Technical Manual for Thermal Power Generation Equipment, Volume 2: Steam Turbine

Author: Chief Editor: Wang Lin et al
Publisher:
Publish Date: 1999-06-01
Features: The Technical Handbook of Power Generation Equipment is a comprehensive technical reference book that systematically summarizes the main contents of various specialized technologies in power generation equipment. The entire handbook summarizes the practical experience in developing power generation equipment in China since the 1980s, featuring rich, practical, and advanced technology. The handbook is divided into four volumes: Boiler, Steam Turbine, Automatic Control, and Power Station System with Auxiliary Equipment. This volume is the second, focusing on Steam Turbine. The main contents include the overall design of steam turbines, thermodynamic cycles, thermal and aerodynamic design, structure and main body system, strength and vibration, regulation and protection and control systems, unit life management, performance testing, reliability analysis, manufacturing processes, and material selection. This handbook is primarily intended for science and technology personnel engaged in design, manufacturing, operation, research, and management in the power generation industry, as well as for reference by faculty and students of relevant higher education institutions.
Excerpt: When the initial steam pressure remains constant but the initial temperature is increased, the cycle thermal efficiency can also be improved. At the same time, due to the increase in steam specific volume at the inlet and the decrease in exhaust steam humidity, the efficiency of the steam turbine is also enhanced, which is more beneficial for the economic performance of the power station. The increase in initial temperature is primarily limited by the allowable temperature of heat-resistant steel. When the initial temperature is excessively high, the steam inlet section of the steam turbine, especially the superheater tubes of the boiler, must use austenitic alloy steel with high thermal strength at high temperatures. This material is expensive and has poor processability, which will increase production costs. The inlet temperature for supercritical and subcritical steam turbine units is generally selected between 535°C and 565°C. Selecting an inlet temperature of 565°C can reduce the thermal consumption of the unit by approximately 80 kJ/(kW·h) compared to 538°C, which will increase the material requirements for the unit, particularly for the boiler superheater tubes. The matching of steam parameters for different types of steam turbines is shown in Tables 2-1-1 and 2-1-2.
II. Steam Turbine Parameter Series
In some countries, steam turbine parameters do not have a fixed standard, and manufacturers can adjust the steam inlet parameters of the steam turbine according to user requirements. Due to historical reasons, steam turbine inlet parameters in China have been standardized, and the capacity and parameter specifications of commonly used fixed power generation steam turbines are listed in Table 2-1-2. To save energy, China primarily focuses on developing high-thermal-economy cogeneration steam turbines for small and medium-power units (below 25 MW). Among these types, steam turbines with secondary high-pressure inlet parameters are particularly economical. The additional investment for secondary high-pressure units compared to medium-pressure units can generally be recovered within two years.
III. Selection of Intermediate Reheating Parameters
To improve the economy of large-capacity units and increase the enthalpy drop of steam in the steam turbine while reducing steam consumption, a single intermediate reheating can be adopted. The exhaust steam from the high-pressure cylinder of the steam turbine is reheated in the boiler and then enters the medium-pressure cylinder to continue performing work. This increases the average absorption temperature of the cycle and reduces thermal consumption. Intermediate reheating can also reduce the humidity of the low-pressure exhaust steam, improve the efficiency of the steam turbine, and extend the service life of the low-pressure blade rows. After adopting single intermediate reheating, the unit's economy can be improved by approximately 5% compared to units without intermediate reheating. Dual reheating can further improve the economy by approximately 2% compared to single intermediate reheating. China has not yet produced steam turbines with dual intermediate reheating. The impact of intermediate reheating temperature on economy is consistent with the impact of initial steam temperature. Under material constraints, a higher reheating temperature is more favorable for economy. Typically, the reheating temperature is chosen at the same level as the initial steam temperature, commonly 538°C/538°C. Considering the lower steam inlet pressure of the medium-pressure cylinder and the need to reduce the humidity of the low-pressure exhaust steam (especially for supercritical parameter units), the reheating temperature may be higher than the initial temperature, selected as 538°C/550°C or 538°C/565°C. When the initial steam parameters and reheating temperature are fixed, there is an optimal reheating pressure for economy. Typically, if there is one extraction for regeneration before reheating, the reheating pressure is set to 18%–22% of the initial pressure; if there is no extraction before reheating, the reheating pressure is set to 22%–26% of the initial pressure. Most intermediate reheating steam turbines produced in China have one extraction before reheating, so the former should be selected.
IV. Back Pressure Selection
Lowering the back pressure of the steam turbine (or increasing the vacuum degree of the condenser) can improve the cycle thermal efficiency. The vacuum degree of the condenser is determined by the heat transfer area of the condenser, the cooling water temperature, and the cooling water flow rate. Different regions have approximately determined cooling water temperatures and corresponding appropriate condenser vacuum degrees. In engineering design, it is typically assumed that there is a 12–15°C temperature difference between the cooling water temperature and the condenser condensate temperature (a smaller value is used for open-cycle cooling, while a larger value is used for closed-cycle cooling with cooling towers). This temperature difference includes the temperature rise of the cooling water and the temperature difference at the end of the condenser tube wall. From different cooling water temperatures, the condensate temperature and corresponding condenser vacuum degree can be obtained. The average cooling water temperature in most regions of China is 20°C, and the condenser pressure is typically 4.9 kPa (for open-cycle cooling using river, lake, or sea water) to 5.4 kPa (for closed-cycle cooling using cooling tower water). When the average cooling water temperature is 25°C, the condenser pressure is approximately 6.4 kPa. Under a fixed cooling water temperature, reducing the condenser pressure increases the output of the steam turbine and improves the unit's economy, but it also requires increasing the cooling water flow rate of the condenser (increasing the power consumption of the circulating water pump) and correspondingly increasing the flow passage size of the low-pressure blade rows, the size of the low-pressure cylinder, and the area of the condenser. This will increase equipment costs and operating expenses. Therefore, the benefits of reducing the condenser pressure must be determined through a comprehensive technical and economic comparison involving factors such as the steam turbine, condenser, and cooling water flow rate. For thermal power plants in severely water-scarce regions, the back pressure of steam turbines using air-cooled condensers is generally higher than those using water-cooled condensers, depending on different climatic and meteorological conditions and the type of air-cooled condenser used. In fact, the exit pressure of the low-pressure blade rows of the steam turbine does not necessarily equal the condenser pressure.