Page 434 - Compression Machinery for Oil and Gas

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High efficiency of large LNG compressors is an important parameter.
The efficiency of a centrifugal stage is a function of capacity and head and most
impellers in LNG service fall in the polytropic efficiency range of 82%–86%.
Further improvements in efficiency over the short term can be attained by
reducing internal impeller flow recirculation by utilizing low-clearance abrad-
able seals, the use of vaned diffusers on the discharge of the centrifugal com-
pressors in place of the vaneless (free-vortex) can also improve efficiency. For
axial compressors, efficiency improvement can be attained by reducing the
radial clearances on the rotor blade tips by the utilization of special seals.
As more impellers are added to the compressor, “off-design” operation gets
affected and stability inherently reduced. This occurs because the individual
impeller performance maps dictate the overall surge point. The situation is fur-
ther complicated by the large side load flows and the complexities of uniform
mixing at the side stream mixing plane. Impeller selection directly affects the
compressor’s overall operating range. Careful staging will allow the designer to
take advantage of each impeller’s best operating range.

Aerodynamic Efficiency—Range Considerations

The total stage efficiency is a function of impeller efficiency, stator component
efficiency, return channel efficiency, and exit scroll efficiency and all of these
play a part in the optimization of the machine. In the case of sidestream
machines that are typical in LNG service, the way in which the side load flow
is introduced and the induced flow and temperature distribution are also very
important and the subject of much analytical and experimental study. The injec-
tion volute design can be optimized for sidestream mixing by going with radial
injection rather than the traditional tangential injection. The radial injection has
much lower pressure losses coefficient. The advantage of tangential injection is
the reduced space required and therefore the reduced bearing span.

Compressor Design Trade-offs and Compromises
The design complexities, risks, and compromises involved in the selection and
design of large refrigeration compressors include aerodynamic and mechanical
issues and constraints. The final compressor design involves several interrelated
trade-offs between aerodynamics, rotordynamics, impeller stress, efficiency,
and operating range. Understanding the complexities requires an appreciation
of these interactions. It is not advisable to set absolute limits on certain param-
eters as one might do for more traditional compressors and therefore a case-by-
case study has to be made of each compressor service. A valuable discussion of
the trade-offs involved in compressor design is provided by Sorokes [6].