Page 72 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 63

stay pressurized and can be restarted without having to vent gas into the
atmosphere. Modern centrifugal compressors with DGSs are capable of this
“pressurized hold.”

Structural
Pressure Vessel
The compressor casing along with its nozzles acts as a pressure vessel that is
designed to seal in the process gas without fugitive emissions. Controlled leak-
age consists of properly working end seals where the emissions are captured.
Fugitive emissions consist of leakage across joints, flanges, endwalls, and
uncontrolled emissions across end seals. The compressor casing serves many
additional purposes acting to support all other components of the compressor
including diaphragms and endwalls. Bearing housings are attached to either
the endwall or the casing. The bearing housings in turn support the bearings
and the rotor.
The casing main inlet and discharge connections are flanged. These main
inlet and discharge flange connections typically coincide with ASME/ANSI
B16.1, B16.5, B16.42, or B16.47 series A or B as specified within the API
617 standard. Raised face (RF) flange designs are commonly used with either
a flat gasket Fig. 3.30 or spiral wound gasket typically with a solid inner ring to
prevent spiral wound gasket extrusion. Ring type joints (RTJs) flanges are also
used and are often preferred for their ease of assembly. Pressures in excess of
the standard ratings require more specialized piping connections such as API
6A design or Grayloc fittings. All flanges are designed to handle the hydrotest
pressure.
Many different casing materials have been applied throughout the years
including ductile iron, cast iron, and cast, forged or fabricated steels. Carbon
steel is commonly used for most simple applications as fabrication expenses
are less than for stainless varieties. On the other hand, corrosive environments

FIG. 3.30 Flange and gasket [5].