I was referred an abnormal noise arising out of a motor driven screw compressor (4 lobe compressors) during unloading. Typically these compressors run at the same speed (1450 rpm in this case) for loading and unloading conditions. The operating pressure of these compressor under loading is 7 bar. I could see the 8th harmonic (spectra of sound pressure as attached)picking up in unloading. The operational excitation frequencies: Motor speed=1450rpm, Screw meshing freq=96.7Hz & its harmonics and the responses corresponds to them.

I was wondering why this 8th harmonic ( a couple of others as well) is picking up in unloading condition. The speed in both loading & unloading are the same, barring the slip at unloading condition.

Kindly provide your thoughts on this.

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I am assuming that you have an oil flooded screw compressor.  The unload noise is a result of too much oil between the rotors, because there is not any air.  If the inle t valve does not have any bypass on it, I would install one, sized for 10% of the compressor flow.  Put a check valve in to prevent backflow on shutdown.  If this is an older unit that has a bypass, clean out the check valve.

There is not any requirement to bypass air to maintain lubrication.  I ran a unit for a year unload with the inlet sealed with no detrimental effect as a test of this point.  The bypass air is there solely to eliminate the transition noise from load to unload.  You have a minimum pressure valve to protect the air/oil separator and there is a blowdown valve in the control system that tries to keep the sump a zero during unload.

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