Page 426 - Compression Machinery for Oil and Gas

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Dry gas seals can have internal diameters of 350mm and journal bearing sizes
can be as high as 320mm. A representation of the current technology limits for
large LNG compressors is provided in Table 10.2.
Most of the LNG compressor casings are low-pressure horizontally split
designs. As per API 617 [4], when a relief valve is not specified, the maximum
allowable working pressure (MAWP) of the casing is 1.25 times the maximum
discharge pressure. Sometimes, off-design process conditions can exceed 1.25
times maximum discharge pressure. This can potentially cause problems with

TABLE 10.2 Current State-of-the-Art Technology Boundaries for LNG
Compressors
Casing
Weight of barrel, tons 120 tons
Power in single casing 100MW
Inner casing diameter, mm 3600mm
Shaft
Length, mm More than 7000mm
Bearing span More than 5800mm
Amplification factor Greater than 5
Log dec Greater than 0.1
Dry gas seal dia 350mm
Journal bearing dia 320mm
Impeller
Inlet relative Mach number 1.05 model test
Inlet relative Mach number 0.975 full scale
Machine Mach number 1.24 model test
Machine Mach number 1.20 full scale
Head coefficient Higher than 0.54
Head per impeller More than 4000m
Impeller diameter Up to 2000mm
Coupling
Size 350mm
Power transmitted 145MW