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Once considered an occasional “necessary evil,” fuel gas booster (FGB) compressors in turbine power generation facilities have become a common requirement. FGBs are installed because of insufficient gas pressure onsite or from nearby utility gas pipelines.
Historically, pressures up to 300 psig (~20 barA) were adequate for most industrial gas turbines and were often available onsite or at an economical distance. But modern gas turbines tend to run at much higher compression ratios, thus raising the inlet fuel gas pressure requirement.
Aeroderivative gas turbines, for example, can require inlet gas pressures that exceed 900 psig (~60 barA) to operate efficiently. To meet these higher gas pressure needs, a FGB compressor can be deployed to raise and ensure adequate fuel gas pressure for the turbine.
The requirements of these compressors are a wide turndown range and the ability to handle supply gas pressure fluctuations, while delivering constant discharge gas pressure to the turbine fuel system. In addition, FGB compressor systems must be able to deal with turbine load changes (part load or start-up) and gas composition changes which alter flow rate needs.
Gas turbines are frequently matched with a fuel gas booster compressor. Centrifugal compressors were first used for low compression ratio requirements and were popular for a variety of reasons. For instance, they required lower maintenance because of lack of wearing parts, lighter weight design of their rotating parts and because their gas filter and oil filter were easily maintained and replaced. Other advantages included being almost free of pulsation, vibration and oil. On the downside, centrifugal compressors do not respond well to changing gas conditions or gas turbine load reductions.
More in January/February 2013 issue of Turbomachinery International