Chemical Process Equipment Area: [Weekly Topic] Vacuum issues with the turbine reheat Apparatus? (2011.6.20~6.26)
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I invite everyone to start discussing vacuum levels in the new week. Turbines have high requirements regarding vacuum levels; what are the impacts of good or poor vacuum conditions on the unit? It is best to illustrate with actual examples. I hope everyone will participate actively. Special note: Those who post excellently will receive a reward of 10 to 50 Wealth points or an equivalent amount of Charm.
Analysis of the Factors Affecting the Thermal Efficiency of Large-Capacity Units Lanzhou Xigu Thermal Power Co., Ltd. Lin Changhong, Yang Qian 【Abstract】 With the rapid development of the power industry and the increase in the number of large-scale units, it has become increasingly important to improve the thermal efficiency of these units. This paper analyzes the factors affecting the thermal economy of units, and proposes some suggestions and methods for improving these factors. 1 Introduction With the rapid development of China’s power industry and the increasing number of large-scale units with capacities of 300MW, 600MW and above, it has become increasingly important and urgent to improve the thermal efficiency of these units. Generally speaking, the factors that affect the thermal efficiency of a unit include the following: a) the unit’s load; b) the operating condition of the unit’s regenerative system; c) the unit’s vacuum level; d) the parameters of the main and reheat steam being within their designed values or corresponding to a sliding pressure regime; e) the efficiency of the unit’s flow components; f) the analysis of the unit’s leakage conditions; g) the boiler’s efficiency. 2 Unit Load 2.1 The unit should operate at its rated load, as it is designed for this load level. The unit achieves its best efficiency at the rated load, and when all operating parameters remain at their designed values, the high-pressure control valves operate at the “three-valve point” setting – that is, valves #1 to #3 are fully open while valve #4 is closed. This results in minimal throttling losses, thereby ensuring the highest possible efficiency of the unit. 2.2 The unit operates in a combined sliding pressure mode. With the development of the power industry, it is an inevitable fact that large-capacity units must be used for peak shaving. To ensure that these units maintain high thermal efficiency under various load conditions, a combined sliding pressure operation mode is employed: in high-load areas (such as above 80%–95% of the rated load), the turbine operates at a constant pressure, with the load being adjusted by opening and closing control valves. This approach results in a higher initial pressure for the turbine unit, higher cycle thermal efficiency, and a relatively high internal efficiency, as the load remains close to the design value. In the lower load range (such as between 80%–95% and 25%–50% of the rated load), operation in slip mode with all four valves open, all three valves open, or all two valves open is carried out; at this time, no valves are partially opened, so the throttling losses are relatively minimal. The main steam temperature remains constant, the volumetric flow rate of the fresh steam stays roughly the same under various loads, the exit flow velocities at each stage of nozzles and blades remain unchanged, the specific enthalpy drop and internal efficiency also remain unchanged, and the overall relative internal efficiency is close to the design value. Currently, the feed water pumps in large-scale units all use hydraulic couplings for speed regulation; variable pressure operation reduces the feed water pressure, thereby decreasing the power consumption of the feed water pumps. When the unit load changes rapidly, the control valve can be opened or closed for emergency regulation. At load levels below the lowest point of slip operation (such as below 25%–50% of rated load), operate at a constant pressure with a relatively low initial pressure, in order to avoid excessive losses in efficiency. 3 Operation status of the unit’s regenerative system 3.1 Impact of the operation status of the unit’s regenerative system on economic efficiency A regenerative system is one that extracts a portion of the steam that has already done work from certain stages of the turbine, using it to heat the feedwater sent to the boiler in order to increase its temperature; it represents the earliest and most common method used to improve the efficiency of units. Abnormal operation of the regenerative system is manifested in aspects such as a decrease in feedwater temperature and abnormal parameters of steam extracted at various stages. For a unit mass of extracted steam, the work generated by low-pressure steam reheat is greater than that produced by high-pressure steam. Therefore, in multi-stage reheat systems, low-pressure steam should be utilized as much as possible to replace high-pressure steam. If the reheat system does not function properly and some of the steam from a certain stage flows into the steam from the lower stage, the high-pressure steam will displace the low-pressure steam, resulting in a decrease in the thermal efficiency of the unit. The flow of extraction steam into the condenser also leads to an increase in the loss of cooling capacity of the unit; a decrease in feedwater temperature results in an increased heat absorption by the feedwater within the boiler, all of which reduce the thermal efficiency of the unit. The main factors causing abnormal operation of the regenerative heating system include an increased temperature difference at the heater, shutdown of the heater, operation without water level on the steam side of the heater, increased pressure loss due to steam extraction, and leaks in the bypass of the high-pressure heater. 3.1.1 The main factors affecting the terminal difference of the heater include: the properties of the heat transfer tubes inside the heater, the dimensions of these tubes, the convective heat transfer coefficient inside the tubes, the condensation heat transfer coefficient outside the tubes, as well as the temperatures of the working fluids inside and outside the tubes. For heaters that are already in operation, the main influencing factor is the heat transfer coefficient both inside and outside the tubes. The primary factors affecting this heat transfer coefficient include the degree of fouling on the heater’s heat transfer tubes and the presence of non-condensable gases such as air within the heater. An increase in the heater terminal difference directly leads to a decrease in the outlet water temperature, resulting in an increased amount of steam extracted at the next stage or an increased heat absorption in the boiler. 3.1.2 The general reason for shutting down a heater is to address defects in it, requiring isolation. Apart from affecting the thermal efficiency of the unit, the shutdown of low-pressure heaters results in a decrease in the inlet water temperature to the deaerator; if this temperature becomes too low, the deaerator may vibrate. The shutdown of high-pressure heaters, on the other hand, leads to serious consequences such as increased moisture content in the last-stage blades of the turbine, overheating of the boiler superheater, and overpressure in the reheater. 3.1.3 An abnormal steam trap control system in the heater will cause the heater to operate without water level, and the most obvious symptom of this is a decrease in the outlet water temperature. Data from a power plant indicate that when the high-pressure heater is operating with water level, the feedwater temperature is 4–6°C higher than when it operates without water level. Operating the heater without water level also causes the extracted steam to enter the next stage of the heater in vapor form, before it can release its latent heat; this displaces the low-pressure steam from that stage, thereby reducing the thermal efficiency of the unit. Additionally, the entry of steam-water mixtures into the drain cooling sections, drain pipes, and drain valves can lead to tube bundle leaks, vibrations in the drain pipes, and erosion of the drain valves, all of which pose threats to the safety of the equipment. This situation is quite common on site, as most of the steam trap control valves for heaters in power plants are pneumatic valves. Issues such as sticking of the valve stems, large fluctuations in regulation, and changes in the set values due to vibrations in the heater’s steam traps can occur. This is especially true for the control valve that regulates the flow from #1 high-pressure heater’s steam traps to the deaerator; such valves are usually installed on the deaerator platform, while the controller itself is located at the 0-meter mark of the high-pressure heater, resulting in slow response times and making it difficult to operate these pneumatic control valves automatically. 3.1.4 An increase in extraction pressure loss is usually caused by the check valves or isolation valves in the extraction pipelines being closed incorrectly or not being opened enough. This leads to a reduction in the amount of extraction steam at this stage, which in turn causes more steam to flow to the next stage, displacing the low-pressure extraction steam. At the same time, the reduced extraction of steam results in a lower temperature of the water output. 3.1.5 High-pressure heater bypasses are also common in various power plants. This is due to leaks in the large bypass electric valves or incomplete opening of the inlet control valves, which results in leaks in the small bypass. As a consequence, the feedwater temperature on the turbine side (at the point where the outlet of the last high-pressure heater does not meet the large bypass) is higher than that on the boiler side. This not only leads to a reduction in high-pressure steam extraction due to the decreased feedwater flow, but it also results in a lower final feedwater temperature. 3.2 How to ensure the normal operation of the unit’s regenerative heating system
3.2.1 Strengthen the recording and analysis of the temperature difference at the heater ends. When an increase in this temperature difference is detected, it should be analyzed and addressed promptly. If there are non-condensable gases such as air inside the heater, the air vent valve of the heater can be opened wider until the temperature difference returns to normal. If the heat transfer tubes of the heater are dirty, they can be cleaned while the heater is isolated from service. 3.2.2 Strengthen the monitoring of the heater’s operating condition, and make use of downtime to conduct inspections whenever possible. High-pressure heaters have high parameters and large heat capacity; if the steam extraction pipeline is equipped only with pneumatic check valves and electric isolation valves, they cannot be closed tightly and reliably. During unit operation, the isolation time for high-pressure heaters is long, so it is advisable to install manual isolation valves on the steam extraction pipeline to reduce maintenance time. 3.2.3 Increase the automatic activation rate of heater water levels to ensure that heaters operate with adequate water levels. Where feasible, replace some pneumatically operated control valves that cannot be automated with electrically operated control valves or jet regulators. 3.2.4 Regularly record the parameters of the heater and extraction steam. Since the lift of the stem of each extraction steam pneumatic check valve varies, the maximum stem lift for each check valve should be recorded for reference during analysis. 3.2.5 Strengthen the comparison of feedwater temperatures on the turbine and boiler sides; the electric valve of the high-pressure heater’s main bypass, which is currently controlled by intermission control to be closed in place, should be switched to torque control, and it is necessary to ensure that the inlet interlock valve is closed properly. 4 Unit vacuum 4.1 Impact of unit vacuum on economic efficiency The proper operation of the vacuum system has a significant impact on the economic efficiency of steam turbine operation. On the one hand, as the vacuum level decreases, the effective enthalpy drop of the steam is reduced; consequently, the generator output drops when the steam flow rate remains constant. When the generator output stays unchanged, the steam flow rate through the unit increases, resulting in a decline in the unit’s efficiency ； On the other hand, as the vacuum in the unit decreases and the temperature of the exhaust cylinder rises, the loss of cooling capacity in the unit increases, resulting in a decrease in the cycle thermal efficiency. Under normal circumstances, a 1% change in vacuum level can cause a 0.7–1% change in the heat rate, and a change of about 1 g/kW·h in coal consumption. 4.2 How to Improve Unit Vacuum 4.2.1 Conduct vacuum tightness tests in accordance with the regulations, carry out comprehensive analysis of operating parameters such as the water temperature at the inlet and outlet of the condenser, the terminal difference, vacuum level, and subcooling, identify the main factors affecting the unit’s vacuum, and formulate corresponding corrective measures. 4.2.2 Carefully carry out leak detection in the vacuum system using methods such as helium mass spectrometry and water flooding; promptly and thoroughly address any detected leaks. 4.2.3 Strengthen the maintenance and management of the condenser ball cleaning system to improve the cleaning efficiency. 4.2.4 Pay attention to adjusting the steam pressure of the shaft seal. On-site, in order to prevent the shaft seal steam from escaping, the pressure of the low-pressure shaft seal steam is often set quite low. Additionally, since the control valves in the overflow control system of the self-sealing system are pneumatic valves, their adjustments can result in significant fluctuations. All these factors contribute to leaks at the low-pressure shaft seal, and this is a fairly common problem. 4.2.5 Maintaining the water temperature in the spray tank at a low level: Tests on a 300MW unit have shown that when the operating water temperature varies within the range of 26–42°C, for every 1°C increase in water temperature, the vacuum decreases by approximately 0.065–0.133 kPa, which results in an increase in coal consumption of about 0.24 grams per kilowatt-hour. 5 Main and reheat steam parameters of the unit
5.1 Influence of main and reheat steam parameters on the unit’s economic efficiency
When the temperature and pressure of the main and reheat steam decrease, the effective enthalpy drop of the steam is reduced. With a constant steam flow rate, the generator output declines; conversely, when the generator output remains unchanged, the steam flow rate through the unit increases, thereby reducing its overall economic efficiency. Theoretically speaking: increasing the temperatures and pressures of the main and reheat steam in a power unit can improve its economic efficiency. However, higher temperatures also raise the temperatures of the equipment and pipes, accelerating the creep rate of the materials and reducing their creep limits ； The increase in main and reheat steam pressure raises the internal stresses on equipment and pipelines; it also increases the final moisture content of the steam, thereby exacerbating corrosion on the last-stage blades of the turbine and seriously jeopardizing the operational safety of the unit. Although the water injection for cooling in the boiler and reheaters is designed to be used as an auxiliary fine-tuning method or in emergency situations, due to advantages such as low inertia in response to water injection and a large temperature adjustment range, operators on site often use it as a regular tuning method, which has a significant impact on the thermal efficiency of the unit. In the case of spray desuperheating in superheaters, since most of the desuperheating water is taken from the outlet of the feed pump and the inlet of the high-pressure heater without passing through the high-pressure heater, it reduces the regenerative extraction steam and the degree of regeneration, thereby decreasing the thermal efficiency of the unit. In the case of reheat steam spray desuperheating, its thermodynamic process involves heat absorption and evaporation at constant pressure along the reheat pressure line, followed by superheating; thereafter, the steam enters the intermediate- and low-pressure cylinders of the turbine to expand and do work. The cycle it completes is a non-reheat cycle operating at medium or lower parameters. Compared to the main cycle (a high- or ultra-high-parameter reheat cycle), its thermal efficiency is significantly lower. 5.2 How to ensure that the main and reheat steam parameters of the unit remain at rated values 5.2.1 Ensuring that the unit operates at the specified steam parameter values under the fixed-sliding-fixed regime. 5.2.2 Ensure the normal operation of the regenerative heating system and maintain the feedwater temperature at normal levels. 5.2.3 Improve the efficiency of the unit’s flow path as much as possible, while ensuring the parameters of the main and reheat steam. 5.2.4 Boiler operators should make frequent adjustments to keep the parameters of main and reheat steam within the specified limits. 5 Flow path of the unit 6.1 Impact of the efficiency of the unit’s flow path on economic performance The efficiency of the flow path refers to the ratio of the actual enthalpy drop in each cylinder to the ideal isentropic enthalpy drop. Factors such as scaling or blockage in the flow path, as well as excessive gaps in the shaft seals and steam seals, can lead to a decrease in the efficiency of the unit’s flow path, which directly affects its thermal efficiency; in severe cases, it can also impact the unit’s output. Due to constraints in design and manufacturing at the time, the flow-through components of older units generally had low efficiency. For instance, the #9 and #10 steam turbines at the Xigu Power Plant in Lanzhou are 165 MW turbines of the ЛТ—140/165-130/15-2 type manufactured in Russia. Under rated operating conditions, the efficiency of their low-pressure cylinders was 75.28% and 77.31%, respectively. After adjustments to the steam seal clearances, replacement of the steam seals, and maintenance of the flow path, these efficiencies increased to 87.45% and 89.52%. Consequently, the heat rate of the units decreased by 587.42 kJ/kW·h and 543.21 kJ/kW·h, while the coal-free power output rose by 15.22 MW and 14.26 MW (all aforementioned indicators include the efficiency gains resulting from major overhauls). It is evident that the efficiency of the flow path has a significant impact on the thermal economy of the units. More and more power units are now equipped with DEH (electronic control systems), which enable valve management. Single-valve control is used during the initial stages of startup, and valve sequencing control is activated under certain conditions, thereby increasing the startup speed of the unit and ensuring higher economic efficiency. If single-valve control is still used during the normal operation of the unit, throttling losses will occur, affecting the unit’s thermal efficiency. In one unit, excessive vibration occurred due to valve control; therefore, single-valve control was used throughout its normal operation. Tests showed that the efficiency of the high-pressure cylinder was 3.48% and 3.77% lower at output levels of 300MW and 270MW, respectively, compared to units of the same type. 6.2 How to improve the efficiency of the flow-through components of the unit 6.2.1 During major overhauls of the unit, the flow-through components should be carefully inspected; any scaling or blockages should be addressed promptly. 6.2.2 After understanding the characteristics of the unit, the gaps such as those in the shaft seals and steam seals can be adjusted to the lower and middle limits specified by the manufacturer. 6.2.3 Older units can consider undergoing modifications to their flow-related components, provided that funding and schedule constraints permit it. Leakage conditions of Unit 7. 7.1 Impact of unit leakage on economic efficiency. Unit leakage can be categorized into two types: external leakage and internal leakage. External leakage of the unit refers to the leakage of steam and water from the thermal system due to imperfections in the pipes or systems. With the loss of these working fluids, there is a loss of energy associated with various flavors. Internal leakage refers to the situation where, due to improper sealing of valves, steam and water leak from a high-parameter section to a low-parameter section within a thermal system. Although no energy escapes the thermal system as in the case of external leakage, these working fluids only participate in the low-parameter thermal cycle, thereby reducing their ability to perform work and decreasing the thermal efficiency of the unit. 7.2 How to reduce unit leaks 7.2.1 Improve the quality of maintenance; use methods such as raising the pressure rating of valves to ensure they close tightly and prevent repeated leaks. 7.2.2 Due to the complexity of the steam turbine drain system and the large number of valves involved, as well as the difficulty in operating some valves, it is necessary to thoroughly check after each normal startup whether all valves are tightly closed. In particular, nowadays an increasing number of units employ SCS (Sequential Control); in such cases, electric valves are used to automatically control the opening and closing of drains. Since electric valves may not close tightly, extra care must be taken during inspection. 7.2.3 Strengthen the inspection and patrol of equipment systems. Use various methods such as touch, listening, and smelling to check for any system leaks; promptly and thoroughly address any leak points found. For issues that cannot be resolved during the unit’s operation, take measures to isolate them as much as possible. 8 Change the starting method of the feed water pump. For units with a capacity of over 300MW, a combination of steam-driven feed water pumps and electric feed water pumps is used. According to the manufacturer’s guidelines, during the initial stages of startup, the electric feed water pump is used to supply water; when the load reaches 100MW, the first steam-driven feed water pump is started, and when the load reaches 150MW, the second feed water pump is started, while the electric feed water pump is switched off and put on standby. This mode of operation has the following two drawbacks: During the cold start of Unit 8.1, it takes a long time to go from starting the electric feedwater pump to reaching a load of 100 MW (that is, starting the steam feedwater pump), and this process consumes a large amount of plant electricity. 8.2 When a steam-driven pump is started, it takes some time to warm up; if the pump needs to be warmed up further, even more time is required. Therefore, before the turbine-driven feed water pump of the unit is started, if a fault occurs in the electric feed water pump, the turbine-driven feed water pump cannot be put into service immediately. This may result in an interruption of boiler feed water supply, thereby causing the entire unit to fail to start. Therefore, the startup method can be improved: at the initial stage of unit startup, the electric feed pump is not activated; instead, the pre-pump of the steam-driven feed pump is used to supply water to the boiler in place of the electric feed pump. Before the pressure in the boiler drum rises to the point where the feed pump can no longer draw in water, the steam-driven feed pump is started directly (with steam supplied by the plant’s own new steam as well as steam from adjacent units). The opportunity provided by using the small steam turbine to increase its speed and warm up the system is taken advantage of to further raise the feed water pressure in order to meet the boiler’s water supply requirements. When a certain load is reached, the steam source for the small turbine is switched, that is, the steam supply is changed from fresh steam or steam from an adjacent unit to the fourth-stage extraction steam of this turbine. This method of starting has the following advantages: 8.2.1 During the initial stage of unit startup, water is supplied by electric feedwater pumps. Since the efficiency of the hydraulic coupling is much lower at low loads compared to that of a small steam turbine, and there are also mechanical and electrical losses as well as losses in power transmission, a significant amount of energy is lost ； In the improved starting method, since the efficiency of the small steam turbine changes little with load variations and it drives the feed water pump directly, there are few intermediate energy conversion steps, resulting in good thermal efficiency. 8.2.2 In the traditional startup method, when it is necessary to start the steam-driven feed water pump at a load of 100 MW, warm-up of the turbine and pump must be carried out (if required). This inevitably consumes a portion of the fourth-stage extraction steam, resulting in a waste of thermal energy. 8.2.3 If the small steam turbine is started using the main steam of the unit itself, the live steam is discharged to the condenser via a bypass. Therefore, the steam utilized by the small steam turbine can be regarded as “waste steam” being put to use. This helps increase the boiler’s evaporation rate, facilitates boiler combustion, and accelerates the rate at which the system heats up and pressure rises. 4. Throughout the startup process, the electric feed water pump remains in a standby state, and due to its fast startup speed, it enhances the reliability of unit startup. 9 Effects on boiler efficiency: There are many factors that affect boiler efficiency, mainly including the oxygen level in the boiler, flue gas temperature, fly ash, the air leakage rate of the boiler, the type of coal used, and the cleanliness of the boiler’s heating surfaces. How to improve boiler efficiency 9.1 Ensure an appropriate excess air coefficient for the boiler, with oxygen levels controlled at 4.0±0.5% (adjust as appropriate depending on the type of coal used). 9.2 The flue gas temperature is related to the cleanliness of the boiler’s heating surfaces and the boiler air leakage rate, as well as to the fineness of the pulverized coal. During normal operation, it is necessary to maintain an appropriate fineness and uniformity of the pulverized coal, while also adjusting the ratio between primary and secondary air. 9.3 In terms of boiler fly ash control, it should be carried out simultaneously with the adjustment of flue gas temperature; regular testing of fly ash and slag is necessary to obtain first-hand data for adjusting boiler combustion. 10 Summary: There are many factors that affect the thermal efficiency of power units; above, a brief analysis has been provided on a few of the main factors. The thermal economy of the unit is not only related to the operators and maintenance personnel specializing in turbines, but also closely associated with other professionals involved in the unit as well as its operating mode. Only through collaborative efforts, careful operation and maintenance, and meticulous servicing can the thermal economy of the unit be maximized. References: 1. Steam Turbine Operation Procedures for Plant 2 of Xigu Thermal Power Co., Ltd. 2. Centralized Control Operation Procedures for Plant 3 of Xigu Thermal Power Co., Ltd. 3. Zheng TiKuan • Thermal Power Plants • Water Resources and Electric Power Press
3.2.1 Strengthen the recording and analysis of the temperature difference at the heater ends. When an increase in this temperature difference is detected, it should be analyzed and addressed promptly. If there are non-condensable gases such as air inside the heater, the air vent valve of the heater can be opened wider until the temperature difference returns to normal. If the heat transfer tubes of the heater are dirty, they can be cleaned while the heater is isolated from service. 3.2.2 Strengthen the monitoring of the heater’s operating condition, and make use of downtime to conduct inspections whenever possible. High-pressure heaters have high parameters and large heat capacity; if the steam extraction pipeline is equipped only with pneumatic check valves and electric isolation valves, they cannot be closed tightly and reliably. During unit operation, the isolation time for high-pressure heaters is long, so it is advisable to install manual isolation valves on the steam extraction pipeline to reduce maintenance time. 3.2.3 Increase the automatic activation rate of heater water levels to ensure that heaters operate with adequate water levels. Where feasible, replace some pneumatically operated control valves that cannot be automated with electrically operated control valves or jet regulators. 3.2.4 Regularly record the parameters of the heater and extraction steam. Since the lift of the stem of each extraction steam pneumatic check valve varies, the maximum stem lift for each check valve should be recorded for reference during analysis. 3.2.5 Strengthen the comparison of feedwater temperatures on the turbine and boiler sides; the electric valve of the high-pressure heater’s main bypass, which is currently controlled by intermission control to be closed in place, should be switched to torque control, and it is necessary to ensure that the inlet interlock valve is closed properly. 4 Unit vacuum 4.1 Impact of unit vacuum on economic efficiency The proper operation of the vacuum system has a significant impact on the economic efficiency of steam turbine operation. On the one hand, as the vacuum level decreases, the effective enthalpy drop of the steam is reduced; consequently, the generator output drops when the steam flow rate remains constant. When the generator output stays unchanged, the steam flow rate through the unit increases, resulting in a decline in the unit’s efficiency ； On the other hand, as the vacuum in the unit decreases and the temperature of the exhaust cylinder rises, the loss of cooling capacity in the unit increases, resulting in a decrease in the cycle thermal efficiency. Under normal circumstances, a 1% change in vacuum level can cause a 0.7–1% change in the heat rate, and a change of about 1 g/kW·h in coal consumption. 4.2 How to Improve Unit Vacuum 4.2.1 Conduct vacuum tightness tests in accordance with the regulations, carry out comprehensive analysis of operating parameters such as the water temperature at the inlet and outlet of the condenser, the terminal difference, vacuum level, and subcooling, identify the main factors affecting the unit’s vacuum, and formulate corresponding corrective measures. 4.2.2 Carefully carry out leak detection in the vacuum system using methods such as helium mass spectrometry and water flooding; promptly and thoroughly address any detected leaks. 4.2.3 Strengthen the maintenance and management of the condenser ball cleaning system to improve the cleaning efficiency. 4.2.4 Pay attention to adjusting the steam pressure of the shaft seal. On-site, in order to prevent the shaft seal steam from escaping, the pressure of the low-pressure shaft seal steam is often set quite low. Additionally, since the control valves in the overflow control system of the self-sealing system are pneumatic valves, their adjustments can result in significant fluctuations. All these factors contribute to leaks at the low-pressure shaft seal, and this is a fairly common problem. 4.2.5 Maintaining the water temperature in the spray tank at a low level: Tests on a 300MW unit have shown that when the operating water temperature varies within the range of 26–42°C, for every 1°C increase in water temperature, the vacuum decreases by approximately 0.065–0.133 kPa, which results in an increase in coal consumption of about 0.24 grams per kilowatt-hour. 5 Main and reheat steam parameters of the unit
5.1 Influence of main and reheat steam parameters on the unit’s economic efficiency
When the temperature and pressure of the main and reheat steam decrease, the effective enthalpy drop of the steam is reduced. With a constant steam flow rate, the generator output declines; conversely, when the generator output remains unchanged, the steam flow rate through the unit increases, thereby reducing its overall economic efficiency. Theoretically speaking: increasing the temperatures and pressures of the main and reheat steam in a power unit can improve its economic efficiency. However, higher temperatures also raise the temperatures of the equipment and pipes, accelerating the creep rate of the materials and reducing their creep limits ； The increase in main and reheat steam pressure raises the internal stresses on equipment and pipelines; it also increases the final moisture content of the steam, thereby exacerbating corrosion on the last-stage blades of the turbine and seriously jeopardizing the operational safety of the unit. Although the water injection for cooling in the boiler and reheaters is designed to be used as an auxiliary fine-tuning method or in emergency situations, due to advantages such as low inertia in response to water injection and a large temperature adjustment range, operators on site often use it as a regular tuning method, which has a significant impact on the thermal efficiency of the unit. In the case of spray desuperheating in superheaters, since most of the desuperheating water is taken from the outlet of the feed pump and the inlet of the high-pressure heater without passing through the high-pressure heater, it reduces the regenerative extraction steam and the degree of regeneration, thereby decreasing the thermal efficiency of the unit. In the case of reheat steam spray desuperheating, its thermodynamic process involves heat absorption and evaporation at constant pressure along the reheat pressure line, followed by superheating; thereafter, the steam enters the intermediate- and low-pressure cylinders of the turbine to expand and do work. The cycle it completes is a non-reheat cycle operating at medium or lower parameters. Compared to the main cycle (a high- or ultra-high-parameter reheat cycle), its thermal efficiency is significantly lower. 5.2 How to ensure that the main and reheat steam parameters of the unit remain at rated values 5.2.1 Ensuring that the unit operates at the specified steam parameter values under the fixed-sliding-fixed regime. 5.2.2 Ensure the normal operation of the regenerative heating system and maintain the feedwater temperature at normal levels. 5.2.3 Improve the efficiency of the unit’s flow path as much as possible, while ensuring the parameters of the main and reheat steam. 5.2.4 Boiler operators should make frequent adjustments to keep the parameters of main and reheat steam within the specified limits. 5 Flow path of the unit 6.1 Impact of the efficiency of the unit’s flow path on economic performance The efficiency of the flow path refers to the ratio of the actual enthalpy drop in each cylinder to the ideal isentropic enthalpy drop. Factors such as scaling or blockage in the flow path, as well as excessive gaps in the shaft seals and steam seals, can lead to a decrease in the efficiency of the unit’s flow path, which directly affects its thermal efficiency; in severe cases, it can also impact the unit’s output. Due to constraints in design and manufacturing at the time, the flow-through components of older units generally had low efficiency. For instance, the #9 and #10 steam turbines at the Xigu Power Plant in Lanzhou are 165 MW turbines of the ЛТ—140/165-130/15-2 type manufactured in Russia. Under rated operating conditions, the efficiency of their low-pressure cylinders was 75.28% and 77.31%, respectively. After adjustments to the steam seal clearances, replacement of the steam seals, and maintenance of the flow path, these efficiencies increased to 87.45% and 89.52%. Consequently, the heat rate of the units decreased by 587.42 kJ/kW·h and 543.21 kJ/kW·h, while the coal-free power output rose by 15.22 MW and 14.26 MW (all aforementioned indicators include the efficiency gains resulting from major overhauls). It is evident that the efficiency of the flow path has a significant impact on the thermal economy of the units. More and more power units are now equipped with DEH (electronic control systems), which enable valve management. Single-valve control is used during the initial stages of startup, and valve sequencing control is activated under certain conditions, thereby increasing the startup speed of the unit and ensuring higher economic efficiency. If single-valve control is still used during the normal operation of the unit, throttling losses will occur, affecting the unit’s thermal efficiency. In one unit, excessive vibration occurred due to valve control; therefore, single-valve control was used throughout its normal operation. Tests showed that the efficiency of the high-pressure cylinder was 3.48% and 3.77% lower at output levels of 300MW and 270MW, respectively, compared to units of the same type. 6.2 How to improve the efficiency of the flow-through components of the unit 6.2.1 During major overhauls of the unit, the flow-through components should be carefully inspected; any scaling or blockages should be addressed promptly. 6.2.2 After understanding the characteristics of the unit, the gaps such as those in the shaft seals and steam seals can be adjusted to the lower and middle limits specified by the manufacturer. 6.2.3 Older units can consider undergoing modifications to their flow-related components, provided that funding and schedule constraints permit it. Leakage conditions of Unit 7. 7.1 Impact of unit leakage on economic efficiency. Unit leakage can be categorized into two types: external leakage and internal leakage. External leakage of the unit refers to the leakage of steam and water from the thermal system due to imperfections in the pipes or systems. With the loss of these working fluids, there is a loss of energy associated with various flavors. Internal leakage refers to the situation where, due to improper sealing of valves, steam and water leak from a high-parameter section to a low-parameter section within a thermal system. Although no energy escapes the thermal system as in the case of external leakage, these working fluids only participate in the low-parameter thermal cycle, thereby reducing their ability to perform work and decreasing the thermal efficiency of the unit. 7.2 How to reduce unit leaks 7.2.1 Improve the quality of maintenance; use methods such as raising the pressure rating of valves to ensure they close tightly and prevent repeated leaks. 7.2.2 Due to the complexity of the steam turbine drain system and the large number of valves involved, as well as the difficulty in operating some valves, it is necessary to thoroughly check after each normal startup whether all valves are tightly closed. In particular, nowadays an increasing number of units employ SCS (Sequential Control); in such cases, electric valves are used to automatically control the opening and closing of drains. Since electric valves may not close tightly, extra care must be taken during inspection. 7.2.3 Strengthen the inspection and patrol of equipment systems. Use various methods such as touch, listening, and smelling to check for any system leaks; promptly and thoroughly address any leak points found. For issues that cannot be resolved during the unit’s operation, take measures to isolate them as much as possible. 8 Change the starting method of the feed water pump. For units with a capacity of over 300MW, a combination of steam-driven feed water pumps and electric feed water pumps is used. According to the manufacturer’s guidelines, during the initial stages of startup, the electric feed water pump is used to supply water; when the load reaches 100MW, the first steam-driven feed water pump is started, and when the load reaches 150MW, the second feed water pump is started, while the electric feed water pump is switched off and put on standby. This mode of operation has the following two drawbacks: During the cold start of Unit 8.1, it takes a long time to go from starting the electric feedwater pump to reaching a load of 100 MW (that is, starting the steam feedwater pump), and this process consumes a large amount of plant electricity. 8.2 When a steam-driven pump is started, it takes some time to warm up; if the pump needs to be warmed up further, even more time is required. Therefore, before the turbine-driven feed water pump of the unit is started, if a fault occurs in the electric feed water pump, the turbine-driven feed water pump cannot be put into service immediately. This may result in an interruption of boiler feed water supply, thereby causing the entire unit to fail to start. Therefore, the startup method can be improved: at the initial stage of unit startup, the electric feed pump is not activated; instead, the pre-pump of the steam-driven feed pump is used to supply water to the boiler in place of the electric feed pump. Before the pressure in the boiler drum rises to the point where the feed pump can no longer draw in water, the steam-driven feed pump is started directly (with steam supplied by the plant’s own new steam as well as steam from adjacent units). The opportunity provided by using the small steam turbine to increase its speed and warm up the system is taken advantage of to further raise the feed water pressure in order to meet the boiler’s water supply requirements. When a certain load is reached, the steam source for the small turbine is switched, that is, the steam supply is changed from fresh steam or steam from an adjacent unit to the fourth-stage extraction steam of this turbine. This method of starting has the following advantages: 8.2.1 During the initial stage of unit startup, water is supplied by electric feedwater pumps. Since the efficiency of the hydraulic coupling is much lower at low loads compared to that of a small steam turbine, and there are also mechanical and electrical losses as well as losses in power transmission, a significant amount of energy is lost ； In the improved starting method, since the efficiency of the small steam turbine changes little with load variations and it drives the feed water pump directly, there are few intermediate energy conversion steps, resulting in good thermal efficiency. 8.2.2 In the traditional startup method, when it is necessary to start the steam-driven feed water pump at a load of 100 MW, warm-up of the turbine and pump must be carried out (if required). This inevitably consumes a portion of the fourth-stage extraction steam, resulting in a waste of thermal energy. 8.2.3 If the small steam turbine is started using the main steam of the unit itself, the live steam is discharged to the condenser via a bypass. Therefore, the steam utilized by the small steam turbine can be regarded as “waste steam” being put to use. This helps increase the boiler’s evaporation rate, facilitates boiler combustion, and accelerates the rate at which the system heats up and pressure rises. 4. Throughout the startup process, the electric feed water pump remains in a standby state, and due to its fast startup speed, it enhances the reliability of unit startup. 9 Effects on boiler efficiency: There are many factors that affect boiler efficiency, mainly including the oxygen level in the boiler, flue gas temperature, fly ash, the air leakage rate of the boiler, the type of coal used, and the cleanliness of the boiler’s heating surfaces. How to improve boiler efficiency 9.1 Ensure an appropriate excess air coefficient for the boiler, with oxygen levels controlled at 4.0±0.5% (adjust as appropriate depending on the type of coal used). 9.2 The flue gas temperature is related to the cleanliness of the boiler’s heating surfaces and the boiler air leakage rate, as well as to the fineness of the pulverized coal. During normal operation, it is necessary to maintain an appropriate fineness and uniformity of the pulverized coal, while also adjusting the ratio between primary and secondary air. 9.3 In terms of boiler fly ash control, it should be carried out simultaneously with the adjustment of flue gas temperature; regular testing of fly ash and slag is necessary to obtain first-hand data for adjusting boiler combustion. 10 Summary: There are many factors that affect the thermal efficiency of power units; above, a brief analysis has been provided on a few of the main factors. The thermal economy of the unit is not only related to the operators and maintenance personnel specializing in turbines, but also closely associated with other professionals involved in the unit as well as its operating mode. Only through collaborative efforts, careful operation and maintenance, and meticulous servicing can the thermal economy of the unit be maximized. References: 1. Steam Turbine Operation Procedures for Plant 2 of Xigu Thermal Power Co., Ltd. 2. Centralized Control Operation Procedures for Plant 3 of Xigu Thermal Power Co., Ltd. 3. Zheng TiKuan • Thermal Power Plants • Water Resources and Electric Power Press
The turbine we use is fed with steam generated as a by-product in the synthesis tower drum. Due to leaks in the synthesis tower, this steam contains a large amount of non-condensable gases, posing significant challenges to the safe operation of the turbine. Currently, after putting both ejectors into service, the vacuum level can only reach -0.052 MPa. There are concerns that the last-stage blades of the turbine might be damaged. As a result, we have no choice but to monitor its operation and wait for an opportunity to carry out maintenance.
This post was last edited by YTH-Gavin on 2011-6-20 14:53. 1. What is the function of a rehydrator? (1) A high vacuum is established and maintained at the turbine exhaust port to increase the available enthalpy drop of the steam, thereby enhancing the turbine’s output power and thermal efficiency ； (2) Recover the condensed water from the exhaust steam to use as boiler feedwater. 2. What is the principle behind the creation of vacuum in the rewatering unit? The waste steam, which has done its work in the turbine, enters the condenser where it exchanges heat, releasing a large amount of energy; as a result, it condenses into water, and its volume decreases sharply while its pressure drops significantly. At a pressure of 1 kg/Cm2, when steam condenses into water, its volume decreases by a factor of 1725, thereby creating a vacuum. 3. What are the reasons for not being able to create a vacuum? (1) Steam supply at the end of the steam seal ； (2) The exhaust safety valve is not supplied with water ； (3) The inlet valve of the extraction pump is not open ； (4) The bypass valve of the drain steam trap in the extraction condenser is not closed ； (5) The discharge valves of the steam extraction pipeline and the steam injection pipeline are not closed ； (6) Steam extractor nozzle blockage ； (7) Internal leakage in the safety valve of the extraction condenser ； (8) Leakage at the expansion joint of the rehydrator or at other connection flanges or welds in the vacuum system. 4. What are the reasons for vacuum descent? (1) Insufficient air supply from the steam seal, with air leaking in ； (2) Insufficient water supply to the atmospheric safety valve ； (3) The line connected to the vacuum condenser drain valve is open, allowing air to leak in ； (4) Internal leakage in the safety valve of the vacuum condenser caused abnormal operation of the condenser ； (5) Air leaks into the condensate pump, resulting in insufficient vaporization; the liquid level in the reclaimer rises and submerges the tubes, reducing the cooling area ； (6) A drop in the cooling water pressure of the rehydrator leads to a reduction in the amount of cooling water flow and an increase in the cooling water temperature, which affects the condensation efficiency ； (7) Steam extraction, cylinder body, and steam injection drain valves not closed ； (8) Accumulation of non-condensable gases such as ammonia and formaldehyde carried along by the main steam and steam injection ； (9) The jet pump is clogged and not working properly ； (10) Drop in the power steam pressure of the ejector ； (11) There are leaks at the connections of various components in the vacuum system, such as the rehydrator expansion joints, as well as at flanges and welds ； (12) The vacuum-breaking valve at the bottom of the atmospheric safety valve was accidentally opened.
A vacuum should be created before starting up, by removing the air from the exhaust chamber; otherwise, after the turbine begins to operate, the inability of the air to condense will cause the exhaust temperature to rise and the pressure to increase, thereby affecting the normal operation of the turbine. The vacuum level of the rehydrator in our facility is 672 mmHg during normal operation, and it is 300 mmHg during warm-up. To reduce startup time, a starting pump is used; once stable operation is achieved, pumps of level 1 and level 2 are employed. If the vacuum level drops, the result in our workshop will definitely be a chain shutdown.
When inspecting and repairing the feedwater pump, special attention should be paid to changes in the vacuum level in the feedwater regenerator. Leakage in the mechanical seal of the feedwater pump can easily allow air to enter the feedwater regenerator. Additionally, when switching or repairing the feedwater pump, it is important to ensure that the inlet valve of the pump closes properly; otherwise, the vacuum level may drop during maintenance, leading to a chain reaction that results in shutdown. Our workshop has experienced this on one occasion
If the vacuum level drops for some reason, I have some work experience to share； As the vacuum level decreases, the exhaust temperature rises, which in turn can lead to a further drop in the vacuum level. My approach is to lower the exhaust temperature. When the rehydrator is in operation, there is usually a make-up water line, but it is not in use under normal circumstances. When the vacuum level drops, open this valve to add water to the rehydrator in order to lower the temperature; even if there is significant air leakage, the vacuum level can at least be maintained. Note: The size to be set depends on the situation; if the pump cannot keep up, drain the water on site first to maintain the vacuum level. Empty it first before adding water; do not keep the water inlet open frequently as there is a lot of dirt, to avoid contaminating the condensate and blocking the water inlet on the atmospheric safety valve. In the cases I’ve encountered where the efficiency really drops, it’s usually due to low steam pressure and low temperature; once these issues are addressed, there’s no problem.
Before the turbine is started, there is air inside it, and the pressure within the turbine is equal to atmospheric pressure. Without vacuuming, air cannot be condensed, which results in high exhaust pressure. In such a situation, a large amount of steam is required when starting up to overcome the frictional forces and inertial forces in the bearings of the turbine and compressor, in order to rotate the rotor; this increases the steam impact force on the blades. Additionally, due to the presence of air in the condenser, the heat exchange between the exhaust steam and the cooling water is weakened, leading to an increase in exhaust temperature. This causes deformation of the components inside the turbine’s rear cylinder, raises the backpressure in the condenser, and may also trigger the safety mechanisms of the condenser.
The vacuum problem is mainly due to insufficient vacuum. What are the causes of insufficient vacuum? (1) Insufficient air supply from the steam seal, with air leaking in； (2) Insufficient water supply to the atmospheric safety valve ； (3) The line connected to the vacuum condenser drain valve is open, allowing air to leak in ； (4) Internal leakage in the safety valve of the vacuum condenser caused abnormal operation of the condenser ； (5) Air leaks into the condensate pump, resulting in insufficient vaporization; the liquid level in the reclaimer rises and submerges the tubes, reducing the cooling area ； (6) A drop in the cooling water pressure of the rehydrator leads to a reduction in the amount of cooling water flow and an increase in the cooling water temperature, which affects the condensation efficiency ； (7) Steam extraction, cylinder body, and steam injection drain valves not closed ； (8) Accumulation of non-condensable gases such as ammonia and formaldehyde carried along by the main steam and steam injection ； (9) The jet pump is clogged and not working properly ； (10) Drop in the power steam pressure of the ejector ； (11) There are leaks at the connections of various components in the vacuum system, such as the rehydrator expansion joints, as well as at flanges and welds ； (12) The vacuum-breaking valve at the bottom of the atmospheric safety valve was accidentally opened.
The level of vacuum affects the dissolved oxygen content in the condensate water, which reduces the economic efficiency of the unit and accelerates corrosion of the unit and its pipelines