CO2 Compression Using an Eight Stage, Integrally Geared, Centrifulgal Compressor

This paper discusses the design, specification, and installation of an integrally geared, 8-stage centrifugal compressor for CO2 pipeline service in North America. To the authors’ knowledge, this was the first, and remains the only, such application of this compression technology in a cross-country pipeline in North America. The facilities were installed in 1999 and became operational in 2000. Design considerations included compressing process CO2 from near atmospheric pressure to the super critical phase that allows for efficient transport of large volumes over long distances in a pipeline. In this phase, the CO2 behaves much like a liquid, but its physical properties (e.g. density and viscosity) vary much like a gas in response to temperature and pressure changes. In order to function properly in this application, the compressor must be flexible enough to compress the CO2 under varying inlet temperature and pressure conditions. Inlet guide vanes were used to regulate flow to the suction of alternating stages. The controls system for these vanes was required to closely monitor interstage conditions in order to make sure that no stage entered surge. Furthermore, the interstage coolers also required precise control to prevent overheating in summer or liquid formation in winter. Ambient design temperatures ranged from −50°F (−45°C) to +104°F (+40°C). A simplified system diagram is shown in Fig. 1. If the air-cooled heat exchangers remove too much heat, liquids will form and the subsequent compressor stage could be severely damaged. If the compressor unit, or any stage, enters surge (stalls), the machine can also be damaged. Consequently, bypass and recirculation schemes (see Fig. 1) were used to enhance safe operations. Ambient airflow to the coolers was controlled by louvers, which in turn were regulated by the compressor control system. These heat exchangers also employed recirculation plenums to permit recirculation of warm air during low ambient temperature conditions. The problem solved by the application of this technology was to efficiently compress large volumes of very low pressure CO2 into the super critical phase for pipeline transportation. The overall compression ratio across these units exceeded 180. In the past, CO2 compression for pipeline service has been accomplished by using numerous, multi-stage reciprocating compressors, over much narrower compression ratios (typically < 15).