Centrifugal Compressor Applications—upstream and Midstream

Centrifugal compressors are used in a large number of different compression applications. This tutorial addresses applications for centrifugal compressors in the upstream and midstream sector, such as LNG compression, refrigeration, pipeline compression, compression for gas injection, gas lift, gas gathering, export compression, air compression, and others. The focus is on the specific compressor requirements for these upstream and midstream applications, not only regarding the design of the compressor, but also its performance characteristics and control requirements. Specific challenges from different process gases are addressed. To understand the interaction of the compressor with the system, different control mechanisms (speed, variable vanes, recycle, throttling) are explained, as well as the requirements for antisurge control. GENERAL CHARACTERISTICS OF CENTRIFUGAL COMPRESSORS The performance of centrifugal gas compressors is best displayed in a map showing isentropic efficiency and isentropic head as a function of the actual inlet flow. (The authors prefer isentropic head and efficiency to polytropic head and efficiency because the isentropic head can be directly derived from station design parameters [i.e., gas composition, suction temperature and pressure, discharge pressure], whereas the definition of the polytropic head requires the additional knowledge of the compressor efficiency or the discharge temperature. In particular, two compressors for the same process conditions will show the same isentropic head, but different polytropic head if their efficiency is different.) The means of control are added parameters. Figure 1 shows the map of a speed controlled compressor. Such a map also needs to define the operating limits of the compressor. Figure 1. Typical Compressor Map (Variable Speed). The limitation for lower flow is the surge limit. Some manufacturers also limit the operation of their machines in the choke region, while others allow the operation of their machines anywhere in choke, as long as the head remains positive. Other limits include the maximum and minimum speed, limits of vane settings, temperature limits and others. The head-flow map does not automatically define the temperature limits of the compressor because the discharge temperature depends also on the gas composition and the suction temperature. With the information in the head-flow-efficiency map and known suction conditions, it can be calculated. The speed limits are either rotor dynamic limitations or stress limits. It must be noted that a performance map as described will not change even if the inlet conditions are changed within limits. THERMODYNAMICS OF GAS COMPRESSION For a compressor receiving gas at a certain suction pressure and temperature, and delivering it at a certain output pressure, the isentropic head represents the energy input required by a reversible, 169 CENTRIFUGAL COMPRESSOR APPLICATIONS—UPSTREAM AND MIDSTREAM