High noise and vibration was found to occur on a compressor oil separator module following the rework of the internal diffuser and addition of a nozzle with blanking flange to allow for inspection.
An investigation revealed that the blanking flange on vessel wall had a natural frequency close to the compressor operating frequency which was causing resonance. The levels of vibration exceeded the EI ‘problem’ line.
Due to the critically of the machine an urgent temporary solution was required and therefore 3 No. possible options were designed to include adding mass to reduce the natural frequency, adding stiffness to increase the natural frequency and a Dynamic Vibration Absorber (DVA) to effectively cancel of the excitation energy. A temporary solution was required until the next shutdown in 2017.
Temporary DVA Solution
The DVA proved to be the most effective solution reducing the dominant vibration at 199.8Hz from 111mm/s to 12mm/s rms.
AVT Concern Factor* Before = 5.5
AVT Concern Factor* After = 0.6
*Concern factor is defined as the closeness of the worst case peak in the vibration spectra to the EI Concern line which is independent of frequency. A concern factor of 1 is on the EI Concern line and CF of 5 is on the problem line.