Vibration Analysis Reveals Poor Motor Mounting
Structure
At the installation of a new 150 gal/min centrifugal pump
driven by a 125 HP motor, high vibration levels were found immediately
upon first start.
After several days of alignment checks, pump teardowns, bearing inspections,
motor uncoupled runs and vendor technical support, no solution was found.
The plant maintenance manager asked the company's vibration analyst to take
readings to troubleshoot the problem. After measuring extremely high vibration
levels,
resonance
was
suspected
and was found at the motor support
structure due to inadequate mounting design for the new installation.
Figure 1
The company’s vibration analyst took readings on the horizontal
pump/motor unit and found extremely high vibration levels focused in the horizontal
direction at the motor bearings on the order of 1 in/sec velocity with only
0.2 in/sec vertically. Directional resonance was immediately suspected.
The
analyst shutdown the unit immediately and performed a visual inspection
of the motor support structure. Twin "C" channel beams were set
to the foundation parallel, supporting one front and one back
foot
on the motor
on either side. No cross support was present and the six inches under the
motor was wide open.
Seeing this, the analyst performed a simple bump test with the
unit off and found a natural frequency at 3,600 cycles per minute in a
horizontal direction on the motor. The running speed of the motor was 3,585
RPM fully
loaded, which provides the driving force necessary to excite this natural frequency
resulting in a resonant condition. The bump test involves placing a vibration
sensor in the direction of interest and exciting the system with a three-pound
rubber hammer to excite random vibration in the range of motor/pump driving
frequencies. A good kick will do the same thing.
The vendor-designed motor support was
then temporarily stiffened through the use of a porta-power ram and the unit
was
started to confirm this resonance condition existed. The resulting vibration
levels in the horizontal direction dropped to 0.085 in/sec, confirming
an inadequate support design for the motor.
The final fix involved welding
bracing
across
the "C" channel at both the front and back feet areas of
the motor, creating a box support. This resulted in an even further
reduction
in vibration levels and eliminated the resonant condition
after only three hours of troubleshooting and repair, compared to the
three days of repeated teardowns and reassembly
upon initial installation due to high vibration.
The lack
of detailed specifications for installation of new machinery provided
to a vendor permits poor installation designs which have repeatedly resulted
in
resonance situations and early failure of associated components
in numerous installations.
Acceptance testing of all new installations and repairs
by vendors
is an integral piece in the care of company assets which, though
not in place at this time, was used by plant personnel who refused to accept
the extreme
vibration felt by hand and requested vendor action.
Our company
is now actively pursuing new installation guidelines (on the order of API
Recommended Practice
686), repair guidelines with root cause analysis findings and
acceptance criteria on new and rebuilt equipment and installations.
|
