Integrally Geared Centrifugal Compressors: Control Methods, Applications and Process Control

Integrally geared centrifugal compressors (IGC) are multi shaft centrifugal process compressors, that use multiple shafts (‘pinions’) driven by a central gear (‘Bullgear’). Each pinion drives one or two impellers. These machines are used in many gas compression processes, such as air separation, natural gas liquefaction, sCO2 bottoming cycles, fuel gas compression and plant air compression. The design allows to drive several (up to 10) compressor or expander stages via a central helical gear. The layout allows for different gear ratios between each pinion and the bullgear, so the speed can be changed after every other impeller, and therefore the speed of the impellers can be optimized based on the aerodynamic requirements. The stages have axial inlet and radial discharge. Therefore, it is easily feasible to cool the gas after every stage, and to use adjustable inlet and diffuser vanes for every stage when needed. Control methods include variable inlet vanes, adjustable diffuser vanes, speed variation, recycle and inlet throttling. The multitude of options allows to select control methodologies based on the characteristics of the process, and impacts the selection of anti surge control methods. Impact of control methods on the compressor maps, and the effect of combining multiple methods are discussed.