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 large units with high pressure heaters, 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 high efficiency and low consumption of large units. With the deepening of internal potential tapping 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 of faults 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 and stop 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. During the commissioning and decommissioning of the high pressure heater, the opening and closing of the electric valve are completed in a short time. Due to frequent start and stop of the unit, the temperature change rate during start and stop exceeds the allowable value, resulting in rapid temperature changes of the internal tubes and tube sheets of 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 level, causing a so-called "false water level." When the indication is too high, the emergency drain electric valve automatically opens, leading to low or no water level operation of the high pressure heater; when the indication is too low, the emergency drain electric valve automatically closes, and the drain water level gradually rises, causing the high water level protection to act, and the emergency drain electric valve opens again, or even the high pressure heater is decommissioned due to misoperation of the measured water level signal. Whether the measured water level is too high or too low, 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 with the tube sheet.
1.3 Loose emergency drain regulating valve of high pressure heater. To improve the safety and reliability of unit operation, 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 Loose high pressure steam inlet valve. When the high pressure heater is decommissioned, due to the loose steam inlet valve, some heating steam still leaks in, causing the tubes to overheat and reduce 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, swinging freely under the impact of high-speed airflow, constantly rubbing and impacting the surrounding tubes, expanding the rupture and leakage of surrounding tubes.
1.6 Vibration of high pressure heater. Steam flowing outside the heater tubes, transversely or longitudinally scouring 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 expel air during commissioning, 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 high pressure heater feedwater tubes. 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 (30 mg/kg N2H2 + 10 mg/kg NH3) or the feedwater in the high pressure heater should be completely drained; the steam side should be protected with deoxygenated feedwater containing 300 mg/kg hydrazine and 10 mg/kg ammonia, with pH=10; or under warm conditions, dry with dry air 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 been corroded to a certain extent.
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 High Pressure Heater Tube System Leakage. During operation of the high pressure heater, if the feedwater terminal temperature difference increases while the feedwater temperature rise of the high pressure heater decreases under basically normal steam parameters and inlet water temperature, 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 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 high pressure heater tube system. The high pressure heater should be shut down 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 welding technology of high pressure heaters;
