Page 175 - Compression Machinery for Oil and Gas

Integrally Geared Compressors Chapter 4 163

Power Generation
Fuel gas compressors are often employed in gas turbine-driven power genera-
tion applications. In such plants, low-pressure gas delivered from the pipeline
must be pressurized to match the gas turbine requirements. Often, a multistage
IGC is used, and dry gas seals are required to minimize the amount of flamma-
ble process fluid escaping to the atmosphere. These applications frequently
have requirements for explosion-proof motors.
Ongoing research is focused on the development of high-pressure/high-
temperature IGC expanders for supercritical carbon dioxide power generation
systems. These applications have the advantage of extremely high efficiency
cycles, but place extreme challenges on sealing technologies.

Basic Industries
IGCs are used for different processes in the basic industries. For ammonia plants,
IGCs are employed as process air compressors and natural compressors. In urea
processes, IGCs are used as high-pressure carbon dioxide compressors at pres-
sures up to 20MPa. In nitric acid plants, IG turbomachinery can be used as pro-
cess air compressors, nitrous gas compressors, and waste gas expanders. All of
these duties can be combined on a single gearbox, which is referred to as a com-
pander. The largest IGCs to date (driver power up to 60MW) are used as process
air compressors for terephthalic acid plants. These trains consist of a steam tur-
bine driver, the IGC, a motor/generator, and a waste gas expander. The IGC dou-
bles as a reduction gear between the steam turbine and the motor/generator, and
the waste gas expander can be a multistage IG expander as well.

Summary
An IGC is a compressor architecture that incorporates a gearbox into the main
body of the compressor unit. This chapter discussed the differences between
IGCs and other compressor architectures. Advantages of IGCs include the abil-
ity to operate different compression stages at respectively optimal design
speeds, intercool between stages to reduce compressor work, increase peak per-
formance and operating range with VIGVs and VDVs, and the fact that IGCs
tend to be more compact than inline centrifugal or reciprocating compressors
sized for a similar duty. In addition, IGCs offer a highly modular architecture
and can handle multiple process streams in a single unit.
Some of the main disadvantages of IGCs are the result of having multiple
pinion shafts and separate inlets/exits for each stage. Specifically, each pinion
requires its own bearings, and each stage requires its own shaft seal. This can
add significant cost and can adversely affect the reliability of IGCs when com-
pared to inline compressors, which only have two bearings and two shaft seals
regardless of the number of stages.