The impact of varying power steam pressure on the GV valve of the compressor

During the operation of a turbine, when parameters such as the inlet and outlet steam pressures, steam temperature, and main steam flow rate are all equal to the design values, this operating condition is referred to as the design condition. Efficiency is at its highest under these conditions, which is why it is also called the economic condition. When all various parameters are at their rated values during operation, this operating condition is referred to as the rated condition. At present, the thermal calculation conditions for large steam turbine units are mostly set at the rated conditions; as a result, the design conditions of these units coincide with their rated conditions. In actual operation, it is difficult to maintain the parameters at their designed values precisely; such operating conditions that do not correspond to the design conditions are referred to as variable conditions of the turbine. At this point, changes in the steam parameters, flow rate entering the turbine, and the vacuum in the condenser will cause changes in the pressure, temperature, enthalpy drop, efficiency, reaction degree, and axial thrust at each stage. This not only affects the economic efficiency of turbine operation but also its safety. Therefore, during normal operation, changes in the initial and final parameters of the turbine should be carefully monitored.   1. Increase in main steam pressure: When the main steam temperature and the condenser vacuum remain constant, but the main steam pressure increases, the enthalpy drop of the steam within the turbine increases, resulting in an increase in the humidity of the exhaust steam from the last stage.   When the main steam pressure increases, even if the total opening degree of the unit’s throttle valve remains unchanged, the main steam flow will increase as well, resulting in a higher unit load, which is beneficial for operational efficiency. However, if the main steam pressure rises beyond the specified range, it will directly threaten the safe operation of the unit. Therefore, the unit operation procedures specify clearly that operation is not allowed when the main steam pressure exceeds the limit value.   Excessively high main steam pressure poses the following hazards: (1) When the main steam pressure rises, in order to maintain a constant load, it is necessary to reduce the total opening degree of the throttle valves; this can only be achieved by closing the already fully open throttle valves. When it is reduced to the point where the first throttle valve is fully open and the second throttle valve is about to open, the enthalpy drop of the steam in the regulating stage is at its maximum, which can cause overload of the moving blades in that stage and may even lead to damage to them.   (2) The final stage blades may be overloaded. After the main steam pressure increases, the specific volume of the steam decreases; as a result, even if the opening degree of the throttle valve remains unchanged, the flow rate of main steam increases. Coupled with the increased total enthalpy drop of the steam, this leads to overloading of the final stage blades. Therefore, it is necessary to pay attention to controlling the load of the unit at this time.   (3) If the main steam temperature remains unchanged while only the main steam pressure increases, the steam humidity in the last stages will increase, thereby exacerbating the erosion of the moving blades in those final stages by water droplets.   (4) The internal stresses in the pressure-bearing components and fastening components will increase. When the main steam pressure increases, the internal stresses in components such as the main steam pipes, automatic main steam valves, throttle valve chambers, cylinders, flanges, and bolts all increase. This reduces their service life and can even cause damage to these components.        Since an increase in main steam pressure can cause many hazards, operation at such pressures is not permitted when the main steam pressure exceeds the allowable range of variation. If the main steam pressure exceeds the specified value, the boiler operator should be contacted promptly to bring it back to the normal range as soon as possible ； When boiler adjustments are ineffective, the electric main gate valve should be used to throttle and reduce pressure. If the above pressure reduction measures are ineffective and the main steam pressure continues to rise, the machine should be shut down immediately.   2. Drop in main steam pressure: When the main steam temperature and the condenser vacuum remain constant, a decrease in main steam pressure results in a reduced enthalpy drop of the steam within the turbine, and the specific volume of the steam increases. At this point, even if the total opening degree of the speed control valve remains unchanged, the main steam flow will decrease, resulting in a reduction in the unit’s load ； If the steam pressure drops too much, the unit cannot operate at full load, resulting in reduced operational efficiency ； At this point, the enthalpy drop in the regulation stage remains close to the design value, while the enthalpy drops in the other stages are all below the design value; therefore, it has no adverse effect on the safety of the unit’s operation. If the main steam pressure drops while the unit is still required to maintain its rated load, it is necessary to open the throttle valve wider to increase the main steam flow. This will cause overloading of the last stages of the turbine, especially the very last blades, thereby affecting the safe operation of the unit. When the main steam pressure drops below the allowable value, the boiler operator should be contacted as soon as possible to restore the steam pressure ； When the steam pressure drops to its minimum level, it is necessary to reduce the load and decrease the amount of steam supplied in order to bring the pressure back to normal. However, it is important to ensure that the pressures required for steam extraction for heating and for use in the deaerator are met, so as not to reduce the unit’s load too much.  3. Increase in main steam temperature In actual operation, the main steam temperature can vary significantly. The impact of such changes on the safety and economic efficiency of the unit is more severe than that caused by changes in main steam pressure; therefore, special attention must be paid to monitoring the main steam temperature. For high-temperature and high-pressure units, the main steam temperature is usually allowed to be only about 5°C higher than the rated temperature. As the main steam temperature increases, the total enthalpy drop of the main steam within the turbine, the turbine’s relative internal efficiency, and the cycle thermal efficiency of the thermal system all improve; this results in reduced heat consumption and higher operational economic benefits. However, if the main steam temperature rises above the allowable level, it can be very harmful to the safety of the equipment.   The hazards of an increase in main steam temperature are as follows: (1) The control stage blades may be overloaded. When the main steam temperature rises, the enthalpy drop of the first stage increases first ； With the load remaining constant, especially in high-speed steam valves where only the first governing steam valve is fully open while the other governing steam valves are closed, the blades of the regulating stage will experience overload.   (2) The mechanical strength of metal materials decreases, and the creep rate increases. When the main steam temperature is too high, the mechanical strength of the high-temperature metal components such as the main steam pipes, automatic main steam valves, throttle valves, cylinders, and control stage steam admission chambers decreases, and the creep rate increases. Components such as cylinders, steam valves, and high-pressure shaft seals are prone to loosening, which can lead to equipment damage or a reduced service life. If the temperature changes significantly and frequently, these high-temperature components will suffer from fatigue damage due to alternating thermal stresses, resulting in crack formation. These phenomena experience an accelerated rate of degradation as the operating time at high temperatures increases.   (3) Vibration may occur in the unit. Excessively high steam temperature can cause thermal deformation and increased thermal expansion in various metal components that are exposed to heat; if this expansion is restricted, the unit may experience vibration.   In the unit’s operating procedures, strict regulations should be established for the limits of main steam temperature as well as the number of hours that operation is permitted under certain over-temperature conditions. The general principle for handling this situation is as follows: when the main steam temperature exceeds the specified range, the boiler operator should be contacted to make adjustments and reduce the temperature as soon as possible. The turbine operator must carry out thorough monitoring and inspections. If the steam temperature remains below the maximum operating temperature allowed by the cylinder material, operation for a short period is permissible; however, once the specified operating time is exceeded, the machine should be shut down immediately ； If the steam temperature exceeds the maximum allowable operating temperature for the cylinder material, the machine should be shut down immediately. For example, the rated main steam temperature for units with medium parameters is 435°C; when the main steam temperature exceeds 440°C, the boiler operator should be contacted to reduce the temperature ； When the main steam temperature rises to between 445 and 450°C, the continuous operation time is limited to no more than 30 minutes, and the cumulative operation time for the entire year must not exceed 20 hours ； When the main steam temperature exceeds 450°C, an immediate fault shutdown should be initiated.   4. Decrease in main steam temperature: When the main steam pressure and condensation vacuum remain constant, a decrease in the main steam temperature results in a reduced total enthalpy drop of the main steam within the turbine. To maintain the rated load, it is necessary to increase the opening degree of the throttle valve, thereby increasing the amount of main steam supplied. Generally, for a typical power unit, for every 10°C decrease in the main steam temperature, the steam consumption increases by 1.3% to 1.5%.   A decrease in the main steam temperature not only affects the economic efficiency of the unit but also threatens its operational safety. Its main hazards are: (1) the final stage blades may become overloaded. Because when the main steam temperature decreases, in order to maintain the rated load unchanged, the main steam flow rate must increase, resulting in a greater enthalpy drop at the last stage; this may cause the blades in that stage to be overloaded.   (2) The steam humidity at the last few stages of the blades increases. When the main steam pressure remains constant and the temperature decreases, the steam humidity at the last few stages of the turbine blades increases. This not only raises the wet steam loss in those last stages but also exacerbates droplet erosion on the blades there, thereby shortening their service life.   (3) The reactivity at all levels increases. As the main steam temperature decreases, the reactivity at each stage increases, resulting in a significant rise in the axial thrust on the rotor. This leads to an increase in the temperature of the thrust bearings, thereby reducing the safety and reliability of the unit’s operation.   (4) High-temperature components will experience significant thermal stress and thermal deformation. If the main steam temperature drops rapidly, the internal wall temperatures of high-temperature components such as the automatic main steam valve casing, control stages, and cylinders will drop sharply, resulting in significant thermal stress and thermal deformation. In severe cases, this can cause cracks in the metal components or lead to wear issues in the moving and stationary parts of the turbine ； When the main steam temperature drops to the limit value, the plant should be shut down.   (5) Water hammer is possible. When the main steam temperature drops by more than 50°C suddenly, it is often a precursor to a water hammer accident; the turbine operator must pay close attention. If the main steam temperature continues to drop, the machine should be stopped immediately to ensure its safety.

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