Solution of steam flow induced vibration of the ho

2022-10-24
  • Detail

Solution to the problem of steam flow induced vibration of 800MW supercritical steam turbine

1 Characteristics of shaft vibration of supercritical units

improving the initial parameters of steam turbine according to Rankine cycle principle can relatively improve the thermal efficiency of units. In recent years, with the increase of unit capacity, in order to save energy and make full use of national energy resources, China's large thermal power units have gradually developed to supercritical pressure

the shaft system of large capacity supercritical steam turbine generator becomes longer and the number of supporting bearing pads increases. For example, Suizhong 800MW supercritical steam turbine generator is a single shaft five cylinder generator with a total rotor length of 59.5m. In this way, the shafting vibration of large units becomes more complex and has certain characteristics

1.1 low frequency vibration: the so-called low-frequency vibration is that the vibration frequency is lower than the shaft rotation frequency. For supercritical steam turbine generator units, low-frequency vibration caused by steam vortex force is easy to occur, which can also be called steam self-excited vibration, also called steam flow excited vibration. It mainly comes from the steam whirl force generated in the steam seal and rim shroud area. This force is always perpendicular to the offset direction of the rotor axis, driving the rotor to produce half speed whirl

1.2 oil film vibration: due to the long rotor of large capacity units, the critical speed is low, especially the critical speed of generator rotor is lower. The following table shows the critical speeds (RPM) of each stage of suidian 800MW steam turbine generator unit:

generator rotor medium pressure rotor, № 1, 3 low-pressure rotor high-pressure rotor, № 3 low-pressure rotor № 2, 3 low-pressure rotor generator rotor medium pressure rotor, № 2 low-pressure rotor

748 1700 1853 1910 2282 4326

it can be seen from the above table that the normal working speed of the rotor (3000rpm) exceeds the first-order critical speed of the generator by three times, This creates basic conditions for oil film oscillation. Oil film oscillation is different from steam flow oscillation. It occurs when the unit is started, and its speed is the speed increased at that time, and the vibration frequency is about half of this speed. The characteristic of oil film oscillation is that it intensifies with the increase of rotating speed

1.3 friction vibration: due to the long rotor of large units, it is easy to have thermal residual deformation. Improper unit startup may cause steam seal friction and rotor vibration

2. Characteristics, causes and elimination measures of steam flow excitation

2.1 characteristics of steam flow excitation

2.1.1 when the load of the unit increases to a certain value, the steam self-excited oscillation will occur. If no measures are taken, the vibration will disappear only when the load drops to this value

2.1.2 it usually occurs on high-pressure rotors with large capacity, and the steam flow excitation at the regulating stage of high-pressure cylinder is the most serious

2.1.3 the vibration frequency is lower than the rotor working frequency. Vibration frequency f=nc/60 (NC is the first critical speed of rotor)

2.2 causes of steam flow excitation

2.2.1 due to the large capacity of the unit and the long rotor, the rotor deflection is also large, the rigidity is relatively poor, and the natural vibration frequency of the rotor is low. When the excitation frequency of steam on the rotor is close, it will cause vibration

2.2.2 supercritical steam turbine has high inlet steam pressure and high steam density. Because the energy provided by the steam vortex interference force depends on the excitation point on the trajectory and the density of steam. Therefore, the greater the steam density, the easier it is to produce steam flow excitation

2.2.3 the rotor is poorly installed, and there is deviation in the center of the dynamic and static parts. In some parts of the whole circumference, some steam passes through the larger gap between the rotor's rotor blade ring and the steam choke ring, and some steam passes through the smaller gap parts. In this way, due to the imbalance of the pressure of the whole circumference, the possibility of steam exciting the rotor is increased

2.3 measures to eliminate steam flow excitation

2.3.1 improve the type of internal sealing device of steam turbine, reduce the inlet spacing, control the leakage at the top of moving blade, and reduce the excitation force of steam on rotor

2.3.2 adjust the cylinder and rotor center to avoid obvious deviation between the rotor and cylinder center during operation

2.3.3 increase the axial clearance between the rotor and the diaphragm. With the increase of axial clearance between nozzle, stationary blade and rotating blade, the exciting force of steam flow whirl can be significantly reduced, but the internal efficiency of steam turbine will be reduced

2.3.4 change the opening sequence of the governing valve of the high-pressure cylinder, so as to avoid the obvious radial offset of the rotor under the action of unilateral steam force and the unbalanced torque on the rotor

2.3.5 increase the bearing pad damping and reduce the bearing load. For example, reduce the bearing clearance, increase the bearing length, and use lubricating oil with high viscosity

2.3.6 adopt bearing shells with good stability, and add squeeze film dampers, etc

2.3.7 increase the critical speed of the rotor

2.3.8 turn the unit for a long time before starting to reduce the rotor deflection

2.3.9 the bearing adopts multi pad tilting pad

2.3.10 the new structure of steam seal at the top of moving blade can solve the problem of low-frequency vibration of unit shafting

3. The first phase project of Suizhong Power Generation Co., Ltd. is two 800MW supercritical coal-fired units supplied by Russia, which is the thermal power unit with the largest single unit capacity in China at present. The steam turbine generator is produced by Leningrad metal factory, single shaft, one intermediate reheat, five cylinder, six exhaust, condensing type. The high-pressure cylinder is a double-layer reflux structure. The high-pressure steam flows from the middle through the regulating stage to the five pressure stages in front of the machine, and then turns 180 ° to enter the six pressure stages on the exhaust side through the interlayer. The shafting is composed of one high-pressure rotor, one medium pressure rotor, three low-pressure rotors, generator rotor and exciter rotor. Each rotor has two support bearings. The rear bearing of the high-pressure cylinder is a combined thrust bearing. The front and rear support bearings of the high-pressure cylinder are tilting pad bearings composed of six sector blocks, and the rest are elliptical boring bearings. Before the installation of the unit, the shaft diameters of the three low-pressure rotors were planed in Shanghai steam turbine factory, and high and low speed dynamic balancing was done

high and intermediate pressure cylinders are respectively equipped with 2 main throttle valves and 4 speed regulating valves. High and medium pressure main throttle valves and high-pressure speed regulating throttle valves are controlled by their respective servo motors. The four speed regulating valves of the intermediate pressure cylinder are controlled by two servo motors. The normal opening sequence of the governing valve of the high-pressure cylinder is #1 and 2 are opened at the same time, #3 and 4 are opened in turn# 3. 4 high pressure regulating steam valve is equipped with a reset device. After putting #3 and 4 high pressure regulating valve reset mechanism into operation before 400MW in cold state, four high pressure regulating steam valves can be opened at the same time. After the unit load is greater than 400MW, exit #3 and 4 high pressure regulating valve reset mechanism. With the increase of load, the high pressure regulating valve opens according to the normal characteristic curve

the following figure shows the layout of the front bearing (#1 bearing) of the high-pressure cylinder and the high-pressure governing valve (viewed from the machine head to the generator side)

3.2 unit vibration characteristics

on May 9, 2000, when 1 unit load was 650 ~ 700MW, the vibration of 1 bearing of the main engine had a sudden phenomenon, which immediately affected the adjacent bearings. The main component of the sudden increase of vibration is 21 ~ 22Hz, and the amplitude is from 15% of the normal value μ Increase to 50 instantly below M μ M above

on May 16th, 2000, according to the suggestions of Russian experts, the main engine #1 bearing was disassembled and inspected. After disassembly, it was found that the upper and lower oil blocking edges of #1 bearing were worn to varying degrees. At the same time, it was found that there was a gap of 0.15 ~ 0.20mm between the back arc of #2 and #6 pads and the bearing pad recess. During maintenance, the oil blocking edge was turned to ensure that the oil blocking edge was 0.10 ~ 0.15mm lower than the surface of the tilting pad. Repair welding was carried out on the back arc of the pads to ensure that there was no gap between the back arc of #2 and #6 pads and the bearing pad recess. After this treatment, the vibration of unit restart has not been eliminated

on June 12, 2000, when unit 1 was started again and the load was increased to about 700MW, the sudden vibration of the main engine 1 bearing reappeared. With the increase of load, the vertical and horizontal vibration reached the tripping value (11.2mm/s), and affected other bearings at the same time. After reducing the load to below 680mw, the vibration will return to the original state

3.3 analysis of vibration causes

from the analysis of vibration mechanism and characteristics, the sudden vibration occurred in suidian unit 1 is a typical steam flow excitation, which belongs to the category of self-excited vibration, and conforms to the following characteristics:

3.3.1 the vibration amplitude is related to the unit load, and has good reproducibility

3.3.2 sudden vibration occurs in the front bearing of high-pressure rotor, and the force causing rotor whirl is closely related to steam flow

3.3.3 the measured vibration frequency during sudden vibration is close to the vibration frequency during steam flow excitation, f=nc/60=1853/60=30.9hz (NC is the first critical speed of high-pressure cylinder)

3.3.4 when vibration occurs, the waveform is sine wave

3.4 preparations before operation

according to the actual operation situation, the relevant technicians of Northeast Electric Power Research Institute and suidian company have decided after many studies that they can only adopt the scheme of changing the opening sequence and opening of the speed regulating valve of the high-pressure cylinder to eliminate the steam flow excitation

3.4.1 disclose to the operators according to the established safety measures, technical measures and organizational measures, and strictly implement them in the implementation process

3.4.2 the boiler keeps 7 Coal Mills running, with unit load of 680 ~ 700MW, and the boiler side is ready for rapid load reduction

3.4.3 remove the boiler ВЗ Low front pressure protection, reduce the main steam pressure of the boiler side outlet crossover to 23.5mpa, and then put the front pressure regulator into operation

3.4.4 check and confirm that the protection of turbine shaft string and bearing vibration is put into operation

3.4.5 the lubricating oil temperature of steam turbine shall be maintained at 40 ~ 42 ℃

3.4.6 unit state before operation: unit load 700MW, lubricating oil temperature 40.6 ℃

number of bearing pads 1 2 3 4 5 6 7 8 9 10 11 13 14

bearing pad oil return temperature ℃ 52 48 50 53 54 63 54 57 56 62 63 58 50 52

oil film pressure MPa 1.02 2.02 1.89 2.50 2.68 2.53 2.24 2.66 2.41 1.71 2.86 3.30

vertical vibration mm/s 3.2 3.6 3.5 1.6 3.5 2 4 4.9 5.0 3.9 4.5 2 2 2.7 4.1 <./p>

horizontal vibration mm/s 4.9 2.9 3.2 2 1.4 2.3 1.6 1.2 3.0 1.9 3.5 1.8 2.4 4 4.2

front of high-pressure cylinder (#1 pad) The bearing pads are 6-pad tilting pads, and the metal temperature is ℃ (#1 ~ 6 pads). The innovative adhesive products and solutions developed to meet the future needs of the market are 56.8, 59.0, 63.3, 53.2, 65.0 and 57.6 respectively. The opening of HP cylinder #1 ~ 4 regulating valve is 73mm, 73mm, 15mm, 4mm

3.5 safety measures:

3.5.1 during the load raising process of the unit, when the vibration reaches the protection action value and the protection does not act, it is necessary to immediately stop the machine with loose and scattered sight

3.5.2 at 1 Watt, the master control shall closely monitor the vibration of the unit, and each monitoring point shall ensure good communication

3.5.3 during the loading process of the unit, pay close attention to the changes of the parameters of the unit. If there is any abnormality, stop loading immediately, and quickly reduce the load until the abnormal vibration is eliminated

3.6 specific operation steps

3.6.1 cut off the power of 3 and 4 regulating valve reset motor of steam turbine, manually input 4 high pressure regulating valve reset device locally, and manually control and open it for 6~10mm

3.6.2 slowly turn down the #2 high-pressure regulating valve locally, so that the #4 high-pressure regulating valve can be opened for 15~20mm

3.6.3 slowly increase the load of the unit and pay attention to the vibration of the unit. In case of sudden vibration, immediately remove the pressure regulator in front of the unit and manually reduce the load quickly

3.6.4 put the pressure regulator in front of the unit into automatic operation and maintain the main steam pressure of the boiler side crossover pipe at 23.5 MPa. At the same time, the boiler side slowly increases the combustion rate from 700MW to increase the unit load, and pay close attention to the vibration of each bearing

3.7 results of steam flow excitation treatment

at 15:20 on June 19, 2000, the load of #1 unit increased from 700MW to 800MW, sharing 38min. In this process, the vibration value of #1 bearing gradually decreased, the vibration of other bearing shells of the unit also decreased to varying degrees, and the metal temperature of each bearing block of #1 bearing shell of the main engine also decreased to varying degrees, and gradually became consistent. The following is the unit

Copyright © 2011 JIN SHI