Seeking advice on the increased vibration of the drive side shaft after maintenance of a centrifugal compressor unit
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Our unit consists of an electric motor coupled with a gearbox driving a centrifugal compressor (operating at 5,450 rpm); the medium being processed is air. Following major maintenance, the rotor and other internal components were cleaned; the wear-prone last-stage gas seal was replaced, while no other components were replaced. During the test run after maintenance, the maximum vibration value on the drive side shaft was 54 μm (compared to 20 μm before maintenance), while the maximum X-direction vibration was 22 μm. On the non-drive side, the maximum Y-direction vibration was 8.4 μm and the maximum X-direction vibration was 2.8 μm. There were no vibration measurement points on the gearbox side. The vibration level was 30 at the start of operation, then it increased gradually; after about 1–2 hours, it stabilized at 54 μm (the changes in load during this period had little impact on the vibration levels). Subsequently, an inspection of the cylinder was carried out, and no signs of wear were found in the replaced gas seals. The clearances between the shaft bearings and the oil seals were within acceptable limits. There was some misalignment, but since there were vertical pins under both the gearbox and the compressor bearing housings, it was not possible to adjust the alignment left or right. Ten shims were added to the back of the shaft bearings, which eliminated that 10 units of misalignment. However, during the subsequent test run, there was no improvement in the vibration situation; it continued to increase gradually and stabilized at 54 μm. The gradual increase in temperature seems to be related to the operating temperature, but it’s not clear where the problem lies. I hope that some experienced friends will kindly offer their guidance and help!
Spectrum analysis and shaft orbit analysis must be performed to make a further judgment.
Well, can that single-channel measurement device used for spectral analysis be used to measure shell vibration as well? Is it necessary to use only the device designed for measuring shaft vibration online? Will the results be different?
Test the quality of the lubricating oil; if necessary, use an oil filter. Check the temperature of the lubricating oil by testing both heating and cooling conditions. Spectrum analysis is also very important
Do you mean a gradual increase in vibration as the speed rises? It might be worth checking the coupling. Is there a mass imbalance in the coupling bolts?
Well, the lubricant wasn’t analyzed, as the oil temperature remained the same as before the maintenance, so the oil temperature wasn’t changed. We have a single-channel analyzer. We used it to measure the shell vibration. The resulting spectrum plots vary depending on the direction and units used; however, most of them show that the 2nd harmonic is higher in amplitude, while the 1X and 3X harmonics are slightly lower. We’re not quite sure what’s going on.
The vibration does not increase as the rotational speed rises; the electric drive quickly reaches its rated speed, and then it takes another hour or two for the vibration to stabilize. There was a problem with the coupling: an original bolt was lost at that time, so we replaced it with a bolt of the same weight. We’re not sure if this had any impact; during the next maintenance session, we plan to send the coupling back to the manufacturer so that it can be fully balanced and the bolts can be adjusted properly.
The 2nd harmonic is caused by imbalance; check the alignment, or consider other sources of imbalance – it may be necessary to use a filtered filter machine in such cases
Well, making adjustments to the alignment didn’t work; I’ll take the time to do a dynamic balance adjustment. At first I thought that the dynamic balance would be related to the rotational speed, but the vibration continued to increase even after reaching the rated speed.
1. After removing the coupling during compressor maintenance, it’s necessary to recheck the alignment. If no adjustments have been made, then alignment is probably not the cause of the problem.
2. According to what you’ve said, the air seal was worn out, but the gap in the air seal should theoretically be larger than the gap in the radial bearing shells. Please verify whether the values are correct.
3. During maintenance, you cleaned the dirt from the impeller; this could have changed the impeller’s balance again, or there might have been wear on the impeller that wasn’t properly checked – did you check whether there was any play in the impeller’s end face? At such a high rotational speed, dynamic balancing is necessary; it’s not easy to carry out this maintenance task. Why not do it now? Just wait until the next major overhaul. . . .
2. According to what you’ve said, the air seal was worn out, but the gap in the air seal should theoretically be larger than the gap in the radial bearing shells. Please verify whether the values are correct.
3. During maintenance, you cleaned the dirt from the impeller; this could have changed the impeller’s balance again, or there might have been wear on the impeller that wasn’t properly checked – did you check whether there was any play in the impeller’s end face? At such a high rotational speed, dynamic balancing is necessary; it’s not easy to carry out this maintenance task. Why not do it now? Just wait until the next major overhaul. . . .
Check the slip pin system next time you park the vehicle