Page 459 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 439

2.0
y
Head (−)
Stable
reverse flow
1.5

1.0
Normal
operation
Transitional
flow
0.5
MQ U

0.0
−0.015 0.000 0.015 0.030 0.045
j (−)
Flow
FIG. 11.6 Behavior of a single-stage centrifugal compressor at positive and negative flow, show-
ing isentropic head coefficient vs. flow for machine Mach numbers from 0.57 to 0.85[15].

differential pressure transmitter across the compressor flow meter, an algorithm
in the control system, and a surge control valve with corresponding accessories.
System dynamic analyses are performed to evaluate centrifugal compressors
stations and/or pipelines that are subject to transient events, such as emergency
shutdowns, line packing, or pressure equalization or relief events. Various com-
mercially available modeling tools are available to predict the transient system
behavior. A transient analysis has to include the effects of mass storage, pipeline
friction losses, speed of sound, heat transfer in pipes and coolers, the dynamic
behavior of valves, the compressors, and the behavior of the relevant control
systems. Transient analysis can be applied to the surge control system of a cen-
trifugal compressor, the dynamics of the entire station, a single vent line, or
extend to cover an entire pipeline.

Transient Surge in Centrifugal Compressor Systems
The compressor control system protects the compressors, and will initiate rapid
shutdown or trip upon the occurrence of high vibrations, overspeed, loss of lube
oil, the presence of explosive gas mixtures, or other alarm conditions. During
these rapid shutdown events, it is preferred that the compressor stays out of
surge, to avoid damaging the close clearance components or the bearings of
the compressor.