CASE STUDY
High vibration of De-dusting Fan and Motor
As planned, yesterday the machine was stopped for further investigation and the
followings noticed.
Coupling issues
We took one set of reading before stopping the machine and high axial
vibration was noticed with 2X fundamental frequency for both fan and motor.
Alignment was rechecked and no severe deviation observed. The coupling
,model 80SPLX 225-140, found over stretched in position and measurement
revealed that the coupling assembly length is lesser by 3 mm than the design
hub to hub distance 140 mm. Further inspection of the coupling element showed
that the adapter ring got shifted radially from its normal position, forcing coupling
bolts to touch the spacer holes where it is supposed to be free to transfer the
torque through the shim pack as per this coupling design. Inspection of the
coupling bushings confirmed that the clearance between the machined bolts and
the bushings are higher than normal and due this higher clearance, the adapter
ring got shifted during normal operation resulted high axial vibration.
Overstretching and low coupling quality are suspected as the reasons for
previous coupling failure also, when the coupling hub to hub distance was
observed 144 mm against the coupling element length 137mm with an abnormal
overstretch of 7mm.
High axial vibration observed before stopping the machine for inspection
2X RPM frequency vibrations, due to coupling fault
Structural vibrations
The machine was stopped and one set of reading was taken for the motor
vertical direction in monitor mode.
Modulating vibration at 0.6X RPM ( 16.6 Hz)sub synchronous frequency was
noticed with max. 3 mm/s amplitude at MOV at stopped condition. This frequency
was observed during the normal running condition also along with the rotational
speed frequency ( see the previous mail below).
Subsequently solo run of the motor was carried out, no other fault frequencies
observed except 0.6X RPM (16.6 Hz) dominancy with 3 mm/s amplitude induced
by structural /pipe line vibrations.
Further analysis showed that the structural vibration is transferred to the machine
via the fan suction line, which is connected to many highly vibrating top floors
through piping connections.
There is a bellow in between the fan suction flange and the suction line.
However, the amplitude of vibration on both sides of the bellow found same,
confirms that the performance of the bellow is not adequate at present and the
vibration is transferring from the line to the machine through this bellow without
any change in amplitude..
Vibration analysis showed that no induced vibration is transferred to the machine
from the first floor steel structure ( where the machine is mounted) which is
isolated with resilient pads at the machine base frame.
Motor solo run, dominancy at 0.6X RPM ( 16.6Hz), no other faults observed
Suction pipe line vibration measured at 6th floor where it is connected to roll
crusher .Dominancy at 16.5 Hz
Base frame excitation
An offline bump test was carried out for the base frame vertical plate below motor
back foot and very high excitation of amplitudes with multiple frequencies are
noticed.
Further inspection showed that the stiffness of this vertical plate is very poor
compared to other 3 vertical plates of the same base frame. One of the reasons
for motor free end vertical high vibration could be the low stiffness of the vertical
plate.
Load run after corrections
Alignment was corrected to 0.03 mm in axial and radial (Rim and face method),
coupling dismantled and re assembled after centering( due to high clearances of
coupling bushings) the spacer and adapter ring, suction line was cleaned with
water wash, impeller washing carried out to remove deposits and load run was
carried out.
Overall vibrations are reduced but the motor vertical vibrations are slightly high,
modulating with 0.6X rotational frequency generated from structural vibrations.
The overall vibration measured at motor free end vertical is 6.3 mm/s, which is
slightly high.
Signature after alignment corrections, 0 .6X rotational frequency is still dominant
Following recommendations are made based on the above observations and
analysis ,
1. Replace the coupling ( The existing one can twist in its position any time
during normal operation , lead to high axial vibration)
2. Ensure the hub distance according to coupling length. ( Yesterday M/M
could not do this due to motor bolt bound)
3. Arrest the suction line vibration.
4. Check and replace the suction bellow if required.
5. Stiffen the motor side vertical base plate.
6. Check and replace the base resilient pads if required.
Regards
Sasikumar
Sr.Reliability Engineer
FERTIL
Mobile: 00971505112524
Tel: 009712 60 26673
Web site: www.fertil.com