[Weekly Topic] Why are time and temperature limits set for filling the boiler before starting it up? ? (5.15--5.22)
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This post was last edited by zgj2405 on 2011-5-14 at 18:13. The water inlet speed before starting the boiler should not be too fast; generally, it should take no less than 4 hours in winter, and 2–3 hours in other seasons. The speed of water inflow should especially be slow at the beginning. The feedwater temperature of cold boilers is generally not higher than 100°C, so that the difference between the feedwater temperature entering the drum and the temperature of the drum wall does not exceed 40°C. For a boiler that has not yet cooled down completely, the inlet water temperature can be set based on the wall temperature of the steam drum; generally, the difference should be kept within 40°C. Otherwise, the inlet water flow rate should be reduced. What is the reason for this?
Because the wall of the steam drum is too thick, large temperature differences can result in excessive thermal stress, which may even cause the steam drum to deform; therefore, it is necessary to control these temperature differences
It is mainly to extend the lifespan and safety of the boiler. Before starting the boiler, fill it with water slowly to prevent excessive temperature differences from causing thermal expansion and contraction, which could affect the boiler’s lifespan ； If water is added too quickly, the pressure exerted on the boiler will increase suddenly, which can easily cause damage to some of its components.
Reply 1# zgj2405: Heat stress is generated, which affects the equipment’s lifespan!
Reply to 1# zgj2405: 1. Since the wall of the steam drum is thick, its expansion is slow, whereas the walls of the tubes connected to the steam drum are thin, resulting in faster expansion. If the inlet water temperature is too high or the flow rate is too fast, it will lead to uneven expansion, causing cracks in the welds and resulting in equipment damage ； 2. When feedwater enters the drum, it first comes into contact with the lower half of the drum’s wall. If the temperature difference between the feedwater and the drum wall is large, and the flow rate of the feedwater is high, a significant difference in expansion will occur between the upper and lower walls of the drum, as well as between its inner and outer walls. This creates substantial additional stress on the drum, leading to deformation; in severe cases, cracks may form.
Reply to 1# zgj2405: 1) Since the wall of the steam drum is thick, its expansion is slow, whereas the walls of the tubes connected to the steam drum are thin, resulting in faster expansion. If the inlet water temperature is too high or the flow rate is too fast, it will lead to uneven expansion, causing cracks in the welds and resulting in equipment damage. (2) When feedwater enters the drum, it first comes into contact with the lower half of the drum’s wall. If the temperature difference between the feedwater and the drum wall is large, and the flow rate of the feedwater is high, a significant difference in expansion will occur between the upper and lower walls as well as between the inner and outer walls of the drum. This creates considerable additional stress on the drum, leading to deformation; in severe cases, cracks may form.
This post was last edited by zgj2405 on 2011-5-20 at 19:09. The process of feeding water into the drum when the boiler is started and the process of draining water from the drum after the boiler is stopped are similar; in both cases, it is necessary to control the rate of change of the drum’s wall temperature. And during the process of feeding water into the boiler’s drum, when the drum pressure rises to 0.2 MPa, it is necessary to close the vent valve on the drum ； During the drainage of the boiler drum, when the drum pressure drops to 0.2 MPa, it is necessary to open the drain valve on the drum. Their purpose is the same: to protect the equipment and ensure the service life of the steam drum.
As long as the water feeding time and temperature are taken into account to manage the thermal stress on the drum, too rapid water feeding or a large difference between the water temperature and the drum temperature can cause significant changes in the drum’s thermal stress, which can cause serious damage to it. According to the latest national standards, it is required that the outlet water temperature of the economizer and the wall temperature of the steam drum shall not exceed 90 degrees. During the water feeding and ignition processes, the temperature difference between the upper and lower walls of the steam drum shall not be greater than 40 or 50 degrees.
(1) Since the wall of the drum is thick, its expansion is slow, whereas the walls of the tubes connected to the drum are thin and expand more rapidly. If the inlet water temperature is too high or the flow rate is too fast, it will lead to uneven expansion, causing cracks in the welds and resulting in equipment damage. (2) When feedwater enters the drum, it first comes into contact with the lower half of the drum’s wall. If the temperature difference between the feedwater and the drum wall is large, and the flow rate of the feedwater is high, a significant difference in expansion will occur between the upper and lower walls as well as between the inner and outer walls of the drum. This creates considerable additional stress on the drum, leading to deformation; in severe cases, cracks may form.