Page 125 - Compression Machinery for Oil and Gas

112 SECTION II Types of Equipment

Natural gas can be transported over large distances in pipelines. Many trans-
mission pipelines require periodic booster compression, increasing pressure to
overcome pressure drop caused by friction in the pipeline. Optimal pipeline
pressures, depending on the length of the pipe, as well as the cost of steel,
are in the range of 4–16MPa balancing the amount of power required to pump
the gas with the investment in pipe. Most interstate or intercontinental pipeline
systems operate at pressures between 6 and 10MPa, although the pressures for
older systems might be lower. The gas usually has to be compressed to pipeline
pressure in a head station (usually coming from a gas plant). This head station
often sees pressure ratios near 3. The pipeline compressors are arranged in reg-
ular distances along the pipeline, usually spaced for pressure ratios between 1.2
and 1.8. The distinction is sometimes made between mainline stations (that
basically operate continuously) and booster stations that are only in operation
sporadically to assist mainline compression.
Subsea pipelines often only have a headstation, but no stations along the
line. They are either used to transport gas to shore from an offshore platform
(see export compression), or to transport gas through large bodies of water.
In either case, relatively high pressures (10–25MPa) are common.
A few onshore pipelines worldwide make use of the added super compress-
ibility of the gas at pressures above 14MPa (depending on gas composition) and
operate as “dense phase” pipelines at pressures between 12.5 and 18MPa. Not
only is natural gas transported in pipelines, but also CO 2 .CO 2 is noncorrosive as
long as it is dehydrated. Most applications transport CO 2 in its dense phase at
pressures above 14MPa, in particular to avoid two-phase flows when ambient
temperatures drop.
If the throughput of a pipeline has to be increased, two possible concepts can
be used: building a parallel pipe (looping), or adding power to the compressor
station (i.e., adding one or more compressors to the station), or a combination of
both. These means can also be combined. If power is added to the station, the
discharge pressure can be increased (assuming this is not already limited by the
pipeline maximum operating pressure). The station will therefore operate at a
higher pressure ratio. The added compressors can either be installed in parallel,
or in series with the existing machines. If the pipeline is looped, the pressure
ratio for the station typically is reduced, and the amount of gas that can be
pumped with a given amount of power is increased. In either scenario, the exist-
ing machines may have to be restaged (for more pressure ratio and less flow per
unit in the case of added power, for more flow and less pressure ratio in the
other case).
In general, pipelines that have many takeoffs, and interconnects (like in the
Unites States) tend to operate more frequently in transient, nonsteady-state con-
ditions. Long, transcontinental pipelines that basically transport gas from points
A to B, tend to operate closer to steady-state conditions. In most cases, compres-
sors experience a significant range of operating conditions (Fig. 3.73) [24].