Page 243 - Intro Predictive Maintenance

234 An Introduction to Predictive Maintenance

Replacing vanes before they break is extremely important. Breakage during operation
can severely damage the compressor, which requires a complete overhaul and realign-
ment of heads and clearances.

Bearings. In normal service, bearings have a relatively long life. Replacement after
about six years of operation is generally recommended. Bearing defects are usually
displayed in the same manner in a vibration profile as for any rotating machine-train.
Inner- and outer-race defects are the dominant failure modes, but roller spin may also
contribute to the failure.

Rotary Screw
The most common reason for compressor failure or component damage is pro-
cess instability. Rotary-screw compressors are designed to deliver a constant volume
and pressure of air or gas. These units are extremely susceptible to any change in
either inlet or discharge conditions. A slight variation in pressure, temperature, or
volume can result in instantaneous failure. The following are used as indices of
instability and potential problems: rotor mesh, axial movement, thrust bearings,
and gear mesh.

Rotor Mesh. In normal operation, the vibration energy generated by male and female
rotor meshing is very low. As the process becomes unstable, the energy caused by the
rotor-meshing frequency increases, with both the amplitude of the meshing frequency
and the width of the peak increasing. In addition, the noise floor surrounding the
meshing frequency becomes more pronounced. This white noise is similar to that
observed in a cavitating pump or unstable fan.

Axial Movement. The normal tendency of the rotors and helical timing gears is to
generate axial shaft movement, or thrusting; however, the extremely tight clear-
ances between the male and female rotors do not tolerate any excessive axial move-
ment and, therefore, axial movement should be a primary monitoring parameter.
Axial measurements are needed from both rotor assemblies. If the vibration ampli-
tude of these measurements increases at all, it is highly probable that the compressor
will fail.

Thrust Bearings. Although process instability can affect both fixed and float bearings,
thrust bearings are more likely to show early degradation as a result of process insta-
bility or abnormal compressor dynamics. Therefore, these bearings should be moni-
tored closely, and any degradation or hint of excessive axial clearance should be
corrected immediately.

Gear-Mesh. The gear-mesh vibration profile also indicates prolonged compressor
instability. Deflection of the rotor shafts changes the wear pattern on the helical gear
sets. This change in pattern increases the backlash in the gear mesh, results in higher
vibration levels, and increases thrusting.