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I. Factors affecting main steam pressure and temperature. 1. Increase in main steam pressure: At the rated power of the unit, an increase in the initial pressure leads to a decrease in the steam flow rate; the pressures in various monitoring sections decrease accordingly. The enthalpy drop across each intermediate stage remains essentially unchanged. As a result of the reduced steam flow rate, the stress on the moving blades in each intermediate stage decreases, and both the pressure difference across the diaphragms and the axial thrust also decrease. Although the pressure difference before and after the control stage increases, its critical operating condition is not at the rated load; therefore, both the control stage and the intermediate stages remain safe as the main steam pressure rises. For the last few stages of blades, as the pressure difference before and after decreases (the pressure before the stage drops), the enthalpy drop of the stage is reduced; from a strength perspective, these last stages of blades are also safe. Of course, the main steam pressure cannot be too high either; otherwise, it may lead to overloading of the unit, overloading of the diaphragms and moving blades, significant axial displacement of the unit, and failures in the thrust bearings, among other safety issues. 2. Drop in main steam pressure: If the unit is required to generate rated power despite a drop in main steam pressure, the main steam flow will increase accordingly. As a result, the pressure before each stage of the unregulated stages increases, while the enthalpy drop in the last few stages rises. Consequently, the load on each stage of the unregulated stages increases, with the last few stages suffering the most severe overload; the axial thrust of the entire machine also increases accordingly. Therefore, when the main steam pressure drops during operation, the unit should be operated at an appropriate load. 3. Increase in main steam temperature: An increase in the main steam temperature is beneficial for the unit from an economic perspective, as it not only improves the thermal efficiency of the cycle but also reduces the exhaust humidity of the turbine. However, from a safety perspective, an increase in the main steam temperature can lead to a deterioration in the properties of metal materials, thereby reducing the service life of certain components such as main steam valves, control valves, shaft seals, flanges, bolts, and high-pressure pipes. For ultra-high parameter units, even a small increase in main steam temperature can cause rapid metal creep, resulting in a significant reduction in the allowable stress. Therefore, in the vast majority of cases, operating at an elevated initial temperature is not allowed. 4. Drop in main steam temperature: A decrease in the main steam temperature at the unit’s rated load will result in an increase in steam flow and an rise in the pressure in various monitoring sections. At this point, the regulating stages are safe, but in the non-regulating stages, especially in the last few stages, an increase in both the enthalpy drop and the main steam flow can lead to overload, which is quite dangerous. At the same time, a decrease in steam temperature leads to an increase in the humidity of the last stages of the blades, which increases wet steam losses and also exacerbates erosion in those last stages of blades, directly threatening the safe operation of the reverse steam turbine. Therefore, as the main steam temperature decreases, the pressure should also be reduced, so that the thermodynamic process curve of the turbine aligns as closely as possible with that under design conditions, thereby increasing the exhaust dryness of the unit. Therefore, the power limit of the unit is relatively high, and load reduction operation should be requested if necessary. II. How do the rapid load reduction during unit operation affect the main steam pressure and temperature? 1. During normal operation, the main gas pressure and temperature of the generator set tend to reach a stable state; under such conditions, the combustion in the boiler also becomes stable, with the amount of coal used, the volume of air supplied, and the amount of water used for temperature reduction all reaching a dynamic equilibrium. 2. When the unit encounters a fault and its load drops rapidly, first, the steam inlet valves of the turbine close quickly, which leads to an increase in the main steam pressure. Secondly, the control system reduces the amount of coal, water, and air supplied. As a result, the combustion process inside the boiler weakens, causing the main steam temperature to drop. If the cooling water is not used properly or the control valves are not closed in time during this process, this decline in temperature will be further exacerbated. 3. A rapid increase in the pressure of the boiler main unit causes the steam inlet valve of the turbine to close further, thereby intensifying the rise in pressure. If this pressure increase happens too quickly, it can even trigger the operation of the PC∨ valve. During this process, the control system will attempt to reduce combustion by lowering the amount of coal used. If not controlled properly, this can lead to a too rapid drop in the temperature of the main steam or the reheat steam, thus triggering an emergency shutdown. III. How to prevent a rapid drop in the main gas temperature and the reheat temperature during rapid load reduction. 1. During the rapid reduction of the unit’s load, it is necessary to closely monitor the changes in the temperature drop before and after the primary, secondary, and tertiary temperature reducers. An appropriate rate of load reduction should be selected to prevent too rapid an increase in main steam pressure. If the amount of load reduction required is large and the time frame is short, one coal grinder should be stopped immediately. 2. Shut down the cooling water control valves for the superheater and reheater in a timely manner, so as to control the changes in steam temperature and reheated steam temperature in advance. Since the combustion in the furnace has already weakened, adjustments should be made ahead of time, even if the temperature has not changed yet.