Page 424 - Compression Machinery for Oil and Gas

Downstream Chapter 10 403

Main Refrigeration Compressors
The design of LNG compressors involves large casing sizes, optimized impeller
designs, high inlet relative Mach numbers, three-dimensional (3D) flows, and
complex side stream mixing. Depending on the process technology used, the
refrigerants may be pure components such as propane, ethylene, and methane
or mixed refrigerants or a combination of these. The propane compressor, which
is typically used to precool the gas, is technically the most challenging machine
in terms of flow coefficient and inlet relative Mach number. Details of LNG
turbines and compressor technology have been presented by Meher-Homji
et al. [3] and much of the material is drawn from this reference.
Process optimization during compressor selection must be done in cooper-
ation with the compressor designer to ensure that compressor selections are
aerodynamically and mechanically robust while meeting process performance
and operability requirements. This is an iterative process involving the
compressor OEM, the process licensor, and the EPC team.

Compressor Configurations and Compressor Technology
Milestones
Compression of large flows of gas can be accomplished by the use of either axial
compressors that are more suited to lower pressure ratios or centrifugal com-
pressors that enable higher pressure ratios. Some LNG processes use both axial
and centrifugal compressors while others use only centrifugal compressors. A
multistage axial flow compressor is shown in Fig. 10.1 and a typical horizon-
tally split centrifugal compressor is shown in Fig. 10.2. There is a trend toward
the use of centrifugal machines due to their inherent robustness.

FIG. 10.1 Axial flow compressor used in LNG service.