Constant pressure and variable pressure operation

The heat storage capacity of the boiler refers to the ability to adapt to changes in load requirements by allowing the pressure and saturation temperature of the steam drum to change in response to sudden variations in steam pressure, when the throttle valves of the turbine are quickly closed or opened, thereby causing a sudden change in the evaporation rate from the steam drum. Thermal power units usually operate in either constant-pressure or variable-pressure modes. During slip operation, the furnace loading parameters change with the load, in the same direction as the load demand. When it is necessary to increase the load, the boiler must absorb some heat in order to raise its parameters and thus enhance its heat storage capacity; conversely, when the load needs to be reduced, the parameters must be lowered to release the stored heat. This precisely hinders the unit’s ability to respond to external load demands, reducing the load response rate. The constant-pressure mode does not change the heat storage capacity of the boiler, which facilitates rapid response to load changes. Compared to operation at constant pressure, the advantages of sliding-pressure operation are as follows: 1. There are almost no throttling losses, which results in a higher relative internal efficiency of the turbine at part-load conditions compared to constant-pressure operation; 2. Since the relative internal efficiency of the turbine remains almost unchanged across different load levels under sliding-pressure operation, the exhaust temperature of the high-pressure cylinder remains essentially constant (whereas it decreases under constant-pressure operation at low loads). This reduces the amount of heat absorbed by the steam in the reheat apparatus, thereby improving the cycle thermal efficiency; 3. As the relative internal efficiency of the turbine remains almost unchanged under sliding-pressure operation, while it decreases under constant-pressure operation at lower loads, the operational economy of sliding-pressure operation is better than that of constant-pressure operation; 4. When the load decreases, the main steam pressure drops, which in turn lowers the outlet pressure of the feed water pump, reducing the electrical or steam consumption of the feed water pump. The disadvantage is that a decrease in main steam pressure due to reduced load leads to a lower cycle thermal efficiency. Therefore, whether sliding-pressure operation should be used at a certain load level depends on a comprehensive assessment of the unit’s operational economy based on the factors mentioned above. Advantages of sliding-pressure operation: 1) As the pressure decreases with lower loads, the specific heat of the steam decreases, as does the superheating required. Therefore, the superheated steam temperature remains stable over a wide range of loads (for example: it can be maintained at the rated value within 40–100% MCR) ； 2) Due to the low throttling losses in the turbine, the exhaust temperature of the high-pressure cylinder remains stable. In subcritical units, when the load is reduced from 100% to 50% MCR, the exhaust temperature of the high-pressure cylinder drops by only about 60 degrees; therefore, it is also easy to maintain stability in the reheat steam temperature ； 3) Due to the low throttling loss of the turbine, the pressure ratio before and after each stage remains almost unchanged compared to the rated load ； And the volumetric flow rate of the steam inside the machine is also essentially the same as that at rated load (due to the reduced pressure) ； Therefore, the stage efficiency of the turbine remains high. Compared to operation at constant pressure, the internal efficiency of the turbine increases during operation at variable pressure ； 4) Since the temperature change of the metal inside the turbine is small during load variations (usually not more than 78 degrees), the thermal stress on the turbine metal is low, and the speed of load variation is not restricted by cylinder stress ； 5) When the unit operates at low load, both the pressure and flow rate of the feedwater decrease; therefore, compared to operation at constant pressure, energy consumption is significantly reduced ； Disadvantages: 1) When the load changes, the pressure and temperature inside the drum also change; as a result, the stress issues associated with the drum become more severe under variable-pressure operation, and this constitutes a major factor limiting the speed at which the unit’s load can be changed ； 2) Changes in the unit load are achieved by adjusting combustion and feedwater in the boiler; since boilers are devices with high thermal inertia, their response to load changes is slow ； 3) At low load, the main steam pressure is reduced, thereby lowering the cycle thermal efficiency of the unit ； Currently, most units operate at constant pressure between 100-90% MCR ； The 90-50% MCR operates at variable pressure, while it operates at constant pressure below 50% (subject to the limit set by the minimum speed of the steam-driven feed water pump)