Foothills Decompression/Recompression Facilities—Unique Power Recovery Application in Gas Pipelines

Foothills Pipe Lines Ltd. is the Canadian sponsor of the Alaska Natural Gas Transportation System (ANGTS), a pipeline project selected in Canada and the United States as the means for transporting Alaskan gas reserves to the lower 48 United States. Currently, certain Prebuild portions of the ANGTS are in operation delivering Canadian gas to U. S. markets. A recent system expansion of the Eastern Leg Prebuild to accommodate increased Canadian gas exports entailed the construction of Decompression/Recompression facilities at Empress, Alberta to enable high-pressure operation of the Foothills pipeline while maintaining gas stripping at existing lowpressure extraction plants. The general process of the Decompression/Recompression facilities involves the expansion of high-pressure pipeline gas to conditions acceptable to the low-pressure extraction plants, then the recompression of the residue gas for return to the pipeline at original pressure. By directing the inlet gas through turbo expanders coupled to brake compressors, a substantial portion of the expansion energy is captured and used in providing the first stage of gas recompression. Including supplemental conventional compression, the Decompression/Recompression facilities are capable of providing approximately 37MW (50,000 HP) for continuous gas recompression. Although power recovery with turbo expanders is relatively common in the gas processing industry, such an application for gas recompression in large gas pipelines is unique. This technical paper describes the Foothills Decompression/Recompression facilities with their utilization of turbo expanders for pipeline recompression service, emphasizing the process design as well as the characteristics of the rotating equipment.