Daily Question (April 16)

During the shutdown of a boiler, how does a temperature difference between the upper and lower walls of the steam drum arise? How to reduce the temperature difference between the upper and lower walls of the steam drum? During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Adopt a sliding-parameter shutdown.

During the shutdown of a boiler, how does a temperature difference between the upper and lower walls of the steam drum arise? How to reduce the temperature difference between the upper and lower walls of the steam drum? During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Adopt a sliding-parameter shutdown.

During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Adopt a sliding-parameter shutdown.

During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Sliding-parameter shutdown is adopted at Unit 1 in Dongfang

During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally adopted: 1. Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. 2. Use a sliding parameter shutdown.

During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Adopt a sliding-parameter shutdown.

During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Adopt a sliding-parameter shutdown.

During the shutdown of a boiler, how does a temperature difference between the upper and lower walls of the steam drum arise? How to reduce the temperature difference between the upper and lower walls of the steam drum? During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Adopt a sliding-parameter shutdown.

During the shutdown of a boiler, how does a temperature difference between the upper and lower walls of the steam drum arise? How to reduce the temperature difference between the upper and lower walls of the steam drum? During the shutdown of the boiler, the steam pressure gradually decreases and the temperature drops as well; the wall of the steam drum cools down gradually due to the cooling effect of the fluid inside it. As the pressure decreases, the saturation temperature also drops. The vapor in contact with the upper wall of the drum is saturated vapor; heated by the wall of the drum, it forms a layer of slightly superheated vapor. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, and thus the temperature of the drum wall decreases slowly. The water in contact with the lower wall of the drum is saturated water; when the pressure drops, some of this water vaporizes on its own due to the decrease in saturation temperature, allowing the water to quickly reach the saturation temperature corresponding to the new pressure. This results in a high convective heat transfer coefficient and excellent cooling effects, enabling the lower wall of the drum to reach the new saturation temperature rapidly. As a result, compared to the startup process, the temperature of the upper wall of the steam drum becomes higher than that of the lower wall. The lower the pressure, the faster the rate of pressure reduction, and the more pronounced this temperature difference becomes. The control criteria for the temperature difference between the upper and lower walls of the drum during shutdown are the same as those during startup. To keep the temperature difference between the upper and lower walls within limits, the following measures are generally taken: (1) Strictly control the rate of pressure reduction in accordance with the pressure reduction curve. (2) Use sliding-parameter shutdown

During the startup process, the drum wall absorbs heat from the working fluid, causing its temperature to rise gradually. At the beginning of operation, the boiler water circulation has not yet been established properly; the water in the drum is at rest, resulting in a very low convective heat transfer coefficient on the drum walls, which means heating occurs very slowly. The upper part of the drum is in contact with saturated steam; as the pressure increases, some of the steam in contact with the wall will condense, releasing heat through this condensation process. The convective heat transfer coefficient at this area is many times higher than that of the water in the lower part. As the pressure rises, the upper wall of the drum can approach the saturation temperature at that pressure more quickly, while the lower wall heats up more slowly. This results in a situation where the upper wall of the steam drum is at a high temperature while the lower wall is at a low temperature. The faster the boiler pressure rises, the greater the temperature difference between the upper and lower walls. The temperature difference between the upper and lower walls of the drum causes compressive stress on the upper wall and tensile stress on the lower wall. The greater the temperature difference, the greater the stress; in severe cases, this causes the steam drum to deform in an arched shape. To control the temperature difference between the upper and lower walls of the drum, the following measures should be taken: (1) Strictly control the pressure increase rate in accordance with the boiler’s pressure rise curve. The heating rate should be controlled such that the temperature rise at the lower wall of the drum is 0.5–1°C/minute, and the rise in the saturation temperature of the drum should not exceed 1.5°C/minute. (2) Forced-circulation boilers and natural-circulation boilers with drum systems can use steam at the bottom of the boiler to drive the feed process, utilizing steam to heat the water in the boiler and ensuring even distribution of it to the burners; natural-circulation boilers can also employ methods such as releasing an appropriate amount of water from the lower header of the water wall. (3) Sliding parameter start is adopted.