Analysis and treatment of abnormal vibration of th

2022-08-24
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Analysis and treatment measures of abnormal vibration of steam turbine

in a large synthetic ammonia plant, promote local research and introduction of tax, credit and other supporting policies for express delivery and green packaging of e-commerce enterprises. The driving machine of synthetic gas compressor adopts medium pressure condensing steam turbine designed and produced by nuovo Pignone, Italy. From the beginning of 2002 to August, the vibration of the steam turbine gradually increased. Under the same working conditions, the vibration at the front bearing slowly increased from 25 m to nearly 70 M. especially in the process of startup and shutdown, the shaft vibration at the front bearing reached more than 125 M. in addition, the vibration of the bearing housing and field noise also increased significantly, seriously affecting the safe, stable and high-load operation of the unit itself and the whole synthetic system

1 introduction to steam turbine support system

the steam turbine model of synthetic gas compressor unit is enk32/45. The cylinder is supported on the rear frame by cat claws on both sides and support plates on both sides of the rear end. A horizontal pin is set at the support plates on both sides of the rear end of the cylinder body, and a longitudinal pin is set at the middle part below the front and rear ends of the cylinder body, so that the positioning dead center of the whole cylinder body is at the rear end. When starting and stopping, the rear end of the cylinder body is the positioning end, which expands or contracts forward

the turbine rotor is supported on the front and rear radial bearings, which are installed in the front and rear bearing seats respectively, and thrust bearings are also installed in the front bearing seats. The front bearing seat of the steam turbine is installed on the front frame. The four corners are supported by four spherical washers, and the height of the spherical washer can be adjusted by the lower adjusting nut. Both sides of the bearing seat are pressed with two fixing bolts, and the fixing bolts are taken with anti loosening measures. Both sides of the front bearing seat are connected with the cylinder block through two cantilever rods extending from the front end of the cylinder. When the cylinder block expands forward, the cantilever screw also pushes the front bearing seat forward on the spherical washer. Therefore, during cold installation, the fixing bolts of the bearing seat are not crushed, and an expansion gap a of 0.02 ~ 0.03mm is reserved between its head and the pressing plate

in order to ensure the left and right positioning of the bearing seat, a longitudinal guide pin is also set under the bearing seat

2 vibration cause finding and analysis

is to find the cause of abnormal vibration of steam turbine of synthetic gas compressor unit. Before the annual overhaul in 2002, the steam turbine was comprehensively inspected for many times, and it was found that there was a large gap between the head of the four fixing bolts of the front bearing seat and the pressing plate, and the distribution was uneven. Looking along the direction of the steam turbine, the gaps of the four bolts were: 0.50mm from the front left, 0.60mm from the front right, 1.10mm from the rear left, and 1.20MM from the rear right

during normal operation of the steam turbine, due to the thermal expansion of the bearing seat, the expansion gap reserved between the head of the fixing bolt of the bearing seat and the pressing plate in the cold state is compensated, and the gap between the two should be basically in the state of no gap. It can be seen from figure L that the two fixing bolts on each side of the bearing seat have taken the anti loosening measures of connection spot welding, so the bolts will not loosen and exit by themselves. Therefore, the reason for the large gap between the head of the fixing bolt and the pressing plate can only be that the bearing seat sinks, and the downward displacement of the corresponding part of the bearing seat and the fixing bolt is roughly equal to the actual gap at this part. In the front bearing pedestal, the installation position of the radial bearing is close to the two support points at the rear end. Due to the large settlement at the rear end of the bearing pedestal, the actual settlement of the radial bearing is greater than the average settlement of the bearing pedestal (0.85mm)

the front radial bearing of the turbine sinks with the bearing seat. When it bears static low stress/tension, the front end of the turbine rotor supported on it sinks, and the sinking amount of the rotor is roughly equal to the sinking amount of the front radial bearing. After checking the data, the radial clearance at the front shaft seal of the steam turbine, the steam seal of the balance plate and the steam seal of the first several stages of the impeller is 0.20 ~ 0.40mm, so the downward displacement of the rotor is enough to make the front end of the rotor rub with the lower part of the front steam seal of the cylinder and the lower part of the diaphragm steam seal of the first several stages, causing the vibration of the rotor and the cylinder, and the vibration of the rotor drives the vibration of the bearing seat. Because the cylinder quality is higher than 9. Aluminum alloy impact block: it is made according to the standard provided by your company, and the impact is relatively small. Since the friction mainly occurs at the front end, the vibration rise of the front end of the rotor and the front bearing pedestal is larger than that of the rear end. Friction and vibration between dynamic and static parts also increase the noise on site

the uneven sinking of the four corners of the bearing seat also destroys the parallel state between the center line of the bearing seat and the center line of the rotor, making the working condition of the radial and thrust bearing pads worse; In addition, the bearing seat drives the rotor to sink, which also causes the radial position of the rotor in the cylinder to be seriously eccentric, which are important reasons for the rotor vibration

according to the analysis, there are three reasons for the sinking of the bearing seat: ① the wear between the lower part of the spherical pad and the adjustable support nut; ② Wear occurs between the upper part of the spherical pad and the lower bearing surface of the bearing seat; ③ The adjustable support nut is loose and moves down

3 troubleshooting

during the overhaul in 2002, the maintenance of the steam turbine of the synthetic gas compressor unit was focused on the front bearing seat. Lift out the front bearing seat for inspection, and it is found that the four adjustable support nuts in the four corner support system are well fastened. However, at the contact between the lower bearing surface of the bearing seat and the spherical pad, four annular pits with different depths were ground, with the depth of 0.20 mm at the front left, 0.40 mm at the front right, 0.50 mm at the rear left and 0.80 mm at the rear right. Check the radial pad and thrust pad of the front bearing, and there is eccentric wear. The steam seal teeth of the shaft seal at the front end of the steam turbine cylinder block, the balance plate seal and the impeller seal at all levels in the front half of the cylinder block are seriously worn. The front shaft seal teeth embedded on the rotor and the interstage steam seal teeth of the previous impellers are seriously worn

in the process of startup and shutdown, the front bearing seat of the turbine should move back and forth on its spherical bearing pad, but when the lower bearing surface of the bearing seat is worn out of the pit by the spherical pad, the upper part of the spherical pad will be partially embedded in the pit, which will seriously affect the normal front and rear sliding of the bearing seat

during the maintenance, the lower bearing surface of the front bearing seat was finished, the pits were eliminated, the worn steam seal teeth in the lower half cylinder were repaired, four sets of spherical pads and the lower adjustment system were replaced, and the front end shaft seal sleeve, balance disk steam seal sleeve, rotor and bearing were also replaced. After the front bearing seat is in place, readjust the height, levelness and left-right position of the bearing seat based on the cylinder block according to the original installation standard, so that the center of the bearing seat is concentric with the center of the cylinder block

in the more than one year since the start-up after maintenance, the shaft vibration value of the steam turbine of the synthetic gas compressor unit is less than 30 m. The type II sample is a rectangular strip sample with equal cross-section, and the shaft vibration value at the front bearing is not more than 20 m when running at full load and over rated speed. The shaft vibration value during the start-up and shutdown process is less than 50 m. The vibration of the shell of the front bearing pedestal of the steam turbine and the on-site noise are also greatly reduced

4 conclusion

the bearing pedestal of the four steam turbines used in this ammonia plant is supported by four spherical pads at four corners. The advantages of this support mode are that the bearing pedestal moves flexibly and the upper and lower positions are adjusted conveniently. Its disadvantages are that the bearing surface is small, and the supporting elements are prone to wear due to vibration and other reasons, causing the bearing pedestal to sink. Therefore, in the daily inspection and maintenance of large units, it is necessary to focus on the inspection of the bearing support system of the steam turbine, find problems in time, and take preventive measures. If necessary, measures can be taken to add a spare support plate between the supporting elements on both sides, as shown in Figure 1 (0.05 ~ 0.06 mm gap is reserved between the upper part of the spare support plate and the lower bearing surface of the coaxial bearing seat), which can effectively prevent the friction between the dynamic and static parts caused by the excessive lowering of the bearing seat

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