The increased size of Liquefied Natural Gas (LNG) plants worldwide has led to an increase in boil-off gas (BOG) flows. The BOG can be either liquefied again to LNG or compressed to higher pressure levels for use as fuel gas. Single shaft multistage centrifugal compressors are used to compress large volume of BOG at high pressures. This paper reviews design considerations for synchronous motor driven BOG centrifugal compressors operating at high discharge pressures. Several design features including compressor selection and sizing, auxiliary system, performance characteristics and testing are reviewed. The use of leading power factor synchronous motors to improve the power factor of the LNG plant is discussed. Capability curves of API 546 synchronous motors for operation in VAR control mode — for maintaining constant reactive power — are explained. The choice between the use of speed control or adjustable guide vanes for BOG compressors is discussed.

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