Thread Content
Has anyone noticed that during the process of filling the boiler with water, if the difference between the water temperature and the wall temperature of the steam drum, or if the water filling speed is not properly controlled, it can easily lead to an excessive temperature difference across the walls of the steam drum. This is because when water is filled into the boiler, it first comes into contact with the lower wall after entering the drum. When the water temperature is higher than the wall temperature of the drum, the inner bottom wall of the drum heats up first; its temperature rises, and as a result the temperature of the inner bottom wall is higher than that of the inner top wall, creating a temperature difference. When the water temperature is lower than the temperature of the drum wall, the inner wall at the bottom of the drum cools down first, resulting in a decrease in its temperature; thus, the temperature of this inner wall becomes lower than that of the upper inner wall, creating a temperature difference. When the difference between the temperature of the water entering the drum and the temperature of the drum wall is too large, or when the flow rate of the water is too fast, it can lead to an excessive temperature difference across the drum wall, even exceeding the specified limits. In such cases, the temperature differences across various points on the drum wall tend to change simultaneously. During the boiler startup process, improper control of factors such as the rate of heating and pressure increase, the activation and deactivation of oil guns as well as their switching, the startup time of the coal grinding system, the selection of the initial coal quantity, and the rate of load increase in the later stages of startup can also lead to excessive temperature differences across the boiler drum wall. Due to the increased wall thickness and length of the boiler drum, as well as the extremely high pressures it must withstand, temperature differences between the upper and lower walls on the inside and outside of the drum can easily occur during the startup and shutdown processes of the unit. These temperature differences generate significant thermal stresses, which affect the safe operation and lifespan of the drum. Therefore, it is very important to effectively control the temperature difference across the drum wall during unit operation. Why shouldn’t the temperature difference in the boiler drum exceed 50°C? The lower part of the steam drum contains a certain amount of water, which is evaporated by the water wall, while the upper part is filled with steam. During the cooling process after the boiler is shut down, since the upper part of the drum is in contact with steam while the lower part is in contact with water, the upper part cools more slowly than the lower part, resulting in a temperature difference between the upper and lower walls of the drum. According to the regulations, the temperature difference must not exceed 50 degrees; this prevents the steam pressure from dropping sharply, as otherwise the lower part of the steam drum would cool down too quickly, ultimately resulting in a temperature difference of more than 50 degrees between the upper and lower parts of the steam drum. Why is it specified that the temperature difference between the upper and lower levels should not exceed 50°C? Due to the large temperature difference, significant thermal stress is generated. Meanwhile, the pressure inside the drum remains high during the shutdown cooling period. Under the combined effect of thermal stress and internal pressure, the stress on the material may exceed its allowable limit, leading to damage to the drum. During the boiler ignition and temperature rise period, there is also a significant temperature difference between the upper and lower parts of the steam drum, resulting in considerable thermal stress. However, at this time the internal pressure of the steam drum is relatively low. As long as the temperature difference between the top and bottom of the steam drum during heating does not exceed 50 degrees, the stress on the drum wall will still remain within the allowable limits. It can be seen that during the cooling period after shutting down the boiler, if the pressure drops too rapidly, it will cause the lower wall of the drum to cool down too quickly; the thermal stress generated between the upper and lower walls of the drum becomes even more dangerous than during the heating process when the boiler is started up. It is caused by an upward external force: When bending deformation occurs due to an upward external force, the upper part of the steam drum is stretched ； The lower half of the drum is compressed. The principle is illustrated in the schematic diagram: During ignition and heating or shutdown and cooling, excessive thermal stress occurs on the steam drum. Since the wall temperature in the upper part is higher than that in the lower part, the upper section of the drum expands more, while the lower section expands less. As a whole, the drum means that the upper part is constrained by the lower part and cannot expand fully, resulting in compression in the upper part. Under the influence of the large expansion in the upper half, the lower half of the drum is pulled and expands together with it, resulting in tension being applied to the lower half. Therefore: the upper part of the steam drum is compressed while the lower part is stretched, causing the steam drum to bend upward. It can be seen that in both cases of upward bending deformation of the drum, one is caused by external forces, while the other is caused by thermal stress. Reasons for the temperature difference on the drum wall: The water entering the boiler has a certain temperature. Once it enters the drum, it first comes into contact with the lower part of the drum. If the water temperature is lower than the temperature of the drum wall, this can cause the lower wall of the drum as well as its inner surface to cool down first. As a result, the upper wall of the drum becomes warmer while the lower wall is cooler; the inner surface is cooler than the outer surface. This phenomenon often occurs during the startup of the unit when it is at normal temperature, at operating temperature, or at very high temperature. If the water temperature is higher than the wall temperature of the drum, the lower wall of the drum as well as the inner surface of that wall heat up first; as a result, the upper wall of the drum has a lower temperature while the lower wall has a higher temperature, and the inner surface has a higher temperature compared to the outer surface. During the ignition and pressure-rising phase, as the pressure increases, the temperature of both boiler water and steam also rises. The lower half of the steam drum is heated by the boiler water, while the upper half is heated by the steam. Although the temperatures of the boiler water and steam remain roughly the same during this process, since the heat transfer coefficient of steam is 2 to 3 times greater than that of water, the temperature rise of the upper part of the drum wall is significantly greater than that of the lower part. The faster the pressure rises, the greater the resulting temperature difference between the two sections of the drum wall. Furthermore, after the boiler is shut down for cooling, as the pressure in the drum decreases, the saturated steam inside the drum is heated by the upper wall of the drum and turns into superheated steam. Superheated steam has a lower density than saturated steam, and it forms a protective layer of superheated steam on the inner upper wall of the drum. Superheated steam has poor thermal conductivity and does not allow for convective heat transfer; as a result, the temperature of the upper wall of the drum cools down slowly. In contrast, the lower wall of the drum is in contact with the boiler water, which continues to facilitate natural convection, leading to faster cooling. Therefore, during the shutdown process, the temperature is high in the upper part and low in the lower part. The wall thickness of the boiler drum in domestic subcritical 300MW units ranges from 140 to 160 mm. The saturation temperature of the water and steam inside the drum increases as pressure rises; the temperature of the inner wall of the drum in contact with the water and steam is close to this saturation temperature, while the outer wall temperature rises due to heat conduction through the metal of the drum wall, resulting in a temperature difference between the inner and outer walls. At the beginning of boiler startup, the water circulation within the boiler is weak; the flow of water in the drum is slow, and in areas where the furnace receives less heat, circulation may even come to a standstill. The temperature of water in these stagnant areas is significantly lower, while heat transfer in the steam space inside the drum is relatively uniform, which further increases the temperature difference between the upper and lower walls of the drum. When the economizer recirculation valve is not tight, during the startup process when water is supplied to the boiler, some of the low-temperature water enters the drum directly without passing through the economizer, resulting in an increased temperature difference across the drum wall. Control of drum wall temperature difference: During the water filling stage of the boiler, the drum wall temperature difference is primarily controlled by regulating the water filling temperature and speed. When the boiler is cold, it is necessary to strictly control the temperature difference between the feedwater temperature and the metal wall temperature of the steam drum, ensuring that it does not exceed 40°C. When conditions permit, positive temperature differences should be used as much as possible; this is beneficial for reducing the time required for expansion and contraction, and more importantly, it helps to compensate for the drop in the temperature of the feedwater after it passes through the economizer, which occurs due to the supercooling of the economizer during shutdown. The temperature drop of some feed water passing through the economizer can reach 40°C to 60°C. Especially in cases of emergency shutdown due to tube failures in the economizer or when the wall temperature of the steam drum is high, forced air is continuously blown into the economizer for maintenance purposes, resulting in the economizer located in the tail flue being completely cooled. When water is supplied, the air temperature around the economizer is only 10°C to 30°C; therefore, the temperature of the water supplied must take into account this temperature drop in the economizer. Additionally, the faster the water is fed into the boiler, the greater the resulting temperature difference. Therefore, the water feed rate must also be controlled. Generally, it is stipulated that the water feeding time should be no less than 2 hours in summer and no less than 4 hours in winter. If the temperature difference between the feedwater temperature and the drum wall is less than 40°C, the feedwater flow rate can be increased appropriately; otherwise, it should be reduced. As long as it is possible to accurately monitor the temperature differences between the upper and lower walls, as well as between the inner and outer walls, at several sections along the length of the drum during operation, to control the rate of change in drum pressure, and to monitor the feedwater temperature, the drum can operate safely and stably. Large domestic units generally install temperature sensors only on the downcomer and saturated steam pipe, and the temperature difference across the drum wall is inferred by monitoring the temperature of their outer walls. (1) During the initial startup phase and the stable operation phase, the temperature difference between the outer wall of the steam extraction pipe and the inner wall of the drum is very small; therefore, the former can be used in place of the latter, or the saturated steam temperature corresponding to the pressure of the drum at that time can be used to replace the temperature of the inner wall of the drum. (2) During the initial start-up phase and the stable operation phase, especially in the early stage of ignition start-up with heating from bottom steam, the temperature difference between the outer wall temperature of the concentrated downcomer and the inner surface temperature at the lower part of the drum is very small, allowing the former to be used in place of the latter. (3) During the boiler shutdown process, the difference between the outer wall temperature of the steam outlet pipe and the saturation temperature at the prevailing pressure is very small; thus, the former can be used in place of the latter. (4) From shutdown to boiler water drainage, the temperature difference between the outer wall of the downcomer and the inner wall at the bottom of the drum is very small, so the former can be used in place of the latter. Practice has shown that as long as the temperature difference between the upper and lower walls of the steam drum, as well as the temperature difference between its inner and outer walls, does not exceed 50°C, the additional thermal stress resulting from these differences will not cause damage to the steam drum. During the shutdown process, sliding parameter shutdown should be used as much as possible to avoid full-pressure shutdown. During the sliding-parameter shutdown process, the rate of pressure drop is controlled based on the temperature difference across the drum wall. If it is detected that this temperature difference exceeds the allowable limit or that its rate of increase is accelerating, the rate of pressure reduction is slowed down; depending on the circumstances, efforts are made to reduce the parameters further before shutting down the plant. Before shutting down the boiler, if the temperature difference across the drum wall is large, operation at reduced parameters should be maintained for a period of time before shutdown in order to reduce the partial temperature difference across the drum wall that occurs during a gradual shutdown. Avoid releasing water under pressure or reducing the pressure during drainage. The drum drain pressure is between 0.5 and 0.8 MPa. During drainage, water is released first and then steam; that is, the economizer drain valve is opened first, followed by the regular drain and heating valves, and finally the steam trap and air valve are opened. Furthermore, not forcing ventilation during water discharge can effectively keep the temperature difference across the drum wall within 35°C. Depending on the actual conditions, the pressure during water release in winter can take the upper limit, while the pressure during water release in summer can take the lower limit. In some units, the temperature difference between the upper and lower walls of the drum rises rapidly 30–40 minutes after water discharge, and can even reach 70–90°C. Since the wall temperature difference that occurs after water is drained from the drum is related to the pressure at the time of drainage, reducing the drainage pressure can effectively decrease this wall temperature difference. However, an excessively low drain pressure is not conducive to drying and corrosion protection of the boiler; therefore, it is very important to select an appropriate drain pressure. Generally, the drainage pressure is specified to be between 0.49 and 0.78 MPa; each unit can determine the appropriate drainage pressure based on the actual temperature of the drum wall after shutdown and the actual drying condition following drainage. Furthermore, the drainage rate also has a certain impact on the temperature difference across the drum wall; a higher drainage rate tends to cause such a temperature difference. For units where the drum pressure is between 0.5 and 0.8 MPa and water drainage is insufficient to meet the requirements regarding the drum wall temperature, while low pressure is not adequate for drying purposes, nitrogen filling for maintenance or introducing hot flue gas from an adjacent furnace after low-pressure water drainage should be employed for drying. Ensure the tightness of the boiler’s steam and water system as well as the flue gas system, and prevent rapid cooling of the boiler. After the boiler is shut down, the temperature inside the furnace remains quite high. The boiler has a large capacity to store heat, and its cooling relies primarily on convective heat transfer between the heated surfaces and the air; the cold air that enters the furnace is heated and then discharged into the atmosphere through the chimney. The greater the heat storage capacity of the boiler and the higher the ventilation rate, the faster the cooling and temperature reduction will occur. Therefore, after the boiler is shut down, once it has been adequately ventilated (usually for 3–5 minutes) to clean the heating surfaces, the induced draft fan and forced draft fan should be turned off, and all dampers in the air, flue gas, and pulverized coal systems should be closed ； Check that all access doors in the boiler, such as the viewing ports and coke-pulling holes, are closed ； Check that the water seals of the slag skimmer and those in the upper water seal groove are functioning properly; otherwise, top up the water to the appropriate level ； Check that all valves in the steam system (including steam traps) are tightly closed. Try to keep the drum operating at a high water level to reduce the frequency of make-up water addition. After the boiler is shut down, as it continues to cool down, the steam pressure gradually decreases. Meanwhile, as the working fluid is consumed, the water level in the steam drum drops. For 300MW units, the inner diameter of the boiler drum is between 1600 and 1800 mm, while its length is around 20,000 to 22,000 mm. The zero point of the water level gauge is usually set at or below the center line of the drum. Therefore, even when the drum is filled to its maximum capacity – which is typically 300 mm – there is still approximately 500 mm of space at the upper part of the drum. By maintaining a feedwater flow rate of 300 t/h for 2 to 3 minutes (the exact flow rate and time vary depending on the unit), it is possible to ensure that the water inside the drum comes into full contact with the upper wall of the drum without overflowing into the superheater. When the water level drops to -50mm, refilling the boiler in the same manner can effectively reduce the number of times the boiler needs to be filled. After the boiler is shut down, since both the furnace temperature and the temperature of the boiler water remain high, the deaerator heating device should continue to be operated during boiler water filling. If the auxiliary steam pressure permits, the feedwater temperature should be raised as much as possible to minimize the temperature difference between the water and the drum walls. If the unit cannot continue to operate after tube failure in the superheater and reheater, the furnace is shut down. One set of supply and exhaust fans is kept running to maintain a total air flow of around 20%, with the negative pressure in the furnace maintained at normal levels. The water level in the steam drum is also kept at normal levels, and efforts are made to minimize the time spent on forced ventilation and natural ventilation. After tube failures in the economizer and water wall, if the water level cannot be maintained, the furnace should be shut down. One set of supply and exhaust fans should be kept running to maintain a total air flow of around 20%, with the negative pressure in the furnace maintained at normal levels; efforts should be made to minimize the time spent on forced ventilation and natural ventilation. Hazards of excessive wall temperature difference in the drum and stages when it occurs: (For reference only) Hazards of excessive wall temperature difference in the drum: An increase in the wall temperature at the upper part of the drum causes the metal there to want to stretch, but this is restricted by the lower part, resulting in axial compressive stress on that upper metal, while the metal in the lower part experiences axial tensile stress. This will cause the steam drum to deform in a arch-back shape. The occurrence of large wall temperature differences leads to increased thermal stress in the drum; the greater the temperature difference between the upper and lower parts, the higher the stress, which in turn causes damage to the drum and reduces its service life. Phases when a large temperature difference between the upper and lower walls of the drum is likely to occur: the early stages of boiler startup, as well as during the cooling and pressure reduction process after the boiler is shut down, are all phases when a large temperature difference between the upper and lower walls of the drum is likely to occur. The saturated temperature of water varies nonlinearly with pressure. At low pressures, the saturated temperature changes significantly with pressure, while at high pressures this change is smaller. Therefore, during the initial startup of the unit and the process of cooling and reducing pressure after the boiler is shut down, it is necessary to strictly control the changes in the pressure of the steam drum. Analysis of the reason for the large temperature difference between the upper and lower walls of the drum during boiler startup. At the initial stage of boiler startup, the temperature of the boiler water gradually rises; no steam is generated before pressure is built up. Since the temperature of the water supplied to the boiler is higher than that of the lower wall of the drum, the temperature of the lower wall ends up being higher than that of the upper wall. After the boiler starts to generate pressure, certain amounts of saturated steam are produced. Due to the temperature difference between the saturated steam and the upper wall of the drum, the saturated steam releases heat to the wall of the drum, as well as its latent heat of vaporization; as a result, the temperature of the upper wall of the drum gradually rises above that of the lower wall. As the drum pressure increases, the change in saturation temperature becomes gradually slower; the temperature of the upper wall of the drum also rises, and the temperature difference between the upper and lower walls decreases over time. Control measures to reduce the temperature difference between the upper and lower walls of the steam drum during boiler startup: Strictly control the rate of temperature and pressure rise, especially during the initial stage of ignition; gradually increase the combustion intensity in the furnace to avoid large fluctuations; and strive to maintain uniform combustion within the furnace. During startup, strive to maintain uniform combustion intensity on the front and rear walls; carry out appropriate regular drainage to promote water circulation inside the furnace. Analysis of the reasons for large temperature differences on the drum wall after the boiler is shut down: Significant differences in heat dissipation conditions: The drum is located outside the furnace and is insulated; coupled with its large heat capacity, this causes the temperature of the drum wall to gradually rise above the temperature of the steam and water inside the drum. In the upper half of the drum of the steam drum, some of the heat is dissipated outside the furnace, some is dissipated inside the steam drum, and some is dissipated to the lower half of the steam drum. In the lower half of the drum, some of the heat is dissipated outside the furnace, some is dissipated inside the steam drum, and it also receives heat transferred from the upper half. There are significant differences in the cooling methods: after the boiler is shut down, it enters a pressure-reduction and cooling phase. The steam drum is primarily cooled by the internal working fluid. As the pressure of the water inside the drum and its corresponding saturation temperature gradually decrease, the lower wall of the drum releases heat to the water, causing it to cool rapidly. In contrast, the upper wall of the drum is in contact with steam; during the pressure-reduction process, its heat transfer coefficient is relatively low, resulting in slower cooling of the metal. Consequently, the temperature of the upper wall becomes higher than that of the lower wall, leading to a temperature difference. The faster the pressure is reduced, the greater the temperature difference; especially when the pressure drops to low levels, a large temperature difference occurs. Control measures to prevent large temperature differences on the boiler drum wall after the boiler is shut down but before water is drained: 1. Control the ventilation volume and duration after shutdown to avoid rapid cooling of the boiler. After the boiler is shut down, one set of supply and exhaust fans continues to operate to maintain an air flow rate of 600 t/h; sufficient ventilation is used to clean the heated surfaces. After 15 minutes of purging, the supply and exhaust fans are stopped, and the boiler operates in negative pressure, with natural ventilation being used for cooling based on the principle of maintaining a slight positive pressure in the furnace. Excessive ventilation volume or prolonged ventilation time will cause the pressure in the steam-water system to drop rapidly; the pressure inside the drum decreases as well, and the saturation temperature of the steam and water in the drum drops accordingly. Due to the differences in heat dissipation conditions between the upper and lower parts of the steam drum, it is inevitable that a relatively large temperature difference occurs across the drum wall after the boiler is shut down. Furthermore, the lower the steam pressure, the faster the saturation temperature drops, and the faster the temperature difference across the drum wall is established. 2. Increase the feedwater temperature. After the boiler is shut down, since the temperature inside the furnace and the temperature of the boiler water remain high, the auxiliary steam heating device for the deaerator should be activated when water is supplied to the boiler. If the pressure of the auxiliary steam permits, the water temperature in the deaerator should be kept above 100 degrees; the feedwater temperature should also be increased as much as possible in order to reduce the temperature difference between the water temperature and the wall of the steam drum. 3. Maintain the drum at a high water level during operation. Use the high water level in the drum to control the wall temperature difference of the drum. The purpose of doing this is to raise the water level in the drum, reduce the volume of the steam-side shell of the drum, and feed water from the economizer that has a lower temperature than that in the drum into the drum. This helps to lower the water temperature inside the drum and, at the same time, balance the temperature difference between the upper and lower walls. 4. Apply hydrophobic treatment before using the machine, and reduce pressure according to the pressure reduction curve. Strictly control the pressure drop rate of the steam drum at 0.05 MPa/min, the saturation temperature drop rate at 1/min, and the temperature difference between the upper and lower walls of the steam drum at 56; otherwise, the pressure drop rate should be reduced. The specific rate of pressure reduction is as follows (over approximately 12 hours): it takes 1.5 hours to reduce the pressure from 9.0~6.0 MPa; another 1.5 hours to go from 6.0~4.0 MPa; another 1.5 hours for the reduction from 4.0~3.0 MPa; 2 hours are required to reduce the pressure from 3.0~2.0 MPa; 3.5 hours are needed to reach 2.0~1.0 MPa; and finally, 1 hour is required to drop the pressure from 1.0~0.8 MPa. 5. Close the series connection, chemical dosing, and sampling doors in a timely manner to reduce the discharge of boiler water; water must not be added to or removed from the steam drum arbitrarily. Conditions to be met before draining water under pressure after shutting down the boiler: Before draining the water from the boiler, notify the chemical department to start the sewage pump in the drainage tank, and activate the cooling water supply for that tank. Pay attention to monitoring the water level and temperature in the unit’s drainage tank (which should not exceed 60). When the drum pressure drops to 0.8 MPa, the wall temperature of the drum falls to around 180 degrees; the temperature difference between the upper and lower walls of the drum is at most less than 15 degrees (the lower, the better). Before starting to drain water, the temperature difference between the upper and lower walls of the drum should be gradually reduced to maintain a small value; once drainage begins, it becomes difficult to control the temperature difference between these two walls. Operation procedure for draining water under pressure after shutting down the boiler: 1. Stop the electric pump, open the main drain valve as well as all the primary and secondary valves for drainage, the manual primary and secondary valves for draining water from the lower header, and the drain valves for the economizers on sides A and B, thereby allowing water to be drained from the boiler under pressure. When the temperature and level of the unit’s drainage tank meet the requirements, and the temperature difference between the upper and lower walls of the steam drum is not significant, the water discharge rate should be increased as much as possible, so that the boiler can retain sufficient residual heat to dry out any remaining steam. 2. When the drum pressure drops to 0.5 MPa, contact the thermal control team to activate the logic for locking the feedwater temperature reduction valves; open the valves for superheater and reheater steam temperature reduction, as well as all the drain valves in the feedwater and temperature reduction pipelines. Flush the temperature reduction pipelines, and inform the thermal control personnel to drain water from the instrument pipelines to prevent freezing. 3. When the drum pressure drops to 0.4 MPa, fully open the main steam pipeline drain valve and the exhaust valve to the atmosphere in order to increase the water drainage rate. Opening the electric valve for venting steam to the atmosphere will cause a rapid drop in the pressure inside the drum, as well as a sharp decrease in the temperature of the steam and water within it. This accelerates the formation of a temperature difference between the upper and lower walls of the drum; it is therefore important to monitor this temperature difference at such times. Throttle the exhaust valve to the atmosphere if necessary. 4. When the drum pressure drops to 0.1 MPa, close the main steam pipeline and the high-pressure bypass drain. Pay attention to monitoring the back pressure of the host; after the main steam pipeline and the high-pressure bypass drain valves are closed, it is not allowed to reopen them before the turbine loses its vacuum. 5. When the drum pressure drops to 0, open all drain and air valves in the superheater system. Also open all bottom heating valves of the furnace and the drain valve at the bottom of the heating header to drain all accumulated water. 6. Open the emergency drain valve of the drum, the electric valves from the drum to the fixed drain pipe, the control valves, and the drain valves of the various level gauges in the drum; after draining all the water from the pipes, close these valves. 7. After the boiler water has been completely drained (ensuring that there is no water in the drain check valves), continue drying for 4 hours. Contact the chemistry department to conduct tests at the air valves to confirm that there is no moisture left; thereafter, close all air valves, drain valves, and release valves in the boiler’s steam and water systems (including those used for regular drainage). Also, close all dampers in the wind and flue gas systems to seal the boiler. If maintenance work is to be carried out on the boiler, forced ventilation for cooling can be employed to reduce the temperature inside the boiler and prepare it for the maintenance. 8. When the drum pressure reaches 0, the vacuum is broken; steam supply to the shaft seal is stopped, the auxiliary steam is shut off, and the steam supply valves to various users are closed. For emergency cooling during boiler repairs, changing the water in the boiler can be used to reduce the temperature difference across the drum wall. During emergency repairs following an accident, changing the water can help accelerate the cooling process. That is, while maintaining a high water level in the drum, it is necessary to keep the feedwater temperature above 100 degrees, and while adding water to the boiler, the boiler’s fixed drain valves should be opened gradually; by using an appropriate amount of water exchange, the boiler can be kept continuously cooled. In natural circulation boilers, the drum serves as the connecting point for the three processes of heating, evaporation, and superheating within the boiler. In practical operation, by strengthening adjustments and carrying out thorough maintenance, and by properly controlling the rate of temperature and pressure increase during boiler startup, as well as the rate of temperature and pressure decrease after the boiler is shut down, along with ensuring proper waterproofing, it is possible to keep the temperature difference across the drum wall within specified limits, thereby extending the drum’s service life.