During the startup of a boiler, how is the temperature difference between the upper and lower walls of the steam drum generated? How to reduce the temperature difference between the upper and lower walls of the steam drum?

This post was last edited by pzhmotor on 2018-2-12 at 15:51. Does the original poster mean the temperature difference between the upper and lower walls of the steam drum? Heated tube surfaces such as the water wall and convective tube banks may experience expansion and cracking due to large temperature differences, which can also cause the flexible connections in the air ducts to break. It is usually caused by too rapid heating when starting the furnace, which is also the main reason why it is necessary to control the startup time. Another possibility is that poor water circulation makes it unsuitable to continue use; the furnace should be shut down urgently for inspection and repair.

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 of the drum. As pressure increases, the upper wall of the steam 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 has a high temperature, while the lower wall has 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 drum to deform in an arched shape. To control the temperature difference between the upper and lower walls of the steam 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 a drum can be started by using steam from the bottom of the boiler to heat the water; the steam is then evenly distributed to the burners. For natural-circulation boilers, it is also possible to release an appropriate amount of water from the lower header of the water wall. (3) Slip parameter starting is adopted.