This paper explains the impact of the interaction between system characteristics and compressor characteristics, both under steady state and transient conditions, and the concepts to optimize and control the units. Process requirements for compression systems require the adjustment of pressures and flows through these compressors. Control concepts need to consider both the characteristics of the individual compressor, as well as the characteristic of the compression system. Multiple unit installations, or installations with multiple compressors per train require specific process control considerations to match the compressors with the process system behavior and the objectives of the station or system operator.
There are two objectives for compressor control: meeting the external process requirements and keeping the compressor within its operational boundaries. Typical control scenarios that have to be considered are process control, starting and stopping of units, and fast or emergency shutdowns. The interaction between a compressor and a compression system, in conjunction with control mechanisms and the compressor characteristic determine the operating point of the compressor in a given situation. For the single compressor the application of these control functions is fairly simple. For compressor applications with multiple compressors in series or parallel, multiple compressors driven by a single driver, multiple compressor trains operating together, or multiple suction or discharge headers the combinations of these control strategies can become very complex. External process objectives can be minimum suction pressure, maximum discharge pressure, or delivered flow. Compressor operational boundaries include surge, minimum speed, maximum speed, and in some instances minimum pressure rise (choke). The operating envelope of a centrifugal compressor is limited by the maximum allowable speed (or, for other control means, the maximum guide vane angle), the minimum flow (surge flow), and the maximum flow (choke or stonewall), and the minimum speed (Figure 1).