Aircraft-Derivative Gas Turbines - What Is It?
Aero-derivative. As the name indicates, these are power generation units, which originated in the aerospace industry as the prime mover of aircraft. These units have been adapted to the electrical generation industry by removing the bypass fans, and adding a power turbine at their exhaust. These units range in power from 2.5 MW to about 50 MW. The efficiencies of these units can range from 35–45%.
Aero-derivative gas turbines consist of two basic components: an aircraft derivative gas generator, and a free-power turbine. The gas generator serves as a producer of gas energy or gas horsepower. The gas generator is derived from an aircraft engine modified to burn industrial fuels. Design innovations are usually incorporated to ensure the required long-life characteristics in the ground based environment.
In case of fan jet designs, the fan is removed and a couple of stages of compression are added in front of the existing low-pressure compressor. The axial-flow compressor in many cases is divided into two sections: a low pressure compressor followed by a high-pressure compressor.
In those cases, there are usually a high-pressure turbine and a low-pressure turbine, which drive the corresponding sections of the compressor. The shafts are usually concentric thus the speeds of the high-pressure and low-pressure sections can be optimized.
In this case, the power turbine is separate and is not mechanically coupled; the only connection is via an aerodynamic coupling. In these cases, the turbines have three shafts, all operating at independent speeds. The gas generator serves to raise combustion gas products to conditions of around 45–75 psi (3–5 Bar) and temperatures of 1300–1700 ◦F (704–927 ◦C) at the exhaust flange.
Figure 1-10 shows a cross section of an aero-derivative engine.
Both the Power Industry and the petrochemical industries use the aircraft-type turbine. The Power Industry uses these units in a combined cycle mode for power generation especially in remote areas where the power requirements are less than 100 MW.
The petrochemical industry uses these types of turbines on offshore platforms especially for gas re-injection, and as power plants for these offshore platforms, mostly due to their compactness and the ability to be easily replaced and then sent out to be repaired.
The aero-derivative gas turbine also is used widely by gas transmission companies and petrochemical plants, especially for many variable speed mechanical drives. These turbines are also used as main drives for Destroyers and Cruise Ships. The benefits of the aero-derivative gas turbines are:
1. Favorable installation cost. The equipment involved is of a size and weight that it can be packaged and tested as a complete unit within the manufacturer’s plant. Generally, the package will include either a generator or a driven pipeline compressor and all auxiliaries and control panels specified by the user. Immediate installation at the job site is facilitated by factory matching and debugging.
2. Adaptation to remote control. Users strive to reduce operating costs by automation of their systems. Many new offshore and pipeline applications today are designed for remote unattended operation of the compression equipment.
Jet gas turbine equipment lends itself to automatic control, as auxiliary systems are not complex, water cooling is not required (cooling by oil-to-air exchanges), and the starting device (gas expansion motor) requires little energy and is reliable. Safety devices and instrumentation adapt readily for purposes of remote control and monitoring the performance of the equipment.
3. Maintenance concept. The off-site maintenance plan fits in well with these systems where minimum operating personnel and unattended stations are the objectives. Technicians conduct minor running adjustments and perform instrument calibrations.
Otherwise, the aero-derivative gas turbine runs without inspection until monitoring equipment indicates distress or sudden performance change. This plan calls for the removal of the gasifier section (the aero-engine) and sending it back to the factory for repair while another unit is installed.
The power turbine does not usually have problems since its inlet temperature is much lower. Downtime due to the removal and replacement of the Gasifier turbine is about eight hours.
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