MODERN COMPACT GAS TURBINE
Figure 1. This 30-MW aero-derivative unit is destined to drive a compressor at a process plant. Source: GE.Figure 1
shows an example of a modern compact ≈30-MW gas turbine for a compressor drive in a chemical processing unit.AN IMPORTANT TREND
The chemical industry, like other manufacturing sectors, increasingly is opting for aero-derivative gas turbines instead of traditional heavy-frame industrial-type gas turbines. The aero-derivative units offer higher efficiencies, greater flexibility, more compact design, lighter weight, better on-condition maintenance and faster parts replacement. They permit efficient control of speed and torque together with potential to use integrated and intelligent control and operation concepts. Aero-derivative gas turbines take advantage of advanced aircraft-engine technologies including modular design. This modularity enables replacing major components without removing the gas turbine from its support mounts. Aero-derivative gas turbines use antifriction bearings in contrast to the hydrodynamic bearings found in heavy-frame industrial-type gas turbines.Today, aero-derivative gas turbines can offer 10–15% greater efficiency than traditional heavy-frame industrial-type units. In a hot-end-drive gas-turbine configuration, the output shaft is at the gas turbine end, where the exhaust gas can reach high temperatures. In this design, the load transmission should be fitted through the exhaust duct. This raises several potential issues: long output-shaft length, high-temperature effects on the transmission shaft, exhaust-duct turbulence and pressure drop, and maintenance accessibility. Insufficient attention to any of these details could result in power loss, high vibration, shaft or coupling failure, and increased downtime for maintenance.In the cold-end-drive configuration, the output shaft extends from the front of the air-compressor. Thus, the driven equipment is accessible and relatively easy to service. However, this design also poses serious drawbacks. The inlet-air system should be configured to accommodate the output shaft and the driven equipment. The inlet duct should be turbulence free. The inlet-air system should provide vortex-free, uniform airflow throughout the operating speed range and various operating conditions. A poor design can lead to catastrophic problems. For example, inlet turbulence can induce surge in the axial air compressor that could result in complete destruction of the gas turbine. So, today, a hot-end-drive usually is chosen. Single-spool integral-output-shaft gas turbines primarily are used to drive an electric generator. This kind of turbine can’t supply the high torque required to start a large pump or compressor under full pressure. However, providing an electric motor with a variable speed drive (VSD) to help during startup can enable use of such a gas turbine in a train with a large pump or compressor.A split-output-shaft gas turbine could be considered as a single-spool gas turbine driving a free power turbine. The air-compressor-turbine shaft (air compressor and its turbine shaft assembly) isn’t physically connected to the power turbine shaft. Instead, they are coupled aerodynamically. This makes starting easier in a mechanical drive application. The gas turbine can attain self-sustaining operation before it picks up the load of the driven equipment. The power turbine could be designed to operate at the same speed as the driven equipment, potentially eliminating the need for a gear unit and its losses, which typically run 2–4%. This design is limited to the hot-end-drive configuration. In a dual-spool split-output-shaft gas turbine, independent low- and high-pressure air compressors and turbines generate the hot gases that drive the free power turbine.A gas turbine should be capable of supplying sufficient torque to drive a particular compressor or pump over its whole specified speed range under all ambient air conditions and operating situations. For example, increasing the fuel and air flow rates and anticipating the turbine inlet-guide-vane opening could help for a gas turbine startup.QUALIFYING A GAS TURBINE
To use a gas turbine in a mechanical drive application, particularly as a compressor driver, demands an extensive qualification process that includes studies and shop tests. Pay specific attention to the following components for a variable-speed-range application:• axial-compressor rotating and stationary blades;• turbine rotor and buckets; and• combustion systems.Gas turbine components are subject to a number of failure mechanisms such as low-cycle fatigue crack initiation, cycling crack propagation, disc burst and creep, all of which should be evaluated over the speed variation range. The ability of the air-compressor rotor and stator blades (and assemblies) to operate in the variable-speed range is important, particularly for single-shaft gas turbines, where the air-compressor shaft is directly coupled to the turbine shaft and driven-equipment rotor assemblies. Figure 2
shows an example of a single-shaft gas turbine rotor.