Turbine vacuum level

The exhaust steam from the turbine enters the steam side of the condenser. A large volume of circulating water is directed to the inside of the condenser’s copper tubes. Heat exchange between the circulating water and the exhaust steam causes the steam to condense into water; its specific volume decreases drastically—by about 30,000 times. As a result, a vacuum is created in the space previously occupied by the steam. Non-condensable gases are then removed via a vacuum pump, thereby maintaining the vacuum state. There are many reasons that can cause a drop in the condenser vacuum. Here, we introduce the common causes of such drops encountered in actual production operations, along with the corresponding solutions. I. Reasons for the decrease in condenser vacuum during the startup of a steam turbine unit: Abnormal shaft seal pressure of the turbine. (1) Reason: During unit startup, if the steam supply pressure for the shaft seals is abnormal, the condenser vacuum level will gradually decline. When the shaft seal pressure is low, the front and rear shaft seals of the high- and low-pressure cylinders of the turbine will allow air to flow back into the cylinders due to insufficient pressure, which raises the temperature of the turbine’s exhaust cylinder and leads to a decrease in condenser vacuum. The cause of low shaft seal pressure may be a malfunction of the shaft seal pressure control valve ； The valve on the shaft seal steam supply system is not open or is not opened far enough. (2) Phenomenon: The reading of the vacuum gauge decreases, while the reading of the temperature in the turbine’s exhaust cylinder increases. (3) Handling: When it is confirmed that the slow decline in the condenser vacuum is caused by insufficient steam pressure for the shaft seal, the operator must immediately check whether the shaft seal pressure and the steam supply are normal. Under normal circumstances, it is sufficient to adjust the shaft seal pressure to its normal value. If the pressure of the steam supply for the shaft seal is insufficient, the steam source should be switched immediately to ensure that the shaft seal pressure remains within the normal range; if this does not work, further inspections should be carried out. 2. Rise in the water level in the hot water well of the condenser: (1) Reason: When the water level in the hot water well of the condenser is too high, it may submerge the copper tubes of the condenser or its exhaust ports, thereby altering the internal operating conditions of the condenser; as a result, the heat exchange efficiency decreases, and the vacuum level will slowly drop. The reason for the increase in the water level in the hot water well of the condenser could be a) a failure of the condensate pump ； b Excessive make-up water volume for demineralized water ； c. Condenser copper tube leakage ； d. Poor drainage of water released during condensate startup ； e. It is caused by insufficient opening of the valves in the condensate water system. (2) Phenomenon: The vacuum gauge reading decreases; the temperature of the steam turbine’s exhaust casing rises; meanwhile, the water level indicated by the condenser level gauge and the local level gauge also rises. (3) Handling: When it is confirmed that a gradual decrease in the condenser vacuum is caused by a rise in the water level in the hot well of the condenser, the operator must immediately determine what exactly is causing the condenser water level to rise, and promptly take measures to bring the water level back to its normal value. 3. Insufficient circulating water volume in the condenser: (1) Cause: When the circulating water volume is insufficient, the amount of steam generated by the turbine that can be cooled in the condenser decreases, which in turn leads to an increase in the temperature of the exhaust cylinder and a drop in the vacuum level of the condenser. The reason for this insufficient circulating water volume could be a malfunction in the circulating water pump ； The low water level in the circulating water inlet chamber causes vaporization of the circulating water pump, resulting in an insufficient amount of circulating water ； The electric inlet and outlet valves on both sides of the unit’s condenser did not open fully ； When circulating water is passed through the condenser, the air in the system has not been completely removed. (2) Phenomena: The reading on the vacuum gauge decreases; the indicated temperature of the turbine’s exhaust casing rises; the inlet and outlet pressures of the condenser’s circulating water fluctuate; and the inlet and outlet water temperatures of the condenser’s circulating water change (the inlet temperature remains normal, while the outlet temperature increases). (3) Handling: When it is confirmed that the gradual decline in the condenser vacuum is caused by insufficient circulating water volume in the condenser, the duty operator should promptly check whether the circulation pump is operating properly and whether the water level in the water intake area is normal. Rapidly go to the site to check whether the inlet and outlet electric valves on both sides of the unit’s condenser are fully open, and whether there are any fluctuations in the pressures at those inlets and outlets (if there are fluctuations, carry out air removal operations until water is discharged from the air pipes). 4. Valves within the negative pressure area are inadvertently opened (or closed). (1) Cause: During the unit startup process, there is a high volume of operations performed by personnel; it is inevitable that some operations may be omitted or performed incorrectly during this period. This is the main reason for such a decrease in vacuum levels. (2) Phenomenon: A drop in vacuum and an increase in the temperature of the turbine’s exhaust cylinder occur just after the crew has completed the operations related to the vacuum system. (3) Handling: When it is confirmed that the slow decline in the condenser vacuum is caused by an accidental opening (or closing) of a valve located in a negative pressure area, the duty personnel should promptly reverse the operation that was just performed. 5. The shaft seal heater is either full of water or has no water. (1) Causes: During unit startup, improper adjustments or issues inherent to the shaft seal system can cause the shaft seal heater to become full of water or to have no water at all. This will result in a decrease in the condenser vacuum. The possible cause for this situation is a leak in the copper tubes of the shaft seal heater ； The drain valve from the shaft seal heater to the condenser hot well is not fully open, or the drain valve is malfunctioning ； Insufficient opening of the steam inlet and outlet valves on the steam side of the shaft seal heater, or accidental opening of the bypass valve, results in a reduced amount of drain water, leaving the shaft seal heater without water. (2) Phenomenon: The reading on the vacuum gauge decreases; there is no negative pressure applied to the shaft. The temperature reading of the turbine’s exhaust cylinder rises. If the shaft seal heater is filled with water, large amounts of white steam will emerge from the front and rear shaft seals of the high- and low-pressure cylinders of the turbine. At this time, the shaft seal pressure increases. In severe cases, water accumulates in the return pipe of the shaft seal heater, altering its operating conditions and resulting in a decrease in vacuum ； If the shaft seal heater is devoid of water, a large amount of steam used for the shaft seal is discharged directly into the condenser without undergoing heat exchange in the shaft seal heater, increasing the thermal load on the condenser and resulting in a decrease in vacuum. (3) Handling: When it is confirmed that the slow decline in the condenser vacuum is caused by either an overfilled or underfilled shaft seal heater, promptly inform the duty officer to check whether the water level in the shaft seal heater is normal. If it is overfilled, open the drain valve of the shaft seal heater; at the same time, check whether the water injection valve is closed, and open the drain valve on the return steam pipe to remove any accumulated water, adjusting the water level in the shaft seal heater to 1/2. If the shaft seal heater is dry, fill it with water first, adjusting the water level in the shaft seal heater to 1/2. During the startup of a steam turbine unit, these are the main reasons for the slow drop in condenser vacuum that is often encountered. Of course, this is not absolute, but the following principle should be followed: when the vacuum of the condenser decreases slowly, the operator should make a comprehensive assessment based on relevant instruments, indicators, and operating conditions, and then take appropriate action. II. During the normal operation of the steam turbine unit, the reasons for the gradual decrease in the condenser vacuum are as follows: 1. An increase in the water temperature of the water tank, leading to abnormal operation of the extractor. (1) Reason: During the operation of the steam turbine unit, changes in season or other factors can cause the water temperature in the water tank to rise; vaporization may occur at the nozzle of the extractor, resulting in abnormal extraction performance. As a result, the gases that cannot be condensed in the condenser are not removed in time, which leads to a decrease in vacuum. The reason for the rise in water temperature in the shooting pool may be the influence of summer ambient temperatures ； A heat source within the thermal system is discharged into the water tank, raising the water temperature. (2) Phenomenon: The vacuum level of the condenser decreases compared to a certain period, or there is a difference in the vacuum levels between morning and evening. If the water temperature in the water jet tank is high when measured with a thermometer or felt by hand, the temperature of the drain pipe of the water jet pump will also be high. (3) Treatment: When it is confirmed that the gradual decrease in the condenser vacuum is caused by an increase in the water temperature in the spray tank, the make-up water valve on the inlet pipe of the spray pump should be opened appropriately to replace the water in the spray tank and lower its temperature. If necessary, simply check whether the overflow pipe of the water shooting pool is unobstructed. 2. Severe water accumulation in the steam return pipe of the shaft seal heater. (1) Cause: When water accumulates in the steam return pipe of the shaft seal heater, the flow area for steam return is reduced. This causes the shaft seal steam supply system to malfunction, resulting in a drop in vacuum. The cause of water accumulation in the return steam pipe of the shaft seal heater may be an increase in the water level of the shaft seal heater ； The water injection valve was forgotten to be closed ； Water is present in the shaft seal steam main. (2) Phenomenon: When water accumulates in the return steam pipe, the outer wall temperature of the exhaust pipe of the shaft seal heater is low. In severe cases, large amounts of white steam will emerge from the shaft seals at the front and rear of the high-pressure and low-pressure cylinders, and water will leak from the casing of the shaft seal heater fan. (3) When it is confirmed that the gradual decrease in the condenser vacuum is caused by water accumulation in the steam return pipe of the seal heater, the plant personnel must promptly fully open the drain valve on the main steam return pipe of the seal heater to drain the water until all water has been removed, and then adjust the water level in the seal heater accordingly. 3. Rise in condensate level (1) Cause: During normal operation, the increase in the condenser level of the unit may be due to an excessive amount of make-up water for desalination ； Condenser copper tube leakage ； The electric valve for condensate recirculation opens accidentally or does not close properly ； Excessively high outlet pressure of the low-pressure heater drain pump and excessively high deaerator pressure (displacing condensate). (2) Phenomenon: The indicator on the condenser level gauge rises, and the current of the operating condensate pump increases. The subcooling of the condensate water increases. (3) Handling: When it is confirmed that the gradual decline in the condenser vacuum is caused by an increase in the condenser water level, the operator should promptly identify the cause of this increase and reduce the condenser water level to its normal value. 4. Mistakes made by operators or maintenance personnel during their work result in a gradual decrease in the condenser vacuum. (1) Causes: Due to mistakes made by operators or maintenance personnel during their work, the condenser vacuum decreases gradually or abruptly. The reasons for this may include operators inadvertently opening or closing valves related to the vacuum system during normal operations, or other factors ； While performing maintenance work related to the vacuum system, maintenance personnel inadvertently opened or closed valves without authorization. (2) Phenomenon: When similar situations occur, the rate of decline in the reading on the condenser vacuum gauge can exhibit two patterns: ① The condenser vacuum decreases slowly, while the temperature of the turbine’s exhaust cylinder rises ； ②When the condenser vacuum drops sharply, the temperature of the turbine’s exhaust cylinder rises rapidly, and the sound emitted by the unit changes suddenly ； If the valve of the circulating water system is closed by mistake, the circulating water pressure in the condenser of the unit will change. (3) Handling: When it is confirmed that a mistake by operating or maintenance personnel has caused a slow or rapid drop in the condenser vacuum, the duty personnel must calmly and promptly revert all operations performed prior to the incident. If it is determined that the maintenance work was carried out while maintenance personnel were present, promptly go to the site and close the valves that were accidentally opened or closed by those personnel. 5. When safety measures related to the vacuum system are being implemented, the vacuum level in the condenser decreases slowly. (1) Reason: During the implementation of such safety measures, if the valves in the vacuum system do not close properly, the vacuum level in the condenser will decline slowly. This may be caused by air from devices or valves located in the negative pressure area being drawn into the condenser, thereby causing a gradual decrease in vacuum level. (2) Phenomenon: The condenser vacuum decreases slowly, and the temperature of the turbine’s exhaust casing rises. (3) Handling: Once it is confirmed that the gradual drop in condenser vacuum is caused by safety measures, the operator should promptly restore all of those safety measures. 6. No water on the steam side of the low-pressure heater during unit operation (1) Cause: During normal operation of the unit, due to human negligence or failure to adjust the water level in the low-pressure heater in a timely manner when operating conditions change, the low-pressure heater operates without any water. As a result, the extracted steam is discharged directly into the hot water tank of the condenser without undergoing heat exchange, which increases the thermal load on the condenser and leads to a decrease in vacuum. (2) Phenomenon: The vacuum of the condenser decreases slowly, and the temperature of the turbine’s exhaust cylinder rises. On-site inspection reveals that the water level gauge of the low-pressure heater in operation shows no water level indication, and the terminal difference increases. (3) Handling: Once it is confirmed that the gradual drop in the condenser vacuum is caused by a lack of water in the low-pressure heater of the operating unit, the operator only needs to adjust the low-pressure heater so that a water level indication is available. III. In the handling of turbine unit accidents, the reasons for the gradual decrease in the condenser vacuum are as follows: 1. Low shaft seal pressure. (1) Reason: When an accident occurs in the unit, various factors can lead to a decrease in shaft seal pressure. For example, when operating as a single unit or with two units, improper handling during an accident can cause a drop in shaft seal pressure, resulting in a gradual decrease in the condenser vacuum. (2) Phenomenon: The vacuum of the condenser decreases slowly, the temperature of the turbine’s exhaust cylinder rises, and the readings of the instruments related to the shaft seal pressure drop. (3) Handling: Handle it according to the following situations: ① In the event of an accident during single-unit operation, if the shaft seal pressure drops and the condenser vacuum decreases slowly, the shaft seal steam supply should be switched to the cold section to maintain normal shaft seal pressure. ②When both units are in operation, should an accident occur to one unit, it is necessary to determine whether to switch the shaft sealing steam source to the auxiliary steam supply based on whether the pressure in the deaerator is high or low, so as to maintain normal shaft sealing pressure. 2. The hot water well of the condenser is filled with water. (1) Cause: Under abnormal conditions, factors related to equipment or personnel can lead to the hot water well of the condenser becoming filled with water, and the reason for this filling may be the tripping of the condenser pump ； After the condensate pump tripped, the non-secure operation of the check valve led to water filling up in the hot water well within the condensate system ； The makeup water volume to the deaerator is excessive. (2) Phenomena: The condenser vacuum decreases slowly; the temperature of the turbine exhaust casing rises; the reading on the condenser water level gauge increases. (3) Handling: When it is confirmed that the slow decline in the condenser vacuum is caused by a full water level in the condenser hot water well, the operator should promptly find a way to reduce the water level in that well to its normal level. 3. Delayed adjustment of the manual valve for steam leakage from the high-pressure shaft seal in the sixth extraction stage. (1) Reason: When an accident occurs in the unit, changes in the main steam flow lead to changes in the amount of steam leaking from the shaft seal, which causes air to be drawn into the condenser at the high-pressure shaft seal area, resulting in a decrease in vacuum. (2) Phenomenon: The condenser vacuum drops slowly, and the temperature of the turbine exhaust cylinder rises. (3) Handling: When it is confirmed that the slow decline in the condenser vacuum is caused by steam leakage from the high-pressure shaft seal, resulting from untimely adjustment of the manual valve for steam discharge to stage 6, the on-duty personnel should promptly go to the site and appropriately reduce the opening of the manual valve for steam leakage from the high-pressure shaft seal to stage 6. 4. Faults in the demineralized water system, or maintenance work on the demineralized water makeup pipelines and valves, can also cause a gradual decrease in the condenser vacuum. During normal operation, there have been instances where faults in the demineralized water system led to such a gradual decline in the condenser vacuum. (1) Reason: In most cases, this is due to the desalination water pump tripping ； Incorrect shutdown (or failure) of valves in the demineralized water system ； It is caused by maintenance work. This is caused by air being drawn into the condenser, provided that the inlet and outlet control valves for the deaerator’s demineralized water supply, as well as the other control valves, are in the open position. (2) Phenomenon: The condenser vacuum drops slowly or rapidly, and the temperature of the turbine exhaust cylinder rises. (3) Handling: When it is confirmed to be a fault in the demineralized water system, or when maintenance work is being carried out on the demineralized water supply pipeline, the shift supervisor should be informed immediately so that they can go to the site to check things out; if necessary, the relevant valves can be closed. The above outlines the common causes, symptoms, and treatment methods for the gradual decline in condenser vacuum under various operating conditions of the unit. Of course, these are not absolute causes, phenomena, or solutions; therefore, it is necessary for us all to continuously summarize and improve our knowledge and skills in the course of our work. IV. Measures to improve the condenser vacuum: 1. Strengthen the operation and maintenance of the circulating water cooling tower; when damage to the packing or water distribution pipes is detected, contact maintenance personnel promptly to replace and repair them. By promptly replacing the damaged filler in the water tower and repairing any broken water supply pipes during minor repairs, the efficiency reduction caused by excessive water spray in certain areas is eliminated, thereby improving the efficiency of the cooling tower. 2. Properly schedule the operation of the circulation water pumps, adjusting the number of pumps in use according to climate changes (two pumps must be in operation during winter) to ensure a proper flow rate of circulating water. 3. Reduce the condenser end difference, ensure the efficiency of the rubber ball cleaning system, and improve the condenser vacuum. 4. Improve the tightness of the turbine vacuum system. If excessive air leaks into this system, it will create an air film on the surface of the copper tubes, reducing the heat transfer coefficient of those tubes. Additionally, this can cause the vacuum pump to operate under overload, thereby affecting the vacuum level in the condenser. 5. Ensure the quality of the circulating water to prevent scaling in the condenser tube bundles. Adjust the recirculating water concentration ratio in a timely manner based on chemical monitoring data. 6. Improve the operational efficiency of the vacuum pump, preventing uncondensable gases from failing to be discharged in time and thus affecting the vacuum level of the unit.

Thank you very much. We are using a steam-driven pump; is it possible that its internal structure could affect the pumping efficiency?

Check whether the water seal in the pipeline is damaged, whether the vent valve for steam seal leakage is properly closed, and whether the pressure of the power steam is normal

The water seal is good. What do you mean by the air leakage vent valve? I’m a bit confused; please advise.

For the air-cooled type: 1. The more obvious factors include air cooling with variable frequency, rotational speed, the amount of steam supplied to the turbine, weather conditions, and the cleanliness of the air-cooling fins; usually, the condition of these fins has a significant impact. 2. Less obvious factors are leaks, particularly issues related to the water seal safety valve; it’s also important to check whether the vacuum-breaking valve in the hot well liquid collection tank is properly sealed when starting up, as well as the vacuum-breaking lines of the water pumps. 3. Actual leaks are not common. Based on experience, the most effective solution here is to clean the air-cooling fins, and this can be done without using chemical cleaners – just a high-pressure water gun is sufficient

Steam ejectors generally do not leak; in essence, they are heat exchangers. Problems that can occur include blockage of the secondary vent (caused by freezing), failures of the steam traps, and delayed drainage, which can lead to water hammer and affect the efficiency of vapor extraction