Page 266 - Root Cause Failure Analysis

COMPRESSORS

Compressors can be divided into three classifications: centrifugal, rotary, and recipro-
cating. This section identifies the common failure modes for each.

CENTRIFUGAL
The operating dynamics of centrifugal compressors are the same as for other centrifugal
machine trains. The dominant forces and vibration profiles typically are identical to
pumps or fans. However, the effects of variable load and other process variables (e.g.,
temperatures, inleddischarge pressure) are more pronounced than in other rotating
machines. Table 22-1 identifies the common failure modes for centrifugal compressors.

Aerodynamic instability is the most common failure mode for centrifugal compres-
sors. Variable demand and restrictions of the inlet airflow are common sources of this
instability. Even slight variations can cause dramatic changes in the operating stability
of the compressor.

The operating life also can be affected by entrained liquids and solids. When dirty air
must be handled, open-type impellers should be used. An open design provides the
ability to handle a moderate amount of dirt or other solids in the inlet-air supply.
However, inlet filters are recommended for all applications and controlled liquid
injection for cleaning and cooling should be considered during the design process.

ROTARY, POSITIVE DISPLACEMENT
Table 22-2 lists the common failure modes of rotary-type, positive-displacement
compressors. This type of compressor can be grouped into two types: sliding vane and
rotary screw.

254