Gas Turbine Driven Generators

2.1 CLASSIFICATION OF GAS TURBINE ENGINES
For an individual generator that is rated above 1000 kW, and is to be used in the oil industry, it
is usual practice to use a gas turbine as the driving machine (also called the prime mover). Below
1000 kW a diesel engine is normally preferred, usually because it is an emergency generator running
on diesel oil fuel.
Gas turbines can be classified in several ways, common forms are:-
• Aero-derivative gas turbines.
• Light industrial gas turbines.
• Heavy industrial gas turbines.
2.1.1 Aero-derivative Gas Turbines
Aircraft engines are used as ‘gas generators’, i.e. as a source of hot, high velocity gas. This gas is
then directed into a power turbine, which is placed close up to the exhaust of the gas generator. The
power turbine drives the generator. The benefits of this arrangement are:-
• Easy maintenance since the gas generator can be removed as a single, simple module. This can be
achieved very quickly when compared with other systems.
• High power-to-weight ratio, which is very beneficial in an offshore situation.
• Can be easily designed for single lift modular installations.
• Easy to operate.
• They use the minimum of floor area.
The main disadvantages are:-
• Relatively high costs of maintenance due to short running times between overhauls.
• Fuel economy is usually lower than other types of gas turbines.
• The gas generators are expensive to replace.Aero-derivative generators are available in single unit form for power outputs from about
8 MW up to about 25 MW. These outputs fall conveniently into the typical power outputs required
in the oil and gas production industry, such as those on offshore platforms.
2.1.2 Light Industrial Gas Turbines
Some manufacturers utilize certain of the advantages of the aero-derivative machines, i.e. high powerto-
weight ratio and easy maintenance. The high power-to-weight ratios are achieved by running
the machines with high combustion and exhaust temperatures and by operating the primary air
compressors at reasonably high compression ratios i.e. above 7. A minimum of metal is used and so
a more frequent maintenance programme is needed. Easier maintenance is achieved by designing the
combustion chambers, the gas generator and compressor turbine section to be easily removable as a
single modular type of unit. The ratings of machines in this category are limited to about 10 MW.
2.1.3 Heavy Industrial Gas Turbines
Heavy industrial gas turbines are usually to be found in refineries, chemical plants and power utilities.
They are chosen mainly because of their long and reliable running times between major maintenance
overhauls. They are also capable of burning most types of liquid and gaseous fuel, even the heavier
crude oils. They also tend to tolerate a higher level of impurities in the fuels. Heavy industrial
machines are unsuitable for offshore applications because:-
• Their poor power-to-weight ratio means that the structures supporting them would need to be much
larger and stronger.
• Maintenance shutdown time is usually much longer and is inconvenient because the machine must
be disassembled into many separate components. A modular concept is not possible in the design
of these heavy industrial machines.
• The thermodynamic performance is usually poorer than that of the light and medium machines.
This is partly due to the need for low compression ratios in the compressor.
They do, however, lend themselves to various methods of heat energy recovery e.g. exhaust
heat exchangers, recuperators on the inlet air.
Figures 2.1 and 2.2 show the relative costs and weights for these types of machines.
2.1.4 Single and Two-shaft Gas Turbines
There are basically two gas turbine driving methods, known as ‘single-shaft’ and ‘two (or twin) shaft’
drives. In a single-shaft gas turbine, all the rotating elements share a common shaft. The common
elements are the air compressor, the compressor turbine and the power turbine. The power turbine
drives the generator.
In some gas turbines, the compressor turbine and the power turbine are an integral component.
This tends to be the case with heavy-duty machines.
The basic arrangement is shown in Figure 2.3.