Page 269 - Root Cause Failure Analysis

Compressors 257

Vanes
Vanes wear continuously on their outer edges and, to some degree, on the faces that
slide in and out of the slots. The vane material is affected somewhat by prolonged
heat, which causes gradual deterioration. The typical life expectancy of vanes in 100
psig service is about 16,000 hours of operation. For low-pressure applications, life-
times may reach 32,000 hours.

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 generally is recommended. Bearing defects usually are 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 also may contribute
to the failure.

Rotary Screw

The most common reason for compressor failure or component damage is process
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 mesh-
ing is very low. As the process becomes unstable, the energy due to 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 clearances between the male
and female rotors do not tolerate any excessive axial movement, and therefore, axial
movement should be a primary monitoring parameter. Axial measurements are
needed from both rotor assemblies. If there is any increase in the vibration amplitude
of these measurements, it is highly probable that the compressor will fail.