Gas turbines and centrifugal compressors are the preferred means of compressing the gas in pipeline systems. Both gas turbines and centrifugal compressors exhibit performance characteristics that depend on the operating point imposed on them by the pipeline operation. It is, thus, necessary for a pipeline simulation model to determine the performance of the gas compressor depending on the head and flow requirement of the operating point, and, subsequently, the performance of the gas turbine as a function of the compressor speed and absorbed power. Parameters such as site elevation, different gas and ambient temperatures, operating speed and changes in gas composition may have to be considered. This paper addresses the functional relationships of turbomachines and ways to easily implement them into simulations. It also addresses frequently encountered problems related to the simulation of turbomachinery.
Simulation of gas turbine-driven centrifugal compressors is necessary for a variety of applications in the general area of pipeline simulation, both for existing pipelines (e.g., for performance monitoring, predictive capability, optimization of the operation) and for planning or designing new pipeline systems (e.g., station sizing, transient simulations, capacity simulations, analysis of contracts (Brown and Rahman, 2002; Bowlin et. al., 2002). In order to simulate a system, it is necessary to understand its function under a variety of conditions. We will, therefore, start the treatment of this subject with an introduction to the operational characteristics of gas turbines and centrifugal gas compressors (Figure 1) (Brun and Kurz, 2000).
The pipeline compressor is typically a centrifugal compressor, either a single stage with overhung rotor or single, two or three staged with a beam style rotor. A stage consists of the inlet system (for the first stage) or a return channel (for subsequent stages), the impeller, the diffuser (either vaneless or with vanes), and after the last stage a discharge collector or (in more modern machines) a discharge volute (Figure 2). Beyond the quest for higher compressor peak efficiencies, the operating requirements imposed by typical pipeline duties (Kurz and Cave, 2000) require a compressor capable of operating over a wide operating range at high efficiency. Wide operating range in a centrifugal compressor can be achieved by a combination of means. Aerodynamic theory suggests a strong relationship between operating range, efficiency and impeller backsweep. However, there is a practical limit to the amount of backsweep. In particular, increasing backsweep reduces the capability of an impeller of given tip speed to make head. With the capability to use two impellers in a casing, this perceived disadvantage could be eliminated. The operating range is further increased by the use of a vaneless diffuser. The amount of backsweep allows for the control of the surge margin at the best efficiency point.