The high pressure heater system is one of the main thermal systems of thermal power units. For a long time, due to reasons in design, manufacturing, installation, and operation, heater leakage has occurred frequently, especially in high pressure heaters of large units, where the situation is particularly severe. The number of forced outages caused by leakage in the high pressure heater system accounts for more than 60% of all high pressure heater forced outages, making it the second factor affecting the equivalent availability factor of large units, second only to boiler tube bursts. This not only affects the stable and full-load operation of large units, but also reduces the thermal efficiency of the entire unit due to the decrease in feedwater temperature, hindering the normal performance of the high efficiency and low consumption advantages of large units. With the deepening of internal cost reduction and efficiency enhancement work in power enterprises, it is one of the urgent tasks currently facing us to detect leakage in the high pressure heater system early during operation, take measures as soon as possible, and minimize the loss caused by faults, so as to improve the economic efficiency of the entire thermal power plant cycle.
1. Cause Analysis of High Pressure Heater Leakage
1.1 Excessive thermal shock during start-up and shutdown of high pressure heater. Some units, due to the inability of the high pressure heater to start with the unit, experience significant thermal shock during each start-up process, leading to leakage of the heater water chamber partition plate. According to regulations, the steam inlet electric valve of the high pressure heater should be opened and closed intermittently, but in actual operation, the electric valve does not have this function. When the high pressure heater is put into operation or taken out of service, the opening and closing of the electric valve are completed within a short time. Due to frequent start-up and shutdown of the unit, the temperature change rate during start-up and shutdown exceeds the allowable value, resulting in rapid temperature changes of the tubes and tube sheet inside the high pressure heater, thereby generating alternating thermal stress. Under this stress, the tubes suffer fatigue damage.
1.2 Unstable drain water level of high pressure heater. During operation of the high pressure heater, the thermal measurement signal of the drain water level does not match the actual water level. That is, the actual water level is within the required range, but the measured water level signal indicates a higher or lower value, causing a so-called "false water level." When it indicates a higher value, the emergency drain electric valve automatically opens, leading to low or no water level operation of the high pressure heater; when it indicates a lower value, the emergency drain electric valve automatically closes, the drain water level gradually rises, causing the high water level protection to act, and the emergency drain electric valve opens again. Even the high pressure heater may be taken out of service due to misoperation of the measured water level signal. Whether the measured water level indicates a higher or lower value, the emergency drain electric valve frequently opens and closes, causing unnecessary erosion, vibration, and overheating of the tube sheet, thereby accelerating the damage of the tubes. Through observation, the fracture points of the high pressure heater tubes are all at the connection position with the tube sheet.
1.3 Poor sealing of the emergency drain regulating valve of the high pressure heater. To improve the safety and reliability of the unit, the high pressure heater is equipped with an emergency drain system. However, due to the poor quality of domestic drain regulating valves, internal leakage occurs, and a certain drain water level cannot be maintained, causing the tubes to be subjected to steam-water erosion, vibration, and tube sheet overheating for a long time.
1.4 Poor sealing of the high pressure steam inlet valve. When the high pressure heater is taken out of service, due to poor sealing of the steam inlet valve, some heating steam still leaks in, causing overheating of the tubes and resulting in reduced strength.
1.5 Damage to surrounding tubes by broken tubes. The ends of broken tubes inside the high pressure heater are in a free state. Under the impact of high-speed airflow, they swing freely, constantly rubbing and impacting the tubes around the broken tubes, expanding the rupture and leakage of surrounding tubes.
1.6 Vibration of high pressure heater. Steam flowing outside the heater tubes, crosswise or lengthwise, and passing through the tube bundle is the main factor causing vibration of the high pressure heater. Some high pressure heaters, due to the lack of an air vent device on the water side, cannot discharge air during operation, causing water hammer and vibration. Vibration causing leakage and damage of heaters has occurred in many cases in domestic power plants.
1.7 Leakage of feedwater tubes in high pressure heater. High-pressure feedwater sprays out at high speed from the tube leakage point, eroding the surrounding tubes.
1.8 No anti-corrosion measures when high pressure heater is out of service. When the high pressure heater is out of service for more than two weeks, the water side should be protected with deoxygenated water containing (30mg/kg N2H2 + 10mg/kg NH3), or the feedwater in the high pressure heater should be completely drained. The steam side should be protected with deoxygenated feedwater containing 300mg/kg hydrazine and 10mg/kg ammonia, with pH=10. Alternatively, under warm conditions, dry air should be used for drying to keep the relative humidity below 40% to effectively prevent corrosion. However, some high pressure heaters have never taken any anti-corrosion measures after commissioning, so the tubes have suffered a certain degree of corrosion.
1.9 Poor manufacturing quality. During the design and manufacturing of high pressure heater equipment, the tube sheet is designed too thin, causing deformation. The welding and expansion between the tube sheet and tubes are poor, which easily leads to tube system leakage during operation.
2. Monitoring and Diagnosis of Tube System Leakage in High Pressure Heater. During operation of the high pressure heater, if the feedwater terminal temperature difference increases while the steam parameters and inlet water temperature are basically normal, and accompanied by a decrease in the feedwater temperature rise of the high pressure heater, it is necessary to check the pressure loss on the feedwater side of the high pressure heater (feedwater inlet and outlet pressure difference). If the feedwater inlet and outlet pressure difference of the high pressure heater decreases, it indicates a short circuit between the water inlet side and the water outlet side inside the high pressure heater, i.e., leakage exists. If during operation of the high pressure heater, the extraction steam parameters are basically normal, and the drain water level is found to have increased, the opening of the drain regulating valve should be checked immediately. If the opening is significantly larger than the normal opening of the drain regulating valve for the corresponding unit, or if the drain water level increases significantly while the opening of the drain regulating valve remains unchanged, it indicates leakage in the high pressure heater system, i.e., a short circuit between the steam and water sides. At this time, the drain water temperature and the feedwater inlet and outlet pressure difference both decrease to varying degrees. In the former case, the drain water level of the high pressure heater increases slightly; in the latter case, the drain regulating valve may be stuck. If during operation of the high pressure heater, the steam side pressure and drain water level increase significantly, the opening of the drain regulating valve increases significantly, and the feedwater inlet and outlet pressure difference, drain water temperature, and outlet feedwater temperature decrease significantly, it indicates a large amount of leakage in the tube system of the high pressure heater. The high pressure heater should be taken out of service immediately to prevent damage to the high pressure heater shell and the malignant accident of water ingress into the steam turbine.
3. Prevention Measures for High Pressure Heater Leakage
3.1 Improve the manufacturing quality of high pressure heaters and strengthen research on assembly and welding technology of high pressure heaters.
