Example of Experience AC Centrifugal Air Compressor Rotor Fault Diagnosis The comparison between Wang Bin's shape and spectrogram of Lin Limao Petrochemical Ethylene Industry Co., Ltd. of China Petroleum Second Construction Machinery Repair Branch found that the phase of the vibration wave changes with the increase of temperature. The conclusion that the rotor is defective, and the part where the defect may exist is inferred.
1 Introduction After the new main steam generator of Yan'an Refinery was put into operation, the standby main fan of the 800,000-ton/year re-heating device was put into operation. The vibration value of the unit gradually increased. The cumulative operation was about half a year, and the vibration value was reported and the parking was stopped. After several driving, it was unsuccessful and the vibration situation worsened. Before the overhaul, we conducted a test and monitoring of the equipment to find out possible problems and provide guidance and basis for the maintenance work.
2Monitoring and analysis 2.1 After the oil, water, electricity system and instrument are put into use normally, the electric disk car is used, and the electric car speed is 99r/min. There are two at the coupling end and the free end of the machine. A set of magnetic induction displacement probes that are perpendicular to each other. When the vehicle is cranked, the Bentley table shows that the vibration values ​​of the coupling end are 2μm and 0μm, and the vibration values ​​of the free end are 20μm and 25μm. This is obviously abnormal, and the free end vibration waveform The figure and the spectrogram are shown in Figure 1 (25 μm measurement point).
From the vibration waveform diagram, the upper part of the waveform is smooth, the displacement is almost zero, and there is a periodic pulse downward. The shape is short and sharp. The displacement value is about 25μm, which is equal to the full reading of the Bentley meter. The period is about 600ms. That is, the crank cycle is viewed from the spectrogram, and the vibration displacement is mainly composed of the main frequency component and has a small amount of frequency doubling component.
The above phenomenon has the typical characteristic of the flaw at the shaft diameter of the vibration measuring shaft, and almost no vibration is detected on the bearing shell of the free end of the compressor, which further confirms this inference. Moreover, the pulse period is the same as the crank cycle, and the frequency spectrum is dominated by the main frequency component, indicating that the number of shaft diameter flaws is one.
During the subsequent test run, the pulse generated by this flaw will be superimposed on the vibration waveform, and its influence on the Bentley reading will depend on the phase difference between the sine wave and the main frequency vibration, if the pulse appears near the peak position. It will weaken the vibration peak-peak, making the Bentley reading smaller than it actually is. If it appears near the trough position, it will make the trough deeper, thus increasing the Bentley reading.
2.2 After the low-load starting system has the test conditions, the compressor outlet is completely vented, and the low-load test is carried out. During the process from the start to the steady state to the rated speed of 5708r/min, the vibration value of the unit is large. At the critical speed, the coupling end vibration is 70μm and 60μm, the free end vibration value is 70μm and 72μm. Under the rated speed, the vibration value of the coupling end is 65μm and 63μm, and the vibration value of the free end is 65μm and 75μm. The maximum distance from the unit vibration low reporting point (85μm) is only 10μm. The waveform and spectrum of the measuring point are shown in Figure 2.
The vibration value of the coupling end is 65 μm, and the waveform and spectrum of the measuring point are shown in Fig. 3.
It can be seen from Figures 2 and 3 that the vibration frequency is mainly concentrated at about 95 Hz, whether it is the coupling end or the free end. The frequency is the rotation frequency of the compressor, that is, the main frequency, while the components of other frequencies are very small. The waveform of the measuring point is basically a sine wave with a period of about 10 ms, which is the rotation period of the compressor rotor of 10.5 ms. These reasons for the vibration are the rotor dynamic imbalance.
As a result of the superposition of the flaw pulse and the vibration waveform, the pulse appears near the peak position, weakening the vibration peak-peak. This indicates that the center angle between the rotor unbalance weight and the flaw is small, that is, the two are substantially on the same side of the rotor. As shown in Figure 4.
2.3 During the low load test of the load test, the vibration value increases slightly. After a certain period of time, the compressor outlet valve is gradually closed. Each time the load is increased by 25, it is stable for a period of time, and the back pressure is increased stepwise to carry out the load test.
In the initial stage of increasing the load, the vibration value did not change significantly, but as the back pressure continued to increase, the vibration value also began to increase.
When the load reaches 75, the vibration value is no longer stable, and it is directly shocked to the high-report line of 120 μm. Before the emergency stop, the vibration value of the coupling end has reached 92μm and 85μm, while the free end vibration value is 95μm and μm. The waveform and spectrum of the μm measuring point can be seen from Figure 5, the dynamic imbalance is still caused. The main factor of vibration is large, and the amount of imbalance increases.
And the spectrogram, except for the difference in amplitude, the vibration waveform also changes significantly, and the pulse caused by the flaw moves to the vicinity of the valley of the sine wave, and the phase difference is about 180°. Obviously, the position of the scar relative to the axis is impossible to change. The only reasonable explanation is that the center of the rotor has changed, which changes the phase of the sine wave. See Figure 2.4. The parking is too loud due to the vibration of the unit, and the vibration value is still rising. Therefore, it is urgent to stop when it fails to reach full load. It is worth mentioning that when the unit decelerates over the resonance frequency, the vibration value exceeds the full range of the Bentley table, and the vibration value reaches 1000 μm or more. The base, the pipeline and the pipe groove cover around the unit have strong vibration, and the rotor is visible. The amount of unbalance has reached a very serious level, and the vibration situation when the unit has just started the critical section is completely different.
3 Fault diagnosis During the test run, as the load gradually increases, the pressure and temperature of the medium gradually increase. As the test time increases, the center of mass of the rotor shifts. The pressure distribution is approximately axisymmetric with respect to the rotor axis, and its effect on the center of mass of the rotor is weak. The possibility of significantly affecting the deformation of the rotor is temperature. The temperature of the outlet medium has reached 120 °C when parking.
According to the test phenomenon and test data analysis, we conclude that the air compressor rotor may have defects, the defects are not exposed at normal temperature, and when the medium temperature rises, heat transfer to the rotor causes the rotor to deform and balance. Destroyed, manifested as strong main frequency vibration. The defect may be the following two situations: (1) The rotor set is matched with too small interference. The rotor is equipped with impellers, bushings, balance drums, etc. These parts are generally hot-mounted on the main shaft. Big overweight. It must be ensured that at the rated working temperature, there is still sufficient interference between the kit and the shaft to withstand the centrifugal force, axial pressure, shear force, thermal stress, etc., without relative movement of the parts. Maintain the necessary stiffness and strength of the shafting.
Once the fit of the kit is too small, during the heating process of the medium, the kit is first heated, the temperature rise of the spindle lags, and the interference disappears, and even a gap is generated, which destroys the strength, rigidity and integrity of the shaft. In addition, the set is a dynamic balance with the main shaft, and the single piece itself does not necessarily maintain balance. If there is a gap between the main shaft and the main shaft, the main shaft cannot hold the radial position of the set, and the set will shift toward the center of gravity. In turn, the dynamic balance of the rotor is destroyed.
(2) Memory effect of metal Many metals and alloys have a memory effect, that is, internal stress is generated in a specific temperature range, and the stress has a tendency to return the part to the original shape at the temperature. If the spindle heat treatment process is not well mastered, this phenomenon may exist.
4 Inspection and repair situation During the maintenance process, the rotor was subjected to a cold dynamic balance experiment, and the dynamic imbalance amount exceeded the standard. Remove the kit one by one, review the dimensions, and find that the interference on the sleeve of the rotor is too small, and there is only one single button on the sleeve to prevent rotation. The center of mass of the sleeve is not on the axis. After the heat is generated, the sleeve is sleeved. The rotor is balanced by the other side of the keyway, as shown in Figure 7.
In addition, in the free end of the vibration measuring shaft diameter, in the same direction as the sleeve keyway, there is indeed a scar, it is understood that the last time the bearing was injured.
In summary, this monitoring skillfully utilizes a flaw on the shaft diameter of the vibration measurement, monitors the vibration phase of the equipment without phase measurement points, infers the defects of the rotor, and reduces the blindness of the detection. Successfully guided the inspection work.
Lin Li 730060 Gansu Lanzhou Lanzhou Refining and Petrochemical General Plant, China Petroleum Second Construction Machinery Repair Branch, Third Workshop
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