Author:Site Editor Publish Time: 2018-05-22 Origin:Site
Since water vapor has a certain saturation temperature under a certain pressure, when the pressure changes, the temperature of saturated water and saturated steam also changes accordingly. If the pressure drops too quickly after the boiler is shut down, the temperature of saturated water and saturated steam will also drop significantly. Because at a lower pressure, the saturated temperature has a higher rate of change of pressure, and because the upper wall of the steam drum is in contact with saturated steam, and the lower wall is in contact with saturated water, the thermal conductivity coefficient of water is larger than steam, and the heat storage rate of the lower wall of the steam drum is The water is quickly transferred to the water, so that the temperature of the bottom wall of the drum is close to the saturation temperature under the new pressure after the pressure drop, and the heat transfer effect of the upper wall of the drum is maintained at a relatively high temperature, and the temperature of the drum wall is higher than the temperature of the bottom wall. The faster the vapor pressure drops, the greater the temperature difference between the upper and lower walls of the drum. At the same time, the vapor pressure drops too fast, the corresponding saturation temperature also decreases, and the wall temperature of the water wall, economizer, and header drops faster. Due to rapid cooling and shrinkage, large temperature stress will occur. Welds are prone to cracks, so the boiler should be naturally depressurized after normal shutdown. When the boiler is normally shut down and shut down, all the steam trap doors should be closed, and the flue damper and manhole doors should be closed so that the boiler can be kept in a closed state and naturally cooled and reduced in pressure.
The normal cooling and emergency cooling after furnace shutdown is exactly the same within the first 6 hours after the furnace shutdown. All smoke, air furnace doors and baffles must be closed. The difference between the two is that in normal cooling, natural ventilation can be performed after the boiler is stopped for 6 hours, and when the emergency cooling is allowed, the induced draft fan is allowed to be ventilated after 6 hours of stopping the furnace, and the water supply and discharge are accelerated to accelerate the cooling. .
The main factor that restricts the shutdown cooling rate is that the steam drum must not generate excessive thermal stress after the furnace is shut down. Contrary to the heating of the steam drum and boiler water on ignition and boosting, after the boiler is shut down, there is an insulation layer outside the steam drum. The wall temperature of the steam drum decreases at a slower rate than the saturation temperature of the steam and the boiler water. It is the upper steam and The lower furnace water cools the drum wall. Due to the large heat release coefficient of boiler water to the drum wall, the wall temperature of the lower half of the drum decreases rapidly, and the saturated steam becomes superheated steam under the heating of the upper half of the drum. The superheated steam not only has a small thermal conductivity, but because its temperature is higher than that of the steam and its steam, the density is lower than the saturated steam, and natural convection cannot be performed with the saturated steam. Therefore, the exothermic coefficient of steam on the upper wall of the steam drum is very small, and the temperature of the upper half of the steam drum drops slowly. The upper and lower halves of the steam drum generate heat stress due to the upward banana deformation caused by the temperature difference.
When the steam drums form a large thermal stress in the initial period of shutdown, the pressure of the steam drums is still high, and the conversion stress resulting from the superposition of the two is large. Therefore, excessive thermal stress at the beginning of the shutdown can endanger the safety of the steam drum.
Due to the thermal stress of the steam drum, it mainly depends on the steam and the speed at which the furnace water saturation temperature drops. Therefore, the most effective way to reduce the thermal stress of the drum is to slow down the pressure drop of the drum. Shut off all smoke during the first 6 hours after furnace shutdown. The blast furnace door and baffle are the best and easiest way to prevent the drum pressure from falling too fast.
Within 6 hours after furnace shutdown, due to the cooling capacity of the furnace wall and the chimney still have the ability to induce wind, cold air leaks from the imperfections such as smoke, wind furnace doors, baffles, and furnace tubes into the furnace, absorbs heat and becomes hot air, discharge. Therefore, the pressure of the steam drum is still slowly decreasing even if all smoke and air door dampers are closed. After 6 hours of shutdown, the pressure of the steam drum has dropped to a very low level. Even if the induced draft fan is started to ventilate and the upstream water and discharge water are accelerated to cool, the thermal stress of the steam drum is also small, and at this time, the pressure of the steam drum is very low. The superimposed reduced stress is also no longer a threat to the safety of the steam drum.