Page 136 - Root Cause Failure Analysis

124 Root Cause Failure Analysis

Overhung or Cantilever
The cantilever design is more susceptible to process instability than centerline centrif-
ugal compressors. Figure 10-1 illustrates a typical cantilever design.

The overhung design of the rotor (Le., no outboard bearing) increases the potential for
radical shaft deflection. Any variation in laminar flow, volume, or load of the inlet or
discharge gas forces the shaft to bend or deflect from its true centerline. As a result,
the mode shape of the shaft must be monitored closely.

Centerline

Centerline designs @e., horizontal and vertical split case) are more stable over a
wider operating range but should not be operated in a variable-demand system.
Figure 10-2 illustrates the normal airflow pattern through a horizontal split-case com-
pressor. Inlet air enters the first stage of the compressor, where pressure and velocity
increases occur. The partially compressed air is routed to the second stage, where the
velocity and pressure are increased further. This process can be continued by adding
additional stages until the desired final discharge pressure is achieved.

lko factors are critical to the operation of these compressors: impeller configuration
and laminar flow, which must be maintained through all of the stages.

Figure 10-1 Cantilever centrifugal compressor is susceptible to instability (Gibbs 1971).