Page 134 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 121

FIG. 3.83 Elliott 15MB11 hydrogen recycle compressor (DWG 1046291 suggested).

compressed air. Therefore we find air compressors, Nitrogen compressors and
Oxygen compressors. Compressing air or nitrogen usually does not put special
requirements on the seals, since the compressed gas is nontoxic. The amount of
leakage is therefore strictly based on economic and performance considerations.
However, being relatively heavy gases, the Mach number levels are often in the
high subsonic range. Machines either use adjustable inlet guide vanes or speed
control to adapt to the required process conditions. In all applications, keeping
the process gas oil free is of great importance Nitrogen compressors, besides in
air separation, are also used in small scale LNG applications, where Nitrogen is
used as the refrigerant in a reverse Brayton cycle. In other words, Nitrogen is
compressed, cooled and expanded through turboexpanders to reach the low
temperatures necessary for liquefying natural gas.

Maintenance Practices
Design for Maintenance
A centrifugal compressor is a long-term capital investment that is generally
depreciated over a 30-year span. As such, a compressor and its installation
are designed not just for long-term operation, but also for long-term
maintenance.
Serviceability is highly dependent on being able to access the compressor.
Compressor accessibility starts with the plant layout and installation plan. An