Technical Manual for Thermal Power Generation Equipment (Volume 2: Steam Turbines)

Author: Wang Lin
Publisher:
Publish Date: 1999-06-01
Features: The Technical Handbook of Thermal Power Generation Equipment is a systematic summary of the main technical contents of various specialized fields in thermal power generation equipment. The entire handbook summarizes the practical experience in developing thermal power generation equipment in China since the 1980s. It is rich in content, practical, and technologically advanced. The handbook is divided into four volumes: Boiler, Steam Turbine, Automatic Control, and Thermal Power Station System and Auxiliaries. This is the second volume, Steam Turbine. The main contents include the overall design of steam turbines, thermodynamic cycles, thermodynamic 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 thermal 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 while 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 and the decrease in exhaust steam humidity, the efficiency of the steam turbine is also improved, 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 too high, the steam admission part 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 process performance, which will increase production costs. The steam admission temperature for supercritical and subcritical steam turbine units is generally selected between 535°C and 565°C. Selecting a steam admission 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, especially for the superheater tubes of the boiler. 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 standard, and manufacturers can adjust the steam admission parameters of the steam turbine according to the requirements of the user. Due to historical reasons, the steam admission parameters of steam turbines in China have been standardized, and the capacity and parameter specifications of commonly used fixed steam turbines for power generation are listed in Table 2-1-2. To save energy, China primarily focuses on developing high thermal economy steam turbines for combined heat and power generation for small and medium-sized units with capacities below 25 MW. Among these types of units, those using sub-high-pressure steam admission parameters are particularly economical. The additional investment required for sub-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, thereby reducing steam consumption, a single intermediate reheating is typically used. The exhaust steam from the high-pressure cylinder of the steam turbine is introduced into the boiler for reheating and then enters the medium-pressure cylinder to continue doing work. This can increase the average absorption temperature of the cycle and reduce thermal consumption. Using intermediate reheating can also reduce the humidity of the exhaust steam from the low-pressure final stage, improve the efficiency of the steam turbine, and extend the life of the final-stage blades. After adopting single intermediate reheating, the economy of the unit can be improved by approximately 5% compared to units without intermediate reheating. The economy of double reheating units is approximately 2% higher than that of single intermediate reheating units. China has not yet produced any double intermediate reheating steam turbines. 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 beneficial for economy. Typically, the reheating temperature is selected at the same level as the initial steam temperature, commonly 538°C/538°C. Considering the lower steam admission pressure level of the medium-pressure cylinder and the need to reduce the humidity of the low-pressure exhaust steam (especially for supercritical parameter units), the intermediate reheating temperature may also 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, the reheating pressure has an optimal value for economy. Typically, when there is one reheat extraction before reheating, the reheating pressure is set to 18%–22% of the initial pressure; when there is no reheat 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 taken for open-cycle cooling, and a larger value is taken for closed-cycle cooling using cooling towers). This temperature difference includes the cooling water temperature rise 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 certain cooling water temperature, lowering the condenser pressure will increase the output of the steam turbine and improve the unit's economy, but it will also require 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 final-stage blades, the size of the low-pressure cylinder, and the area of the condenser. This will increase the cost and operating expenses of the equipment. Therefore, the benefits of lowering the condenser pressure must be determined through a comprehensive technical and economic comparison of factors such as the steam turbine, condenser, and cooling water flow rate. For thermal power plants in areas with severe water shortages, the back pressure of the steam turbine using air-cooled condensers is generally higher than that 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 final-stage moving blades of the low-pressure cylinder of the steam turbine does not necessarily equal the condenser pressure.