Page 141 - Root Cause Failure Analysis

Compressors 129

sions occur in a given span of time, resulting in a higher pressure. Air compressors are
designed to generate particular pressures to meet specific application requirements.

Other Pe~ormance indicators
Centrifugal compressors are governed by the same performance indicators as centrif-
ugal pumps or fans.

Installation

Dynamic compressors seldom pose serious foundation problems. Since moments and
shaking forces are not generated during compressor operation, there are no variable
loads to be supported by the foundation. A foundation or mounting of sufficient area
and mass to maintain the compressor level and alignment and to assure safe soil load-
ing is all that is required. The units may be supported on structural steel if necessary.
The principles defined in the section on Operating Dynamics in Chapter 8 for centrif-
ugal pumps also apply to centrifugal compressors.

It is necessary to install pressure-relief valves on most dynamic compressors to pro-
tect them, due to restrictions placed on casing pressure, power input, and to keep out
of its surge range. Always install a valve capable of bypassing the full-load capacity
of the compressor between its discharge port and the first isolation valve.

Operating Methods
The acceptable operating envelope for centrifugal compressors is very limited. There-
fore, care should be taken to minimize any variation in suction supply, back-pressure
caused by changes in demand, and frequency of unloading. The operating guidelines
provided in the compressor vendor’s O&M manual should be followed to prevent
abnormal operating behavior or premature wear or failure of the system.

Centrifugal compressors are designed to be baseloaded and may exhibit abnormal behav-
ior or chronic reliability problems when used in a load-following mode of operation. This
is especially true of bullgear and cantilever compressors. For example, a I-psig change in
discharge pressure may be enough to cause catastrophic failure of a bullgear compressor.
Variations in demand or back pressure. on a cantilever design can cause the entire rotating
element and its shaft to flex. This not only a€fects the compressor’s efficiency but also
accelerates wear and may lead to premature shaft or rotor failure.

All compressor types have moving parts, high noise levels, high pressures, and high-
temperature cylinder and discharge-piping surfaces.

POSITIVE DISPLACEMENT
Positive-displacement compressors can be divided into two major classifications:
rotary and reciprocating.