Question ---- Two probes at the same location, 90 degrees to each other, with their vibration levels alternating between increasing and decreasing
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Dear friends: Our company has a centrifugal compressor, on which two vibration probes installed at the same location are angled at 90° to each other. The vibration level of one probe is on the rise, while that of the other is on the decline. What could be the cause of this? Note: The lubricating oil in this compressor has been oxidized by the material.
First, I would like to ask: is the trend of vibration rising and falling obvious? Which one is faster? You also mentioned that the lubricant is oxidized by the materials; if there are problems with the lubricant, the compressor’s performance will decline due to reduced lubrication and cooling effects, which in turn leads to poorer operational performance. The resulting effect is generally only an increase in vibration levels; a decrease is definitely not accurate
Take a look at the bearing bush temperature as a reference. If the temperature doesn’t change while the vibration levels rise and fall, it means that at least one of the vibration readings is incorrect! Just have an instrument checked!
By how much does each of the two values change, and at what rate does the change occur if it happens gradually? The magnitude of vibration is related to both the excitation force and the stiffness. For two different directions at the same measurement point, if they are unbalanced, the excitation force is the same, but the stiffness differs. So your situation might be caused by a rigid change. If it is oil film vortexing, the excitation force may also differ in the two directions. Actually, you don’t need to worry about whether their trends are aligned; it’s sufficient as long as the overall value doesn’t exceed the limit, and the change rates of the two aren’t too large.
Thank you all for your answers. I’d like to add some further details regarding the issues that weren’t clearly explained. Both probes are positioned at 45° relative to the horizontal direction. This compressor features 3 compression stages, with one low-speed shaft driving two high-speed shafts; a total of 8 vibration probes are installed on these two high-speed shafts. The problem I mentioned occurred in one of these probes – its vibration level increased from 25μm to 30μm over the course of a week, while the level of the other probe decreased from 43μm to 39μm. The remaining 6 probes showed almost no change, with only very slight decreases in their vibration levels. Additionally, I have a question for the person who posted on floor 4: What do you mean by \"change in stiffness\"?
Our machines have experienced the same issue before, especially when the speed changes; at both ends of the turbine, one pressure decreases while the other increases. It worked fine after being turned on and off a few times; I don’t know what caused the issue. Now, the two points that form a 90° angle with each other both increase or decrease in value, but one of them has already exceeded the threshold value, while the other is still quite low.
Since you are measuring shaft vibration, it is the stiffness of the oil film between the shaft and the rotor that has an impact. A change of a few micrometers doesn’t matter; there’s no need to worry about it. Let alone something big becoming small, hehe.
The data obtained from monitoring is not static; it changes over time. This variation is related to factors such as load and speed, as well as the installation location. As long as its trend of change is observed, and it does not keep increasing, that’s sufficient.
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Thank you all for your enthusiasm. Also, two pieces of equipment broke down in the past couple of days; I was busy dealing with those issues, which is why I couldn’t come and inform you of the results in a timely manner. I’m truly sorry for that. Here are the results: 1. We didn’t perform any repairs on this compressor. 2. The upward trend in vibration levels has basically stopped, with the values now fluctuating at around 29μm. 3. I consulted a professional testing company, and they said it might be due to fluctuations in the oil film. Once again, I apologize for not being able to inform you of the results sooner, and I thank you all once more for your help.