Problems And Countermeasures In The Double-flow Ring Sealing Oil System Of Generators

1 Introduction to dual-flow ring sealing oil system
The turbine generator sealing oil system consists of two independent but interconnected oil circuits on the air side and hydrogen side. The airside and hydrogen side sealing oils simultaneously supply oil to the double-flow annular sealing tiles at both ends of the generator. The return oil from the bearings of the turbine generator set is boosted by the air-side sealing oil pump, and passes through the air-side sealing oil cooler and filter to the air-side oil ring of the double-flow annular sealing shoe at the steam and excitation ends of the generator. The sealing oil pressure is controlled by the leakage of the differential pressure valve. When the hydrogen pressure in the generator changes or the airside sealing oil pressure fluctuates, the differential pressure valve will adjust the airside sealing oil leakage amount to maintain the airside sealing oil pressure. Greater than the hydrogen pressure in the generator 0.085Mpa. The return oil of the airside sealing oil is discharged to the oil return system of the generator support bearing. After the hydrogen side sealing oil is boosted by the hydrogen side sealing oil pump, it passes through the hydrogen side sealing oil cooler and filter, and then is divided into two paths, which pass through the generator steam and excitation end balancing valves to the hydrogen side of the generator steam and excitation sealing tiles. In the oil ring, the function of the steam and excitation balance valve is to track the pressure in the air-side oil ring in the steam and excitation end sealing tiles, and adjust the pressure in the hydrogen-side oil ring in the steam and excitation end sealing tiles so that the difference between the pressure in the air-side oil ring is not greater than ± 50mm water column, the hydrogen side sealing oil returns to the sealing oil tank. The oil level of the sealing oil tank is controlled by replenishing oil at the outlet of the airside sealing oil pump or discharging oil to the inlet of the airside sealing oil pump.

 2 Analysis of the reasons for the large amount of hydrogen replenishment and oil inflow of the dual-flow sealed oil structure turbine generator
 2.1 In actual operation, it is difficult to control the balance of air-side sealing oil and hydrogen-side sealing oil pressures.
 According to the design principle of the dual-flow sealing oil structure sealing tile, only by maintaining the pressure of the air-side sealing oil and the hydrogen-side sealing oil in the sealing tile to be basically equal and reducing the exchange of air-side and hydrogen-side sealing oil can the entrained air in the air-side oil system be prevented. Wait until it enters the hydrogen side sealing oil system. However, in actual operation, it is difficult to control the balance of air-side sealing oil and hydrogen-side sealing oil pressure due to equipment structure and other aspects.

 When the air-side sealing oil pressure is greater than the hydrogen-side sealing oil pressure, the air-side sealing oil flows toward the hydrogen side in the sealing tile, and the air entrained by the air-side sealing oil enters the hydrogen-side sealing oil. When the hydrogen-side sealing oil pressure is greater than the air-side sealing oil pressure, the hydrogen-side sealing oil will flow into the airside in the sealing tile, which will cause the hydrogen-side sealing oil tank oil level to decrease, the hydrogen-side sealing oil tank float valve will open, and the airside seal will The pressure oil at the outlet of the oil pump is replenished into the hydrogen side sealed oil tank through the float valve. Therefore, whether the air-side sealing oil pressure is greater than the hydrogen-side sealing oil pressure, or the hydrogen-side sealing oil pressure is greater than the air-side sealing oil pressure, the oil smoke, water vapor, etc. entrained by the air-side sealing oil returning from the bearing will pass through and The hydrogen-side sealing oil is exchanged and enters the hydrogen-side sealing oil system, and is then sucked into the generator through the oil gear in the sealing oil, causing hydrogen pollution in the generator, a decrease in hydrogen purity, and an increase in the amount of hydrogen replenishment.

 There are two main reasons for the imbalance in the pressure of the air-side sealing oil and the hydrogen-side sealing oil. One is the poor adjustment accuracy of the balance valve of the hydrogen-side sealing oil system. The current required accuracy of the balance valve is ±50 mm water column (±490Pa). During operation, due to the small gap between the balance valve piston and the oil cylinder, slight impurities may cause the piston to increase the movement resistance or even get stuck, causing the balance to be compromised. The valve adjustment accuracy becomes worse and cannot effectively maintain the balance of air and hydrogen side sealing oil pressure, which in turn causes hydrogen pollution and increases the amount of hydrogen replenishment.
   The second main reason causing the pressure imbalance between the air-side sealing oil and the hydrogen-side sealing oil is the measurement error of the air-side and hydrogen-side sealing oil pressures. During the operation of the unit, only by maintaining the oil pressure balance between the sealing tile and the rotating shaft can the mutual movement of the air and hydrogen side sealing oil be reduced. However, due to the structure of the equipment, currently only the air and hydrogen side sealing oil on the sealing tile can be measured. The pressure at the oil inlet is used as the adjustment signal of the balance valve, so it will inevitably cause measurement errors. The balance valve cannot effectively maintain the balance of the air and hydrogen side sealing oil pressures, thereby causing the generator's hydrogen supply to increase.

 2.2 The gap between the sealing tile and the generator rotor increases
 The oil flow flowing from the sealing shoe to the rotating shaft along the axial direction of the rotating shaft to the air side and hydrogen side is called axial flow. When the pressure difference between the air and hydrogen side sealing oils remains constant, the exchange volume of the air and hydrogen side sealing oils is different from the sealing shoe. is proportional to the gap. For a 300MW steam turbine, the sealing shoe diameter gap is 0.15-28mm. When the sealing shoe gap increases from 0.15mm to 0.28mm during operation, the sealing oil flow will increase greatly, and due to the inevitable existence between the air and hydrogen side sealing oil The increase in pressure difference and sealing oil flow will cause the exchange volume of the air and hydrogen side sealing oil to double. The air, moisture, etc. carried in the airside sealing oil enter the hydrogen side sealing oil through exchange, and then pass between the hydrogen side sealing oil and the hydrogen side sealing oil. The contact of hydrogen into the generator hydrogen will pollute the hydrogen and reduce the purity of the hydrogen. The increase in the amount of sealing oil will cause the static pressure oil return pipeline to be blocked. When the oil level in the hydrogen side oil return chamber (defoaming box) of the generator rises above the lowest position of the journal, oil will enter the generator.

 2.3 The temperature of generator sealing oil is high
 The viscosity of the sealing oil decreases as the oil temperature increases. Within the same circulation area, a certain sealing oil pressure must be maintained. When the sealing oil temperature is high, a larger flow of sealing oil is required. Similarly, the increase in sealing oil temperature will cause the sealing tile gap to increase, which also requires an increase in sealing oil flow to maintain a certain sealing oil pressure. Generator manufacturers generally stipulate that the normal temperature of the air and hydrogen side sealing oil of hydrogen-cooled generators is between 27-50°C. For the 300MW turbine-generator packaged sealing oil system, the outlet oil temperature of the oil cooler of the air and hydrogen side sealing oil systems is controlled by a water withdrawal regulating valve, which is generally maintained at around 42°C. The viscosity of the oil at 42°C is smaller than that at 27°C. To maintain a certain sealing oil pressure, a larger sealing oil flow rate is required. Similarly, due to the increase in sealing oil temperature, the inner diameter of the sealing tile will increase. In order to ensure that the hydrogen in the generator does not leak out, it is also necessary to increase the sealing oil flow to maintain a certain pressure. Therefore, if the sealing oil temperature is too high, the sealing oil flow will increase. According to the analysis in 2.2, it will also cause the purity of hydrogen in the generator to decrease or the generator to enter oil.

 2.4 The output of the smoke exhaust fan is small
   Looking at the sealing oil system of the 300MW turbine generator, the oil source of the airside sealing oil pump is taken from the hydrogen oil separator. The main function of the exhaust gas of the hydrogen oil separator is to extract trace amounts of hydrogen in the airside oil to prevent hydrogen from returning with the lubricating oil. to the main tank. Increase the output of the exhaust fan of the hydrogen-oil separator to form a large negative pressure in the hydrogen-oil separator, so that the air in the airside oil will be extracted together with the hydrogen. In this way, the air content in the airside sealing oil will be reduced. According to 2.1 According to the analysis of the article (generator hydrogen pollution is mainly due to the air carried by the air-side sealing oil entering the hydrogen-side sealing oil through exchange with the hydrogen-side sealing oil, and then polluting the hydrogen through the exchange of hydrogen-side sealing oil and hydrogen), hydrogen pollution will be reduced.

 3 Measures to prevent oil from entering the generator, reduce hydrogen pollution, and reduce the amount of hydrogen replenishment
 3.1 Ensure the appropriate gap between the sealing shoe and the rotating shaft
 3.1.1 Ensure that the sealing tile clearance meets the requirements during maintenance
 For 300MW steam turbines, the diameter gap between the sealing tile and the rotating shaft is required to be 0.105-205mm. During maintenance, the gap between the sealing tiles should be strictly in accordance with the standards to ensure that it meets the requirements and should be as close as possible to the lower limit. This can not only reduce the flow of sealing oil, but also prevent excessive sealing tile clearance. Small size may cause defects such as high sealing tile temperature, sealing tile wear and even excessive vibration of the generator shaft.
 3.1.2 Use high-precision sealing oil filter
 At present, the air-hydrogen side sealing oil of the 300MW sealing oil system uses a scraper-type filter. However, in fact, this scraper-type filter can only be regarded as a coarse filter and cannot effectively filter out the tiny particles in the sealing oil. It is precisely due to the grinding caused by the relative flow between the tiny particles in the sealing oil flow and the sealing pad and journal that aggravates the wear of the sealing pad and journal, resulting in an increase in the gap between the running sealing pads. It is reported that scraper filters have been eliminated abroad, and there are examples of domestic power plants replacing scraper filters with fiber filters with a filtration accuracy of 0.01mm or less.

 3.2 Improve the adjustment accuracy and operational reliability of the balance valve
   Improving the adjustment accuracy of the balance valve can effectively reduce the amount of movement of the air and hydrogen side sealing oil and prevent hydrogen pollution. This can be done from the following two aspects:
 3.2.1 Prevent the balance valve from jamming and adjustment failure
 3.2.1.1 In the early stage of operation of the sealing oil system after maintenance, the balance valve bypass valve can be manually adjusted to prevent the balance valve components from becoming stuck due to uncleanness of the system after maintenance.
 3.2.1.2 Use new balancing valve
 It is reported that a domestic unit has successfully developed a balancing valve with a continuously rotating valve core. This balancing valve uses sealing oil as the power oil to push the valve core to rotate at a certain speed, which can prevent impurities in the sealing oil from causing the valve core to jam.
 3.2.2 After maintenance, perform a balance valve adjustment test to ensure the pressure balance of the air and hydrogen side sealing oils.
 The purpose of the balance valve is to control the air in the sealing tile, the air in the hydrogen side sealing oil ring, and the hydrogen sealing oil without exchanging. Based on this principle, the sealing oil tank fill and drain valves can be closed, and the balance valve can be observed and adjusted according to the oil level changes in the sealing oil tank. Make adjustments to finally make the oil level in the sealed oil tank basically stable, thereby reducing the exchange of air and hydrogen side sealing oil in the sealing tile. The rules can be found through experiments. During normal operation of the unit, fine-tuning the balance valve according to whether the sealed oil tank is replenishing or draining oil can also reduce the exchange of air and hydrogen side sealing oil in the sealing tile.

 3.3 Control the temperature of sealing oil
 Tests can be carried out where the sealing oil temperature changes between the upper and lower limits within the standard requirement range. When the vibration of the generator shaft does not increase, try to keep the sealing oil temperature operating at the lower limit of the standard, thereby reducing the flow of sealing oil and reducing power generation. The purpose is to feed oil into the machine and reduce hydrogen pollution.

 3.4 Increase the wind pressure of the smoke exhaust fan
 Increasing the wind pressure of the exhaust fan of the hydrogen-oil separator can increase the negative pressure of the hydrogen-oil separator and reduce the air and water content in the air-side sealing oil, thus reducing the pollution of hydrogen caused by the exchange of air-side and hydrogen-side sealing oils.
 4 Conclusion 
 In summary, in view of the hydrogen pollution and oil inlet defects of the generator, taking corresponding preventive measures according to the characteristics of the double-flow ring sealing oil system can effectively reduce the hydrogen pollution and oil inlet defects of the generator and improve the operation of the generator. safety and reliability.