Page 410 - Compression Machinery for Oil and Gas

Midstream Chapter 9 389

Pipeline Transmission Compression
Pipeline Design
A pipeline is more than just the piping itself. Compressor stations, spaced about
every about 40–100miles along the pipeline, are necessary to make a pipeline
work. The compressors pressurize the gas to ensure the gas continues to flow at
a constant rate.
Safe transmission pressures for interstate or intercontinental lines are typi-
cally considered to be 4–10MPa (600–1500 psi) with the majority of pipelines
operating near the bottom end to the middle of that pressure range. These lim-
itations are based on piping material strength, installation cost considerations,
flow velocities, noise concerns, and volume of vented gas for maintenance. The
hoop stress in the piping due to internal pressure is typically limited to between
40% and 72% of the yield strength of the material (typically 240–410MPa or
35,000–60,000 psi). This hoop stress typically limits the maximum allowable
operating pressure (MAOP). Operating temperatures range from ambient tem-
perature up to 128.93 °F, however, compressor station and processing plant pip-
ing can vary from 116 to 450K (-250 °F to + 350 °F) or more.
In long distance pipelines, gases typically travel at 4.6–12m/s (16–43km/h).
Liquids typically travel at 1.5–3.7m/s (1.6–13km/h). Traveling from Houston
to New York City ( 2660km) would take a batch of natural gas 3–7days and
a batch of oil or gasoline 9–23days.
As the gas travels through the pipe, it will have pressure losses directly cor-
related with the flow velocity and length of pipe. To boost the gas pressure,
compressor stations are installed every 48–112km throughout the transmission
system. A typical compressor station would include multiple reciprocating
or centrifugal compressors, filtering, gas cooling, and a significant amount of
above ground or buried piping. Compressor station total horsepower can
range from 7500 to 37,500kW. The distance between compressor stations is
carefully optimized between the capital cost of a compressor station and the
power requirement cost of compression. This trade-off is shown graphically
in Fig. 9.2 with varying pressure ratio curves relating the cost in terms of num-
ber of stations to the pipe diameter. Typical compression ratios are between
1.2 and 1.8.
The installation cost of a new pipeline can be very expensive. The average
cost of installation is roughly $40,000–$80,000/in. of diameter per mile of
length (as shown in Fig. 9.3). For a 36-in. diameter pipeline, this translates into
a cost of roughly $1.5–3.0 million dollars per mile of pipeline, or $1.5–3.0 bil-
lion dollars for a 1000-mile long pipeline. A compressor station can cost any-
where from $10 to 100 million each.
Most gas transmission companies have centralized control facilities for
managing the flow of fluids through these extensive piping networks to cus-
tomers. The central gas control stations collect and manage data from monitor-
ing equipment along the pipeline. Flow rate through the pipeline, operational