Why is there a large temperature difference across the drum wall when a boiler is shut down for water drainage? How can this be prevented?

Control of the temperature difference between the upper and lower walls of the drum after shutdown: While we generally pay close attention to controlling the temperature difference between these two walls when the boiler is in operation, and take appropriate measures for this purpose, there are aspects related to the methods and precautions for controlling this temperature difference after shutdown that merit further discussion. Our factory stipulates that the temperature difference between the upper and lower walls of the steam drum should not exceed 56°C, but in actual operation, values higher than this limit have been observed. The operating procedures specify that water release under pressure should begin at 1 MPa, and the process should be stopped if the temperature difference increases during water release. The regulations are based on normal circumstances. The insulation performance of Boilers #3 and #4 in our plant is not very good (during normal operation of the unit, the temperature of the insulation layer at the viewing ports is extremely high; it’s impossible to touch it with one’s hand, which is a significant difference from the situation in Phase 1). As a result, it’s not possible to follow the regulations, as otherwise the temperature difference would exceed the allowed limits. The manufacturer’s instructions for Dongguo boilers also recommend that it is better to drain the boiler water while there is still a slight pressure in the steam drum, as the residual heat helps to dry the inner surfaces of the components. It has been explained here that an appropriate micro-pressure is sufficient. It is recommended that the pressure during water drainage be between 0.5 MPa and 0.8 MPa; considering the conditions of our plant, the lower limit can be adopted. In actual operation, it is important to ensure that the metal temperature at the highest point does not exceed 170°C when starting water drainage under pressure, and also to keep the temperature difference between the upper and lower walls below 20°C at the start of drainage. Once water release begins, the temperature difference will lose control. Practice has shown that stopping the water release does not help control the temperature difference. Therefore, the most important thing for us is to take preventive measures in advance. It is also worth noting that the wall temperature of the steam drum lags behind the pressure; at 1 MPa, the corresponding saturated temperature is around 170 degrees, but the metal temperature of the steam drum in our plant is often still around 210 degrees. As a result, it’s difficult to avoid excessive temperature differences.

This post was last edited by zgj2405 on 2011-7-14 at 21:30. The wall of the drum cools down gradually due to the cooling effect of the fluid inside it. As pressure decreases, the saturation temperature also drops. The saturated steam in contact with the upper wall of the drum is heated by the wall of the drum, resulting in a layer of slightly superheated steam. This layer has a low convective heat transfer coefficient, which means its cooling effect on the drum wall is poor, leading to a slow decrease in the temperature of the drum wall. In contact with the lower half of the wall is saturated water; when the pressure drops, the saturation temperature decreases, causing part of this water to vaporize into steam (simultaneously, the lower half of the wall releases some of its stored heat to warm the saturated water), which lowers the temperature of the saturated water. This results in high convective heat transfer and an effective cooling effect – that is, the lower half of the drum cools down rapidly. Therefore, when starting or stopping the furnace, it is necessary to strictly control the speed in accordance with the pressure variation curve.