Page 461 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 441

Station Dynamics
The pipe system within which the compressor operates (regardless of compres-
sor type) will impose its characteristic on the compressor. There are three fun-
damental steady-state system characteristics that need to be considered
(Fig. 11.8):
A. strong head-flow relationship
B. weak head-flow relationship
C. integrative relationship
The case of strong head-flow relationship is, for example, seen in gas pipelines.
Under steady-state conditions, the pressure loss in the pipeline that imposes the
suction and discharge pressure on the compressor station increases significantly
when the flow through the pipeline has to be increased. The pressure levels are
thus dictated by friction losses, which depend on the gas velocity in the pipe.
In a weak head-flow relationship, the head requirement for the compressor
head stays more or less constant with changes in flow. This behavior is found
not only in refrigeration compressors, but also for situations where the process
dictates a constant suction pressure (e.g., separator pressure), while the dis-
charge gas is fed via a short pipe into a larger flowing pipeline, so the compres-
sor discharge pressure is more or less dictated by the pressure in the large
pipeline. Friction losses have therefore a very small effect, and this results in
very little changes in the pressure losses with flow.
In an integrative relationship, which exists, for example, in storage applica-
tions (Kurz and Brun [18]), the compressor fills a large cavity. That means, the

FIG. 11.8 System characteristics and compressor map.