This study defines a pipeline system under typical conditions and discusses the system behavior for two compressors piped either in parallel or in series configuration. This paper discusses the behavior of centrifugal compressors in a two-unit gas pipeline station without a standby unit, if one of the units has to be shut down. The compressors are driven by variable speed drivers such as gas turbines or variable frequency electric motors. Both series and parallel arrangements are analyzed in a transient simulation mode to determine which operation mode is more advantageous. The simulation involves the modeling of the station and compressors, as well as the pipeline segments upstream and downstream of the station. The behavior of the compressors, as well as the flows and the storage effects in the pipeline, are described in detail. Significant differences between the two layouts become apparent. The paper also addresses the performance characteristics of the compressor and how these performance characteristics influence the conclusions.
The pressure and flow characteristics of pipelines as well as other factors may influence the arrangement of compressors in a station. This study defines a pipeline system under typical conditions and discusses the system behavior for two compressors piped either in parallel or in series configuration (Figure 1). At the design point, it is possible to achieve higher station efficiency with either the series or the parallel arrangement of the two compressors. If circumstances require the shutdown of one of the compressors, significant differences between the two layouts become apparent. The simulation results confirm the expectations that the series compressor design is the better selection, due to the fact that, with one unit out of service, the remaining unit will operate in a more efficient region of its performance map compared to the parallel design compressor. If one compressor is out of service, it will be shown that the decay in pipeline outlet pressure at the city gate (i.e., the transfer point for the pipeline) will be slower with the series compressor design and higher flow will be available for load distribution at this point.
The simulation was performed with the aid of the Unsteady-State Module (USM) to the Stoner Workstation Service (SWS) for Windows (SWSg-USM). This program provides digital computer simulation of the transient gas flow behavior in a piping system (Stoner, 1996; Stoner, 1999). The analysis procedure allows for the calculation of the time-varying flows, pressures, power and other variables and can determine the response of the system due to these and other variables. Compressor maps for the relationships between flow, head and efficiency, based on actual centrifugal compressor performance, can be implemented into the simulation (Kurz and Ohanian, 2003). The compressor models specifically cover the traditional and active portions of the compressor map from surge to choke. The program has capabilities to recognize surge and choke conditions (recycle flow, generate excess head, etc.) within bounds of available power and speed, but does not include specific performance data beyond surge and choke limits.