Page 120 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 107

FIG. 3.72 Organic coating (left) and electroless nickel coating (right). (Photo courtesy of
Elliot Group.)

and fouling resistance, but they have low durability. Electroless nickel has good
corrosion and fouling resistance, and good durability, which offers additional
advantages where aggressive wash oil and water injections are utilized.

Driver-Specific Concerns With Centrifugal Compressors
Gas Turbine Driven
Gas turbine driven compressor systems are often employed because they can
often employ process gas as fuel, neglecting the need for high wattage electrical
requirements from the grid. The downside to this approach is that two turboma-
chines are coupled and must operate at the speed, thus changes in speed affect
the performance in both the driver and the driven machines. When a compressor
system does not require substantial speed variation, a gas turbine drive often
makes economic sense.

VFD Motor Driven
VFDs are utilized when an electric motor must operate over a speed range, as
opposed to a fixed speed. These devices can generate significant dynamic tor-
ques, which have the potential to excite torsional critical speeds. The frequency
content of the dynamic torques can vary substantially, and can occur at either
integer or noninteger multiples of running speed. Please see the section on
Torsional Rotordynamics or Chapter 7 (drivers) for further information related
to VFDs.

Multitrain Configuration
Parallel/Serial
Centrifugal compressors are often laid out in parallel or serial configurations.
Parallel systems provide extra capacity as well as inherent redundancy
(or sparing), while serial applications provide greater range of pressure ratios.