We have ID FAN with the specification Double-Stage Axial Flow Fan, Speed 750 RPM no variable speed drive. Each stage has 20 Blades, and each blade can rotate to control flow.
This is newly installed ID FAN, Before comissioning we did balancing, the 1xRPM reduce from 2.6 mm/s to 0.1 mm/s (Horz NDE, Opening Blade 0%) and when commissioning the vibration was fluctuative depends on opening blade pitch (See Attached File 1).
When there is Load the Vibration also very fluctuative (See Attached File 2), We see it as problem because the other ID FAN with same specification have stable vibration even the load change (opening blade change).
We suspect structure problem or flow problem, and the we try to take measurement on the casing and open the insulation. then suddenly the vibration dropped from 2.3 mm/s to 1.3 mm/s and it become stable even the load change. After we install back the insulation the vibration increase and it became unstable again (See Attached File 3).
Because there is no change on parameter process and the other id fan also stable, we don’t suspect flow problem anymore.
So what do you guys think? Is it Resonance?