Page 132 - Intro Predictive Maintenance

Vibration Monitoring and Analysis 123

Machines with rotating elements are designed to generate vertical lift of the rotating
element when operating within normal parameters. This vertical lift must overcome
gravity to properly center the rotating element in its bearing-support structure;
however, because gravity and atmospheric pressure vary with altitude and barometric
pressure, actual lift may not compensate for the downward forces of gravity in certain
environments. When the deviation of actual lift from designed lift is significant, a rotor
may not rotate on its true centerline. This offset rotation creates an imbalance and a
measurable level of vibration.

Flow Instability and Operating Conditions
Rotating machines subject to imbalance caused by turbulent or unbalanced media flow
include pumps, fans, and compressors. A good machine design for these units incor-
porates the dynamic forces of the gas or liquid in stabilizing the rotating element. The
combination of these forces and the stiffness of the rotor-support system (i.e., bearing
and bearing pedestals) determine the vibration level. Rotor-support stiffness is impor-
tant because unbalanced forces resulting from flow instability can deflect rotating ele-
ments from their true centerline, and the stiffness resists the deflection.

Deviations from a machine’s designed operating envelope can affect flow stability,
which directly affects the vibration profile. For example, the vibration level of a cen-
trifugal compressor is typically low when operating at 100 percent load with laminar
airflow through the compressor; however, a radical change in vibration level can result
from decreased load. Vibration resulting from operation at 50 percent load may
increase by as much as 400 percent with no change in the mechanical condition of
the compressor. In addition, a radical change in vibration level can result from turbu-
lent flow caused by restrictions in either the inlet or discharge piping.

Turbulent or unbalanced media flow (i.e., aerodynamic or hydraulic instability) does
not have the same quadratic impact on the vibration profile as that of load change, but
it increases the overall vibration energy. This generates a unique profile that can be
used to quantify the level of instability present in the machine. The profile generated
by unbalanced flow is visible at the vane- or blade-pass frequency of the rotating
element. In addition, the profile shows a marked increase in the random noise gener-
ated by the flow of gas or liquid through the machine.

Mechanical Motion and Forces
A clear understanding of the mechanical movement of machines and their components
is an essential part of vibration analysis. This understanding, coupled with the forces
applied by the process, is the foundation for diagnostic accuracy.

Almost every unique frequency contained in the vibration signature of a machine-train
can be directly attributed to a corresponding mechanical motion within the machine.
For example, the constant endplay or axial movement of the rotating element in a
motor-generator set generates elevated amplitude at the fundamental (1X), second har-