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MODULARIZATION AND PREFABRICATION HAVE IMPROVED ACCESS AS WELL AS OPERATIONS AND MAINTENANCE
Over the last few decades, gas turbine packaging has evolved due to improvements in energy efficiency, performance, capacity and power density. The development of new technologies along with tighter health, safety and environmental standards has raised reliability and availability. In addition, more compact package designs make for easier access, operation and maintenance.
The energy efficiency of gas turbines has made great progress over the past 40 years, attributed mainly to a better integration within the plants, particularly as regards heat recovery. In the last four decades, power and heat integration accounted for more than 60% of efficiency improvements in gas turbine packages.
While this is a rough figure, it highlights that the deep integration of power and heat in modern plants must be respected when it comes to gas turbine packaging.
There is a renewed interest in small-scale power and process plants, particularly in developing countries. As turbines scale down to below 100 kW in the case of small microturbines (at an installed cost of about 1,000 $/kW and efficiency of around 20%), packing of small turbines should be done in such a way as to minimize equipment requirements and the overall item count (as well as the number of process steps involved) to optimize capital costs and minimize footprints while obtaining high efficiency and reliability.
But the latest generation of large plants offers its own unique challenges. There can be single-shaft and multi-shaft variations, for example. Two-bearing designs are preferred for single-shaft gas turbines. Three-bearing solutions could cause alignment or others issues — in any case, they are only available for large single-shaft machines. The preferred casing type for large gas turbines is a horizontally split design with easy access.
But the importance of packaging has risen to even greater prominence as the time allotted for construction and commissioning has been reduced. This has occurred despite the fact that some areas of the process and power industries have seen throughput per train multiply from a factor of five to ten in the last few decades, and an overall efficiency leap of about 100%. As a consequence, greater modularization and prefabrication are becoming the norm for large and complex gas turbine packages.
The fuel system is another important part of a gas turbine package. Natural gas is the best fuel option if it is available, as this reduces maintenance costs, reliability issues and pollution. However, about 30% to 40% of all gas turbines operate on liquid fuels, which can vary from light liquid fuels (naphtha, kerosene and others) to heavy fuels. Fuel selection typically involves regional dependencies and preferences. For example, crude oil would be more attractive fuel for gas turbine units in oil pipeline services.
Whatever fuel is to be used within the package, it should be injected at a pressure of about 3 to 5 bar above the air-compressor discharge pressure. A well-designed fuel system with all required separators, multiple filtration stages, knock-out drum, heaters, accessories, auxiliaries, instruments and monitoring devices is a wise investment in any package. The payback is swift in terms of trouble-free operation.
Proper filtration for the inlet air, too, is a necessary part of reliable packaging, otherwise erosion cases and contamination should be expected. The minimum requirement could be a high-efficiency multi-stage air filter system to remove particles down to below 2 microns. Additionally, the starter system should be selected and sized with care. A properly designed hydraulic system is usually the best option for gas turbine packages.
Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.