fan’s moving blades at the value appropriate for the current load, reduce the opening degree of the intake fan’s stationary blades, and keep the negative pressure in the furnace at +200~+300 Pa. 3. If the negative pressure in the furnace fluctuates greatly, an increased number of oil injectors should be used to ensure that the boiler does not go out. 4. Increase the amount of primary and secondary feedwater cooling water to ensure, to the greatest extent possible, that the wall temperatures of the heating surfaces in all parts of the boiler do not exceed the specified limits. If the tube wall temperature remains out of limit and is difficult to control, reduce the unit load and shut down the upper coal mills. 5. When it is confirmed that the water seal is lost or the water level in the slag skimmer is low, immediately notify the ash removal operation team to increase the speed of the slag removal water pump (Phase 2), open the water supply valve for the slag skimmer, and if necessary, use the boiler’s fire hose at the zero-meter mark to supply water to the slag skimmer, aiming to restore the water level to normal as quickly as possible. The shift supervisor shall, if necessary, instruct the ash removal team to close the furnace bottom hydraulic shut-off valve. 6. As the water seal at the bottom of the boiler gradually recovers, it is necessary to reduce the amount of water used for cooling the main reheated steam in a timely manner. At the same time, the air supply volume should be increased based on the rising oxygen level, in order to prevent fuel coking due to insufficient air supply and sudden drops in the temperature of the main reheated steam. 7. Once combustion stabilizes, remove the oil gun, return to normal operation mode, and conduct a comprehensive inspection of the boiler. III. Prevention: 1. Improve the quality of monitoring by conducting more thorough analysis of various parameters in the powder production and combustion systems, in order to detect any abnormalities as early as possible. 2. All centralized control operators must be able to determine on-site whether the furnace bottom water seal is functioning properly, as well as whether the slag remover and hydraulic shut-off valves are operating correctly. The department will conduct on-site tests for each shift and team by January 10th. 3. The assistant operator in charge of the furnace must check the water level of the slag skimmer and the water seal at the bottom of the furnace twice per shift. (Under normal conditions, there should be a small amount of overflow water from both the overflow pipe of the water seal at the bottom of the furnace and the overflow pipe of the slag skimmer.) Any abnormal conditions must be reported to the shift leader immediately. The pilot’s shift log should record the inspection results, and any abnormalities should be reported to the ash removal operators.

Phenomenon: 1. The opening increases and the current rises when induced draft ventilation is activated automatically； 2. The reheat steam temperature and superheated steam temperature rise, and the flow rate of desuperheating water increases ； 3. The oxygen level in the boiler increases, leading to unstable combustion that may worsen. Treatment: 1. Determine whether oil injection is needed to stabilize combustion based on the fluctuations in combustion ； 2. Immediately inspect the water level in the water seal on site, and restore the water seal at the bottom of the furnace as appropriate ； 3. Reduce the air supply volume to keep the oxygen level within the normal range ； 4. Adjust the amount of cooling water and fuel supply to ensure that the main and reheat steam temperatures do not exceed limits. If the steam temperature cannot be controlled, operation at reduced load can be adopted. Principle of water seal: A water seal is an annular water tank that remains fixed; during normal operation, water must be supplied continuously. The water that overflows from this tank is used as the water for flushing the cold ash hopper. At the very bottom of the water wall, an annular water seal baffle is welded and inserted into the water seal groove. It moves as the water wall expands upward and downward; its vertical movement prevents the water surface of the water seal from being exposed, thus preventing air from entering the furnace and thus creating a water seal.