Page 462 - Compression Machinery for Oil and Gas

442 SECTION III Applications

compressor discharge pressure is increased as a function of the cumulative flow
into the cavity, as a result of filling the cavity with gas. Similar conditions can be
found in gas gathering applications where (on a much slower scale) the field
pressure (and with it the compressor suction pressure) declines as a function
of the cumulative flow out of the gas field. These fields additionally also have
a strong head-flow relationship, that is, increasing the flow at any given time
would lower the compressor suction pressure.
Often, rather than maintaining steady-state conditions, pipelines are oper-
ated under slowly or rapidly changing operating conditions. There are many
cases in which the transient system behavior must be considered. For example,
a pipeline can be operated by feeding more gas into the pipeline than what is
taken out of the system—otherwise referred to as line packing. While pipelines
in steady-state conditions have a unique station pressure ratio for a given flow,
in transient conditions, the steady-state relationships are no longer valid. For
example, if a centrifugal compressor receives more driver power, and increases
its speed and throughput rapidly, the station pressure ratio will react very slowly
to this change (Fig. 11.9). This is due to fact that initially the additional flow has
to pack the pipeline (with its considerable volume) until changes in pressure
become apparent. Thus, the dynamic change in operating conditions would lead

30,000

25,000

20,000

Head 15,000

10,000

5000

0
0 2000 4000 6000 8000 10,000 12,000 14,000
Flow
FIG. 11.9 Typical operating points if transient conditions are considered, in this case due to a fast
engine acceleration from 50% to 100% load (Kurz et al. [19]).