Page 126 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 113

FIG. 3.73 Operating points collected over a 6-month period in a gas compression station with
three units in parallel. (Figure 23 from R. Kurz, S. Ohanian, M. Lubomirsky, On compressor station
layout, ASME GT2003-38019, 2003.)

As part of the transportation process in pipelines, gas can be stored in storage
facilities, which often use former gas fields, or salt caverns. This allows to bal-
ance differences in supply and demand on a seasonal or daily basis. The com-
pressors for storage applications have to be capable to compress high gas flows
at low-pressure ratios when the facility is empty, and the cavity pressure is close
to the gas pipeline supply pressure; in addition, it must compress low gas flows
at high-pressure ratios when the facility is filled, and thus the storage pressure is
much higher than the pipeline supply pressure. This requirement is often met by
the capability to operate compressors either in parallel or in series.
Usually, all applications upstream and including a gas plant are considered
“upstream” applications, while the applications related to bringing gas to the
ultimate users are referred to as “midstream.” Applications in refineries, chem-
ical, and processing plants are considered “downstream” applications [24].

Gas Transmission Compressor
Natural gas is transported over large distances via pipelines. This pipelines are
typically operated at pressures of about 100 bar, but there also pipelines at
higher pressures (up to about 240 bar in the case of subsea pipelines), and many
older pipelines at lower pressures (40 bar). In any case the gas will experience a
loss in pressure due to friction effects in the pipe. Therefore, we find gas com-
pressor stations every 50 to 100 miles along the pipeline which recompress the
gas. In order to minimize energy consumption along the pipeline, these com-
pressor stations operate at relatively low pressure ratios, typically in the range
of 1.2–1.7 [24a]. In many instances, the fluctuation in gas demand (on an
hourly, daily or seasonal basis) requires the compressors to operate over a wide