Generator-loaded expanders

Fig ure 3-10. One of eight generator-loaded Rotoflow expanders used to convert geothermal energy to electricity. 

Temperature ehanges for the flue gas to the expander produee the effeets shown in Figure 4-63. The expander inlet temperature at design is 1,200°F. As the expander inlet temperature rises, the expander horsepower eurve moves to the left and upward while the ehange in the blower eurve is insignifieant. The results are that the lower horsepower balanee point moves to the left and down, the peak of the expander eurve moves to the left and up, the peak generator load inereases to G, and the expander bypass valve opens at a lower feed rate. 

Note that the inerease in peak generator load is not simply the amount of inerease in the peak expander horsepower. Sinee the peak has moved to the left as well as up, the eoineident horsepower required by the blower has been redueed and the equivalent of this reduetion is also added to the peak generator load. 

For FCC applications, a rigorous analysis typically involves transient evaluations of expander coupling failures, generator load drops, compressor and surge system operation, and control valve malfunctions. The results of these evaluations permit optimum selection of control valves and control strategies. 

Prevent runaway of the string in the case of a generator load loss. The large amount of available energy upstream of the expander must be dissipated. 

While many of the applications use the expander to drive a generator, a compressor is a good alternative candidate for the load. Expanders are generally custom-sized and can, therefore, be readily matched to the centrifugal or axial compressor. It also will match the screw compressor of the dry type, at least in the larger frames, Basic sizing to the compressor should follow the same guidelines as for steam turbines. 

Most ethylene plants operate continuously with the expanders operating at or near design conditions. If necessary, due to their unique design characteristics, radial inflow turboexpanders can accommodate a wide range of process conditions without significant losses in thermal or mechanical efficiency. Expanders may be loaded with booster compressors, gear-coupled generators, dynamometers, or other in-plant mechanical equipment such as pumps. In ethylene plants, turboexpanders are typically used in eitlier post-boost or pre-boost applications. 

During the normal mode (normal running operation), the brake is rotating at the speed of the generator. However, it is completely deenergized and, therefore, presents no load to the expander. 

Load sharing or selective load shedding is of interest to many users of hot gas expanders. A particularly successful European FCC application is illustrated in Figure 6-43. The addition of an expander-generator set to the FCC unit at a major refinery presented a challenge because a trip of the expander could upset the process. The company that is the subject of this application case study, GHH Borsig, solved this problem with the installation of a computerized control system and through computer simulation of trips. 

In the event that eertain faults oeeur in the eleetrieal equipment of the generator, the load eireuit breaker must be opened immediately. The result is that the maehine train is aeeelerated with the full power of the expander. Only if the inlet valves are elosed within 0.6 see ean exeessive overspeed be avoided. Eor this reason, both inlet valves must be able to elose within this time window in the event of an emergeney trip. 

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