Axial vibration problems in air separation centrifugal air compressors
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This centrifugal air compressor is used as a compressor for our company’s 3200 air separation unit; it features 4 stages of compression. At the beginning of November, operators noticed fluctuations in the vibration signal of the compressor’s shaft as shown on the DCS trend chart (there are 8 measurement points in total across the 4 stages, 2 points per stage). All 8 points showed a slight increase, by around 2 μm, before returning to their normal values after brief spikes. Since then, the system has remained under monitoring. After nearly a month of monitoring, it was found that similar fluctuations mainly occurred between 5 p.m. and 8 a.m. the following day, with the maximum amplitude reaching 4 μm; moreover, the value always returned to its original level after each sudden fluctuation. At around 7 a.m. on December 16, the largest simultaneous fluctuation was observed at all 8 measurement points, with a fluctuation amplitude of 20 μm. The entire period of this sudden change lasted about 1 minute, after which the values returned to normal. The manufacturer was contacted, and it was suggested that the issue might be caused by a change in the rotor’s dynamic balance or wear on the gear surfaces. However, if there is indeed a change in the rotor’s dynamic balance or gear surface wear, the vibration patterns should follow a certain pattern, with the vibration levels increasing after each fluctuation. The vibrations should not occur only between 5 p.m. and 8 a.m. the following day, nor should the vibration levels return to their values before the fluctuations. Upon inspection, no obvious problems were found with the contact points of the measuring devices or the DCS system. I would appreciate it if experts could help analyze the cause of the vibration in our company’s air compressors. Thank you! Note: Between 5 p.m. and 8 a.m. the next day, there can be up to 3-4 sudden spikes, with usually 1-2 spikes.
Are there any changes in the process during these fluctuations, such as gas flow rate or pressure? These occurrences usually take place at night; is there a large difference in temperature between day and night, which could lead to variations in air density and thus affect the compressor? Based on the described results, I believe there is no problem with the equipment itself!
During fluctuations, there are hardly any changes in parameters such as gas volume and pressure. If changes in air density are caused by temperature differences, then the load on the equipment should also change. Based on the operation records from recent years, the performance of the equipment during winter remains relatively consistent; therefore, we can rule out problems with the equipment itself. However, it is not clear whether there are any other factors at play.
Are there any fluctuations in DCS and signal measurement instruments?
The DCS shows fluctuations, but it is not possible to rule out the possibility of an incorrect display. What is certain at present is that when the site lighting is turned on, both the DCS and the on-site secondary meters show fluctuations, but the amplitude is no greater than 2μm.
There is similar experience, which is closely related to the activation of lighting switches.
If the on-site lighting fixtures are turned on and this causes fluctuations in the instruments related to the air compressor’s shaft system, then the electromagnetic compatibility is quite poor~~~~ One could try turning the lighting on and off to see what happens~~~~ If such an effect indeed occurs, then shielding measures will be necessary