Principles for handling major turbine accidents

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Principles for dealing with major turbine accidents The basic principle in power generation is \"safety first,\" as accidents in power plants, especially those that result in severe damage to generating equipment, can cause significant economic losses to the company. This requires the operating staff to have a thorough understanding of the equipment’s structure and performance, as well as familiarity with systems such as those related to steam and water and the procedures for handling accidents. They must regularly conduct accident simulations and training so that, in the event of an accident, they can make quick and accurate judgments and carry out appropriate actions skillfully. General Provisions for Accident Handling 1. When operation crew members detect abnormalities during monitoring and routine inspections, they shall conduct a comprehensive analysis and assessment based on these abnormal signs, in conjunction with relevant indicators and signals. They must report such findings to the shift leader, team supervisor, and the respective workshop as soon as possible, so that a joint analysis can be carried out and coordinated actions taken to address the issue. If the shift leader or supervisor is not at the accident site, appropriate actions should be taken promptly in accordance with the relevant provisions of the operating procedures ； If the conditions for an emergency shutdown have been met, the switch must be turned off promptly to ensure the safety of the main equipment; a vacuum breakdown should be induced as a means of shutting down the system. One must not hesitate out of hope for the best or fear of taking responsibility, as such hesitation will delay action and lead to an escalation of the accident. II. In the event of an accident, the team leader is the organizer and commander responsible for handling the accident within their field of expertise ； The shift supervisor is the overall commander for handling accidents, and the team leaders must obey the supervisor’s orders ； The shift leader shall, under the unified command of the shift supervisor, lead the personnel on duty in his/her shift to handle accidents swiftly and decisively in accordance with their respective responsibilities. The workshop supervisors shall provide necessary guidance based on the actual conditions at the scene; they have the authority to direct the shift leader and the relevant professionals during accident handling, but such directives must not conflict with those of the shift supervisor. In the event of any conflict, the instructions of the shift supervisor shall take precedence. All orders given by the shift supervisor should be strictly followed, except those that pose a direct threat to personnel safety or equipment integrity. And handle it calmly and steadily in accordance with the following principles. (1) To quickly eliminate threats to personnel and equipment, personal safety must be ensured first. (2) Minimize the scope of the accident and ensure the normal operation of non-faulty equipment. (3) Once the fault is resolved, restore the normal operation of the unit as soon as possible to meet the system’s load requirements and ensure power supply to external users. The unit can only be shut down when it is indeed no longer in a condition to operate, or when continuing to operate poses an immediate threat to human safety and equipment safety. (3) All maintenance and testing activities must be halted when an accident occurs. The crew has the right to prevent unauthorized persons from entering the accident site. (4) In the event of an accident not covered by these procedures, the operators shall rely on their experience to make correct judgments based on the specific circumstances, and take proactive measures to handle it promptly. (4) In the event of a failure in an automatic device, the operators should make correct judgments, promptly switch the relevant automatic device to manual mode, make timely adjustments, and maintain normal parameters of the unit in order to prevent the accident from escalating. (5) Once the accident has been dealt with, the operators should truthfully record in the duty log the time of the accident, its symptoms, and the measures taken. Immediately after finishing their shift, they should convene the relevant personnel to carefully discuss and analyze the causes of the accident, the responsibilities involved, and the measures that need to be taken going forward. Summarize the experience and draw lessons from it. (6) In the event of an accident during shift handover, the personnel involved should assist one another, but they must follow the unified instructions of the on-duty supervisor or shift leader. Shift handover can only take place after the accident has been resolved. Handling of Several Typical Accidents in Turbines I. Emergency Shutdown An emergency shutdown should be carried out under the following circumstances: 1. When the turbine experiences severe vibration, or when obvious sounds of metal friction, impact, and other abnormal noises are heard from within it. 2. The turbine speed increased to 3330 r/min, yet the emergency shutdown device did not activate. 3. The main steam temperature drops suddenly, exceeding the specified limit values, or water hammer occurs. 4. The main bearing or end shaft seal emits strong sparks or thick smoke. 5. The temperature of the babbitt in any main bearing or thrust bearing, as well as the temperature of the return oil, rise rapidly and exceed the specified limit values. 6. The turbine oil system caught fire, and local measures were insufficient to extinguish it. 7. The oil pressure in the oil system or the oil level in the main oil tank drops below the specified values. 8. The axial displacement of the turbine rotor exceeds the specified limit value, yet the axial displacement protection does not activate. 9. Rupture occurs in the main steam pipeline, the pipeline ahead of the extraction check valve, and the oil system pipelines or accessories, resulting in a severe leak. 10. Intense smoking or fire inside the generator and exciter. Standard procedure for emergency shutdown: 1. Manually activate the emergency safety device, ensuring that the automatic main steam valve, governor valve, and extraction check valve close promptly, and adjust the rotating baffle of the extraction unit to close as well. 2. Send “Attention” and “Machine in Danger” signals to the control room, disconnect the generator; at this point the speed decreases, and the coasting time is recorded. 3. Start the circulating oil pump and pay attention to changes in oil pressure. 4. Activate the vacuum breakdown device, stop the steam extractor, and break the vacuum in the condenser. 5. Open the condensate recirculation valve, close the low-pressure heater outlet water valve, and maintain the condenser water level. 6. To adjust a extraction-type unit, the medium and low pressure electric steam supply valves should be closed, and the voltage regulator should be disconnected. 7. Contact the shift supervisor as needed to activate the temperature reduction valve. 8. Other operations shall be carried out in accordance with the general shutdown procedures. 9. After completion of the processing, report to the shift supervisor and workshop leader. II. Blade damage incidents: During the operation of turbines, blade damage incidents can occur, including blade cracks, breakage, water erosion, detachment of the shroud, or welding failure or breakage of the struts. Leaf damage is a common type of equipment failure encountered in power plants. 1. Symbol of damaged blades: (1) When a single blade or surrounding strip breaks off and flies away, a metallic striking sound will be produced. (2) When the shroud of the regulating stage becomes detached, if it gets stuck on the blades of the next stage, or if a blade from some stage downstream of the regulating stage breaks off and blocks the flow path, this will cause the pressure in the steam chamber of the regulating stage or certain extraction pressures to rise ； When the low-pressure final-stage blades or shrouds fall into the condenser, there is a knocking sound inside the condenser. If the copper tubes of the condenser are damaged, this leads to a sudden increase in the hardness and conductivity of the condensate water, an elevation in the level of the hot well water, as well as an increased degree of subcooling of the condensate water ； When a large number of blades fall off asymmetrically, it causes the rotor to become unbalanced, leading to a significant increase in vibration of the unit. 2. Causes of turbine blade damage and fracture: There are various reasons for blade damage incidents, which are also related to factors such as design, manufacturing, installation, maintenance procedures, and operation and upkeep. Improper operation during the startup and shutdown of the unit resulted in water hammer ； Leaf overload ； Overload operation ； Operation at low grid frequency ； Or the escalation of other accidents, which further causes mechanical damage to the blades, can all easily lead to blade breakage incidents. 3. Measures to prevent blade damage: (1) The power grid should operate at a normal frequency, avoiding low-frequency operation to prevent the blades from operating within the resonance range. (2) When the initial and final steam parameters of the turbine as well as the extraction steam pressure exceed the specified ranges, the load should be reduced accordingly ； (3) Do not operate for extended periods under a load with only the adjustment steam valve fully open ； (4) When there are knocking sounds inside the turbine and the vibration of the unit increases suddenly, the machine should be stopped immediately for inspection to prevent the accident from worsening ； (5) During the major overhaul of the unit, a thorough inspection should be carried out on the damage to the flow-through components, and any defects present in the blades must be addressed promptly. Frequency measurement of the blade is carried out; if the vibration characteristics are not satisfactory, frequency tuning is required. III. Fires in the turbine oil system Fires in the turbine oil system tend to be intense and difficult to control. If the source of oil and heat is not cut off promptly, the fire will spread rapidly, leading to the destruction of equipment and buildings as well as posing a threat to human life. 1. Phenomena of oil system fire: There is bright flames or thick smoke at the turbine bearings, oil tanks, oil system pipes, etc. 2. Causes of oil system fires: Oil system fires generally occur when oil leaks from parts where the system is not airtight, and this oil comes into contact with hot components such as cylinders or steam pipes that lack proper insulation. If not dealt with promptly, it often leads to a fire. 3. Handling of oil system fires: During the operation of the turbine, if a fire breaks out in the oil system, the operators must promptly issue an alarm signal, inform the firefighters, and take active measures to extinguish the fire. When extinguishing a fire, wet cloths or dry extinguishing agents should be used; water or sand is not permitted. To extinguish the fire quickly, it is necessary to find a way to cut off the oil supply and the power to the faulty equipment. If the fire cannot be extinguished immediately and it poses a threat to the safety of the crew, the vacuum emergency shutdown should be activated, and the emergency oil drain valve should be opened to discharge the oil into the accident oil collection pit. Additionally, it is also necessary to prevent the fire from spreading to adjacent units. 4. Measures to prevent fires in the oil system: (1) Preventing oil leaks in the system – From a design and installation perspective, the pipes of the turbine oil system should be placed as low as possible below the steam pipes. Flanges and bolts should be used sparingly for pipe connections; welding should be preferred wherever possible. The layout of the pipes should take full account of their contraction when heated or cooled. A nested structure with the high-pressure oil pipe on the inside and the lubrication return pipe on the outside should be adopted as much as possible. From the perspective of operation and maintenance, operators should conduct thorough routine inspections, paying attention to monitoring whether the oil pressure, the oil returning to the bearings, and the conditions at the bearing oil seals are normal. When the control system experiences significant fluctuations or the oil pipes of the unit vibrate, it is necessary to promptly check for any leaks in the oil system pipelines, and address any leaks that are found immediately. (2) Isolate heat sources. The lowest ignition point of turbine oil is around 200 degrees Celsius; hydraulic components of the control system, such as pilot valve spools and oil pipelines, should be kept away from high-temperature heat sources ； For the main steam pipes or other high-temperature steam and water pipes located near the oil system, iron or aluminum sheets should be installed outside the insulation layer. Additionally, smoking while walking around the factory premises should be prohibited ； Do not perform open-flame operations around the oil system ； During the operation of the turbine, it is necessary to prevent bending of the main shaft and avoid static and dynamic friction at the shaft seal. (3) Complete fire-fighting facilities: Sufficient fire-fighting equipment should be provided in the turbine room and placed in visible locations; no miscellaneous items should be piled up around them, and the passages within the plant must remain unobstructed. Above areas with dense piping such as fuel tanks, it is advisable to install smoke detection alarms and fire nozzles, so that in the event of a fire in the oil system, an automatic alarm can be triggered and extinguishing agents can be sprayed at the source of the fire. In addition, operators are also required to conduct regular accident prevention drills for fire prevention and extinguishment. IV. Shaft bearing burnout accidents: Shaft bearing burnout accidents in steam turbines refer primarily to the bearings of the thrust bearings and support bearings. 1. Accident signs and hazards of bearing bush burnout: (1) The temperature of the bearing bush material and the temperature of the lubricating oil returning to the system increase significantly; once the oil film is damaged, the vibration of the unit increases and smoke begins to emanate from the bearing bushes ； (2) The axial displacement of the turbine increases; if it exceeds the value specified in the regulations, the axial displacement protection or the thrust bearing wear protection will activate, causing the turbine to be shut down via interlock. (3) Vibration of the unit intensifies, and in severe cases, abnormal noises are heard along with an increase in noise level. In the event of a bearing burnout accident, it can cause the bearing material to melt, damage the rotor shaft journals, and lead to contact friction between the moving and stationary parts of the turbine; in severe cases, this can result in damage to the turbine equipment. 2. Reasons for bearing wear: (1) Water hammer in the turbine, or the loss of balancing function of the turbine’s balance piston, or improper handling of a drop in steam temperature, can cause water to enter the turbine along with the steam. Additionally, poor quality of the steam or issues related to the blade structure can lead to a significant increase in the axial thrust on the turbine, resulting in overloading of the thrust bearings ； (2) Lubricating oil pressure drops, oil volume is low, or there is a lack of oil supply ； (3) Impurities enter the oil system, and the quality of the lubricating oil is substandard, resulting in the destruction of the bearing oil film ； (4) Excessively high lubricating oil temperature ； (5) Abnormal vibration occurs in the unit; the breakdown of the oil film causes wear and damage to the bushing materials ； (6) Poor grounding of the turbine rotor, causing shaft current to break down the oil film ； (7) An error occurred during the oil system switchover while the system was in operation, resulting in the bearings running out of oil and being damaged ； (8) The oil pump is not functioning properly, or the plant power supply is interrupted ； 3. Reasons to prevent bearing bush burnout: (1) Ensure proper supply of oil to the bearing lubrication system. A. Operators should regularly monitor the lubricating oil pressure, temperature, and return flow volume, and ensure that the oil purification system is functioning properly, so as to maintain a continuous supply of oil to the bearing bushes ； B. The power supply for the lubricating oil pump must be safe and reliable ； C. During operation, it is necessary to prevent misoperations when the oil system is switched ； D. When the turbine is in operation, the shaft seal system must function properly to prevent lubricating oil from containing water. (2) The turbine bearings should be equipped with devices to prevent shaft current, ensuring proper grounding of the unit rotor. (3) The temperature of the bearing bush babbitt and the readings from various oil temperature sensors in the lubricating oil system are accurate and reliable. When the bearing bush bismuth temperature exceeds 90 degrees Celsius ； The machine should be stopped immediately if the oil return temperature of either bearing exceeds 75 degrees Celsius, or if it rises suddenly and continuously to 70 degrees Celsius. (4) Prevent water hammer in the turbine as well as friction between the moving and stationary parts of its flow passages, in order to avoid excessive axial thrust or abnormal rotor vibration. V. Turbine water hammer: Turbine water hammer accidents are severe incidents; if not handled promptly, they can easily damage the turbine itself. A sudden water hammer during the operation of a turbine causes the metal components such as the steam chamber, cylinders, and rotor, which are operating at high temperatures, to cool down rapidly. This results in significant thermal stress and thermal deformation; the cylinders may bend upward, developing cracks, and the joint surfaces of the cylinder flanges may start to leak steam. The negative expansion difference increases, leading to wear and damage to the moving and stationary parts of the turbine ； Large shaft bending of the rotor also causes friction between the moving and stationary parts, all of which lead to severe vibration in the unit. When water hammer occurs, due to the large amount of moisture present in the steam, the speed of the water is greater than that of the steam, resulting in a water plug effect in the steam passage. This increases the pressure difference before and behind the impeller, leading to a sharp increase in axial thrust. If an emergency shutdown is not carried out promptly, the thrust bearings will become overloaded and burned out, thereby causing severe dynamic and static friction within the turbine and resulting in its damage. Furthermore, when water hammer occurs, the water entering the turbine exerts a braking effect on the rapidly rotating rotor blades, especially the long blades in the low-pressure stages, whose tip linear velocity can reach 300–400 m/s or more; the impact force of the water droplets on these blades is quite significant, and in severe cases it can bend or even break the blades. In short, water hammer will cause severe damage to the turbine. 1. Causes of water hammer: (1) Excessive evaporation in the boiler or uneven evaporation leads to gas-water bubbling. (2) Leakage in the boiler desuperheater or improper adjustment, operator errors, or failure of the automatic feedwater control system lead to a full water condition in the boiler. (3) Insufficient pipe warming or poor drainage of condensate during turbine startup ； An imperfect drain system in the main steam pipeline or the boiler’s superheater may allow water to enter the turbine. (4) During a sliding-parameter shutdown, improper control leads to an excessive rate of temperature drop, causing the steam temperature to fall below the saturation temperature at that pressure, resulting in wet steam containing water. (5) During the startup of the turbine, the steam supply pipeline for the steam seal was not properly warmed up, and water drainage was insufficient, resulting in a mixture of steam and water being sent to the steam seal. (6) During shutdown, when switching to the backup steam seal steam source, water accumulated in the backup system was sent to the steam seal without being properly drained. (7) Rupture of the water tubes in the high and low pressure heaters, failure of the secondary protection devices, and improper sealing of the extraction check valve cause water to flow back into the turbine through the extraction pipes. (8) After shutdown, supervision of the condenser water level was neglected, resulting in the condenser filling with water, which then flowed into the cylinder. 2. Water hammer phenomenon: (1) The main steam temperature drops sharply, and white steam may appear at the valve stems, flanges, shaft seals, etc. of the main steam valve and control steam valves. (2) The vibration of the unit gradually increases until it becomes severe. (3) The temperature of the thrust bearing’s babbitt rises rapidly, and the sound of the unit’s rotation becomes abnormal. (4) The temperature difference between the upper and lower parts of the cylinder increases, and the temperature of the lower cylinder needs to be reduced significantly. 3. Handling methods: Water hammer is one of the most hazardous accidents that can occur during turbine operation. Operators must quickly and accurately determine whether water hammer has occurred; generally, a sharp drop in the main steam temperature is used as an indicator (white vapor may not appear at the valve stems and flanges of the main steam valve and control valves when water hammer occurs). At the same time, it is necessary to check the temperature difference between the upper and lower parts of the cylinder, as the temperature of the lower cylinder inevitably drops significantly when water enters the turbine. Upon confirmation of a water hammer incident, the vacuum should be immediately broken to trigger an emergency shutdown. (1) Vacuum breakdown emergency shutdown. (2) Open all drain valves on the turbine casing and main steam pipes to ensure thorough drainage. (3) Correctly record the rotor coasting time and vacuum value. (4) Listen carefully to the sounds inside the cylinder during idle operation. (5) Check and record the bearing bush u-joint temperature and axial displacement data. (6) Pay attention to the rotating sound of the unit during coasting and the operation of the thrust bearings. If the coasting time is normal, and after sufficient drainage of water vapor and the main steam temperature has returned to normal, the unit can be restarted. But this requires special care in listening carefully for any abnormal noises inside the cylinder, as well as measuring whether the vibration of the unit has increased; if any abnormalities are detected, startup should be stopped immediately and the cylinder opened for inspection. (7) If water enters the machine due to a rupture in the heater’s steel pipe, quickly close the extraction check valve manually and also shut off the heating steam valve of the heater; ensure that all water is drained from the extraction pipe. VI. Severe overspeed accident of steam turbine generators: When the speed of the steam turbine exceeds the speed at which the emergency shutdown device activates, N (N=111%–112% of n, where n is the rated speed), and continues to rise, this is referred to as a severe overspeed. The rotating components of a turbine are generally subjected to strength checks at 112% of n. During operation, if the turbine speed exceeds this limit, these rotating components will experience stresses beyond their design limits and break, resulting in severe vibration of the unit and damage to the equipment. In severe cases, it can cause the turbine to run away, leading to the failure of the unit’s shafting and rendering the entire unit unusable. 1. Phenomenon of turbine overspeed: The turbine speed gauge and frequency meter indicate values that exceed the upper limits and continue to rise, while the pressure oil and lubricating oil levels also increase proportionally ； Increased unit vibration ； The operating sound is abnormal ； The unit suddenly lost its load to zero. 2. Reasons for turbine overspeed: The main reasons for turbine overspeed are failures in the control and safety oil systems or equipment faults, which cause the system to malfunction and thus fail to regulate the speed of the turbine. (1) Poor quality of turbine oil, such as the presence of impurities or water in the oil, along with the failure to operate the purification system as required, can cause corrosion and sticking in the speed control and safety systems. (2) The control system is not properly adjusted, and it is unable to maintain the unit in idle mode ； Or excessive variation in rotational speed, or high retardation rate, etc ； (3) The emergency safety device is stuck or has insufficient travel, or its operating speed is too high; the settings of additional protection devices (such as electrical overspeed protection) are incorrect, or these devices fail to function ； (4) Due to poor steam quality, scaling occurs on the stems of the automatic main steam valve and the control steam valve; as a result, when these valves need to be closed, they fail to operate due to sticking, which leads to overspeed ； (5) Sticking or incomplete closing of the extraction check valve and the high-pressure cylinder exhaust check valve, etc. 2. Measures to prevent overspeeding: To avoid overspeed accidents in turbines, it is first necessary for the turbine control system to have good static and dynamic characteristics. Secondly, prevention is key during operation; to this end, the following technical measures should be taken: (1) General requirements for control and safety systems. All overspeed protection devices must be in good condition and functioning properly. The main steam valve, control steam valves, and extraction check valves should be able to close quickly and tightly without any sticking. Regardless of the operating conditions of the unit, the control system must be able to maintain stability and function properly even when the load is reduced partially or completely. (2) Strengthen oil quality supervision: Conduct regular tests and analyses of oil quality. Oil purification devices must be in proper operation to prevent water and impurities from entering the oil, thereby avoiding corrosion and jamming of the control components. (3) Strengthen supervision of steam quality: During operation, enhance supervision of steam quality to prevent salt from being carried in the steam, which could lead to scaling on the steam valve stems and cause sticking. (4) Regular testing of the control safety system: A. Regular testing of the control safety system is one of the main methods for checking whether the system is in good condition, whether it can operate quickly and accurately in abnormal situations, and for preventing severe overspeeding of the unit. B. Protection device experiment. After a major overhaul of the turbine, and after the emergency shutdown device or control system has been disassembled or adjusted and has operated continuously for 2000 hours, prior to the load-shedding test, as well as when restarting the turbine one month after it has been shut down, two speed-raising tests should be conducted; the difference between the speeds achieved in these two tests should not exceed 0.6%. The emergency stop device is calibrated at (1.11～1.12)n; if its operating speed is too high or too low, adjustments should be made. C. Valve tightness test and shut-off test. To prevent excessive overspeeding of the turbine when it is taken off full load or during an emergency shutdown, as well as to enable effective control of the speed at low speeds, valve tightness tests should be conducted regularly. The valve tightness test is a test conducted to check the closing degree of the main steam valve and control steam valves, as well as to examine the tightness of the extraction check valve. VII. Friction between the rotating and stationary parts of the turbine and bending of the main shaft 1. Causes of accidents (1) Reasons for friction between the rotating and stationary parts [1] Improper installation, maintenance, or adjustment of the clearance between these parts. [2] When the moving and stationary components are heated or cooled, they expand or contract unevenly. ［3］The mechanical deformation of the stressed part exceeds the allowable value. [4] The thrust bearing or spindle bearing is damaged. [5] Severe vibration of the unit. ［6］The rotor assembly components are loose and displaced. [7] Damage to the components in the flow passage, or hard debris entering the flow passage. [8] Forcefully turn the rotor when the rotor is bent or the cylinder is severely deformed. (2) Main causes of major shaft bending [1] Friction between the moving and stationary parts causes localized overheating of the rotor. ［2］After shutdown, when the cylinder temperature is still high, cold water enters the cylinder for some reason, causing the lower side of the rotor, which is in a high-temperature state, to come into contact with the cold water. This leads to sudden local cooling, resulting in a large temperature difference between the upper and lower parts and thus thermal deformation, which in turn causes the main shaft to bend. According to the calculation results, when the temperature difference between the upper and lower parts of the rotor reaches 150–200 degrees Celsius, it will cause the main shaft to bend. The higher the rotor metal temperature, the more likely it is to cause bending of the main shaft. [3] The raw material of the rotor contains excessive internal stress; after operating at higher temperatures for a period of time, this internal stress is gradually released, resulting in bending deformation of the rotor. 2. Accident symbolism: Since such accidents occur inside the cylinder and cannot be observed directly, judgments can only be made based on the causes and characteristics of the accident. It generally has the following characteristics: (1) Increased vibration of the unit, even severe shaking. (2) Sparks may occur at the front and rear steam seals. (3) There is a metallic grinding sound inside the cylinder. (4) For units equipped with an oil main shaft deflection indicator, the indicated value will increase or exceed the limit. (5) If the thrust bearing is damaged, the temperature of the thrust pad will rise, the axial displacement reading may exceed the limit, and a signal will be generated. (6) The temperature difference between the upper and lower cylinders may increase rapidly. 3. Accident handling methods: By observing various signs such as excessive vibration of the turbine units, metallic friction sounds inside the cylinders, or sparks at the steam seals, and by considering the changes in readings from relevant instruments, it can be determined that such an accident has occurred. In such cases, an immediate shutdown is necessary; avoiding measures such as reducing load or speed to continue warming up the equipment would only delay the shutdown process, thereby exacerbating the accident and causing greater damage to the equipment. During shutdown, the rotor’s coasting time must be recorded, and manual turning of the rotor should be performed after it has come to rest. If it cannot be rotated, do not force it; a thorough analysis and investigation are necessary, along with appropriate measures, until the cylinder can be opened for inspection. VIII. Drop in Turbine Vacuum: There are two types of drops in turbine vacuum – sudden drops and gradual drops. 1. Causes of the incident: (1) Reasons for a sudden drop in vacuum: [1] Interruption of the circulating water supply: A power outage in the plant, tripping of the circulating water pump motor, damage to the check valve of the pump, or rupture of the circulating water pipes can all lead to an interruption in the circulating water supply. ［2］ Interruption of steam supply to the shaft seal: Malfunctions of the shaft seal pressure regulator, interruption of the steam supply source, or water ingress into the shaft seal system can all lead to an interruption in the steam supply to the shaft seal. This results in a large amount of air entering the exhaust cylinder, causing a sharp drop in the vacuum level of the condenser. ［3］Vacuum pump failure: A malfunction of the water injection pump in a jet-type vacuum pump, or a breakdown in the water injection system, can both cause the vacuum pump to fail to function properly. This requires switching to the backup exhaust equipment as soon as possible. [4] Full water in the condenser: Leaks in the copper tubes of the condenser, failures in the condensate pumps, or improper maintenance by operators can all lead to an excess of water in the condenser, thereby causing a drop in vacuum. [5] Severe leakage in the vacuum system: Due to cracks or damage in the pipes or valves of the vacuum system, a large amount of air leaks into the condenser. In such cases, it is necessary to locate the source of the leakage as quickly as possible and take emergency repair measures to stop the leak; otherwise, the machine must be shut down for maintenance. (2) Reasons for the gradual decline in vacuum: A gradual decrease in vacuum occurs frequently; it generally poses little threat to the safe operation of the unit. However, it is difficult to identify the causes. The main reasons can be summarized as follows: [1] Leaks in the vacuum system: This is usually manifested by a vacuum level in the turbine that is lower than normal at the same load, and this level remains constant. As the load increases, the vacuum level in the condenser actually rises (an increase in load reduces the range of vacuum levels within the unit’s vacuum system). The tightness of the vacuum system can be checked through regular vacuum system tightness tests. If it is confirmed that the vacuum system is not airtight, the leak location must be carefully identified; a candle flame or specialized leak detection equipment can be used for this purpose, and the leak should be repaired promptly. After major and minor overhauls of the unit, the vacuum system should be filled with water to detect leaks, in order to eliminate them and ensure that the vacuum system remains airtight during operation. ［2］High condenser water level: An increase in the condenser water level is often caused by abnormal operation of the condensate pump or a fault in the pump, which leads to a decrease in the pump’s load. If necessary, start the backup water pump to shut down the faulty pump for inspection and repair. If an increase in condensate hardness or a rise in the heater level is detected, an increase in the condenser level can also be caused by leaks in the condensate recirculation valves. ［3］ Insufficient circulating water volume: Under the same load (i.e., the same amount of steam discharged), if the outlet temperature of the condenser’s circulating water rises, meaning the temperature difference between the inlet and outlet increases, it indicates that there is an insufficient amount of circulating water in the condenser. It is necessary to check whether there are any abnormalities in the operation of the circulating water pump, as well as to examine the outlet pressure of the pump, the water pressure at the inlet of the condenser’s water chamber, and the water level at the inlet of the circulating water. Additionally, it is important to check whether the inlet filter is clogged. ［4］Abnormal operation or reduced efficiency of the extractor: In such cases, an increase in the terminal difference of the condenser can be observed. It is necessary to check whether the water pressure in the extractor is normal, whether the water level and temperature in the water tank of the jet water extractor are normal, and what the integrity of the extractor’s vacuum system is like. If possible, it is advisable to test the extractor’s operating capacity and efficiency. [5] Fouling of condenser copper tubes: The structure of the condenser copper tubes leads to a decrease in vacuum, which necessarily results in an increase in the terminal difference. Any malfunction in the water tower’s watering device or the water distribution channels will cause the temperature of the circulating water to rise, thereby reducing the vacuum level of the condenser. 2. Accident symbols: (1) Drop in condenser vacuum, increase in exhaust steam temperature. (2) The unit load decreases, or the main steam flow increases at the same load. (3) Rise in condenser water level. (4) Abnormalities occur in the circulating water pump, condensate water pump, exhaust equipment, circulating water cooling equipment, etc. 3. Accident handling methods: Depending on whether the vacuum level of the condenser drops rapidly or slowly, different handling methods are employed based on the underlying causes. The unit load must be reduced in accordance with the regulations outlined in the operating procedures, taking into account the degree of decline in the condenser’s vacuum level (the operating procedures provide a table showing the relationship between vacuum levels and unit load). During the load reduction process, if a fault cannot be resolved immediately and the condenser vacuum level continues to drop even when it reaches the allowable minimum value, it is necessary to shut down the machine for repairs. IX. Load shedding of steam turbine generators: During operation, if the electrical load suddenly drops to zero, this type of incident is known as load shedding of steam turbine generators. There are four situations for load shedding: (1) The generator is disconnected, and the unit’s speed remains below the speed at which the emergency shutdown device activates. (2) The generator is disconnected, and the emergency protection device activates. (3) The generator is disconnected, the unit’s speed is higher than the speed at which the emergency shutdown device activates, yet the emergency shutdown device does not operate. (4) The load was reduced to zero, but the generator was not disconnected. The symbols and handling methods for these four types of load shedding are different, as described below. 1. The generator is disconnected, and the unit’s speed remains below the speed at which the emergency shutdown device activates. (1) Reason: Due to a fault in the electrical system, the generator’s oil circuit breaker tripped, causing the load to be disconnected. The dynamic characteristics of the control system were satisfactory, and the speed was kept below the level at which the emergency protection device would activate; as a result, the emergency protection device did not operate. (2) Symbol ［1］ indicates that the electrical load is zero. [2] The tachometer reading increases and stabilizes at a value above the rated speed of 3000 r/min; this value is determined by the magnitude of the speed variation in the control system and the position of the synchronizer corresponding to the load carried by the unit before the load is removed. [3] The stroke of the oil motor decreases, and the speed control valve is closed to the no-load position. [4] The extraction check valves in each section close and a “closed” signal is issued. (3) Processing: [1] Set the synchronizer to the no-load position and maintain the unit’s speed at 3000 r/min. [2] Adjust the steam supply to the shaft seal in a timely manner to maintain the condenser vacuum, monitor the pressure inside the deaerator, and make necessary adjustments. [3] For the extraction turbine unit, close the electric steam supply valve that controls the extraction steam and deactivate the pressure regulator. [4] Open the condensate recirculation valve (pay attention to the condensate main pipe pressure), maintain the condenser water level, and add demineralized water if necessary. [5] Switch the steam traps of each heater: direct the steam trap of the high-pressure heater to the low-pressure heater, direct the steam trap of the low-pressure heater to the condenser, and stop the pump for the steam trap of the low-pressure heater. [6] Check operational parameters such as the unit’s expansion, expansion difference, vibration, and the temperatures or temperature differences in various parts of the cylinders. Once it is confirmed that everything is normal, send a “normal” signal to the control room, connect the generators in parallel, and adjust the electrical load accordingly based on the cylinder temperatures. All other operations shall be carried out in accordance with the unit’s operating procedures. 2. The generator is disconnected from the grid; the control system is unable to regulate the speed, and the emergency protection device activates. (1) Causes: Electrical faults cause the unit to lose its load, and the dynamic characteristics of the turbine’s control system are poor, resulting in an excessive increase in speed and thus activation of the emergency protection device. (2) Symbol ［1］ indicates that the electrical load meter reads zero. [2] The automatic main steam valve, throttle steam valve, and extraction check valve are closed, and a signal is sent. [3] The turbine speed rises to the speed at which the critical safety device activates, and then drops. [4] The emergency safety device activates and sends a signal. (3) Handling method: [1] When the speed drops to 3050 r/min, quickly move the synchronizer to the no-load position, reapply the brake, open the automatic main steam valve, and use the synchronizer to adjust the speed to 3000 r/min. [2] Start and stop the auxiliary oil pump according to changes in oil pressure and requirements. [3] The other operations are the same as those in items [2] to [6] of the handling for the first type of load shedding. [4] Report to the relevant supervisors; the system can be connected in parallel under load again only after it is functioning properly. 3. The generator is disconnected from the grid; the control system is unable to regulate the speed, resulting in over-speed, and the emergency shutdown device does not activate. (1) Cause: A fault in the electrical system causes the unit to lose its load, while the turbine’s control system is unable to control the speed. As a result, the speed rises rapidly and exceeds the speed at which the emergency shutdown device would activate, yet this device still fails to function. (2) Symbol ［1］ indicates that the electrical load meter reads zero. [2] The turbine speed rises above 3300–3360 r/min, and the sound emitted by the unit is abnormal. [3] The oil pressure at the outlet of the main oil pump rises rapidly. [4] Unit vibration increases. [5] The extraction check valves in each section close and send a signal. (3) Handling method [1]: Immediately press the manual emergency stop button to interrupt the vacuum-based emergency shutdown; the automatic main steam valve and throttle valve will close promptly while a signal is generated. [2] For the extraction unit, close the electric valve for adjusting the extraction steam supply and deactivate the pressure regulator. ［3］Move the synchronizer to the no-load position. [4] Start the auxiliary oil pump based on oil pressure. [5] Complete other shutdown operations. [6] Report to the management for a comprehensive inspection; operation can be resumed only after confirming that all units are functioning properly. The emergency protection devices must pass testing and adjustment before the units can be connected in parallel under load. 4. Load is reduced to zero, but the generator remains connected. (1) Cause: Misoperation of the turbine protection device or control system leads to an interruption in the steam supply to the turbine, resulting in the load being reduced to zero, yet the generator stays connected. (2) Symbol: [1] The electrical load meter and main steam flow meter indicate zero. [2] The automatic main steam valve, the speed-regulating steam valve, and the check valves for extraction at each stage are closed, and a signal is sent. [3] The tachometer still indicates 3000 r/min (the same as the grid frequency). [4] The main oil pressure remains unchanged. [5] Exhaust temperature increases. [6] A certain protection signal triggers an alarm. (3) Handling method [1]: Quickly check various protection signals of the unit, verify the values shown on the protection indicator instruments that have sent out signals, and at the same time inspect the condition of the unit itself. If it is confirmed that a fault in the equipment has caused the protection to activate, the generator should be disconnected from the system immediately to shut it down. [2] If it is confirmed that the unit itself and related meters are functioning normally, and that the malfunction was caused by a faulty protection system or control system, the protection switch can be turned off and then reactivated to quickly restore the unit’s load. At the same time, the thermal engineering team should be contacted to determine the cause of the faulty protection and take measures to resolve it as soon as possible. [3] During the handling process, rapid analysis and judgment are necessary, as the generator remains connected, and the turbine operates at 3000 r/min without steam supply. This operation must not exceed the time specified by the manufacturer, usually not more than 3 minutes; otherwise, the manual generator disconnection button should be pressed to shut down the machine and identify the cause of the issue. [4] The turbine shall not be started up and connected to the grid immediately until the cause of the defect has been identified or the defect has been eliminated. [5] Report to the management that the unit can only be restarted after the defects are resolved. Throughout the startup, paralleling, and load-bearing processes, strict monitoring of all aspects of the turbine is required, along with conducting relevant tests and adjustments as needed. X. Abnormal vibration of the generator set 1. Phenomenon: (1) Audio-visual alarm for \"high rotor vibration\". (2) On-site inspection showed a significant increase in unit vibration. 2. Reasons: (1) Dynamic and static rubbing between the turbine generator set components or bending of the main shaft. (2) Broken turbine blades or unbalanced rotor mass. (3) Water entering the cylinder or cold air causes deformation of the cylinder. (4) The bearing is not working properly or the bearing housing is loose. (5) The steam turbine rotor is out of alignment or the coupling is loose. (6) Jamming in the sliding system causes uneven expansion. (7) Large fluctuations in lubricating oil pressure and temperature. (8) Steam turbine overload. (9) Steam shaft seal damaged. (10) The small shaft at the front end of the shaft (in front of the thrust bearing) breaks; in this case, in addition to vibration, sparks may emerge from the shaft seal area of the high-pressure cylinder. (11) Debris has fallen between the rotating and fixed parts of the turbine generator and exciter. (12) The clearances on both sides of the shaft bearings of the steam turbine generator set are not within the specified range. (13) Mechanical looseness in the generator section. (14) Friction occurs within the flow passage of the turbine and at the shaft seals on both ends; friction also takes place between the diaphragms, shaft seals, steam baffle plates, oil retaining rings, and the shaft. 3. Handling: (1) If large vibrations occur due to significant changes in the unit’s load and parameters, it is necessary to stabilize these values as soon as possible, while simultaneously monitoring changes in the turbine’s expansion difference and the temperature difference between the upper and lower cylinders. (2) Check whether the lubricating oil pressure, oil temperature, and bearing temperature are changing normally; if not, adjust them to normal levels. (3) Listen to the sounds inside the turbine on site. (4) Check that the expansion difference and axial displacement are normal. (5) If the shaft vibration of the unit exceeds 0.07 mm, it should be shut down due to a fault. XI. Power outage for plant use 1. Symptoms: (1) Accident signal alarm, accident horn sounds. (2) All operating AC motors tripped and stopped, the ammeter readed “0”, and the standby AC auxiliaries did not activate. (3) The turbine trips, and the unit is disconnected from the load to “0”. (4) The steam temperature, steam pressure, and vacuum drop rapidly. (5) When the normal lighting is turned off, the emergency lighting should turn on. 2. Handling: 1. After all plant power is lost, the shutdown should be carried out in such a way as not to disrupt the vacuum. 2. Start the turbine DC lubricating oil pump. If the DC oil pump fails to start properly, perform an emergency shutdown. 3. It is prohibited to discharge steam or water into the condenser; manually close the valves through which steam or water may enter the condenser. 4. If the DCS power supply is lost, contact the electrical department; the DCS power supply must be restored first before the plant power can be restored. 5. Restore the plant power supply as soon as possible; once it is restored, proceed to start and switch on each type of oil pump and water pump one by one. A comprehensive inspection of the unit is also carried out. 6. The unit will shut down automatically in the event of a power loss in 505E. XII. Deaerator Vibration 1. Causes of vibration (1) Excessive water inflow to the deaerator or too low temperature of the chemical make-up water. (2) For newly put into operation deaerators, when placed in parallel with existing deaerators, the feedwater temperature is more than 15 degrees Celsius lower than that in the existing deaerators. (3) After the maintenance of the deaerator’s downcomer is completed, excessive and rapid air discharge into the deaerator takes place when it is put back into use. (4) If the water level in the deaerator tank is too high, water hammer can occur as a result of direct contact with the steam at the steam inlet pipe, or water hammer can arise when water flows into the pipe through the steam extraction pipe. 2. Treatment method: Vibration occurs in the deaerator; sometimes it is mild, and at other times it is quite severe. In the event of severe vibration, decisive action must be taken promptly to prevent the accident from escalating. If, based on the interpretation of the vibrations as indicating an issue caused by an excessive amount of water in the deaerator, it is determined that this is the cause of the vibrations, then the amount of deaerated water should be adjusted to a level consistent with that of the other deaerators. If vibration is caused by an excessive amount of chemical make-up water or too low a temperature, in addition to making adjustments by redistributing it to other deaerators, some of the chemical make-up water can be fed into the condenser for heating, or other heating measures can be employed. If the vibration is caused by an excessive water level in the deaerator tank, measures should be taken immediately to drain water from the deaerator tank. If the deaerator vibrates due to water hammer in its heating steam pipe, steam extraction can be temporarily stopped; it can be restarted once the water hammer has subsided. When putting the jelly tube from the deaerator back into service after maintenance, it is essential to proceed slowly to allow the air inside the tube to be gradually expelled through the deaerator. One must never act in haste in pursuit of speed. The temperature difference between the water temperature of the newly put into operation deaerator and that of the operating deaerator must not exceed the specified 15 degrees Celsius. Vibration caused by the paralleling of new deaerators should be addressed immediately by stopping the paralleling operation; paralleling is permitted only once the water temperature meets the standards. 13. Failures in the turbine control system and oil system 1. Identification and handling of oil leaks in the oil system: (1) Both the oil pressure and the oil level in the tank decrease: Cause: Oil is leaking from the pressure oil pipes outside the tank, or there is a leak in the cooler. Handling: Quickly locate the oil leak and try to eliminate it while the system is running. If it cannot be resolved in time and the oil level drops to –200 mm with no possibility of raising it, the system should be shut down due to the fault. (2) The oil level remains unchanged when the oil pressure drops ； Reason: Pressure oil leaks into the fuel tank or return chamber (such as due to a faulty check valve on the oil pump), or the inlet of the oil pump or the filter on the filler is clogged. Handling: Check the operation of the main oil pump, verify that the check valve is airtight, and find ways to resolve the issue while it is still in operation. When the lubricating oil pressure drops, the AC oil pump should be started. (3) Oil level is low, but oil pressure remains normal ； Reason: Oil generally leaks from various connection pipes of the tank or the oil cooler, etc., and accumulates at the bottom of the tank. Handling: After confirming that the gauge readings are indeed correct, immediately locate the source of the oil leak; if necessary, top up the oil tank. If the problem cannot be resolved during operation, shut down the machine immediately as an emergency measure. 2. Judgment and handling of increased oil temperature (1) Increased oil temperature in individual bearings: Causes include blocked lubrication oil pipes, debris blocking them, incorrect installation of bearing shells, etc. Handling: Check whether oil is returning to this bearing. If smoke is observed from the bearing or if the oil return temperature rises sharply by more than 75°C, the vacuum should be broken to initiate an emergency shutdown. (2) Generally increased bearing temperature. Solution: Adjust the water volume in the cold oil cooler and remove dirt from the copper tubes of the cold oil cooler. 3. Abnormal operation of the main oil pump: Symptoms include fluctuations in the speed control oil pressure and the oil pressures at the inlet and outlet of the main oil pump, as well as abnormal noises from the pump. Start the circulating oil pump to maintain the lubricating oil pressure, report to the shift leader and the plant manager, and confirm the abnormality ； An emergency shutdown should be initiated in case of a fault; if a distinct metallic screeching sound is heard, the vacuum should be broken immediately to trigger an emergency shutdown. 4. Abnormal operation of the auxiliary oil pump: During shutdown, if the gasoline pump fails, the AC oil pump should be started; if the AC oil pump also fails, then the DC oil pump should be used. During normal operation, the AC oil pump is tested, and any issues found must be repaired promptly. XIV. Feed water pump failures (including vaporization, motor smoking, pump set vibration, lubrication oil interruption, fluid coupling failure) Vaporization 1. Indicators of vaporization: (1) Sudden fluctuations in the inlet and outlet pressures of the water pump. (2) A loud rustling sound is produced at the water pump inlet. (3) The pump current also fluctuates with pressure. 2. Reasons for vaporization: (1) The feed water pump operates under overload, resulting in an excessive flow rate and reduced pressure in the inlet pipe of the feed water pump, which leads to vaporization. (2) Sudden drop in pressure inside the deaerator. (3) The filter screen at the inlet of the feed water pump is clogged with debris, resulting in a pressure drop at the pump inlet and thus vaporization. (4) Air has entered the feed water pump inlet pipeline. 3. Handling methods: If severe vaporization at the inlet of the feed water pump is detected, the following actions should be taken: (1) Start the standby pump to reduce the flow rate of the operating feed water pump ； In the absence of a backup feed water pump, the operator and furnace operator should be contacted immediately to appropriately reduce the unit load in order to eliminate vaporization at the feed water pump inlet. (2) Analyze and identify the causes of vaporization at the inlet of the feed water pump, and take appropriate measures to eliminate them promptly. (3) When air enters the inlet pipe of the feed water pump, operation should be stopped promptly to remove the air from within the pump. When the vaporization at the inlet of the water pump persists even after starting a backup feed water pump or reducing the unit’s load, the cause of this vaporization may be air entering the feed water pump inlet; this situation usually occurs as a result of improper drainage of air from the pipeline system at the feed water pump inlet. (4) If vaporization occurs due to the blockage of the feed pump inlet filter, the feed pump should be stopped, and the inlet filter should be cleaned, provided that the capacity of the feed pump is sufficient to meet the requirements. Under normal conditions, the inlet filter of the feed water pump should be cleaned regularly on a scheduled basis. Methods for dealing with power outage failures in operating feed water pumps 1. Causes of power outage in feed water pumps (1) Faults in the power supply circuit of the feed water pump. (2) Power supply to the entire plant is interrupted. (3) Local interruption of the plant power supply. 2. Handling methods: (1) If there is a disruption in the power supply to the electrical busbar or if there is a fault in the power supply system of the feed water pump itself, the backup pump connected to another busbar should be started immediately to ensure proper water supply to the boiler ； (2) If there is a power outage for the entire plant, the interlock switch of the standby feed water pump should be disengaged and the switch of the operating pump should be turned off to keep the unit idling (for condensing units, measures should be taken based on the vacuum level of the condenser); wait until the electrical staff restore power supply before restarting the feed water pump. Methods for dealing with the abnormal drop in pressure in the feedwater main 1. Causes of the drop in feedwater main pressure (1) A sudden increase in the turbine load leads to a sharp rise in the amount of water required by the boiler. (2) The boiler feedwater regulator fails to function properly during operation. (3) Severe leakage from the feedwater heater, boiler economizer, superheater, or water wall tubes. (4) The power supply frequency of the electrical system decreases. (5) Severe leakage in the water supply pipeline. 2. Handling methods: (1) When an increase in feedwater flow rate and a decrease in the pressure in the feedwater main are detected, the standby feedwater pump should be started in accordance with the minimum pressure requirement for the feedwater main ； If there is no backup pump, the operator should be notified to reduce the unit load. The main pipe feedwater system should reduce the overall plant load. (2) If it is detected that the pressure in the feedwater main decreases due to leaks in the boiler feedwater pipes, in addition to increasing the feedwater pressure, it is also necessary to increase the amount of water supplied to the deaerator for desalination, in order to prevent the deaerator water level from dropping too low. (3) If the pressure in the feed water main decreases while the feed water flow rate and current are low, and the indoor lighting becomes dimmer, it indicates that the decrease in pressure in the feed water main is caused by a drop in the power supply frequency. In this case, using the method of starting the feed water pump will not increase the feed water pressure. The driver and furnace operator should be informed to operate at a reduced main steam pressure in order to ensure proper water supply to the boiler. (4) Due to the drop in pressure in the feedwater main, the operations following the activation of the feedwater pump are as follows: Turn on the power switch for the auxiliary pump, and eliminate the light and sound signals indicating the drop in pressure in the feedwater main ； Adjust or shut off the recirculation of the auxiliary pump based on the flow rate of the feed water pump ； Check the reasons why the feed water pump is activated ； If the interlocking is caused by excessive fluctuations in the unit load or improper adjustment of the boiler feedwater regulator, the operating mode should be selected based on the unit load and the feedwater main pressure. The specific operations shall be carried out in accordance with the provisions in the operating procedures. 15. Pipe rupture before the main steam and extraction check valves 1. In the event of a rupture in the main steam pipe or the pipe leading to the extraction check valve, the following actions should be taken: (1) Start the AC oil pump and handle it as an emergency shutdown ； (2) Isolate the steam pipe at the faulty section, and at the same time open the windows in the turbine room to release the steam. Be careful not to run around randomly, to avoid being injured by the air currents. 2. When the gasket at the flange joint of the main steam pipe is damaged, the following should be done: (1) Try to reduce steam leakage ； (2) If steam is ejected violently, the section of pipe where the leakage occurs should be isolated; when steam accumulates at the work site, windows should be opened ； (3) It is not possible to quickly isolate the damaged section of the pipe; when the damage to the steam pipe flange is severe enough to threaten the safe operation of the unit, the machine should be shut down immediately. 3. In the event of a rupture in other pipes: (1) Quickly cut off or switch off the faulty pipe, try to maintain operation, and inform the relevant personnel to carry out emergency repairs promptly ； (2) According to the connection system of each pipeline, stop the relevant operating equipment of the faulty pipeline and start the backup equipment; if switching is not possible or the fault cannot be resolved, the system should be shut down due to the fault. 16. DCS power loss or shutdown: When the power supply to the DCS system is lost: 1. If both power supplies of the DCS system are unavailable, the operators lose control over the unit; the main unit’s protection mechanism should activate to shut down the unit, otherwise manual shutdown should be carried out immediately. 2. DCS microcomputer statuses are not synchronized: (1) Phenomenon: When the status displays for power, valves, protection, or interlocks in the same screen on the microcomputers of Operator Stations #1 and #2 differ from each other. (2) Handling: [1] Report to the shift supervisor immediately and contact the thermal engineering team for prompt handling. [2] Generally, the microcomputer should not be operated until the fault is resolved. If it is necessary to operate the drive or valve during this period, it is essential to ensure that the status displayed on the microcomputer for that drive or valve matches its actual status at that time; otherwise, operation is strictly prohibited. DCS freeze 1. Symptoms: (1) The time display inside the microcomputer stops. (2) The parameters displayed for all measurement points will no longer be refreshed. 2. Handling: (1) In the event of a system crash, it is necessary to report to the shift supervisor immediately and contact the thermal engineering team for handling. (2) Before the deadlock is resolved, the unit loses microcomputer control; therefore, enhanced monitoring using conventional instruments is necessary, operations on the microcomputer must be prohibited, and plans for potential accidents should be prepared. Normal operation mode is resumed once the fault is resolved. (3) If the faulty microcomputer cannot return to normal operation within a short time and this endangers the safety of the unit, consult the shift supervisor and use the 505E control panel to set the load to “0”, start the DC oil pump, shut down the unit, and perform electrical disconnection. Complete the other shutdown procedures as required by the regulations. Once the microcomputer returns to normal, restart the unit operation.