Integrate Compressor Performance Maps into Process Simulation

Variable-speed centrifugal compressors are used in a wide variety of industries and applications, including natural gas pipelines and processing plants, oil refineries and chemical plants, air-separation units, refrigeration and air conditioning equipment refrigerant cycles, gas turbines and auxiliary power generators, and many more. Some natural gas plants, for example, employ a propane-precooled, mixed-refrigerant natural gas liquefaction process consisting of a classical propane liquefaction cycle that precools both the natural gas feed and the mixed refrigerant followed by a mixed-refrigerant liquefaction cycle that provides the lowtemperature refrigeration needed to liquefy the natural gas. Compression is a key element of both refrigeration cycles. Dynamic process simulation offers many tangible and intangible benefits (1). Integrating the turbomachinery controls (TMC) into the dynamic simulation of a compressor system enables the engineer to select properly sized equipment that can cope with transient conditions (surge conditions in particular), reduces commissioning time through pre tuning of the regulatory controls, improves operator training by exposing operators to realistic simulated operating scenarios, and provides verified startup and shutdown procedures during site acceptance testing before startup takes place (2). Dynamic simulators for modeling processes that involve gas compression require a rigorous unit-operation model of a centrifugal compressor. The performance maps provided by compressor manufacturers are a critical element of this model. This article explains how to use the performance maps for variable-speed, fixed-inlet guide-vane centrifugal compressors for the dynamic simulation of a compressor system. Performance maps and their use in simulation A performance map describes how a compressor’s polytropic head and power vary with volumetric suction flowrate and rotational speed for a specific set of suction conditions (i.e., fluid molecular weight, pressure, temperature, compressibility, and isentropic exponent at the inlet of the compressor). Manufacturers typically provide performance maps as two sets of performance curves — a series of plots of polytropic head vs. volumetric suction flowrate, and a corresponding series of power vs. flow plots. Each pair of curves corresponds to a different rotational speed, and all pertain to the same suction conditions. Performance maps may be constructed for more than one set of suction conditions. Figure 1 presents two performance maps for a high-pressure mixed-refrigerant compressor for a natural gas liquefaction process. The top performance map corresponds to design operation, and the bottom map corresponds to derime operation, which removes rime (the granular ice that forms when supercooled droplets freeze rapidly on contact with a cold surface) from the compressor. These maps were generated from data obtained by manually digitizing the performance maps provided by the manufacturer; the irregularities in the curves are a result of the manual digitization process. Process simulators combine the performance maps with material and energy balances and thermodynamic relationships to predict the performance of the compressor under various operating scenarios. However, the performance maps supplied by the manufacturer are not broadly useful for process simulation because, strictly speaking, they apply only at the suction conditions and rotational speeds for which they Transforming the performance maps to a reduced coordinate system that is independent of suction conditions and rotational speeds allows these curves to be accurately incorporated into a process simulator. Grant Stephenson Honeywell Process Solutions Integrate Compressor Performance Maps into Process Simulation