Case – Adre meting: Surge
During startup a motor driven compressor (integrated gears) sometimes switches off because it reaches overcurrent.
The following charts show the trend:
Again we mainly see high vibrations during phase 1 & 2. On that axis there is unfortunately no keyphasor.
Zooming in on this event results in the following chart:
- An alternating number of revolutions per minute, from 13440 to 12550 cpm.
- A rising direct value axially.
• High radial vibration values, above the monitor’s full scale.
- High casing vibrations (20,1g pk and/or 7mm/s)
This phenomenon reminds us of a case of surge
A spectral analysis leads to the following image:
We can see that the frequency of the high peak drops along when the number of revolutions per minute increases.
That phenomenon can be attributed to the presumed pump behaviour.
The zooming noise drops in frequency (the pumps are slowing down) until it disappears completely.
This phenomenon is strongest when the axis of phase 3 & 4 revolves around 13000 cpm, as depicted on the following chart.
We understand surge/pumping as the machine’s behaviour at left hand side of the compressor map.
There is strong counterpressure and the compressor doesn’t succeed in pushing away its full flow. As a result some gas flows back, followed by another surge. This leads to a process of pulsation.
The change in flow direction and therefore also in force results in an axial movement of the axis. Because of the variation in forces the number of revolutions per minute will on those moments also be unstable.
The cause of the startup problems was clarified with this measurement.
In consultation with the manufacturer some adaptations were made to the inlet guide vanes to avoid this problem.